Minerals In Peridotite Xenoliths Research Articles

Multi-stage metasomatism revealed by trace element content and Fe isotopic composition of minerals in peridotite xenoliths from Niutoushan in the Cathaysia Block, South China

This paper reports petrology, major and trace element content, and Fe isotopic composition of minerals from a suite of spinel lherzolites entrained in the Cenozoic Niutoushan basalt from the Cathaysia block, South China. The aim is to constrain the nature and evolution of the lithospheric mantle, as well as the factors controlling the fractionation of Fe isotopes beneath the Cathaysia block, South China. Based on the petrographic features and mineral compositions, the Niutoushan spinel lherzolite can be subdivided into two groups. Group 1 lherzolite has a high olivine Fo content (89.3 to 90.6) and both LREE-depleted and LREE-enriched chondrite-normalized REE patterns in clinopyroxene grains, reflecting variable degrees of partial melt extraction (up to 20%) followed by a multistage metasomatic history. The Fe isotopic composition of Group 1 lherzolite minerals shows little variation (δ57/54Fe = −0.05‰ to 0.03‰) with an average of 0.00 ± 0.06‰ (2SD, n =7), close to the suggested δ57/54Fe value of 0.04 ± 0.04‰ for the upper mantle, and there is no significant inter-mineral disequilibria. Compared with the Group 1 lherzolite, the Group 2 lherzolite experienced melt-rock interaction with significant enrichment of Fe as evidenced by relatively lower Mg# (84.9 to 88.2) in olivine and pyroxene grains and LREE-enriched REE patterns in clinopyroxene. Importantly, the Group 2 lherzolite exhibits a large range in Fe isotopic compositions, with δ57/54Fe ranging from −0.48‰ to 0.27‰, and disequilibrium Fe isotope fractionation is noted between most of coexisting Group 2 minerals (Δ57/54FeOpx–Ol = −0.16‰ to 0.25‰ and Δ57/54FeCpx–Ol = −0.16‰ to 0.30‰), reflecting kinetic fractionation during melt-rock interaction. Such processes play an important role in producing inter-mineral Fe isotopic disequilibrium and local Fe isotopic heterogeneity of the subcontinental mantle lithosphere beneath the Cathaysia block, South China, as noted for the subcontinental lithospheric mantle under the North China Craton.

Metasomatism-controlled hydrogen distribution in the Spitsbergen upper mantle

Abstract Hydrogen concentrations in minerals of peridotite xenoliths in alkali basaltic rocks from Quaternary volcanoes in northwest Spitsbergen were measured using polarized Fourier transform infrared spectroscopy (FTIR) to trace the effects of geologic processes on hydrogen distribution in the continental lithospheric mantle. The mineral grains show hydrogen profiles with lower concentrations at rims suggesting diffusive hydrogen loss during the entrapment and transport of the xenoliths in magma. However, hydrogen concentrations in the centers of the grains are uniform and appear to represent hydrogen abundances in the Spitsbergen upper mantle. The olivine, orthopyroxene, and clinopyroxene contain 1–10, 130–290, and 350–560 ppm H2O, respectively. Hydrogen abundances away from metasomatic melt conduits recorded by Type 1 xenoliths are correlated with the concentrations of incompatible trace elements, indicating that hydrogen distribution is related to mantle metasomatism. By contrast, hydrogen near the melt conduits, recorded by Type 2 xenoliths, shows no regular correlations with incompatible trace elements (except Nb in clinopyroxene) and may be affected by fractional crystallization of amphibole in the conduits. Hydrogen contents decrease away from the melt conduits and are controlled by the interaction between the depleted host mantle and percolating metasomatic melts. Therefore, the metasomatic melt could have variably hydrated the Spitsbergen upper mantle via different processes. The H2O/Ce ratios of the melt in equilibrium with clinopyroxene near the metasomatic melt conduits range from 93 to 218, i.e., within the oceanic island basalt (OIB) range. This is consistent with that the metasomatic melt could have been derived from OIB-type sources evidenced by the Sr-Nd isotope compositions of the xenoliths.

