Olivine Inclusions Research Articles

A siliceous komatiitic source of Bushveld magmas revealed by primary melt inclusions in olivine

Our experimental thermobarogeochemical study of the primary melt inclusions in olivine (Fo91) from poikilitic harzburgite to dunite of the Uitkomst Complex in the Bushveld Igneous Province revealed a series of melt compositions that are the result of combined fractional crystallization and assimilation. The primary melt inclusions up to 50 μm in size are fully crystallized and composed of predominantly olivine and orthopyroxene with highly subordinate chromite, sulfide, amphibole and plagioclase accompanied by a fluid phase in shrinkage voids. The most primitive melt within the series has a re-homogenization temperature of >1450 °C and contains 22.47 wt% MgO, 12.57 wt% FeO, 51.90 wt% SiO2, 2.50 wt% CaO, 8.27 wt% Al2O3 and 1.69 wt% Na2O recalculated to equilibrium with the host olivine. This is the first direct measurement of the komatiitic parental magma of the Bushveld magmas, inferred in previous studies. The estimated maximum PT parameters of the melt corresponds to 2.5 GPa and 1590 °C implying that the primitive melt inclusions in the olivine were trapped while still in the upper mantle and before further crustal assimilation. This primitive melt shows the steepest slope (La/Yb)PM, the highest Ni and is depleted in volatile components (168 ppm H2O, 264 ppm Cl, 258 ppm F and 114 ppm S according to secondary ion mass spectrometry measurements) compared to more evolved compositions, which were progressively enriched in volatiles up to 7665 ppm Н2О, 1728 ppm Cl, 2288 ppm S, and 549 ppm F as the rehomogenization temperature fell to 1335 °C. The progressive assimilation of crustal material responsible for magma modification is also manifested by the occurrence of CaNa volatile-rich minerals (such as amphibole, phlogopite, albite, sodalite, natrolite, pectolite, Cl-apatite and carbonate) in the interstitial assemblage and in “exotic” polyphase inclusions in cumulus olivine.The concentrations and ratios of the incompatible elements in the melt inclusions in Uitkomst olivine are similar to those in the model compositions of the ultramafic Bushveld parental magma and support a common source of ultramafic melts in the Bushveld Province. The main components of the melts, however, are not in accord with either boninitic or komatiitic lineages indicating a unique nature for the Bushveld magmas.

Cation Disorder Caused by Olivine-Ringwoodite Phase Transition Mechanism, Possible Explanation for Blue Olivine Inclusion in a Diamond

Synchrotron X-ray diffraction, as well as visual observations, in a diamond anvil cell (DAC) using soft metal gaskets or slightly reducing gas environment, have revealed that the olivine-ringwoodite transition in olivines of several compositions take place in two steps: step 1: displacive restacking of the oxygen layers, followed by step 2: diffusive reordering of the cations. The initiation of the phase transition was observed at temperatures as low as 200 °C below the reported temperature for the phase transition under hydrostatic conditions. These observations, especially residual disordered cations, have important implications for deep-focus earthquakes, the ability of ringwoodite to host surprising amounts of water, and possibly the observation of a blue olivine inclusion in a natural diamond from Brazil and in a pallasitic meteorite from Russia.

High-Ca boninitic melt inclusions in lavas of the Troodos ophiolite and a reappraisal of genetic relationships between different lava types

Abstract Boninites in many supra-subduction zone ophiolites are thought to mark the early stage of subduction. The Troodos ophiolite in Cyprus contains a variety of volcanic rocks including the lower pillow lavas and the boninite-bearing upper pillow lavas (UPL). Here we present major, trace element and Pb isotope data for melt inclusions in olivine from the UPL on the northern flank of the Troodos ophiolite. Melt inclusions hosted by high-Mg (Fo = 89.4–93.5) olivine grains have typical high-Ca boninitic compositions with high MgO (11.2–18.4 wt%) and SiO2 (51.2–55.0 wt%) and low TiO2 (0.2–0.6 wt%) contents, in line with the existence of boninites in the northern part of the ophiolite. The melt inclusions have trace element compositions similar to the Troodos boninitic lavas and can be regarded as representing the parental magma of the boninites. The boninitic magmas were derived from high-degrees of partial melting of a refractory source at 1400 °C and 1.5 GPa based on thermobarometers and phase equilibria. The melt inclusions have significantly variable Pb isotopic compositions (208Pb/206Pb = 2.059–2.122; 207Pb/206Pb = 0.828–0.862) and are enriched in fluid-mobile trace elements (e.g., U, Sr, and Pb), indicating that the refractory source was metasomatized by slab fluids. In comparison, recalculation of literature data suggests that the lower pillow lavas were derived from a less refractory and shallower source. We propose a subduction initiation model in which melting of two unique sources was needed to form the chemostratigraphy of lavas in the Troodos ophiolite.

