Melt- versus fluid-induced metasomatism in spinel to garnet wedge peridotites (Ulten Zone, Eastern Italian Alps): clues from trace element and Li abundances

The peridotite bodies of the Ulten Zone (Upper Austroalpine, Italian Eastern Alps) are enclosed in Variscan migmatites and derive from a mantle wedge environment. They display the progressive transformation of porphyroclastic spinel peridotites (T=1,200°C; P=1.5 GPa) into fine-grained garnet–amphibole peridotites (T=850°C; P=3 GPa). Detailed bulk-rock and mineral trace element analyses of a sample suite documenting the entire metamorphic evolution of the peridotites revealed several stages of metasomatism. The spinel peridotites derive from a depleted mantle that became enriched in some large ion lithophile element (LILE) and light rare earth elements (LREE). The same signature pertains to clinopyroxene and orthopyroxene, indicating that this metasomatic signature was acquired at the recorded temperature of 1,200°C. Such a temperature is considerably above the wet peridotite solidus and hence the metasomatic agent must have been a hydrous melt. Moreover, the Li-enrichment of the spinel-facies pyroxenes (up to 24 ppm Li) reflects disequilibrium distribution after exchange with a presumably mafic melt. ^cpx/opx D _Li=3–7 and ^cpx/ol D _Li=2.7–8 indicate that the spinel-facies clinopyroxene hosts higher Li amounts than the coexisting minerals. LREE fractionation, variable LREE enrichment, LILE enrichment with respect to HFSE (average clinopyroxene Pb _N /Nb _N =16–90) in spinel lherzolites can be related to chromatographic effects of porous melt flow. The significant enrichment of pyroxenes from the spinel lherzolites in Pb, U and Li indicates that the metasomatic melt was subduction-related. All these features suggest that the spinel lherzolites formed a mantle wedge layer percolated by melts carrying recycled crustal components and rising from a deeper source of subduction magmas. The garnet + amphibole peridotites equilibrated at temperatures well below the wet solidus in the presence of an aqueous fluid. Bulk-rock trace element patterns display pronounced positive anomalies in Cs, Ba, Pb and U and moderate enrichment in Li, indicating addition of a crustal component to the mantle rocks. Amphibole hosts most of these trace elements. Clinopyroxene displays high LILE/HFSE (Pb _N /Nb _N =300–600), low Ce/Pb (1.4–2.7 in garnet-facies clinopyroxene compared with 2.6–24.5 in the spinel-facies one) and variable LILE and LREE enrichments. The coupled increase of modal amphibole, Sr and Pb, together with positive Pb–Sr and Pb–U correlations, further indicate that incompatible element influx in these samples was fluid-mediated. In the garnet-facies samples, amphibole and, interestingly, olivine have similarly high Li concentrations as clinopyroxene, leading to ^cpx/amph D _Li=0.7 and ^cpx/ol D _Li=0.7–0.8, the latter being up to ten times lower than in the spinel-facies rocks. Due to its high modal abundance, olivine is the main host of Li in the garnet–amphibole peridotites. The observed metasomatic features provide evidence for the infiltration of an aqueous fluid in the mantle wedge above a subducting slab. This fluid most likely derived from subducted crustal rocks that underwent partial melting. Successive retrograde re-equilibration during exhumation of the garnet peridotite is accompanied by garnet and clinopyroxene breakdown and amphibole formation. This process produced minor changes, such as an increase of HREE and Li in amphibole, and an increase of Li in olivine. The general trace element signature remains essentially unchanged during retrogression and further hydration, indicating that fluids with a similar composition to the one present at the garnet–amphibole peridotite formation, were responsible for increased amphibole formation. The combined evidence from the metamorphic and metasomatic evolution indicates that the peridotites experienced first corner flow in a mantle wedge, followed by subduction and finally entrapment and exhumation within a crustal slab. During their entire history the Ulten peridotites were percolated first by melts and then by aqueous fluids, which added recycled crustal components to the mantle wedge.     

Lithium enrichment and isotopic variation in minerals from peridotite xenoliths from northwestern Ethiopian plateau

We report Lithium (Li) concentrations and isotopic compositions for co-existing olivine, orthopyroxene (opx), and clinopyroxene (cpx) mineral separates from depleted and metasomatised peridotite xenoliths hosted by basaltic lavas from northwestern Ethiopian plateau (Gundeweyn area). The peridotites contain five lherzolites and one harzburgite and are variably depleted and enriched in LREE relative to HREE. In both depleted and enriched lherzolites, Li is preferentially incorporated into olivine (2.4-3.3 ppm) compared to opx (1.4-2.1 ppm) and cpx (1.4-2.0 ppm) whereas the Li contents of olivines (5.4 ppm) from an enriched harzburgiteare higher than those of lherzolites. Olivines from the samples show higher Li abundances than normal mantle olivines (1.6-1.9 ppm) indicating the occurrence of Li enrichments through melt-preroditite interaction. The average δ⁷ Li values range from +2.2 to +6.0‰ in olivine, from -0.1 to +2.0‰ in opx and from -4.4 to -0.9‰ in cpx from the lherzolites. The Li isotopic composition (3.5‰) of olivines from harzburgite fall within the range of olivine from lherzolites but the opxs show low in δ⁷Li (-2.0‰). Overall Li isotopic compositions of olivines from the peridotites fall within the range of normal mantle olivine, δ⁷Li values of ~+4±2‰ within uncertainty, reflecting metasomatism (enrichment) of the peridotites by isotopically heavy Li-rich asthenospheric melt. Li isotope zonation is also observed in most peridotite minerals. Majority of olivine grains display isotopically heavy cores and light rims and the reverse case is observed for some olivine grains. Orthopyroxene and clinopyroxene grains show irregular distribution in δ⁷Li. These features of Li isotopic compositions within and between grains in the samples reflect the effect of diffusion-driven isotopic fractionation during meltperidotite interaction and cooling processes.     

