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Petrologic origin of exsolution textures in mantle minerals: Evidence in pyroxenitic xenoliths from Yakutia kimberlites

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Exsolution lamellae in pyroxene and garnet porphyroblasts in pyroxenite xenoliths from the Mir, Udachnaya, and Obnazhennaya kimberlites (Siberian Craton) reveal a diverse suite of exsolved phases, including oxides (spinels, ilmenite, rutile, and chromite), pyroxene, and garnet. Textural characteristics suggest that exsolved phases progressively increased in volumetric proportions, and in some cases, the bulk xenoliths transformed from a lithology dominated by coarse grains (i.e. > 2 cm; megacrystalline) to a significantly finer-grained texture (i.e. < 1cm). These exsolved lamellae are the result of a complex and protracted sub solidus history following magmatic crystallization. Equilibrium pressure–temperature estimates place these xenoliths at low-to-moderate pressure–temperature conditions (690–910◦C and 2.0–4.5 GPa) in the lithospheric mantle at the time of entrainment in the kimberlite. However, reconstructed compositions of initial pyroxene and garnet crystals suggest that this suite of pyroxenites formed at considerably higher temperatures and pressures that, in some instances, may have approached the majorite stability field. Pyroxenites that do not contain primary garnet may have been derived from shallower depths. Progressive exsolution in these pyroxenites is of importance inasmuch as such processes can permit localized changes in rheological properties and may also accommodate strain within portions of lithospheric mantle. Because most xenolith studies focus on peridotites and eclogites, the pyroxenite sample suite studied in this work represents an important contribution towards a greater understanding of the Siberian lithospheric mantle.
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Petrologic origin of exsolution textures in mantle
minerals: evidence in pyroxenitic xenoliths from
Yakutia kimberlites
Taisia Aleksandrovna Alifirova a , Lyudmila Nikolaevna Pokhilenko a , Yuriy Ivanovich
Ovchinnikov a , Cara Lyhn Donnelly b , Amy J.V. Riches b & Lawrence August Taylor b
a V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of
Science, Novosibirsk, Russia, 630090
b Department of Earth and Planetary Sciences, Planetary Geosciences Institute, University
of Tennessee, Knoxville, TN, 37996, USA
Available online: 30 Nov 2011
To cite this article: Taisia Aleksandrovna Alifirova, Lyudmila Nikolaevna Pokhilenko, Yuriy Ivanovich Ovchinnikov, Cara Lyhn
Donnelly, Amy J.V. Riches & Lawrence August Taylor (2012): Petrologic origin of exsolution textures in mantle minerals:
evidence in pyroxenitic xenoliths from Yakutia kimberlites, International Geology Review, 54:9, 1071-1092
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International Geology Review
Vol. 54, No. 9, July 2012, 1071–1092
Petrologic origin of exsolution textures in mantle minerals: evidence in pyroxenitic
xenoliths from Yakutia kimberlites
Taisia Aleksandrovna Alifirovaa*, Lyudmila Nikolaevna Pokhilenkoa, Yuriy Ivanovich Ovchinnikova, Cara Lyhn
Donnellyb, Amy J.V. Richesband Lawrence August Taylorb
aV.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Science, Novosibirsk, Russia 630090;
bDepartment of Earth and Planetary Sciences, Planetary Geosciences Institute, University of Tennessee, Knoxville, TN 37996, USA
(Accepted 25 July 2011)
Exsolution lamellae in pyroxene and garnet porphyroblasts in pyroxenite xenoliths from the Mir, Udachnaya, and
Obnazhennaya kimberlites (Siberian Craton) reveal a diverse suite of exsolved phases, including oxides (spinels, ilmenite,
rutile, and chromite), pyroxene, and garnet. Textural characteristics suggest that exsolved phases progressively increased
in volumetric proportions, and in some cases, the bulk xenoliths transformed from a lithology dominated by coarse grains
(i.e. >2 cm; megacrystalline) to a significantly finer-grained texture (i.e. <1cm).
These exsolved lamellae are the result of a complex and protracted sub solidus history following magmatic crystalliza-
tion. Equilibrium pressure–temperature estimates place these xenoliths at low-to-moderate pressure–temperature conditions
(690–910C and 2.0–4.5 GPa) in the lithospheric mantle at the time of entrainment in the kimberlite. However, reconstructed
compositions of initial pyroxene and garnet crystals suggest that this suite of pyroxenites formed at considerably higher
temperatures and pressures that, in some instances, may have approached the majorite stability field. Pyroxenites that do not
contain primary garnet may have been derived from shallower depths.
Progressive exsolution in these pyroxenites is of importance inasmuch as such processes can permit localized changes in
rheological properties and may also accommodate strain within portions of lithospheric mantle. Because most xenolith stud-
ies focus on peridotites and eclogites, the pyroxenite sample suite studied in this work represents an important contribution
towards a greater understanding of the Siberian lithospheric mantle.
Keywords: Yakutia; lithospheric mantle; exsolution lamellae; pyroxenite; lherzolite; garnet
Introduction
Whereas garnet-bearing mafic materials, including
eclogites and pyroxenites, represent only minor pro-
portions of the compositionally heterogeneous cratonic
lithospheric mantle, they have been the focus of consider-
able study inasmuch as they provide critical information
on mantle evolution. However, despite several decades of
research, the origin of eclogite and pyroxenite xenoliths
remains contentious, with opposing camps describing
them as relics of subducted oceanic crust or as originating
from a purely mantle source as high-pressure cumulates
that crystallized from mantle melts (e.g. see differing
reviews by Jacob 2004 and Griffin and O’Reilly 2007).
Petrographic and compositional information from
pyroxene and garnet porphyroblasts in mantle-derived
pyroxenite xenoliths, combined with knowledge of their
exsolution features, provides compositional data regarding
the primary phases crystallized at magmatic temperatures,
as well as the sub solidus processes that have modified
them. This information places important constraints on the
*Corresponding author. Email: taa@igm.nsc.ru
origin, evolution, and pressure–temperature (PT)history
of lithospheric mantle. In addition, better knowledge of
pyroxenite origins may be useful for assessing the char-
acter, depth, and timing of melting and the proportion of
subducted materials that may have been incorporated into
the lithospheric mantle.
Among the known mantle rocks, exsolution features
are common in garnets and pyroxenes in eclogites (e.g.
Smyth and Caporuscio 1984; Haggerty and Sautter 1990);
megacrysts (e.g. Clarke and Pe-Piper 1983; Wang et al.
1999); peridotites (e.g. van Roermund and Drury 1998;
Dawson 2004; Song et al. 2004); pyroxenites and web-
sterites (e.g. Garrison and Taylor 1981; Zhang and Liou
2003); mineral inclusions in diamonds (e.g. Moore and
Gurney 1985; Brenker et al. 2002); and garnet pyroxenites
in several ultramafic massifs [e.g. Sundal Grubse, Norway
(Lappin 1973, 1974); Ariège-group peridotites, Pyrenees
(Conquéré and Fabriès 1984). In particular, exsolution fea-
tures in xenolith minerals from kimberlite pipes from the
Siberian Craton have been described in several publications
ISSN 0020-6814 print/ISSN 1938-2839 online
© 2012 Taylor & Francis
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1072 T.A. Alifirova et al.
(e.g. Sobolev and Sobolev 1964; Jerde et al. 1993; Taylor
et al. 2003).
Large pyroxene crystals displaying notable exsolu-
tion features are regularly identified in mantle materi-
als (e.g. Boyd and Nixon 1973; Schulze et al. 1978;
Schmickler et al. 2004). In a study of exsolution in
two-pyroxene megacrysts from kimberlite pipes in South
Africa, Meyer and McCallister (1984) demonstrated that
the limited development of exsolution plates of clinopy-
roxene in orthopyroxene is a function of the steep slope of
the orthopyroxene limb of the two-pyroxene miscibility gap
(Lindsley and Dixon 1976). Single- and multi-phase oxide
exsolution features (ilmenite, rutile, spinel) have also been
reported for kimberlite-hosted clinopyroxene and orthopy-
roxene megacrysts and xenocrysts, and in mantle xenolith
minerals from several localities worldwide (e.g. Kaapvaal
Craton, Dawson and Reid 1970; Ringwood and Lovering
1970; Siberian Craton, Laz’ko 1979; Roden et al. 2006;
Slave Craton, Kopylova et al. 1999). In addition, kyan-
ite and corundum platelets occur in mantle xenoliths from
Siberian and South African kimberlites (e.g. Carswell et al.
1981; Jerde et al. 1993; Qi et al. 1997). Compared to pyrox-
ene, garnet exsolution from pyroxenes has received rela-
tively minor attention. Studies of clinopyroxene megacrysts
from South African kimberlites have shown that garnet
exsolution is generally located along lamellae parallel to
the {100} plane (Aoki et al. 1980). Similar exsolution
textures have been found not only in kimberlite-derived
xenoliths (e.g. Schmickler et al. 2004), but also in eclog-
ites and pyroxenites from ultramafic complexes metamor-
phosed at ultra-high pressure (UHP) (e.g. Sobolev and
Shatsky 1990). Garnets in mantle rocks present in UHP
terranes, kimberlites, and lamprophyres typically contain
mineral inclusions oriented along {111} and {110} planes,
and such oriented needle-shaped inclusions are probably of
exsolution origin (e.g. Sobolev et al. 1973; Laz’ko 1979).
This study presents new petrographic and mineral
major-element data to constrain the order and crystal chem-
istry of the exsolution features in pyroxene and garnet
porphyroblasts in mantle xenoliths from diamondiferous
and barren kimberlites from the Yakutia region of the
Siberian Craton. We use these data to constrain the PT
conditions attending these rocks and to reconstruct pri-
mary phase compositions, thus placing constraints on their
mantle paragenesis.
Geologic setting
The Siberian Craton comprises several blocks or terranes,
each composed of many smaller terranes with ages typi-
cally between 2.5 and 3.5 thousand milliion years, which
were amalgamated during the mid-Proterozoic (2.1–1.8
Ga; Rosen et al. 2005). The basement rocks have been sub-
divided into seven provinces, based on their order of accre-
tion onto the craton (Rosen et al. 1994). The most extensive
basement outcrops occur in the Anabar and Aldan Shields,
which are located in the northern and south-eastern parts
of the craton (Rosen et al. 1994).
The Siberian Craton has experienced a minimum of
three intense cycles of kimberlite magmatism: (1) Upper
Devonian to Lower Carboniferous (367–345 Ma); (2)
Triassic (245–215 Ma); and (3) Upper Jurassic (160–149
Ma) (e.g. Davis et al. 1980; Kinny et al. 1997; Griffin
et al. 1999; Kostrovitsky et al. 2007). Geographically, the
Palaeozoic kimberlites are located in a zone trending N–
NE across the craton, whereas the younger Triassic and
Jurassic kimberlites are located in the northern part of
the craton. This spatial distribution has been suggested to
result from the migration of the Siberian Craton over a
hotspot (e.g. Pokhilenko et al. 1999; Griffin et al. 2005;
Tychkov et al. 2008). The Siberian Craton has also experi-
enced intense intraplate magmatism, which is expressed at
the surface as the 400,000 km2Siberian Flood Basalts
that erupted at 250 ±1 Ma (Courtillot and Renne 2003;
Carlson et al. 2006). Two of the kimberlites from this
study, Mir (362 Ma; Davis et al. 1980) and Udachnaya
(367 ±5 Ma; Kinny et al. 1997), are diamondiferous
pipes that are located in the central part of the Siberian
Craton (Figure 1). The non-diamondiferous Obnazhennaya
kimberlite (148 ±3 Ma; Davis et al. 1980) was emplaced
after the Siberian Flood Basalts event, near the north-east
cratonic margin (Figure 1).
Methods
Petrographic examination and geochemical analysis of
the xenoliths in this study, listed in Table 1, were per-
formed on 30 µm polished sections at the V.S. Sobolev
Institute of Geology and Mineralogy, Siberian Branch of
the Russian Academy of Sciences in Novosibirsk (IGM).
A Carl Zeiss LEO 1430 scanning electron microscope
was used to obtain back-scattered electron (BSE) images
and X-ray energy-dispersive system compositional data.
Selected samples were analysed using Raman spectroscopy
on a Jobin Yvon U-1000 RAMANOR and a WiTec con-
focal Raman microscope Alpha300R. Both spectroscopic
techniques were conducted with a 532 nm wavelength laser.
Major-element compositions of minerals were deter-
mined by electron microprobe analysis using a Cameca
Camebax-micro and a JEOL JXA-8100 electron micro-
probe at IGM. Analyses were conducted using an accel-
eration voltage of 20 kV, a focused beam with a current of
10 nA, counting times of 20–30 s, and standard PAP correc-
tion procedures (e.g. Lavrent’ev et al. 1987; Korolyuk et al.
2008). Precision and accuracy were monitored using natu-
ral and synthetic standards that were measured at regular
intervals during each analytical session. Intra- and inter-
grain homogeneity within individual samples was assessed
by measuring 3–10 points in each grain. Detection limits
are typically <0.04 wt.% for SiO2,TiO
2,Al
2O3,MgO,
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International Geology Review 1073
Figure 1. Simplified geological map of the Siberian plat-
form showing the boundary of the platform (1), surface expo-
sures of Precambrian rocks (2), and location of Mesozoic
(3) and Palaeozoic (4) kimberlite fields. The locations of the
Obnazhennaya, Mir, and Udachnaya kimberlites (5) from which
the studied xenoliths are derived are also shown (modified after
Pokhilenko et al. 1999).
CaO, Na2O, and K2O and <0.05–0.07 wt.% for FeO,
MnO, and Cr2O3.
Petrographic characteristics
Pyroxenitic (n=18) and garnet-lherzolite (n=2) man-
tle xenoliths were collected from three Yakutian kimber-
lite pipes (Figure 1): the Udachnaya-East (n=6) and
Mir (n=4) diamondiferous pipes and the diamond-
free Obnazhennaya pipe (n=10). This collection of
xenoliths is generally coarse grained (commonly con-
taining crystals >1 cm in maximum dimension) and
includes garnet- and garnet-spinel websterites, olivine web-
sterites, lherzolites, orthopyroxenites, and clinopyroxenites
(Table 1). In general, the xenoliths from the Obnazhennaya
and Udachnaya-East pipes are exceptionally fresh and
have been little altered by low-temperature weathering.
Samples from the Mir kimberlite pipe are variably altered,
with patchy replacement of olivine and orthopyroxene
by low-temperature alteration assemblage comprised of
serpentine, carbonates, and sulphates (Table 1). Most of the
rocks are inequigranular, with large pyroxene (up to 3 cm)
and garnet (up to 2 cm) grains set in a finer-grained matrix
(with grain sizes <1 mm) of the same mineral assemblage
±olivine, phlogopite, and oxide grains of various sizes.
Large pyroxene crystals generally show evidence of strain
in the form of undulose extinction. All studied xenoliths
contain silicates exhibiting a number of exsolution features,
which are the focus of this study.
Petrography of xenolith minerals and their exsolution
features
The silicate population of the studied xenolith suite is com-
posed of orthopyroxene (opx), clinopyroxene (cpx), and
garnet porphyroblasts, and these minerals host exsolution
phases (Table 1). Large clinopyroxene (up to about 3 cm
in length, e.g. sample O-125), orthopyroxene (2–3 cm in
length, e.g. O-436 and M5/01), and garnet (up to 2 cm
in maximum dimension) grains contain exsolution features
in the form of lamellae and inclusions of varying crystal
form.
Exsolution features in pyroxenes
Exsolved phases hosted by pyroxene occur in three forms:
(1) thin lamellae up to 20 µm thick (usually 3–5 µm)
that form as elongate plates with rectilinear or curved
boundaries; (2) large tabular and lenticular inclusions or
plates that are generally up to 500 µm thick, occasion-
ally extending across the host mineral (several millimetres),
and form as a single exsolved grain or as granular aggre-
gates; and (3) thickened lamellae that have transformed
into well-defined grains (recrystallized), and blebs which
sometimes form chains of aggregated minerals within the
host mineral or adjacent to the grain boundary. Exsolution
features generally form parallel to {100}, {010}, and {001}
planes of the host mineral. Phases exsolved from large
pyroxene crystals include oxides (spinels, ilmenite, rutile,
and chromite), pyroxene, and garnet (Figure 2). Exsolved
oxides generally form as needle- or rod-like crystals that
are 1–5 µm thick (occasionally up to 10–20 µm in width)
with variable lengths (tens to hundreds of micrometres
long) and/or as crystals that generally have lenticular or
tabular crystal shapes. Long and short oxide lamellae have
rectilinear boundaries, and lenticular plates with lobate
grain boundaries are also observed.
In detail, large clinopyroxene grains commonly contain
exsolved garnet in the form of rounded inclusions (blebs)
and faceted lamellae (e.g. UV201/09). Garnet exsolu-
tion is often associated with orthopyroxene lamellae (e.g.
O-173, O-207, and O-332). In addition, curved inter-
growths of orthopyroxene and garnet are observed in
clinopyroxene porphyroblasts from olivine websterite (e.g.
O-207). Exsolved garnet is also observed as lamellae (up
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1074 T.A. Alifirova et al.
Table 1. Petrographic summary of the Yakutian mantle xenoliths.
