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Mineralogical and geochemical constraints on the shallow origin, ancient veining, and multi-stage modification of the Lherz peridotite

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  • University of Edinburgh School of Geoscience and SETI Institute

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New major- and trace-element data of bulk-rocks and constituent minerals, and whole-rock Re–Os isotopic compositions of samples from the Lherz Massif, French Pyrenees, reveal complex petrological relationships between the dominant lithologies of lherzolite ± olivine-websterite and harzburgite. The Lherz peridotite body contains elongate, foliation parallel, lithological strips of harzburgite, lherzolite, and olivine-websterite cross-cut by later veins of hornblende-bearing pyroxenites. Peridotite lithologies are markedly bimodal, with a clear compositional gap between harzburgites and lherzolites ± olivine-websterite. Bulk-rock and mineral major-element oxide (Mg–Fe–Si–Cr) compositions show that harzburgites are highly-depleted and result from ∼20-25 wt.% melt extraction at pressures <2 GPa. Incompatible and moderately-compatible trace-element abundances of hornblendite-free harzburgites are analogous to some mantle-wedge peridotites. In contrast, lherzolites ± olivine-websterite overlap estimates of primitive mantle composition, yet these materials are composite samples that represent physical mixtures of residual lherzolites and clinopyroxene dominated cumulates equilibrated with a LREE-enriched tholeiitic melt. Trace-element compositions of harzburgite, and some lherzolite bulk-rocks and pyroxenes have been modified by; (1) wide–spread interaction with a low-volume LREE-enriched melt +/− fluid that has disturbed highly-incompatible elements (e.g., LREEs, Zr) without enrichment of alkali- and Ti-contents; and (2) intrusion of relatively recent, small-volume, hornblendite-forming, basanitic melts linked to modal and cryptic metasomatism resulting in whole-rock and pyroxene Ti, Na and MREE enrichment.
Bulk-rock Fe/Si and Mg/Si values. Magnesium-Fe-Si systematics of the studied samples are compared to the composition of residues resulting from isobaric anhydrous melting experiments (a) performed with fertile peridotite starting materials over a pressure range of 1-5 GPa (Herzberg (2004) after Walter (1998)). Binary mixing analogues of igneous refertilisation (b) were derived from the constant addition of experimental melts (analogous to primary mantle malts) reported by Hirose and Kushiro (1993) and Walter (1998) to harzburgite 04LH33 of this study. Representative mineral compositions (c) were derived from those reported in Appendix B, and error bars represent 2r stdev calculated directly from the population analysed (in each case n ) 30). The range of observed compositions is also compared to; (1) estimates of primitive mantle composition, J79 = Jagoutz et al. (1979), G79 = Green et al. (1979), HZ86 = Hart and Zindler (1986), R91 = Ringwood (1991), MS95 = McDonough and Sun (1995), A95 = Allègre et al. (1995); and (2) a representative CI-chondrite value, TM85 = Taylor and McLennan (1985). The thick grey arrow delineates the linear-array of Mg/Si-Fe/Si compositions reported for lherzolite ± Ol-websterite of site A, and this array of data tends toward the low Mg/Si-Fe/Si values of Ol-websterites. Hb-hazburgite = hornblendite-bearing harzburgite, Hb-Lherzolite = hornblendite-bearing lherzolite, Ol-Websterite = olivine-websterite, and CLherzolite = composite-lherzolite, refer to the main text for details.
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Mineralogical and geochemical constraints on the shallow
origin, ancient veining, and multi-stage modification
of the Lherz peridotite
Amy J.V. Riches
, Nick W. Rogers
Department of Earth and Environmental Sciences, The Open University, Milton Keynes MK7 6AA, United Kingdom
Received 29 November 2010; accepted in revised form 25 July 2011; available online 31 July 2011
Abstract
New major- and trace-element data of bulk-rocks and constituent minerals, and whole-rock Re–Os isotopic compositions
of samples from the Lherz Massif, French Pyrenees, reveal complex petrological relationships between the dominant lithol-
ogies of lherzolite ± olivine-websterite and harzburgite. The Lherz peridotite body contains elongate, foliation parallel, lith-
ological strips of harzburgite, lherzolite, and olivine-websterite cross-cut by later veins of hornblende-bearing pyroxenites.
Peridotite lithologies are markedly bimodal, with a clear compositional gap between harzburgites and lherzolites ± olivine-
websterite. Bulk-rock and mineral major-element oxide (Mg–Fe–Si–Cr) compositions show that harzburgites are highly-
depleted and result from 20-25 wt.% melt extraction at pressures <2 GPa. Incompatible and moderately-compatible
trace-element abundances of hornblendite-free harzburgites are analogous to some mantle-wedge peridotites. In contrast,
lherzolites ± olivine-websterite overlap estimates of primitive mantle composition, yet these materials are composite samples
that represent physical mixtures of residual lherzolites and clinopyroxene dominated cumulates equilibrated with a LREE-
enriched tholeiitic melt. Trace-element compositions of harzburgite, and some lherzolite bulk-rocks and pyroxenes have been
modified by; (1) wide–spread interaction with a low-volume LREE-enriched melt +/fluid that has disturbed highly-incom-
patible elements (e.g., LREEs, Zr) without enrichment of alkali- and Ti-contents; and (2) intrusion of relatively recent, small-
volume, hornblendite-forming, basanitic melts linked to modal and cryptic metasomatism resulting in whole-rock and pyrox-
ene Ti, Na and MREE enrichment.
Rhenium-Os isotope systematics of Lherz samples are also compositionally bimodal; lherzolites ± olivine-websterite have
chondritc to suprachondritic
187
Os/
188
Os and
187
Re/
188
Os values that overlap the range reported for Earth’s primitive upper
mantle, whereas harzburgites have sub-chondritic
187
Os/
188
Os and
187
Re/
188
Os values. Various Os-model age calculations
indicate that harzburgites, lherzolites, and olivine-websterites have been isolated from convective homogenisation since the
Meso-Proterozoic and this broadly coincides with the time of melt extraction controlled by harzburgite Os-isotope composi-
tions. The association between harzburgites resulting from melting in mantle-wedge environments and Os-rich trace-phases
(laurite–erlichmanite sulphides and Pt–Os–Ir-alloys) suggests that a significant portion of persistent refractory anomalies in
the present-day convecting mantle of Earth may be linked to ancient large-scale melting events related to wide-spread sub-
duction-zone processing.
Ó2011 Elsevier Ltd. All rights reserved.
1. INTRODUCTION
The origin of orogenic massifs (c.f. Den Tex, 1969; Wyl-
lie, 1969), including the Lherz peridotite, has been the sub-
ject of much debate and it has been suggested that these
layered assemblages of peridotite and pyroxenite may rep-
0016-7037/$ - see front matter Ó2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.gca.2011.07.036
Corresponding author. Present address: Department of Earth
and Atmospheric Sciences, The University of Alberta, Edmonton,
Alberta, Canada T6G 2E3. Tel.: +1 780 492 2676.
E-mail address: ariches@ualberta.ca (A.J.V. Riches).
www.elsevier.com/locate/gca
Available online at www.sciencedirect.com
Geochimica et Cosmochimica Acta 75 (2011) 6160–6182
Author's personal copy
resent; (1) exhumed sub-continental lithospheric mantle
(e.g., Menzies and Dupuy, 1991; Reisberg and Lorand,
1995; Burnham et al., 1998; Downes, 2001) from the
mechanical boundary layer (MBL; White, 1988), or (2)
inherited heterogeneities from asthenospheric upper-mantle
(c.f. Peate et al., 1997; Parkinson and Pearce, 1998; Parkin-
son et al., 1998; Brandon et al., 2000; Bizimis et al., 2005;
Harvey et al., 2006; Mu
¨ntener and Manatschal, 2006; Bizi-
mis et al., 2007; Liu et al., 2008; Simon et al., 2008; Liu
et al., 2009; Warren et al., 2009; Dijkstra et al., 2010; Ishik-
awa et al., 2011 for examples of isotopic and mineralogical
heterogeneity present in Earth’s upper-mantle) that may
even be linked to upwelling diapirs (e.g., Bodinier et al.,
1988; Fabrie
`s et al., 1991). Several authors have used the
chemistry of mantle materials to provide estimates of the
composition of primitive upper mantle (PUM) and bulk-sil-
icate Earth (BSE; e.g., Palme and Nickel, 1985; Zindler and
Hart, 1986; McDonough and Sun, 1995; Becker et al.,
2006), and some authors have integrated information from
ultramafic bodies into models of the spatial distribution of
chemical diversity in Earth’s upper mantle (e.g., Alle
`gre and
Turcotte, 1986; Morgan and Morgan, 1999).
The Re–Os isotope system is widely considered to record
high-temperature processes (e.g., Shirey and Walker, 1998;
Carlson, 2005; Pearson et al., 2007; Carlson et al., 2008),
and Re–Os isotopic compositions of peridotites have been
used to estimate the timing of major mantle differentiation
events; methods of applying this tool to obtain osmium-
model ages (Alumino-chron and Sulphur-chron approaches)
were founded on studies of the Lherz peridotite and other
Arie
`ge-group massifs (e.g., Reisberg and Lorand, 1995;
Burnham et al., 1998). These Os-dating methods have been
applied to a number of other massif peridotite, ophiolite,
abyssal peridotite, and xenolith suites (e.g., Ronda and
Horoman massifs, Reisberg et al., 1991; Saal et al., 2001;
Southeast Australian Xenoliths, Handler et al., 1997; North
China Xenoliths, Gao et al., 2002; Liu et al., 2010, 2011;
Gakkel Ridge peridotites, Liu et al., 2008) where mantle
depletion ages have tentatively been linked to periods of
significant crustal generation and lithosphere stabilisation
(c.f. review by Rudnick and Walker, 2009).
As a large, well exposed, relatively fresh expanse of peri-
dotite that can be subjected to field investigations on all
length scales up to 1 km
2
(e.g., Conque
´re
´and Fabrie
`s,
1984; Bodinier et al., 1988; Fabrie
`s et al., 2001; Le Roux
et al., 2007) the Lherz massif has occupied a prominent po-
sition in debates concerning mantle evolution. Lherz is one
of 40 ultramafic bodies that crop out in the North Pyrenean
Metamorphic Zone (NPMZ; Fig. 1), which marks the meet-
ing of the Iberian plate to the south and European plate to
the north (e.g., Choukroune, 1992). The peridotite–pyroxe-
nite assemblage of Lherz covers a wide-range of major-,
minor-, and trace-element abundances (e.g., Bodinier
et al., 1988; Burnham et al., 1998; Le Roux et al., 2007),
and lithophile-element isotope compositions of this body
overlap the spectrum of
147
Sm/
144
Nd,
87
Sr/
86
Sr, and Pb–
Pb isotopic values reported in mantle peridotites world-
wide (Polve
´and Alle
`gre, 1980; Downes et al., 1991; Mukasa
et al., 1991; Zanetti et al., 1996; Henry et al., 1998; Le Roux
et al., 2009). Within the Lherz massif harzburgites distal
from cross-cutting veins are isotopically enriched with high
87
Sr/
86
Sr (0.70475 ± 4), high
87
Rb/
88
Sr (0.0075), low eNd
(+0.6), and contrast to lherzolites that commonly display
low
87
Sr/
86
Sr (0.70202 ± 2 to 0.70274 ± 4), low
87
Rb/
88
Sr
(0.0011–0.0042), and high eNd (+7.2 to +11.9); these isoto-
pic characteristics are not the result of melt extraction pro-
cesses alone (Downes et al., 1991; Le Roux et al., 2009).
Two contrasting petrogenetic models may account for
the compositional diversity of the Lherz massif; (1) harz-
burgites and lherzolites may result from variable degrees
of ancient melt extraction, and isotope systems based on
incompatible lithophile-elements may record later events
linked to thermal perturbation and/or incompatible-ele-
ment modification by moderate amounts of exotic melts
and/or fluids (e.g., Burnham et al., 1998; Henry et al.,
1998); or (2) Le Roux et al. (2007, 2008, 2009) recently
proposed that the peridotite–pyroxenite assemblage of
Lherz could reflect refertilisation of ancient depleted peri-
dotites by substantial volumes of percolating basaltic melt
(30–60 wt.% websterite, where these authors view webste-
rites as frozen melt-fronts) during the Variscan orogeny.
It is important to distinguish between these models as re-
cent suggestions of Variscan-age igneous refertilisation
may cast doubt on the significance of osmium model-age
determinations for the Lherz massif and other global peri-
dotite suites. If correct, the model proposed by Le Roux
et al. (2007, 2008, 2009) has important geodynamic impli-
cations if mineralogically fertile LREE-depleted lherzolites
are secondary products, and if large-volumes of basaltic
melt can ascend through mantle lithosphere by wide-
spread percolative flow (defined herein as igneous refertil-
isation) that; (1) crystallises substantial amounts of sec-
ondary clinopyroxene ± spinel ± sulphide ± amphibole;
and (2) is characterised by pronounced fractionation of
trace-elements at a chromatographic melt-front (c.f. Ver-
nie
`res et al., 1997).