Tectonic implications of garnet-bearing mantle xenoliths exhumed by Quaternary magmatism in the Hangay dome, central Mongolia

Garnet-bearing mantle xenoliths have been recovered from Quaternary alkali basalts, both within and peripheral to the Hangay dome of central Mongolia. Microfabric analysis and thermobaromery, combining empirical thermobarometers and the self-consistent dataset of THERMOCALC, indicate that garnet websterites from the Shavaryn-Tsaram volcanic centre at the dome core were formed in the spinel-lherzolite upper mantle at pressures of 17–18 kbars and temperatures of 1,070–1,090°C, whereas garnet lherzolites were derived from greater depths (18–20 kbars). Garnet lherzolites from the Baga Togo Uul vents near the dome edge were formed at 18–22 kbars under significantly cooler conditions (960–1,000°C). These xenoliths reveal reaction coronas of (1) orthopyroxene, clinopyroxene, plagioclase and spinel mantling garnets; (2) spongy rims of olivine replacing orthopyroxene and (3) low-Na, low-Al clinopyroxene replacing primary clinopyroxene. Trace-element abundances indicate that clinopyroxene from these coronas is in chemical equilibrium with the host magma. The thermobarometric and textural data suggest that lherzolite xenoliths from both sites were derived from depths of 60–70 km and entrained in magma at 1,200–1,300°C. The average rate of ascent, as determined by olivine zoning, lies in the range 0.2–0.3 m s⁻¹. The contrast in thermal profiles of the upper mantle between the two sites is consistent with a mantle plume beneath the Hangay dome with elevated thermal conditions beneath the core of the dome being comparable to estimates of the Pleistocene geotherm beneath the Baikal rift.     

Origin of the Tertiary basaltic volcanism in the northern Hessian Depression

Volcanic activity started about 20 Ma before present with quartz tholeiites (QTh), had a climax with alkali olivine basalts (AOB) 13 to 14 Ma ago and ended 7 Ma ago with nepheline basanites (NB) and olivine nephelinites (ON). AOB covers 73% of the volcanic area. About 250 basalts and peridotite xenoliths were sampled for investigation. An upper mantle layer ranging from about 90 to 60 km depth has been conditioned for a preferential alkali basalt production by advection of H₂O-CO₂-fluids containing Si, Al, Ca, K, Na, P as major constituents beside numerous incompatible minor elements. At the onset of the geodynamically triggered mantle conditioning locally restricted diapirism into shallow depth has caused formation of olivine tholeiite magmas (OTh) at about 1,300° C by partial melting. All of these OTh primary melts intruded due to a favourable compressibility into granulites of the lower crust. The rare QTh basalts are their derivative magmas which have been slightly contaminated in the crust. Magmas of the subsequent alkali basaltic volcanism (AOB, bAOB, NB, ON, MON) formed by in-situ partial melting at about 75 to 90 km depth after depression of the peridotite solidi by fluids to temperatures 1,200° C. Except many AOB these magmas are primary melts as characterized by olivine/melt distribution coefficients of Mg/Fe²⁺ (K _D=0.29 to 0.34), by Ni concentrations (260 to 330 ppm) and the occurrence of peridotite xenoliths. Rapid rise of gas charged melts due to saturation in CO₂ prevented separation of olivine etc. and of xenoliths. The sequence of magmas from OTh to ON (or MON) is formed from decreasing proportions of orthopyroxene (opx) and increasing contributions of clinopyroxene (cpx) and phlogopite (ph) at almost equal proportions of spinel (sp). Incongruent melting of opx (and cpx) for OTh, AOB, NB and ON is correlated with precipitation of olivine. The average xenolith composition (73% ol, 18% opx, 7% cpx, 1.1% sp and 1.3/0.5% ph) was used to model the sources of the investigated melts by 9 incompatible elements and to calculate degrees of partial melting. The occurrence of garnet cannot be reliably excluded by modelling on the basis of HREE distribution coefficients. The average xenolith composition was used for modelling because of its resemblance with worldwide sampled depleted mantle inclusions. For avoiding to exhaust at least one mineral of the model mantle in the support of the norm composition of OTh, AOB, NB and MON magmas the degrees of partial melting cannot exceed 12.5%, 6%, 6% and 4% respectively. Mantle containing about 500 ppm K (and the correlated incompatible elements), like the average of 36 xenoliths, allows to explain the formation of OTh magmas. AOB, NB and ON melts require peridotite with slightly less than 1,500 ppm K, 670 ppm P and proportions of the correlated elements LREE, Sr, Ba, Zr, Rb, Cs, Ta, Th, Hf, U, which are higher than their abundance in primitive mantle rocks. About 20% of the xenoliths have this composition. Metasomatism of fluids with these elements must have been an immediate precursor of the alkali basaltic volcanism. Otherwise the preservation of a local disequilibrium in ⁸⁷Sr/⁸⁶Sr ratios between cpx cores and total rock at upper mantle temperatures cannot be explained.     

