Showing papers in "Contributions to Mineralogy and Petrology in 2006"

Crystallization thermometers for zircon and rutile

Abstract: Zircon and rutile are common accessory minerals whose essential structural constituents, Zr, Ti, and Si can replace one another to a limited extent. Here we present the combined results of high pressure–temperature experiments and analyses of natural zircons and rutile crystals that reveal systematic changes with temperature in the uptake of Ti in zircon and Zr in rutile. Detailed calibrations of the temperature dependencies are presented as two geothermometers—Ti content of zircon and Zr content of rutile—that may find wide application in crustal petrology. Synthetic zircons were crystallized in the presence of rutile at 1–2 GPa and 1,025–1,450°C from both silicate melts and hydrothermal solutions, and the resulting crystals were analyzed for Ti by electron microprobe (EMP). To augment and extend the experimental results, zircons hosted by five natural rocks of well-constrained but diverse origin (0.7–3 GPa; 580–1,070°C) were analyzed for Ti, in most cases by ion microprobe (IMP). The combined experimental and natural results define a log-linear dependence of equilibrium Ti content (expressed in ppm by weight) upon reciprocal temperature: $$\log ({\text{Ti}}_{{{\text{zircon}}}}) = (6.01 \pm 0.03) - \frac{{5080 \pm 30}}{{T\;(\hbox{K})}}.$$ In a strategy similar to that used for zircon, rutile crystals were grown in the presence of zircon and quartz (or hydrous silicic melt) at 1–1.4 GPa and 675–1,450°C and analyzed for Zr by EMP. The experimental results were complemented by EMP analyses of rutile grains from six natural rocks of diverse origin spanning 0.35–3 GPa and 470–1,070°C. The concentration of Zr (ppm by weight) in the synthetic and natural rutiles also varies in log-linear fashion with T −1: $$\log ({\text{Zr}}_{{{\text{rutile}}}}) = (7.36 \pm 0.10) - \frac{{4470 \pm 120}}{{T\;(\hbox{K})}}.$$ The zircon and rutile calibrations are consistent with one another across both the synthetic and natural samples, and are relatively insensitive to changes in pressure, particularly in the case of Ti in zircon. Applied to natural zircons and rutiles of unknown provenance and/or growth conditions, the thermometers have the potential to return temperatures with an estimated uncertainty of ±10 ° or better in the case of zircon and ±20° or better in the case of rutile over most of the temperature range of interest (∼400–1,000°C). Estimates of relative temperature or changes in temperature (e.g., from zoning profiles in a single mineral grain) made with these thermometers are subject to analytical uncertainty only, which can be better than ±5° depending on Ti or Zr concentration (i.e., temperature), and also upon the analytical instrument (e.g., IMP or EMP) and operating conditions.

TitaniQ: a titanium-in-quartz geothermometer

Abstract: Titanium is one of many trace elements to substitute for silicon in the mineral quartz. Here, we describe the temperature dependence of that substitution, in the form of a new geothermometer. To calibrate the “TitaniQ” thermometer, we synthesized quartz in the presence of rutile and either aqueous fluid or hydrous silicate melt, at temperatures ranging from 600 to 1,000°C, at 1.0 GPa. The Ti contents of quartz (in ppm by weight) from 13 experiments increase exponentially with reciprocal T as described by: $$ {\text{Log}}{\left( {X^{{{\text{qtz}}}}_{{{\text{Ti}}}} } \right)} = (5.69 \pm 0.02) - \frac{{(3765 \pm 24)}} {{T(K)}}. $$ Application of this thermometer is straightforward, typically requiring analysis of only one phase (quartz). This can be accomplished either by EPMA for crystallization temperatures above 600°C, or by SIMS for temperatures down to at least 400°. Resulting temperature estimates are very precise (usually better than ±5°C), potentially allowing detailed characterization of thermal histories within individual quartz grains. Although calibrated for quartz crystallized in the presence of rutile, the thermometer can also be applied to rutile-absent systems if TiO2 activity is constrained.

Trace element partitioning between mica- and amphibole-bearing garnet lherzolite and hydrous basanitic melt : 1. Experimental results and the investigation of controls on partitioning behaviour

Abstract: Thirty five minor and trace elements (Li, Be, B, Sc, Cu, Zn, Ga, Ge, As, Rb, Nb, Mo, Ag, Cd, In, Sn, Sb, Cs, Ba, La, Ce, Nd, Sm, Tb, Ho, Tm, Lu, Hf, Ta, W, Tl, Pb, Bi, Th and U) in experimentally produced near-liquidus phases, from a primitive nelpheline basanite from Bow Hill in Tasmania (Australia), were analysed by LAM ICP-MS. A number of halogens (F, Cl and I) were also analysed by electron microprobe. The analyses were used to determine mineral/melt partition coefficients for mica, amphibole, garnet, clinopyroxene, orthopyroxene and olivine for conditions close to multiple saturation of the basanite liquidus with garnet lherzolite (approximately 2.6 GPa and 1,200°C with 7.5 wt% of added H2O). A broader range of conditions was also investigated from 1.0 GPa and 1,025°C to 3.5 GPa and 1,190°C with 5–10 wt% of added H2O. The scope and comprehensiveness of the data allow them to be used for two purposes, these include the following: an investigation of some of the controlling influences on partition coefficients; and the compilation of a set partition coefficients that are directly relevant to the formation of the Bow Hill basanite magma by partial melting of mantle peridotite. Considering clinopyroxene, the mineral phase for which the most data were obtained, systematic correlations were found between pressure and temperature, mineral composition, cation radius and valence, and ΔGcoulb (the coulombic potential energy produced by substituting a cation of mismatched valence into a crystallographic site). ΔGcoulb is distinctly different for different crystallographic sites, including the M2 and M1 sites in clinopyroxene. These differences can be modelled as a function of variations in optimum valence (expressed as 1 sigma standard deviations) within individual M1 and M2 site populations.