In this study we report new field and petrographic char-
acteristics, mineral and bulk-rock major-, minor-, and
trace-element abundances, and whole-rock Re–Os concen-
trations and isotopic compositions of adjacent harzburgite
and lherzolite bodies (± olivine-websterite) at two sample
traverses within the Lherz massif. We demonstrate that
the peridotites and (diopside-bearing) olivine-websterites
of the Lherz massif were created in low-pressure environ-
ments (spinel–facies) where melt migration was dominated
by channel-flow during the Proterozoic. In addition, we
show that; (1) magnesian harzburgites with low-Ti clinopy-
roxenes, analogous to some mantle-wedge peridotites, may
result from low-pressure (<2 GPa) melting in the presence
of fluid; (2) incompatible-element abundances for this sam-
ple suite are decoupled from major-element and Os-isotope
systematics, recording two distinct post-differentiation
metasomatic events linked to; (a) wide–spread interaction
with a LREE-enriched melt +/fluid; and (b) short-
length-scale modal and cryptic modification adjacent to
hornblendite-veinlets. These results cast serious doubt on
the igneous refertilisation model of Le Roux et al. (2007,
2008, 2009), and indicate that the oldest portions of the
Lherz massif represent tectonically juxtaposed lithologies
that formed in a mantle-wedge environment.
Petrogenesis of the Lherz peridotite 6161
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2. FIELD, PETROGRAPHIC AND MINERALOGICAL
CHARACTERISTICS
2.1. Field constraints
The Lherz peridotite body is a layered ultramafic sequence
dominated by foliated peridotite in which the broadly NE-
SW striking fabric (Fig. 1b) is defined by elongate silicate
grains (olivine + pyroxene). Concordant harzburgites form
subordinate tabular features in direct contact with lherzolite
lithologies ± olivine-websterite. Some lherzolites contain
bands of olivine-websterite (up to 10 cm wide) that parallel
the foliation plane with a broad NE–SW strike. Field rela-
tionships indicate that the banded series of harzburgite,
lherzolite, and concordant olivine-websterite is old relative
to volumetrically minor anhydrous layered-pyroxenite (±
garnet) sequences (up to 4 m thick; Bodinier et al., 1987a)
that cross-cut the peridotite foliation at angles of 20°(Con-
que
´re
´and Fabrie
`s, 1984; Fabrie
`s et al., 2001). In addition,
Sautter and Fabrie
`s (1990) indicated that the peridotite
assemblage, olivine-websterite bands, and layered anhy-
drous–pyroxenites are isoclinally folded within Arie
`ge-group
peridotite bodies. Late-stage intrusive features dated at
100 Ma (e.g., Montigny et al., 1986; Henry et al., 1998) in-
clude amphibole-bearing pyroxenites and hornblendite-veins
that cross-cut the dominant foliation within spinel perido-
tites at angles of 30°(Conque
´re
´and Fabrie
`s, 1984; Fabrie
`s
et al., 2001). More recent studies by Le Roux et al. (2007,
2008, 2009) suggested an alternative view in which the con-
tacts between harzburgites and lherzolites are convoluted
and the foliation within harzburgites is over-printed by later
mineral growth (secondary pyroxene ± spinel ± amphi-
bole ± sulphide) linked to the generation of secondary lherz-
olites during a pervasive igneous refertilisation process.
Fig. 1. (A) Geological map of the Eastern Pyrenees, with the location of the study area shown in the inset where the grey-shaded area
corresponds to the Alpine mountain chain (after Choukroune, 1992). The Lherz peridotite is associated with metamorphic rocks of the North
Pyrenean Zone (NPZ). NPFT = North Pyrenean Frontal Thrust. (B) A detailed geological map of the Lherz Massif after Fabrie
`s et al. (2001)
and Le Roux et al. (2007).
6162 A.J.V. Riches, N.W. Rogers / Geochimica et Cosmochimica Acta 75 (2011) 6160–6182
Author's personal copy
In this study, two sample traverses were undertaken
across contacts between harzburgite and lherzolite bodies
that contain contrasting abundances of pyroxenite. Site A
contains a high pyroxenite abundance evident in the form
of numerous olivine-websterite bands (1.5–8 cm wide) with-
in the lherzolite body, and this traverse coincides with site 4
of Lorand et al. (2010). Site B, which corresponds to tra-
verse 2 of Le Roux et al. (2007, 2008, 2009) and site 2 of
Lorand et al. (2010), has a low pyroxenite abundance with
no clear exposure of olivine-websterite bands (where bands
are continuous features distinct from websteritic pods
<30 cm in length) in the lherzolite body over the sampling
scale of this work. In contrast to Le Roux et al. (2007,
2008, 2009) and Lorand et al. (2010) we observed hornblen-
dite–veinlets (<2 cm in width, Fig. A1 of Appendix A) in
several samples proximal to the harzburgite–lherzolite con-
tact at site B. Le Roux et al. (2007) suggested that harzburg-
ites record a foliation of N40–60°E that is over-printed by a
later and more steeply dipping foliation in lherzolites. Field
observations confirm that the foliation of harzburgites and
lherzolites proximal to the compositional boundary at site
B is weak, and that the foliation deviates from the N40–
60°E orientation that is dominant across the massif. Site
A is structurally distinct from site B (Fig. A2, Appendix
A). At site A, the foliation within the harzburgite and
banded-lherzolite body is coincident and conforms to the
N40–60°E plane observed at many outcrops across this
massif. Olivine-websterite bands within the lherzolite body
at site A are generally concordant with the foliation defined
by elongate silicates of the adjacent peridotite. In addition,
the contact between harzburgite and lherzolite at sites A
and B is sharp (at the 10 cm and thin section scale), and
these observations are consistent with previous field studies
and structural maps of Lherz in which harzburgite and
lherzolite lithologies represent elongate lithological strips
(e.g., Conque
´re
´and Fabrie
`s, 1984; Fabrie
`s et al, 2001 and
references therein) juxtaposed during plastic deformation
(e.g., Fig. 2 of Sautter and Fabrie
`s, 1990).
2.2. Petrographic features
Samples collected during this study are up to
20 30 20 cm in size and are heterogeneous at the
hand-specimen scale. All studied samples are coarse-
grained peridotites and olivine-websterites (silicates are typ-
ically 5–10 mm at the long axis), dominated by olivine,
orthopyroxene, clinopyroxene, spinel, minor amphibole,
and minor sulphide in an interlocking crystalline matrix
with a porphyroclastic texture (porphyroclasts are up to
25 mm at their long axis). Many samples from site A and
several samples from site B are hybrid formations of peri-
dotite + olivine-websterite (bands and pods), and these
composite samples are distinct from relatively homoge-
neous, mineralogically fertile, peridotites that have been
incorporated into previous studies that addressed the com-
position of Earth’s primitive mantle (e.g., McDonough and
Sun, 1995). Olivine, orthopyroxene, and clinopyroxene
crystals are elongate and show a preferred alignment, defin-
ing a clear foliation within the samples. Isolated areas (gen-
erally <3 mm wide) of later crystallised
pyroxene ± spinel ± sulphide ± minor amphibole are volu-
metrically minor (generally <5 vol.%, particularly in sam-
ples distal from cross-cutting hornblendite veinlets).
Similar features have been identified in previous petro-
graphic and mineralogical studies of the Lherz peridotites
(e.g., Lorand, 1989, 1991; Woodland, 1992; Woodland
et al., 1996). Minor trails (<0.5 mm wide) of spinel parallel
the penetrative foliation in some places, but are often in-
clined to it, consistent with observations by Woodland
(1992), and Woodland et al. (1996). All studied specimens
have experienced relatively low degrees of serpentinisation
Fig. 2. Bulk-rock major-element compositions. Literature data for
massif peridotites of the Lherz ultramafic body are from Bodinier
et al. (1988) and Burnham et al. (1998). Estimates of primitive
mantle compositions shown here were reported by McDonough
and Sun (1995). Hb-hazburgite = hornblendite-bearing harzburg-
ite, Hb-Lherzolite = hornblendite-bearing lherzolite, Ol-Webste-
rite = olivine-websterite, and C-Lherzolite = composite-lherzolite,
the classification scheme is described in the main text.
Petrogenesis of the Lherz peridotite 6163
Author's personal copy
(610 vol.%). Samples are classified on the basis of modal
content following the IUGS scheme of Streckeisen (1973)
and are reported in Appendices A and B.
Harzburgites contain <5 wt.% clinopyroxene, 10–
15 wt.% orthopyroxene, up to 75 wt.% olivine, <5 wt.%
chromite, minor sulphide, Fe-hydroxide, and calcite. In
addition, a few samples contain minor amounts of intersti-
tial amphibole (generally 60.5 mm in maximum dimension).
Harzburgites often have a fragmentary/mosaic texture of
heavily-cracked olivine crystals with sub-grain boundaries
inclined to the direction of elongation. Minor amounts of
serpentine form along cracks through olivine and at some
sub-grain boundaries. Many harzburgites contain a fine-
grained groundmass (<0.5 mm) in which heterogeneous
shearing is evident, and flow-lines defined by silicate neo-
blasts (<0.1 mm, commonly olivine) are present around a
number of silicate porphyroclasts. Silicates (olivine, clino-,
and orthopyroxene), particularly olivine, predominantly
display undulose extinction and this is evident in porphyro-
clastic and matrix grains. In places, orthopyroxene and oliv-
ine crystals cross-cut the harzburgite foliation, indicating a
later crystallisation phase after the older foliation-forming
event. Harzburgites contain isolated clusters of pyrox-
ene + Al-chromite (up to 5 5 cm, many of which are sym-
plectic), and lesser amounts of Al-chromite forming thin
trails (<1 cm wide) generally no more than 2 cm in length.
A limited number of harzburgites from site B contain nar-
row (0.5–1.5 cm) veinlets of hornblendite (kaersutitic
amphibole ± phlogopite ± pyroxene), which are associated
with modal metasomatism characterised by kaersutite for-
mation within the adjacent peridotite matrix (Appendix
A). Samples containing visible hornblendite–veinlets are
classified as hornblendite–harzburgites.
Lherzolites contain >5 wt.% clinopyroxene, 10–25 wt.%
orthopyroxene, <65 wt.% olivine, minor amounts of Al-
chromite (generally <3 wt.%), interstitial amphibole
(<0.5 mm), sulphide, calcite, and Fe-hydroxide. These
coarse-grained peridotites are often heterogeneous (on 10 s
cm-scales) with localised areas of low pyroxene abundance
creating portions that are almost harzburgitic (<5 cm diam-
eter); the foliation in these domains is parallel to that of
adjacent lherzolitic areas. A limited number of polished sec-
tions contain isolated regions (<3 mm wide) where silicate
alignment is not well developed, and the dominant foliation
is cross-cut in places by later silicate grains that approach an
equant character. Lherzolite samples from site A often con-
tain bands of olivine-websterite (>60 wt.% clino- and ortho-
pyroxene, 3 to 8 wt.% chromite, minor interstitial sulphide
and amphibole) that are generally 1.5–4 cm in width, and
reach a maximum thickness of 8 cm. These hand-specimens
have a penetrative foliation parallel to the orientation of
olivine-websterite bands, and contain silicates with undulose
extinction. Regions adjacent to olivine-websterite bands are
lherzolitic with a gradational edge (<2 cm) displaying a
modest increase in clino- and orthopyroxene mode
(5 wt.%), and concomitant decrease in olivine abundance
adjacent to olivine-websterites. Samples containing broad
regions (8–20 cm wide) of lherzolite and clearly-defined
bands of olivine-websterite are classified as composite-lherz-
olites (Appendices A and B).
2.3. Mineralogical characteristics
Major- and minor-element compositions of constituent
minerals are reported with trace-element abundances of cli-
no- and orthopyroxenes in Appendix B. Porphyroclastic
and matrix phases commonly cover a limited range of ma-
jor-element contents, and some crystals <0.5 mm in diame-
ter display irregular compositional zoning. In general,
constituent phases of peridotites and olivine-websterites
within the Lherz massif have Mg# (where Mg# = 100Mg/
[Mg + Fe
total
]) values of the order spinel olivine <
orthopyroxene < clinopyroxene.
2.3.1. Olivine and spinel compositions
Peridotites and olivine-websterites of the Lherz massif
contain forsteritic olivine, with the most magnesian compo-
sitions present in harzburgites (Mg# = 91–92, and
Mg# = 90–91 at sites A and B, respectively), and olivines
with lower Mg# (89–90) present in lherzolites, composite-
lherzolites, and olivine-websterites. Harzburgites associated
with hornblendite-veinlets contain olivine with Mg# values
(91) similar to those of hornblendite-free harzburgites.
The Mg# of olivines generally corresponds to the bulk-rock
composition, and the variation of olivine mode with Mg#
in several peridotites of the Lherz massif overlaps the oce-
anic trend of Boyd (1989); (Fig. A3 of Appendix A). In de-
tail, several harzburgite samples from site A and B contain
lower modal abundances of olivine and greater concentra-
tions of orthopyroxene than predicted by the oceanic trend
(Fig. A3, Appendix A), and only one of these samples con-
tains visible hornblendite. Nickel contents of olivines of site
A and B generally range from 0.38 to 0.46 wt.% NiO, and
extend to lower values (reaching 0.26 wt.%) in composite-
lherzolites of site A, with a maximum NiO content
(0.54 wt.%) in a lherzolite spatially associated with hornb-
lendite at site B (04LH04). This range of Mg# and Ni con-
tents is similar to those reported for in olivine-websterites
and peridotites of abyssal peridotites, forearc peridotites,
ophiolite sequences, and cratonic xenoliths (e.g., Kelemen
et al., 1998; O’Hara and Ishii, 1998; Suhr et al., 2003; Dick
et al., 2010; Warren and Shimizu, 2010).