Feldspar-bearing lherzolite xenoliths in alkali basalts from Hamar-Daban,southern Baikal region,Russia

 Lherzolite xenoliths in Miocene to Pleistocene basalts from five sites in the Hamar-Daban range in southern Siberia provide sampling of the mantle close to the axis of the Baikal rift. These anhydrous spinel lherzolites commonly have foliated fabrics and spongy rims around clinopyroxene, and many contain accessory feldspar. The feldspar occurs in reaction zones adjacent to spinel and orthopyroxene (where it appears to have been formed by the reaction: spl+opx+cpx+fluid →fs+ol) and less commonly as thin, irregular veins. The feldspars have variable compositions but are generally alkali-rich; their K₂O content ranges from 0.3 to 11.2% and is much higher than in plagioclase from orogenic lherzolites (usually <0.1% K₂O). The temperature range for the Hamar-Daban xenolith suite (950–1010° C) is more restricted than for spinel peridotite xenoliths from other occurrences in the Baikal area. The feldspar-bearing lherzolites yield equilibration temperatures similar to or slightly lower than feldspar-free ones. The majority of the Hamar-Daban lherzolites are fertile and clinopyroxene-rich, as for most other occurrences in the Baikal region. Trace element compositions of selected xenoliths and their clinopyroxenes were determined by ICP-MS, INAA and proton microprobe. Feldspar-bearing xenoliths are enriched in alkalies indicating that feldspar formation is associated with addition of material and is not simply due to isochemical phase changes. Most xenoliths and their clinopyroxenes studied are depleted in light REE and have contents of Sr, Zr and Y common for fertile or moderately depleted mantle peridotites. Few are moderately enriched in LREE, Sr, Th and U. Sr-Nd isotope compositions of clinopyroxenes indicate long-term depletion in incompatible elements similar to unmetasomatised xenoliths from other occurrences south and east of Lake Baikal. The formation of feldspar and of spongy aggregates after clinopyroxene, and the enrichment in alkalies appear to be recent phenomena related to infiltration of an alkali-rich, H₂O-poor fluid into spinel peridotites. Received: 20 March 1995 / Accepted: 26 June 1995     

Significance of rare hydrous alkaline melts in Hawaiian xenoliths

 Two types of melt pockets occur in Hawaiian mantle xenoliths: amphibole-bearing (AMP) and spinel-bearing (SMP). AMPs contain amphibole (kaersutite), olivine (Fo₉₂), clinopyroxene (with 7–11% Al₂O₃), vesicles and glass. SMPs contain olivine, clinopyroxene, spinel, glass, and vesicles. The glasses in SMPs (SiO₂=44–45%, 11–12% alkalis, La=90–110 ppm) and AMPs (SiO₂=49–54%, 6–8.5% alkalis, La=8–14 ppm) are distinct in color and composition. Both glasses are generally characterized by LREE-enriched (chondrite-normalized) patterns. Amphibole and clinopyroxene have gently convex upward-to-moderately LREE-enriched patterns. Mineral/glass trace element abundance ratio plots show a strong negative Ti anomaly and a gentle negative Zr anomaly for clinopyroxene/glass; whereas amphibole/glass patterns show a distinctive positive Ti spike. The amphibole/glass trace element ratios are similar to published megacryst/lava values. An earlier study showed that the Hawaiian spinel lherzolites (lithosphere) have largely been metasomatized during post-erosional Honolulu magmatic activity. REE abundances of SMP glasses (melts) overlap the REE abundances calculated for such metasomes. The occurrence of hydrous, alkaline, mafic melt pockets in Hawaiian upper mantle xenoliths implies that (1) such hydrous liquids are generated in the upper mantle, and (2) water plays a role in magmatic activity associated with the Hawaiian plume. Although we are uncertain about the source (plume, lithosphere, or asthenosphere) of this water, we speculate that such melts and other alkalic lavas erupted on Oahu and on the sea-floor over the Hawaiian arch were generated from a broad „wet“ rim of a radially layered Hawaiian plume, whose hot and „dry“ core supplied the shield-forming magmas. Received: 6 February 1995 / Accepted: 28 August 1995     

Geochemical syntheses among the cratonic,off-cratonic and orogenic garnet peridotites and their tectonic implications

Garnet-bearing mantle peridotites, occurring as either xenoliths in volcanic rocks or lenses/massifs in high-pressure and ultrahigh-pressure terrenes within orogens, preserve a record of deep lithospheric mantle processes. The garnet peridotite xenoliths record chemical equilibrium conditions of garnet-bearing mineral assemblage at temperatures (T) ranging from ~700 to 1,400°C and pressures (P) > 1.6–8.9 GPa, corresponding to depths of ~52–270 km. A characteristic mineral paragenesis includes Cr-bearing pyropic garnet (64–86 mol% pyrope; 0–10 wt% Cr₂O₃), Cr-rich diopside (0.5–3.5 wt% Cr₂O₃), Al-poor orthopyroxene (0–5 wt% Al₂O₃), high-Cr spinel (Cr/(Cr + Al) × 100 atomic ratio = 2–86) and olivine (88–94 mol% forsterite). In some cases, partial melting, re-equilibration involving garnet-breakdown, deformation, and mantle metasomatism by kimberlitic and/or carbonatitic melt percolations are documented. Isotope model ages of Archean and Proterozoic are ubiquitous, but Phanerozoic model ages are less common. In contrast, the orogenic peridotites were subjected to ultrahigh-pressure (UHP) metamorphism at temperature ranging from ~700 to 950°C and pressure >3.5–5.0 GPa, corresponding to depths of >110–150 km. The petrologic comparisons between 231 garnet peridotite xenoliths and 198 orogenic garnet peridotites revealed that (1) bulk-rock REE (rare earth element) concentrations in xenoliths are relatively high, (2) clinopyroxene and garnet in orogenic garnet peridotites show a highly fractionated REE pattern and Ce-negative anomaly, respectively, (3) Fo contents of olivines for off-cratonic xenolith are in turn lower than those of orogenic garnet and cratonic xenolith but mg-number of garnet for orogenic is less than that of off-cratonic and on-cratonic xenolith, (4) Al₂O₃, Cr₂O₃, CaO and Cr# of pyroxenes and chemical compositions of whole rocks are very different between these garnet peridotites, (5) orogenic garnet peridotites are characterized by low T and high P, off-cratonic by high T and low P, and cratonic by medium T and high P and (6) garnet peridotite xenoliths are of Archean or Proterozoic origin, whereas most of orogenic garnet peridotites are of Phanerozoic origin. Taking account of tectonic settings, a new orogenic garnet peridotite exhumation model, crust-mantle material mixing process, is proposed. The composition of lithospheric mantle is additionally constrained by comparisons and compiling of the off-cratonic, on-cratonic and orogenic garnet peridotite.     

