Silica-rich Melts Research Articles

Experimental melting of phlogopite websterite in the upper mantle between 1.5 and 4.5 GPa

Abstract Reaction experiments have confirmed that phlogopite websterite can be formed by interaction of peridotite with hydrous alkaline or silica-rich melts. Phlogopite websterites commonly occur as xenoliths in orogenic and intraplate volcanism but do not receive much attention. We have experimentally investigated the melting behaviour of a phlogopite websterite at 1.5 GPa (1050–1300 °C), 3.0 GPa (1100–1500 °C), and 4.5 GPa (1200–1500 °C) to contribute to understanding the sources of ultrapotassic rocks that occur in different settings. The solidus temperature rises with increasing pressure, bracketed between 1050 and 1100 °C at 1.5 GPa, 1100 and 1150 °C at 3.0 GPa and between 1200 and 1250 °C at 4.5 GPa. At 1.5 GPa, phlogopite websterite melts incongruently to form olivine and melt, whereas orthopyroxene, garnet and melt are formed at 3.0 and 4.5 GPa. The transition of orthopyroxene from reactant to product with increasing pressure results in changes in the SiO2-content of melts. The experimental melts reach a maximum K2O content when phlogopite is consumed completely at temperatures ~150 °C above the solidus. The melting reactions are similar to those of phlogopite lherzolite, but the low-Al2O3 starting materials result in lower Al2O3 in the melt than in melts of phlogopite lherzolite. Comparison with natural ultrapotassic rock compositions reveals that the sources of ultrapotassic rocks in convergent settings may be dominated by phlogopite websterite, phlogopite lherzolite and phlogopite harzburgite. Sources of ultrapotassic rocks in intraplate settings are more likely to include phlogopite clinopyroxenite ± CO2 and K-richterite. In all melting experiments on phlogopite-bearing rocks, K2O from phlogopite passes into the melt, and hence the highest K2O contents in ultrapotassic rocks must be an indication of the minimum stoichiometric coefficient of phlogopite in the reaction. In cases where phlogopite websterite or phlogopite lherzolite is identified as the source, the minimum modal percentage of phlogopite in the source can be inferred from the highest K2O content. When applied to the Milk River minettes and New South Wales leucitites, the estimated modal proportion of phlogopite in the sources is greater than 20 wt.%. Phlogopite can survive the subduction process and melt later in the post-collisional environment, whereas thermal perturbations are necessary to trigger melting of phlogopite-bearing assemblages at the base of the lithosphere in intraplate settings.

A redox effect on the viscosity of molten pyrolite

The viscosity of molten Earth mantle has been determined for various redox states using very high temperature viscometry at 1 bar. The viscosity-temperature relationship of the most oxidised pyrolite studied here is comparable to that of a previous determination of the viscosity of a peridotite melt. For the first time, the effect of iron redox state on molten mantle viscosity has been determined by calorimetric analysis of the glass transition on extremely carefully characterised glassy samples quenched from conditions of variable fO2 using gas-mixing levitation. There is a clear trend of decreasing viscosity with increasing redox state over the range of Fe3+/ΣFe. = 0.07–0.29. We also observe an increase in glass transition temperatures with increasing melt depolymerisation (i.e., higher NBO/T ratios). Both the negative oxidation dependence of viscosity and glass transition behaviour upon depolymerisation are contradictory to the trends observed from redox viscometry of more silica-rich melts but may be consistent with the broadly diminishing positive redox viscometry trends of melts with increasing basicity.

Silica-rich melts originating from melt-hydrated peridotite reactions

Interactions between melts and hydrated peridotite may control production of silica-rich melts, fluids and magmas in the upper mantle and may be predominant mechanism for producing felsic crust in shallow ultramafic settings of rocky planets. This research investigated composition of olivine-hosted mineral, melt and multi-phase inclusions in dunite xenoliths from the 1800–1801 Kaupulehu flow of Hualalai Volcano. The host olivine (forsterite Fo 82–89 , mol. %) appears with mineral inclusions of crystalline chromite having compositions similar to those found in alkaline series rocks. Abundant silica-rich glasses (SiO 2 = 63 ± 2.9 wt%, Al 2 O 3 = 21 ± 1.2 wt%; K 2 O = 4.3 ± 0.5 wt%, Na 2 O = 3.6 ± 0.7 wt%, 38 analyses) found in the olivine-hosted melt and multiphase inclusions are interpreted as quenched silicate liquids entrapped due to nephelinite melt percolation in hydrated harzburgite in the presence of free aqueous fluid at pressures of 0.7 to 0.1 GPa and a melt-to-peridotite mass ratio of 0.2 to 0.3. This work also re-interprets the metasomatic origin of global examples of silica-rich glass inclusions in peridotite xenolith minerals according to results from recent experiments on melt-hydrated peridotite interaction processes. As a planetary process, production of silica-rich melts and fluids within series of associated magmas and their crystallized differentiates can occur due to reaction of primitive mafic (e.g., ocean island basalt, OIB) melts with hydrated peridotite (or serpentinite) in the presence of aqueous fluids at pressures below 1 GPa. • Si-rich glasses are produced due to reaction of mafic melts with hydrated peridotite.