Several types of spinel were identified on the basis of
their textural locations and major-element compositions
by Woodland et al. (1996), and definitions suggested by
these authors have been adopted and expanded in this
work. We recognise four spinel-types: (1) P-type spinels
(<1 mm) are common, occur interstitially, and are generally
anhedral with dark-brown to mid-brown body colour, (2)
S-type spinels have similar crystal form and textural associ-
ations to P-type spinels, but are less common and have
varying body colour that is olive-green and red-brown in
some cases, (3) rare G-type spinels (<0.5 mm) occur in a
limited number of harzburgite samples, have olive-green
body colour, and often form as isolated crystals at oliv-
ine-olivine contacts or adjacent to elongate orthopyroxene,
and (4) C-type spinels (0.5–1 mm) occur with clino- and
orthopyroxene in symplectic clusters and have irregularly
varying body colour ranging from olive-green to dark-
brown. In general, spinels are of chromite to Al-chromite
composition and have major-element characteristics similar
6164 A.J.V. Riches, N.W. Rogers / Geochimica et Cosmochimica Acta 75 (2011) 6160–6182
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to those reported for unmetasomatised abyssal spinel-peri-
dotites and orogenic massifs (e.g., Type-1 peridotites of
Dick and Bullen, 1984, and spinels of abyssal peridotites re-
viewed by Barnes and Roeder, 2001), which overlap the
range of spinel compositions suggested for four-phase spi-
nel-facies melting residues (olivine + orthopyroxene + spi-
nel + clinopyroxene; Dick and Fisher, 1983; Pearce et al.,
2000). In detail, spinels of site B harzburgites tend to have
lower Cr# (where Cr# = 100Cr/[Cr + Al]) at a given Mg#
compared to spinels of site A harzburgites (Fig. A3 of
Appendix A). Spinels of site A harzburgites overlap the
range of compositions previously reported in massif perido-
tites of the Arie
`ge-group ultramafic bodies. In contrast, spi-
nels of site B harzburgites overlap the range of major-
element compositions reported for Arie
`ge-group peridotites
that crop out adjacent to amphibole-pyroxenites (Fig. A3,
Appendix A). The olivine and spinel compositions of the
studied samples overlap the range reported for abyssal per-
idotites (e.g., Dick and Bullen, 1984), and differ from supra-
subduction zone (SSZ) peridotites associated with boninite
magmatism that contain spinels with Cr# >60 (e.g., Parkin-
son and Pearce, 1998; Choi et al., 2008).
2.3.2. Pyroxene and amphibole compositions
The studied samples contain diopside and enstatite cover-
ing a range of jadeite contents (<1 to 14.3 mol.%). Clinopy-
roxene compositions can be divided into three groups on
the basis of their major-element contents. Two clinopyroxene
groups dominate the studied samples and these fall on trend 1
of Supplementary Fig. A4 (Appendix A); hornblendite-free
harzburgites contain group-1 clinopyroxenes with the lowest
Na
2
O and highest Al
2
O
3
contents of all studied diopsides,
and group-2 pyroxenes hosted by hornblendite-free lherzo-
lites and olivine-websterites have moderate Na
2
O and
Al
2
O
3
contents. Lesser amounts of Group-3 clinopyroxenes
are present in the studied samples, and these are found in
hornblendite-bearing harzburgites and hornblendite-free
harzburgites that crop out proximal to the compositional
boundary at sites A and B; group-3 diopsides have higher
Na
2
O contents at equivalent Al
2
O
3
abundances (trend 2 of
Fig. A4, Appendix A) compared to clinopyroxene groups 1
and 2. Pyroxenes with relatively high Na
2
O abundances also
have high Cr
2
O
3
and lower CaO contents at a given Al
2
O
3
.
Group-3 clinopyroxenes from each sample site differ when
Ti-abundances are compared (Appendix B). Group-3 clino-
pyroxenes of site B, often associated with visible hornblen-
dite, contain up to 1.2 wt.% TiO
2
, and are analogous to
clinopyroxenes previously reported for Lherz peridotites
that; (1) crop out adjacent to amphibole-pyroxenites (Wood-
land et al., 1996); and (2) have been linked to the refertilisa-
tion trend defined by Le Roux et al. (2007). Group-3
clinopyroxenes of site A (samples 04LH34A and 04LH35)
have much lower Ti-contents (<150 ppm, Appendix B) than
abyssal peridotite clinopyroxenes (Ti >240 ppm; Dick et al.,
2010; Warren and Shimizu, 2010) and are analogous to low-
Ti clinopyroxenes of subduction-zone peridotites (e.g., Ti
<150 ppm; Parkinson et al., 1992; Bizimis et al., 2000).
Harzburgites of the Lherz massif contain minor
amounts of interstitial amphibole and previous studies sug-
gested that pargasitic compositions are common (e.g.,
Downes et al., 1991; Fabrie
`s et al., 1991). Lherzolites, com-
posite-lherzolites, and olivine-websterites contain greater
abundances of interstitial amphibole than harzburgites,
and distinct compositional groups are present at each sam-
ple site. Amphiboles of site A lherzolites ± olivine-webste-
rite are generally sodic and of eckermannite composition
(Appendix B). In contrast, amphiboles of site B harzburg-
ites and lherzolites ± hornblendite are generally calcic with
pargasitic to Mg-gedrite compositions. Amphiboles present
in hornblendite–veinlets are kaersutitic, and are optically
and compositionally analogous to amphiboles reported in
previous studies of hornblendite lithologies within the
Lherz massif (e.g., McPherson et al., 1996; Fabrie
`s et al.,
2001; Lorand and Gregoire, 2010).
2.3.3. Geothermometry
Equilibration temperatures calculated from core compo-
sitions of pyroxene, olivine, and spinel phases in a number
of textural locations (e.g., porphyroclasts, typical matrix,
and pyroxene-spinel symplectites) are reported in Appendix
B, and are generally 800–900 °C, 720–900 °C, and
650–900 °C for two-pyroxene, orthopyroxene-spinel, and
olivine-spinel thermometry, respectively. This range of equil-
ibration temperatures is similar to those reported in earlier
studies of Ca–Al–Fe–Mg thermometry within the Arie
`ge-
group peridotites (e.g., Conque
´re
´and Fabrie
`s, 1984), and re-
flects cooling to lithospheric conditions substantially below;
(1) the peridotite solidus (c.f. Hirschmann, 2000; Wasylenki
et al., 2003; Herzberg, 2004; for a compilation of anhydrous
and H
2
O under-saturated solidus curves); and (2) the poten-
tial temperature thought to be typical of MORB mantle (e.g.,
1280 °C; McKenzie and Bickle, 1988). Conque
´re
´and Fabrie
`s
(1984) studied a number of Arie
`ge-group peridotite bodies
that crop out at different points along the North Pyrenean
Metamorphic Zone and suggested that two distinct episodes
of subsolidus re-equilibration are recorded across this region.
The first re-equilibration episode proposed by these authors,
R
1
–D
1
, has been linked to peridotites with coarse-granular
textures (e.g., Fonte
ˆte Rouge peridotites), and may reflect
cooling to temperatures of 900–1000 °C at pressures of 12–
15 kbar. The second re-equilibration event, R
2
–D
2
, is associ-
ated with porphyroclastic textures (like those reported for the
Lherz massif), development of a penetrative foliation, forma-
tion of cm-thick ultra-mylonite bands in some places (Ave
`
Lallemant, 1967), and records equilibration extending to
lower temperatures and pressures (down to 750 °C and 8–
13 kbar; Ve
´til et al., 1988). The second of these regionally
recognised mantle re-equilibration events is consistent with
thermometric results reported here.
3. WHOLE-ROCK AND PYROXENE CHEMISTRY
3.1. Bulk-rock major-element, Cr, and Ni abundances
The studied sample suite displays highly systematic vari-
ations in major-element compositions (Fig. 2 and Appendix
A) with pronounced negative correlations between MgO
(29.5–46.4 wt.%) and CaO (0.34–7.53 wt.%), Al
2
O
3
(0.47–
7.80 wt.%), Na
2
O (<0.05–0.65 wt.%) and SiO
2
(42.9–
Petrogenesis of the Lherz peridotite 6165
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46.8 wt.%). By contrast, abundances of MgO and FeO are
not well correlated (not shown) in that lherzolites and harz-
burgites covering a range of MgO contain up to 8.32 wt.%
FeO, while the minimum harzburgite FeO content is 7.67
wt.%, and composite-lherzolites and olivine-websterites ex-
tend to lower FeO values. (6.33 wt.%). This range of values
coincides with those reported in previous studies of the
Lherz massif (e.g., Bodinier et al., 1988; Burnham et al.,
1998), and linear correlations between bulk-rock CaO,
Al
2
O
3
, MgO, and SiO
2
contents are a common feature of
peridotite–pyroxenite assemblages in other ultramafic
bodies such as Beni-Bousera, Horoman, and Ronda (e.g.,
Frey et al., 1985; Bodinier and Godard, 2003). This range
of compositions overlaps estimated compositions of BSE
(e.g., Figs. 2and 3) and depleted MORB mantle (DMM;
Workman and Hart, 2006).
In more detail, the studied sample suite is broadly bimo-
dal with harzburgite MgO abundances generally between
45 and 47 wt.%, whereas lherzolites, composite-lherzolites
and olivine-websterites contain <40 wt.% MgO. These
harzburgites have low Al
2
O
3
(61 wt.%), CaO (61 wt.%),
and high Ni contents (2142–2379 ppm) and Mg# (91.7–
92.1), consistent with P70 wt.% olivine and <5 wt.% clino-
pyroxene resulting from 20-25 wt.% melt removal
(Appendix A). In contrast, lherzolites ± olivine-websterite
bands have higher Al
2
O
3
(1.5–5.8 wt.%), CaO (2.0–
5.6 wt.%), and lower MgO (34–43 wt.%), Ni (1542–
1918 ppm) contents, and Mg# (90.3 to 91.6) corresponding
to higher pyroxene abundances. Olivine-websterite bands
have the highest Al
2
O
3
(6.6–7.8 wt.%), CaO (6.9–
7.5 wt.%), lower MgO (31–29 wt.%), Mg# (90.2–91.5) with-
in the range of enclosing lherzolites, and the lowest Ni
abundances (1412–1381 ppm) consistent with P60 wt.%
pyroxene and 630 wt.% olivine. The majority of studied
lherzolites ± olivine-websterite bands are composite sam-
ples with Al
2
O
3
values in excess of primitive mantle esti-
mates, and these specimens are not simple residues of
melt extraction.
Despite the strong systematic co-variations in abun-
dances of many major-element oxides (e.g., Fig. 2a), there
are significant differences in the major-element systematics
of the two sample sites that are exemplified by plots of
Mg/Si against Fe/Si, and Cr/Al against distance from the
compositional boundary (Figs. 2c and 3). Harzburgite
and lherzolite samples generally have Cr/Al values that de-
fine distinct groups related to each lithology. Harzburgites
Fig. 3. Bulk-rock Fe/Si and Mg/Si values. Magnesium-Fe-Si
systematics of the studied samples are compared to the composition
of residues resulting from isobaric anhydrous melting experiments
(a) performed with fertile peridotite starting materials over a
pressure range of 1–5 GPa (Herzberg (2004) after Walter (1998)).
Binary mixing analogues of igneous refertilisation (b) were derived
from the constant addition of experimental melts (analogous to
primary mantle malts) reported by Hirose and Kushiro (1993) and
Walter (1998) to harzburgite 04LH33 of this study. Representative
mineral compositions (c) were derived from those reported in
Appendix B, and error bars represent 2r
stdev
calculated directly
from the population analysed (in each case n30). The range of
observed compositions is also compared to; (1) estimates of
primitive mantle composition, J79 = Jagoutz et al. (1979),
G79 = Green et al. (1979), HZ86 = Hart and Zindler (1986),
R91 = Ringwood (1991), MS95 = McDonough and Sun (1995),
A95 = Alle
`gre et al. (1995); and (2) a representative CI-chondrite
value, TM85 = Taylor and McLennan (1985). The thick grey arrow
delineates the linear-array of Mg/Si–Fe/Si compositions reported
for lherzolite ± Ol-websterite of site A, and this array of data tends
toward the low Mg/Si–Fe/Si values of Ol-websterites. Hb-hazburg-
ite = hornblendite-bearing harzburgite, Hb-Lherzolite = hornblen-
dite-bearing lherzolite, Ol-Websterite = olivine-websterite, and C-
Lherzolite = composite-lherzolite, refer to the main text for details.
"
6166 A.J.V. Riches, N.W. Rogers / Geochimica et Cosmochimica Acta 75 (2011) 6160–6182
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commonly have high Cr/Al values that range from 0.50 to
0.85, whereas lherzolites, composite lherzolites, and olivine-
websterites generally have Cr/Al values <0.20. A number of
samples at site B have Cr/Al values that are intermediate
(0.28–0.45) between harzburgite and lherzolite end-mem-
bers; these generally crop out close (<1.5 m) to the compo-
sitional boundary between adjacent lherzolite and
harzburgite bodies at site B. Samples with intermediate
Cr/Al contain visible, or crop out in close proximity to
mm- to cm-scale hornblendite veins. Samples associated
with hornblendite-veinlets at site B have Mg#’s that are
intermediate between; (1) distal harzburgite and lherzolite
samples of site B; and (2) all samples of site A.
On the basis of Mg–Fe–Si systematics peridotites of site
A are separated into two distinct compositional groups cor-
responding to harzburgite and lherzolite bodies (Fig. 3).
Harzburgites define a tightly-clustered group with a narrow
range of Mg/Si (1.298–1.326) and Fe/Si values (0.286–
0.298). Lherzolites and olivine-websterites of site A define
a striking linear trend with Mg/Si varying from 0.809 to
1.099 as Fe/Si increases from 0.224 to 0.309. This trend
coincides with the estimated composition of BSE given by
McDonough and Sun (1995), and the DMM-value of
Workman and Hart (2006). In contrast, peridotites of site
B do not show such a clear division between lherzolites
and harzburgites and cover a narrower range of Fe/Si val-
ues. Harzburgites define a clustered group with Fe/Si values
between 0.298 and 0.314 and Mg/Si values of 1.341–1.389.