Generation of Calc-Alkaline Magmas during Crystallization at High Oxygen Fugacity: An Experimental and Petrologic Study of Tephras from Buldir Volcano, Western Aleutian Arc, Alaska, USA

Abstract Despite agreement that calc-alkaline volcanism occurs at subduction zones and is responsible for the genesis of continental landmasses, there is no consensus on the source of the Fe-depleted signature hallmark to calc-alkaline volcanism. In this study, we utilize mafic tephras collected from Buldir Volcano to address the genesis of strongly calc-alkaline volcanic rocks (those with a low Tholeiitic Index; ≤0·7) in a segment of the western Aleutian Arc to determine if the eruptions are plausibly part of a liquid line of descent, if they are mixtures of crustal melts and parental magmas, or if they are mixtures of melts of the mantle and the subducting slab. We conducted a series of H2O-saturated phase equilibrium experiments (1175–1000°C; 100 MPa) in a rapid-quench cold-seal (MHC) apparatus on the most primitive natural lava from Buldir (9·34 wt % MgO) at oxidizing conditions near the Re–ReO2 buffer. We confirmed that all experiments equilibrated 0·3 ± 0·23 log units above the Re–ReO2 buffer (ΔQFM ∼ +2·8) using X-ray Absorption Near Edge Structure (XANES) spectroscopy. Chromite is the liquidus phase, followed by olivine, then plagioclase, then clinopyroxene, and finally hornblende. Once clinopyroxene saturates, spinel composition shifts to magnetite. We compared our experimental results to the major element geochemistry and petrology of six tephras (51·9–54·8 wt % SiO2) from Buldir collected during the 2015 field season of the GeoPRISMS shared platform. Tephras contain olivine + plagioclase + clinopyroxene + spinel ± hornblende; plagioclase comprises most of the crystalline volume, followed by either olivine or hornblende. Spinel is ubiquitous; with Cr-rich spinel inclusions in olivine and hornblende, and magnetite in the groundmass. Variations in phenocryst assemblages and compositions between samples can be attributed to differences in pre-eruptive temperatures, where hotter samples are devoid of hornblende, and contain Fo-rich olivine and plagioclase with lower An-contents, owing to the position of the mineral-in curves at fluid-saturated conditions. Experimental glasses match the depletion in FeOT observed in the tephra whole rock compositions. The continuous depletion in FeOT is attributable to saturation of spinel as a liquidus phase (initially as chromite) and continuous crystallization through the experimental series (changing to magnetite at colder temperatures). In contrast to the natural samples, the experiments show enrichment in TiO2 with decreasing MgO, suggesting that differentiation did not occur at 100 MPa on Buldir. The TiO2 depletion in volcanic rocks from Buldir can be accounted for if hornblende crystallization occurs close to the liquidus of a parental magma; a condition that is met at higher pressures and hydrous conditions. The emerging picture for Buldir Island is that (1) oxidizing conditions are required to drive the observed depletions in FeOT via crystallization of spinel, and (2) elevated H2O contents and high pressures are required to saturate hornblende close to the liquidus to reproduce the entire suite of major elements. Our study provides a mechanism to generate the calc-alkaline trends observed at Buldir without requiring mixing of slab and mantle melts. We conclude that calc-alkaline volcanic rocks with extremely low Tholeiitic Indices (0·7), like those from Buldir, cannot be generated in absence of high oxygen fugacity, even at high pressure and/or elevated water pressures.

Evidence for superhydrous primitive arc magmas from mafic enclaves at Shiveluch volcano, Kamchatka

Mafic enclaves preserve a record of deep differentiation of primitive magmas in arc settings. We analyze the petrology and geochemistry of mafic enclaves from Shiveluch volcano in the Kamchatka peninsula to determine the differentiation histories of primitive magmas and to estimate their pressures, temperatures, and water contents. Amphibole inclusions in high forsterite olivine suggest that the primitive melt was superhydrous (i.e., > 8 wt% H2O) and was fractionating amphibole and olivine early on its liquid line of descent. We find that the hydrous primitive melt had liquidus temperatures of 1062 ± 48 °C and crystallized high Mg# amphibole at depths of 23.6–28.8 km and water contents of 10–14 wt% H2O. The major and trace element whole-rock chemistry of enclaves and of published analyses of andesites suggest that they are related through fractionation of amphibole-bearing assemblages. Quantitative models fractionating olivine, clinopyroxene, and amphibole reproduce geochemical trends defined by enclaves and andesites in variation diagrams. These models estimate 0.2–12.2% amphibole fractionated from the melt to reproduce the full range of enclave compositions, which overlaps with estimates of the amount of amphibole fractionated from parental melts based on whole-rock dysprosium contents. This contribution extends the published model of shallow processes at Shiveluch to greater depths. It provides evidence that primitive magmas feeding arc volcanoes may be more hydrous than estimated from other methods, and that amphibole is an important early fractionating phase on the liquid line of descent of superhydrous, primitive mantle-derived melts.

Compositional variations, thermometry, and probable parental magmas of Archean chromite from the Sargur greenstone belt, Western Dharwar Craton (India)