Exsolutions
Sample Rock name
T
(C)
P
(GPa)
Mineral assemblage (number
refers to vol.%) Orthopyroxene Clinopyroxene Garnet
Obnazhennaya
O-125 grt
clinopyroxenite
810 3.2 11grt, 84cpx, 4opx, trace: phl, ap,
sf, srp, clt
grt grt, opx cpx
O-173 grt websterite 830 2.8 18grt, 64cpx, 17opx, trace: ilm,
rt, sf
ilm, cpx, rt, grt ilm, grt, opx, rt rt, ilm, ctn,
cpx
O-207 grt ol websterite 830 3.4 24grt, 41cpx, 26opx, 8ol, trace:
ilm, rt, sf, srp
cpx, ilm, rt opx, grt, ilm, rt rt, ilm, opx,
ctn, cpx
O-436 spl-grt ol websterite 820 2.6 9grt, 8cpx, 56opx, 24ol, 2chr,
trace: phl, ap, sf, srp, crb, clt
grt, chr, cpx grt, chr
O-107 grt websterite 800 3.2 25grt, 66cpx, 8opx, trace: phl, ap,
sf
grt, opx cpx, rt
O-264 grt websterite 51grt, 24cpx, 24opx, trace: ilm, sf,
srp
cpx, rt, ilm opx, ilm, rt cpx, rt, ilm
O-332 grt websterite 39grt, 51cpx, 9opx, trace: phl, rt,
sf, srp, clt
ilm, rt, cpx grt, opx, rt rt, cpx, opx,
ilm
O-571 spl-grt lherzolite 49grt, 10cpx, 11opx, 22ol, 7chr,
trace: rt, srp
ilm, rt opx, ilm, rt rt, cpx, ilm
O-301 grt websterite 31grt, 41cpx, 25opx, 2ol, trace: rt,
sf, srp, clt
cpx, ilm, rt opx, ilm, rt rt, opx, ilm,
cpx
O-550 grt websterite 780 2.4 36grt, 53cpx, 10opx, trace: sf cpx, rt, ilm, grt opx, rt, grt rt, cpx, ilm
Mir
M4/01 spl-grt ol websterite 18grt, 58cpx, 10opx, 12ol, 1chr,
trace: phl, ap, sf, srp, crb
cpx, chr opx, chr rt, ilm
M5/01 grt websterite 690 2.0 23grt, 21cpx, 55opx, trace: phl,
ilm, sf, srp, crb, clt
cpx, rt, ilm, grt rt, opx, ilm rt, ol, opx,
ilm, plag,
cpx
M31/01 grt ol websterite 890 4.3 15grt, 45cpx, 20opx, 8ol, 10phl,
1ilm, trace: sf, srp, crb, sph, clt
cpx opx rt, cpx
M34/01 spl-grt lherzolite 740 2.4 17grt, 8cpx, 35opx, 38ol, 1chr,
trace: ilm, rt, sf, srp
rt, cpx, chr grt, opx rt, chr, ilm,
opx, cpx
Udachnaya
UV41/03 grt websterite 4grt, 59cpx, 36opx, trace: rt, sf cpx, grt, ilm, rt opx, rt, grt
UV70/03 grt orthopyroxenite 47grt, 36opx, 15phl, 1rt, trace:
ilm, ap, zrn, sf, mnz
cpx, ilm, rt rt, ilm, ap,
ctn
UV201/09 grt clinopyroxenite 46grt, 53cpx, trace: phl, ap, sf, crb grt, opx rt
UV223/09 grt ol websterite 910 4.5 18grt, 26cpx, 28opx, 27ol, trace:
phl, ap, sf, srp, clt
rt, cpx grt, opx, rt rt, ilm, opx,
cpx
UV127/09 grt ol websterite 850 3.7 19grt, 16cpx, 35opx, 29ol, trace:
phl, rt, ap, sf, srp, clt
cpx, rt, ilm opx, ilm, rt rt, ilm, chr,
ctn, cpx
UV345/08 spl-grt
orthopyroxenite
6grt, 6cpx, 86opx, 1chr, trace:
phl, srp
cpx, grt, chr, rt opx, grt, chr, rt rt, chr, opx,
ctn
Note: ap, apatite; chr, chromian spinel; clt, chlorite; cpx, clinopyroxene; clt, chlorite; ctn, crichtonite; crb, carbonates; grt, garnet; ilm, ilmenite; mnz,
monazite; opx, orthopyroxene; ol, olivine; phl, phlogopite; pl, plagioclase; rt, rutile; sf, sulphides; spl, spinel; srp, serpentine; sph, sulphates; and zrn, zircon.
to5µm thick) in a number of well-preserved orthopyrox-
ene porphyroblasts in xenoliths from the Mir kimberlite.
In addition, these orthopyroxene crystals contain isolated
lenticular, rounded, and euhedral inclusions of clinopy-
roxene and oxides in locations distal and proximal to the
porphyroblast grain boundary.
Exsolution features in garnet
Compared to pyroxene porphyroblasts, large garnet
crystals generally contain lower exsolution proportions
(generally <3.6 vol.%; Figures 3A and 3B). Within
garnet, exsolved phases occur as (1) thin needles (gener-
ally <5µm wide and <100 µm long) and (2) lamellae
that often parallel the {111} plane and commonly exceed
100 µm in length, with constant or variable thickness
that is generally <30 µm wide. Rutile needles are often
twinned (Figures 3C and 3D).
Common exsolution assemblages in garnet include
clinopyroxene, rutile, ilmenite, orthopyroxene, and
chromite (Figures 4, 5; Table 1). Exsolution lamellae in
garnet may also contain uniformly distributed needles
and rods of crichtonite (generally 1–3 µm in diameter;
Figure 6A and 6B). More rarely, orientated prisms of
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International Geology Review 1075
Figure 2. Exsolution lamellae of orthopyroxene (Opx) and garnet (Grt) in a clinopyroxene (Cpx) in websterite O-322, with smaller
orthopyroxene crystals at the grain margin (A) and exsolved clinopyroxene, garnet, and Cr-spinel (Spl) hosted by an orthopyroxene in
olivine websterite O-436 (B). Exsolution features in olivine websterite O-207 include lamellae of orthopyroxene accompanied by ilmenite
(Ilm) and rutile (Rt) platelets in a clinopyroxene (C) and intergrown ilmenite and rutile platelets in an orthopyroxene host (D).
Figure 3. Exsolution textures in garnet showing common lamellae scales in websterite O-550 (A) and smaller ‘embryonic’ exsolution
features in clinopyroxenite UV201/09 (B). Twinned platelet (C) and twinned elongate ‘needle’ (D) of rutile (Rt) in websterite O-264.
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1076 T.A. Alifirova et al.
Figure 4. Oxide exsolution features identified in garnets include rutile (Rt) +ilmenite (Ilm) +crichtonite (Ctn) assemblages in olivine
websterite O-207: (A) transmitted light image, (B) BSE image; and chromite (Chr) +rutile associations in lherzolite M34/01: (C and D)
BSE images.
Figure 5. Clinopyroxene (Cpx) exsolution features hosted by garnet in websterite M5/01 (A and B – BSE images) and websterite O-264
(C and D – transmitted light). These images illustrate the association of rutile (Rt) and ilmenite (Ilm) with exsolved clinopyroxene.
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International Geology Review 1077
Figure 6. Crichtonite (Ctn) and apatite (Ap) prisms in garnets of olivine websterites O-207 (A) and UV127/09 (B) (BSE images) and
orthopyroxenite UV70/03 (C and D) which are associated with elongate rutile (Rt).
Figure 7. Uncommon exsolution of orientated olivine (Ol) platelets associated with rutile (Rt) lamellae in garnet in websterite M5/01
(A and B) and a complex lamella of clinopyroxene (Cpx) with, plagioclase (Pl) and an unidentified phase hosted by a garnet in M5/01
(C–E): C – BSE image of lamella; D – combined false-colour image of an XYplane 1 µm deeper than shown in (C); E – Raman spectrum
of the plagioclase showing labelled wavelength bands characteristic of the oligoclase–albite system.
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1078 T.A. Alifirova et al.
F-apatite (up to 400 µm long and 40–50 µm wide) are
observed associated with ilmenite and rutile lamellae in
garnet (e.g. UV70/03; Figure 6C and 6D), and monazite
is occasionally found with these apatite crystals. Garnet
porphyroblasts from xenolith M5/01 show several unusual
exsolution features, including oriented olivine plates up
to 65 µm long and 2–3 µm thick; exsolved olivine that
is associated with rutile ±orthopyroxene (Figure 7A
and 7B); and an unusual lamella that is comprised of
clinopyroxene, plagioclase, and an unidentified silicate-
phase is also present in a garnet in M5/01 (Figure 7C–E).
Most of the xenoliths in this study (n=14) contain
complex exsolution textures in garnet porphyroblasts,
comprising composite lamellae assemblages and cross-
cutting relationships that differ between lamellae and
inclusion populations. For example, websterite O-264
contains a lamella dominated by clinopyroxene that also
contains rutile and ilmenite, and this feature cross-cuts
pre-existing lamellae (Figure 5C and 5D). Another
anomalous, complex exsolution texture is observed in a
single xenolith from Udachnaya (UV223/09) that contains
garnet porphyroblasts with composite lamellae of rutile +
ilmenite ±clinopyroxene, which appear to have narrow
margins (<50 µm) of symplectite-like spinel +olivine ±
phlogopite rims.
Major-element compositions of porphyroblasts and
their exsolution features
Clinopyroxene
Clinopyroxene has Ca–Fe–Mg compositions within the
diopside range (Wo43–48.9En44.3–51.6Fs2.1–9.7; Figure 8;
Table 2). The range of Mg# values (where Mg# =
100 Mg/[Mg +Fetotal ]) observed in clinopyroxene porphy-
roblasts varies within each xenolith group from 85.8 to 89.0
in clinopyroxenites, 84.1 to 96.0 in websterites, 91.5 to 95.2
in olivine websterites, 93.5 to 93.8 in an orthopyroxenite,
and 94.9 to 95.1 in garnet lherzolites (Table 2; Figure 9).
Within individual clinopyroxene porphyroblasts, signifi-
cant compositional variation is evident; for example, the
Mg# of clinopyroxene in sample O-107 generally increases
from the core (Mg# =86.9) to the rim (Mg# =88.5;
Figure 9). In addition, Al2O3and Na2O contents of
clinopyroxenes generally have an antithetic relationship
with CaO and MgO concentrations. Clinopyroxene por-
phyroblasts cover a range of Cr2O3contents, with the
highest Cr2O3abundances observed in olivine websterites
and garnet lherzolites (up to 2.69 and 2.32 wt.%, respec-
tively; Figure 9). Core–rim compositional relationships
show significant variations in Al2O3and Na2O contents
in clinopyroxenes from websterite (e.g. O-301, O-107,
UV41/03), clinopyroxenite (O-125), and olivine websterite
(O-207) xenoliths (Table 2). The TiO2content in clinopy-
roxene in websterite xenolith O-301 decreases from the
core (0.56 wt.%) to the rim (0.31 wt.%; Figure 9).
Figure 8. Host mineral pyroxene (A) and garnet (B) composi-
tions. Di, diopside; En, enstatite; Fs, ferrosilite; Hd, hedenbergite;
Fe, Fe-components; Mg, Mg-components; Ca, Ca-components;
Cpx, clinopyroxene; Opx, orthopyroxene; Grt, garnet.
Most of the clinopyroxene lamellae within orthopy-
roxene and garnet have compositions that are similar to
those of clinopyroxene porphyroblasts. However, lamel-
lae in orthopyroxenes from three samples (O-173, O-
550, and UV41-03) have higher MgO and CaO contents
and lower Al2O3and Na2O abundances when compared
to clinopyroxene porphyroblasts from the same sample
(Table 2). Other notable variations between porphyrob-
last and exsolved clinopyroxene compositions include TiO2
contents that are lower in the lamellae in some websterites
(e.g. O-173 and UV41/03) and Cr-rich lamellae in some
olivine websterites (e.g. M4/01 and O-436).
Orthopyroxene
Orthopyroxenes from the xenoliths cover a range of
major-element contents and generally have enstatite com-
positions (Wo0.2–1.2En87.5–94.6Fs5–12.2 ); however, occa-
sional bronzites (Wo0.2–0.7En75.2–86Fs13.7–24.4 ; Figure 8)
have been identified (samples UV70/03, O-125, O-332,
UV41/03). The Mg#s of orthopyroxene porphyroblasts are
not consistently lower than those of coexisting clinopy-
roxene porphyroblasts when the entire sample suite is
considered. Within individual rock types, a range of
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International Geology Review 1079
Table 2. Major-element compositions of clinopyroxene from Siberian xenoliths.
Sample # EMP site SiO2TiO2Al2O3Cr2O3Fe OMnO MgO CaO Na2OK
2OTotalMg#
Orthopyroxenites
UV345/08 C 54.45 0.08 1.86 1.26 2.06 <0.06 16.73 22.94 1.08 <0.03 100.48 93.5
R 54.57 0.07 1.61 1.19 1.98 <0.06 16.87 23.02 1.09 <0.03 100.42 93.8
LP 54.55 0.07 1.67 1.31 2.08 <0.06 16.77 22.98 1.13 <0.03 100.58 93.5
Clinopyroxenites
O-125 C 54.72 0.12 7.23 0.16 3.61 <0.06 12.22 17.28 3.93 <0.03 99.32 85.8
R 55.07 0.11 5.97 0.17 3.79 <0.06 13.11 17.55 3.62 <0.03 99.44 86.0
LG 55.34 0.13 4.91 0.12 3.15 <0.06 13.79 18.94 3.02 <0.03 99.46 88.6
UV201/09 C 55.01 0.20 1.82 0.69 3.66 0.08 16.56 20.45 1.83 <0.03 100.31 89.0
R 55.09 0.20 1.75 0.69 4.01 0.08 16.43 20.10 1.96 <0.03 100.31 88.0
LG 54.65 0.18 1.70 0.66 4.12 0.09 16.67 19.91 1.98 <0.03 99.96 87.8
Websterites
O-107 C 54.92 0.14 6.15 0.14 3.42 <0.06 12.74 18.15 3.43 <0.03 99.15 86.9
R 55.35 0.15 4.63 0.17 3.32 <0.06 14.37 18.98 2.93 <0.03 99.92 88.5
LP 55.59 0.07 4.20 0.13 3.57 <0.06 13.88 18.77 2.74 <0.03 99.02 87.4
O-173 C 53.50 0.53 6.75 0.28 2.13 0.07 13.79 19.71 2.68 <0.03 99.46 92.0
R 53.88 0.62 7.08 0.33 2.08 <0.06 13.80 19.86 2.74 <0.03 100.46 92.2
LP 55.20 0.35 3.66 0.32 1.97 <0.06 16.00 21.38 1.95 <0.03 100.84 93.5
M5/01 C 55.04 0.30 5.92 0.43 1.47 <0.06 13.64 18.66 3.77 <0.03 99.23 94.3
R 56.01 0.31 5.42 0.81 1.32 <0.06 13.87 19.03 3.62 <0.03 100.44 94.9
LP 54.21 0.32 5.77 0.57 1.44 <0.06 13.65 18.78 3.69 <0.03 98.49 94.4
O-301 C 53.76 0.56 5.59 1.40 1.25 <0.06 14.57 20.18 2.31 <0.03 99.65 95.4
R 55.02 0.31 3.36 1.56 1.44 0.07 15.62 21.49 1.73 <0.03 100.58 95.1
O-332 C 54.75 0.13 3.96 0.49 4.58 0.24 14.03 18.68 3.34 <0.03 100.19 84.5
R 54.69 0.13 3.61 0.39 4.83 0.19 14.36 18.96 3.06 <0.03 100.20 84.1
LG 53.95 0.13 3.87 0.59 5.24 0.17 14.16 18.39 3.16 <0.03 99.64 82.8
O-264 C 53.28 0.52 5.09 0.95 1.21 <0.06 15.33 20.84 2.54 <0.03 99.81 95.7
R 53.66 0.53 4.60 0.96 1.16 <0.06 15.46 20.88 2.49 <0.03 99.77 96.0
O-550 C 54.28 0.47 6.91 0.32 1.75 <0.06 13.97 19.08 2.91 <0.03 99.75 93.4
R 54.50 0.47 6.71 0.24 1.63 0.10 14.16 18.96 2.96 <0.03 99.75 93.9
LP 55.18 0.42 4.93 0.35 1.68 <0.06 15.17 19.87 2.43 <0.03 100.10 94.1
UV41/03 C 52.79 0.43 6.50 0.63 3.97 0.10 12.95 19.58 2.23 <0.03 99.16 85.3
R 52.96 0.40 5.86 0.63 4.29 0.10 13.64 18.95 2.27 <0.03 99.09 85.0
LP 53.69 0.21 4.07 0.84 3.85 0.08 14.11 19.82 2.14 <0.03 98.81 86.7
Olivine websterites
M4/01 C 55.50 0.19 3.28 2.41 1.33 <0.06 14.73 20.02 2.76 <0.03 100.28 95.2
R 54.73 0.17 2.90 2.69 1.40 <0.06 15.06 20.14 2.79 <0.03 99.92 95.0
Inc 54.71 0.17 2.91 2.89 1.31 0.07 14.60 20.04 2.90 <0.03 99.61 95.2
M31/01 C 54.14 0.12 2.19 0.82 2.59 0.07 15.68 20.74 2.16 <0.03 98.53 91.5
R 54.64 0.09 2.08 0.98 2.42 <0.06 15.85 21.17 1.97 <0.03 99.24 92.1
O-436 R 54.15 0.19 2.51 1.36 1.83 <0.06 16.19 20.72 1.72 0.04 98.76 94.0
LP 54.00 0.21 2.78 1.88 1.86 <0.06 15.41 20.23 2.08 <0.03 98.51 93.7
O-207 C 54.27 0.49 6.37 0.40 1.91 <0.06 13.57 18.67 3.11 <0.03 98.82 92.7
R 54.04 0.52 5.54 0.37 2.06 <0.06 14.03 19.35 2.81 <0.03 98.74 92.4
UV223/09 C 56.08 0.29 3.09 0.90 2.13 0.07 15.74 19.44 2.70 <0.03 100.45 93.0
R 55.71 0.29 3.03 0.92 2.14 0.07 15.73 19.43 2.73 <0.03 100.04 92.9
LP 55.64 0.35 3.01 0.91 2.17 <0.06 15.90 19.31 2.75 <0.03 100.11 92.9
UV127/09 C 55.35 0.28 3.11 2.37 1.90 <0.06 15.26 18.44 3.36 <0.03 100.13 93.5
R 55.32 0.30 3.10 2.38 1.94 <0.06 15.23 18.33 3.37 <0.03 100.03 93.3
LP 55.38 0.27 3.26 2.63 1.90 <0.06 14.97 18.23 3.58 <0.03 100.28 93.4
Lherzolites
M34/01 C 55.04 0.22 3.20 2.32 1.34 <0.06 14.57 20.14 2.78 <0.03 99.67 95.1
R 55.28 0.19 3.36 2.29 1.34 <0.06 14.37 20.11 2.74 <0.03 99.71 95.0
O-571 C 53.31 0.42 4.23 1.81 1.46 <0.06 15.36 20.27 2.91 <0.03 99.82 94.9
R 53.22 0.41 3.95 1.96 1.47 <0.06 15.38 20.35 2.82 <0.03 99.61 94.9
Note: C, core; R, rim; LP, lamella in pyroxene; LG, lamella in garnet; Inc, inclusion. Total Fe as FeO.
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1080 T.A. Alifirova et al.
Figure 9. Compositional features of garnets (A) and clinopyroxenes (B–D) in studied xenoliths showing core–rim relationships. Legend
as in Figure 8.
Mg# values is observed for orthopyroxene porphyrob-
lasts from clinopyroxenites (86.0–86.2), orthopyroxenites
(75.5–90.1), websterites (80.7–95.0), olivine websterites
(91.4–92.8), and garnet lherzolites (92.9–93.5; Table 3).