Site B lherzolites cover a range of Fe/Si and Mg/Si values
(0.273–0.314 and 1.031–1.271, respectively) defining an
elongate field sub-parallel to the linear correlation reported
for the websterite-banded lherzolite body of site A (Fig. 3).
Taken together, the harzburgite samples of site A and site B
describe an elongate field that is sub-parallel to the lherzo-
lite field of site B.
3.2. Bulk-rock and pyroxene trace-element characteristics
Lherz peridotites (± olivine-websterite) and their con-
stituent pyroxenes cover a range of trace-element abun-
dances (Figs. 4 and 5, Appendices A and B), and the
rare-earth-element (REE) profiles of bulk-rocks are re-
flected by the REE-patterns of constituent clinopyroxenes.
In general, lherzolites and olivine-websterites are light-
rare-earth-element (LREE) depleted and have higher abun-
dances of heavy- and middle-rare-earth-elements (HREE
and MREE) than harzburgite samples. Europium anoma-
lies ([Eu/Eu
*
]
N
, where [Eu
*
]
N
= [(Gd + Sm)/2]
N
, and N de-
notes normalisation to CI-chondrite) of the studied samples
are 0.44–0.74 (Appendix B), and constitutent pyroxenes
generally have (Eu/Eu
*
)
N
of 0.24–0.32; these relatively
small negative values differ from positive Eu-anomalies
(>1) reported for bulk-rocks and clinopyroxenes of selected
ophiolite peridotites from the Eastern Central Alps (e.g.,
Fig. 4. Rare-earth-element (REE) abundances (with an interpolated space on the x-axis for Pm) of bulk-rocks (BR) and constituent
clinopyroxenes (CPX) normalized to the CI-chondrite estimate of McDonough and Sun (1995). Hb-hazburgite = hornblendite-bearing
harzburgite, Hb-Lherzolite = hornblendite-bearing lherzolite, Ol-Websterite = olivine-websterite, and C-Lherzolite = composite-lherzolite,
the main text contains details of lithological classifications.
Petrogenesis of the Lherz peridotite 6167
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Totalp and Platta ultramafics; Mu
¨ntener et al., 2010).
Hornblendite-free harzburgites of site A are characterised
by U-shaped REE-patterns, and samples with the greatest
degree of LREE-enrichment also have elevated chondrite-
normalised U and Th relative to Ba and Nb. The trace-ele-
ment systematics of peridotites at site B, differ from those of
site A, and show a wide-range of REE-patterns, including
convex-upward and sinusoidal shapes (e.g., Fig. 4); the lat-
ter are generally found in hornblendite-bearing rocks or
spatially associated samples, whereas U-shaped REE-pat-
terns are present in samples that lack hornblendite and have
low bulk Ti-contents (<30 ppm). In general, the studied
samples lack the selective enrichment of large-ion-litho-
phile-elements (LILE; Cs, Rb, Ba, and Sr) relative to
high-field-strength-elements (HFSE; Th, U, Nb, Ta, Zr,
and Hf) reported for SSZ-peridotites of the Izu–Bonin–
Mariana arc sequence (Fig. 5), but hornblendite-free harz-
burgites have chondrite-normalised trace-element patterns
similar to harzburgite xenoliths recovered from the Avacha
volcano, Kamchatka arc (Ionov, 2010).
The range of trace-element abundances in the studied
bulk-rocks and clinopyroxenes is similar to the composi-
tional range reported in previous investigations of the
Lherz massif (e.g., Bodinier et al., 1988, 1990; McPherson
et al., 1996; Burnham et al., 1998; Bodinier et al., 2004;
Le Roux et al., 2007, 2009). No extremely depleted clinopy-
roxenes, analogous to that reported for harzburgite 06LI15
(REE 6CI-chondrite abundances; Le Roux et al., 2009) are
Fig. 5. Spider-diagram of bulk-rock trace-element abundances normalized to the CI-chondrite value of McDonough and Sun (1995).
Proximal harzburgites of site A crop out within 25 cm of the contact between harzburgite and lherzolite bodies, whereas distal samples crop
out >1 m from this compositional boundary. For comparison, the range of bulk-rock trace-element compositions reported for Torishima
forearc samples and peridotites of the Conical seamount, Izu–Bonin–Mariana arc (Parkinson and Pearce, 1998), are shown by the grey field.
SSZ = supra-subduction zone. Hb-hazburgite = hornblendite-bearing harzburgite, Hb-Lherzolite = hornblendite-bearing lherzolite, Ol-
Websterite = olivine-websterite, and C-Lherzolite = composite-lherzolite, details of the classification scheme applied here are given in the text.
6168 A.J.V. Riches, N.W. Rogers / Geochimica et Cosmochimica Acta 75 (2011) 6160–6182
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present in our sample set. Bulk-rock HREE contents and
ratios determined in this study overlap the range previously
reported for Lherz peridotites and for peridotite and pyrox-
enite assemblages of other Arie
`ge-group massifs, Ronda,
Horoman, and the Beni-Bousera ultramafic complexes
(Fig. 6a).
Trace-element partition coefficients between co-existing
clino- and orthopyroxene (Kd
CPX/OPX
) phases are broadly
analogous to those reported in previous studies of mantle
peridotites (e.g., Stosch, 1982; Bodinier et al., 1988; Eggins
et al., 1998; Witt-Eickschen and O’Neill, 2005; Lee et al.,
2007; Harvey et al., 2010; Appendix B). Absolute values
of REE Kd
CPX/OPX
are relatively high in the studied sample
suite that has equilibration temperatures of 800 °C or less.
The elevated REE Kd
CPX/OPX
values of Lherz samples with
lower equilibration temperatures than reported for several
xenolith suites (refer to Appendix B) probably reflect the
temperature dependence of subsolidus REE redistribution
that is thought to be linked to Ca-exchange between clino-
and orthopyroxene during cooling (e.g., Witt-Eickschen
and O’Neill, 2005 and references therein), and this is the
subject of ongoing study.
Fig. 6. Moderately-incompatible (Dy, Yb, V), incompatible (Ti), and highly incompatible (Zr) trace-element compositions of bulk-rocks
(Bulk; (a) and (b) and constituent clinopyroxenes (CPX; (c) and (d) Bulk-rock models of spinel- (Sp) and garnet- (Gt) facies fractional melting
assume a starting composition equivalent to that of primitive mantle (PUM; McDonough and Sun, 1995) and incorporate melting modes and
partition coefficients reported by Johnson et al. (1990) that are relevant for anhydrous conditions. These melting curves reflect dynamic
melting in which a critical porosity retains 1% of the melt in all residues (after McKenzie, 1985). The melting modes of Kinzler (1997) are used
in our calculations. The blue curve in (a) describes compositions expected for melting residues produced under hydrous conditions in the
mantle-wedge where HREEs are expected to be more incompatible (Gaetani et al., 2003; McDade et al., 2003). Vanadium is multi-valent on
Earth, occurring as V
3+
,V
4+
, and V
5+
; models of bulk-rock abundances resulting from variable degrees of fractional melting are redox
sensitive and are shown over a range of oxidation conditions (FMQ 1 to FMQ + 1; after Parkinson and Pearce, 1998). Data fields delineate
the range of compositions reported for spinel-bearing peridotites devoid of garnet ± plagioclase (Bodinier et al., 1987b; Bodinier et al., 1988;
Fabrie
`s et al., 1989; Bodinier et al. 1990; McPherson et al., 1996; Van der Wal and Bodinier, 1996; Burnham et al., 1998; Fabrie
`s et al., 1998;
Parkinson and Pearce, 1998; Garrido et al., 2000; Lenoir et al., 2000, 2001; Downes, 2001; Beccaluva et al., 2004; Bianchini et al., 2007; Ionov,
2010), and clinopyroxene compositions determined by in-situ methods (Parkinson et al., 1992; Bizimis et al., 2000;Johnson et al., 1990;
Johnson and Dick, 1992; Warren and Shimizu, 2010). SSZ = supra-subduction zone, MOR = mid-ocean ridge. Hb-hazburgite = hornblen-
dite-bearing harzburgite, Hb-Lherzolite = hornblendite-bearing lherzolite, Ol-Websterite = olivine-websterite, and C-Lherzolite = composite-
lherzolite, see the main text for details. FMM = fertile MORB mantle (Pearce and Parkinson, 1993), DMM = depleted MORB mantle
(Workman and Hart, 2006).
Petrogenesis of the Lherz peridotite 6169
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3.3. Bulk-rock S, Cu, Re and Os concentrations and Re–Os
isotope compositions
Bulk-rock S, Cu, Re, and Os abundances and Re–Os
isotopic compositions generally fall into distinct composi-
tional groups with no clear gradation across the composi-
tional boundary between harzburgites and lherzolites at
sites A and B (Fig. 7). Harzburgites have low bulk S and
Cu contents (35–52 ppm and 1.12–2.90 ppm, respectively),
whereas lherzolites and composite-lherzolites have high S
and Cu contents (163–415 ppm and 16.2–42.1 ppm, respec-
tively). Olivine-websterites measured in this study have S
contents (334 ppm; 04LH38) within the range observed in
spatially associated lherzolites, but higher Cu concentra-
tions (77.6 ppm; 04LH14A) than adjacent peridotites. Sul-
phur and Re abundances define broad negative
correlations with increasing bulk-MgO content (Fig. A8
of Appendix A), and this range of values coincides with
those previously reported for the Lherz peridotite body
(Reisberg and Lorand, 1995; Burnham et al., 1998), passing
close to estimates of primitive mantle composition (Appen-
dices A and B). A limited number of samples (04LH07A,
04LH37B) have relatively high S contents at a given MgO
concentration, and these rocks also have elevated Re con-
tents. The olivine-websterite analysed in this work
(04LH38) has low S and Re abundances when compared
to the dominant negative correlation between these ele-
ments and bulk-MgO (Fig. A8, Appendix A). Harzburgites
generally have high osmium contents (4012–5312 ppt) with
few Lherz harzburgites containing <4100 ppt (an exception
includes 04LH34A of site A with 3017 ppt Os). Lherzolites
and composite-lherzolites have Os concentrations that gen-
erally range from 3624 to 4112 ppt, and these lherzolites
rarely have Os abundances in excess of 4100 ppt. Lherzolite
04LH07A, associated with hornblendite-veinlets and char-
acterised by relatively high Re and S abundances, has the
highest Os content (4538 ppt) of the studied sample suite.
Olivine-websterite 04LH38 has the lowest osmium concen-
tration (2830 ppt) of all studied samples, and this value is
significantly greater than Os concentrations generally re-
ported for basaltic materials (e.g., 1–50 ppt range of
MORB; Shirey and Walker, 1998). Measured Os abun-
dances in this sample suite coincide with the range of values
reported in other investigations of Lherz peridotites, and
these data do not define a strong positive correlation with
bulk-MgO (Appendices A and B).
Harzburgites generally have low
187
Re/
188
Os
(0.023 ± 0.002 to 0.066 ± 0.007) and sub-chondritic
187
Os/
188
Os values that are within uncertainty of one an-
other (0.117; Fig. 7 and Appendix B), and higher
187
Os/
188
Os values than reported for 3 of the 4 low-S harz-
burgites studied by Luguet et al. (2007). Few studied harz-
burgites (04LH34A of site A is an exception) have
concomitant elevations of
187
Re/
188
Os and
187
Os/
188
Os
above values of 0.1 and 0.120, respectively. Lherzolites
and composite-lherzolites have
187
Re/
188
Os compositions
ranging from 0.32 ± 0.003 to 0.55 ± 0.006, and chondritic
to supra-chondritic
187
Os/
188
Os values (0.1246–0.1324).
Sample 04LH07A, associated with hornblendite-veinlets,
Fig. 7. Bulk-rock Re–Os isotope compositions, bulk- Al
2
O
3
abundances, and distance from compositional boundaries between
harzburgite and lherzolite bodies at sites A and B. Correlations are
calculated using the isoplot 3.00 tool (Ludwig, 2001) and all
literature data is combined with that reported here during
regression calculations that assume a model-1 fit (where assigned
uncertainties are 2rinternal precision values). Model-ages calcu-
lated via the ‘aluminochron’ method compare an initial
187
Os/
188
Os
value (taken from the Os-isotope value of the linear regression at
an Al
2
O
3
content of 0.7 wt.%) to a mantle evolution curve that
assumes a
187
Os/
188
Os Solar System initial of 0.09524 ± 0.00011
(IIIA irons, 95% confidence interval; Smoliar et al., 1996), a Re-
decay constant of 1.666 10
11
(Smoliar et al., 1996) and a
present-day chondritic value of 0.1274 ± 0.0034 (Walker et al.,
2002). PME = Proterozoic melt extraction, AME = Achaean melt
extraction, after Becker et al. (2006). Data reported for Lherz
samples of previous studies were taken from by Reisberg and
Lorand (1995), Burnham et al. (1998), Becker et al. (2006), and
Luguet et al. (2007). Primitive mantle estimates are taken from
McDonough and Sun (1995) and Meisel et al. (2001). Ol-Webste-
rite = olivine-websterite, and C-Lherzolite = composite-lherzolite,
refer to the main text for classification details.
6170 A.J.V. Riches, N.W. Rogers / Geochimica et Cosmochimica Acta 75 (2011) 6160–6182
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has unusual Re–Os isotope compositions compared to
other lherzolites analysed in this work; the bulk
187
Re/
188
Os
value of 0.36 ± 0.004 falls within the range of other lherzo-
lite samples, but 04LH07A has the lowest
187
Os/
188
Os com-
position (0.11258 ± 0.00010) of all studied specimens. The
highest
187
Re/
188
Os (0.73 ± 0.007) and
187
Os/
188
Os
(0.1387) isotope compositions yet reported for the Lherz
massif (Fig. 7) were determined for olivine-websterite
04LH38.