Deciphering metasomatic events beneath Mindszentkálla (Bakony-Balaton Highland Volcanic Field, western Pannonian Basin) revealed by single-lithology and composite upper mantle xenoliths

Single-lithology and composite xenoliths from Mindszentkálla (Bakony-Balaton Highland Volcanic Field) in the Carpathian-Pannonian region record geochemical evolution of the subcontinental lithospheric mantle. The dominant single-lithology xenoliths are orthopyroxene-rich (22 vol% on average) harzburgites. Three composite xenoliths contain either two or more domains including dunite, olivine-orthopyroxenite, orthopyroxenite, apatite-bearing websterite and amphibole-phlogopite-bearing vein. The presence of different lithologies is a result of at least two metasomatic events that affected the lithospheric mantle. The first event resulted in orthopyroxene enrichment thus formed harzburgitic mantle volumes (Group I xenoliths). Major- and trace element distributions of the bulk harzburgites differ from the geochemical trends expected in residues of mantle melting. In contrast, petrographic and geochemical attributes suggest that the harzburgite was formed by silica-rich melt - peridotitic wall rock interactions in a supra-subduction zone. Within the Group I xenoliths, two subgroups were identified based on the presence or lack of enrichment in U, Pb and Sr. Since these elements are fluid mobile, their enrichment in certain Group I xenoliths indicate reaction with a subduction-related fluid, subsequent to the harzburgite formation. The effect of a second event overprints the features of the Group I xenoliths and is evidenced in all domains of two composite xenoliths (Group II xenoliths). The general geochemical character involves enrichment of basaltic major and minor elements (Fe, Mn, Ti, Ca) in the rock-forming minerals and convex-upward rare earth element (REE) patterns in clinopyroxenes. We suggest that the different domains represent reaction products with variably evolved basaltic melts of a single magmatic event. The tectonic background to the formation of Group I xenoliths is likely linked to the subduction of oceanic crust during the Mesozoic–Paleogene. This happened far from the current position of Mindszentkálla, to where the lithosphere, including the metasomatized mantle volume, was transferred via plate extrusion. The Group II xenoliths appear to bear the geochemical signature of a younger (Neogene) basaltic magmatic event, likely the same that produced the host basalt transporting the xenoliths to the surface.

Decoupled Trace Element and Isotope Compositions Recorded in Orthopyroxene and Clinopyroxene in Composite Pyroxenite Veins from the Xiugugabu Ophiolite (SW Tibet)

Abstract Pyroxenite veins and dikes are commonly observed in the mantle section of ophiolites. Because of their mantle occurrence, these pyroxenites are free from crustal contamination and offer a unique opportunity for studying mantle compositions and melt–rock interaction processes. We conducted an integrated petrological and geochemical study of a suite of composite orthopyroxenite, websterite, and pyroxene-bearing dunite veins from the Xiugugabu ophiolite located on the western segment of Yarlung–Zangbo Suture Zone. The dunite is separated from the host peridotite by a layer of pyroxenite, forming a composite vein system. Systematic variations in major, minor, and trace element compositions in minerals across the composite veins are observed. Two generations of orthopyroxenes in the pyroxenites are characterized by high Mg#, low TiO2 concentrations, and depleted patterns of incompatible trace elements. Clinopyroxenes in the pyroxenites are characterized by high Mg#, low contents of TiO2 and Na2O, spooned shaped REE patterns, and a negative Zr anomaly. Through major and trace element modeling, we showed that both orthopyroxene and clinopyroxene were in equilibrium with melts with different compositions. This hypothesis is further confirmed by distinct initial Nd and Hf isotope ratios in the two pyroxenes. A model for the formation of composite pyroxenite veins is developed, whereby hydrous and silica-rich melts percolate along the margins of a dunite channel. The orthopyroxenite was formed by the reaction between a hydrous, silica-rich melt and the surrounding peridotite. The websterite is formed by reactive crystallization of a hybrid melt produced by mixing silica-rich melt and the melt formed by remelting of previously depleted peridotite in the deeper part of the mantle column. The extremely enriched Nd–Hf isotope compositions of the pyroxenite veins (εNd = −20.3 to +11.5 and εHf = −13.2 to +25.3, 125 million years ago) can be explained by the addition of ancient, recycled sediments to the mantle source in a supra-subduction setting. Based on the low-Cr# spinel in the Xiugugabu dunites (Cr# = 19–50) and the depleted nature of the parental melt of the Xiugugabu pyroxenites, we deduced that the formation of pyroxenites postdate the formation of the Xiugugabu ophiolite at ~125–130 Ma. Collectively, results from this study have provided support to the hypothesis that the Xiugugabu ophiolite experience a two-stage evolution, i.e., firstly formed in a mid-ocean ridge setting and subsequently modified in a supra subduction zone.