Water contents of the Cenozoic lithospheric mantle beneath the western part of the North China Craton: Peridotite xenolith constraints

Nominally anhydrous phases (clinopyroxene (cpx), orthopyroxene (opx), and olivine (ol)) of peridotite xenoliths hosted by the Cenozoic basalts from Beishan (Hebei province), and Fansi (Shanxi province), Western part of the North China Craton (WNCC) have been investigated by Fourier transform infrared spectrometry (FTIR). The H₂O contents (wt.) of cpx, opx and ol are 30–255 ppm, 14–95 ppm and ~ 0 ppm, respectively. Although potential H-loss during xenolith ascent cannot be excluded for olivine, pyroxenes (cpx and opx) largely preserve the H₂O content of their mantle source inferred from (1) the homogenous H₂O content within single pyroxene grains, and (2) equilibrium H₂O partitioning between cpx and opx. Based on mineral modes and assuming a partition coefficient of 10 for H₂O between cpx and ol, the recalculated whole-rock H₂O contents range from 6 to 42 ppm. In combination with previously reported data for other two localities (Hannuoba and Yangyuan from Hebei province), the H₂O contents of cpx, opx and whole-rock of peridotite xenoliths (43 samples) hosted by the WNCC Cenozoic basalts range from 30 to 654 ppm, 14 to 225 ppm, and 6 to 262 ppm respectively. The H₂O contents of the Cenozoic lithospheric mantle represented by peridotite xenoliths fall in a similar range for both WNCC and the eastern part of the NCC (Xia et al., 2010, Journal of Geophysical Research). Clearly, the Cenozoic lithospheric mantle of the NCC is dominated by much lower water content compared to the MORB source (50–250 ppm). The low H₂O content is not caused by oxidation of the mantle domain, and likely results from mantle reheating, possibly due to an upwelling asthenospheric flow during the late Mesozoic–early Cenozoic lithospheric thinning of the NCC. If so, the present NCC lithospheric mantle mostly represents relict ancient lithospheric mantle. Some newly accreted and cooled asthenospheric mantle may exist in localities close to deep fault.     

Geochemical and O-isotope constraints on the evolution of lithospheric mantle in the Ross Sea rift area (Antarctica)

Peridotite xenoliths found in Cenozoic alkali basalts of northern Victoria Land, Antarctica, vary from fertile spinel-lherzolite to harzburgite. They often contain glass-bearing pockets formed after primary pyroxenes and spinel. Few samples are composite and consist of depleted spinel lherzolite crosscut by amphibole veins and/or lherzolite in contact with poikilitic wehrlite. Peridotite xenoliths are characterized by negative Al₂O₃–Mg# and TiO₂–Mg# covariations of clino- and orthopyroxenes, low to intermediate HREE concentrations in clinopyroxene, negative Cr–Al trend in spinel, suggesting variable degrees of partial melting. Metasomatic overprint is evidenced by trace element enrichment in clinopyroxene and sporadic increase of Ti–Fe_tot. Preferential Nb, Zr, Sr enrichments in clinopyroxene associated with high Ti–Fe_tot contents constrain the metasomatic agent to be an alkaline basic melt. In composite xenoliths, clinopyroxene REE contents increase next to the veins suggesting metasomatic diffusion of incompatible element. Oxygen isotope data indicate disequilibrium conditions among clinopyroxene, olivine and orthopyroxene. The highest δ¹⁸O values are observed in minerals of the amphibole-bearing xenolith. The δ¹⁸O_cpx correlations with clinopyroxene modal abundance and geochemical parameters (e.g. Mg# and Cr#) suggest a possible influence of partial melting on oxygen isotope composition. Thermobarometric estimates define a geotherm of 80°C/GPa for the refractory lithosphere of NVL, in a pressure range between 1 and 2.5 GPa. Clinopyroxene microlites of melt pockets provide P–T data close to the anhydrous peridotite solidus and confirm that they originated from heating and decompression during transport in the host magma. All these geothermometric data constrain the mantle potential temperature to values of 1250–1350°C, consistent with the occurrence of mantle decompressional melting in a transtensive tectonic regime for the Ross Sea region.     

Oxygen isotope evidence for subduction and rift-related mantle metasomatism beneath the Colorado Plateau–Rio Grande rift transition

Spinel lherzolite and pyroxenite xenoliths from the Rio Puerco Volcanic Field, New Mexico, were analyzed for oxygen isotope ratios by laser fluorination. In lherzolites, olivine δ¹⁸O values are high (+5.5‰), whereas δ¹⁸O values for pyroxenes are low (cpx=+5.1‰; opx=+5.4‰) compared to average mantle values. Pyroxenite δ¹⁸O values (cpx=+5.0‰; opx=+5.3‰) are similar to those of the lherzolites and are also lower than typical mantle oxygen isotope compositions. Texturally and chemically primary calcite in pyroxenite xenoliths is far from isotopic equilibrium with other phases, with δ¹⁸O values of +21‰. The isotopic characteristics of the pyroxenite xenoliths are consistent with a petrogenetic origin from mixing of lherzolitic mantle with slab-derived silicate and carbonatite melts. The anomalously low δ¹⁸O in the pyroxenes reflects metasomatism by a silicate melt from subducted altered oceanic crust, and high δ¹⁸O calcite is interpreted to have crystallized from a high δ¹⁸O carbonatitic melt derived from subducted ophicarbonate. Similar isotopic signatures of metasomatism are seen throughout the Rio Puerco xenolith suite and at Kilbourne Hole in the southern Rio Grande rift. The discrete metasomatic components likely originated from the subducted Farallon slab but were not mobilized until heating associated with Rio Grande rifting occurred. Oxygen diffusion modeling requires that metasomatism leading to the isotopic disequilibrium between calcite and pyroxene in the pyroxenites occurred immediately prior to entrainment. Melt infiltration into spinel-facies mantle (xenoliths) prior to eruption was thus likely connected to garnet-facies melting that resulted in eruption of the host alkali basalt.     