Multi-stage metasomatism revealed by trace element and Li isotope distributions in minerals of peridotite xenoliths from Allègre volcano (French Massif Central)

The modal, chemical, and isotopic compositions of mantle peridotite are largely modified by metasomatic processes, which may affect them repeatedly. Xenoliths are commonly used to characterize those metasomatic processes along with the structure, and chemical and isotopic compositions of mantle domains. Nevertheless, the original mantle signatures born by mantle xenoliths are potentially obscured by the interactions occurring between the host magma and the xenolith itself. Here we attempt to identify to which degree the original Li content and isotopic composition, as well as other trace element contents of mantle xenoliths, can be modified by interaction with the host magma. Peridotite xenoliths that have suffered extensive exchange with the entraining magma were sampled in the solidified lava lake of Allègre, Southern French Massif Central, in order to decipher the signature related to peridotite-melt interaction, and to further unravel the evolution of the sub-continental lithospheric mantle. In-situ trace element analyses of clinopyroxene (Cpx) were performed via LA–ICP–MS, and the Li content and isotopic composition of pyroxene and olivine (Ol) via SIMS. Negative HFSE anomalies (Ti/Eu ratios as low as 437) and markedly high LREE/HREE ratios ((La/Yb)N as high as 79) are characteristic of mantle metasomatism at depth. Lithium isotope systematics indicates that at least two different metasomatic events affected the peridotite. Exceptionally high Li contents in Cpx (up to 50ppm) and slight Li enrichment of Ol rims are ascribed to diffusive Li influx with a positive δ7Li value (+3.2‰) from the host magma after entrainment. Conversely, Ol cores preserve extremely light Li isotopic compositions (δ7Li as low as −25‰) with high Li contents (up to 4.4ppm) compared to normal mantle, indicating a metasomatic event that occurred before xenolith entrainment. The negative δ7Li signature of this early metasomatism may be related to subduction-related fluids released during the Variscan orogeny. Trace element distributions in minerals reveal that the HFSE and REE composition of Cpx and the negative δ7Li signature in Ol cores were not acquired simultaneously. Therefore at least three successive metasomatic events affected the Allegre peridotites, as revealed through the use of detailed in-situ Li isotopic analyses to trace melt-rock interactions.

Regional heterogeneity in the water content of the Cenozoic lithospheric mantle of Eastern China

AbstractThe major and trace elements and H2O contents of minerals in peridotite xenoliths hosted by the Cenozoic basalts in Northeast China (NEC) were evaluated using electron microprobe, laser‐ablation inductively coupled plasma–mass spectrometry and Fourier transform infrared spectroscopy, respectively. Although a potential loss of H during the xenoliths' ascent cannot be excluded for olivine, orthopyroxene (opx) and clinopyroxene (cpx) largely preserved the H2O contents of their mantle source in all of the samples, as inferred from (1) the homogenous H2O contents within single pyroxene grains and (2) the equilibrium H2O partitioning between cpx and opx. No OH was detected for pyroxenes of peridotite xenoliths from the north part of NEC (NNEC). Combined with previously published data from the North China Craton (NCC) and the South China Block (SCB), the regional heterogeneity in the water contents in the Cenozoic lithospheric mantle beneath the whole Eastern China has been revealed. The lithospheric mantle beneath the NNEC is completely dry. The “bulk” water contents of the lithospheric mantle of the south part of NEC and the NCC have similar ranges and average values, whereas those of the SCB are much higher (12–195 ppm, average 90 ± 45 ppm for whole rock). The regional variations in the H2O content of the Cenozoic lithospheric mantle of Eastern China cannot be caused by partial melting, mantle metasomatism, or variations in redox state. We propose that the lithospheric mantle beneath the different regions of Eastern China may have distinct origins and may have undergone distinct geodynamic processes.