Regional studies of Arie
`ge-group peridotites identified
broad linear correlations between
187
Os/
188
Os and
187
Re/
188
Os, and relatively strong positive correlations be-
tween
187
Os/
188
Os, S and Al
2
O
3
contents (Reisberg and
Lorand, 1995; Burnham et al., 1998) that overlap the range
of Os–Al values reported here. New results for the Lherz
massif alone show that bulk-rock
187
Os/
188
Os values of
the studied sample suite do not correlate linearly with
187
Re/
188
Os, S, Al
2
O
3
, MgO, Yb (e.g., Fig. 7 and Appendix
B). Harzburgites generally cluster at low
187
Os/
188
Os,
187
Re/
188
Os, S, Al
2
O
3
, MgO, and Yb values, whereas lherz-
olites ± olivine-websterite form a cluster at higher S, Al
2
O
3
,
MgO, and Yb contents with
187
Os/
188
Os values overlapping
the range of Os-isotope compositions reported for recently
exhumed abyssal peridotites that sample present-day con-
vecting upper mantle (e.g., Fig. 9b), and similar observa-
tions have been described for Middle-Atlas peridotite
xenoliths (Wittig et al., 2010).
4. PETROGENETIC MODELS
The Lherz body lacks dunite and contains relatively
small volumetric proportions of harzburgite, thus it differs
from harzburgite dominated massifs associated with ophio-
lite sequences that generally contain cross-cutting dunite
channels, which have been linked to melt generation at high
melt-flux rates (e.g., Canyon Mountain, Oregan; Josephine,
Oregan, Semail, Oman; Troodos, Cyprus, Red Mountain,
New Zealand; Dick and Sinton, 1979; Boudier and Nicolas,
1985; Kelemen et al., 1995). The entire range of bulk-rock
and spinel major-element compositions reported for Lherz
differ from SSZ-peridotites that may have equilibrated with
boninitic magmas (e.g., spinel Cr# >60 reported for perido-
tites of the Izu–Bonin–Mariana Arc, Parkinson and Pearce,
1998) and their experimental equivalents (e.g., Gaetani and
Fig. 8. Models of bulk-rock compositions resulting from binary mixing between a basaltic melt akin to MORB picrite and harzburgites
04LH33 are constructed at selected points in time (t), and the proportion of melt (wt.%) added is marked by vertical dashed lines. Models
simulate incremental addition MORB-like melt (16 wt.% Al
2
O
3
, 10 ppt Os, 2000 ppt Re, and a present-day
187
Os/
188
Os value of 0.134, after
Shirey and Walker, 1998 and BVSP, 1981). Data retrieved from previous studies of Lherz peridotites were taken from Reisberg and Lorand
(1995), Burnham et al. (1998), Becker et al. (2006), and Luguet et al. (2007). Primitive mantle estimates are taken from McDonough and Sun
(1995) and Meisel et al. (2001). Ol-Websterite = olivine-websterite, and C-Lherzolite = composite-lherzolite, classification details are given in
the main text.
Petrogenesis of the Lherz peridotite 6171
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Grove, 1998; Parman and Grove, 2004; Grove et al., 2006).
However, highly-depleted (whole-rock Al
2
O
3
<0.85 wt.%)
hornblendite-free harzburgites of site A, with bulk-rock
trace-element contents analogous to mantle-wedge perido-
tites (e.g., Avacha peridotite xenoliths; Ionov, 2010), con-
tain low-Ti clinopyroxenes that are similar to those
reported for subduction-zone peridotites of the Hellenic
ophiolite and Izu–Bonin–Mariana arc peridotite suites
(Fig. 6c and d). Many of the studied peridotites from the
Lherz body have bulk-rock and mineral major-, minor-,
and trace-element compositions that are transitional be-
tween abyssal and SSZ-peridotites (e.g., Fig. 6; Appendices
A and B), and for these reasons the following discussion
examines whether the studied range of peridotite ± oliv-
ine-websterite compositions can be accounted for by; (1)
percolative refertilisation of refractory peridotites with
basaltic melt; (2) variable degrees of melt depletion fol-
lowed or accompanied by physical mixing of peridotite
and pyroxenite during mechanical mingling (± short-length
scale melt-interaction at pyroxenite margins); and (3) short-
length scale metasomatism unrelated to postulated Ca, Al,
Ti, Fe enrichment envisaged in models of igneous-refertili-
sation. We also evaluate the tectonic setting in which
melting and olivine-websterite formation may have taken
place.
4.1. Assessment of percolative igneous refertilisation
Models of igneous refertilisation constructed by two-
component mixing between harzburgite 04LH33 and exper-
imental melts (analogous to primary melts that could be
envisaged in a zone-refining scenario) produce a broad field
of products overlapping the range of lherzolite Mg–Fe–Si
compositions at site B, and the majority of compositions
observed in the banded-lherzolite body at site A (Fig. 3b).
However, the orientation of the elongate fields of harzburg-
ite at site A and B, and the field of site B lherzolite Mg–Fe–
Si compositions cross-cut the trajectory of these igneous
refertilisation models. In this context, the range of melt
compositions required to satisfy the observed bulk-rock
compositions does not follow a single refertilisation trajec-
tory and requires interaction with substantial melt volumes
(P5-10 wt.% of the product) ranging from basanite to tho-
leiite. The requirement of significant melt volumes of dis-
tinct major-element compositions coexisting in closely
spaced samples (10 s cm scales) during a percolative igneous
refertilisation process is difficult to envisage as a physically
realistic aspect of melt infiltration. Hand-specimen, miner-
alogical, and bulk compositional data show that site A
lherzolites represent physical mixtures of lherzolite + oliv-
ine-websterite, and field observations indicate that this pro-
cess is structurally old and took place under conditions
where plastic-deformation operated. Site B lherzolites are
heterogeneous and may also contain a websteritic compo-
nent, but the interpretation of this group of samples is com-
plicated by modal-metsomatism that formed secondary
pyroxene ± spinel ± amphibole ± sulphide accompanied
by cryptic-metasomatism that elevated LREE and MREE
abundances of bulk-rocks and clinopyroxenes during the
formation of late-stage cross-cutting hornblendite-veinlets.
Fig. 9. Whole-rock Al
2
O
3
contents and
187
Os/
188
Os compositions (a) and a histogram of Os-isotope compositions (b). These diagrams
incorporate data for two European continental xenolith suites (Massif Central, Harvey et al., 2010; and Spitsbergen, Svalbard Archipelago,
Choi et al., 2010), peridotite xenoliths of the Middle-Atlas, Morocco (Wittig et al., 2010), and four oceanic peridotite suites; Izu–Bonin–
Mariana SSZ-peridotites (Parkinson et al., 1998); Mid-Atlantic Ridge abyssal periodtites, ODP Site 920 (Brandon et al., 2000) and ODP Hole
1274A (Harvey et al., 2006); and abyssal peridotites of Gakkel Ridge (Liu et al., 2008). The primitive mantle estimate corresponds to that
reported by Meisel et al. (2001). Literature data are sourced from Reisberg and Lorand (1995), Burnham et al. (1998), Becker et al. (2006), and
Luguet et al. (2007). SSZ = supra-subduction zone, MOR = mid-ocean ridge. Ol-Websterite = olivine-websterite, and C-Lherzolite = com-
posite-lherzolite, refer to the main text for details.
6172 A.J.V. Riches, N.W. Rogers / Geochimica et Cosmochimica Acta 75 (2011) 6160–6182
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4.2. Spinel-facies harzburgite formation
Bulk-rock MgO, Mg#, Al
2
O
3
, and HREE abundances
of the studied peridotites correlate with one another and
trace the extent of melt extraction in relatively homoge-
neous samples unaffected by recent interaction with basan-
itic magmas associated with hornblendite formation. For
these reasons moderately-compatible and compatible-ele-
ment compositions of harzburgites reflect the conditions
of melt generation. The highest degrees of melting are re-
corded by harzburgites (up to 25 wt.% melt loss; Appen-
dix B), which have Mg–Fe–Si abundances and relatively
high olivine/orthopyroxene values (olivine/orthopyrox-
ene = 2.08–5.76) that cannot be accounted for by anhy-
drous melting alone (Fig. 3a and Fig. A3 of Appendix A).
Low-Al harzburgites of site A contain clinopyroxenes with
low-alkali contents; the low-Ti abundances of these clino-
pyroxenes differ from residual clinopyroxenes of abyssal
peridotites (Fig. 6c and d), but are broadly analogous to
clinopyroxenes of SSZ-peridotites. However, clinopyroxene
and bulk-rock trace-element data for these rocks lack pro-
nounced enrichment of Cs, Rb, Ba, and Pb, suggesting that
these materials did not form in an environment that experi-
enced high-rates of fluid flux from subducted lithosphere
(i.e., not within 100 km of a slab-wedge interface; Pearce
and Parkinson, 1993; though this parameter is dependent
on nature of subducting materials, the angle of dip of the
subducting slab, and the maturity of the subduction-zone
system). The lack of correlation between harzburgite
incompatible-element abundances and orthopyroxene/oliv-
ine values (not shown) suggests that it is unlikely that orth-
opyroxene is of secondary origin, and significant degrees of
interaction with Si-bearing melts/fluids are doubtful. Min-
eralogical affinities between peridotite bands of the Lherz
massif, abyssal (Fig. A3, Appendix A) and SSZ-peridotites
(e.g., Fig. 6), which are thought to be derived from melting
over an interval that does not exceed 2 GPa (c.f. Kinzler,
1997; Walter, 2003 and references therein), lead us to exam-
ine trace-element constraints on the depth at which melting
took place to produce the peridotites of the Lherz massif.
The studied sample set displays a wide-range of trace-
element characteristics, which indicates that Lherz perido-
tites have a complex metasomatic history (appraised in Sec-
tion 4.3). We focus on constraining the genesis of site A
harzburgites as the origin of site B harzburgites is obscured
by Al, Fe, Ti, Na, LREE, and MREE enrichment resulting
from interaction with hornblendite forming magmas. Mod-
erately-incompatible element contents of peridotites may be
significantly less susceptible to post-melting modification
than incompatible elements, and for this reason HREEs
are used to constrain melting style. Previous authors have
used bulk-rock Tm–Yb compositions to suggest that the
Lherz peridotites result from variable degrees of garnet-fa-
cies melting (i.e., in excess of 2 GPa; Bodinier et al., 1988;
Burnham et al., 1998). New trace-element data for the
Lherz peridotites, combined with that of previous studies,
show that harzburgites generally have a positive slope be-
tween HREEs, and bulk-rock Dy–Yb and Lu–Hf contents
(Fig. 6 and Fig. A7 of Appendix A) define arrays analogous
to residue compositions expected for spinel-facies melting.
Hornblendite-free harzburgites and constituent clinopyrox-
enes generally have positive MREE–HREE slopes, and this
contrasts to negative MREE–HREE slopes of clinopyrox-
enes of abyssal peridotites exhumed at slow-spreading
ridges where the earliest stages of melting take place in
the presence of garnet followed by significant degrees of
fractional melting in spinel-facies mantle (e.g., Johnson
et al., 1990; Johnson and Dick, 1992; Hellebrand et al.,
2002, 2005). For these reasons Lherz harzburgites may have
experienced melting under slightly shallower conditions (or
over a narrower melting interval) than many of the abyssal
peridotites studied to date.
Models were constructed for fractional melting of a prim-
itive-mantle (McDonough and Sun, 1995) source, and the
precise composition of predicted residues is sensitive to the
selected partition coefficients, melting modes, and starting
composition. The range of Dy–Yb compositions in Lherz
peridotites defines a fractionation curve sub-parallel to melt-
ing curves calculated for residue compositions expected for
fractional melting of fertile peridotite (analogous to primitive
mantle (McDonough and Sun, 1995), fertile-MORB mantle
(FMM; Pearce and Parkinson, 1993), or DMM (Workman
and Hart, 2006)) in the spinel-facies (Fig. 6a). Compared to
residue compositions expected for spinel-facies melting un-
der anhydrous conditions, Lherz harzburgites have consis-
tently lower Yb-abundances over a range of Dy/Yb values.
Experimental studies have suggested that HREEs may be less
compatible during fluid-present melting (e.g., Gaetani et al.,
2003; McDade et al., 2003, and for this reason the relatively
low HREE-contents of Lherz harzburgites indicate that they
are residual after fluid-present melting (Fig. 6a). Abundances
of V–Yb–Sc in these harzburgites are broadly consistent with
models of melt depletion under oxidation conditions of FMQ
to FMQ+1 (Fig. 6b and Fig. A7c of Appendix A), but the
highest V-abundances determined for site A harzburgites,
while overlapping the range reported for subduction-zone
peridotites of Avacha and Izu–Bonin–Mariana, correspond
to samples displaying the greatest degree of LREE-enrich-
ment that may be linked to secondary processes. Earlier stud-
ies of oxidation conditions, which used olivine-
orthopyroxene-spinel major-element equilibria (Woodland,
1992; Woodland et al., 1996, 2006), suggested that spinel-per-
idotites of the Lherz massif have oxidation states ranging
from QFM 1.5 to QFM + 1; which are transitional be-
tween fo
2
values reported for abyssal peridotites and conti-
nental xenolith suites. Differences between fo
2
conditions
indicated by V-abundances determined in this study com-
pared to previous studies of olivine–orthopyroxene–spinel
equilibria (e.g., Woodland, 1992; Woodland et al., 1996,
2006) may reflect partial or complete resetting of oxidation
records after melt extraction, and resolving the cause of these
differences is beyond the scope of this work. Importantly, re-
sults of modelling the behaviour of moderately compatible
trace-element compositions (Dy, Yb, Lu, Hf, V, and Sc) dur-
ing melting are consistent with bulk-rock major-element and
mineralogical characteristics that suggest harzburgites result
from 20–25 wt.% melt removal (Appendix B) in the absence
of residual garnet (Fig. 6a and b, Fig. A7b and c of Appendix
A), and a starting composition broadly analogous to FMM
and/or DMM is in agreement with previous results for melt-
Petrogenesis of the Lherz peridotite 6173
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ing in mantle-wedge environments, particularly those located
beneath relatively thin lithosphere such as that expected for
back-arc basin settings (e.g., Pearce and Parkinson, 1993;
Woodhead et al., 1993; Peate et al., 1997; Ewart et al.,
1998; Parkinson et al., 1998; Langmuir et al., 2006).