The chromite and magnesite deposits from Talur, Sindhuvalli, and Doddakanya in the layered ultramafic-mafic plutonic sequence are a part of the Archean Sargur greenstone belt within the Western Dharwar Craton of the Indian shield. The ultramafic bodies in these three areas are highly sheared and comprise of serpentinized dunite with chromitite bodies along with harzburgite and pyroxenite. Detailed electron microprobe studies reveal intense compositional variability of the chromite grains in silicate-rich chromitite (~ 50 vol% of chromite) and serpentinite (~ 2 vol% of chromite) throughout the entire ultramafic sequence. In Talur, the chromites in massive chromitites (~ 60–70 vol% of chromite) are characterized by relatively lower Mg# (0.28–0.39) compared to chromites from massive chromitites in Sindhuvalli (Mg# = 0.63–0.69). The accessory chromite in dunite from Talur shows higher Cr#, Fe2O3, TiO2, and MnO content than other localities. The range of Fo content of cumulus olivine in dunites from Talur (Fo91-92) is relatively lower than the dunite from Doddakanya (Fo92-93) and net-textured chromitite from Sindhuvalli (Fo95-96). The range of Mg# in the intercumulus clinopyroxene in Talur net-textured chromitite varies between 94 and 99, whereas, in the Sindhuvalli net-textured chromitite, the intercumulus clinopyroxene shows a lower range of Mg# (= 96–97). Chromites in massive and silicate-rich chromitites, and from dunite contain several rounded to subhedral inclusions of silicate minerals of various sizes (~ 4–20 μm). Spherical inclusions of olivine (Fo94) are present in accessory chromites of dunite (Fo87). The Mg-rich clinopyroxene inclusions have Cr2O3 content from 1.5 to 2.3 wt%. The clinopyroxene inclusions in the Talur schlieren-banded chromitite are more magnesian (Mg# = 95–98) compared to the inclusions in chromites of the Sindhuvalli net-textured chromitite (Mg# = 96–97). The euhedral phlogopite inclusions in chromites from the Sindhuvalli net-textured chromitite are Mg-rich (~ 31.09 wt% MgO) with a significant amount of K2O (~ 9.28 wt%). The chlorite inclusion in chromites from the Talur schlieren-banded chromitite is also Mg-rich (~ 39.32 wt% MgO) with Al2O3 ~ 21.01 wt% and a significant amount of Cr2O3 (~ 3.51 wt%). Clinopyroxene (along with phlogopite) inclusions always occur as independent grain testifying their primary nature and might be genetically linked with the chromite crystallization from a primary high-Mg silicate magma. Chlorite and dolomite inclusions may be related to secondary hydrothermal alteration events. The average re-equilibration temperature of the olivine-chromite pairs in the Sindhuvalli net-textured chromitite and Doddaknaya dunite of around 545 °C represents the first stage of metamorphism under reducing conditions. Subsequently, the second hydrothermal episode involved fluids evolving to an average temperature of about 327 °C calculated from the accessory chromite and olivine pairs from Talur dunite, consistent with the abundance of magnesite veins throughout the Talur mines. The computed values of Al2O3, TiO2, and FeO/MgO ratio of the melt show that the parental melt associated with the chromitites in the Sargur greenstone belt has a close resemblance with the parental melt of the chromitites in the Nuggihalli schist belt, which is low-Al komatiitic. In the tectonic discrimination diagrams, most of the primary chromite compositions from Sargur plot within the field of chromites from komatiites from the Archean greenstone belts. Parental komatiitic magma fractionated in the subvolcanic conduits within the thin lithospheric crust and formed the sill-like layered ultramafic-mafic bodies with chromite deposits. These conduits fed the overlying extrusive system that developed the ultramafic-mafic volcanic sequence of the Sargur greenstone belt in the Western Dharwar Craton.

Diamond resorption and immiscibility of C-O-H fluid in kimberlites: Evidence from experiments in H2O – CO2 – SiO2 – MgO – CaO system at 1–3 GPa

Kimberlites are the deepest sourced magmas emplaced at the Earth's surface. They provide a “window” into the processes at the base of the subcratonic mantle. A better understanding of the origin, composition, and emplacement mechanisms of kimberlites is hampered by uncertainties in the contents of the two main volatiles, H2O and CO2. Diamond dissolution in H2O and in CO2 fluids produces distinct resorption features offering an opportunity to determine the composition of the magmatic fluid in kimberlites. Here we examined the relationship between H2O:CO2 ratio of the fluid and the style of diamond resorption by conducting experiments in C-O-H fluid saturated with silicates with variable H2O:CO2 ratios at the conditions of kimberlite ascent of 1–3 GPa and 1150–1350 °C. Our results showed that the geometry of etch pits on diamond and the resorption style evolve consistently as bulk CO2 content of the fluid changes from 0 to 50 to 50–90 and 90–100 mol%. The fluid composition at the run conditions was monitored by entrapment of synthetic fluid inclusions in olivine and quartz. The inclusions demonstrated the existence of a fluid miscibility gap at 1–3 GPa and 1250 °C with two fluid endmembers, an aqueous and a carbonic phase, which H2O:CO2 ratio at 1 GPa determined with confocal Raman microscopy is (H2O)0.62(CO2)0.38 and (H2O)0.12(CO2)0.88 respectively. Hence, diamond resorption morphology depends on the proportions of the end-member aqueous and carbonic fluids, which vary with the bulk composition of the fluid. The different density and ability of aqueous and carbonic fluids to dissolve silicates (olivine) would promote their separation in the rising magma column. Concentration of the lower density aqueous fluid towards the tip of the propagating dyke would facilitate more efficient fracturing of the country rocks and faster ascent of the kimberlite magma causing explosive eruption. We propose that preferential attachment of aqueous fluid bubbles would help to increase the buoyancy of olivine xenocrysts and possibly diamond in the kimberlite magma offering a mechanism for transporting the heavy mantle cargo.