Transformation of the Sub-Continental Lithospheric Mantle Beneath the North China Craton (NCC): Constraints from the Geochemical Characteristics of Olivine Websterite Xenoliths and Their Minerals in the Cenozoic Basalts from Hannuoba

The sub-continental mantle beneath North China Craton (NCC) has attracted extensive attention in the past decades because of its dramatic transformation from an old, cold, thick, and refractory mantle to a juvenile, hot, thinner, and fertile mantle. However, the transformation mechanism remains largely controversial. The mantle xenoliths entrapped in basalts, as petrogenetic indicators, can provide an important window to reveal the evolution of the sub-continental lithospheric mantle. In this study, we present a systematical study on the geochemical characteristics of the olivine websterite xenoliths and their minerals in the Cenozoic basalts in the Hannuoba region located at the central orogenic belt of the NCC. The results, compared with the geochemical data of Paleozoic and Meosozic peridotites, the Cenozoic composite pyroxenites as well as the global cumulate pyroxenites, demonstrate that: (1) The source of the websterite is probably the lithospheric mantle, which is mainly newly accreted, but with small amounts of ancient mantle residues. The source may be contaminated by different degrees of crustal materials. The high Nb/Ta ratios (11.36–20.57) of the websterite indicate that the Ti-bearing minerals (such as rutile) are probably involved in the source; (2) The websterite is more likely to be produced as a result of interaction of mantle peridotite with the silica-rich melts that are mainly derived from the asthenospheric mantle and also contributed by the crustal materials; (3) The metasomatic crustal melts might be derived from the subducted Paleo-Pacific plate. These melts interacted with the lithospheric mantle can significantly transform the chemical composition of the lithospheric mantle, and consequently play an important role in the destruction of the NCC. An important implication for the destruction of the NCC is further discussed.

Chemical and Structural Alterations in the Amorphous Structure of Obsidian due to Nanolites.

Obsidian is volcanic glass that results from the rapid cooling of silica-rich melt. Nanoscale crystallites precipitate out of the melt prior to solidification and remain embedded in the amorphous matrix. These crystallites provide information on the flow kinetics and composition of the melt. Due to the sparsity and size of nanolites, studies often focus on supramicron crystallites. This research takes advantage of the conchoidal fracture of obsidian by knapping samples with nanometer-thin edges for transmission electron microscopy characterization. Nanolites in the amorphous matrix are studied using energy-dispersive spectroscopy (EDS) and electron diffraction. Certain alkali and alkaline-earth cations exhibit patterns of depletion near Fe-oxide nanolites. EDS is used to identify nanolites and variations in the composition of the matrix. Parallel beam diffraction and radial distribution function analysis of nearest-neighbor distances determine average bond lengths in the matrix near nanolites, showing that nanolites influence the nearby short-range ordering and atomic character of the matrix. Analysis reveals decreased mean nearest-neighbor distances in the matrix adjacent to nanolites compared to the bulk. Our methods exhibit the required sensitivity to detect variations in the composition and structure near nanolites, and our findings indicate that obsidian nanolites contribute to quantifiable localized changes in the amorphous structure.

Crustal melting and suprasolidus phase equilibria: From first principles to the state-of-the-art

Partial melting is the fundamental process by which juvenile crust was produced from the mantle and subsequently reworked to become the stable, compositionally-differentiated continents on which we live and which host most of the elemental resources required by our modern technological society. Irreversible differentiation of the continental crust occurs principally through the production, segregation and migration of silica-rich (felsic) melts from deeper source rocks to shallower sinks where they erupt or, more commonly, crystallise as granite sensu lato. Here we provide for both novices and professionals a comprehensive but accessible account of the processes involved in crustal melting and suprasolidus phase equilibria from first principles to the forefront of modern research. To begin, we introduce the tectono-metamorphic context for crustal melting before considering the evidence at outcrop and in thin section for partial melting, then briefly review melt extraction from crustal rocks. As a prerequisite to understanding the physicochemical basis for crustal melting, we summarize the essential thermodynamics that underpin calculated phase equilibria, distinguish different types of melting reaction, and review the requirements for, methodology behind, and limitations of a phase equilibrium modelling approach based on equilibrium thermodynamics. We explain the various types of phase diagram used to investigate partial melting and assess open versus closed system processes, including internal and external buffering of H2O. Those crustal sources that partially melt to produce granite are considered in detail, namely basic rocks such as basalts and gabbros, clastic sedimentary rocks such as greywackes, siltstones and mudstones (pelites), and granites themselves. We concentrate mainly on intracrustal partial melting in convergent-margin settings, and on anatexis during exhumation of deeply-subducted continental crust from mantle depths. We discuss the behaviour of trace elements and accessory minerals during melting, and consider the implications for isotope geochemistry. To close we include a brief summary of some of the important points and offer a few suggestions for future lines of research on crustal melting.