Earth's moderately volatile element composition may not be chondritic: Evidence from In, Cd and Zn

Current models assume that siderophile volatile elements (SVE) are depleted in bulk Earth to the same extent as lithophile elements of similar volatility. The observed additional depletion of many SVE relative to lithophile elements in the bulk silicate Earth (BSE) is ascribed to partitioning of SVE into Earth's core. However, the assumption of similar volatility of moderately volatile elements during Earth formation processes as in solar gas is quite uncertain. Here, these assumptions will be tested by assessing abundances and ratios of indium and cadmium in the BSE using new data on mantle rocks, and the application of high- and low-pressure–temperature metal–silicate partitioning data.New bulk rock abundance data of In and Cd obtained on bulk rocks of peridotite tectonites and xenoliths by isotope dilution refine previous results inferred from basalts and in-situ analyses of silicate minerals in peridotite xenoliths. The CI chondrite-normalized abundance of In in the BSE is similar to zinc and is 3–4 times higher than Cd. New and published low- and high-P–T metal–silicate partitioning data indicate that, during core formation at a range of conditions, In is always more siderophile than Zn and Cd. Adding the fraction of these elements in Earth's core to the BSE results in bulk Earth compositions that yield higher CI chondrite normalized abundances of In in the bulk Earth compared to Zn and Cd. Because In is more volatile than Zn and Cd in gas of solar composition, suprachondritic In/Zn and In/Cd in the bulk Earth suggest that during formation of Earth or its building materials, the volatilities of these elements and perhaps other volatile elements likely have changed significantly (i.e. In became less volatile). The results also suggest that known carbonaceous chondrites likely did not deliver the main volatile element-rich fraction of the Earth. Various arguments suggest that the loss of moderately volatile elements during planetary accretion should be limited, thus, their abundances in the bulk Earth likely reflect the average composition of Earth's building materials. Combined with evidence from nucleosynthetic isotope anomalies, the data suggest that Earth's main building materials originated from compartments of the inner solar system where volatile element abundances evolved differently from the formation area of known chondrites. The materials with nonchondritic volatile element composition may have been used up for building the terrestrial planets.

Lithium elemental and isotopic variations in rock-melt interaction

Despite the occurrence of highly variable lithium (Li) elemental distribution and isotopic fractionation in mantle mineral, the mechanism of Li heterogeneity and fractionation remains a controversial issue. We measured Li contents and isotopic compositions of olivine and clinopyroxene xenocrysts and phenocrysts from kamafugite host lavas, as well as minerals in melt pockets occurring as metasomatic products in peridotite xenoliths from the Western Qinling, central China. The olivine xenocrysts in the kamafugites show compositional zonation. The cores have high Mg# (100×Mg/(Mg+Fe); 91.0–92.2) and Li abundances (5.63–21.7ppm), low CaO contents (≤0.12wt%) and low δ7Li values (−39.6 to −6.76‰), which overlap with the compositional ranges of the olivines in the melt pockets as well as those in peridotite xenoliths. The rims of the olivine xenocrysts display relatively low Mg# (85.9–88.2), high CaO contents (0.19–0.38wt%) and high δ7Li values (18.3–26.9‰), which are comparable to the olivine phenocrysts (Mg#: 86.4–87.1; CaO: 0.20–0.28wt%; Li: 12.4–36.8ppm; δ7Li: 18.1–26.0‰) and the silicate-melt metasomatized olivines. The clinopyroxene phenocrysts and clinopyroxenes in the melt pockets have no distinct characteristics with respect to the Li abundances and δ7Li values, but show higher and lower CaO contents, respectively, than the clinopyroxenes from silicate and carbonatite metasomatized samples. These features indicate that Li concentration and isotopic signatures of the cores of the xenocrysts recorded carbonatite melt-peridotite reaction (carbonatite metasomatism) at mantle depth, and the variations in the rims probably resulted from xenocryst–host magma interaction during ascent. Our results reveal that the interaction with carbonatite and silicate melts gave rise to an increase in Li abundance in minerals of peridotite xenoliths at mantle depth or during transportation. In terms of δ7Li, the carbonatite and silicate melts produced remarkably contrasting δ7Li variations in olivine. Based on the systematic variations of Li abundances and Li isotopes in olivines, we suggest that the δ7Li value of olivine is a more important indicator than that of clinopyroxene in discriminating carbonatite and silicate melt interaction agents with peridotites.