Concentrations of CaO, Cr2O3, and Na2O are generally low
in orthopyroxene porphyroblasts (typically <0.50 wt.%),
and Al2O3contents vary from 0.42 to 4.36 wt.%. Within
individual orthopyroxene grains, compositional zoning is
generally low: <0.5 wt.% variation of all major elements,
from core to rim, in the most xenoliths. However, five xeno-
liths (O-107, O-207, O-301, O-332, and UV70/03) have
variations of up to 1.0 wt.% in Al2O3, FeO, and/or MgO
contents. Orthopyroxenes from websterite sample O-301
also show a significant decrease in Cr2O3content from core
(1.08 wt.%) to rim (0.34 wt.%).
In websterite xenoliths, exsolved orthopyroxene in
clinopyroxene porphyroblasts has compositions that can
vary significantly (e.g. up to 2.0 wt.% Al2O3, 0.6 wt.%
FeO, and 1.8 wt.% MgO), whereas little variation is
observed in the orthopyroxenite and clinopyroxenite
xenoliths (Table 3). CaO contents generally demonstrate
uniform distribution in orthopyroxene porphyroblasts
except for lherzolite O-571, where CaO decreases from
the centre (0.66 wt.%) to the edge (0.24 wt.%). Websterite
sample O-332 also contains orthopyroxene that is exsolved
from garnet and this crystal has CaO contents up to
0.52 wt.%, a value that is higher than that of orthopyroxene
porphyroblasts (0.31–0.33 wt.%), and the orthopyroxene
lamellae (0.37 wt.%) in clinopyroxene from the same
sample. Additionally, the orthopyroxene that exsolved
from garnet has high Al2O3(1.43 wt.%) and MnO
contents (0.47 wt.%).
Garnet
The major-element compositions of garnet porphyrob-
lasts cover a range of pyrope and almandine contents
(Prp46.3–76.1Alm11.4–44.3) and have low Ca components
(grossular <9.3, uvarovite <12.9 mol.%, andradite
0.4–5.4 mol.%, Ti-andradite <1.4 mol.%) in all samples.
Interrelations of Ca-, Mg-, and Fe-components in garnet
porphyroblasts are shown in Figure 8. Garnets with the
highest pyrope contents are found in websterites (up to 76.1
mol.%), olivine websterites (up to 72.8 mol.%), and gar-
net lherzolites (up to 71.8 mol.%), and these garnets also
have high Cr contents (up to 1.98, 4.54, and 4.62 wt.%,
respectively; Figure 9A). High pyrope contents are typi-
cal of samples that contain olivine (e.g. O-301, O-571, and
M34/01), but are also observed in some samples contain-
ing no observable olivine (e.g. O-264, O-550, and M5/01).
In nearly all samples, garnets have near-uniform intra- and
inter-mineral Prp–Alm–Grs proportions. Cr2O3contents
vary within garnet porphyroblasts from a limited number
of xenoliths (e.g. O-571 and M34/01), with Cr-rich cores
(2.73 and 4.62 wt.%) and lower Cr contents in the corre-
sponding rim (2.3 and 4.41 wt.%; Figure 8A). All garnets
have low Na2O(<0.1 wt.%) and TiO2(<0.51 wt.%)
abundances (Table 4).
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International Geology Review 1081
Table 3. Major-element (wt.%) compositions of orthopyroxene from Siberian xenoliths.
Sample #
Electron
microprobe
analysis site SiO2TiO2Al2O3Cr2O3Fe OMnO MgO CaO Na2OK
2OTotalMg#
Orthopyroxenites
UV70/03 C 52.40 0.06 4.36 <0.06 15.53 0.08 26.81 0.17 <0.03 <0.03 99.48 75.5
R 53.62 0.05 3.65 <0.06 15.00 0.07 27.58 0.12 <0.03 <0.03 100.14 76.6
Inc 53.24 0.05 2.94 <0.06 15.48 0.08 27.67 0.11 <0.03 <0.03 99.60 76.1
UV345/08 C 56.72 <0.04 1.44 0.48 6.70 0.12 34.28 0.15 <0.03 <0.03 99.94 90.1
R 56.86 <0.04 1.09 0.41 6.72 0.10 34.46 0.14 <0.03 <0.03 99.81 90.1
LP 56.89 <0.04 0.96 0.42 6.60 0.10 34.79 0.15 <0.03 <0.03 99.96 90.4
Clinopyroxenites
O-125 C 56.51 <0.04 0.71 <0.06 9.31 0.07 32.72 0.21 0.05 <0.03 99.62 86.2
R 57.02 <0.04 0.62 <0.06 9.48 0.10 32.70 0.21 0.06 <0.03 100.22 86.0
LP 56.92 <0.04 0.60 <0.06 9.15 0.08 32.99 0.21 0.05 <0.03 100.01 86.5
Websterites
O-107 C 57.29 <0.04 0.68 <0.06 8.26 0.07 33.31 0.19 0.06 <0.03 99.91 87.8
R 57.74 <0.04 0.81 <0.06 7.86 <0.06 34.02 0.20 0.07 <0.03 100.82 88.5
LP 57.16 <0.04 0.67 <0.06 8.87 0.08 33.08 0.23 0.05 <0.03 100.18 86.9
O-173 C 58.87 0.09 1.10 <0.06 5.96 0.10 34.99 0.26 0.05 <0.03 101.48 91.3
R 58.53 0.08 0.84 0.07 5.76 0.11 35.49 0.26 0.04 <0.03 101.19 91.7
LP 56.36 0.06 2.73 0.11 6.31 0.11 34.30 0.20 0.05 <0.03 100.24 90.6
M5/01 C 57.62 <0.04 1.21 0.07 4.67 0.07 35.52 0.13 0.06 <0.03 99.37 93.1
O-301 C 55.70 0.07 2.50 1.08 4.55 0.11 35.73 0.16 <0.03 <0.03 99.94 93.3
R 57.96 0.06 1.48 0.34 4.27 0.09 36.20 0.17 <0.03 <0.03 100.59 93.8
O-332 C 55.52 <0.04 0.58 0.22 12.54 0.27 29.92 0.31 0.11 <0.03 99.50 81.0
R 55.73 <0.04 0.62 0.14 12.49 0.20 29.34 0.33 0.10 <0.03 98.98 80.7
LP 55.66 <0.04 0.66 0.23 12.12 0.34 29.83 0.37 0.13 <0.03 99.39 81.4
LG 54.76 <0.04 1.43 0.31 12.50 0.47 28.88 0.52 0.12 <0.03 99.03 80.5
O-264 C 57.68 0.10 1.11 0.24 3.50 0.07 37.14 0.21 0.05 <0.03 100.08 95.0
R 57.39 0.09 1.47 0.31 3.65 0.07 37.00 0.20 0.04 <0.03 100.21 94.8
O-550 C 58.22 0.08 1.38 0.17 4.68 <0.06 36.05 0.22 0.05 <0.03 100.87 93.2
R 58.25 0.09 0.95 <0.06 4.50 0.07 36.37 0.21 <0.03 <0.03 100.53 93.5
LP 56.77 0.06 3.41 <0.06 4.92 0.13 34.62 0.20 0.07 <0.03 100.24 92.6
UV41/03 C 55.22 <0.04 1.96 0.25 11.79 0.21 30.47 0.17 <0.03 <0.03 100.13 82.2
R 55.35 <0.04 1.63 0.21 11.96 0.18 30.49 0.18 <0.03 <0.03 100.05 82.0
LP 53.79 <0.04 3.62 0.26 12.40 0.22 29.01 0.18 <0.03 <0.03 99.55 80.7
Olivine websterites
O-436 C 56.86 0.09 1.22 0.44 5.04 0.10 35.14 0.25 0.07 <0.03 99.21 92.6
R 57.21 0.08 0.83 0.31 4.86 0.08 35.36 0.23 0.07 <0.03 99.01 92.8
O-207 C 58.05 0.07 0.84 0.18 5.14 <0.06 35.30 0.22 0.04 <0.03 99.89 92.4
R 57.32 0.06 1.32 0.11 5.79 0.07 34.73 0.20 0.06 <0.03 99.65 91.4
M31/01 C 57.82 <0.04 0.43 0.10 5.61 0.16 35.20 0.25 0.08 <0.03 99.68 91.8
UV223/09 C 58.54 0.10 0.43 0.09 5.48 0.10 35.17 0.29 0.10 <0.03 100.30 92.0
R 58.62 0.09 0.42 0.08 5.45 0.11 34.85 0.26 0.11 <0.03 99.99 91.9
UV127/09 C 58.42 0.09 0.52 0.24 4.90 0.10 35.44 0.24 0.11 <0.03 100.07 92.8
R 58.20 0.09 0.45 0.21 4.93 0.10 35.57 0.24 0.11 <0.03 99.91 92.8
Lherzolites
M43/01 C 58.33 <0.04 0.82 0.39 4.78 0.10 35.34 0.16 0.06 <0.03 100.02 92.9
R 57.80 <0.04 0.61 0.22 4.81 0.14 35.23 0.17 0.05 <0.03 99.05 92.9
O-571 C 56.69 0.08 1.37 0.39 4.57 0.10 36.35 0.66 0.14 <0.03 100.34 93.4
R 57.09 0.09 1.15 0.31 4.51 0.10 36.61 0.24 0.07 <0.03 100.17 93.5
Note: C, core; R, rim; LP, lamella in pyroxene; LG, lamella in garnet; Inc, inclusion. Total Fe as FeO.
Garnets exsolved from pyroxenes are generally char-
acterized by lower Cr2O3contents, compared to garnet
porphyroblasts from the same sample. For instance, garnet
lamellae in clinopyroxene (UV201/09) and orthopyroxene
(O-436) have 1.58 and 1.94 wt.% of Cr2O3, respectively,
whereas garnet porphyroblasts in these samples contain
2.38–2.39 and 2.66–2.81 wt.%, respectively. A rare inclu-
sion of garnet in phlogopite has the highest almandine (48.1
mol.%) and the lowest pyrope (42.1 mol.%) contents.
Geothermobarometry
Xenolith PTconditions were calculated using the Ca-
in-orthopyroxene thermometer (Brey and Kohler 1990),
in combination with the garnet–orthopyroxene barometer
of Brey and Kohler (1990). The preferential use of the
Ca-in-orthopyroxene thermometer, relative to the two-
pyroxene thermometer of Brey and Kohler (1990), is
that PTconditions previously reported for pyroxenites
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1082 T.A. Alifirova et al.
from these kimberlites yielded low-temperature estimates
(e.g. Taylor et al. 2003; Roden et al. 2006), and that
the Ca-in-orthopyroxene is more accurate at temperatures
below 900C (e.g. Smith 1999). However, temperatures
were also calculated using the two-pyroxene ther-
mometer, as agreement between results obtained using
different thermometers is often taken as proof of reliable
thermobarometry (e.g. Franz et al. 1996a,b; Woodland and
Table 4. Major-element compositions of garnet from Siberian xenoliths.
Sample #
Electron
microprobe
analysis site SiO2TiO2Al2O3Cr2O3FeOMnO MgO CaO Na2OK
2OTotalMg#
Orthopyroxenites
UV70/03 C 39.81 0.06 22.05 <0.06 21.61 0.33 12.37 3.21 <0.03 <0.03 99.49 50.5
R 40.01 <0.04 22.12 <0.06 21.33 0.33 12.43 3.25 <0.03 <0.03 99.55 50.9
Inc 39.47 <0.04 21.77 <0.06 23.32 0.34 11.13 3.31 <0.03 <0.03 99.45 46.0
UV345/08 C 40.73 <0.04 19.77 4.13 11.25 0.55 16.72 6.62 <0.03 <0.03 99.80 72.6
R 40.90 <0.04 19.95 3.95 11.38 0.55 16.89 6.68 0.04 <0.03 100.35 72.6
LP 40.66 <0.04 19.70 4.03 11.41 0.53 16.63 6.63 <0.03 <0.03 99.63 72.2
Clinopyroxenites
O-125 C 41.40 <0.04 22.75 0.16 13.40 0.33 17.25 3.87 <0.03 <0.03 99.23 69.7
R 41.03 0.05 22.77 0.15 14.27 0.32 17.03 3.88 <0.03 <0.03 99.52 68.0
LP 41.28 <0.04 22.66 0.13 14.38 0.35 16.86 3.81 <0.03 <0.03 99.51 67.6
UV201/09 C 41.10 0.51 20.13 2.38 11.00 0.50 17.54 6.32 0.10 <0.03 99.57 74.0
R 40.74 0.51 19.85 2.39 11.85 0.55 16.93 6.40 0.10 <0.03 99.31 71.8
LP 41.51 0.48 20.82 1.58 11.32 0.49 17.37 6.27 0.09 <0.03 99.92 73.2
Websterites
O-107 C 41.85 <0.04 22.79 0.14 13.12 0.30 17.89 3.91 <0.03 <0.03 100.05 70.9
R 41.63 <0.04 22.67 0.11 13.13 0.33 18.15 3.87 <0.03 <0.03 99.94 71.1
LP 41.64 <0.04 22.68 0.11 13.94 0.33 17.30 3.83 <0.03 0.05 99.93 68.9
O-173 C 42.92 0.07 23.51 0.35 9.44 0.38 20.24 4.48 0.04 <0.03 101.43 79.3
R 42.47 0.06 23.48 0.39 9.65 0.35 19.92 4.39 0.04 <0.03 100.74 78.6
M5/01 C 42.58 0.08 23.06 0.73 7.86 0.32 21.17 3.79 0.06 <0.03 99.66 82.8
R 42.31 0.07 22.96 0.67 7.67 0.30 21.52 3.71 0.05 <0.03 99.27 83.3
O-301 C 42.73 0.10 22.19 1.98 7.13 0.35 21.09 4.82 <0.03 <0.03 100.41 84.1
R 42.40 0.09 22.30 1.95 7.13 0.34 20.99 4.87 <0.03 <0.03 100.09 84.0
O-332 C 40.41 0.07 22.28 0.60 17.26 0.63 14.54 4.19 0.05 <0.03 100.05 60.0
R 40.24 0.06 22.29 0.68 17.07 0.71 14.57 4.11 0.05 <0.03 99.77 60.3
LP 40.44 0.05 22.36 0.51 16.88 0.74 14.82 3.96 0.05 <0.03 99.82 61.0
O-264 C 42.14 0.11 22.82 1.33 6.15 0.27 21.93 4.85 0.04 <0.03 99.65 86.4
R 42.08 0.09 22.81 1.32 6.11 0.26 21.92 4.77 0.04 <0.03 99.38 86.5
O-550 C 42.72 0.12 23.44 0.44 7.01 0.25 21.78 4.15 0.04 <0.03 99.96 84.7
R 42.74 0.09 23.51 0.45 7.14 0.30 21.53 4.09 <0.03 <0.03 99.88 84.3
LP 42.62 0.07 23.49 0.29 8.06 0.34 20.98 3.98 <0.03 <0.03 99.87 82.3
UV41/03 C 40.03 <0.04 21.73 0.57 18.46 0.78 13.03 4.74 <0.03 <0.03 99.39 55.7
R 39.98 <0.04 21.75 0.58 18.27 0.80 12.95 4.76 <0.03 <0.03 99.13 55.8
LP 40.05 <0.04 21.73 0.61 18.91 0.86 12.73 4.48 <0.03 <0.03 99.41 54.5
Olivine websterites
M4/01 C 41.75 0.15 20.21 4.46 7.83 0.49 19.79 5.00 0.05 <0.03 99.74 81.83
R 41.72 0.08 20.14 4.54 8.04 0.48 19.49 4.97 0.07 <0.03 99.52 81.20
O-436 C 41.72 0.10 21.54 2.66 8.30 0.38 19.63 4.73 0.05 <0.03 99.11 80.8
R 41.47 0.10 21.36 2.81 8.23 0.42 19.77 4.89 0.04 <0.03 99.10 81.1
LP 41.79 0.08 22.05 1.94 8.02 0.37 20.62 4.44 0.04 <0.03 99.32 82.1
O-207 C 42.39 0.11 23.07 0.34 9.20 0.41 20.04 4.11 0.05 <0.03 99.71 79.5
R 42.25 0.11 23.26 0.36 9.34 0.37 19.93 3.98 0.05 <0.03 99.64 79.2
M31/01 C 41.49 0.07 22.07 1.50 9.66 0.60 19.48 4.29 0.06 <0.03 99.21 78.2
R 41.58 0.06 21.79 1.48 9.59 0.56 19.43 4.30 0.06 <0.03 98.85 78.3
UV223/09 C 42.35 0.24 22.24 1.49 8.88 0.40 20.22 4.08 0.06 <0.03 99.95 80.2
R 42.36 0.26 22.07 1.57 8.83 0.39 20.20 4.14 0.07 <0.03 99.90 80.3
UV127/09 C 41.99 0.31 20.30 3.70 8.21 0.42 20.10 4.51 0.09 <0.03 99.63 81.4
R 41.87 0.26 20.25 3.85 8.16 0.42 20.08 4.46 0.09 <0.03 99.45 81.4
Lherzolites
M34/01 C 41.52 0.18 19.79 4.62 8.44 0.53 18.61 5.57 0.06 <0.03 99.32 79.7
R 42.07 0.14 20.07 4.41 8.72 0.56 18.29 5.39 0.06 <0.03 99.67 78.9
O-571 C 41.29 0.12 21.52 2.73 7.90 0.45 20.57 5.10 0.05 <0.03 99.72 82.3
R 41.41 0.10 21.86 2.30 8.01 0.45 20.79 4.93 0.05 <0.03 99.89 82.2
Note: C, core; R, rim; LP, lamella in pyroxene; LG, lamella in garnet; Inc, inclusion. Total Fe as FeO.
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International Geology Review 1083
Figure 10. Pressure–temperature estimates for xenoliths from
Mir (filled symbols), Udachnaya (half-filled symbols), and
Obnazhennaya (open symbols). Symbols: circles, websterites;
squares, olivine websterites; triangles, clinopyroxenites; and dia-
mond, spinel-garnet lherzolite. Small crosses show the PTesti-
mates of eclogites and pyroxenites from Obnazhennaya of Taylor
et al. (2003), and stars show pyroxenite PTdata from Mir
of Roden et al. (2006). Conductive geotherms of Pollack and
Chapman (1977) and the diamond (D)–graphite (G) stability line
(Kennedy and Kennedy 1976) are shown for reference.
Koch 2003; Lazarov et al. 2009). The close approach to
equilibrium of most samples is also supported by the homo-
geneity of the pyroxenes within these xenoliths, where
the compositions of pyroxene porphyroblasts are similar
to those of lamellae within garnet and the other pyrox-
ene (Tables 2–4). PTestimates of six samples (spinel-
garnet orthopyroxenite UV345/08; four garnet websterites:
O-301, O-332, O-264 and UV41/03; and spinel-garnet
lherzolite O-571) were excluded due to large tempera-
ture discrepancies (i.e. >100C) between thermometers.