Mineralogy and Geochemistry of Ultramafic Rocks from Rachoni Magnesite Mine, Gerakini (Chalkidiki, Northern Greece)

The importance of magnesite for the EU economy and industry is very high, making the understanding of their genesis for the exploration for new deposits a priority for the raw materials scientific community. In this direction, the study of the magnesite-hosting ultramafic rocks can be proved very useful. For the present study, ultramafic rock samples were collected from the magnesite ore-hosting ophiolite of the Gerakini mining area (Chalkidiki, Greece) to investigate the consecutive alteration events of the rocks which led to the metallogenesis of the significant magnesite ores of the area. All samples were subjected to a series of analytical methods for the determination of their mineralogical and geochemical characteristics: optical microscopy, XRD, SEM, EMPA, ICP–MS/OES and CIPW normalization. The results of these analyses revealed that the ultramafic rocks of the area have not only all been subjected to serpentinization, but these rocks have also undergone carbonation, silification and clay alteration. The latter events are attributed to the circulation of CO2-rich fluids responsible for the formation of the magnesite ores and locally, the further alteration of the serpentinites into listvenites. The current mineralogy of these rocks was found to be linked to one or more alteration event that took place, thus a significant contribution to the metallo- and petrogenetic history of the Gerakini ophiolite has been made. Furthermore, for the first time in literature, Fe inclusions in olivines from Greece were reported.

Isotopic Evidence for Multiple Recycled Sulfur Reservoirs in the Mangaia Mantle Plume

AbstractMangaia, an ocean island in the Cook‐Austral volcanic chain, is the type locality for the HIMU mantle reservoir and has also been shown to exhibit evidence for recycled sulfur with anomalous δ34S and Δ33S that has been attributed an Archean origin. Here we report bulk S‐isotope data from sulfide inclusions in olivine and pyroxene phenocrysts from one of the previously analyzed and four additional Mangaia basalts to further test for the prevalence of anomalous S in the HIMU mantle source feeding Mangaia. We document compositions that range from −5.13‰ to +0.21‰ (±0.3 2σ), +0.006‰ to +0.049‰ (±0.016 2σ), −0.81‰ to +0.69‰ (±0.3 2σ) for δ34S, Δ33S, and Δ36S, respectively. These data extend the range of measured compositions and suggest S‐isotope heterogeneity in the HIMU mantle source at Mangaia. We show that S‐isotope compositions of bulk sulfide in olivine is not in isotopic equilibrium with bulk sulfide in pyroxene from the same samples and that samples from a confined area (M4, M10, M12, and M13) in the northern central part of the island show a distinct covariation for δ34S and Δ33S. This isotopic variation (forming an array) suggests mixing of sulfur from two sources that were captured at different stages of crystallization by phenocrysts in the Mangaia HIMU sulfur endmember.

Applications and Limitations of Elastic Thermobarometry: Insights From Elastic Modeling of Inclusion‐Host Pairs and Example Case Studies

AbstractElastic thermobarometry can be used to constrain the pressure and temperature conditions of mineral crystallization by exploiting the difference in the elastic evolution of a mineral inclusion and its host during cooling and decompression. In this work we examine the pressure‐temperature sensitivity of >5,000 untested inclusion‐host pairs. Hosts such as diamond and zircon are ideal host minerals because their low compressibility makes them rigid containment vessels. Highly compressible inclusions such as albite, graphite, and quartz serve as the most reliable barometers. We provide three case studies of inclusion‐host pairs from different geologic settings to demonstrate the advantages and challenges associated with these mineral pairs. Apatite inclusions in olivine from Yellowstone caldera mostly record negative residual pressures (tension) and suggest magmatic crystallization at ~0.4 GPa. Rutile inclusions in garnet from Verpeneset eclogites record near ambient conditions and do not recover reasonable metamorphic conditions of rutile entrapment. These results suggest that stiff inclusions may have a tensile strain limit, a possible limitation of elastic thermobarometry. Albite inclusions in epidote from a blueschist (Syros, Greece) record geologically reasonable entrapment pressures, but a large range of residual pressures that may be caused by the complex anisotropy of both phases. Our theoretical and applied results indicate that elastic thermobarometry has the potential to be used to understand petrologic processes in diverse geologic environments, including mantle, metamorphic, and magmatic settings but that each elastic thermobarometer requires careful evaluation.

Chlorine in the Earth’s Mantle as an Indicator of the Global Recycling of Oceanic Crust

Abstract —Homogenized melt inclusions in olivine were studied in Archean komatiites from the Barberton Greenstone Belt, Weltevreden Formation in South Africa (3.3 Ga), Abitibi Greenstone Belt in Canada (2.72 Ga), and the Belingwe Greenstone Belt in Zimbabwe (2.69 Ga). Contamination of the komatiite melts with crustal material enriched in Rb, Cl, and H2O during the crystallization of olivine is demonstrated. Uncontaminated melts have mantle Rb/Nb ratios but are significantly enriched in Cl and H2O relative to K and Ce, respectively, exhibiting similar incompatibility during crystallization and partial mantle melting. These observations suggest the presence of a chlorine- and water-enriched mantle source before 3.3 Ga. The excess Cl and H2O contents in the komatiites are assumed to result from the interaction of partially molten mantle plumes with the mantle transition zone. The most likely source of Cl and H2O enriching the deep mantle is the oceanic lithosphere that endured a seafloor alteration. We conclude that the recycling of the altered oceanic lithosphere into the mantle, probably via subduction, began in the first billion years of the Earth’s history. Delamination of the Archean crust could not cause transport of chlorine and water into the deep mantle.