4.3. Modification of Lherz peridotites by melt- and fluid-
metasomatism
Incompatible- and highly-incompatible element contents
(including LREE, Zr, and Sr) of bulk-rocks and pyroxenes
cannot be accounted for by models of melt extraction, and
may have been modified after an earlier melting event. Pre-
vious authors have suggested that the range of trace-ele-
ment abundances reported for Lherz peridotites may be
explained by hydrous, carbonatite, and silicate-melt meta-
somatism that may reflect two distinct processes; (1) mul-
ti-stage metasomatic interaction with several generations
of compositionally distinct melts/fluids (e.g., Woodland
et al., 1996; Burnham et al., 1998; Fabrie
`s et al., 1998,
2001); or (2) pervasive melt-infiltration accompanied by a
chromatographic melt-front (e.g., Bodinier et al., 1988;
Bodinier et al., 1990; Le Roux et al., 2007, 2008, 2009).
New trace-element data for harzburgite–lherzolite outcrops
with contrasting pyroxenite and structural characteristics
are consistent with the occurrence of two distinct metaso-
matic events. The first metasomatic event is characterised
by cryptic metasomatism of harzburgites at site A and B;
these samples display LREE-enrichment relative to MREE
and HREE, have relatively high chondrite-normalised U
and Th contents compared to Ba and Nb (pronounced in
site A harzburgites). These samples lack pronounced
enrichment of Cs, Ba, and Pb, that is generally associated
with SSZ-melts that impart a distinctive trace-element sig-
nature linked to fluid release close to the slab-wedge inter-
face (e.g., Pearce and Parkinson, 1993; Parkinson and
Pearce, 1998). The lack of Ti-enrichment (Figs. 5and 6c
and d), absence of apatite, and the lack of a linear correla-
tion between clinopyroxene abundances, orthopyroxene/
olivine values, and LREE-abundances and LREE/HREE
values (of bulk-rocks and constituent pyroxenes; not shown
refer to Appendix B), combined with the range of bulk-rock
and pyroxene Zr abundances (Appendix B) suggest that
LREE-enrichment in hornblendite-free harzburgites is the
result of post-melting interaction with; (1) a low-volume
Si-poor fluid phase, (2) a low-volume volatile-bearing melt,
or (3) a combination of these (c.f., Rudnick et al., 1993;
Schiano and Clocchiatti, 1994; Schiano et al., 1995; Xu
et al., 2003; Bouvier et al., 2010a,b for relevant compari-
sons). No fluid inclusion data are currently available for
the Lherz massif, but at least two generations of CO
2
–H
2
O
bearing fluids thought to have equilibrated at temperatures
of 950 °C and pressures of 6–7 kbar and 8–9 kbar, respec-
tively, have been reported in this region for amphibole-bear-
ing peridotites of the Cassou massif (Fabrie
`s et al., 1989;
Bilal, 1978), and fluids of broadly similar compositions
may be responsible for a significant portion of LREE, and
Sr, enrichment in hornblendite-free harzburgites.
The second metasomatic event identified in this study
postdates earlier LREE-enrichment linked to Si-poor flu-
ids/melts, and involves modal (amphibole ± phlogo-
pite ± sulphide ± secondary pyroxene) and cryptic
metasomatism (MREE-enrichment, intermediate Cr/Al
values, and relatively high bulk-rock and pyroxene Ti and
Na contents in the absence of secondary minerals) linked
to the formation of hornblendite-veinlets at site B. These
mineralogical and chemical enrichment characteristics are
consistent with results reported in earlier studies of melt–
rock interaction adjacent to amphibole-bearing veins (e.g.,
Bodinier et al., 1987b; McPherson et al., 1996; Woodland
et al., 1996; Bodinier et al., 2004). Outcrop relationships
suggest that hornblendite-vein formation is a relatively re-
cent event, and previous studies indicated that hornblen-
dites crystallised at 100 Ma (e.g., Henry et al., 1998)at
relatively low-pressures (e.g., <1.3 GPa; Fabrie
`s et al.,
2001; Lorand and Gregoire, 2010). Conque
´re
´and Fabrie
`s
(1984) indicated that hornblendites intrude along composi-
tional boundaries between harzburgite and lherzolite, and
our own observations suggest that this may be the case at
site B. In addition, new spatial information related to
LREE-enrichment at site A suggests that the lithological
boundary between adjacent harzburgite and lherzolite
bodies represents a conduit that facilitated post-melting
fluid ingress, and both site A and B demonstrate that pre-
existing structural features can focus fluid and/or melt flow
in mantle materials.
4.4. Petrogenesis of Ol-websterite bands and associated
Lherzolites
The majority of the studied lherzolites are composite
samples with distinct textures at the two study sites. Sam-
ples of the banded-lherzolite body of site A contain web-
sterites that are generally thick (up to 8 cm wide),
foliation parallel, and laterally extensive (10 s of metres).
Websterite bands of site A plot on an extension of
bulk-rock major-element covariation trends defined by
Lherz peridotites (e.g., Fig. 2a). Major-element composi-
tions (e.g., Mg#) of site A olivine-websterites do not so-
lely reflect equilibrium and fractional crystallisation of a
basaltic melt (c.f. crystallisation experiments of Villiger
et al., 2004, and references therein for a discussion of
experimental constraints on basalt crystallisation pro-
cesses), and differ from crystallisation products derived
from H
2
O-undersaturated melts (up to 5 wt.% H
2
O) at
1.2 GPa (e.g., Mu
¨ntener et al., 2001). Although modal
abundances and mineralogical compositions of olivine-
websterites cannot be accounted for by products of crys-
tallisation experiments performed to date, published
experimental work has generally used starting materials
with Mg# 670 (c.f. Mu
¨ntener et al., 2001; Villiger
et al., 2004, 2007 and references therein). Major-element
compositions of olivine-websterites tend toward the com-
position of clinopyroxene (e.g., Fig. 3c), do not lay on a
tie-line between clino- and orthopyroxene, and while they
probably represent cumulate bands in the broadest sense
(c.f. Dantes et al., 2007 and a review by Downes, 2007),
it is unclear if pyroxene-segregation during channelled
magma flow (e.g., Irving, 1980) significantly influenced
their compositions.
6174 A.J.V. Riches, N.W. Rogers / Geochimica et Cosmochimica Acta 75 (2011) 6160–6182
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Equilibrium melts calculated from trace-element com-
positions of clinopyroxenes indicate that lherzolites and
olivine-websterites are in equilibrium with a LREE-en-
riched melt (Appendix B). The absolute concentrations
of REE in the calculated melt are dependent upon the
choice of partition coefficients (and the conditions for
which they are relevant), but all calculated melts are con-
sistently LREE-enriched. This melt composition is distinct
from that of bulk-rock olivine-websterites, and REE con-
tents of equilibrium melts are broadly analogous to E-
MORB (Sun and McDonough, 1989), and continental
tholeiites parental to Triassic Pyrenean dolerites (e.g., Ali-
bert, 1985) when anhydrous partition coefficients are ap-
plied. Downes (2001) used lithophile-element isotope
compositions to show that no direct link between the
lherzolites of Lherz and Pyrenean dolerites exists (e.g.,
Downes, 2001).
LREE-enriched basalts and parental melts have been
identified in back-arc basin environments (e.g., Pearcy
et al., 1990; Stern et al., 1990), and for this reason REE
compositions do not provide unique information to con-
strain the environment in which lherzolites and olivine-web-
sterites formed. The presence of amphibole in the studied
specimens, and major and trace-element evidence of frac-
tionation controlled by olivine + clinopyroxene in the ab-
sence of garnet (e.g., Figs. 3c and 6a) may suggest that
partition coefficients determined for mantle-wedge environ-
ments are more appropriate. The application of partition
coefficients reported for mantle-wedge conditions (e.g.,
Pearce and Parkinson, 1993; Gaetani et al., 2003; McDade
et al., 2003) indicates that melts in equilibrium with lherzo-
lite and olivine-websterite pyroxenes have Cr and Y abun-
dances that overlap the compositional range reported for
island-arc tholeiites (IAT), back-arc basin basalts (BABB),
and MORB produced by melting of a relatively fertile man-
tle source (c.f. Fig. 8 of Pearce et al., 1984, Appendix B).
Titanium–vanadium ratios of these equilibrium melts are
<20, and this is consistent with Ti/V values expected for
back-arc basin settings (c.f. Shervais, 1982 for a discussion
of Ti/V variation in basalts of distinct provenance). Nio-
bium contents calculated for these equilibrium melts (gener-
ally 1–4 ppm) are broadly analogous to those of N-MORB,
and are lower than Nb concentrations typical of EMORB
and OIB (2.33 ppm, 8.30 ppm and 48 ppm, respectively;
Sun and McDonough, 1989). The similarity of equilibrium
liquid compositions among olivine-websterites and adjacent
lherzolites suggests that these rocks are genetically related.
The simplest explanation of the current major- and
trace-element data set in closely-spaced lherzolites and oliv-
ine-websterites is that olivine-websterites represent
pyroxene-rich cumulates of a tholeiitic LREE-enriched melt
that crystallised in melt-flow channels created during mod-
erate degrees of melting (615 wt.% melt removal; Bodinier
et al., 1988; Burnham et al., 1998), and this differentiation
event may also have formed residual lherzolites. In this
sense site A banded-lherzolites may be broadly analogous
to thin-layer peridotites and pyroxenites of the Horoman
massif where pyroxenite forming melts are not thought to
cause significant refertilisation (i.e., Al, Ca, Ti addition)
of adjacent peridotites (Malaviarachchi et al., 2010).
4.5. Re–Os: timing of lithosphere stabilisation or inherited
upper-mantle heterogeneity?
Samples of the Lherz massif are characterised by Re and
Os abundance variations with bulk-MgO content that indi-
cate Re and Os behave as moderately–incompatible and
compatible elements, respectively, and this observation is
broadly consistent with previous studies of Re–Os behav-
iour in basaltic silicate melt systems (e.g., Pearson et al.,
2004). Rhenium-Os isotope compositions of Lherz perido-
tites ± olivine-websterite do not define a statistically mean-
ingful isochron (e.g. Fig. 7a and b; large MSWD 1), nor
do they coincide with mixing curves that approximate igne-
ous refertilisation (constant addition of basaltic melt to
refractory peridotite) recently, at 100–500 Ma (correspond-
ing to the time of hornblendite formation, and including the
period of late-Variscan thermal events), and at 0.5 Ga inter-
vals between 1 Ga and 3.5 Ga (Fig. 8). The large difference
between the relatively low Os contents of basalts (typically
10–500 ppt) compared to the high Os abundances in mantle
materials (generally on the order of 3000–5000 ppt), where
the majority of the osmium is hosted by Os-rich sulp-
hides ± Os–Ir-alloys associated with olivine (e.g., Luguet
et al., 2007), mean that
187
Os/
188
Os compositions of perido-
tites will not be significantly disturbed by small to modest
degrees of melt–rock interaction, particularly when interac-
tion has taken place within the last 1 Ga (Fig. 8). For these
reasons, osmium isotope compositions of individual bulk-
rock peridotites, while strictly reflecting the sum of a mixed
phase population (e.g., Lorand, 1991; Burton et al., 1999;
Alard et al., 2000; Harvey et al., 2006, 2010, 2011; van Ack-
en et al., 2008, 2010; Lorand et al., 2010; Lorand and Alard,
2011), probably record the approximate time of melt-deple-
tion as volumetrically minor Os-rich trace-phases may ac-
count for >90% of the bulk-rock Os content (c.f. Harvey
et al., 2010 for a study of the effects of variable degrees of
melt/fluid metasomatism on Os-isotope compositions).
The lack of an isochronous bulk-rock Re–Os isotope
relationship reflects the fact that the Lherz body comprises
intercalated units of distinct lithologies where harzburgites
may not be strictly coeval with lherzolite ± olivine-webste-
rite, and these materials may have evolved with variable ini-
tial
187
Os/
188
Os compositions from 1.6 Ga onwards.
Additionally, a comparison of Re–Os isotope data to a
1.6 Ga reference line (Fig. 7) indicates that some samples
have higher Al
2
O
3
and Re abundances at a given
187
Os/
188
Os composition, and this probably reflects recent
Re and Al disturbance that may be linked to hornblen-
dite-interaction that has not significantly disturbed
187
Os/
188
Os values. The
187
Os/
188
Os isotope compositions
of harzburgites generally yield T
RD
ages (representing a
minimum differentiation age; Shirey and Walker, 1998)of
1.4 to 1.5 Ga (Fig. 9b). The period of isolation required
to generate the observed
187
Os/
188
Os values in composite-
lherzolites and olivine-websterites with suprachondritic
Re/Os values is in broad agreement with T
RD
ages of harz-
burgites, assuming that these materials have experienced a
single-stage evolution with respect to Os.