The pyroxenites equilibrated at relatively low pressures
and temperatures (2.0–4.5 GPa and 690–910C), which are
similar to that obtained for the spinel-garnet lherzolite from
Mir (2.4 GPa and 740C; Table 1). Xenoliths from the
Palaeozoic Mir kimberlite span the range of PTcondi-
tions, whereas the younger Obnazhennaya pipe has lower
PTvalues, compared to the Palaeozoic Udachnaya kim-
berlite (Figure 10). Olivine websterites were equilibrated at
the highest temperatures.
Discussion
Petrological significance of exsolution features
Constraining the formation of texturally and mineralogi-
cally diverse exsolution features in mantle pyroxene and
garnet porphyroblasts provides insights for understand-
ing the origin and evolution of this suite of xenoliths.
In this section, we discuss the stoichiometric and non-
stoichiometric substitutions that may describe the forma-
tion of the exsolution lamellae and inclusions observed in
pyroxene and garnet porphyroblasts, and we use this infor-
mation to place constraints on the history of these rocks.
Textural constraints and mineralogical controls on
exsolution morphology
Petrographic characteristics indicate that exsolution fea-
tures in the studied suite of xenoliths generally appear in
a successive order: thin lamellae–large lamellae–lamellae
changing into grains–clusters of exsolved grains–exsolved
grains at the boundary of the host mineral. This pro-
gressive coarsening of exsolution morphologies results
in an increase in the volume of exsolved phases over
time. Similar exsolution systematics have been identified in
pyroxenes from garnet pyroxenite bands in orogenic mas-
sifs (e.g. Sautter and Fabriès 1990), other mantle xenolith
suites (e.g. Mercier and Nicolas 1975), and experimental
studies (e.g. Ried and Fuess 1986; Weinbruch et al. 2003).
Although this general model accounts for the majority of
the exsolution characteristics observed in pyroxene por-
phyroblasts, some garnet, oxide, and pyroxene exsolution
features appear in sequences that differ from one another
in detail. For example, spinel and ilmenite exsolution fea-
tures do not generally coarsen to form large (>500 µm)
grains.
Exsolution lamellae and inclusions in pyroxene
Exsolution features present in pyroxenes may be related to
the redistribution of molecular constituents and/or cations
described by exchange reactions, as outlined in Garrison
and Taylor (1981). Diopside and enstatite components
may exsolve from an initial pyroxene of broadly diopsidic
composition (Pokhilenko 1990). However, the formation
of garnet in pyroxene porphyroblasts is conditioned by
the presence of the tschermakite component in the initial
pyroxene (Herzberg 1978); garnet and pyroxene compo-
sitions are plotted on an A–C–FM diagram (Figure 11),
which provides a visual analogue of this process. Jerde
et al. (1993) note that the processes capable of producing
garnet exsolution through the removal of the Tschermak
component of pyroxene are varied, as the solubility of the
Tschermak component in pyroxene decreases with increas-
ing pressure and decreasing temperature (e.g. Gasparik
1990). Therefore, processes that involve a pressure increase
or temperature decrease (or a combination of the two)
may result in garnet exsolution from pyroxene (Jerde et al.
1993). Although the measured pyroxene porphyroblasts
in this study generally have low tschermakite abundances
(<4.94 mol.%), reconstructed total pyroxene compositions
are enriched in tschermakite (up to 10.9 mol.%), and this
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1084 T.A. Alifirova et al.
Figure 11. A–C–FM diagram showing present host and initial phase compositions of garnet, orthopyroxene, and clinopyroxene. Arrows
represent compositional evolution of these minerals by continued exsolution processes. A =Al2O3+Cr2O3–Na
2O, C =CaO,
F. M . =FeOtotal +MgO +MnO.
evidence supports the notion that garnets may exsolve from
a relatively tschermakite-rich initial pyroxene.
Ilmenite inclusions in pyroxene may not have formed in
response to stoichiometric reactions, as Ti4+does not enter
tetrahedral positions and 2FeTiO3does not completely cor-
respond to pyroxene crystal chemistry (Garrison and Taylor
1981). Previous studies have suggested that the Ti4+in
M1 position enters the pyroxene structure as a R2+TiAl2O6
molecule, where R2+=Fe, Ca, Mg (Onuma and Kimura
1978; Garrison and Taylor 1981). As such, it is possible
to produce ilmenite by exsolution reactions in pyroxenes
through the non-stoichiometric components of pyroxenes
with participation of NATAL molecules (NaTiAlSiO6and
NaTiAlO6, where is vacant in the tetrahedral position;
Garrison and Taylor 1981). In this case, free oxygen either
participates in the reaction (exsolution with oxidation)
or is released (exsolution with reduction; Garrison and
Taylor 1981). The reactions involving oxygen and NATAL
molecules can also account for the presence of the jadeite
component in pyroxenes and the hematite component in
ilmenites.
Minor amounts of spinel are present in pyroxene
porphyroblasts, and these crystals may represent exso-
lution products (Garrison and Taylor 1981). Molecules
MgMgSiO6,MgAl
2O6,Mg
2SiO4, and MgAl2O4may
participate during spinel formation and may be present in
non-stoichiometric pyroxenes as isomorphic components
(Garrison and Taylor 1981). Non-stoichiometric clinopy-
roxenes have been described in natural minerals (e.g.
Sobolev et al. 1968) and are reported in experimental prod-
ucts (e.g. Kushiro 1972). Sobolev et al. (1968) and Jerde
et al. (1993) observed high-Al non-stoichiometric pyrox-
enes in Siberian xenoliths, and the presence of tschermakite
and Ca-eskola molecules in the pyroxene structure may
account for the excess Al that produces kyanite and corun-
dum exsolution. Garrison and Taylor (1981) proposed that
non-stoichiometric pyroxenes associated with spinel exso-
lution may be indicative of a cation deficit (Si and Al) in
the tetrahedral position. Vacant tetrahedral positions may
result in a charge imbalance producing an unstable config-
uration. However, the charge imbalance may be corrected
for if H4replaces IVSi4+(Sclar et al. 1968) or if an OH
group molecule precipitates on the vacancies (Martin and
Donnay 1972; Wilkins and Sabine 1973).
Exsolution lamellae and inclusions in garnet
Exsolution of pyroxene identified in garnet porphyroblasts
may be explained by the breakdown of majoritic garnet to
form clinopyroxene and garnet (e.g. van Roermund et al.
2001). The presence of rutile and ilmenite in garnet may
result from the breakdown of Ti-rich garnet (van Roermund
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International Geology Review 1085
et al. 2000a), where the reaction product TiO3may occur
as TiO32– or TiO2+O2–. Under oxidizing and reduc-
ing conditions, TiO3may react to produce TiO2+O2or
TiO2+OH.
Ti-phase formation in pyroxenes may be due to the
presence of NATAL molecules in primary porphyrob-
lasts, and similar reasoning may be applied to account for
ilmenite and rutile exsolution in garnets (e.g. Yang and
Liu 2004; Song et al. 2005). In this case, the following
scheme describes cation substitutions: M2+Ti4+2Al3+
or Na+Ti4+Ca2+Al3+(Zhang and Liou 2003) com-
bined with Mg2+Si4+2Al3+or Na+Si4+Ca2+Al3+
(Sobolev and Lavrent’ev 1971; Zhang and Liou 2003).
Several poly-phase exsolution features with textural
characteristics indicating that they formed simultaneously
are observed in the garnet porphyroblasts of the studied
xenolith suite. Of particular interest is the exsolution of
rutile +chromite and pyroxene in garnet (e.g. Figure 4),
which likely results from the breakdown of Cr–Ti garnet to
form pyroxene, chromite, and rutile (van Roermund et al.
2000a,b). Also, plagioclase is notably present in a poly-
phase exsolution feature in websterite M5/01 from the
Mir kimberlite (Figure 7). It is unlikely that plagioclase
exsolved directly from garnet (Pokhilenko and Alifirova
2011); rather it may form after an earlier exsolution step
that exsolves Na–Cr–Al pyroxene from the host (Yang and
Liu 2004; Pokhilenko and Alifirova 2011).
As mentioned above, faceted F-apatite prisms that are
closely aligned with elongate rutile and ilmenite inclu-
sions have been identified in phlogopite-bearing orthopy-
roxenite UV70/03 (Figures 6C and 6D). Pokhilenko and
Alifirova (2011) have suggested that the association of
rutile +ilmenite +apatite results from exsolution from
a Ti–P–F garnet. To account for apatite and Ti-oxide
exsolution features in garnet, [TiO4] and [PO4] may poten-
tially substitute for [SiO4] in garnet (e.g. Thompson 1975;
Bishop et al. 1976) and F may substitute for the hydroxyl
group in the garnet structure (e.g. Valley et al. 1983).
Apatite exsolution in garnet has also been identified in
a wide range of lithologies that include mantle eclog-
ites of the Koidu kimberlite complex (Haggerty et al.
1994), ultramafic bodies of the Sulu metamorphic com-
plex (Ye et al. 2000), metapelites of the UHP terrane
of Rhodope Province, Greece (Mposkos and Kostopoulos
2001), and garnet megacrysts from Australian kimberlite
pipes and palaeoalluvial deposits of the Bingara-Copeton
area, Australia (Barron and Barron 2008).
The origin of rare olivine exsolution in garnet is
not well constrained, and olivine in garnets from garnet-
websterite M5/01 differ from wadsleyite observed in
experimental products resulting from high-temperature
decompression of relatively fertile garnet lherzolite
(14–12 GPa; Dobrzhinetskaya et al. 2004). In addition,
Afanas’ev et al. (2001) have suggested that olivine inclu-
sions may form when pre-existing olivine exsolution
recrystallizes. However, it is possible that olivine exsolves
directly from an initial garnet of non-stoichiometric com-
position, but we are not able to distinguish between these
two scenarios with the information that is currently avail-
able. As an alternative, we suggest a feasible path of
Ti–Si garnet breakdown, with the formation of pyrox-
ene, olivine, and rutile lamellae: Mg3(Mg,Ti)Si3O12 (Ti-
garnet) +Mg3(Mg,Si)Si3O12 (Si-garnet) 3Mg2Si2O6
(pyroxene) +Mg2SiO4(olivine) +TiO2(rutile).
Figure 7B illustrates the result of this process, which is
expressed by the close spatial distribution of the reaction
products.
Sub solidus history of Siberian pyroxenites
The compositions of xenolith porphyroblasts, and their
associated populations of exsolved phases, can be used
to place constraints on the sub solidus chemical and
PThistory. The abundant exsolution lamellae in garnets
and pyroxenes from these pyroxenites require mantle res-
idence at relatively low pressures and temperatures for
extended periods of time. The length of this residence
period is not constrained, but based on Nd-isotope dise-
quilibrium between phases in the pyroxenites from Mir, it
was probably in excess of several hundred million years
(Roden et al. 1999, 2006; Agashev et al. 2001). Mineral
Nd-isotope disequilibrium in xenoliths containing garnets
with exsolution lamellae from Africa was also reported
by Macdougall and Haggerty (1999), and these authors
also suggest that such xenoliths were stored at moderate
pressures in the lithosphere for several hundred million
years. The mineral compositions of the pyroxenite xeno-
liths in this study are very similar to those of the lherzolites
(Tables 2–4), which also equilibrated at relatively low pres-
sures and temperatures, suggesting that regardless of the
history of the pyroxenites, they last equilibrated at upper
mantle conditions.
Variations in molecular Ca and Al in silicate porphy-
roblasts (e.g. Figure 10) indicate significant sub solidus
redistribution of cations. Brey et al. (1990) have suggested
that Ti contents in garnets may correlate positively with
pressure, which is consistent with experimental work by
Zhang et al. (2003). Experimental observations indicate
that Ti phases exsolve from garnet as pressure decreases.
In addition, it has been suggested that Ti is more ‘comfort-
able’ in the pyroxene structure, compared to that of garnet,
over the temperature interval of 1100–1300C, while at
lower temperatures Ti may prefer oxide (ilmenite and
rutile) structures (Zhang et al. 2003). In addition, textural
relationships among exsolution features of several studied
garnets support the notion that ilmenite and rutile may have
exsolved from an earlier pyroxene exsolution. These obser-
vations provide important evidence that some portions of
the Siberian lithospheric mantle may have experienced
cooling to <1100C prior to kimberlite entrainment, and
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1086 T.A. Alifirova et al.
this cooling was potentially accompanied by a decrease in
pressure.
Mineralogical constraints on the origin of pyroxenitic
xenoliths of Yakutia
Coarse-grained pyroxenitic xenoliths from Yakutia contain
paragenetic phase assemblages that are indicative of a com-
plex and long-lived sub solidus history. To constrain the
magmatic conditions that may have crystallized the pyrox-
enitic protolith, reconstructed porphyroblast major-element
compositions (Table 5) have been calculated by combining
the volume proportions of the host mineral with exsolved
phases of known major-element compositions and assumed
densities.
Reconstructed clinopyroxene compositions
Reconstructed clinopyroxene compositions show
Wo38–48.6En45–54.4 Fs2.4–12 variations for all studied
xenoliths and have a general trend towards decreasing
wollastonite and enstatite components, with increasing
ferrosilite proportions. These compositions have higher
Mg and Al contents (MgO =13.1–16.7 wt.%; Al2O3=
2.0–10.1 wt.%) than clinopyroxenes in the matrix of the
corresponding xenoliths. Additionally, some samples also
yield reconstructed clinopyroxene porphyroblast composi-
tions that have relatively high TiO2concentrations (up to
1.51 wt.%). The highest TiO2values (0.95–1.51 wt.%) are
in reconstructed clinopyroxenes from websterite (O-264,
O-173, M5/01), olivine websterite (O-207), and lherzolite
(O-571) xenoliths. Compared to measured clinopyroxene
compositions, the reconstructed clinopyroxenes contain
lower jadeite (up to 20.5 mol.% vs. 1.2–26.2 mol.%)
and higher Ca-tschermakite (up to 10.9 mol.% vs.
0–4.9 mol.%) contents, suggesting that the aluminium
cations may have redistributed in clinopyroxenes from
IV to VI coordination during exsolution process. This is
well demonstrated in clinopyroxenite O-125, where the
measured composition has 1.8 mol.% of Ca-tschermakite
and 26.2 mol.% of jadeite, whereas the reconstruction
produces 10.9 mol.% and 20.5 mol.% of these compo-
nents, respectively. Some of olivine websterites (O-436
and M4/01) have reconstructed clinopyroxene porphy-
roblast compositions with Cr2O3contents higher than the
corresponding matrix compositions (1.43 and 3.25 wt.%
vs. 1.36 and 2.41 wt.%, respectively).
Reconstructed orthopyroxene compositions
Reconstructed orthopyroxene compositions have
En75.7–93.8Fs5.1–23.9 contents that are similar to matrix
orthopyroxene compositions, with higher wollastonite
proportions (0.4–4.2 mol.%). The reconstructed compo-
sitions suggest that these phases may have crystallized
as clinopyroxene at high pressures and temperatures, and
that these pyroxenes may have been characterized by
relatively high Al2O3contents (up to 6.29 wt.%), with up
to 12.3 mol.% of tschermakite. CaO and TiO2contents
are up to 2.13 and 0.96 wt.%, respectively (Table 5).
Reconstructed orthopyroxenes from olivine websterite
and orthopyroxenite (O-436 and UV345/08) are char-
acterized by relatively high Cr2O3contents (1.3 and 1.2
wt.%, respectively). Sodium contents increase slightly in
reconstructed orthopyroxenite compositions (e.g. from
0.06 wt.% in matrix composition to 0.24 wt.% of Na2Oin
the reconstruction for sample M5/01).
Reconstructed garnet compositions
The reconstructed garnet major-element composi-
tions did not significantly differ from measured
garnet porphyroblast compositions and have a simi-
lar range of pyrope, almandine, and uvarovite contents
(Prp45.7–75.4Alm12–43.7Uvr0.1–12 ). Grossular (<7.9 mol.%)
and andradite (<4.9 mol.%) contents are slightly lower
than those measured in matrix garnets. Reconstructed
compositions generally have higher TiO2(up to 1.9 wt.%),
and consequently have higher Ti-andradite contents (up
to 5.1 mol.%), with the highest TiO2values observed for
garnets from olivine websterite UV223/09 and UV127/09
(TiO21.58–1.9 wt.%, 4.3–5.1 mol.% of Ti-andradite). The
host garnet in UV70/03, which contains 1.06 vol.% of
apatite rods, has lower measured CaO contents compared
to the reconstructed values (3.21 vs. 3.63 wt.%). Despite
the high-volume portion of pyroxene lamellae in most
garnets (up to 1.85 vol.% in garnets from UV223/09),
the corresponding reconstructed garnet compositions
still maintain normal Si/O ratios (about 3 Si atoms per
12 oxygens), which likely results from a balancing of
majoritic and titaniferous components within precursor
garnet compositions.
Affinities between primary minerals and magma
products
Many authors have suggested that exsolution features in
pyroxene and garnet porphyroblasts in pyroxenite man-
tle xenoliths can largely be accounted for by decreasing
temperature and pressure conditions subsequent to mag-
matic crystallization (e.g. Sobolev and Sobolev 1964;
Kirby and Etheridge 1981; Sen and Jones 1988; Becker
1997). Observations from this study of Siberian xenoliths
can generally be explained by a broadly analogous P
Thistory. Additionally, the reconstructed compositions of
pyroxenes and garnets often contain significant NATAL
components, and many initial garnets may have had high
Si contents consistent with majorite compositions, fea-
tures which would support initial crystallization at elevated
pressure conditions.
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International Geology Review 1087
Table 5. Reconstructed mineral major-element (wt.%) compositions.