Tracing magma mixing in granite genesis: in situ U–Pb dating and Hf-isotope analysis of zircons

Abstract: In situ zircon U–Pb and Hf-isotopic data have been determined for mafic microgranular enclaves and host granitoids from the Early Cretaceous Gudaoling batholith in the Liaodong Peninsula, NE China, in order to constrain the sources and petrogenesis of granites. The zircon U–Pb age of the enclaves (120 ± 1 Ma) is identical to that of the host monzogranite (120 ± 1 Ma), establishing that the mafic and felsic magmas were coeval. The Hf isotopic composition of the enclaves [e Hf(t) = +4.5 to −6.2] is distinct from the host monzogranite [e Hf(t) = −15.1 to −25.4], indicating that both depleted mantle and crustal sources contributed to their origin. The depleted mantle component was not previously revealed by geochemical and Nd and Sr isotopic studies, showing that zircon Hf isotopic data can be a powerful geochemical tracer with the potential to provide unique petrogenetic information. Some wall-rock contamination is indicated by inherited zircons with considerably older U–Pb ages and low initial Hf isotopic compositions. Hafnium isotopic variations in Early Cretaceous zircons rule-out simple crystal–liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks. Instead, mixing of mantle-derived mafic magmas with crustal-derived felsic magmas, coupled with assimilation of wall rocks, is compatible with the data.

Effect of water on tholeiitic basalt phase equilibria: an experimental study under oxidizing conditions

Abstract: To investigate the effect of water on phase relations and compositions in a basaltic system, we performed crystallization experiments at pressures of 100, 200 and 500 MPa in a temperature range of 940 to 1,220°C using four different water contents. Depending on the water activity, the oxygen fugacity varied between 1 and 4 log units above the quartz-magnetite-fayalite buffer. Addition of water to the dry system shifts the solidus > 250°C to lower temperatures and increases the amount of melt drastically. For instance, at 1,100°C and 200 MPa, the melt fraction increases from 12.5 wt% at a water content of 1.6 wt% to 96.3% at a water content of 5 wt% in the melt. The compositions of the experimental phases also show a strong effect of water. Plagioclase is shifted to higher anorthite contents by the addition of water. Olivine and clinopyroxene show generally higher MgO/FeO ratios with added water, which could also be related to the increasing oxygen fugacity with water. Moreover, water affects the partitioning of certain elements between minerals and melts, e.g., the Ca partitioning between olivine and melt. Plagioclase shows a characteristic change in the order of crystallization with water that may help to explain the formation of wehrlites intruding the lower oceanic crust (e.g., in Oman, Macquarie Island). At 100 MPa, plagioclase crystallizes before clinopyroxene at all water contents. At pressures > 100 MPa, plagioclase crystallizes before clinopyroxene at low water contents (e.g. 3 wt%. This change in crystallization order indicates that a paragenesis typical for wehrlites (olivine–clinopyroxene–without plagioclase) is stabilized at low pressures typical of the oceanic crust only at high water contents. This opens the possibility that typical wehrlites in the oceanic crust can be formed by the fractionation and accumulation of olivine and clinopyroxene at 1,060°C and > 100 MPa in a primitive tholeiitic basaltic system containing more than 3 wt% water. The comparison of the experimental results with evolution trends calculated by the thermodynamic models “MELTS” and “Comagmat” shows that neither model predicts the experimental phase relations with sufficient accuracy.

U-Pb systematics of the McClure Mountain syenite: thermochronological constraints on the age of the 40 Ar/ 39 Ar standard MMhb

Abstract: Recent advances in U–Pb geochronology allow unprecedented levels of precision in the determination of geological ages. However, increased precision has also illuminated the importance of understanding subtle sources of open-system behavior such as Pb-loss, inheritance, intermediate daughter product disequilibria, and the accuracy of the model assumptions for initial Pb. Deconvolution of these effects allows a much richer understanding of the power and limitations of U–Pb geochronology and thermochronology. In this study, we report high-precision ID-TIMS U–Pb data from zircon, baddelleyite, titanite and apatite from the McClure Mountain syenite, from which the 40Ar/39Ar hornblende standard MMhb is derived. We find that excess 206Pb in zircon due to inclusions of high-Th minerals and elevated Th/U in titanite and apatite jeopardize the utility of the 238U–206Pb system in this rock. Strongly air-abraded zircons give dates that are younger than chemical-abraded zircons, which yield a statistically robust 207Pb/235U date of 523.98±0.12 Ma that is interpreted as the crystallization age. We explore the best method of Pbc correction in titanite and apatite by analyzing the U–Pb isotopes of K-feldspar and using 2-D and 3-D regression methods—the latter of which yields the best results in each case. However, the calculated compositions of Pbc for titanite, apatite and K-feldspar are different, implying that using a single Pbc correction for multiple U–Pb thermochronometers may be inaccurate. The U–Pb thermochronological results are used to predict a closure time for Ar in hornblende of 522.98±1.00 Ma. Widely cited K–Ar and 40Ar/39Ar dates overlap with the U–Pb date, and relatively large errors make it impossible to verify whether U–Pb dates are systematically ≤1% older than K–Ar and 40Ar/39Ar dates.

The development and refinement of continental arcs by primary basaltic magmatism, garnet pyroxenite accumulation, basaltic recharge and delamination: insights from the Sierra Nevada, California

Abstract: The lower crust of the Mesozoic Sierra Nevada batholith was made up of high MgO, garnet-poor and low MgO, garnet-rich pyroxenites. Both groups are genetically linked and are collectively complementary to the mafic to intermediate Sierran plutons. High MgO pyroxenites represent high pressure cumulates from a mantle-derived hydrous basalt or basaltic andesite, resulting in derivative magmas having unusually low MgO for a given SiO2 as represented by the numerous mafic enclaves found in many Sierran plutons. The low MgO pyroxenites are either (1) shallow pressure cumulates from these derivative magmas or (2) partial melting residues (restites) of these derivative magmas after they were emplaced and solidified at lower crustal levels. In both cases, the complementary melt to the low MgO pyroxenites is driven to higher SiO2 contents, generating diorites and granodiorites. However, this simple two-stage scenario for the origin of Sierran granitoids cannot explain the observation that the Mg# of Sierran intermediate magmas remains roughly constant at ∼0.45–0.50 with increasing SiO2. Basaltic recharge/mixing with the lower crust is suggested as one means of buffering Mg#s and re-melting the lower crust to generate granitic melts, the latter of which mix with more juvenile magmas to complete the Sierran differentiation series.