Kinetic partitioning of major-minor cations between olivine and Hawaiian tholeiitic basalt under variable undercooling and cooling rate conditions

In order to elucidate the kinetic partitioning of cations between olivine and basalt, we performed undercooling (−ΔT) and cooling rate (CR) experiments at atmospheric pressure and QFM-2 buffer. Starting from the superliquidus temperature of 1250 °C, a Hawaiian tholeiitic basalt was cooled at the rates of 4 (CR4), 20 (CR20), and 60 (CR60) °C/h to the final target temperatures of 1175 °C (−ΔT = 35 °C; −ΔT35) and 1125 °C (−ΔT = 85 °C; −ΔT85). Results show that polyhedral olivine morphologies are obtained at -ΔT35, whereas strong disequilibrium skeletal and/or dendritic textures form at -ΔT85. The amount of forsterite in olivine decreases from to 85% to 78% with increasing both -ΔT and CR. A diffusive boundary layer also develops in the melt next to the olivine surface and its composition becomes progressively enriched in Ca, owing to its incompatible behavior with the lattice site. Residual melts are progressively depleted in silica and enriched in alkali from CR4 to CR60, but silica-rich melts are observed with increasing -ΔT. In terms of Fe2+-Mg exchange, olivines obtained at -ΔT35 are always in equilibrium with the diffusive boundary layer, comprising both the interface melt next to the olivine surface and the far-field melt where all chemical gradients cease. At -ΔT85, however, the Fe2+-Mg exchange indicates two distinct equilibration stages between olivine core and far-field melt, and between olivine rim and interface melt. Partition coefficients (Kd) of Mg, Fe, Mn, Ca, and Cr calculated at the olivine-melt interface preferentially change as a function of -ΔT rather than CR. From -ΔT35 to -ΔT85, KdMg, KdFe, KdMn, and KdCr remarkably increase, whereas the opposite applies to KdCa. Through the application of equilibrium partitioning models, we found that Mg, Fe, Mn, and Ca are incorporated into the olivine lattice site at near-equilibrium proportions. This generally good agreement with modeling data demonstrates that diffusive mass transport of cations in our experiments occurred under the conditions of local equilibrium at the olivine surface. In contrast, marked deviations from the expected equilibrium are found for KCr in response to the major influence of crystal field stabilization energy on cation incorporation.

Experiment constraints on orthopyroxene enrichment in the Kaapvaal craton lithospheric mantle

Peridotite xenoliths from the Kaapvaal cratonic lithosphere are characterized by high modal orthopyroxene (Opx), which is inconsistent with the mineralogy of residues of partial melting of primitive mantle. The high orthopyroxene content has been explained by the reaction between Opx-poor peridotite and silica-rich melts. To assess and further constrain this model, we conducted a series of experiments on the interaction of Si-rich melt with Opx-poor harzburgite at 1.5 GPa and 1100 to 1250 °C. Orthopyroxene is enriched in all experiments. In the low temperature run at 1100 °C, melt is consumed and phlogopite is precipitated with the generation of orthopyroxene. Mg# (atomic 100*Mg/(Mg+Fe)) values in olivines also exhibit a decrease compared with the starting olivines. At temperatures of ≥1200 °C, phlogopite is absent as the temperatures are higher than its upper stability field. The residues are composed of high modal orthopyroxene and high Mg# olivine. Only the residues in the high temperature runs are consistent with Kaapvaal cratonic mantle xenoliths. Combined our experiments with the distribution, geochronological and geochemical data of the xenoliths, we propose a two-stage ‘metasomatism-depletion’ model to explain the generation of Opx-rich xenoliths in Kaapvaal craton. In the early Archean, high-degree melting of primitive mantle produced a highly depleted lithospheric mantle, which was Opx-poor. Subsequently, Si-rich melts/fluids were released from the subducting oceanic slabs to interact with the Opx-poor mantle, generating an Opx-rich, but hydrous and fertile mantle. The metasomatic mantle is similar to the result of the run at 1100 °C. In the late Archean, the metasomatic mantle was re-melted in post-collisional extensional environments, which produced voluminous lavas with residues of highly depleted, Opx-rich peridotites. The Opx-rich residual peridotites are consistent with the runs at ≥1200 °C. Thus, Opx enrichment in the Kaapvaal craton lithospheric mantle can be well explained by the two-stage ‘metasomatism-depletion’ model.