Sample # SiO2TiO2Al2O3Cr2O3FeO MnO MgO CaO Na2OK
2OTotal Mg#
Reconstructed garnet compositions
O-125 41.43 0.04 22.71 0.16 13.37 0.33 17.25 3.91 0.02 0.00 99.23 69.68
O-173 42.50 0.95 23.28 0.36 9.37 0.37 20.05 4.44 0.04 0.01 101.38 79.22
O-207 41.94 1.10 22.79 0.35 9.12 0.40 19.85 4.06 0.05 0.00 99.65 79.50
O-107 41.66 0.49 22.68 0.13 13.05 0.30 17.81 3.89 0.03 0.00 100.04 70.85
O-264 41.85 0.75 22.65 1.33 6.11 0.27 21.79 4.82 0.04 0.00 99.61 86.40
O-332 40.07 0.91 22.00 0.61 17.10 0.62 14.45 4.17 0.06 0.00 100.00 60.08
O-571 40.88 0.97 21.31 2.71 7.86 0.44 20.39 5.05 0.05 0.00 99.67 82.21
O-301 42.40 0.77 22.02 1.97 7.10 0.35 20.95 4.78 0.02 0.00 100.36 84.02
O-550 42.38 0.81 23.26 0.45 6.98 0.25 21.63 4.12 0.04 0.00 99.92 84.67
M4/01 41.56 0.55 20.12 4.44 7.81 0.48 19.71 4.98 0.05 0.01 99.72 81.81
M5/01 42.20 0.93 22.79 0.74 7.81 0.32 20.98 3.79 0.07 0.00 99.61 82.72
M31/01 41.48 0.10 22.06 1.50 9.65 0.60 19.47 4.28 0.06 0.01 99.21 78.24
M34/01 41.16 1.04 19.51 4.57 8.39 0.52 18.48 5.53 0.06 0.01 99.27 79.69
UV70/03 39.31 0.48 21.77 0.04 21.35 0.32 12.21 3.63 0.02 0.00 99.11 50.47
UV201/09 41.08 0.54 20.12 2.37 10.99 0.50 17.53 6.31 0.10 0.00 99.56 73.98
UV223/09 41.76 1.90 21.49 1.47 8.70 0.38 19.88 4.18 0.10 0.00 99.85 80.28
UV127/09 41.48 1.58 19.79 3.64 8.15 0.41 19.81 4.60 0.13 0.00 99.58 81.25
UV345/08 40.57 0.38 19.69 4.12 11.21 0.54 16.65 6.59 0.02 0.00 99.78 72.59
Reconstructed clinopyroxene compositions
O-125 52.15 0.10 10.10 0.15 5.80 0.10 13.50 14.35 3.10 0.00 99.37 80.58
O-173 52.41 0.97 7.89 0.29 2.78 0.09 14.46 18.22 2.44 0.00 99.55 90.26
O-207 53.40 0.99 6.91 0.38 2.75 0.06 15.65 16.13 2.65 0.00 98.92 91.02
O-436 53.19 0.18 3.98 1.43 2.30 0.06 16.52 19.49 1.59 0.04 98.80 92.74
O-107 54.76 0.12 6.04 0.12 4.38 0.05 15.23 15.63 2.94 0.01 99.29 86.09
O-264 52.72 1.51 4.96 0.94 1.32 0.05 15.59 20.21 2.47 0.00 99.76 95.48
O-332 54.10 0.32 4.52 0.48 5.44 0.27 14.83 17.12 3.03 0.01 100.12 82.92
O-571 53.01 0.95 4.17 1.78 1.53 0.06 15.55 19.90 2.85 0.00 99.79 94.75
O-301 53.49 1.08 5.47 1.37 1.34 0.06 14.93 19.65 2.25 0.00 99.64 95.19
O-550 53.94 0.62 7.24 0.31 2.09 0.07 15.15 17.67 2.68 0.01 99.77 92.82
M4/01 54.62 0.20 3.38 3.25 1.65 0.05 14.96 19.43 2.68 0.01 100.23 94.17
M5/01 54.58 1.06 5.73 0.42 1.65 0.02 14.19 17.95 3.62 0.00 99.20 93.89
M31/01 54.16 0.12 2.18 0.82 2.61 0.07 15.78 20.63 2.15 0.01 98.54 91.52
M34/01 54.88 0.48 3.19 2.32 1.34 0.06 14.53 20.08 2.77 0.00 99.65 95.09
UV41/03 52.62 0.62 6.52 0.63 4.15 0.10 13.14 19.19 2.18 0.00 99.16 84.95
UV201/09 54.32 0.22 2.79 0.73 4.06 0.10 16.66 19.65 1.73 0.02 100.29 87.96
UV223/09 55.82 0.57 3.21 0.89 2.22 0.08 15.96 19.02 2.64 0.02 100.42 92.77
UV127/09 55.08 0.71 3.06 2.33 1.97 0.06 15.43 18.14 3.30 0.00 100.11 93.31
UV345/08 54.09 0.46 2.03 1.28 2.17 0.04 16.74 22.57 1.05 0.00 100.44 93.23
Reconstructed orthopyroxene compositions
O-125 52.63 0.01 6.29 0.06 10.60 0.14 28.68 1.12 0.04 0.00 99.59 82.82
O-173 58.06 0.39 1.75 0.06 5.93 0.10 33.69 1.29 0.14 0.01 101.42 91.01
O-207 57.42 0.68 1.04 0.19 5.06 0.05 34.17 1.05 0.16 0.00 99.81 92.32
O-436 54.50 0.09 3.54 1.30 5.49 0.12 32.89 1.14 0.11 0.00 99.20 91.43
O-264 57.00 0.96 1.16 0.26 3.50 0.07 36.39 0.60 0.09 0.00 100.03 94.88
O-332 55.45 0.11 0.64 0.22 12.42 0.27 29.64 0.61 0.15 0.00 99.50 80.97
O-571 56.31 0.46 1.36 0.39 4.53 0.10 36.10 0.66 0.14 0.00 100.05 93.42
O-301 55.19 0.84 2.50 1.09 4.45 0.11 34.87 0.76 0.08 0.00 99.91 93.31
O-550 57.83 0.53 1.47 0.17 4.61 0.03 35.32 0.74 0.11 0.01 100.82 93.17
M4/01 57.32 0.02 0.24 0.82 5.03 0.00 35.63 0.49 0.07 0.00 99.61 92.66
M5/01 55.61 0.57 3.44 0.15 4.86 0.09 32.92 1.41 0.24 0.00 99.30 92.35
M31/01 57.81 0.02 0.44 0.11 5.60 0.16 35.12 0.34 0.09 0.01 99.68 91.79
M34/01 57.70 0.54 0.95 0.66 4.69 0.09 34.36 0.83 0.15 0.01 99.97 92.89
UV41/03 53.79 0.58 3.31 0.29 12.08 0.24 28.85 0.80 0.07 0.00 100.01 80.98
UV70/03 52.95 0.05 3.89 0.03 15.32 0.08 27.19 0.18 0.03 0.00 99.72 75.98
UV223/09 58.16 0.59 0.49 0.12 5.37 0.10 34.54 0.73 0.16 0.00 100.26 91.97
UV127/09 57.84 0.82 0.59 0.30 4.84 0.09 34.68 0.68 0.19 0.00 100.03 92.73
UV345/08 54.83 0.45 2.70 1.20 6.74 0.13 31.61 2.13 0.10 0.00 99.92 89.31
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1088 T.A. Alifirova et al.
The parent phase of the pyroxene ±garnet ±oxide
±kyanite ±corundum assemblage, which is analogous
to the studied sample suite, is generally considered to
be an initial pyroxene that contained significant portions
of tschermakite +jadeite ±Ca-eskola molecules, and
such a primary phase has been interpreted as the product
of high-pressure magmatism at high pressures (generally
>25 kbar, Harte and Gurney 1975). However, individual
garnet pyroxenites may differ from one another in detail
and this illustrates the need for caution when interpreting
their depth of origin and the linkages between these mantle
samples, intrusive features, and surface magma suites. For
example, some garnet pyroxenites in peridotite massifs, for
which field constraints are available, have been central to
debates concerning the depth of magma genesis, models of
isobaric and polybaric cooling, and the degree of vertical
transport experienced by mantle portions associated with
collisional terranes (e.g. garnet pyroxenite bands of the
Ariège-group massifs, Pyrenees; Herzberg 1978; Conquéré
1979; Conquéré and Fabriès 1984). Conquéré (1979)
argued that the depth of melt generation and crystallization
that is recorded by garnet pyroxenites from the Ariège-
group massifs may be limited to <25 kbars, due to the
fact that garnet occurs only as an exsolution phase in pri-
mary pyroxene. As such, the Yakutian pyroxenites reported
in this study (e.g. UV41/03) which lack primary garnet
could potentially be derived from shallower sources than
pyroxenites containing garnet porphyroblasts of magmatic
origin. Furthermore, Jerde et al. (1993) have suggested that
kyanite ±corundum-bearing xenoliths from Yakutia may
yield reconstructed Al2O3abundances >24 wt.%, compo-
sitions that may be unrealistic for terrestrial pyroxenes; thus
these materials may have been derived from transformed
plagioclase-bearing cumulates.
Similar to the garnet porphyroblasts reported in this
study, garnets from kimberlite-hosted peridotite and pyrox-
enite xenoliths (e.g. Kaapvaal Craton, Sautter et al. 1991)
and ultramafic portions of UHP terranes (c.f. Liou et al.
2007; Pandey et al. 2010) contain pyroxene lamellae that
can be accounted for by exsolution from an initial garnet
of majorite composition. High-pressure experiments sug-
gest that majorite garnet is stable at pressures in excess of
60–100 kbars (e.g. Takahashi 1986; Irifune 1987; Gasparik
1990; Ohtani 1990). Majoritic garnets have more than 3
cations of Si per 12 oxygens in response to the substitu-
tion of Si for Al in the sixfold site. Si substitution increases
with pressure and results in extensive, or even complete,
dissolution of pyroxene into garnet in the mantle transition
zone (e.g. Irifune 1987; Ringwood, 1991). Si excesses (up
to 3.02 cations) are observed within this suite of Yakutian
pyroxenites, suggesting that xenoliths containing these gar-
net porphyroblasts may have originated at great depth.
Taken together, primary pyroxene and garnet phases
in the Yakutian pyroxenite suite may be explained by
magmatism occurring over a wide range of pressures,
some of which may have taken place in the presence
of majoritic garnet. Furthermore, exsolution systematics
observed in these pyroxenites suggest that the grain size
and phase proportions of these lithologies change over
time, likely due to decreasing temperature ±pressure con-
ditions, and these modifications may have consequences for
the rheological properties of pyroxenites and strain local-
ization in lithospheric mantle (e.g. Vissers et al. 1997;
Toy et al. 2010). These findings have important implica-
tions for the conditions under which alkaline, tholeiitic,
and basanitic magmas were generated beneath the Siberian
Craton, and further studies of these samples are required to
assess the possible links between cumulate pyroxenites and
known Siberian magma products, such as the large-volume
Siberian flood basalts (e.g. Hawkesworth et al. 1995; Horan
et al. 1995; Arndt et al. 1998; Elkins-Tanton and Hager
2000).
The formation of most pyroxenites can be explained
by crystallization from mantle-derived melts (Bodinier
and Godard 2003; Kaeser et al. 2009). However, some
globally distributed suites of pyroxenite xenoliths that con-
tain multi-phase exsolution assemblages have trace ele-
ment and radiogenic- and stable-isotope compositions that
may indicate the involvement of recycled oceanic crust
and depleted mantle in regions of melt generation (e.g.
Solov’eva 1986; Xu 2002; Bizimis et al. 2005, 2007; Day
et al. 2010; Downes 2007; Gonzaga et al. 2010). This evi-
dence may suggest that subducted lithosphere is present
in asthenospheric environments, and when combined with
geochemical studies of basalts (e.g. Zindler and Harte
1986; Hofmann 2003), the geochemical and chronolog-
ical characteristics indicate that these compositional and
isotopic heterogeneities persisted over long time periods
(>1 thousand million years) in Earth’s upper mantle.
These observations have important implications for man-
tle dynamics and the composition and mass balance of
Earth’s internal reservoirs. As such, trace element abun-
dances, stable- and radiogenic-isotope compositions, and
chronological data for this suite of pyroxenite xenoliths are
integral for constraining petrogenetic links between crust
and mantle portions of the Siberian lithosphere, informa-
tion that is vital for understanding the evolution of mag-
matism in this region. Thus, the sample suite reported here
may represent an important contribution towards a greater
understanding of Siberian lithospheric mantle, and addi-
tional geochemical studies of pyroxenitic xenoliths may
provide important information on the magmatic history
and evolution of the diamondiferous Siberian lithospheric
mantle.
Summary
Exsolution lamellae in pyroxene and garnet porphyroblasts
from Siberian pyroxenite mantle xenoliths display a com-
plex and protracted sub solidus history following magmatic
Downloaded by [Taisia Aleksandrovna Alifirova] at 22:19 21 May 2012
International Geology Review 1089
crystallization. Textural features demonstrate that the pro-
gressively exsolved phases increased volumetrically, and
in some cases, the bulk xenolith changed from a coarse-
grained (i.e. >2 cm; megacrystalline) to a fine-grained
(i.e. <1 cm) lithology. Further, reconstructed compositions
of initial pyroxene and garnet crystals suggest that several
of the studied pyroxenites formed at PTconditions that
may approach those of the majorite stability field. However,
pyroxenites that do not contain primary garnet may have
been derived by melting at shallower depths.
Progressive exsolution in these pyroxenites may be
of great importance inasmuch as such processes attend
localized changes in rheological properties and may also
accommodate strain within portions of lithospheric man-
tle. The range of conditions under which these pyroxenites
were generated provides additional constraints on the gen-
esis of alkaline, tholeiitic, and basanitic magmas in the
Siberian mantle. In addition, previous studies have sug-
gested that subducted materials may be involved in the
generation of magmas that crystallize pyroxenites with
multi-phase exsolution assemblages, providing evidence of
persistent compositional and isotopic heterogeneities in the
Earth’s upper mantle.
Acknowledgements
We express our sincere appreciation to U. Schmidt, A.T. Titov,
M.V. Khlestov, L.V. Usova, V.N. Korolyuk, A.P. Shebanin,
A.V. Korsakov, and E.V. Petrushin for their assistance with ana-
lytical work and preparation of samples. The sample UV70/03
was put at our disposal by A.V. Golovin. Partial support, particu-
larly for the writing of this paper, was provided by the Planetary
Geosciences Institute at the University of Tennessee.
References
Afanas’ev, V.P., Zinchuk, N.N., and Pokhilenko, N.P.. 2001.
Morphology and morphogenesis of indicator minerals in kim-
berlites: Novosibirsk, Izdatel’stvo, SO RAN, Filial “Geo”, p.
276 (in Russian).
Agashev, A.M., Watanabe, T., Kuligin, S.S., Pokhilenko, N.P.,
and Orihashi, Y., 2001, Rb–Sr and Sm–Nd isotopes in
garnet pyroxenite xenoliths from Siberian kimberlites: an
insight into lithospheric mantle: Journal of Mineralogical and
Petrological Sciences, v. 96, p. 7–18.
Aoki, K., Fujimaki, H., and Kitamura, M., 1980, Exsolved garnet-
bearing pyroxene megacrysts from some South African kim-
berlites: Lithos, v. 13, p. 269–279.
Arndt, N., Chauvel, C., Czamanske, G., and Fedorenko, V., 1998,
Two mantle sources, two plumbing systems: Tholeiitic and
alkaline magmatism of the Maymecha River Basin, Siberian
flood volcanic province: Contributions to Mineralogy and
Petrology, v. 133, p. 297–313.
Barron, B.J., and Barron, L.M., 2008, Phanerozoic diamonds
and decompression microstructures in eclogitic garnet and
clinopyroxene from eastern Australia: Implications for con-
tinental crust-mantle development, in Extended Abstracts
of the 9th International Kimberlite Conference: Frankfurt,
Germany, abstract #: 9IKC-A-00025.
Becker, H., 1997, Petrological constraints on the cooling his-
tory of high-temperature garnet peridotite massifs in lower
Austria: Contributions to Mineralogy and Petrology, v. 128,
p. 272–286.
Bishop, F.C., Smith, J.V., and Dawson, J.B., 1976, Na, P, Ti and
coordination of Si in garnet from peridotite and eclogite
xenoliths: Nature, v. 260, p. 696–697.
Bizimis, M., Griselin, M., Lassiter, J.C., Salters, V.J.M., and
Sen, G., 2007, Ancient recycled mantle lithosphere in the
Hawaiian plume: Osmium-hafnium isotopic evidence from
peridotite mantle xenoliths: Earth and Planetary Science
Letters, v. 257, p. 259–273.
Bizimis, M., Sen, G., Salters, V.J.M., and Keshav, S., 2005, Hf-
Nd-Sr isotope systematic of garnet pyroxenites from Salt
Lake Crater, Oahu, Hawaii: Evidence for a depleted compo-
nent in Hawaiian volcanism: Geochimica et Cosmochimica
Acta, v. 69, p. 2629–2646.
Bodinier, J.L., and Godard, M., 2003, Orogenic, ophiolitic, and
abyssal peridotites, in Holland, H.D., and Turrekian, K.K.,
eds., Treatise on geochemistry, v. 2: Amsterdam, Elsevier,
p. 103–170.
Boyd, F.R., and Nixon, P.H., 1973, Origin of the ilmenite-silicate
nodules in kimberlites from Lesotho and South Africa:
Lesotho kimberlites, Maseru Press, p. 254–268.
Brenker, F.E., Stachel, T., and Harris, J.W., 2002, Exhumation of
lower mantle inclusions in diamond: A TEM investigation of
retrograde phase transitions, reactions and exsolution: Earth
and Planetary Science Letters, v. 198, p. 1–9.
Brey, G.P., Koehler, T., and Nickel, K.G., 1990,
Geothermobarometry in four-phase lherzolites I.
Experimental results from 10 to 60 kbar: Journal of
Petrology, v.31, p.1313–1352.
Brey, G.P., and Kohler, T., 1990, Geothermobarometry in four-
phase lherzolites II: New thermobarometers and practi-
cal assessment of existing thermobarometers: Journal of
Petrology, v. 31, p. 1353–1378.
Carlson, R.W., Czamanske, G., Fedorenko, V., and Ilupin, I.,
2006, A comparison of Siberian meimechites and kimber-
lites: implications for the source of high-Mg alkali magmas
and flood basalts: Geochemistry, Geophysics, Geosystems,
v. 7, doi:10.1029/2006GC001342.
Carswell, D.A., Dawson, J.B., and Gibb, F.G.F., 1981,
Equilibration conditions of upper-mantle eclogites:
Implications for kyanite-bearing and diamondiferous
varieties: Mineralogical Magazine, v. 44, p. 79–89.
Clarke, D.B., and Pe-Piper, G.G., 1983, Multiply exsolved
clinopyroxene megacrysts from the Frank Smith pipe, Cape
Province, South Africa: Lithos, v. 16, p. 75–84.
Conquéré, F., 1979, Comments on ‘the bearing of phase-equilibria
in simple and complex systems on the origin and evolu-
tion of some well-documented garnet websterites’ by C.T.
Herzberg: Contributions to Mineralogy and Petrology, v. 219,
p. 219–222.
Conquéré, F., and Fabriès, J., 1984, Chemical disequilibrium and
its thermal significance in spinel peridotites from the Lherz
and Freychinede ullramafic bodies (Ariege, French Pyrenees),
in Kornprobst, J., ed., Kimberlites II: The mantle and crust-
mantle relationships: Amsterdam, Elsevier, p. 319–331.
Courtillot, V., and Renne, P., 2003, On the ages of flood basalt
events: Comptes Rendus Geoscience, v. 335, p. 113–140.