Experimental evidence of decompression melting during exhumation of subducted continental crust

Abstract: Experiments have been carried out on a metagreywacke at 800, 850 and 900°C, in the pressure range 0.5–5 GPa to locate the solidus and the eclogite/amphibolite facies transition in felsic rocks, identify the nature of the reactions responsible for major mineralogical changes, and determine the proportions of phases as a function of pressure. The mineral assemblage phengite + clinopyroxene + garnet + quartz/coesite is stable above 2.3 GPa while biotite + plagioclase + garnet + quartz is stable below 2 GPa. The model reaction for the eclogite/amphibolite facies transition in metagreywackes is: $$ {\text{Phe}} + {\text{Cpx}} + {\text{Qtz}} = {\text{Bt}} + {\text{Pl}} + {\text{Grt}} + {\text{melt}} $$ with melt on the low pressure–high temperature side of the reaction. The modal proportion and calcium content of garnet change with pressure. Both decrease from 5 to 2.5 GPa, then increase at the eclogite/amphibolite facies transition, and finally decrease with decreasing pressure below 2.3 GPa. The grossular content in garnet is thus a potential marker of the eclogite/amphibolite facies transition during retrogression. The modal proportion of melt progressively increases with decreasing pressure from 5 to 2.5 GPa, then shows a sudden and marked increase between 2.5 and 2.3 GPa, and finally decreases between 2.3 and 1 GPa. Thus, a melting pulse occurs at the eclogite/amphibolite facies transition during decompression of subducted continental crust. A survey of the main UHP metamorphic regions and the P–T paths followed during their geotectonic history indicates that partial melting may have played a role during their exhumation. A striking feature of retrogressed UHP felsic rocks is that garnet rims are commonly enriched in grossular. Our experiments explain this observation and demonstrate that a grossular-rich growth zone in garnet is not necessarily indicative of highest pressures reached during metamorphism but may correspond to a decompression stage.

Temperature-dependent isotopic fractionation of lithium between clinopyroxene and high-pressure hydrous fluids

Abstract: The fractionation of lithium isotopes between synthetic spodumene as representative of Li-bearing clinopyroxene and Cl- and OH-bearing aqueous fluids was experimentally determined between 500 and 900°C at 2.0 GPa. In all the experiments, 7Li was preferentially partitioned into the fluid. The fractionation is temperature dependent and approximated by the equation Δ7Li(clinopyroxene–fluid)=−4.61×(1,000/T [K]) + 2.48; R 2=0.86. Significant Li isotopic fractionation of about 1.0‰ exists even at high temperatures of 900°C. Using neutral and weakly basic fluids revealed that the amount of fractionation is not different. The Li isotopic fractionation between altered basalt and hot spring water (350°C) in natural samples is in good agreement with our experimentally determined fractionation curve. The data confirm earlier speculations drawn from the Li isotopic record of dehydrated metamorphic rocks that fluids expelled from a dehydrating slab carry heavier Li into the mantle wedge, and that a light Li component is introduced into the deeper mantle. Li and Li isotopes are redistributed among wedge minerals as fluids travel across the wedge into hotter regions of arc magma production. This modifies the Li isotopic characteristics of slab-derived fluids erasing their source memory, and explains the absence of cross-arc variations of Li isotopes in arc basalts.

Fractionation of the noble metals by physical processes

Abstract: During partial melting in the earth’s mantle, the noble metals become fractionated. Os, Ir, Ru, and Rh tend to remain in the mantle residue whereas Pt, Pd, and Re behave mildly incompatible and are sequestered to the silicate melt. There is consensus that sulfide plays a role in the fractionation process; the major noble metal repository in the mantle is sulfide, and most primitive mantle melts are sulfide-saturated when they leave their mantle sources. However, with sulfide–silicate partitioning, the fractionation cannot be modeled properly. All sulfide–silicate partition coefficients are so extremely high that a silicate melt segregating from a mantle source with residual sulfide should be largely platinum-group elements free. We offer a physical alternative to sulfide–silicate chemical partitioning and provide a mechanism of generating a noble metal-rich melt from a sulfide-saturated source: Because sulfide is at least partially molten at asthenospheric temperature, it will behave physically incompatible during melt segregation, and a silicate melt segregating from a mantle residue will entrain molten residual sulfide in suspension and incorporate it in the basaltic pool melt. The noble metal abundances of a basalt then become independent of sulfide–silicate chemical partitioning. They reflect the noble metal abundances in the drained sulfide fraction as well as the total amount of sulfide entrained. Contrary to convention, we suggest that a fertile, sulfide-rich mantle source has more potential to generate a noble metal-enriched basaltic melt than a refractory mantle source depleted by previous partial melting events.

Diffusion-limited REE uptake by eclogite garnets and its consequences for Lu–Hf and Sm–Nd geochronology

Abstract: Garnets from the Zermatt-Saas Fee eclogites contain narrow central peaks for Lu + Yb + Tm ± Er and at least one additional small peak towards the rim. The REE Sm + Eu + Gd + Tb ± Dy are depleted in the cores but show one prominent peak close to the rim. These patterns cannot be modeled using Rayleigh fractionation accompanied by mineral breakdown reactions. Instead, the patterns are well explained using a transient matrix diffusion model where REE uptake is limited by diffusion in the matrix surrounding the porphyroblast. Observed profiles are well matched if a roughly linear radius growth rate is used. The secondary peaks in the garnet profiles are interpreted to reflect thermally activated diffusion due to temperature increase during prograde metamorphism. The model predicts anomalously low 176Lu/177Hf and 147Sm/144Nd ratios in garnets where growth rates are fast compared to diffusion of the REE, and these results have important implications for Lu–Hf and Sm–Nd geochronology using garnet.

