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

Abstract

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
(690–910◦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 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 (P–T)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
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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 P–T
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 X–Yplane 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 O∗MnO 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 O∗MnO 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 P–Tconditions 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 P–Tconditions 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 900◦C (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 SiO2TiO2Al2O3Cr2O3FeO∗MnO 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 P–Testi-
mates of eclogites and pyroxenites from Obnazhennaya of Taylor
et al. (2003), and stars show pyroxenite P–Tdata 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). P–Testimates 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. >100◦C) between thermometers.
The pyroxenites equilibrated at relatively low pressures
and temperatures (2.0–4.5 GPa and 690–910◦C), which are
similar to that obtained for the spinel-garnet lherzolite from
Mir (2.4 GPa and 740◦C; Table 1). Xenoliths from the
Palaeozoic Mir kimberlite span the range of P–Tcondi-
tions, whereas the younger Obnazhennaya pipe has lower
P–Tvalues, 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
NaTiAl∇O6, 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
MgMgSi∇O6,MgAl
2∇O6,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
P–Thistory. 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 1100–1300◦C, 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 <1100◦C 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
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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 P–Tconditions 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.
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