Davis, G.L., Sobolev, N.V., and Khar’kiv, A.D., 1980, New data
on the age of Yakutian kimberlites obtained by the uranium-
lead method on zircons: Doklady Earth Sciences v. 254,
p. 53–57.
Dawson, J.B., 2004, A fertile harzburgite–garnet lherzolite
transition: possible inferences fro the roles of strain and
metasomatism in upper mantle peridotites: Lithos, v. 77,
p. 553–569.
Downloaded by [Taisia Aleksandrovna Alifirova] at 22:19 21 May 2012
1090 T.A. Alifirova et al.
Dawson, J.B., and Reid, A.M., 1970, A pyroxene-ilmenite
intergrowth from the Monastery Mine, South Africa:
Contributions to Mineralogy and Petrology, v. 26, p. 296–301.
Day, J.M.D., Pearson, D.G., Macpherson, C.G., Lowry, D., and
Carracedo, J.-C., 2010, Evidence for distinct proportions
of subducted oceanic crust and lithosphere in HIMU-type
mantle beneath El Hierro and La Palma, Canary Islands:
Geochimica et Cosmochimica Acta, v. 74, p. 6565–6589.
Dobrzhinetskaya, L.F., Green, H.W., Renfro, A.P., Bozhilov,
K.N., Sprengler, D., and van Roermund, H.L.M., 2004,
Precipitation of pyroxenes and Mg2SiO4 from majoritic gar-
net: simulation of peridotite exhumation from great depth:
Terra Nova, v. 16, p. 325–330.
Downes, H., 2007, Origin and significance of spinel and gar-
net pyroxenites in the shallow lithospheric mantle: Ultramafic
massifs in orogenic belts in Western Europe and NW Africa:
Lithos, v. 99, p. 1–24.
Elkins-Tanton, L.T., and Hager, B.H., 2000, Melt intrusion as a
trigger for lithospheric foundering and the eruption of the
Siberian flood basalts: Geophysical Research Letters, v. 27,
p. 3937–3940.
Franz, L., Brey, G.P., and Okrusch, M., 1996a, Reequilibration
of ultramafic xenoliths from Namibia by metasomatic pro-
cesses at the mantle boundary: Journal of Geology, v. 104,
p. 599–615.
Franz, L., Brey, G.P., and Okrusch, M., 1996b, Steady-state
geotherm, thermal disturbances, and tectonic development
of the lower lithosphere underneath the Gibeon Kimberlite
Province, Namibia: Contributions to Mineralogy Petrology, v.
126, p. 181–198.
Garrison, J.R., Jr., and Taylor, L.A., 1981, Petrogenesis of
pyroxene-oxide intergrowths from kimberlite and cumu-
late rocks: Co-precipitation or exsolution?: American
Mineralogist, v. 66, p. 723–740.
Gasparik, T., 1990, Phase relations in the transition zone: Journal
of Geophysical Research, v. 95, p. 15751–15769.
Gonzaga, R.G., Lowry, D., Jacob, D.E., LeRoex, A., Schulze,
D., and Menzies, M.A., 2010, Eclogites and garnet pyroxen-
ites: Similarities and differences: Journal of Volcanology and
Geothermal Research, v. 190, p. 235–247.
Griffin, W.L., Natapov, L.M., O’Reilly, S.Y., van Achterbergh,
E., Cherenkov, A.F., and Cherenkov, V.G., 2005, The
Kharamai kimberlite field, Siberia: Modification of the litho-
spheric mantle by the Siberian Trap event: Lithos, v. 81,
p. 167–187.
Griffin, W.L., and O’Reilly, S.Y., 2007, Cratonic lithospheric
mantle: is anything subducted?: Episodes, v. 30, p. 43–53.
Griffin, W.L., Ryan, C.G., Kaminsky, F.V., O’Reilly, S.Y.,
Natapov, L.M., Win, T.T., Kinny, P.D., and Ilupin, I.P., 1999,
The Siberian lithosphere traverse: Mantle terranes and the
assembly of the Siberian Craton: Tectonophysics, v. 310,
p. 1–35.
Haggerty, S.E., Fung, A.T., and Burt, D.M., 1994, Apatite, phos-
phorus and titanium in eclogitic garnet from the upper mantle:
Geophysical Research Letters, v. 21, p. 1699–1702.
Haggerty, S.E., and Sautter, V., 1990, Ultradeep (greater than 300
kilometers), ultramafic upper mantle xenoliths: Science, v.
248, p. 993–996.
Harte, B., and Gurney, J.J., 1975, Evolution of clinopyroxene and
garnet in an eclogite nodule from the Roberts Victor kimber-
lite pipe, South Africa: Physics and Chemistry of the Earth,
v. 9, p. 367–387.
Hawkesworth, C.J., Lightfoot, P.C., Fedorenko, V.A., Blake, S.,
Naldrett, A.J., Doherty, W., and Gorbachev, N.S., 1995,
Magma differentiation and mineralization in the Siberian
continental flood basalts: Lithos, v. 34, p. 61–88.
Herzberg, C.T., 1978, The bearing of phase equilibria in sim-
ple and complex systems on the origin and evolution of
some well-documented garnet websterites: Contributions to
Mineralogy and Petrology, v. 66, p. 375–382.
Hofmann, A.W., 2003, Sampling mantle heterogeneity through
oceanic basalts: Isotopes and trace elements. in R.W. Carlson,
H.D. Holland and K.K. Turekian, eds., Treatise on geo-
chemistry: The mantle, v. 2:, Oxford, Elsevier–Pergamon,
p. 61–101.
Horan, M.F., Walker, R.J., Fedorenko, V.A., and Czamanske,
G.K., 1995, Osmium and neodymium isotopic constraints on
the temporal and spatial evolution of Siberian flood basalt
sources: Geochimica et Cosmochimica Acta, v. 59, p. 5159–
5168.
Irifune, T., 1987, An experimental investigation of the pyroxene–
garnet transformation in a pyrolite composition and its bear-
ing on the constitution of the mantle: Physics of the Earth and
Planetary Interiors, v. 45, p. 324–336.
Jacob, D.E., 2004, Nature and origin of eclogite xenoliths from
kimberlites: Lithos, v. 77, p. 295–316.
Jerde, E.A., Taylor, L.A., Crozaz, G., and Sobolev, N.V., 1993,
Exsolution of garnet within clinopyroxene of mantle eclog-
ites: major and trace element chemistry: Contributions to
Mineralogy and Petrology, v. 114, p. 148–159.
Kaeser, B., Olker, B., Kalt, A., Altherr, R., and Pettke, T.,
2009, Pyroxenite xenoliths from Marsabit (Northern Kenya):
Evidence for different magmatic events in the lithospheric
mantle and interaction between peridotite and pyroxen-
ites: Contributions to Mineralogy and Petrology, v. 157,
p. 453–472.
Kennedy, C.S., and Kennedy, G.C., 1976, The equilibrium bound-
ary between graphite and diamond: Journal of Geophysical
Research, v. 81, p. 2467–2470.
Kinny, P.D., Griffin, B.J., Heaman, L.M., Brakhfogel, F.F., and
Spetsius, Z.V., 1997, SHRIMP U-Pb ages of perovskite
from Yakutian kimberlites. Proceedings of Sixth International
Kimberlite Conference, Vol. 1: Kimberlites, related rocks,
and mantle xenoliths: Russian Geology and Geophysics, v.
38, p. 97–105.
Kirby, S.H., and Etheridge, M.A., 1981, Exsolution of Ca-
clinopyroxene from orthopyroxene aided by deformation:
Physics and Chemistry of the Minerals, v. 7, p. 105–109.
Kopylova, M.G., Russell, J.K., and Cookenboo, H., 1999,
Petrology of peridotite and pyroxenite xenoliths from the
Jericho kimberlite: Implications for the thermal state of the
mantle beneath the Slave Craton, Northern Canada: Journal
of Petrology, v. 40, p. 79–104.
Korolyuk, V.N., Lavrent’ev, Yu G., Usova, L.V., and Nigmatulina,
E.N., 2008, JXA-8100 microanalyzer: Accuracy of analysis
of rock-forming minerals: Russian Geology and Geophysics,
v. 49, p. 165–168.
Kostrovitsky, S.I., Morikiyo, T., Serov, I.V., Yakovlev, D.A., and
Amirzhanov, A.A., 2007, Isotope-geochemical systematics
of kimberlites and related rocks from the Siberian Platform:
Russian Geology and Geophysics, v. 48, p. 272–290.
Kushiro, I., 1972, Determination of liquidus relations in syn-
thetic silicate systems with electron probe analysis: The
system forsterite-diopside-silica at 1 atmosphere: American
Mineralogist, v. 57, p. 1260–1271.
Lappin, M.A., 1973, An unusual clinopyroxene with complex
lamellar intergrowths from an eclogite in the Sunndal-
Grubse ultramafic mass, Almklovdalen, Nordfjord, Norway:
Mineralogical Magazine, v. 39, p. 313–320.
Lappin, M.A., 1974, Eclogites from the Sunndal-Grubse ultra-
mafic mass, Almklovdalen, Norway and the T-P history
Downloaded by [Taisia Aleksandrovna Alifirova] at 22:19 21 May 2012
International Geology Review 1091
of the Almklovdalen masses: Journal of Petrology, v. 15,
p. 567–601.
Lavrent’ev, Yu. G., Usova, L.V., Kuznetsova, A.I., and Letov, S.V.,
1987, X-Ray spectral quantometric microanalysis of main
kimberlite minerals: Soviet Geology and Geophysics, v. 5,
p. 53–58.
Lazarov, M., Woodland, A.B., and Brey, G.P., 2009, Thermal state
and redox conditions of the Kaapvaal mantle: A study of
xenoliths from the Finsch mine, South Africa: Lithos, v. 112S,
p. 913–923.
Laz’ko, E.E., 1979, Indicator minerals of diamond and genesis of
kimberlite rocks: Moscow, Nedra, p.192 (in Russian).
Lindsley, D.H., and Dixon, S.A., 1976, Diopside-enstatite equi-
libria at 850 to 1400C, to 35 kbars: American Journal of
Science, v. 276, p. 1285–1301.
Liou, J.G., Zhang, R.Y., and Ernst, W.G., 2007, Very high-
pressure orogenic garnet peridotites: Proceedings of the
National Academy of Sciences, United States, v. 104,
p. 9116–9121.
Macdougall, J.D., and Haggerty, S.E., 1999, Ultradeep xenoliths
from African kimberlites: Sr and Nd isotopic compositions
suggest complex history: Earth and Planetary Science Letters,
v. 170, p. 73–82.
Martin, R.F., and Donnay, G., 1972, Hydroxyl in the mantle:
American Mineralogist, v. 57, p. 554–570.
Mercier, J.-C.C., and Nicolas, A., 1975, Textures and fabrics
of upper-mantle peridotites as illustrated by xenoliths from
basalts: Journal of Petrology, v. 16, p. 454–487.
Meyer, H.O.A., and McCallister, R.H., 1984, Two-pyroxene
megacrysts from south African kimberlites, in Kornprobst,
J. ed., Kimberlites II: The mantle and crust-mantle relation-
ships: Amsterdam, Elsevier, v. 2, p. 133–144.
Moore, R.O., and Gurney, J.J., 1985, Pyroxene solid solution in
garnets included in diamond: Nature, v. 318, p. 553–555.
Mposkos, E.D., and Kostopoulos, D.K., 2001, Diamond, former
coesite and supersilicic garnet in metasedimentary rocks from
the Greek Rhodope: A new ultrahigh-pressure metamorphic
province established: Earth and Planetary Science Letters, v.
192, p. 497–506.
Ohtani, E., 1990, Majorite fractionation and genesis of komatiites
in the deep mantle: Precambrian Research, v. 48, p. 195–202.
Onuma, K., and Kimura, M., 1978, Study of the system
CaMgSi2O6-CaFe3+AlSiO6-CaAl2SiO6-CaTiAl2O6: II. The
join CaMgSi2O6-CaAl2SiO6-CaTiAl2O6and its bearing on
Ca-Al-rich inclusions in carbonaceous chondrite: Journal of
Faculty of Science, Hokkaido University, Series IV, v. 18,
p. 215–236.
Pandey, A., Leech, M., Milton, A., Singh, P., and Verma, P., 2010,
Evidence of former majoritic garnet in Himalayan eclog-
ite points to 200-km subduction of Indian continental crust:
Geology, v. 38, p. 399–402.
Pokhilenko, L.N., and Alifirova, T.A., 2011, Plagioclase and
apatite from exsolution structures in minerals from mantle
xenoliths: Doklady Earth Sciences, v. 437, Part 2, p. 483–485.
Pokhilenko, N.P., 1990. Mantle parageneses in kimberlites, their
origins and the search value [Doctoral thesis]: Novosibirsk,
Institute of Geology and Geophysics, pp. 450.
Pokhilenko, N.P., Sobolev, N.V., Kuligin, S.S., and Shimizu, N.,
1999, Peculiarities of distribution of pyroxenite paragenesis
garnets in Yakutian Kimberlites and some aspects of the evo-
lution of the Siberian Craton lithospheric mantle, in Gurney,
J.J., Gurney, J.L., Pascoe, M.D., and Richardson, S.H. eds.,
Proceedings of the 7th International Kimberlite Conference:
Cape Town, Red Roof Design, p. 690–707.
Pollack, H.N., and Chapman, D.S., 1977, On the regional vari-
ation of heat flow, geotherms and lithosphere thickness:
Tectonophysics, v. 38, p. 279–296.
Qi, Q., Taylor, L.A., Snyder, G.A., Clayton, R.N., Mayeda,
T.K., and Sobolev, N.V., 1997, Detailed petrology and
geochemistry of a rare corundum eclogite xenolith from
Obnazhennaya, Yakutia: Russian Geology and Geophysics, v.
38, p. 247–260.
Ried, H., and Fuess, H., 1986, Lamellar exsolution systems
in clinopyroxene. Transmission Electron Microscopy obser-
vations: Physics and Chemistry of the Minerals, v. 13,
p. 113–118.
Ringwood, A.E., and Lovering, J.F., 1970, Significance of
pyroxene-ilmenite intergrowths among kimberlite xenoliths:
Earth and Planetary Science Letters, v. 7, p. 371–375.
Ringwood, A.E., 1991, Phase transformations and their bearing
on the constitution and dynamics of the mantle: Geochimica
et Cosmochimica Acta, v. 55, p. 2083–2110.
Roden, M.F., Laz’ko, E.E., and Jagoutz, E.. 1999. The role of gar-
net pyroxenites in the Siberian Lithosphere: Evidence from
the Mir Kimberlite. in Gurney, J.J., Gurney, J.L., Pascoe,
M.D., and Richardson, S.H., eds., Proceedings of the 7th
International Kimberlite Conference, Cape Town, Red Roof
Design, p. 714–720.
Roden, M.F., Patiño-Douce, A.E., Jagoutz, E., and Laz’ko, E.E.,
2006, High pressure petrogenesis of Mg-rich garnet pyroxen-
ites from Mir kimberlite, Russia: Lithos, v. 90, p. 77–91.
Rosen, O.M., Condie, K.C., Natapov, L.M., and Nozhkin,
A.D., 1994, Archean and Early Proterozoic Evolution of
the Siberian Craton: a preliminary assessment, in Condie,
K.C., ed., Archean crustal evolution: Amsterdam, Elsevier,
p. 411–460.
Rosen, O.M., Manakov, A.V., and Suvorov, V.D., 2005, The col-
lisional system in the northeastern Siberian Craton and a
problem of diamond-bearing lithospheric keel: Geotectonics
(English Translation), v. 39, p. 456–479.
Sautter, V., and Fabriès, J., 1990, Cooling kinetics of garnet
websterites from the Freychinede orogenic lherzolite massif,
French Pyrenees: Contributions to Mineralogy and Petrology,
v. 105, p. 533–549.
Sautter, V., Haggerty, S.E., and Field, S., 1991, Ultradeep
(>300 kilometers) ultramafic xenoliths: Petrological evi-
dence from the transition zone: Science, v. 252, p. 827–830.
Schmickler, B., Jacob, D.E., and Foley, S.F., 2004, Eclogite
xenoliths from the Kuruman kimberlites, South Africa:
Geochemical fingerprinting of deep subduction and cumulate
processes: Lithos, v. 75, p. 173–207.
Schulze, D.J., Helmstaedt, H., and Cassie, R.M., 1978, Pyroxene-
ilmenite intergrowths in garnet pyroxenite xenoliths from
a New York kimberlite and Arizona latites: American
Mineralogist, v. 63, p. 258–265.
Sclar, C.B., Carrison, L.C., and Stewart, O.M., 1968, High pres-
sure synthesis and stability of hydroxylated orthoenstatite
in the system MgO-SiO2-H2O: Eos Transactions, American
Geophysical Union, v. 49, p. 356.
Sen, G., and Jones, R.E., 1988, Exsolved silicate and oxide
phases from clinopyroxenes in a single Hawaiian xenolith:
Implications for oxidation state of the Hawaiian upper mantle:
Geology, v. 16, p. 69–72.
Smith, D., 1999, Temperatures and pressures of mineral equilibra-
tion in peridotite xenoliths: Review, discussion, and implica-
tions, in Fei, Y., Bertka, C.M., and Mysen, B.O., eds., Mantle
petrology: Field observations and high pressure experimenta-
tion: A tribute to Francis R. Boyd: The Geochemical Society
Special Publication, v. 6, p. 171–188.
Smyth, J.R., and Caporuscio, F.A., 1984, Petrology of a suite
of eclogite inclusions from the Bobbejaan kimberlite II:
Primary phase compositions and origin, in Kornprobst J. ed.,
Kimberlites II: The mantle and crust-mantle relationships:
Amsterdam, Elsevier, p. 121–131.
Downloaded by [Taisia Aleksandrovna Alifirova] at 22:19 21 May 2012
1092 T.A. Alifirova et al.
Sobolev, N.V., Kuznetsova, I.K., and Zyuzin, N.I., 1968, The
petrology of grospydite xenoliths from the Zagadochnaya
kimberlite pipe in Yakutia: Journal of Petrology, v. 9,
p. 253–280.
Sobolev, N.V., and Lavrent’ev, Yu.G.., 1971, Isomorphic sodium
admixture in garnets formed at high pressures: Contributions
to Mineralogy and Petrology, v. 31, p. pp. 1–12.
Sobolev, N.V., Lavrent’ev, Yu.G., Pospelova, L.N., and
Pokhilenko, N.P., 1973, Isomorphic admixture of tita-
nium in pyrope-almandine garnets: Zapiski Vsesoyuznogo.
Mineralogicheskogo Obschestva v. 102, p. 150–155 (in
Russian).