LA-ICP-MS Analysis of Crystallized Melt Inclusions in Olivine, Plagioclase, Apatite and Pyroxene: Quantification Strategies and Effects of Post-Entrapment Modifications

Abstract Melt inclusions represent a unique tool to reconstruct the composition and chemical evolution of silicate melts in magmatic systems. Laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICP-MS) is the most commonly used microanalytical technique to analyse crystallized melt inclusions without prior re-homogenization. Well-preserved melt inclusions can be quantified by subtracting the contribution of co-ablated host with a carefully selected internal standard. However, post-entrapment compositional re-equilibration commonly renders this task difficult, to the same degree as it would affect any quantification after prior re-homogenization. In this study, we first examine well-preserved, crystallized melt inclusions hosted in olivine, plagioclase, apatite, clinopyroxene and orthopyroxene from porphyry dikes and volcanic rocks to test various quantification strategies and evaluate the associated uncertainties, and then we use these strategies to quantify coarsely crystallized melt inclusions from gabbroic rocks at Marble Canyon (USA) and Laiyuan (China) that experienced severe post-entrapment modifications due to relatively slow cooling rates. The results demonstrate that even for well-preserved melt inclusions hosted in chemically complex minerals the uncertainty related to inclusion–host deconvolution can be rather high (up to 30‒50% for host-incompatible trace elements significantly above their limits of detection), though other uncertainties inherent to LA-ICP-MS analysis are relatively small (typically ≤5‒10%). The deconvolution-related uncertainty can be minimized to ca. 10% by (1) choosing whole rocks that are fresh and representative of magmatic liquids, (2) choosing the smallest possible spot size to ablate the melt inclusions, and (3) choosing a host endmember that is compositionally as similar as possible to the one ablated together with the inclusion. Results of coarsely crystallized melt inclusions from gabbroic rocks suggest that the range of elements affected by post-entrapment re-equilibration varies from intrusion to intrusion. Olivine-hosted melt inclusions from Marble Canyon appear to have diffusively lost Fe, Ti and Ca, whereas those from Laiyuan lost Fe, Na, Al, Ca, Ti and Y and gained V. However, the relative abundances of K, P, Rb, Sr, Zr, Nb, Mo, Cs, Ba, Ce, Ta, Pb, Th, U and ±Cu appear unchanged. Plagioclase-hosted melt inclusions from Marble Canyon are relatively well-preserved, whereas those from Laiyuan lost significant amounts of Fe, K, Mg, Mn, Rb and Co. Apatite-hosted melt inclusions seem well preserved with regard to most elements except for Cu. These results suggest that despite the post-entrapment modification of certain element concentrations and the associated difficulties in melt inclusion quantification, information on the approximate abundances of other elements that are invaluable for petrogenetic and metallogenic studies can still be retrieved from melt inclusions in gabbroic rocks using the LA-ICP-MS technique.

Synthetic fluid inclusions XXIII. Effect of temperature and fluid composition on rates of serpentinization of olivine

Serpentinization, i.e. the hydrothermal alteration of ultramafic rocks, is an important and ubiquitous geologic process that occurs at slow- and ultraslow-spreading mid-ocean ridges, magma-poor passive margins, and subduction zones. While serpentinization occurs over a wide temperature range and involves diverse fluid compositions, few experimental studies systematically examined the effects of temperature and fluid composition on the kinetics of serpentinization of olivine, and published rates diverge greatly. We present results of an experimental study using synthetic fluid inclusions in Mg-rich olivine as micro-batch reactors to monitor the effects of temperature (100–350 °C), fluid composition (H2O-MgCl-NaCl, H2O-NaCl, H2O-MgCl), and total salinity on serpentinization rates of olivine under closed system conditions.Petrographic observations and Raman analyses of the experimental run products revealed the alteration of olivine to produce serpentine minerals, brucite, and magnetite. The salinity of the aqueous fluid in the inclusions increased as H2O was removed from the solution and incorporated into the hydrous product phases and served as a proxy for reaction progress. The fastest serpentinization rates were found at 250 °C in the presence of a seawater-like aqueous solution. This result further suggests that serpentinization of olivine is fastest at lower temperatures (∼250 °C) and shallower depths in the oceanic lithosphere than suggested by previous studies (∼300 °C). Moreover, our experiments show that serpentinization rates decrease by several orders of magnitude as salinity and the concentration of dissolved Mg increase. These effects may reconcile some of the differences observed when comparing results of previously published rates obtained from experiments involving fluids of different compositions. We constructed a quantitative model based on the principle of detailed balance to predict variations in serpentinization rates (J±) from low temperatures to 320 °C. It suggests that at 25 °C, serpentinization rates are 4 to 5 orders of magnitude slower than at 250 °C. Moreover, the temperature dependence model has been coupled with the kinetic effects derived from experiments involving different fluid composition to calculate the amount of time required for serpentinization of olivine having different reactive surface areas. The model shows that between 200 and 300 °C serpentinization is fast on geologic timescales when rocks with large reactive surface areas interact with a seawater-like aqueous solution (complete serpentinization in days to decades).

Origin of corundum within anorthite megacrysts from anorthositic amphibolites, Granulite Terrane, Southern India