Mantle-derived mafic-ultramafic xenoliths and the nature of Indian sub-continental lithosphere

Mantle derived xenoliths in India are known to occur in the Proterozoic ultrapotassic rocks like kimberlites from Dharwar and Bastar craton and Mesozoic alkali igneous rocks like lamrophyres, nephelinites and basanites. The xenoliths in kimberlites are represented by garnet harzburgites, lherzolites, wehrlite, olivine clinopyroxenites and kyaniteeclogite varieties. The PT conditions estimated for xenoliths from the Dharwar craton suggest that the lithosphere was at least 185 km thick during the Mid-Proterozoic period. The ultrabasic and eclogite xenoliths have been derived from depths of 100–180 km and 75–150 km respectively. The Kalyandurg and Brahmanpalle clusters have sampled the typical Archaean subcontinental lithospheric mantle (SCLM) with a low geotherm (35 mW/m²) and harzburgitic to lherzolitic rocks with median X_mg ^olivine > 0.93. The base of the depleted lithosphere at 185–195 km depth is marked by a 10–15 km layer of strongly metasomatised peridotites (X_mg ^olivine > ∼0.88). The Anampalle and Wajrakarur clusters 60 km to the NW show a distinctly different SCLM; it has a higher geotherm (37.5 to 40 mW/m²) and contains few subcalcic harzburgites, and has a median X_mg ^olivine = 0.925. In contrast, the kimberlites of the Uravakonda and WK-7 clusters sampled quite fertile (median X_mg ^olivine ∼0.915) SCLM with an elevated geotherm (> 40 mW/m²). The lamrophyres, basanites and melanephelinites associated with the Deccan Volcanic Province entrain both ultramafic and mafic xenoliths. The ultramafic group is represented by (i) spinel lherzolites, harzburgites, and (ii) pyroxenites. Single pyroxene granulite and two pyroxene granulites constitutes the mafic group. Temperature estimates for the West Coast xenoliths indicate equilibration temperatures of 500–900°C while the pressure estimates vary between 6–11 kbar corresponding to depths of 20–35 km. This elevated geotherm implies that the region is characterized by abnormally high heat flow, which is also supported by the presence of linear array of hot springs along the West Coast. Spinel peridotite xenoliths entrained in the basanites and melanephelinites from the Kutch show low equilibrium temperatures (884–972°C). The estimated pressures obtained on the basis of the absence of both plagioclase and garnet in the xenoliths and by referring the temperatures to the West Coast geotherm is ∼ 15 kbar (40–45 km depth). The minimum heat flow of 60 to 70 mW/m² has been computed for the Kutch xenolith (Bhujia hill), which is closely comparable to the oceanic geotherm. Xenolith studies from the West Coast and Kutch indicate that the SCLM beneath is strongly metasomatised although the style of metasomatism is different from that below the Dharwar Craton.     

Petrology and geochemistry of peridotite xenoliths from the Letlhakane kimberlites, Botswana

The diamondiferous Letlhakane kimberlites are intruded into the Proterozoic Magondi Belt of Botswana. Given the general correlation of diamondiferous kimberlites with Archaean cratons, the apparent tectonic setting of these kimberlites is somewhat anomalous. Xenoliths in kimberlite diatremes provide a window into the underlying crust and upper mantle and, with the aid of detailed petrological and geochemical study, can help unravel problems of tectonic setting. To provide relevant data on the deep mantle under eastern Botswana we have studied peridotite xenoliths from the Letlhakane kimberlites. The mantle-derived xenolith suite at Letlhakane includes peridotites, pyroxenites, eclogites, megacrysts, MARID and glimmerite xenoliths. Peridotite xenoliths are represented by garnet-bearing harzburgites and lherzolites as well as spinel-bearing lherzolite xenoliths. Most peridotites are coarse, but some are intensely deformed. Both garnet harzburgites and garnet lherzolites are in many cases variably metasomatised and show the introduction of metasomatic phlogopite, clinopyroxene and ilmenite. The petrography and mineral chemistry of these xenoliths are comparable to that of peridotite xenoliths from the Kaapvaal craton. Calculated temperature-depth relations show a well-developed correlation between the textures of xenoliths and P-T conditions, with the highest temperatures and pressures calculated for the deformed xenoliths. This is comparable to xenoliths from the Kaapvaal craton. However, the P-T gap evident between low-T coarse peridotites and high-T deformed peridotites from the Kaapvaal craton is not seen in the Letlhakane xenoliths. The P-T data indicate the presence of lithospheric mantle beneath Letlhakane, which is at least 150 km thick and which had a 40mW/m² continental geotherm at the time of pipe emplacement. The peridotite xenoliths were in internal Nd isotopic equilibrium at the time of pipe emplacement but a lherzolite xenolith with a relatively low calculated temperature of equilibration shows evidence for remnant isotopic disequilibrium. Both harzburgite and lherzolite xenoliths bear trace element and isotopic signatures of variously enriched mantle (low Sm/Nd, high Rb/Sr), stabilised in subcontinental lithosphere since the Archaean. It is therefore apparent that the Letlhakane kimberlites are underlain by old, cold and very thick lithosphere, probably related to the Zimbabwe craton. The eastern extremity of the Proterozoic Magondi Belt into which the kimberlites intrude is interpreted as a superficial feature not rooted in the mantle. Received: 19 March 1996 / Accepted: 16 October 1996     

Thermal and redox equilibrium conditions of the upper-mantle xenoliths from the Quaternary volcanoes of NW Spitsbergen,Svalbard Archipelago