Sobolev, N.V., and Shatsky, V.S., 1990, Diamond inclusions in
garnets from metamorphic rocks: A new environment for
diamond formation: Nature, v. 343, p. 742–746.
Sobolev, V.S., and Sobolev, N.V., 1964, Xenoliths in kimber-
lites of North Yakutia and Upper Mantle structure: Doklady
Academii Nauk SSSR, v. 158, p. 22–26 (in Russian).
Solov’eva, L.V., 1986, Heterogeneity of the upper mantle
beneath the Siberian platform. in 4th International Kimberlite
Conference, Perth, Extended Abstracts, p. 340–342.
Song, S., Zhang, L., Chen, J., Liou, J.G., and Niu, Y., 2005, Sodic
amphibole exsolutions in garnet-peridotite, North Qaidam
UHPM belt, NW China: Implications for ultradeep-origin and
hydroxyl defects in mantle garnets: American Mineralogist, v.
90, p. 814–820.
Song, S., Zhang, L., and Niu, Y., 2004, Ultra-deep origin of gar-
net peridotite from the North Qaidam ultrahigh-pressure belt,
northern Tibetan Plateau, NW China: American Mineralogist,
v. 89, p. 1330–1336.
Takahashi, E., 1986, Melting of a dry peridotite KLB-1 up to 14
GPa: Implications on the origin of peridotite upper mantle:
Journal of Geophysical Research, v. 91, p. 367–9382.
Taylor, L.A., Snyder, G.A., Keller, R., Remley, D.A., Anand, M.,
Wiesli, R., Valley, J., and Sobolev, N.V., 2003, Petrogenesis
of group A eclogites and websterites: Evidence from
the Obnazhennaya kimberlite, Yakutia: Contributions to
Mineralogy and Petrology, v. 145, p. 424–443.
Thompson, R.N., 1975, Is upper-mantle phosphorous contained
in sodic garnet?: Earth and Planetary Science Letters, v. 26,
p. 417–424.
Toy, V.G., Newman, J., Lamb, W., and Tikoff, B., 2010, The role
of pyroxenites in formation of shear instabilities in the man-
tle: Evidence from an ultramafic ultramylonite, Twin Sisters
Massif, Washington: Journal of Petrology, v. 51, p. 55–80.
Tychkov, N.S., Pokhilenko, N.P., Kuligin, S.S., and Malygina,
E.V., 2008, Composition and origin of peculiar pyropes
from lherzolites: Evidence for the evolution of the litho-
spheric mantle of the Siberian Platform: Russian Geology and
Geophysics, v. 49, p. 225–239.
Valley, J.W., Essene, E.J., and Peacor, D.R., 1983, Flourine-
bearing garnets in Adirondack calc-silicates: American
Mineralogist, v. 68, p. 444–448.
van Roermund, H.L.M., and Drury, M.R., 1998, Ultra-high pres-
sure (P >6 GPa) garnet peridotites in Western Norway:
Exhumation of mantle rocks from >185 km depth: Terra
Nova, v. 10, p. 295–301.
van Roermund, H.L.M., Drury, M.R., Barnhoorn, A., and De
Ronde, A.A., 2000a, Non-silicate inclusions in garnet from
an ultra-deep orogenic peridotite: Geological Journal., v. 35,
p. pp. 209–229.
van Roermund, H.L.M., Drury, M.R., Barnhoorn, A., and De
Ronde, A.A., 2000b, Super-silicic garnet microstructures
from an orogenic garnet peridotite, evidence for an ultra-deep
(>6 GPa) origin: Journal of Metamorphic Geology, v. 18,
p. 135–147.
van Roermund, H.L.M., Drury, M.R., Barnhoorn, A., and De
Ronde, A.A., 2001, Relict majoritic garnet microstructures
from ultra-deep orogenic peridotites in Western Norway:
Journal of Petrology, v. 42, p. 117–130.
Vissers, R.L.M., Drury, M.R., Newman, J., and Fliervoet, T.F.,
1997, Mylonitic deformation in upper mantle peridotites of
the North Pyrenean Zone (France): Implications for strength
and strain localization in the lithosphere: Tectonophysics, v.
279, p. 303–325.
Wang, L., Essene, E.J., and Zhang, Y., 1999, Mineral inclu-
sions in pyrope crystals from Garnet Ridge, Arizona, USA:
Implications for processes in the upper mantle: Contributions
to Mineralogy and Petrology, v. 135, p. 164–178.
Weinbruch, S., Styrsa, V., and Muller, W.F., 2003, Exsolution
and coarsening in iron-free clinopyroxene during isother-
mal annealing: Geochimica et Cosmochimica Acta, v. 67,
p. 5071–5082.
Wilkins, R.W.T., and Sabine, W., 1973, Water content of some
nominally anhydrous silicates: American Mineralogist., v. 58,
p. 508–516.
Woodland, A.B., and Koch, M., 2003, Variation in oxygen fugac-
ity with depth in the upper mantle beneath the Kaapvaal
craton, Southern Africa: Earth Planetary Science Letters, v.
214, p. 295–310.
Xu, Y., 2002, Evidence for crustal components in the mantle
and constraints on crustal; Recycling mechanisms: pyroxenite
xenoliths from Hannuoba, North China: Chemical Geology, v.
182, p. 301–322.
Yang, J., and Liu, L., 2004, Coupled isomorphic substitu-
tion and exsolution of pyroxene, rutile, apatite and quartz
in supersilicic garnet: Chinese Science Bulletin, v. 49,
p. 70–75.
Ye, K., Cong, B., and Ye, D., 2000, The possible subduction of
continental material to depths greater than 200km: Nature, v.
407, p. 734–736.
Zhang, R.Y., and Liou, J.G., 2003, Clinopyroxenite from the
Sulu ultrahigh-pressure terrane, eastern China: Origin and
evolution of garnet exsolution in clinopyroxene: American
Mineralogist, v. 88, p. 1591–1600.
Zhang, R.Y., Zhai, S.M., Fei, Y.W., and Liou, J.G., 2003,
Titanium solubility in coexisting garnet and clinopyroxene
at very high pressure: The significance of exsolved rutile
in garnet: Earth and Planetary Science Letters, v. 216,
p. 591–601.
Zindler, A., and Harte, S., 1986, Chemical geodynamics: Annual
Reviews in Earth Planetary Sciences, v. 14, p. 493–571.
Downloaded by [Taisia Aleksandrovna Alifirova] at 22:19 21 May 2012
... Available mineralogical data indicate the Cr-pyrope of the lherzolitic paragenesis as being the principal host for a series of Ti-rich oxide inclusions namley: rutile, Mg-rich ilmenite ('picroilmenite'), crichtonite-group minerals, srilankite and carmichaelite (Botkunov et al., 1987, Kostrovitsky and Garanin, 1992, Wang et al., 1999Săbău and Alberico, 2003;Vrana, 2008;Alifirova et al., 2012Alifirova et al., , 2020Ziberna et al., 2013;Nikolenko et al., 2017Nikolenko et al., , 2021Rezvukhin et al., 2018Rezvukhin et al., , 2019. Although there is characteristic enrichment in Ti and Fe, these oxide phases also contain elevated amounts of silicate-incompatible elements, such as high-field-strength elements (HFSE: Nb, Zr and Ta) in rutile, Zr in srilankite, and an exotic array of large ion lithophile elements (LILE: Ba, Sr and K), HFSE, and light rare earth elements (LREE: La, Ce) in the crichtonite-group minerals. ...
... A significant controversy surrounds the origin of Ti-oxide needles in Сr-pyrope as the criteria to distinguish epigenetic inclusions from syngenetic ones are not obvious. As a rule, elongated oxide inclusions in Cr-pyrope are oriented along specific crystallographic directions (Alifirova et al., 2012Wang et al., 1999;Vrana, 2008;Rezvukhin et al., 2018Rezvukhin et al., , 2019. Two mechanisms have been suggested for the origin of oriented inclusions in garnet namely syngenetic (epitaxial co-precipitation) and epigenetic (solid-state isochemical exsolution). ...
... Inclusions in pyropes from the Chompolo lamprophyres represent a typical set of high-Ti and/or Cr-rich oxides, reported previously in Cr-pyrope from kimberlite and lamprophyre bodies of the Siberian craton (Botkunov et al., 1987;Kostrovitsky and Garanin, 1992;Varlamov et al., 1996;Alifirova et al., 2012Alifirova et al., , 2020Ziberna et al., 2013;Rezvukhin et al., 2016aRezvukhin et al., , 2016bRezvukhin et al., , 2018Nikolenko et al., 2017), ultramafic magmatic breccias of the Colorado Plateau, USA (Wang et al., 1999), and European peridotite massifs (Săbău and Alberico, 2003;Vrana, 2008). These minerals are also common as xenocrysts in kimberlites and components of metasomatised peridotite xenoliths worldwide (e.g. ...
Article
Pyrope xenocrysts ( N = 52) with associated inclusions of Ti- and/or Cr-rich oxide minerals from the Aldanskaya dyke and Ogonek diatreme (Chompolo field, southeastern Siberian craton) have been investigated. The majority of xenocrysts are of lherzolitic paragenesis and have concave-upwards (normal) rare earth element (REE N ) patterns that increase in concentration from light REE to medium–heavy REE (Group 1). Four Ca-rich (5.7–7.4 wt.% CaO) pyropes are extremely low in Ti, Na and Y and have sinusoidal REE N spectra, thus exhibiting distinct geochemical signatures (Group 2). A peculiar xenocryst, s165, is the only sample to show harzburgitic derivation, whilst demonstrating a normal-to-weakly sinusoidal REE N pattern and the highest Zr (93 ppm) and Sc (471 ppm). Chromite–magnesiochromite, rutile, Mg-ilmenite and crichtonite-group minerals comprise a suite of oxide mineral inclusions in the pyrope xenocrysts. These minerals are characteristically enriched in Cr with 0.6–7.2 wt.% Cr 2 O 3 in rutile, 0.7–3.6 wt.% in Mg-ilmenite and 7.1–18.0 wt.% in the crichtonite-group minerals. Complex titanates of the crichtonite group enriched in large ion lithophile elements (LILE) are high in Al 2 O 3 (0.9–2.2 wt.%), ZrO 2 (1.5–5.4 wt.%) and display a trend of compositions from the Ca–Sr-specific varieties to the Ba-dominant species (e.g. lindsleyite). In the pyrope xenocrysts the oxides coexist with silicates (clino- and orthopyroxene and olivine), hydrous silicates (talc, phlogopite and amphibole), carbonate (magnesite), sulfides (pentlandite, chalcopyrite, breakdown products of monosulfide and bornite solid solutions), apatite and graphite. P – T estimates imply the inclusion-bearing pyrope xenocrysts have been derived from low-temperature peridotite assemblages that resided at temperatures of ~600–800°C and a pressure range of ~25–35 kbar in the graphite stability field. Pyrope genesis is linked to the metasomatic enrichment of peridotite protoliths by Ca–Zr–LILE-bearing percolating fluid–melt phases containing significant volatile components. These metasomatic agents are probably volatile-rich melts or supercritical C–O–H–S fluids that were released from a Palaeo-subduction slab.
... Although these calculated equilibration temperatures are unreliable, the temperature range falls within the estimated range for orthopyroxenite xenoliths from the Udachnaya-East kimberlite (Ionov et al., 2010(Ionov et al., , 2020. The presence of exsolution lamellae in pyroxenes indicates that temperature estimates cannot be reliably constrained, as pyroxenes compositional changes have undergone sub-solidus modification (e.g., Alifirova et al., 2012). Thus, this orthopyroxenite xenolith may have originated from even greater P-T conditions than estimated here (e.g., Alifirova et al., 2012). ...
... The presence of exsolution lamellae in pyroxenes indicates that temperature estimates cannot be reliably constrained, as pyroxenes compositional changes have undergone sub-solidus modification (e.g., Alifirova et al., 2012). Thus, this orthopyroxenite xenolith may have originated from even greater P-T conditions than estimated here (e.g., Alifirova et al., 2012). ...
... Bivariate plots of Mg# vs. CaO, TiO 2 , Cr 2 O 3 , Na 2 O and Al 2 O 3 for clinopyroxene in xenolith sample UV-50/05 (red diamonds), along with clinopyroxene-I (Cpx -I; yellow diamonds), clinopyroxene-II (Cpx-II; blue diamonds) and clinopyroxene exsolution lamellae in orthopyroxene (purple circles) compared with inclusions of clinopyroxene in xenocrystic olivine (blue triangles;Kamenetsky et al., 2009;Sobolev et al., 2015) and clinopyroxene in mantle-derived xenoliths from the Udachnaya-East (UDE) kimberlite (orange squares;Soloveva et al., 2008;Ionov et al., 2010;Alifirova et al., 2012). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) ...
Article
Reconstructing the original composition of kimberlite melts in the mantle and delineating the processes that modify them during magmatic ascent and emplacement in the crust remains a significant challenge in kimberlite petrology. One of the most significant processes commonly cited to drive initial kimberlite melts towards more Si-Mg-rich compositions and decrease the solubility of CO2 is the assimilation of mantle orthopyroxene. However, there is limited direct evidence to show the types of reactions that may occur between mantle orthopyroxene and the host kimberlite melt. To provide new constraints on the interaction between orthopyroxene and parental kimberlite melts, we examined a fresh (i.e. unmodified by secondary/post-magmatic alteration) orthopyroxenite xenolith, which was recovered from the serpentine-free units of the Udachnaya-East kimberlite (Siberian Craton, Russia). This xenolith is composed largely of orthopyroxene (~ 90%), along with lesser olivine and clinopyroxene and rare aluminous magnesian chromite. We can show that this xenolith was invaded by the host kimberlite melt along grain interstices and fractures, where it partially reacted with orthopyroxene along the grain boundaries and replaced it with aggregates of compositionally distinct clinopyroxene, olivine and phlogopite, along with subordinate Fe-Cr-Mg spinel, FeNi sulphides and djerfisherite (K6(Fe,Ni,Cu)25S26Cl). Primary melt inclusions in clinopyroxene replacing xenolith-forming orthopyroxene, as well as secondary melt inclusion trails in xenolith orthopyroxene, clinopyroxene and olivine are composed of similar daughter mineral assemblages that consist largely of: NaK chlorides, along with varying proportions of phlogopite, Fe-Cu-Ni sulphides, djerfisherite, rasvumite (KFe2S3), Cr-Fe-Mg spinel, nepheline and apatite, and rare rutile, sodalite, barite, olivine, Ca-K-Na carbonates and NaK sulphates. The melt entrapped by these inclusions likely represent the hybrid products produced by the invading kimberlite melt reacting with orthopyroxene in the xenolith. The mechanism that could explain the partial replacement of orthopyroxene in this xenolith by clinopyroxene, olivine and phlogopite could be attributed to the following reaction: Orthopyroxene + Carbonatitic (melt) ➔ Olivine + Clinopyroxene + Phlogopite + CO2. This reaction is supported by theoretical and experimental studies that advocate the dissolution of mantle orthopyroxene within an initially silica-poor and carbonate-rich kimberlite melt. The mineral assemblages replacing orthopyroxene in the xenolith, together with hosted melt inclusions, suggests that the kimberlitic melt prior to reaction with orthopyroxene was likely carbonate-rich and Na-K-Cl-S bearing. The paucity of carbonate in the reaction zones around orthopyroxene and in melt inclusions in clinopyroxene replacing xenolith-forming orthopyroxene and xenolith minerals (orthopyroxene, clinopyroxene and olivine) is attributed to the consumption of carbonates and subsequent exsolution of CO2 by the proposed decarbonation reaction. Concluding, we propose that this orthopyroxenite xenolith provides a rare example of the types of reactions that can occur between mantle orthopyroxene and the host kimberlite melt. The preservation of this xenolith and zones around orthopyroxene present new insights into the composition and evolution of parental kimberlite melts and CO2 exsolution.
... The MgO content of reconstructed bulk rocks of this group is always lower than 16%, and we call this group "low-MgO eclogites". The other group of samples, previously named Group A eclogites (Qi et al. 1994;Taylor et al. 2003;Alifirova et al. 2012Alifirova et al. , 2015 or websterites (Spengler and Alifirova 2019), contain minor orthopyroxene and rutile in addition to garnet and clinopyroxene. This group of samples has higher MgO content in the reconstructed bulk rock, and is referred to as "high-MgO" here. ...
... A large number of high-MgO eclogites and pyroxenites from Obnazhennaya have previously been studied (Qi et al. 1994;Taylor et al. 2003;Alifirova et al. 2012Alifirova et al. , 2015. However, far fewer studies have been carried out on the Obnazhennaya low-MgO eclogites. ...
... The size of most xenoliths was 4 cm × 6 cm × 4 cm (Fig. 2a). Eclogite and pyroxenite xenoliths from Obnazhennaya have previously been investigated for major, trace elements, oxygen isotope, and Sr-Nd isotopes by Qi et al. (1994); Taylor et al. (2003); Alifirova et al. (2012Alifirova et al. ( , 2015 and Spengler and Alifirova (2019). In these studies, nearly all of samples were of group A (or high-MgO eclogites and websterites). ...
... The MgO content of reconstructed bulk rocks of this group is always lower than 16%, and we call this group "low-MgO eclogites". The other group of samples, previously named Group A eclogites (Qi et al. 1994;Taylor et al. 2003;Alifirova et al. 2012Alifirova et al. , 2015 or websterites (Spengler and Alifirova 2019), contain minor orthopyroxene and rutile in addition to garnet and clinopyroxene. This group of samples has higher MgO content in the reconstructed bulk rock, and is referred to as "high-MgO" here. ...
... A large number of high-MgO eclogites and pyroxenites from Obnazhennaya have previously been studied (Qi et al. 1994;Taylor et al. 2003;Alifirova et al. 2012Alifirova et al. , 2015. However, far fewer studies have been carried out on the Obnazhennaya low-MgO eclogites. ...
... The size of most xenoliths was 4 cm × 6 cm × 4 cm (Fig. 2a). Eclogite and pyroxenite xenoliths from Obnazhennaya have previously been investigated for major, trace elements, oxygen isotope, and Sr-Nd isotopes by Qi et al. (1994); Taylor et al. (2003); Alifirova et al. (2012Alifirova et al. ( , 2015 and Spengler and Alifirova (2019). In these studies, nearly all of samples were of group A (or high-MgO eclogites and websterites). ...