The source of granitic gneisses and migmatites in the Antarctic Peninsula: a combined U-Pb SHRIMP and laser ablation Hf isotope study of complex zircons

Abstract: Zircons gneisses and migmatites collected from the Antarctic Peninsula have different core–rim hafnium isotope ratio relationships depending on whether evidence for zircon dissolution is present or absent. Two samples contain inherited zircon that is partially dissolved. In these samples, the 176Hf/177Hf rations of the inherited zircon and new magmatic zircon rims are, on average, indistinguishable and consistent with in situ melting. In such cases the hafnium isotopic composition of the melt was probably strongly influenced by the dissolved zircon component at the source. Variation in 176Hf/177Hf within the magmatic zircon rims from grain to grain suggests that Hf isotopes were only partially homogenized during melt migration; alternatively, zircon growth may have taken place within small volumes of partial melt. Other samples do not preserve textural evidence for zircon dissolution during melt generation; in these samples the 176Hf/177Hf values of the inherited zircon and new magmatic zircon rims are different. The zircon rims apparently suggest a source of less evolved hafnium than that contained within the inherited zircon. Whether this relates to a separate juvenile source or, alternatively, is derived from minerals other than zircon at the source, cannot be resolved. Inherited zircon, irrespective of age, has been strongly influenced by the reworking of a juvenile Late Mesoproterozoic source, suggesting that such crust underlies the Antarctic Peninsula. Our results therefore suggest that Hf isotope analyses provide great potential for future studies investigating the source and processes involved in the generation of crustal melts.

Platinum-group element, Gold, Silver and Base Metal distribution in compositionally zoned sulfide droplets from the Medvezky Creek Mine, Noril’sk, Russia

Abstract: Concentrations of Ag, Au, Cd, Co, Re, Zn and Platinum-group elements (PGE) have been determined in sulfide minerals from zoned sulfide droplets of the Noril’sk 1 Medvezky Creek Mine. The aims of the study were; to establish whether these elements are located in the major sulfide minerals (pentlandite, pyrrhotite, chalcopyrite and cubanite), to establish whether the elements show a preference for a particular sulfide mineral and to investigate the model, which suggests that the zonation in the droplets is caused by the crystal fractionation of monosulfide solid solution (mss). Nickel, Cu, Ag, Re, Os, Ir, Ru, Rh and Pd, were found to be largely located in the major sulfide minerals. In contrast, less than 25% of the Au, Cd, Pt and Zn in the rock was found to be present in these sulfides. Osmium, Ir, Ru, Rh and Re were found to be concentrated in pyrrhotite and pentlandite. Palladium and Co was found to be concentrated in pentlandite. Silver, Cd and Zn concentrations are highest in chalcopyrite and cubanite. Gold and platinum showed no preference for any of the major sulfide minerals. The enrichment of Os, Ir, Ru, Rh and Re in pyrrhotite and pentlandite (exsolution products of mss) and the low levels of these elements in the cubanite and chalcopyrite (exsolution products of intermediate solid solution, iss) support the mss crystal fractionation model, because Os, Ir, Ru, Rh and Re are compatible with mss. The enrichment of Ag, Cd and Zn in chalcopyrite and cubanite also supports the mss fractionation model these minerals are derived from the fractionated liquid and these elements are incompatible with mss and thus should be enriched in the fractionated liquid. Gold and Pt do not partition into either iss or mss and become sufficiently enriched in the final fractionated liquid to crystallize among the iss and mss grains as tellurides, bismithides and alloys. During pentlandite exsolution Pd appears to have diffused from the Cu-rich portion of the droplet into pentlandite.

U–Pb zircon ages, geochemical and isotopic compositions of granitoids in Songpan-Garze fold belt, eastern Tibetan Plateau: constraints on petrogenesis and tectonic evolution of the basement

Abstract: The Songpan-Garze fold belt, located in the eastern part of the Tibetan Plateau, covers a huge triangular area bounded by the Yangtze (South China), the North China and the Tibetan Plateau blocks. In the northeastern part of the Songpan-Garze fold belt, the Yanggon and Maoergai granitoids provide insights into regional tectono-magmatic events, basement nature and tectonic evolution. U-Pb zircon SHRIMP dating shows that the Yanggon and Maoergai granitoids have mag- matic crystallization ages of 221 ± 3.8 Ma and 216 ± 5.7 Ma, respectively. Both the granitoids display adakitic geochemical signatures, suggesting that their magma was derived from partial melting of thickened lower crust. Pb-Sr-Nd isotopic compositions for granitoids reveal that there is an unexposed Proterozoic basement in the Songpan-Garze belt, which has an affinity with the Yangtze block. During development of the Paleo-Tethys ocean, the basement of the Songpan- Garze belt would be a peninsula approaching the Paleo-Tethys ocean from the Yangtze block.

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

Abstract: 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/opxDLi=3–7 and cpx/olDLi=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 PbN/NbN=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 (PbN/NbN=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/amphDLi=0.7 and cpx/olDLi=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.

Zircon U-Pb geochronology and elemental and Sr-Nd isotope geochemistry of Permian mafic rocks in the Funing area, SW China

Abstract: Mafic-layered intrusions and sills and spatially associated andesitic basalts are well preserved in the Funing area, SW China. The 258±3 Ma-layered intrusions are composed of fine-grained gabbro, gabbro and diorite. The 260±3 Ma sills consist of undifferentiated diabases. Both the layered intrusions and volcanic rocks belong to a low-Ti group, whereas the diabases belong to a high-Ti group. Rocks of the high-Ti group have FeO, TiO2 and P2O5 higher but MgO and Th/Nb ratios lower than those of the low-Ti group. They have initial 87Sr/86Sr ratios (0.706–0.707) lower and ɛNd (−1.5 to −0.6) higher than the low-Ti equivalents (0.710–0.715 and −9.6 to −4.0, respectively). The high-Ti group was formed from relatively primitive, high-Ti magmas generated by low degrees (7.3 –9.5%) of partial melting of an enriched, OIB-type asthenospheric mantle source. The low-Ti group may have formed from melts derived from an EM2-like, lithospheric mantle source. The mafic rocks at Funing are part of the Emeishan large igneous province formed by a mantle plume at ∼260 Ma.