Partial melting control of water contents in the Cenozoic lithospheric mantle of the Cathaysia block of South China

Major and trace element and the H2O contents of minerals in peridotite xenoliths hosted by the Cenozoic basalts of Jiande in the Cathaysia block were evaluated using electron microprobe, laser-ablation ICP-MS and Fourier transform infrared spectroscopy, respectively. The correlations among the major elements of the minerals define a melting trend, and modeling of the Y and Yb contents in the clinopyroxenes indicates that the degree of partial melting ranges from 1% to 15%. Most samples (22 out of 29) show depleted chondrite-normalized rare earth element patterns and a degree of partial melting <4%. The H2O contents (weight in ppm) of the clinopyroxenes, orthopyroxenes and olivines are 390–590ppm, 160–330ppm and ∼0ppm, respectively. Although potential H-loss during xenolith ascent cannot be excluded for olivines, pyroxenes largely preserve the H2O content they have in the mantle prior to sampling by the host basalts, as inferred from (1) the homogenous H2O content within single pyroxene grains, and (2) the equilibrium H2O partitioning between the clinopyroxene and orthopyroxene. Based on the mineral modes and assuming a partition coefficient of 10 for H2O between the clinopyroxene and olivine, the calculated whole-rock H2O contents range from 90 to 220ppm, similar to that of the MORB source. When combined with previously reported data for peridotites hosted by Cenozoic basalts at other localities of the Cathaysia block, the correlation of H2O content with melting index (such as Yb content in cpx, Cr# in spinel) suggests that extent of partial melting is the main factor controlling H2O abundance in these rocks. Nevertheless, the variations in whole-rock H2O contents cannot be perfectly modeled as a simple modal melting process using available experimental partition coefficients of H2O between peridotite and melt. The lack of coherent variations between the H2O contents of the whole rocks and the metasomatic index (La/Yb ratio in clinopyroxene) indicates that mantle metasomatism did not modify the initial H2O contents after the melting event(s). Based on the similarities in major and trace elements, and of H2O contents in the Cathaysia block peridotites and those inferred for the MORB source, we propose that the Cenozoic lithospheric mantle of the Cathaysia block is accreted from the upwelled and cooled asthenospheric mantle. In addition, the H2O contents of the Cenozoic lithospheric mantle of the Cathaysia block are much higher than those of the North China Craton of similar fertility (100ppm vs. 20ppm for whole rock H2O contents), where lithospheric thinning occurred during the Mesozoic. This implies that either the Cathaysia block did not undergo similar lithospheric thinning or that the mechanism of the lithospheric thinning was different.

In-situ trace elements and Li and Sr isotopes in peridotite xenoliths from Kuandian, North China Craton: Insights into Pacific slab subduction-related mantle modification

The trace element, Li and Sr isotopic compositions of major minerals in peridotite xenoliths from Kuandian, North China Craton (NCC) were analyzed in situ to investigate Pacific slab subduction-related mantle modification beneath the eastern NCC. The 87Sr/86Sr ratios (0.70282–0.70553) are positively correlated with the La/Nb and U/Nb ratios for clinopyroxenes (Cpx). Based on the trace element distribution patterns, three types of Cpx were identified. Type 1 Cpx are characterized by significant Nb–Ta–Ti depletions and the highest Sr isotopic ratios (up to 0.70553); Type 2 Cpx display Nb (or Ta) depletions and highly variable Nb and Ta fractionation; and Type 3 Cpx show no significant Nb or Ta depletion or even weak Ta enrichment and have the lowest Sr isotopic ratios (0.70282–0.70323). Some Cpx display a pattern of increasing Ca+Mg±Si from the cores to the rims, indicating addition of Ca+Mg±Si-rich fluids derived from a serpentinized peridotite layer above the subducting slab. These features indicate an ancient mantle metasomatic event associated with the subduction of the altered Pacific oceanic crust, which may contribute substantially to the destruction of the eastern NCC.The variations in the Li contents and isotopic compositions of major minerals in the Kuandian peridotite xenoliths encode both the melt/fluid metasomatism and the cooling process. The Li contents in the Cpx (up to 34.8ppm) and orthopyroxenes (Opx) (up to 28.0ppm) are typically higher than those in the coexisting olivines (Ol) (<9.19ppm), suggesting silicate melt metasomatism. Considering the rapid diffusion rate of Li in minerals at mantle temperature, the Li enrichment and low δ7Li values in the rims of most Ol, Cpx and Opx may have arisen from diffusive fractionation due to faster diffusion of 6Li compared to 7Li during a latest Li-rich melt/fluid metasomatic process shortly before the host magma eruption. On the other hand, a few Ol grains show decreased Li contents and higher δ7Li values from the cores to the rims. This could imply Li isotope fractionation caused by cooling-induced Li redistribution during and/or after the host magma eruption, because DLiOl/Cpx decreases significantly with decreasing temperature and 6Li diffuses faster than 7Li.