Osmium isotope compositions do not offer highly-pre-
cise geochronological information (c.f. review by Rudnick
Petrogenesis of the Lherz peridotite 6175
Author's personal copy
and Walker, 2009), but do provide broad constraints on the
timing of ancient differentiation. Meso-Proterozoic Os-ages
determined here are; (1) similar to model ages derived from
the Os–Al correlations generated from our study focused
on constraining the petrogenesis of the oldest portions of
the Lherz massif (foliation parallel lithological strips); (2)
within error of Os-model ages reported for peridotites that
include ultramafic samples from an ESE–WNW traverse of
>300 km of the Pyrenean chain (Reisberg and Lorand,
1995; Burnham et al., 1998); and (3) overlap Nd-model ages
reported for the oldest portions of western European crust
where structurally divided I- and S-type granites have been
linked to ancient subduction systems (e.g., 1.7 to 1.4 Ga;
Liew and Hofmann, 1988). Crustal materials of central and
western Europe for which Liew and Hofmann (1988) re-
ported Meso-Proterozoic Nd-model ages do not have a di-
rect genetic link to ultramafic bodies of the Pyrenees, but
these authors suggested that crustal materials of the Mal-
donubian zone (including central and southern France)
may be linked to a Proterozoic continental mass to the
south, and this crustal mass could potentially be related
to the genesis of Arie
`ge-group massifs (c.f. Burnham
et al., 1998). Further studies are required to constrain the
composition, origin, and evolution of the oldest crustal
materials within the Pyrenean chain, and this information,
combined with further knowledge of peridotite emplace-
ment mechanisms, may then be used to provide a more rig-
orous assessment of crust–mantle relationships in this
region.
Osmium-Al values reported for the Lherz peridotite
body overlap and extend to higher and lower
187
Os/
188
Os
values than those reported for other Arie
`ge-group ultra-
mafic massifs that define a broad positive correlation with
bulk-rock Al
2
O
3
(Fig. 9a), and such trends, when broadly
supported by correlations between
187
Os/
188
Os composi-
tions and Re/Os abundance and isotopic ratios (Appendi-
ces A and B), are generally considered to reflect long-term
isolation from the homogenising environment of the con-
vecting upper mantle (e.g., Reisberg and Lorand, 1995;
Burnham et al., 1998). However, the recent discovery of a
broad Os–Al correlation in modern convecting upper man-
tle (Fig. 9a, spinel-facies peridotites exhumed at the ultra-
slow-spreading Gakkel Ridge (1.4–0.7 cm/year; Michael
et al., 2003) that yields an ancient model-age (2 Ga; Liu
et al., 2008) increases uncertainty about the geological
meaning of Os-ages determined for ultramafic materials
of Proterozoic and Phanerozoic terranes. The range of
187
Os/
188
Os isotope compositions reported for samples
from the Lherz massif overlaps the spread of abyssal peri-
dotite Os-isotope compositions, but Os–Al correlations
and
187
Os/
188
Os frequency distributions of Lherz samples
differ from abyssal peridotites in detail (Fig. 9b). Mid-
Atlantic Ridge (Brandon et al., 2000; Harvey et al., 2006)
and Izu–Bonin–Mariana peridotites (Parkinson et al.,
1998) form discrete data clusters covering a range of
187
Os/
188
Os compositions with limited variation in bulk-
rock Al
2
O
3
abundances, and this is distinct from the distri-
bution of Os-isotope and Al
2
O
3
compositions for samples
of the Lherz massif. When compared to all available
187
Os/
188
Os data for abyssal peridotites (Fig. 9b) a greater
proportion of
187
Os/
188
Os compositions reported for the
Lherz massif are subchondritic, clustering at T
RD
ages of
2 to 1.5 Ga. The broad correlation between
187
Os/
188
Os
isotope compositions and bulk-rock Al
2
O
3
abundances that
encompasses Lherz and many other Pyrenean ultramafic
massifs (sampling an area >300 km in length) exhumed in
a mountain belt with a significant collisional history (c.f.
McCann, 2008a,b; Garcia-Sansegundo et al., 2011), lead
us toward a preferred model in which the oldest portions
of the Lherz massif were created during Meso-Proterozoic
melting linked to the formation of overlying crust. New
mineralogical and trace-element data for Lherz perido-
tites ± olivine-websterite indicate that the oldest portions
of this massif represent materials from mantle-wedge envi-
ronments, and fluid-present spinel-facies melting may have
taken place in a back-arc basin setting. For these reasons
subduction zone processes may have been active at the
boundary between the Iberian and European plates during
the Meso-Proterozoic, and this may be a regionally impor-
tant aspect of crustal growth.
An important outcome of this study is new composi-
tional information that suggests a link between high degrees
of melting in shallow, subduction-influenced environments
that may be responsible for the formation Pt–Ir–Os alloys
identified in harzburgites produced by 20–25 wt.% melt re-
moval (e.g., those studied by Luguet et al., 2007). Pt–Ir–Os
alloys ± laurite-erlichmanite sulphides may form as the
point of S-exhaustion is approached during mantle melting
(c.f. Luguet et al., 2007). Complete removal of S during
low-pressure, subduction influenced melt generation is con-
sistent with experimental evidence that indicates such envi-
ronments probably produce primary melts with high S
contents at sulphur saturation (>1300 ppm) capable of rel-
atively high-degrees of S-removal from the residue when
compared to melting at pressures >2 GPa (e.g., Mavroge-
nes and O’Neill, 1999; Holzheid and Grove, 2002; Jugo,
2009). Isolated minerals with high Os contents created dur-
ing melting to the point of S-exhaustion are essentially de-
void of Re, have relatively high-melting temperatures
(>1300 °C; e.g., Andrews and Brenan, 2002), and may pre-
serve records of ancient mantle differentiation in materials
that have a multi-stage history of magmatism and solid-
state mixing (Luguet et al., 2007). Thus, the geochemical
properties of Os mean that bulk-rock, Pt–Ir–Os alloy, and
primary laurite-erlichmanite Os-isotope compositions pro-
vide a powerful record of ancient, large-volume differentia-
tion events, and statistical assessment of such information
(e.g., Pearson et al., 2007), in the context of well-defined
petrological constraints, is of great importance for deter-
mining; (1) the timing and mechanisms of local, regional,
and global crust–mantle evolution; and (2) the proportion
of refractory material present in Earth’s heterogeneous
upper mantle.
5. SUMMARY AND IMPLICATIONS
New compositional data for closely spaced samples
recovered from two traverses of adjacent harzburgite and
lherzolite bodies ± olivine-websterite show that percolative
igneous refertilisation during the Variscan is not a viable
6176 A.J.V. Riches, N.W. Rogers / Geochimica et Cosmochimica Acta 75 (2011) 6160–6182
Author's personal copy
process to explain the range of major- and trace-element
abundances, and Re–Os isotopic compositions reported
for the Lherz massif. Field, textural, and compositional
data indicate that the Lherz massif is composed of elongate,
foliation parallel, lithological strips that were juxtaposed
during plastic deformation. Major-element oxide,
bulk-rock and clinopyroxene trace-element compositions
show that highly-depleted harzburgites result from
20 – 25 wt.% melt extraction, are analogous to some man-
tle-wedge peridotites (e.g., Avacha xenoliths), and were cre-
ated at pressures <2 GPa. In contrast, the studied
lherzolites ± olivine-websterite are heterogeneous and rep-
resent physical mixtures of residual materials and clinopy-
roxene-dominated cumulates equilibrated with a LREE-
enriched tholeiitic melt that formed in the absence of
residual garnet. Two metasomatic events have modified
peridotite compositions; the first involves wide-spread per-
colation of a Si-poor melt/fluid; and the second is evident at
site B where relatively recent, small-volume melt interaction
is linked with the intrusion of hornblendite-veinlets at the
compositional boundary between adjacent harzburgite
and lherzolite bodies. In addition, new mineral major-ele-
ment data indicate that the intercalated peridotite + pyrox-
enite assemblages of the Lherz ultramafic body equilibrated
at lithospheric conditions (temperatures of 800–900 °Cor
less), and previous studies of lithophile-element isotope
compositions suggested that cooling and recrystallisation
occurred during the Cretaceous (Henry et al., 1998 and ref-
erences therein).
Rhenium-Os systematics in this suite of samples show
that Os behaves compatibly and
187
Os/
188
Os compositions
have not been significantly disturbed by small-volume melt
interaction. Osmium-isotope compositions, combined with
literature data for other Arie
`ge-group ultramfic bodies, de-
fine a broad positive correlation with bulk-rock Al
2
O
3
abundances indicating that harzburgites, lherzolites, and
olivine-websterites have been isolated from convective
homogenisation since the Meso-Proterozoic and this
broadly coincides with the time at which melting created
the residual materials. The association between harzburg-
ites resulting from spinel-facies melting in mantle-wedge
environments and residual, Os-rich, laurite-erlichmanite
sulphides and Pt–Os–Ir-alloys suggests that a substantial
proportion of persistent refractory anomalies in the pres-
ent-day convecting mantle of Earth may be linked to an-
cient large-scale melting events that may in turn be
related to subduction processing.
ACKNOWLEDGMENTS
Dr. B.L.A. Charlier is thanked for his guidance during the doc-
toral study of A.J.V. Riches. In addition, we are grateful to Dr. A.
Gannoun for support during Re–Os isotope analyses, Dr. S. Ham-
mond for guidance during measurement of trace-element concen-
trations, Dr. John Watson for his assistance during XRF
analyses, and we are indebted to Michelle Higgins and Kay Green
for the preparation of many fine polished sections. Dr. H. Downes
and Dr. I.J. Parkinson are thanked for their encouragement and re-
marks on a previous version of this work, and Dr. M. Bizimis, two
anonymous reviewers, and the Associate Editor, Dr. S. Huang, are
thanked for their astute comments on an earlier draft of this man-
uscript. This work was supported by a NERC studentship (Grant
Number NER/S/A/2004/13014) to A.J.V. Riches.
APPENDIX A. SUPPLEMENTARY DATA
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/
j.gca.2011.07.036.
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Associate editor: Shichun Huang
6182 A.J.V. Riches, N.W. Rogers / Geochimica et Cosmochimica Acta 75 (2011) 6160–6182
... The pyroxenites may represent leftover cumulates and reaction products from these processes. However, mechanical mixing of pyroxenite and harzburgite has also been proposed as a mechanism capable of producing the refertilization at Lherz which is commonly attributed to melt reaction (Riches and Rogers 2011). ...
... Abyssal peridotites-see Figs. 1 and 2. Ocean island basalt mantle xenoliths-seeFig. 2. Continent/continent-ocean transition tectonite:Reisberg and Lorand (1995);Meisel et al. (1996);Roy-Barman et al. (1996);Rehkämper et al. (1999);Snow et al. (2000);Saal et al. (2001);Becker et al. (2006);Luguet et al. (2007); vanAcken et al. (2008);Riches and Rogers (2011);. ...
... Fractional crystallisation trend is from Arai (1994). (Riches & Rogers, 2011), Iberia Abyssal Plain peridotites (Cornen et al., 1996), Ronda peridotite (Masaaki, 1980); Supra-subduction zone (SSZ) peridotites -Tonga fore-arc peridotites (Birner et al., 2017), Marum ophiolite complex, Papua New Guinea (Kaczmarek et al., 2015), Massif du Sud peridotites (Pirard et al., 2013), peridotite xenoliths from the New Ireland basin of Papua New Guinea (McInnes et al., 2001). Abyssal peridotite field (dunites excluded) is after the data of (Warren, 2016). ...
... Tick marks indicate 5% melting increments. Also plotted are the ophiolitic peridotites from the settings of ocean-continent-transition (OCT; data from Becker et al., 2006;Fischer-Gödde et al., 2011;Lorand et al., 1999Lorand et al., , 2000Lorand et al., , 2008Lorand et al., , 2010Luguet et al., 2007;Rampone et al., 1995;Reisberg & Lorand, 1995;Riches & Rogers, 2011;Saal et al., 2001;Snow et al., 2000;van Acken, Becker, Hammerschmidt, et al., 2010;van Acken et al., 2008;Wang et al., 2013), mid-ocean-ridge (MOR; data from Aldanmaz et al., 2012;Batanova et al., 2008;Dijkstra et al., 2010;Gong et al., 2020;Hanghøj et al., 2010;Hu et al., 2020;Schulte et al., 2009;Sergeev et al., 2014;Uysal et al., 2012) and supra-subduction-zone (SSZ; data from Agranier et al., 2007;Aldanmaz et al., 2012Aldanmaz et al., , 2020Büchl et al., 2002Büchl et al., , 2004Chen et al., 2020;Haller et al., 2021;C. Z. Liu et al., 2018;O'Driscoll et al., 2012O'Driscoll et al., , 2015O'Driscoll et al., , 2018Prelević et al., 2015;Secchiari, Gleissner, et al., 2020;Snortum & Day, 2020;Uysal et al., 2012;Xu & Liu, 2019). ...
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The Lesvos ophiolite, Greece, was recently identified as an analogue to the External Liguride Unit formed at an ocean-continent-transition (OCT) setting. To further study the evolutionary history of this body, we analyzed highly siderophile element abundances and Re-Os isotopic compositions of 14 Lesvos peridotites, in combination with petrology, bulk-rock major and trace element geochemistry. Almost all the Lesvos peridotites fall within the field of global OCT and mid-ocean-ridge (MOR) peridotites, indicating that these rocks have not experienced subduction-related processes. The near-horizontal 187Os/188Os vs. Al2O3 trend over a wide range of Al2O3 (0.45–3.66 wt. %) may have been caused by recent melt depletion during rifting and thinning of the Lesvos lithosphere. Combining data from available global ophiolitic peridotites, we find that a large proportion of the more Al-depleted supra-subduction-zone (SSZ) peridotites show lower Os/Ir, higher Pd/Ir and remarkably elevated radiogenic Os relative to other tectonic environments (OCT and MOR). By linking this kind of geochemical evolution to the Wilson Cycle, a complete picture emerges: (1) In the OCT to MOR stages, the extensional rifting environment may lead to mild to moderate melt depletion, followed by, or associated with, infiltration of S-saturated (high fS2) basaltic magmas; (2) When progressing into the SSZ stage, more extreme degrees of mantle melt depletion may be driven by aqueous fluids in the sub-arc mantle. During this stage, high-fO2 slab-derived fluids and/or S-undersaturated (low fS2) boninitic magmas may infiltrate the sub-arc mantle, followed by subsequent S-saturated forearc basaltic magma infiltration.