Subduction factory processes beneath the Guguan cross-chain, Mariana Arc: no role for sediments, are serpentinites important?

Abstract: We need to understand chemical recycling at convergent margins and how chemical interactions between subducted slab and the overlying mantle wedge affect mantle evolution and magmagenesis. This requires distinguishing contributions from recycled individual subducted components as well as those contributed by the mantle. We do this by examining magmatic products generated at different depths above a subduction zone, in an intra-oceanic arc setting. The Guguan cross-chain in the intra-oceanic Mariana arc overlies subducted Jurassic Pacific plate lithosphere at depths of ~125--230 km and erupts mostly basalt. Basalts from rear-arc volcanoes are more primitive than those from the magmatic front, in spite of being derived by lower degrees of melting of less-depleted mantle. Rear-arc magmas also show higher temperatures and pressures of equilibration. Coexisting mineral compositions become more MORB- or OIB-like with increasing height above the subduction zone. Trace element and isotopic variations indicate that the subduction component in cross-chain lavas diminishes with increasing depth to the subduction zone, except for water contents. There is little support for the idea that melting beneath the Mariana Trough back-arc basin depleted the source region of arc magmas, but melting to form rear-arc volcanoes may have depleted the source of magmatic front volcanoes. Enrichments in rear-arc lavas were not caused by sediment melting; the data instead favor an OIB-type mantle that has been modestly affected by subduction zone fluids. Our most important conclusion is that sediment fluids or melts are not responsible for the K--h relationship and other cross-chain chemical and isotopic variations. We speculate that an increasing role for supercritical fluids released from serpentinites interacting with modestly enriched mantle might be responsible for cross-chain geochemical and isotopic variations.

Petrogenesis of highly depleted peridotites and gabbroic rocks from the Mayarí-Baracoa Ophiolitic Belt (Eastern Cuba).

Abstract: The Moa-Baracoa and Mayari-Cristal massifs (eastern Cuba) are two ophiolitic complexes mainly constituted by harzburgite tectonites and minor dunites, cut by gabbroic dykes. The Moa-Baracoa massif exhibits a well developed Moho transition zone and an incomplete crustal section made up of layered gabbros and tectonically emplaced pillow basalts. A plutonic crustal section is absent in the Mayari-Cristal massif and mantle tectonites are in tectonic contact with arc-related volcanic rocks. Mantle peridotites are very refractory in terms of modal composition, whole rock major element and HREE contents implying that Moa-Baracoa and Mayari-Cristal harzburgites are residues after high degrees (20–30%) of partial melting. The relative enrichment of Th, Nb, Ta and LREE in peridotites is due to re-equilibration of melting residues with percolating melts. Peridotites lost on average 6 wt% of relative MgO by intense seafloor weathering. REE contents and Mg# of melts in equilibrium with cumulate gabbros from the Moho transition zone and crustal section of the Moa-Baracoa massif coincide with those of the spatially-related pillow basalts. On the other hand, no geochemical relation has been inferred between melt in equilibrium with Mayari-Cristal segregate and the spatially-related arc volcanics. Our results indicate that the Mayari-Baracoa Ophiolitic Belt formed at an original back-arc spreading centre. The Moa-Baracoa massif represents a portion of MORB-like lithosphere located nearby a back-arc mid-ocean spreading ridge, and the Mayari-Cristal massif represents a piece of transitional (MORB to IAT) mantle located closer to the paleo-volcanic arc than Moa-Baracoa.

Mineralogy and geochemistry of microgranular enclaves in Palaeoproterozoic Malanjkhand granitoids, central India: evidence of magma mixing, mingling, and chemical equilibration

Abstract: Palaeoproterozoic (ca 2,480 Ma) felsic magmatism of Malanjkhand region of central Indian Precambrian shield, referred to as Malanjkhand granitoids (MG), contain xenoliths of country rocks and mesocratic to melanocratic, fine-grained porphyritic microgranular enclaves (ME). The shape of ME is spheroidal, ellipsoidal, discoidal, elongated, and lenticular, varying in size from a few centimeters to about 2 m across. The contact of ME with the host MG is commonly sharp, crenulate, and occasionally diffuse, which we attribute to the undercooling and disaggregation of ME globules within the cooler host MG. The ME as well as MG show hypidiomorphic texture with common mineral Hbl-Bt-Kfs-Pl-Qtz assemblage, but differ in modal proportions. The variation in minerals' composition, presence of apatite needles, elongated biotites, resorbed plagiclase, ocellar quartz, and other mafic–felsic xenocrysts strongly oppose the restite and cognate origins of ME. Compositions of plagioclases (An3–An29), amphiboles (Mg/Mg+Fe2+=0.55–0.69), and biotites (Mg/Mg+Fe2+=0.46–0.60) of ME are slightly distinct or similar to those of MG, which suggest partial to complete equilibration during mafic–felsic magma interactions. Al-in-amphibole estimates the MG pluton emplacement at ca 3.4 ± 0.5 kbar, and therefore, magma mixing and mingling must have occurred at or below this level. The $$\hbox{Fe}\rightleftharpoons\hbox{Mg}$$ substitution in biotites of ME and MG largely suggests subduction-related, calc–alkaline metaluminous (I-type) nature of felsic melts. Most major and trace elements against SiO2 produce near linear variation trends for ME and MG, probably generated by the mixing of mafic and felsic magmas in various proportions. Trace including rare earth elements patterns of ME–MG pairs, however, show partial to complete equilibration, most likely governed by different degrees of elemental diffusion. The available evidence supports the model of ME origin that coeval mafic (enclave) and felsic (MG) magmas produced a hybrid (ME) magma layer, which injected into cooler, partly crystalline MG, and dispersed, mingled, and undercooled as ME globules in a convectively dynamic magma chamber.