Abstract Growth of corundum in metamorphosed anorthosites and related basic-ultra-basic rocks is an exceptional feature, and its origin remains elusive. We describe the occurrence of and offer an explanation for the genesis of corundum in anorthositic amphibolites from ~2.5 Ga old basement of the Granulite Terrane of Southern India (GTSI). The studied amphibolites from two localities, Manavadi (MvAm) and Ayyarmalai (AyAm), contain anorthite lenses (An90–99) with euhedral to elliptical outline set in a finer-grained matrix of calcic plagioclase (An85–90) and aluminous amphibole (pargasite-magnesiohastingsite). The lenses, interpreted as primary magmatic megacrysts, and the matrix are both recrystallized under static condition presumably during the regional high pressure (HP) metamorphism (~800 °C, 8–11 kbar) at ~2.45 Ga. Corundum occurs in the core of some of the recrystallized anorthite lenses (An95–99) in two modes: (1) Dominantly, it forms aggregates with magnetite (with rare inclusion of hercynite; in MvAm) or spinel (and occasionally hematite-ilmenite; in AyAm). The aggregates cut across the polygonal grain boundaries of the anorthite and contain inclusions of anorthite. (2) Corundum also occurs along the grain boundaries or at the triple junctions of the polygonal anorthite grains, where it forms euhedral tabular grains, sieved with inclusions of anorthite or forms skeletal rims around the recrystallized anorthite, such that it seems to be intergrown with anorthite. Combined petrological data and computed phase relations are consistent with growth of corundum in an open system during regional metamorphism in the presence of intergranular fluids. Two mechanisms are proposed to explain the formation of the corundum in the amphibolites: (1) corundum + magnetite/spinel aggregates formed dominantly by oxy-exsolution of pre-existing Al-Fe-Mg-(Ti)-spinel. This pre-existing spinel may be primary magmatic inclusions within the anorthite phenocrysts or could have formed due to reaction of primary magmatic inclusions of olivine with the host anorthite. Pseudosections of fO2-nH2O-T-P in the CaO–FeO–MgO–Al2O3–SiO2–H2O (CFMASH) system indicate that fO2 and H2O strongly influence the formation of corundum + amphibole from the initial magmatic assemblage of anorthite (phenocrysts) + spinel ± olivine (inclusions). (2) The corundum with anorthite presumably formed through desilification and decalcification of anorthite, as is indicated by computed phase relations in isobaric-isothermal chemical potential diagrams (µSiO2-µCaO) in parts of the CASH system. Growth of corundum in this mode is augmented by high activity of anorthite in plagioclase, high pressure, and low-to-medium temperature of metamorphism. This study thus presents a new viable mechanism for the origin of corundum in anorthositic amphibolites, and basic-ultra-basic rocks in general, which should provide new insight into lower crustal processes like high-pressure metamorphism.

Evolution of kimberlite magmas in the crust: A case study of groundmass and mineral-hosted inclusions in the Mark kimberlite (Lac de Gras, Canada)

Kimberlites are the surface manifestation of deeply-derived (>150 km) and rapidly ascended magmas. Fresh kimberlite rocks are exceptionally rare, as most of them are invariably modified by pervasive deuteric and/or post-magmatic fluids that overprint the original mineralogy. In this study, we examined fresh archetypal kimberlite from the Mark pipe (Lac de Gras, Canada), which is characterised by well-preserved olivine and groundmass minerals. The sequence of crystallisation of the parental melt and its major compositional features, including oxygen fugacity, were reconstructed using textural relationships between magmatic minerals, their zoning patterns and crystal/melt/fluid inclusions. Crystal and multiphase primary, pseudosecondary and secondary melt/fluid inclusions in olivine, Cr-diopside, spinel, perovskite, phlogopite/kinoshitalite, apatite and calcite preserve a record of different stages of kimberlite melt evolution. Melt/fluid inclusions are generally more depleted in silica and more enriched in alkalis (K, Na), alkali-earth (Ba, Sr) and halogens (Cl, F) relative to the whole-rock composition of the Mark kimberlite. These melt/fluid inclusion compositions, in combination with presence of elevated CaO (up to 1.73 wt%), in Mg-rich olivine rinds, crystallisation of groundmass kinoshitalite, carbonates (calcite, Sr-Ba-bearing) and alkali-enriched rims around apatite suggest that there was progressive enrichment in CO2, alkalis and halogens in the evolving parental melt.The Mark kimberlite groundmass is characterised by the following stages of in-situ crystallisation: (1) olivine rims around xenocrystic cores + Cr-spinel/TIMAC. (2) Mg-rich olivine rinds around olivine rims/cores + MUM-spinel (followed by pleonaste and Mg-magnetite) + monticellite (+ partial resorption of olivine, along with the formation of ferropericlase and CO2 as a result of decarbonation reactions) + perovskite + apatite. (3) Olivine outmost rinds, which are coeval with phlogopite/kinoshitalite + apatite + sulphides + carbonate (calcite, Ba-Sr-Na-bearing varieties). In addition, oxygen fugacity of the Mark kimberlite was constrained by olivine-chromite, perovskite and monticellite oxygen barometry and showed that the parental melt became progressively more oxidised in response to fractional crystallisation. (4) Deuteric (i.e. late-stage magmatic) and/or post-magmatic (i.e. external fluids) alteration of magmatic minerals (e.g., olivine, monticellite, ferropericlase) and crystallisation of mesostasis serpentine, K-bearing chlorite and brucite (i.e. replacement of ferropericlase). The absence of any alkali (Na, K) and halogen (F, Cl) rich groundmass minerals in the Mark kimberlite may be attributed to these elements becoming concentrated in the late-stage melt where they potentially formed unstable, water-soluble carbonates (such as those observed in melt inclusions). Consequently, these minerals were most likely removed from the groundmass by deuteric and/or post-magmatic alteration.