Article
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The petrology, mineral major and trace-element concentrations, and garnet oxygen isotopic composition of low-MgO (11–16 wt%) eclogites from the Obnazhennaya kimberlite, Siberian craton, are used to infer their petrogenesis. These eclogites contain two types of compositionally distinct garnet: granular coarse garnet, and garnet exsolution (lamellae and fine-grained garnet) in clinopyroxene. The former record higher temperatures at lower pressures than the latter, which record the last stage of equilibrium at moderate pressure–temperature conditions 2.3–3.7 GPa and 855–1095 °C in the upper mantle at the time of entrainment. Although derived from the garnet stability field, these rocks have low-pressure cumulate protoliths containing plagioclase, olivine, and clinopyroxene as reflected by pronounced positive Eu and Sr anomalies in all eclogites, and low heavy rare earth element (HREE) contents in both minerals and reconstructed bulk rocks for a number of samples. Major elements, transition metals, and the HREE compositions of the reconstructed whole rocks are analogous to modern oceanic gabbro cumulates. Despite geochemical signatures supporting an oceanic crust origin, mantle-like δ18O of the garnets (5.07–5.62‰) for most samples indicates that the protoliths either did not interact with seawater or have coincidently approximately normal igneous values. Some of the eclogite xenoliths have lower SiO2 contents and depleted light REE ((Nd/Yb)N < 1) compared to modern oceanic gabbros, suggesting that they experienced partial melting. Positively inclined middle to heavy REE patterns ((Dy/Yb)N < 1) of the reconstructed bulk rocks mostly result from repeated partial melting in the eclogite stability field, based on melting model calculations. We, therefore, suggest that the Obnazhennaya low-MgO eclogites may represent the gabbroic section of subducted or foundered basaltic crust that underwent continued partial melting processes at high pressures, where garnet was the main residual phase.
... Ga (Rosen et al., 2005). Three main intervals of kimberlite magmatism on the Siberian craton took place at 367-345 Ma, 245-215 Ma and 160-145 Ma (Alifirova et al., 2012;Griffin et al., 1999). ...
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A significant portion of the Earth’s water is found at trace levels in nominally anhydrous rock forming minerals in the crust and mantle. Its distribution has far-reaching implications in melt-generating processes, mantle rheology and the stability of the lithosphere. We report petrographic, major and trace element, Fourier transform infrared spectroscopy and secondary ion mass spectrometry data on four eclogite xenoliths from the Obnazhennaya kimberlite, Siberia that contain hydrous minerals at upper mantle conditions. Al-rich (> 9.9 wt.% Al2O3) omphacitic clinopyroxene, a major mineral of eclogites, contains crystallographically controlled exsolutions of CaO-rich (up to 23.6 wt.%) garnet (coronae, lenses), garnet-zoisite (lamellae) or amphiboles (needle/blade-like exsolutions). The omphacite hosting the exsolved lamellae shows enrichment or depletion in major (Al, Si, Mg) and trace (e.g., HREE) elements compatible with garnet and zoisite, proportional to the abundance of exsolutions. We argue that zoisite likely formed concomitantly with garnet exsolution from a water- and Al-rich precursory omphacite, without any fluid addition. The compositional gradients are consistent with diffusion-controlled partitioning in a closed system, and their preservation indicates the exsolution likely took place shortly prior to eclogite entrapment by the kimberlite. Large, non-exsolved, omphacite grains contain minimum ~870–1500 ppm wt. H2O (determined as OH). We infer that intertwined lamellae of hydrous garnet (600–960 ppm wt. H2O) and minor (<1%) zoisite (2 wt.% H2O) formed at mantle conditions due to progressive exsolution, without external fluids. Recalculated whole rock water contents range from ~320 to 970 ppm wt. H2O. These estimates exceed by far those for the surrounding peridotitic mantle and suggest that, although a minor component, eclogites may locally be an important water reservoir in the cratonic mantle.
... The formation of spinel Spl after garnet (Fig. 3p) is initiated by a decrease in pressure upon crossing the boundary between the garnet-and spinel-peridotite facies of the upper mantle during the high-temperature removal of xenoliths to the surface. The pressure decrease results in the exsolution of rutile TiO 2 , ilmenite (Fe,Mg)TiO 3 , quartz SiO 2 in garnet and orthopyroxene Opx (Mg,Fe)SiO 3 , rutile TiO 2 , ilmenite (Fe,Mg)TiO 3 in omphacite as well (Alifirova et al., 2012(Alifirova et al., , 2015. The ordered intergranular lamellae of apatite Ca 5 (PO 4 ) 3 (F,Cl) in the exsolution texture of omphacite could be formed with participation of kimberlite metasomatism. ...
Article
Abstract—The conditions of genesis of diamondiferous ultrabasic and basic rocks from xenoliths in kimberlite were studied by combining the data from analytical investigations of their mineral phases and experimental results of the study of melting relations in the diamond-forming mineral systems of the upper mantle. The compositions of minerals in some samples of metasomatized diamondiferous eclogite associated with diamond-free eclogite from kimberlite of the Udachnaya pipe (Yakutia) were studied for the first time. The new results obtained in addition to the literature data were applied for generalization of estimates of genetically important characteristics of the chemical compositions of garnets, Ca-clinopyroxenes, and omphacites from diamond-bearing peridotite, pyroxenite, and eclogite. As a result, it was found that quite “fresh” minerals of diamondiferous rocks have typomorphic differences from the same minerals of diamond-free upper-mantle rocks. At the same time, it is significant that the compositions of minerals from diamondiferous rocks and paragenetic inclusions in diamonds are identical. These peculiarities of mineralogy of diamondiferous rocks are genetically significant; based on the mantle–carbonatite theory of the origin of diamond and associated mineral phases, this provides support for the same physicochemical origin of diamonds, minerals of diamondiferous rocks, and paragenetic inclusions in diamonds. Finally, the following genetic conclusions are made. (1) Completely miscible silicate (±oxide)–carbonate melts with dissolved carbon are the parental medium in petrogenesis of diamondiferous ultrabasic and basic rocks. (2) The physicochemically consistent formation of diamondiferous rocks and paragenetic inclusions of peridotitic and eclogitic minerals in diamonds occurred in the common diamond-forming chambers/reservoirs of parental melts; diamond-free peridotite, pyroxenite, and eclogite were the host mantle rocks for such chambers. (3) The origin of continuous series of diamondiferous peridotite–pyroxenite–eclogite rocks is controlled by the fractional ultrabasic–basic evolution of parental melts with exhaustion of olivine and orthorhombic pyroxene via the peritectic reactions. (4) Ascending f lows of kimberlite magmas destroyed the parental chambers and captured diamonds with inclusions, individual minerals, their intergrowths, diamondiferous ultrabasic and basic rocks; at the entrance and exit from the chambers, they captured differentiated diamond-free host rocks of the mantle as well. (5) With further ascent from the mantle to the Earth’s crust, the material of diamond-forming chambers and diamond-free mantle was mixed in convecting kimberlite magma and was transported from the mantle to cumulative crustal chambers. (6) Kimberlite magmas were gradually solidified in stationary cumulative chambers with the release of highly compressed fluids; with an increase of pressure up to the critical values, they intruded into the rocks of the roof and ejected kimberlite with xenoliths of diamondiferous and mantle rocks to the surface with the formation of explosion pipes.
... Plutonic rocks of the dunite-clinopyroxenite series (dunites, wehrlites, olivine clinopyroxenites, and clinopyroxenites) are especially important as they are the main constituents of the Moho transition zone (MTZ) and their petrogenesis still remains unclear [12]. There have been many studies that have focused on dunites and clinopyroxenite xenoliths [13][14][15][16], but comparatively few studies on dunite-clinopyroxenite series rocks [14]. The main feature of the Nizhne-Derbinsk complex is the presence of ultramafic and subultramafic cumulates. ...
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The Nizhne-Derbinsk mafic-ultramafic complex is located between the Central Asian Orogenic Belt and the Siberian Craton and, is associated with the Ballyk fault. The largest, spatially related to each other, plutons in the central part of the complex are the Burlakski and Nizhne-Derbinsk. Rocks in the main units of these plutons are divided into three groups: peridotites (ultramafic), pyroxenites (sub-ultramafic), and gabbroic rocks (mafic). The ultramafic and sub-ultramafic cumulate series are devoid of plagioclase and contain <3 vol. % chromian spinel. The Fo content of olivine in the sub-ultramafic cumulates from both plutons ranges from Fo79 to Fo86. The En content [= Mg/(Mg + Fe + Ca) × 100 atomic ratio] of clinopyroxenes and orthopyroxenes varies from 46–56, and 63–80, respectively. Plagioclase corresponds to labradorite with An contents between 55 and 57. Hornblende is compositionally similar to pargasite. The sequence of change of rock units corresponds to the paragenesis: olivine − olivine + clinopyroxene (orthopyroxene) − clinopyroxene + orthopyroxene – clinopyroxene + orthopyroxene + plagioclase – orthopyroxene. Petrographic, mineralogical, and mineral chemical features of the Burlakski and Nizhne-Derbinsk plutons suggest that the diversity of the material composition of these plutons is due to the processes of magmatic differentiation in deep-seated conditions. Estimates of crystallization pressures and temperatures of the Burlakski and Nizhne-Derbinsk plutons suggest that they crystallized at high pressures ≥ 10kb and temperatures ranging from 1000–1400 °C. Mineralogical and petrological features suggest that the mafic-ultramafic cumulates were derived from a high-Mg basaltic magma. The presence of magmatic hornblende and hydrous mineral assemblages within the ultramafic cumulates indicates that the parental melts had been enriched in dissolved volatile constituents. Taking into account the age of the gabbronorites of the Burlakski pluton (~490 ± 11.8 Ma), the magmatism likely occurred during the Ordovician collision stage of the evolution of the Central Asian Fold Belt.
... Ga (Kobussen et al., 2006;Rosen et al., 2005). Seismic investigation of the craton and associated lithosphere show a variation in depth of the Lithosphere-Asthenosphere Boundary (LAB) from 200-240 km in the centre (Boyd et al., 1997;Goncharov et al., 2012) to 130-140km in the north and west (Griffin et al., 1999;Rosen et al., 1994).Three main events of kimberlite magmatism mark the Siberian Craton at 367-345Ma, 245-215Ma and 160-145Ma (Alifirova et al., 2012). ...
Thesis
Cette étude est basée sur la collection la plus complète d'éclogites cratoniques (> 180 nodules), provenant des cratons sibériens et sud-africains.Les éclogites non-métasomatisés de type IIA sont plus magnésiennes, enrichis en LREE, avec des valeurs de δ18O, de 3,7-7,5 ‰, des anomalies positives en Sr et correspondant à une couche à basse pression-température (PT). La composition de la roche totale est un protolithe basaltique. Les éclogites de type IIB sont plus sodiques et alumineux, appauvris en terres rares, avec des valeurs de δ18O de 2,3-3,6 ‰, correspondant à l'équilibre à haute PT. La composition de la roche totale correspond au protolithe pyroxénitique. Les éclogites à coésite, disthène ou corindon ont des clinopyroxènes riches en jadéite, avec des anomalies positives en Eu et Sr et des grenats riches en grossulaire, avec des anomalies positive en Eu et négatives en Sr. Les estimations de PT indiquent que les éclogites à co-ky-cor ont été équilibrées dans la partie la plus basse de la quille cratonique et la composition de la roche totale correspondant à un protolithe gabbroïque très appauvri, cohérente avec la subduction d'une séquence basaltique à websteritique d'une croûte océanique altérée, appauvrie en éléments incompatibles.L'estimation minimum d'eau dans l'omphacite d'Obnazhennaya est de ~930-1410 ppm d'H20 et pour la roche totale de ~310-890 ppm H2O, significativement supérieures aux péridotites environnantes. Ainsi, les éclogites du manteau peuvent être un important réservoir d'eau à la base de la racine cratonique, avec des conséquences majeures pour la longévité de la quille cratonique, la rhéologie du manteau et pour le cycle global de l'eau.
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We report the first comprehensive micro‐Raman study of crichtonite group minerals (CGM) as inclusions in pyropic garnet grains from peridotite and pyroxenite mantle xenoliths of the Yakutian kimberlites as well as in garnet xenocrysts from the Aldan shield lamprophyres (Russia). The CGM form (i) morphologically oriented needles, lamellae, and short prisms and (ii) optically unoriented subhedral to euhedral grains, either single or intergrown with other minerals. We considered common mantle‐derived CGM species (like loveringite, lindsleyite, and their analogues), with Ca, Ba, or Sr dominating in the dodecahedral A site and Zr or Fe in the octahedral B site. The Raman bands at the region of 600–830 cm−1 are indicative of CGM and their crystal‐chemical distinction, although the intensity and shape of the bands appear to be dependent on laser beam power and wavelength. The factor‐group analysis based on the loveringite crystal structure showed the octahedral and tetrahedral cation groups with 18f and 6c Wyckoff positions, namely, dominantly TiO6 and to a lower extent CrO6, MgO4, and FeO4 groups, to be the major contributors to the Raman spectral features. The ionic groups with dodecahedral (M0) and octahedral (M1) coordination are inactive for Raman scattering while active in infrared absorption. A number of observed Raman modes in the CGM spectra are several times lower than that predicted by the factor group analysis. The noticed broadening of modes in the CGM Raman spectra may result from a combining of bands at the narrow frequency shift regions. Solid solution behavior, luminescence, and partial metamictization of the CGM may exert additional influence on the Raman band shape. The Raman spectral features showed CGM to be accurately identified and distinguished from other Ti‐, Fe‐, Cr‐, and Zr‐containing oxides (e.g., ilmenite or those of spinel and magnetoplumbite groups) occurring as accessory mantle minerals.
Chapter
The most important chromium-bearing phases in the Earth’s upper mantle are chrome spinel, Cr-bearing pyroxene, and Knr-garnet, which are replaced with Knr-Maj garnet, MgCr2O4 with the calcium titanate-type structure, and (Mg,Fe)2SiO4 wadsleyite/ringwoodite. Ferropericlase (Mg,Fe)O and (Mg,Fe)SiO3 bridgmanite may be considered as the host phases for chromium under the lower mantle conditions. Below we review the information about the high-pressure chromium-bearing phases in the Earth's mantle. Since chromite (Fe,Mg)(Cr,Al)2O4 containing up to ~55 wt% Cr2O3 is the major ore mineral of chromium and is an accessory phase in many ultramafic rocks, the largest chromite deposits and the models of their formation are considered.
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Several grossulars from Adirondack calc-silicates have been found to contain up to 0.76 wt.% (equivalent to 1.5 mole%) of F substituting for O in the structure. These garnets occur with secondary prehnite and form through retrograde reaction of wollastonite after the peak of Grenville metamorphism. Chemical, XRD and optical data are given.-J.A.Z.
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A large (6 kg), rare, corundum eclogite xenolith from the Obnazhennaya kimberlite, Yakutia, was divided into seven portions for detailed petrologic and geochemical studies. The variations observed in minerals, especially in granular garnets, are remarkable. The CaO content of a large garnet grain (∼10 mm) increases from core (8.41%) to rim (19.2%), and MgO and FeO decrease from core (17.4% and 8.41%, respectively) to rim (10.6% and 5.70%, respectively). The garnet core was equilibrated with a precursor clinopyroxene at about 1400°C; the garnet rim and adjacent clinopyroxene rim, as well as garnet exsolution lamellae and adjacent host clinopyroxene were last equilibrated at about 1050°C. The zonation preserved in these garnets requires a rapid cooling rate from 1050°C to a final "frozen" temperature. REE concentrations of clinopyroxene and garnet are much lower compared to other normal eclogues from Obnazhennaya. High Al2O3 and CaO contents in the xenolith and positive Eu anomalies in garnets indicate that the protolith of this eclogite was a noritic anorthosite, possibly apart of the ancient oceanic crust. However, δ18O values of garnet and clinopyroxene separates falls within the normal mantle range. This, combined with a low jadeite component in clinopyroxene, indicates that seawater alteration was not severe. Conversely, later rehomogenization processes in the upper mantle may have significantly changed its low-temperature alteration signatures.
Chapter
Compositional variations of the four essential phases have been examined in spinel peridotite samples from these ultramafic bodies representative of an exceptionally wide modal and chemical range. In addition to this between-sample variation, in each individual sample, spinels and pyroxenes show irregular and sometimes wide chemical variations from a nearly constant composition in the core of porphyroclasts to their margins and to the coexisting neoblasts, whereas olivine composition is invariable. These chemical disequilibria result from the superimposed effects of two episodes of deformation and recrystallization. By applying various geothermometers, two groups of temperatures have been estimated at 950oC and 650o-700oC, respectively.-J.M.H.
Chapter
Some 50 clinopyroxene-orthopyroxene megacrysts, with and without garnet, from Bellsbank, Frank Smith and Koffiefontein kimberlites, have been chemically analysed. The majority of the two-pyroxene megacrysts are believed to have formed as a result of exsolution. Chemically they belong to the Cr-poor suite and also have relatively low Fe contents when compared with other megacrysts. Determinations of an equilibration T and P from the clinopyroxene and orthopyroxene in individual megacrysts gave ranges of 220oC and 18 kbar for Bellsbank and Frank Smith, and approx 100oC and 9 kbar for Koffiefontein. The Bellsbank, and limited Frank Smith data, form a trend suggesting that the megacrysts lie in a P-T space along a palaeogeotherm. In contrast, the Koffiefontein intergrowths appear to cluster more in a group than to form a trend, suggesting that they did not reach equilibrium with the ambient geotherm.-J.M.H.
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The diamond-bearing kimberlite diatremes in the northeastern Siberian Craton allow the tectonic models of the formation of Earth's crust to be compared with the specific features of the underlying upper mantle. The Mesoarchean (3.2-2.8 Ga) stage in the evolution of the craton was characterized by the creation of sialic continental masses that amalgamated into the Pangea-0 supercontinent 2.8-2.6 Ga ago. Pangea-0 subsequently broke up into new microcontinents that amalgamated again 1.9-1.8 Ga ago into the Siberian Craton in its present-day configuration as a part of the Pangea-1 supercontinent. This sequence of events is supported by the isotopic ages of ancient terranes, their subsequent thermal transformation, and the generation of collision-related granitoids. Interpretation of seismic profiles indicates that the attributes inherent to the Phanerozoic collision prisms have been retained in the crust of the Paleoproterozoic fold system. These attributes include the Earth's crust as thick as 58 km, fragments of the lower, high-velocity layer (presumably, underplating basaltic rocks), and deep-seated blocks of high-velocity upper mantle raised to the level of the lower crust. These crustal units are underlain by the high-velocity diamond-bearing lithospheric mantle that has thickened to 260-300 km. The thickness of this mantle layer decreases to approximately 200 km toward the margins of the region, and a local swell has been identified as a lithospheric keel (root). The spatial relationships between the crustal and mantle structures are evident. It would be the simplest to suggest that this keel was formed as a result of accretion of the lithospheric mantle fragments together with the crustal terranes attached to their roofs. Such a suggestion comes into conflict with the commonly accepted views on the exclusively crustal nature of the continental collision and may become a subject of further investigations.