A subduction wedge origin for Paleoarchean peridotitic diamonds and harzburgites from the Panda kimberlite, Slave craton: evidence from Re–Os isotope systematics

Abstract: An extensive study of peridotitic sulfide inclusion bearing diamonds and their prospective harzburgitic host rocks from the 53 Ma Panda kimberlite pipe, Ekati Mine, NWT Canada, has been undertaken with the Re–Os system to establish their age and petrogenesis. Diamonds with peridotitic sulfide inclusions have poorly aggregated nitrogen (<30% N as B centers) at N contents of 200–800 ppm which differs from that of chromite and silicate bearing diamonds and indicates residence in the cooler portion of the Slave craton lithospheric mantle. For most of the sulfide inclusions, relatively low Re contents (average 0.457 ppm) and high Os contents (average 339 ppm) lead to extremely low 187Re/188Os, typically << 0.05. An age of 3.52 ± 0.17 Ga (MSWD = 0.46) and a precise initial 187Os/188Os of 0.1093 ± 0.0001 are given by a single regression of 11 inclusions from five diamonds that individually provide coincident internal isochrons. This initial Os isotopic composition is 6% enriched in 187Os over 3.5 Ga chondritic or primitive mantle. Sulfide inclusions with less radiogenic initial Os isotopic compositions reflect isotopic heterogeneity in diamond forming fluids. The harzburgites have even lower initial 187Os/188Os than the sulfide inclusions, some approaching the isotopic composition of 3.5 Ga chondritic mantle. In several cases isotopically distinct sulfides occur in different growth zones of the same diamond. This supports a model where C–O–H–S fluids carrying a radiogenic Os signature were introduced into depleted harzburgite and produced diamonds containing sulfides conforming to the 3.5 Ga isochron. Reaction of this fluid with harzburgite led to diamonds with less radiogenic inclusions while elevating the Os isotope ratios of some harzburgites. Subduction is a viable way of introducing such fluids. This implies a role for subduction in creating early continental nuclei at 3.5 Ga and generating peridotitic diamonds.

Mantle Pb paradoxes: the sulfide solution

Abstract: There is growing evidence that the budget of Pb in mantle peridotites is largely contained in sulfide, and that Pb partitions strongly into sulfide relative to silicate melt. In addition, there is evidence to suggest that diffusion rates of Pb in sulfide (solid or melt) are very fast. Given the possibility that sulfide melt “wets” sub-solidus mantle silicates, and has very low viscosity, the implications for Pb behavior during mantle melting are profound. There is only sparse experimental data relating to Pb partitioning between sulfide and silicate, and no data on Pb diffusion rates in sulfides. A full understanding of Pb behavior in sulfide may hold the key to several long-standing and important Pb paradoxes and enigmas. The classical Pb isotope paradox arises from the fact that all known mantle reservoirs lie to the right of the Geochron, with no consensus as to the identity of the “balancing” reservoir. We propose that long-term segregation of sulfide (containing Pb) to the core may resolve this paradox. Another Pb paradox arises from the fact that the Ce/Pb ratio of both OIB and MORB is greater than bulk earth, and constant at a value of 25. The constancy of this “canonical ratio” implies similar partition coefficients for Ce and Pb during magmatic processes (Hofmann et al. in Earth Planet Sci Lett 79:33–45, 1986), whereas most experimental studies show that Pb is more incompatible in silicates than Ce. Retention of Pb in residual mantle sulfide during melting has the potential to bring the bulk partitioning of Ce into equality with Pb if the sulfide melt/silicate melt partition coefficient for Pb has a value of ∼ 14. Modeling shows that the Ce/Pb (or Nd/Pb) of such melts will still accurately reflect that of the source, thus enforcing the paradox that OIB and MORB mantles have markedly higher Ce/Pb (and Nd/Pb) than the bulk silicate earth. This implies large deficiencies of Pb in the mantle sources for these basalts. Sulfide may play other important roles during magmagenesis: (1) advective/diffusive sulfide networks may form potent metasomatic agents (in both introducing and obliterating Pb isotopic heterogeneities in the mantle); (2) silicate melt networks may easily exchange Pb with ambient mantle sulfides (by diffusion or assimilation), thus “sampling” Pb in isotopically heterogeneous mantle domains differently from the silicate-controlled isotope tracer systems (Sr, Nd, Hf), with an apparent “de-coupling” of these systems.

Deformation fabrics of the Cima di Gagnone peridotite massif, Central Alps, Switzerland: evidence of deformation at low temperatures in the presence of water

Abstract: We report a new observation of the olivine B-type lattice-preferred orientation (LPO), from the garnet peridotite at Cima di Gagnone, Switzerland. The olivine B-type fabric forms at low temperatures and/or high stress in the presence of water, and is of particular interest because it may be used to explain the trench-parallel shear-wave splitting that is often observed at subduction zones. In conjunction with the olivine B-type fabric, we have found strong orthopyroxene LPO that is identical to those formed under water-free conditions. This suggests that water may not have a significant effect on orthopyroxene fabric. From the olivine microstructure, we determine that a stress of 22 ± 8 MPa was applied during the deformation event that formed the olivine LPO. Using an olivine flow-law, and assuming geological strain-rates, we determine the temperature of deformation to be 800 ± 175°C. This does not preclude an ultra-deep origin for the ultramafic rocks at Cima di Gagnone, but indicates that much of the deformation recorded in the microstructure occurred at modest temperatures.

Immiscible silicate liquid partition coefficients: implications for crystal-melt element partitioning and basalt petrogenesis

Abstract: This study investigates partitioning of ele- ments between immiscible aluminosilicate and boro- silicate liquids using three synthetic mixtures doped with 32 trace elements. In order to get a good spatial separation of immiscible liquids, we employed a high- temperature centrifuge. Experiments were performed at 1,050-1,150� C, 1 atm, in sealed Fe and Pt containers. Quenched products were analysed by electron micro- probe and LA ICP-MS. Nernst partition coefficients (D's) between the Fe-rich and Si-rich aluminosilicate immiscible liquids are the highest for Zn (3.3) and Fe (2.6) and the lowest for Rb and K (0.4-0.5). The plots of D values against ionic potential Z/r in all the composi- tions show a convex upward trend, which is typical also for element partitioning between immiscible silicate and salt melts. The results bear upon the speciation and structural position of elements in multicomponent silicate liquids. The ferrobasalt-rhyolite liquid immis- cibility is observed in evolved basaltic magmas, and may play an important role in large gabbroic intrusions, such as Skaergaard, and during the generation of unusual lavas, such as ferropicrites.