Zoned Pyroxenes as Prospectivity Indicators for Magmatic Ni-Cu Sulfide Mineralization

Small intrusions dominated by olivine- and pyroxene-rich cumulates are well known to be favourable hosts to magmatic Ni-Cu-(Platinum Group Element - PGE) sulfide mineralization. Such intrusions are common in a variety of settings around the world, but only a very small proportion contain economically exploitable sulfides; these tend to be of conduit or chonolith style. If prospectivity could be discriminated from sparse sampling at early exploration stages, then the discovery rate for deposits of this type could be improved. To this end, a number of pyroxene-bearing samples from small intrusions containing magmatic sulphide deposits have been investigated including the Noril’sk-Talnakh camp in Siberia, the Kotalahti nickel belt in Finland, Ntaka Hill in Tanzania, Nova-Bollinger in the Albany-Fraser Orogen of Australia, Savannah in the Halls Creek Orogen of Australia, Jinchuan in central China, Xiarihamu in Tibet and Huangshanxi in the east Tianshan Ni province of NW China. To compare, samples from unmineralised intrusions in four of these regions were also investigated along with four mafic intrusions from other localities that are not associated with any known economic sulfide mineralisation. Using fine-scale (<5 μm/pixel) chemical imaging on the Australian Synchrotron, complex zoning in chromium was found in cumulate and poikilitic pyroxenes within the strongly mineralised intrusions. The zoning patterns can be separated into three distinct types: 1) abrupt zoning: a single change in trace element concentration with a sharp boundary; 2) sector zoning: hourglass style zonation; and 3) oscillatory zoning: small scale oscillations that are usually cyclic. Zoning of all three types can be present in a single grain. The presence of cumulus orthopyroxene with a combination of abrupt zoning, sector zoning and resorbed olivine inclusions has so far only been detected in mineralised intrusions. This combination of zoning patterns is postulated to be an indication of high magma flux and fluctuating cooling rates that accompany wall rock assimilation in dynamic conduits where sulphide liquid forms and accumulates. The distinctive zoning patterns reported here can, in many cases, be easily imaged using desktop microbeam XRF mapping techniques and may provide a useful fertility indicator for the exploration of new magmatic Ni-Cu-(PGE) deposits.

Northwest Africa 8694, a ferroan chassignite: Bridging the gap between nakhlites and chassignites

The origin(s) of the chassignites and nakhlites, closely related martian olivine and augite cumulates, respectively, are much debated. Northwest Africa (NWA) 8694 is the third chassignite to be discovered and the most ferroan, containing 85% olivine (Fo54). Its O-isotope compositions (δ18O ∼ 4.4‰, Δ17O ∼ 0.30‰) are typical of other martian meteorites. It has adcumulate texture and contains cumulus chromite, poikilitic pigeonite (En56Fs37Wo7) and mesostasis (trapped interstitial liquid). The latter contains pyroxene and plagioclase (An23 Ab70 Or8) plus rare K- feldspar (Or74), and has a trachyandesitic to trachytic bulk composition. Melt inclusions in olivine contain a variety of phases including biotite and rare amphibole. Olivine, chromite, and pigeonite compositions are intermediate between those of the other chassignites and those of the nakhlites. Augite, which appears to mantle pigeonite, has a composition overlapping that in nakhlite NWA 998 and some other nakhlites at (En41-40Wo38-39). The augite lamellae in pigeonite 1–2 μm in apparent width, and the survival of Ca zoning in olivine, suggest a near-surface cooling environment. The bulk-rock REE concentrations in the three chassignites do not correlate with Mg# but depend on the abundance of trapped liquid. The form of REE patterns calculated for olivine subtraction is very like those of nakhlite mesostases, but the observed concentrations of LREE in NWA 8694 trapped liquid have a very steep slope. This is explained by undersampling of baddeleyite and zirconolite that occur near olivine contacts with mesostasis. Though pyroxene is unzoned, its trace element variations indicate fractional crystallization. The range of olivine compositions in the three chassignites (Fo79-54) is too large to result from the crystallization sequential growth of olivine from a single magma undergoing fractional crystallization. The Ge/Si ratios show degassing of NWA 8694 which sets this chassignite apart from other chassignites and nakhlites, implying a unique batch of magma for its genesis. Many potential parent liquids are capable of generating the NWA 8694 olivine composition, though not its alkaline mesostasis. We calculated that Nakhla parent liquid NA01a (Stockstill et al., 2005) with 10% Nakhla core olivine added would produce both olivine crystals and alkaline daughter liquids with compositions matching those of NWA 8694. This meteorite is a chassignite cumulate containing nakhlitic mesostasis, a direct link between the chassignites and the nakhlites and the association of dunitic to trachytic compositions is reminiscent of terrestrial shield volcanoes. Chassignites and nakhlites were possibly formed when solidification fronts on chamber walls were disrupted, mainly as side eruptions of olivine-charged magmas from the deeper zones, and augite-charged fractionated magmas from nearer the summit of a volcano resembling Piton de la Fournaise on Earth.

Diamond-rich placer deposits from iron-saturated mantle beneath the northeastern margin of the Siberian Craton

We demonstrate for the first time the presence of iron carbides in placer diamonds from the northeastern region of the Siberian craton. It was found that the inclusions are polycrystalline aggregates, and iron carbides filling the fissures in the diamonds, thus providing clear evidence that the iron melts were captured first. Iron carbides were identified in diamonds containing mineral inclusions of eclogitic (Kfs, sulfide) and peridotitc (olivine) paragenesis. Iron carbides with minor amounts of admixed nickel were detected in a diamond sample containing an olivine inclusion (0.3 wt% Ni), indicating that the iron melt was not in equilibrium with the mantle peridotite.The low nickel contents of the iron carbides provide the best evidence that the subducted crust is a likely source of the iron melt. Diamonds containing carbide inclusions are characterised by a relatively low nitrogen aggregation state (5–35%), which is not consistent with the high temperature of the transition zone. Therefore, we have reason to assume that the studied diamonds are from the lower regions of the lithosphere. Considering all factors, the model for the interaction of the ascending asthenospheric mantle with the subducting slab seems to be more realistic.