Composition gradients in silicate inclusions in chromites from the dunitic mantle-crust transition (Oman ophiolite) reveal high temperature fluid- melt-rock interaction controlled by faulting

The transition between the mantle section and the oceanic crust in the Maqsad area (Oman ophiolite) is mainly made of variably impregnated dunites locally associated with chromitite ore bodies. There, the dunitic transition zone (DTZ) developed above a mantle diapir that fed with MORB the former oceanic spreading centre. However, orthopyroxene and amphibole impregnations in dunites from the DTZ are witnesses of a hydrated magmatism that looks restricted to this interface. The main other piece of evidence is the nature of silicate minerals included in chromite grains scattered in dunite (e.g., amphibole, orthopyroxene, mica), which are mostly issued from a hydrated and silica-rich melt or fluid. Here, we report on a study of such inclusions along a section sampled in detail in the Maqsad DTZ. It brings critical information on the processes involved in the fluid-melt-peridotite reaction below oceanic spreading centres, complementary to the one provided by the interstitial silicates forming the matrix of the dunite. We first show that both the nature and the composition of the inclusions are well-correlated to those of the impregnations in the host dunites, then that the chemical evolution along the cross-section for all materials correlate to the presence of faults that developed at an early, syn-magmatic stage. This confirms that the early tectonics in the deep oceanic lithosphere primarily controls the fluid-melt-rock reactions and can condition chemical cycling, including for halogens (Cl, F), in oceanic spreading centre setting

Theoretical Models of Decompression-Induced Plagioclase Nucleation and Growth in Hydrated Silica-Rich Melts

Microlites crystallization changes the flow properties of ascending magmas, with major implications on eruption dynamics. In order to predict microlite number density and size, we present a theoretical modelling of plagioclase nucleation and growth in rhyolitic melts, aimed at reproducing the decompression-induced isothermal crystallization experiments of Mollard et al. (2012). Thus, the modelling is valid for plagioclase crystallization in haplotonalitic/rhyolitic melts decompressed at 875 °C from 200 MPa to final pressures of 50, 75, or 100 MPa, which represents effective undercooling (Teff) of 110, 80 and 55 °C, respectively. The nucleation-rate calculation using the Classical Nucleation Theory (CNT; case of homogeneous nucleation) and values of crystal-melt interfacial tensions () either from literature or empirically calculated strongly disagree with the experimental nucleation rates. Inverting the CNT calculation by using the experimentally-determined nucleation rates suggests plagioclase-liquid from 0.041 to 0.059 J.m-2 with Teff increasing from 55 to 110 °C, which is about 2-3 times lower than  determined empirically and macroscopically. Further refining  by considering a non-smooth and non-spherical plagioclase nucleus leads to an elongated and rough morphology with a shape parameter () of 500-1350 and a rugosity parameter (ds) of 2.3. The experimental plagioclase growth rates have been modelled by atom diffusion in melt, following Fick’s second law adapted to multicomponent systems. The component with the smallest flux (i.e. limiting growth) is calcium, as determined from the analytical profiles of the experimental glasses. The crystal-growth model considers a crystal-melt interface advancing over time and a chemically-closed finite reservoir. The overall good agreement between the model and the experimental growth laws validates diffusion as the main process controlling isothermal decompression-induced plagioclase growth under moderate Teff <110 °C. The calculated CaO diffusion coefficients in silicic melt at 875 °C and water saturation pressures from 50 to 100 MPa ranges from 10-14 to 10-15 m2/s. The simulations may be relevant to predict nucleation and growth of plagioclase microlites in phenocryst-poor rhyolitic magmas that rapidly ascend from a storage zone and crystallize at shallower. This may be relevant to eruption dynamics such as dome-related blasts.

40Ar/39Ar systematics of melt lithologies and target rocks from the Gow Lake impact structure, Canada

The age of the Gow Lake impact structure (Saskatchewan, Canada) is poorly constrained, with previous estimates ranging from 100 to 250 Ma. Using a combination of step-heating and UV laser in situ40Ar/39Ar analyses we have sought to understand the 40Ar/39Ar systematics of this small impact crater and obtain a more precise and accurate age. This structure is challenging for 40Ar/39Ar geochronology due to its small size (∼5 km diameter), the silicic composition of the target rock, and the large difference in age between the impact event and the target rock (∼1.2 Ga). These factors can serve to inhibit argon mobility in impact melts, leading to retention of ‘extraneous’ 40Ar and anomalously older measured ages. We mitigated the undesirable effects of extraneous 40Ar retention by analysing small volume aliquots of impact glass using step-heating and even smaller volumes via the UV laser in situ40Ar/39Ar technique. Although primary hydration of impact-generated glasses enhanced the diffusivity of 40Ar inherited from silica-rich melts, data still had to be corrected for extraneous 40Ar by using isotope correlation plots to define the initial trapped 40Ar/36Ar components. Our inverse isochron age of 196.8 ± 9.6/9.9 Ma (2σ, analytical/external precision) demonstrates that the Gow Lake event occurred within uncertainty of the Triassic-Jurassic boundary, but there is no evidence that it was part of an impact cluster.