Mixing and differentiation in the Oruanui rhyolitic magma, Taupo, New Zealand: evidence from volatiles and trace elements in melt inclusions

Abstract: Large pyroclastic rhyolites are snapshots of evolving magma bodies, and preserved in their eruptive pyroclasts is a record of evolution up to the time of eruption. Here we focus on the conditions and processes in the Oruanui magma that erupted at 26.5 ka from Taupo Volcano, New Zealand. The 530 km3 (void-free) of material erupted in the Oruanui event is comparable in size to the Bishop Tuff in California, but differs in that rhyolitic pumice and glass compositions, although variable, did not change systematically with eruption order. We measured the concentrations of H2O, CO2 and major and trace elements in zoned phenocrysts and melt inclusions from individual pumice clasts covering the range from early to late erupted units. We also used cathodoluminescence imaging to infer growth histories of quartz phenocrysts. For quartz-hosted inclusions, we studied both fully enclosed melt inclusions and reentrants (connecting to host melt through a small opening). The textures and compositions of inclusions and phenocrysts reflect complex pre-eruptive processes of incomplete assimilation/partial melting, crystallization differentiation, magma mixing and gas saturation. ‘Restitic’ quartz occurs in seven of eight pumice clasts studied. Variations in dissolved H2O and CO2 in quartz-hosted melt inclusions reflect gas saturation in the Oruanui magma and crystallization depths of ∼3.5–7 km. Based on variations of dissolved H2O and CO2 in reentrants, the amount of exsolved gas at the beginning of eruption increased with depth, corresponding to decreasing density with depth. Pre-eruptive mixing of magma with varying gas content implies variations in magma bulk density that would have driven convective mixing.

Geochemical constraints on the origin of the Permian Baimazhai mafic–ultramafic intrusion, SW China

Abstract: The ∼260 Ma Baimazhai mafic–ultramafic intrusion is considered to be part of the Emeishan large igneous province and consists of orthopyroxenite surrounded by websterite and gabbro. The intrusion is variably mineralized with a massive sulfide ore body (∼20 vol.%) in the core of the intrusion. Silicate rocks have Ni/Cu ratios ranging from 0.3 to 46 with majority less than 7 and are rich in LREE relative to HREE and show Nb and Ta anomalies in primitive mantle-normalized trace element patterns, with low Nb/Th (1.0–4.5) and Nb/La (0.3–1.0) ratios. Their ɛ Nd(t) values range from −3.3 to −8.4. Uniform Pd/Pt (0.7–3.5) and Cu/Pd (100,000–400,000) ratios throughout the intrusion indicate that all the sulfides in the rocks were formed in a single sulfide-saturation event. Modeling suggests that the Baimazhai rocks were formed when an Mg-rich magma became crustally contaminated in a deep-seated staging chamber. Crustal contamination (up to ∼35%) drove the magma to S-saturation and forced orthopyroxene (Opx) onto the liquidus. The crystal-bearing magma forced out of the staging chamber was migrated by flow differentiation and consequently, the denser sulfide melt and the Opx crystals became centrally disposed in the flowing magma to form the Baimazhai intrusion.

The origin of reaction textures in mantle peridotite xenoliths from Sal Island, Cape Verde: the case for ''metasomatism'' by the host lava

Abstract: Reaction zones around minerals in mantle xenoliths have been reported from many localities worldwide. Interpretations of the origins of these textures fall into two groups: mantle metasomatic reaction or reaction during transport of the xenoliths to the surface. A suite of harzburgitic mantle xenoliths from Sal, Cape Verde show clear evidence of reaction during transport. The reactions resulted in the formation of olivine–clinopyroxene and Si- and alkali-rich glass reaction zones around orthopyroxene and sieve-textured clinopyroxene and sieve textured spinel, both of which are associated with a Si- and alkali-rich glass similar to that in the orthopyroxene reaction zones. Reaction occurred at pressures less than the mantle equilibration pressure and at temperatures close to the liquidus temperature of the host magma. In addition, there is a clear spatial relation of reaction with the host lava: reaction is most intense near the lava/xenolith contact. The residence time of the xenoliths in the host magma, determined from Fe–Mg interdiffusion profiles in olivine, was approximately 4 years. Our results cannot be reconciled with a recent model for the evolution of the mantle below the Cape Verde Archipelago involving mantle metasomatism by kimberlitic melt. We contend that alkali-rich glasses in the Sal xenoliths are not remnants of a kimberlitic melt, but rather they are the result of reaction between the host lava or a similar magma and xenolith minerals, in particular orthopyroxene. The formation of a Si- and alkali-rich glass by host magma–orthopyroxene reaction appears to be a necessary precursor to formation of sieve textured spinel and clinopyroxene.

Trace element and isotopic variations from Mt. Vulture to Campanian volcanoes: constraints for slab detachment and mantle inflow beneath southern Italy

Abstract: New Sr–Nd–Pb isotopic ratios and trace element data for volcanic mafic rocks outcropping along a E–W transect in southern Italy, from Mt. Vulture to Neapolitan volcanoes, are reported. The variation of LILE/HFSE, HFSE/HFSE and radiogenic isotopes along this transect indicates that all of these volcanoes contain both intra-plate and subduction-related signatures, with the former decreasing from Mt. Vulture to Campanian volcanoes. New data are also reported for the Paleocene alkaline rocks from Pietre Nere (Apulia foreland), which show isotopic ratios mostly overlapping the values for Mediterranean intra-plate volcanoes as well as the Eocene–Oligocene alkaline mafic lavas from the northern Adria plate. Pietre Nere provides evidence for an OIB mantle composition of FOZO-type, free of subduction influences, that is present beneath the Adria plate (Africa) before its collision with Europe. After this collision, and formation of the southern Apennines, westward inflow of mantle from the Adria plate to the Campanian area occurred, as a consequence of slab break off. Interaction of subduction components with inflowing Adria mantle generated hybrid sources beneath the Vulture–Campania area, which can explain the compositional features of both Mt. Vulture and the Campanian mafic rocks. Therefore, mafic magmas from these volcanoes represent variable degrees of mixing between different mantle components.