Suprasubduction-zone origin of the podiform chromitites of the Bir Tuluhah ophiolite, Saudi Arabia, during Neoproterozoic assembly of the Arabian Shield

The ultramafic section of a dismembered ophiolite is exposed at Bir Tuluhah, in the north-central part of the Arabian Shield. It is penetratively serpentinized and locally carbonate-altered to talc‑carbonate and quartz‑carbonate rocks (listvenite) along shear zones and fault planes. Despite the high degree of mineral replacement, preserved mesh and bastite textures and fresh relics of primary Cr-spinel and olivine show that the protoliths were mainly harzburgite with minor dunite, with sparse massive chromitite bodies of various forms and sizes. Olivine inclusions in the chromitite lenses have higher forsterite content and NiO concentrations than fresh olivine relics in the host harzburgites and dunites, due to subsolidus re-equilibration. Cr-spinels in the chromitites have higher Cr# (0.74–0.82) than those hosted in dunite (0.72–0.76) or harzburgite (0.55–0.66). The scarce Cr-spinel crystals in harzburgite that have Cr# < 0.6 are interpreted to represent the population least affected by melt-rock interaction. The chromitite bodies are interpreted to have formed just below the contact between the oceanic crust and mantle sections (i.e., the petrologic Moho). The primary olivine (high Fo and Ni content) and Cr-spinel core compositions (high Cr# and low TiO2 content) of the Bir Tuluhah serpentinized peridotite are typical of modern supra-subduction zone (SSZ) fore-arc peridotites and consistent with crystallization from boninitic magma. The multistage petrogenesis leading to the chromitite bodies begins with moderate to high degrees of melt extraction from the protoliths of the serpentinized harzburgites, followed by reaction with melt compositions that evolved from tholeiite to boninite and left dunite residues. The massive Cr-rich chromitites in the Bir Tuluhah ophiolite are most probably the residues of such interaction between depleted harzburgite and ascending melts; mixtures of the reacted melts formed boninites, which became saturated with chrome-rich spinel and crystallized chromite pods before ascending past the Moho. We offer a novel thermodynamic model of this mixing and reaction process that quantifies the yield of Cr-spinel.

A genetic story of olivine crystallisation in the Mark kimberlite (Canada) revealed by zoning and melt inclusions

Elucidating the composition of primary kimberlite melts is essential to understanding the nature of their source, petrogenesis, rheology, transport and ultimately the origin of diamonds. Kimberlite rocks are typically comprised of abundant olivine (~25–60 vol%), which occurs as individual grains of variable size and morphology, and includes xenocrysts and zoned phenocrysts. Zoning patterns and inclusions in olivine can be used to decipher the petrogenetic history of kimberlites, starting from their generation in the mantle through to emplacement in the crust. This study examines well-preserved, euhedral, zoned olivine crystals from the Mark kimberlite (Lac de Gras, Canada). Olivine typically consists of xenocrystic cores, which are homogeneous in composition but vary widely between grains (Fo88.1–93.6). These cores are in turn surrounded by (in order of crystallisation) magmatic rims and Mg-rich rinds (Fo95.3–98.1). In addition, we document a new type of olivine zone (‘outmost rind’) that overgrows Mg-rich rinds.Crystal and melt/fluid inclusions are abundant in olivine and preserve a record of kimberlite melt evolution. For the first time in the studies of kimberlite olivine, we report primary melt inclusions hosted in Mg-rich olivine rinds. In addition, we observe that pseudosecondary melt/fluid inclusions are restricted to interior olivine zones (cores, rims) and are considered to have formed prior to rind formation. Pseudosecondary melt/fluid inclusions are inferred to have been entrapped at depth, as evidenced by measured densities in thermometric experiments of CO2 and decrepitation haloes, indicating a minimum entrapment pressure of ~200–450 MPa (or ~6–15 km). Both primary and pseudosecondary melt inclusions in olivine have daughter minerals dominated by CaMg and K-Na-Ba-Sr-bearing carbonates, K-Na-chlorides along with subordinate silicates (e.g., phlogopite, monticellite), Fe-Mg-Al-Ti-spinel, perovskite, phosphates and sulphates/sulphides and periclase. In addition to phases reported in primary melt inclusions, pseudosecondary melt inclusions contain more diverse and exotic daughter mineral assemblages, where they contain phases such as tetraferriphlogopite Ba- or K-sulphates, kalsilite and Na-phosphates. The daughter mineral assemblages are consistent with a silica-poor, alkali dolomitic carbonatite melt. We demonstrate that the different types of inclusions in olivine can assist in constraining the timing of multi-stage olivine growth and the composition of the crystallising melt.The large variance in olivine zoning patterns, morphologies and Ni distribution (i.e. both coupling with and decoupling from Fo) indicates that olivine in the studied Mark kimberlite samples represent an accumulation of olivine, where olivine was derived from successive stages of the ascending magma and/or from multiple, but related pulses of magma. Primary and pseudosecondary melt/fluid inclusions in olivine indicate that a variably differentiated silica-poor, halogen-bearing, alkali-dolomitic melt crystallised and transported olivine in the Mark kimberlite.