Multistage melt/fluid-peridotite interactions in the refertilized lithospheric mantle beneath the North China Craton: constraints from the Li–Sr–Nd isotopic disequilibrium between minerals of peridotite xenoliths

Elemental and Li-Sr-Nd isotopic data of minerals in spinel peridotites hosted by Cenozoic basalts allow us to refine the existing models for Li isotopic frac- tionation in mantle peridotites and constrain the melt/fluid- peridotite interaction in the lithospheric mantle beneath the North China Craton. Highly elevated Li concentrations in cpx (up to 24 ppm) relative to coexisting opx and olivine (\4 ppm) indicate that the peridotites experienced metasomatism by mafic silicate melts and/or fluids. The mineral d 7 Li vary greatly, with olivine (?0.7 to ?5.4%) being isotopically heavier than coexisting opx (-4.4 to -25.9%) and cpx (-3.3 to -21.4%) in most samples. The d 7 Li in pyroxenes are considerably lower than the normal mantle values and show negative correlation with their Li abundances, likely due to recent Li ingress attended by diffusive fractionation of Li isotopes. Two exceptional samples have olivine d 7 Li of -3.0 and -7.9%, indicating the existence of low d 7 Li domains in the mantle, which could be transient and gener- ated by meter-scale diffusion of Li during melt/fluid-peri- dotite interaction. The 143 Nd/ 144 Nd (0.5123-0.5139) and 87 Sr/ 86 Sr (0.7018-0.7062) in the pyroxenes also show a large variation, in which the cpx are apparently lower in 87 Sr/ 86 Sr and slightly higher in 143 Nd/ 144 Nd than coexisting opx, implying an intermineral Sr-Nd isotopic disequilib- rium. This is observed more apparently in peridotites having low 87 Sr/ 86 Sr and high 143 Nd/ 144 Nd ratios than in those with high 87 Sr/ 86 Sr and low 143 Nd/ 144 Nd, suggesting that a rela- tively recent interaction existed between an ancient me- tasomatized lithospheric mantle and asthenospheric melt, which transformed the refractory peridotites with highly radiogenic Sr and unradiogenic Nd isotopic compositions to the fertile lherzolites with unradiogenic Sr and radiogenic Nd isotopic compositions. Therefore, we argue that the litho- spheric mantle represented by the peridotites has been het- erogeneously refertilized by multistage melt/fluid-peridotite interactions.

Li, Be, and B abundances in minerals of peridotite xenoliths from Marsabit (Kenya): Disequilibrium processes and implications for subduction zone signatures