Evidence for subduction-related signatures in the southern African lithosphere from the N-O isotopic composition of metasomatic mantle minerals

Current understanding of the fate of subducted material (and related fluids) in the deep Earth can be improved by combining major and trace element geochemistry with stable isotopic compositions of mantle rocks or minerals. Limited isotopic fractionation during high temperature processes means that significant deviations from mantle-like isotope ratios in mantle rocks probably result from recycling of surficial material. To determine the effects and origins of mantle metasomatic fluids/melts, new δ15N and δ18O data have been collected for thirteen mantle xenoliths – harzburgites, wehrlites, lherzolites, and MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) rocks – from the Bultfontein kimberlite (Kimberley, South Africa), which show varying degrees of metasomatism. The δ18O values of olivine and orthopyroxene in phlogopite-free harzburgites match the mantle composition (δ18Oolivine = +5.2 ± 0.3‰; δ18Oorthopyroxene = +5.7 ± 0.3‰; 2 s.d.), consistent with previous inferences that harzburgites were formed by interaction with ancient silica-rich melts unrelated to subduction processes. Wehrlite samples display mineral compositional characteristics (e.g., low La/Zr in clinopyroxene) resembling those of other products of kimberlite melt metasomatism, such as PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks. The inferred interaction with kimberlite melts may be responsible for O isotopic disequilibrium between clinopyroxene and olivine (Δ18O = +0.2‰) in the wehrlites of this study. In contrast with broadly mantle-like δ18O values, the δ15N value of phlogopite in a wehrlite sample (+5.9‰) differs from the mantle composition (δ15N = −5 ± 2‰). This unusual N isotopic composition in kimberlite-related mantle products might indicate that a recycled crustal component occurred in the source of the Kimberley kimberlites, or was assimilated during interaction with the lithospheric mantle.Similar major and trace element characteristics in clinopyroxene from phlogopite-lherzolite and MARID samples suggest metasomatism by fluids of similar composition. Lherzolite and MARID clinopyroxene δ18O values (as low as +4.4‰) extend below those reported in mantle peridotites (i.e. δ18Oclinopyroxene = +5.6 ± 0.3‰; 2 s.d.), and strong negative correlations are found between mineral δ18O values and major element compositions (e.g., Na2O contents in clinopyroxene). Furthermore, phlogopite δ15N values (+4 to +7‰) in the studied lherzolite and MARID samples are higher than mantle values. Combined, the low δ18O-high δ15N isotopic signatures of MARID and lherzolite samples suggest progressive mantle metasomatism by a melt containing a recycled oceanic crust (eclogitic) component. This study demonstrates that progressive enrichment of the subcontinental lithospheric mantle may be inextricably linked to plate tectonics via recycling of subducted crustal material into the deep mantle.

Analysis of shock metamorphic processes in the Zagami meteorite

The study of shock-metamorphic features of the Zagami meteorite revealed pseudotachylite-like melt veins with inhomogeneous chemistry and schlieren structure of silica-glass and alkali feldspar melt glass. The feldspar occurs as diaplectic glass in the interstitial area indicating short-time (few seconds) quenching of shock pressure during the impact event, with post-shock annealing. At several locations, apatite needles were identified, which are formed by fluids (cold water with dissolved ions) after the crystallization of cumulate magmatic minerals. Phosphates also could form in impact melts due to circulation of fluids after the impact event. The other signature for the high shock temperature is the presence of Ca–Ti-rich pyroxenes and titanomagnetite, which indicate temperature above 1,200 °C. The formation of silica-rich melt in interstitial area has two scenarios: (a) fractional melting of the Martian crust or (b) formation by pseudotachylite-like impact melting. According to textural observations (schlieren pattern), we propose an impact origin of the large amount of silica-rich melt in this meteorite. Pseudotachylite-like textures were mentioned earlier in terrestrial impact craters; however, we first propose them to form in a Martian meteorite based on their similarity of texture with terrestrial pseudotachylites.

Pentacoordinated and hexacoordinated silicon cations in a potassium silicate glass: Effects of pressure and temperature

Silicon cations with five or six oxygen first neighbors (SiO5 and SiO6, pentacoordinated and hexacoordinated) are known primarily from alkali and alkaline earth silicate glasses melted at pressures up to about 12 GPa, where their formation is thought to be an important part of the densification of the liquid. These species, especially SiO5 groups, have also been proposed as high-energy intermediate structures in the bond-swapping that controls network cation and anion diffusion, and hence viscous transport, in high temperature, silica-rich melts, as well as in low-temperature liquid-state silicate reactions, and have thus been the subject of numerous recent theoretical studies. In spite of this interest, the amount of experimental data on controls of pressure, temperature, and composition on concentrations of penta- and hexacoordinated silicon is very limited. Here we report new results from 29Si MAS NMR on high pressure glasses with the model composition K2Si4O9, which show that because of transient pressure drop during quench, previous studies probably significantly underestimated SiO5 and SiO6 concentrations. In addition, we refine the earlier results on SiO5 even in 1 bar glasses, and demonstrate a strong increase in its concentration at higher fictive temperature. A positive enthalpy for the reaction forming this species is consistent with its role as an energetically important transition complex in melt dynamics.