Upper-mantle xenoliths in Cenozoic basalts of northwestern Spitsbergen are rocks of peridotite (spinel lherzolites) and pyroxenite (amphibole-containing garnet and garnet-free clinopyroxenites, garnet clinopyroxenites, and garnet and garnet-free websterites) series. The upper-mantle section in the depth range 50–100 km is composed of spinel peridotites; at depths of 80–100 km pyroxenites (probably, dikes or sills) appear. The equilibrium conditions of parageneses are as follows: in the peridotites—730–1180 °C, 13–27 kbar, and oxygen fugacity of − 1.5 to + 0.3 log. un.; in the pyroxenites—1100–1310 °C, 22–33 kbar. The pyroxenite minerals have been found to contain exsolved structures, such as orthopyroxene lamellae in clinopyroxene and, vice versa, clinopyroxene lamella in orthopyroxene. The formation temperatures of unexsolved phases in orthopyroxene and clinopyroxene are nearly 100–150 °C higher than the temperatures of the lamellae–matrix equilibrium and the equilibrium of minerals in the rock. The normal distribution of cations in the spinel structure and the equilibrium distribution of Fe^2 + between the M1 and M2 sublattices in the orthopyroxenes point to the high rate of xenolith ascent from the rock crystallization zone to the surface. All studied Spitsbergen rock-forming minerals from mantle xenoliths contain volatiles in their structure: OH⁻, crystal hydrate water H₂O_cryst, and molecules with characteristic CH and CO groups. The first two components are predominant, and the total content of water (OH– + H₂O_cryst) increases in the series olivine → garnet → orthopyroxene → clinopyroxene. The presence of these volatiles in the nominally anhydrous minerals (NAM) crystallized at high temperatures and pressures in the peridotites and pyroxenites testifies to the high strength of the volatile–mineral bond. The possibility of preservation of volatiles is confirmed by the results of comprehensive thermal and mass-spectral analyses of olivines and clinopyroxene, whose structures retain these components up to 1300 °C. The composition of hypothetic C–O–H fluid in equilibrium (in the presence of free carbon) with the underlying mantle rocks varies from aqueous (> 80% H₂O) to aqueous–carbonic (~ 60% H₂O). The fluid becomes essentially aqueous when the oxygen activity in the system decreases. However, there is no strict dependence of the redox conditions on the depth of formation of xenoliths.     

Behaviour of Li and its isotopes during metasomatism of French Massif Central lherzolites

Li behaviour and distribution in the mantle were investigated by ion microprobe in situ measurements on co-existing olivine (ol), orthopyroxene (opx), clinopyroxene (cpx) and amphibole (amp) in xenoliths from the French Massif Central. The fertile spinel lherzolites of this study record increasing degrees of mantle metasomatism, from unmetasomatised anhydrous samples through cryptically metasomatised samples to highly metasomatised amphibole-rich samples. In anhydrous lherzolites, Li is preferentially incorporated into olivine (1.1-1.4 ppm, average values) compared to pyroxenes (0.2-0.9 ppm). The hydrous samples clearly show enrichment of Li in ol (1.5-5.0 ppm), opx (1.1-2.4 ppm) and cpx (2.4-5.4 ppm), while amphibole incorporates less Li than the co-existing phases (0.8-1.3 ppm). Average δ⁷Li values range from +7.6 to +14.5‰ in ol, from 5.1 to +13.7‰ in opx and from 8.8 to +10.3‰ in cpx from the anhydrous lherzolites. A layered peridotite sample (Sdi) shows higher Li content in all phases, with lighter isotopic composition in opx and cpx (−0.6 and −2‰ average δ⁷Li values, respectively). In the hydrous lherzolites average δ⁷Li values both overlap and extend beyond these ranges in ol (up to 17.5 ‰) and in opx (up to 22.9‰), and vary widely in cpx (−2.7 to +9.7‰). Low δ⁷Li values are observed in some opx (−10.4‰) and cpx (−13‰) from sample Sdi, and in cpx from three hydrous samples (from −9.7 to −5.3‰). The different anhydrous phases from the hydrous samples show large intra-grain variations in Li isotopic ratios (e.g., up to 18‰) compared to the same phases from the anhydrous samples (mostly less than 6‰), excepting sample Sdi which has up to 20.4‰ variation in cpx. Similar to the anhydrous silicates, amphiboles show a wide variation of δ⁷Li values on the intra-grain scale (2-27‰). These variations are interpreted to result from fractionation processes during metasomatism by a silicate melt undergoing compositional changes as it percolates through and reacts with the peridotite phases. Thus Li abundances and isotopic in situ measurements are useful for tracing metasomatic processes but the heterogeneities observed in the samples preclude any identification of a specific mantle source by its Li signature.     

Temperature-dependent distribution of Cr between olivine and pyroxenes in lherzolite xenoliths

The temperature effect on the exchange reaction Cr₂O₃(ol)=Cr₂O₃(px) was studied for coexisting olivine and both clino and ortho pyroxenes. The distribution of Cr between olivine and clinopyroxene in 31 coarse garnet lherzolites and 10 porphyroclastic garnet lherzolites from kinberlites, and in 17 coarse spinel lherzolites from basalts, obeys a van't Hoff relation (c.f. Stosch 1981) with the Wells two-pyroxene temperature: T(Kelvin)=8,787 (In D _Cr+ 2.87) where D _Cr(opx/ol)=wt.% Cr(clinopyroxene)/Cr(olivine). An analogous exchange for olivine and orthopyroxene with 0.7–1.6 wt.% Al₂O₃ in 41 garnet lherzolites from kimberlites shows considerable scatter about the following relation: T(Kelvin)=5,540/(ln D _cr+1.86) where D _cr(opx/ol)= wt.% Cr(orthopyroxene)/Cr(olivine). Spinel lherzolites and a garnet lherzolite from the Malaita alnöite do not obey the second relation. For orthopyroxene with 2.5–5.1 wt.% Al₂O₃, D _cr(opx/ol) is 1.7 to 3 times higher, and for 0.1 wt.% Al₂O₃ is 2 times lower than for the garnet lherzolites. Experimental calibration is needed, especially to check the possible effect of Al on D _cr(opx/ol).     