... 6). This group includes the majority of mantle peridotite bodies found across the Pyrenees (Verschure and others, 1967;Bodinier and others, 1988;Fabriès and others, 1998;Henry and others, 1998;Le Roux and others, 2007;Riches and Rogers, 2011). The petrological and chemical characteristics of inherited mantle domains are interpreted as representing ancient subcontinental lithospheric mantle exhumed to the sea-floor following rifting (Picazo and others, 2016) ( fig. 4). ...
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In this contribution, we review aspects of the petrological, metamorphic and sedimentological characteristics of Neotethyan and Pacific subduction zones and compare them to the Western and Central Alps and the Pyrenees. We argue that the current models of formation of the Western and Central Alps, which invoke the subduction of significant volumes of oceanic lithosphere and spontaneous subduction initiation are unable to explain fundamental characteristics unique to the Pyrenees and European Alps orogens. Their characteristic geodynamic features are surprisingly distinct from features of Wadati-Benioff-type subduction, as the latter typically have large subducted oceanic slabs, near continuous magmatism and specific subduction initiation characteristics. The Pyrenees and the Western to Central Alps however, are characterized by subduction initiation without magmatism at rifted margins and inefficient subduction of hydrated lithologies into the convective upper mantle. The pre-collisional lithosphere from the Pyrenees to the Central Alps, or Western Tethys sensu lato comprised several sub-basins characterized by extremely thinned continental crust, exhumed subcontinental mantle and, locally, minor volumes of embryonic ultra-slow spreading ocean crust. This allows us to distinguish Benioff-type oceanic subduction resulting from the efficient subduction of hydrated oceanic lithosphere from Ampferer-type continental subduction. The latter records the closure of hyperextended continental basins with minor volumes of oceanic crust, and subduction of predominantly dry lithosphere into the convective upper mantle.
... This creates a clear sampling bias, in that PGM that have formed in association with melting of the depleted MORB mantle, which comprise a much larger volume, are not as well-represented as those associated with supra-subduction zone mantle wedges. Mantle source regions associated with arc magmatism and ophiolite formation are thought to be ultra-depleted and are thus far more likely to have sustained dramatic Re-and S-loss ( Luguet et al., 2003;Malavergne et al., 2004;Nowell et al., 2008b;Pearson et al., 2007;Mallmann and O'Neill, 2007;Riches and Rogers, 2011;Fonseca et al., 2012) when compared to MORB sources. Nevertheless, our results are broadly consistent with measurements of the Os isotope composition of natural alloys, some examples of which will be discussed below. ...
Article
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Although Earth’s continental crust is thought to derive from melting of the Earth’s mantle, how the crust has formed and the timing of its formation are not well understood. The main difficulty in understanding how the crust was extracted from the Earth’s mantle is that most isotope systems recorded in mantle rocks have been disturbed by crustal recycling, metasomatic activity and dilution of the signal by mantle convection. In this regard, important age constraints can be obtained from Re-Os model ages in platinum group minerals (PGM), as Re-poor and Os-rich PGM show evidence of melting events up to 4.1 Ga. To constrain the origin of the Re-Os fractionation and Os isotope systematics of natural PGM, we have investigated the linkage between sulfide and PGM grains of variable composition via a series of high-temperature experiments carried out at 1 bar. We show that with the exception of laurite, all experimentally-produced PGM, in particular Pt3Fe (isoferroplatinum) and Pt-Ir metal grains, are systematically richer in Re than their sulfide precursors and will develop radiogenic 187Os/188Os signatures over time relative to their host base metal sulfides. Cooling of an PGM-saturated sulfide assemblage shows a tendency to amplify the extent of Re-Os fractionation between PGM and the different sulfide phases present during cooling. Conversely, laurite grains (RuS2) are shown to accept little to no Re in them and their Os isotope composition changes little over time as a result. Laurite is therefore the PGM that provides the most robust Re-depletion ages in mantle lithologies. Our results are broadly consistent with observations made on natural PGM, where laurites are systematically less radiogenic than Pt-rich PGM. These experimental results highlight the need for the acquisition of large datasets for both mantle materials and ophiolite-derived detrital grains that include measurements of the Os isotope composition of minerals rich in highly siderophile elements at the grain scale (i.e. PGM and base metal sulfides). Only with such datasets is it possible to identify past episodes of mantle melting.
... The major element compositions of clinopyroxene and spinel from Iberia-Newfoundland are plotted in Fig. 5 (triangles), together with data from the Lherz peridotite (French Pyrenees;Le Roux et al., 2007;Riches and Rogers, 2011;reversed triangles). Ultramafic rocks from the Galicia Bank are mainly plagioclase-bearing spinel-lherzolites and minor harzburgites (ODP Site 637, Kornprobst et al., 1988). ...
Poster
New ideas and concepts have been developed to understand and be able to give a simplified large-scale view of the evolution of the mantle lithosphere in hyper-extended magma-poor rifted margins based on the ancient Alpine Tethys rifted margin. In contrast to the classical assumption assuming a simple, isotropic mantle lithosphere, these new models integrate observations from exposed and drilled mantle rocks and propose that the mantle lithosphere evolved and was modified during an extensional cycle from post-orogenic collapse through several periods of rifting to embryonic oceanic (ultra-) slow seafloor spreading. But it is, at present, unclear how far these ideas can be generalized at Atlantic type rifted margins. In our presentation, we review the available mantle data from dredged samples in the North Atlantic and from ophiolite massifs and xenoliths in preserved and reactivated passive margins i.e. the Alpine Tethys, the Pyrenean domain, and the Dinarides and Hellenides. We revisit the available terminology concerning mantle massifs and xenoliths and compile the available data to identify different mantle domains. We define chemical and petrological characteristics of mantle domains based on clinopyroxene and spinel compositions and compile them on present-day and paleo-geographic maps of Western Europe. Finally we link the observed distribution of mantle domains to the post-Variscan extensional cycle and link domains to processes related to the late post-Variscan extension, the rift evolution and refertilization associated to hyper-extension and the development of embryonic oceanic domains.
Article
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To unveil how forearc lithosphere cools and re-equilibrates, we carried out a comprehensive geothermometric investigation of the New Caledonia ophiolite, which represents a rare example of proto-arc section generated during subduction infancy. A large dataset, including more than eighty samples (peridotites and mafic-ultramafic intrusives), was considered. Closure temperatures calculated for the lherzolites using slow (TREE-Y) and fast diffusing (TCa-in-Opx, TBKN, TCa-in-Ol, TOl-Sp) geothermometers provide some of the highest values ever documented for ophiolitic peridotites, akin to modern sub-oceanic mantle. Cooling rates deduced from TREE-Y and TBKN yield values of ≈ 10-3 °C/y, similar to those obtained with TCa-in-Ol. These features are consistent with a post-melting history of emplacement, possibly along a transform fault, and thermal re-equilibration via conduction. Cpx-free harzburgites register a high-T evolution, followed by quenching and obduction. The relatively high TCa-in-Ol, TOl-Sp and cooling rates computed from TCa-in-Ol (≈ 10-3 °C/y) are atypical for this geodynamic setting, mirroring the development of an ephemeral subduction system, uplift and emplacement of the Peridotite Nappe. Temperature profiles across the crust-mantle transect point to high closure temperatures, with limited variations with depth. These results are indicative of injection and crystallization of non-cogenetic magma batches in the forearc lithosphere, followed by thermal re-equilibration at rates of ≈ 10-4-10-3 °C/y. Our study shows that the thermal conditions recorded by forearc sequences are intimately related to specific areal processes and previous lithospheric evolution. Thus, detailed sampling and exhaustive knowledge of the geological background are critical to unravel the cooling mechanisms in this geodynamic setting.
Article
Platinum-group element (PGE) + Re abundances and Os isotopic compositions were determined for metasomatized peridotite and cumulate pyroxenite xenoliths from Kharchinsky volcano, Kamchatka. Two peridotites have moderately high PGE concentrations with 2.9-3.3 ppb Os and smoothly variable PGE patterns similar to abyssal peridotites and encompassing primitive and depleted mantle compositions. One of these has unradiogenic Os (¹⁸⁷Os/¹⁸⁸Os = 0.1148) at the lower limit of compositions observed in abyssal peridotite which results in a time of Re depletion (TRD) model age of ∼1.8 Ga. These contrast Kharchinsky peridotites that are Pt-Ir enriched with sharply fractionated PGE patterns with Pt/Pd=132-168 and Ir/Ru∼4.0. This pattern of enrichment in the heavier and more highly siderophile Pt and Ir over the lighter and more siderophile-chalcophile Ru and Pd (Fleet and Stone, 1991; Fleet et al., 1991) is consistent with sulfur depletion under oxidizing metasomatic conditions that leave a residue with PGE host minerals that are dominantly alloys. A single Ru-enriched peridotite with low PGE abundances is interpreted to reflect advanced stages of paragenesis driven by metasomatism and resulting in the loss of all PGE and Re relative to Ru. All Kharchinsky peridotites included in this study have relatively unradiogenic Os (¹⁸⁷Os/¹⁸⁸Os = 0.1148-0.1314) but some are coupled to PGE and lithophile element characteristics that require oxidizing metasomatic conditions (Ce/Ce*<<1.0) and desulfurization leading to a general loss PGE and data patterns that are consistent overall with low abundances of relatively radiogenic Os in arc peridotites globally (Brandon et al., 1996; Saha et al., 2005; Widom et al., 2003). Kharchinsky pyroxenites, which are relatively undeformed and only weakly metasomatized, have similarly fractionated PGE patterns with enrichments in Pt (5.7-46 ppb) and Pd (0.5-2.7 ppb) but with Os, Ir, and Ru abundances <0.23 ppb. The patterns resemble arc basalts and may reflect preferential mobility of Pt, Pd, and Re in slab fluids. High Pt and Pt/Pd up to 33 in the pyroxenites indicate that a Pt-rich alloy with high Pt/Pd is present in the early (olivine-rich) cumulate assemblage. Several Kharchinsky samples (peridotites and pyroxenites) are Pt-enriched with high Pt/Os and Pt/Re >20 that will produce rapid ingrowth of ¹⁸⁶Os relative to ¹⁸⁷Os. Deep recycling and storage of such subduction-modified rocks might preclude the need for core-mantle exchange to explain ¹⁸⁶Os-enrichments in komatiites and Hawaiian picrites originating in the deep mantle (Brandon and Walker, 2005; Brandon et al., 1998).
Article
Melting experiments on fertile peridotite KR4003, a 'pyrolitic' composition, were made from 3 to 7 GPa in piston-cylinder and multi-anvil apparatus. Temperature gradients across the sample were minimized (<25 degrees C), and the compositions of all phases were determined. Modal abundances of coexisting phases were calculated by mass balance, and the results were used to determine phase relations. Orthopyroxene is not stable at the solidus of garnet peridotite above similar to 3.3 GPa, but crystallizes above the solidus by incongruent melting of cpx. Melt compositions from 3 to 7 GPa (>10% melting) are picritic, komatiitic, and peridotitic. The Al2O3 content of partial melts decreases with increase in pressure because of an increase in garnet stability, providing a barometer for melting. The Al2O3 contents of komatiites indicate secular variation in the average pressure of melt segregation from residues, with early Archean komatiites and Cretaceous komatiites generated at the highest and lowest average pressures, respectively. The high CaO/Al2O3 ratios of Archean alumina undepleted komatiites (similar to 0.9-1.5) require residual garnet if their sources were pyrolitic. Paradoxically, chondrite-normalized Gd/Yb of about unity in these komatiites precludes garnet involvement. Archean komatiite source regions may have had CaO/Al2O3 values of about 1.4 and 1.0 in the early and late Archean, respectively, significantly greater than the pyrolitic ratio of 0.8, whereas the source of Cretaceous komatiites may have had pyrolitic CaO/Al2O3. Thus, secular variations in this ratio are indicated. Chemical differences between coeval alumina undepleted and alumina depleted komatiites can be explained by melting at similar pressures, with alumina undepleted komatiites segregating from a garnet-free residue, and alumina depleted komatiites segregating from a garnet-bearing residue. Depleted, high-temperature peridotites from cratons, and oceanic peridotites, can be melting residues of pyrolitic mantle at low pressures (<3 GPa). Average low-temperature peridotite from the Siberian craton can be generated as a residue of komatiite melt extraction from a near-pyrolitic mantle at similar to 6 GPa and 40% melting. Average southern African low-temperature peridotite cannot be a melting residue of pyrolitic mantle. However, it can be a residue of komatiite melt extraction at >7 GPa from a mantle enriched in SiO2 relative to pyrolite.
Article
Amphibole pyroxenites intruded the upper mantle of lithospheric nature, at 1.0-1.5 GPa and 900-1000oC. The incomplete tectonic transposition of these dykes towards the foliation resulted from intra-lithospheric shearings linked with the develoment of the North Pyrenean transform zone during the Cretaceous. There is an abridged English version.-English summary
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.