Anatomy of a chromitite dyke in the mantle/crust transition zone of the Oman ophiolite

The igneous and mechanical processes controlling the formation of nodular chromite ore have been investigated through the study of a chromitite dyke emplaced in the uppermost part of the 330 m-thick dunitic mantle/crust transition zone that developed at the top of a mantle diapir in the Maqsad area of the Oman ophiolite. The dyke is parallel to the paleo-ridge axis, has a vertical extent of about 30 m and an average thickness of 2 m. It presents spectacular variations in ore texture, offering a unique opportunity to identify the zones of nodule nucleation in the upper parts of the dyke, growth in the intermediate parts and accumulation at the bottom.Nodules grew by progressive accretion of euhedral chromite grains, 100–200 μm in size, around a nucleus made essentially of olivine and plagioclase embedded in skeletal chromite. At a critical size of 2 to 3 cm, the nodules, still poorly consolidated, sunk, accumulated and compacted at the bottom of the dyke. The interstitial silicate matrix between the nodules is essentially troctolitic (high Mg (Fo~92) and high Ni (NiO ~0.35 wt%) olivine and calcic (An83 to An85) plagioclase with minor pargasite). At about mid-height, the dyke broadens significantly, reaching a width of 12 m, the center of this bulge being filled with smaller-sized nodules embedded in an anorthositic matrix. This feature is interpreted to represent a magma pocket where the melt and nodule nuclei accumulated before complete crystallization and cooling of the system. The alteration of the silicate matrix is less intense in this bulge than in the rest of the dyke.Silicate inclusions within chromite grains indicate that the parental melt of the chromite was hybrid between two endmembers: a common MORB-like melt and a silica-rich hydrous fluid or a water-rich trondhjemitic melt, possibly produced by low degree melting of hydrothermally altered gabbro and/or serpentinized peridotite from the country rocks. The Ti content in the chromite (average ~0.5 wt% TiO2) from the dyke is significantly higher than that of chromites emplaced at deeper levels in the mantle/crust dunitic transition zone (DTZ) and in the harzburgitic mantle from the Maqsad area. This points to the progressive evolution of the MORB component (product of decompression melting in the diapir) during its ascent from the mantle diapir to the base of the crust. Fractional crystallization occurred in a context of buffering of compatible element concentrations (Mg, Cr, Ni) around elevated, “primitive” values through exchanges between the percolating melt and the host harzburgite and dunite.The chemical composition of both chromite and silicates is constant (i.e. evenly scattered) from the bottom to the top of the dyke and does not mimic the evolution in the ore texture nor in the size and abundance of the nodules. This implies that the parental melt composition remained globally unchanged during the formation of the ore body arguing for open system conditions during nodule formation and accumulation. The only significant evolution is observed in the central bulge where the chromite Ti content is higher (average ~0.7 wt% TiO2 with spikes reaching several percent) confirming that this magma pocket was filled with more evolved melt.No bottom to top evolution in the XCr of chromite is observed within the dyke but individual nodules show a well-developed zoning in XCr ratio from their nucleus (XCr ~58) to their margins (XCr ~48). An increase in the TiO2 content of chromite toward the nodules' edges is not systematic and, when present, is quite moderate. In the largest nodules, inclusions of hydrated silicates are preferentially distributed in a circle, midway between the nodule's nucleus and its edge. This garland of inclusions coincides with a positive peak in the XCr profile, while the TiO2 profile is not affected. This implies that the zoning in XCr cannot be assigned to fractional crystallization alone. It is tentatively explained by a scenario where the nodules crossed a gradient in the proportion of the MORB vs. more reducing hydrous melt during their growth. The gradient could have been maintained by a density contrast between a buoyant hydrous silica-rich melt and denser MORB.

Occurrence of the Iron–rich Melt in the Heijianshan Iron Deposit, Eastern Tianshan, NW China: Insights into the Origin of Volcanic Rock–hosted Iron Deposits