Evidence from Antarctic mantle peridotite xenoliths for changes in mineralogy, geochemistry and geothermal gradients beneath a developing rift

Garnet and spinel peridotite xenoliths associated with the Phanerozoic Lambert-Amery Rift in eastern Antarctica contain evidence for several stages in the development of the mantle beneath the rift. Despite the fact that equilibria were only partly attained, a combination of petrography, whole-rock geochemistry, mineral chemistry and thermobarometry can be used to decipher four stages prior to entrainment of the xenoliths in the host magma during the initial stages of the breakup of Antarctica, India and Madagascar. The first chronological stage is represented by harzburgitic protoliths represented by rare occurrences of low-Ca olivines and orthopyroxenes in spinel lherzolites: these yield the lowest temperatures of 830-850 °C, and are also characterized by distinct trace element contents; lower Ti, Cr, V and Zn in olivine and orthopyroxene, and additionally lower Cu, Ni, Ga and Li in orthopyroxene. Some garnets are subcalcic, indicating that the spinel-garnet lherzolites also formed from harzburgitic protoliths. The second stage is the formation of garnet due to a pressure increase probably related to collision at 1.1 Ga. The third stage is marked by the growth of clinopyroxene, demonstrably in cpx-poor spinel lherzolites but probably in all xenolith groups: equilibrium of clinopyroxene with olivine and orthopyroxene was not attained in all samples, so that the non-judicious use of thermobarometers can produce bewildering results. The fourth stage is an enrichment episode that affected all spinel-garnet peridotites and about half of the spinel peridotites. During this stage, reaction rims were produced on the clinopyroxenes that formed during stage 3, the modal content of olivine and Mg/(Mg + Fe) in the rocks was reduced, CaO, Al₂O₃ and trace elements were enriched, and garnets were almost completely transformed to kelyphites. A later stage is documented by interstitial glasses and films around spinels related to infiltration of melt from the host magma. These post-date, and are more enriched in alkalies than, partially melted rims on clinopyroxenes, demonstrating that all the three earlier episodes were pre-entrainment events. Pressures indicated by the spinel + garnet lherzolites are restricted to 20-24 kbar at 1040-1180 °C. Early harzburgitic assemblages are interpreted to represent an earlier, cooler geotherm, whereas the kelyphite assemblages indicate temperatures 180-200 °C hotter than the main xenolith geotherm. This event also caused recrystallization of the clinopyroxene rims and is attributed to heating during rifting, but not due to the host magma itself. The preservation of evidence for three progressively hotter geotherms can be related to the upward movement of isotherms during the development of the sub-rift mantle.     

Hydrogen partitioning between melt,clinopyroxene, and garnet at 3 GPa in a hydrous MORB with 6 wt.% H<Subscript>2</Subscript>O

To understand partitioning of hydrogen between hydrous basaltic and andesitic liquids and coexisting clinopyroxene and garnet, experiments using a mid-ocean ridge basalt (MORB) + 6 wt.% H₂O were conducted at 3 GPa and 1,150–1,325°C. These included both isothermal and controlled cooling rate crystallization experiments, as crystals from the former were too small for ion microprobe (SIMS) analyses. Three runs at lower bulk water content are also reported. H₂O was measured in minerals by SIMS and in glasses by SIMS, Fourier Transform infrared spectroscopy (FTIR), and from oxide totals of electron microprobe (EMP) analyses. At 3 GPa, the liquidus for MORB with 6 wt.% H₂O is between 1,300 and 1,325°C. In the temperature interval investigated, the melt proportion varies from 100 to 45% and the modes of garnet and clinopyroxene are nearly equal. Liquid composition varies from basaltic to andesitic. The crystallization experiments starting from above the liquidus failed to nucleate garnets, but those starting from below the liquidus crystallized both garnet and clinopyroxene. SIMS analyses of glasses with >7 wt.% H₂O yield spuriously low concentrations, perhaps owing to hydrogen degassing in the ultra-high vacuum of the ion microprobe sample chamber. FTIR and EMP analyses show that the glasses have 3.4 to 11.9 wt.% water, whilst SIMS analyses indicate that clinopyroxenes have 1,340–2,330 ppm and garnets have 98–209 ppm H₂O. D _H ^cpx−gt is 11 ± 3, D _H ^cpx−melt is 0.023 ± 0.005 and D _H ^gt−melt is 0.0018 ± 0.0006. Most garnet/melt pairs have low values of D _H ^gt−melt, but D _H ^gt−melt increases with TiO₂ in the garnet. As also found by previous studies, values of D _H ^cpx−melt increase with Al₂O₃ of the crystal. For garnet pyroxenite, estimated values of D _H ^{pyroxenite−melt} decrease from 0.015 at 2.5 GPa to 0.0089 at 5 GPa. Hydration will increase the depth interval between pyroxenite and peridotite solidi for mantle upwelling beneath ridges or oceanic islands. This is partly because the greater pyroxene/olivine ratio in pyroxenite will tend to enhance the H₂O concentration of pyroxenite, assuming that neighboring pyroxenite and peridotite bodies have similar H₂O in their pyroxenes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The upper mantle under La Palma,Canary Islands: formation of Si−K−Na-rich melt and its importance as a metasomatic agent