The light elements Li, Be, and B have been analyzed in situ in minerals from three groups of peridotite xenoliths hosted in Quaternary basanites from the Marsabit volcanic field (northern Kenya). Group I and II are fertile lherzolites that experienced deformation, decompression, and cooling in the context of Mesozoic rifting (Group I), followed by heating, static recrystallization, and associated cryptic metasomatism (Group II) as a result of Tertiary‐Quaternary rifting and magmatism. Group III xenoliths are spinel harzburgites and dunites that experienced strong cryptic and modal metasomatism. The Li‐Be‐B systematics in minerals of Group I and II are similar to unmetasomatized subcontinental lithospheric mantle. In contrast, Group III samples are characterized by significant enrichment in all light elements and disequilibrium partitioning between different phases. Light element concentrations levels are similar to that expected for mantle rocks metasomatized by melts and fluids released from subducting slabs, while light element/rare earth element ratios (especially Li/Yb) approach those of typical Island Arc basalts. However, detailed investigation of textures and chemical zoning shows that at least Li concentrations in primary minerals were modified (i.e., decoupled from Yb) during late‐stage melting and/or fluid percolation related to Tertiary‐Quaternary alkaline magmatism in Marsabit (formation of melt pockets consisting of silicate glass, clinopyroxene, olivine, and chromite), ultimately followed by xenolith entrapment and transport to the surface. Mass balance calculations show that the melt pockets formed at the expense of earlier metasomatic phases. During this process the melt pockets mostly preserved the B, Be, and rare earth element budget of the precursor phase assemblage, whereas Li was added. Elevated B/Be and low Ce/B of metasomatic phases prior to late melting could result from metasomatism by a slab fluid. However, similar characteristics are expected for evolved Si‐ and CO2‐rich fluids derived from basanite melt‐peridotite interaction, not related to any subduction zone process. The results of this study imply that the inference of a “slab signature” exclusively based on trace element data of metasomatized peridotite is ambiguous.

Lithosphere formation in the central Slave Craton (Canada): plume subcretion or lithosphere accretion?

Major-element compositions of minerals in peridotite xenoliths from the Lac de Gras kimberlites provide constraints on the mode of lithosphere formation beneath the central Slave Craton, Canada. Magnesia contents of reconstructed whole rocks correlate positively with NiO and negatively with CaO contents, consistent with variable partial melt extraction. Alumina and Cr2O3 contents are broadly positively correlated, suggestive of melt depletion in the absence of a Cr–Al phase. Garnet modes are high at a given Al2O3 content (a proxy for melt depletion), falling about a 7 GPa melt depletion model. These observations, combined with high olivine Mg# and major-element relationships of FeO-poor peridotites ( 3 GPa (residual FeO content being a sensitive indicator of melt extraction pressure), and similar high pressures of last equilibration (∼4.2 to 5.8 GPa), provide multiple lines of evidence that the mantle beneath the central Slave Craton has originated as a residue from high-pressure melting, possibly during plume subcretion. Apparent low melt depletion pressures for high-FeO peridotites (>7.5 wt%) could suggest formation in an oceanic setting, followed by subduction to their depth of entrainment. However, these rocks, which are characterised by low SiO2 contents (<43 wt%), are more likely to be the result of post-melting FeO-addition, leading to spuriously low estimates of melt extraction pressures. They may have reacted with a silica-undersaturated melt that dissolved orthopyroxene, or experienced olivine injection by crystallising melts. A secular FeO-enrichment of parts of the deep mantle lithosphere is supported by lower average Mg# in xenolithic olivine (91.7) compared to olivine inclusions in diamond (92.6).

Geochemistry and origin of sulfide minerals in peridotite xenoliths from the North Island, New Zealand

Geochemistry and origin of sulfide minerals in peridotite xenoliths from the North Island, New Zealand

The MARID (mica-amphibole-rutile-ilmenite-diopside) suite of xenoliths in kimberlite

Within the ‘glimmerite’ nodules occurring within kimberlite pipes we recognize the MARID suite consisting of varying proportions of mica, amphibole, rutile, ilmenite and diopside. Banding of some specimens is interpreted as cumulate layering. All specimens were deformed either before incorporation into the host kimberlite or during intrusion. Compared with minerals in peridotite xenoliths, the MARID ones are lower in Al 2O 3 and Cr 2O 3, but richer in total iron. The MARID micas, amphiboles, diopsides, ilmenites and probably rutiles contain substantial Fe 2O 3 indicative of oxidizing conditions. The amphibole is potassic richterite. Micas of the megacryst suite in kimberlite have less total iron and Fe 2O 3 than micas of the MARID suite. We suggest that the rocks of the MARID suite crystallized under oxidizing conditions from a magma, chemically similar to kimberlite, within the higher parts of the upper mantle: the presence of amphibole restricts the depth to less than ~ 100 km. A xenolith containing olivine and orthopyroxene as well as minerals similar to but not the same compositionally as MARID-types is interpreted as a metasomite, possibly representing wall-rock of a magma body from which MARID-suite rocks crystallized.