AbstractLong‐standing controversy persists over the presence and role of iron–rich melts in the formation of volcanic rock‐hosted iron deposits. Conjugate iron–rich and silica–rich melt inclusions observed in thin‐sections are considered as direct evidence for the presence of iron‐rich melt, yet unequivocal outcrop‐scale evidence of iron‐rich melts are still lacking in volcanic rock‐hosted iron deposits. Submarine volcanic rock‐hosted iron deposits, which are mainly distributed in the western and eastern Tianshan Mountains in Xinjiang, are important resources of iron ores in China, but it remains unclear whether iron‐rich melts have played a role in the mineralization of such iron ores. In this study, we observed abundant iron‐rich agglomerates in the brecciated andesite lava of the Heijianshan submarine volcanic rock–hosted iron deposit, Eastern Tianshan, China. The iron‐rich agglomerates occur as irregular and angular masses filling fractures of the host brecciated andesite lava. They show concentric potassic alteration with silicification or epidotization rims, indicative of their formation after the wall rocks. The iron‐rich agglomerates have porphyritic and hyalopilitic textures, and locally display chilled margins in the contact zone with the host rocks. These features cannot be explained by hydrothermal replacement of wall rocks (brecciated andesite lava) which is free of vesicle and amygdale, rather they indicate direct crystallization of the iron‐rich agglomerates from iron‐rich melts. We propose that the iron‐rich agglomerates were formed by open‐space filling of volatile‐rich iron‐rich melt in fractures of the brecciated andesite lava. The iron‐rich agglomerates are compositionally similar to the wall‐rock brecciated andesite lava, but have much larger variation. Based on mineral assemblages, the iron‐rich agglomerates are subdivided into five types, i.e., albite‐magnetite type, albite‐K‐feldspar‐magnetite type, K‐feldspar–magnetite type, epidote‐magnetite type and quartz‐magnetite type, representing that products formed at different stages during the evolution of a magmatic‐hydrothermal system. The albite‐magnetite type represents the earliest crystallization product from a residual iron‐rich melt; the albite‐K‐feldspar‐magnetite and K‐feldspar‐magnetite types show features of magmatic‐hydrothermal transition, whereas the epidote‐magnetite and quartz‐magnetite types represent products of hydrothermal alteration. The occurrence of iron‐rich agglomerates provides macroscopic evidence for the presence of iron‐rich melts in the mineralization of the Heijianshan iron deposit. It also indicates that iron mineralization of submarine volcanic rock‐hosted iron deposits is genetically related to hydrothermal fluids derived from iron‐rich melts.

Behaviour of niobium during early Earth’s differentiation: insights from its local structure and oxidation state in silicate melts at high pressure

Niobium (Nb) is one of the key trace elements used to understand Earth’s formation and differentiation, and is remarkable for its deficiency relative to tantalum in terrestrial rocks compared to the building chondritic blocks. In this context, the local environment of Nb in silica-rich melts and glasses is studied by in situ x-ray absorption spectroscopy (XAS) at high pressure (P) up to 9.3 GPa and 1350 K using resistive-heating diamond-anvil cells. Nb is slightly less oxidized in the melt (intermediate valence between +4 and +5) than in the glass (+5), an effect evidenced from the shift of the Nb-edge towards lower energies. Changes in the pre-edge features are also observed between melt and glass states, consistently with the observed changes in oxidation state although likely enhanced by temperature (T) effects. The oxidation state of Nb is not affected by pressure neither in the molten nor glassy states, and remains constant in the investigated P-range. The Nb–O coordination number is constant and equal to below 5 GPa, and only progressively increases up to at 9.3 GPa, the maximum P investigated. If these findings were to similarly apply to basaltic melts, that would rule out the hypothesis of Nb/Ta fractionation during early silicate Earth’s differentiation, thus reinforcing the alternative hypothesis of fractionation during core formation on reduced pre-planetary bodies.

Bonding of xenon to oxygen in magmas at depth

The field of noble gases chemistry has witnessed amazing advances in the last decade with over 100 compounds reported including Xe oxides and Xe–Fe alloys stable at the pressure-temperature conditions of planetary interiors. The chemistry of Xe with planetary materials is nonetheless still mostly ignored, while Xe isotopes are used to trace a variety of key planetary processes from atmosphere formation to underground nuclear tests. It is indeed difficult to incorporate the possibility of Xe reactivity at depth in isotopic geochemical models without a precise knowledge of its chemical environment. The structure of Xe doped hydrous silica-rich melts is investigated by in situ high energy synchrotron X-ray diffraction using resistive heating diamond anvil cells. Obtained pair distribution functions reveal the oxidation of Xe between 0.2 GPa and 4 GPa at high T up to 1000 K. In addition to the usual interatomic distances, a contribution at 2.05±0.05 Å is observed. This contribution is not observed in the undoped melt, and is interpreted as the Xe–O bond, with a coordination number of about 12 consistent with Xe insertion in rings of the melt structure. Xe solubility measurements by electron microprobe and particle induced X-rays emission analysis confirm that Xe and Ar have similar solubility values in wt% in silicate melts. These values are nonetheless an order of magnitude higher than those theoretically calculated for Xe. The formation of Xe–O bonds explains the enhanced solubility of Xe in deep continental crust magmas, revealing a mechanism that could store Xe and fractionate its isotopes. Xenon is indeed atypical among noble gases, the atmosphere being notably depleted in elemental Xe, and very strongly depleted in Xe light isotopes. These observations are known as the ‘missing’ Xe paradox, and could be solved by the present findings.