Geochemistry of the Emeishan flood basalts at Yangliuping, Sichuan, SW China: implications for sulfide segregation

Abstract: A well-developed, 1,000 m thick basaltic sequence in the Yangliuping region, northern part of the Emeishan basalt province, includes the Lower and Middle Units of tholeiitic basalts and an Upper Unit of both tholeiites and subalkalic basalts. The basalts contain 42–55 wt% SiO2 and 4.1–8.3 wt% MgO. Most of these lavas have Gd/Yb > 2.0, Zr/Nb < 12, and ɛNd(260 Ma) values from +2.5 to +4.7. The platinum-group elements (PGE) are very mildly depleted in most of the basalts which contain 8–19 ppb Pt and 7–27 ppb Pd. However, a significant proportion of the Middle Unit basalts are strongly depleted in PGE with some samples having concentrations lower than detection limits. They have extremely high Zr/Nb ratios (up to 14.5) and low ɛNd(260 Ma) values (+3.21 to +0.65), features of extensive lower crustal contamination. Some samples in this unit have high Ni/Pd (3,965–61,198) and low Pd/Cr (410,000–3,930,000) ratios, indicating sulfide segregation and PGE depletion prior to eruption. The primary magmas were S-undersaturated and derived from partial melting at variable depths in the upper mantle. The early and late stage magmas, as represented by the Lower and Upper Units, underwent AFC processes which induced mild S-saturation and PGE depletion in some of the basalts, whereas the magmas represented by the Middle Unit experienced more extensive crustal contamination resulting in stronger S-saturation and in most cases significant PGE depletion.

Timescales and significance of high-pressure, high-temperature metamorphism and mafic dike anatexis, Snowbird tectonic zone, Canada

Abstract: New geochronological, isotopic and geochemical data for a spectacular swarm of deep crustal migmatitic mafic dikes offer important insight into processes operative during 1.9 Ga high pressure, high temperature metamorphism along the Snowbird tectonic zone in northern Saskatchewan. High-precision U–Pb zircon dates reveal anatexis of Chipman mafic dikes at 1,896.2 ± 0.3 Ma during syntectonic and synmetamorphic intrusion at conditions of 1.0–1.2 GPa, >750°C. U–Pb zircon dates of 1,894–1,891 Ma for cross-cutting pegmatites place a lower bound on major metamorphism and deformation at the currently exposed crustal levels. The persistence of elevated temperatures for ~14 m.y. following peak conditions is implied by younger U–Pb titanite dates, and by Sm–Nd whole rock isotopic data that suggest the derivation of the pegmatites by melting of a mafic source. Limited melting of the host felsic gneiss at 1.9 Ga despite high temperature is consistent with evidence for their previous dehydration by granulite facies metamorphism in the Archean. Spatial heterogeneity in patterns of mafic dike and tonalitic gneiss anatexis can be attributed to lateral peak temperature and compositional variability. We correlate 1,896 Ma Chipman mafic dike emplacement and metamorphism with substantial 1.9 Ga mafic magmatism over a minimum along-strike extent of 1,200 km of the Snowbird tectonic zone. This suggests a significant, continent-wide period of asthenospheric upwelling that induced incipient continental rifting. Extension was subsequently terminated by hinterland contraction associated with Trans-Hudson accretion and orogenesis. Little activity in the lower crust for ca. 650 m.y. prior to Proterozoic metamorphism and mafic magmatism implies an extended interval of cratonic stability that was disrupted at 1.9 Ga. This episode of destabilization contrasts with the record of long-term stability in most preserved cratons, and is important for understanding the lithospheric characteristics and tectonic circumstances that control the destruction or survival of continents.

Petrogenesis of base metal sulphide assemblages of some peridotites from the Kaapvaal craton (South Africa)

Abstract: Thirty-two peridotite xenoliths from kimberlitic pipes of the Kaapvaal craton were analysed for S and studied in reflected light microscopy and electron microprobe. Correlation between whole-rock S concentrations and sulphide modal abundances has been obscured by kimberlite-related sulphur within the mantle and by low-temperature contamination processes during emplacement. Mantle-derived base metal sulphides (BMS) occur as solitary inclusions (SI) and intergranular blebs. Unfractured SI encloses intergrowths of Ni-poor and Ni-rich monosulphide solid solution (Mss) phases, coexisting with pentlandite (Pn) and Cu-rich sulphides. Textural relationships between Mss phases and Cu-sulphides are consistent with fractional crystallization of Mss from a Cu–Fe–Ni sulphide melt. Pn-rich euhedral SI may have crystallized from a more metal-rich sulphide melt. However, the opaque mineral assemblages of both fractured sulphide inclusions and intergranular BMS point to a progressive desulphurization of Mss, yielding Pn-rich grains, often replaced by Fe-poor heazlewoodite and abundant magnetite, while Cu-sulphides are replaced by native copper. This trend is consistent with reducing conditions generated by low-temperature serpentinization. A residual origin cannot be ruled out for the Mss enclosed in the most refractory peridotites, although their Ni/Fe ratios are too low to be consistent with an equilibration with olivine at magmatic temperatures. Modal abundances of mantle-derived BMS increase in the Fe-enriched metasomatized peridotites. At least two BMS precipitation processes can be recognized: (1) precipitation of Fe–Cu-rich immiscible sulphide melts in phlogopite-(ilmenite–rutile) peridotites and (2) sulphidation reactions from an H2S-rich fluid phase in phlogopite-K richterite peridotites.