 Mantle xenoliths hosted by the Historic Volcan de San Antonio, La Palma, Canary Islands, fall into two main group. Group I consists of spinel harzburgites, rare spinel lherzolites and spinel dunites, whereas group II comprises spinel wehrlites, amphibole wehrlites, and amphibole clinopyroxenites. We here present data on group I xenoliths, including veined harzburgites and dunites which provide an excellent basis for detailed studies of metasomatic processes. The spinel harzburgite and lherzolite xenoliths have modal ol−opx−cpx ratios and mineral and whole rock major element chemistry similar to those found in Lanzarote and Hierro, and are interpreted as highly refractory, old oceanic lithospheric mantle. Spinel dunites are interpreted as old oceanic peridotite which has been relatively enriched in olivine and clinopyroxene (and highly incompatible elements) through reactions with basaltic Canarian magmas, with relatively high melt/peridotite ratio. Group I xenoliths from La Palma differ from the Hierro and Lanzarote ones by a frequent presence of minor amounts of phlogopite (and amphibole). Metasomatic processes are also reflected in a marked enrichment of strongly incompatible relative to moderately incompatible trace elements, and in a tendency for Fe−Ti enrichment along grain boundaries in some samples. The veins in the veined xenoliths show a gradual change in phase assemblage and composition of each phase, from Fe−Ti-rich amphibole+augite+Fe−Ti-oxides+apatite+basaltic glass, to Ti-poor phlogopite+Cr-diopside±chromite+ Si−Na−K-rich glass+fluid. Complex reaction zones between veins and peridotite include formation of clinopyroxene±olivine+glass at the expense of orthopyroxene in harzburgite, and clinopyroxene+spinel±amphibole±glass at the expense of olivine in dunite. The dramatic change in glass composition from the broadest to the narrowest veins includes increasing SiO₂ from 44 to 67 wt%, decreasing TiO₂/Al₂O₃ ratio from >0.24 to about 0.02, and increasing K₂O and Na₂O from 1.8 to >7.0 wt% and 3.8 to 6.7 wt%, respectively. The petrographic observations supported by petrographic mixing calculations indicate that the most silicic melts in the veined xenoliths formed as the result of reaction between infiltrating basaltic melt and peridotite wall-rock. The highly silicic, alkaline melt may represent an important metasomatic agent. Pervasive metasomatism by highly silicic melts (and possibly fluids unmixed from these) may account for the enriched trace element patterns and frequent presence of phlogopite in the upper mantle under La Palma. Received: 15 January 1996 / Accepted 30 May 1996     

New experimental determination of Li and B partition coefficients during upper mantle partial melting

Despite the growing interest for Li and B as geochemical tracers, especially for material transfer from subducting slabs to overlying peridotites, little is known about the behaviour of these two elements during partial melting of mantle sources. In particular, mineral/melt partition coefficients for B and to a lesser extent Li are still a matter of debate. In this work, we re-equilibrated a synthetic basalt doped with ~10 ppm B and ~6 ppm Li with an olivine powder from a spinel lherzolite xenolith at 1 GPa–1,330°C, and we analyzed Li and B in the run products by secondary ion mass spectrometry (SIMS). In our experiment, B behaved as a highly incompatible element with mineral/melt partition coefficients of the order of 10⁻² (D ^ol/melt = 0.008 (0.004–0.013); D ^opx/melt = 0.024 (0.015–0.033); D ^cpx/melt = 0.041 (0.021–0.061)), and Li as a moderately incompatible element (D ^ol/melt = 0.427 (0.418–0.436); D ^opx/melt = 0.211 (0.167–0.256); D ^cpx/melt = 0.246 (0.229–0.264)). Our partition coefficients for Li are in good agreement with previous determinations. In the case of B, our partition coefficients are equal within error to those reported by Brenan et al. (1998) for all the mineral phases analyzed, but are lower than other coefficients from literature for some of the phases (up to 5 times for cpx). Our measurements complement the data set of Ds for modelling partial melting of the upper mantle and basalt generation, and confirm that, in this context, B is more incompatible than previously anticipated.     

Inter-mineral Fe isotope variations in mantle-derived rocks and implications for the Fe geochemical cycle

Iron isotope and major- and minor-element compositions of coexisting olivine, clinopyroxene, and orthopyroxene from eight spinel peridotite mantle xenoliths; olivine, magnetite, amphibole, and biotite from four andesitic volcanic rocks; and garnet and clinopyroxene from seven garnet peridotite and eclogites have been measured to evaluate if inter-mineral Fe isotope fractionation occurs in high-temperature igneous and metamorphic minerals and if isotopic fractionation is related to equilibrium Fe isotope partitioning or a result of open-system behavior. There is no measurable fractionation between silicate minerals and magnetite in andesitic volcanic rocks, nor between olivine and orthopyroxene in spinel peridotite mantle xenoliths. There are some inter-mineral differences (up to 0.2‰ in ⁵⁶Fe/⁵⁴Fe) in the Fe isotope composition of coexisting olivine and clinopyroxene in spinel peridotites. The Fe isotope fractionation observed between clinopyroxene and olivine appears to be a result of open-system behavior based on a positive correlation between the Δ⁵⁶Fe_{clinopyroxene-olivine} fractionation and the δ⁵⁶Fe value of clinopyroxene and olivine. There is also a significant difference in the isotopic compositions of garnet and clinopyroxene in garnet peridotites and eclogites, where the average Δ⁵⁶Fe_{clinopyroxene-garnet} fractionation is +0.32 ± 0.07‰ for six of the seven samples. The one sample that has a lower Δ⁵⁶Fe_{clinopyroxene-garnet} fractionation of 0.08‰ has a low Ca content in garnet, which may reflect some crystal chemical control on Fe isotope fractionation. The Fe isotope variability in mantle-derived minerals is interpreted to reflect subduction of isotopically variable oceanic crust, followed by transport through metasomatic fluids. Isotopic variability in the mantle might also occur during crystal fractionation of basaltic magmas within the mantle if garnet is a liquidus phase. The isotopic variations in the mantle are apparently homogenized during melting processes, producing homogenous Fe isotope compositions during crust formation.