Silicate Mineral Compositions Research Articles

Experimental Investigation on the Transport of Sulfide Driven by Melt‐Rock Reaction in Partially Molten Peridotite

AbstractExtraction of sulfides from the partially molten mantle is vital to elucidate the cycling of metal and sulfur elements between different geochemical circles but has not been investigated systematically. Using laboratory experiments and theoretical calculations, this study documents systematical variations in lithologies and compositions of silicate minerals and melts, which are approximately consistent with the results of the thermodynamically‐constrained model. During a melt‐peridotite reaction, the dissolution of olivine and precipitation of new orthopyroxene generate an orthopyroxene‐rich layer between the melt source and peridotite. With increasing reaction degree, more melt is infiltrated into and reacts with upper peridotite, which potentially enhances the concomitant upward transport of dense sulfide droplets. Theoretical analyses suggest an energetically focused melt flow with a high velocity (∼170.9 μm/hr) around sulfide droplets through the pore throat. In this energic melt flow, we, for the first time, observed the mechanical coalescence of sulfide droplets, and the associated drag force was likely driving upward entrainment of fine μm‐scale sulfide. For coarse sulfide droplets whose sizes are larger than the pore throat in the peridotite, their entrainment through narrow constrictions in crystal framework seems to be physically possible only when high‐degree melt‐peridotite reaction drives high porosity of peridotite and channelized melt flows with extremely high velocity. Hence, the melt‐rock reaction could drive and enhance upward entrainment of μm‐ to mm‐scale sulfide in the partially molten mantle, potentially contributing to the fertilization of the sub‐continental lithospheric mantle and the endowment of metal‐bearing sulfide for the formation of magmatic sulfide deposits.

Trace element composition of silicate minerals in the porphyritic and nonporphyritic chondrules of Elenovka (L5) and Knyahinya (L/Ll5) meteorites

The results of SIMS and EPMA studies on the silicate minerals and bulk compositions (SEM-EDS) of porphyritic and nonporphyritic chondrules from Elenovka and Knyahinya meteorites are reported. The trace element composition of silicate minerals (olivine, low-Са pyroxene) in equilibrated ordinary chondrites (EOC) has not been affected considerably by thermal metamorphism on the chondritic parent bodies. Therefore, equilibrated chondrites can be used for chondrule-forming processes studies. Low-Са pyroxene in nonporphyritic chondrules contains higher REE, Ba, Sr concentrations than that in porphyritic chondrules at similar trace element concentrations in the olivine of chondrules. The data obtained indicate that the formation of non-porphyritic chondrules was triggered by an increase in the cooling rate of chondrules upon the formation of pyroxene, rather than a difference in the initial conditions of chondrule formation. Higher refractory incompatible element (Nb, LREE) concentrations in the olivine of chondrules than those in the olivine of the matrix and contrasting trace element (Zr, Sr, Cr, REE) concentrations in the low-Са pyroxene of the chondrules and the matrix suggest that the matrix and chondrules of the meteorites formed in one reservoir under different physico-chemical conditions (density, redox state, rotation speed, homogeneity, temperature, shocks, electrical discharge, etc.).

Melilite-olivine neрhelinites of Mt. Tabaat (Makhtesh Ramon, Israel): Geological, petrographic and geochemical characteristics and conditions of genesis

Research subject. The melilite-olivine nephelinite subvolcanic body Tabaat, which includes melilite rocks found for the first time on the territory of Levant.Materials and methods. The chemical composition of minerals (about 400 analyzes) was determined out on a CAMECA SX-100 microanalyzer equipped with five wave spectrometers with crystal analyzers TAP, LPET and LLIF. The elemental composition was measured at an accelerating voltage of 15 kV, an electron beam current of 40 nA. The oxygen concentration was calculated from the condition of stoichiometric composition of silicate minerals and chromite. In addition, the results of earlier studies of minerals (150 analyses) and data on melt inclusions were used.Results. The Tabaat subvolcanic body, which is part of the Early Cretaceous olivine-basalt-basanite-nephelinite association Makhtesh Ramona (Negev, Israel), has a complex concentrically-zonal structure, with olivine melanephelinites in the peripheral zone, melilite-olivine melanephelinites in the central and connecting zones normative and, less often, melilite-containing melanephelinites. Prismatic separation is widely manifested in the rocks. The fold-like bending and concave-curved edges of the prisms are a reflection of the plastic state of the cooling body and its ability to compress and accommodate a high fluid pressure, which develops during the formation of melilite nephelinites.Conclusion. All mineral diversity of rocks of the Mt. Tabaat is a derivative of a single portion of magmatic melt under conditions of its adiabatic cooling at the place of stabilization. A special role in the course of crystallization of the massif belongs to minerals with a high water content – analcime, zeolites, iddingsites, bowlingites and saponite-celadonites, which indicate the deuteric stage of its development. The study of melt inclusions in olivine and clinopyroxene showed the continuity of their composition with the composition of host melilite nephelinites and the importance of incongruent melting during the formation of melilite, which is a product of the reaction of nepheline with olivine or clinopyroxene.

Antigorite dehydration fluids boost carbonate mobilisation and crustal CO2 outgassing in collisional orogens

The processes of carbon mobilisation at convergent plate boundaries are hotly debated. Recent findings suggest that carbon release along subduction geothermal gradients may well be more relevant than previously thought; however, it has remained difficult to achieve steady state atmospheric CO2 conditions over geological time scales based on current volatile cycle models. Here, we report on meta-ophicarbonate rocks in the contact metamorphic aureole of the Bergell intrusion, Val Malenco, European Alps, that reached T-P conditions of at least 650 °C at 0.35 GPa. We demonstrate by combined field evidence, geochemistry, and closed-system thermodynamic modelling that over 50% of the rock carbonate has been mobilised locally in response to reactive fluid flow upon progressive isobaric heating from 350 to >650 °C. Despite complete mineral transformation at the antigorite + calcite devolatilisation reaction, the resulting tremolite-ophicarbonate preserves the original ophicarbonate texture by olivine-chlorite clasts, representing the former antigorite-serpentinite fraction, embedded in a monomineralic tremolite matrix formed from the calcite fraction that often exceeded 50 vol%. Closed system thermodynamic modelling based on an ophicarbonate composition of 80 wt% serpentinite +20 wt% calcite reveals that rock-buffered fluid XCO2 values evolve from <0.09 to as high as ∼0.16, and the total H2O-CO2 fluid fraction released may be as high as 14 wt%; values that are highly sensitive to bulk ophicarbonate composition.Major to trace element geochemistry reveals a history of peridotite melt depletion followed by ocean floor hydration and ophicarbonate formation, consistent with the ocean continent transition (OCT) setting of the mantle rocks in the Mesozoic. This is demonstrated by positive anomalies in B, U, As, Sb, Bi, and W, and by bulk rock and notably carbonate primitive mantle normalised REE patterns that are basically identical to that of calcite precipitated from Jurassic seawater except for its negative Eu anomaly. Prograde metamorphic tremolite and diopside possess enrichments notably in Sr and REE inherited from reactant calcite, as is also supported by a good match between measured and modelled silicate mineral compositions. Specific geochemical characteristics of the prograde meta-ophicarbonate rocks imply fluid mediated, open system processes. Antigorite dehydration fluid ingress, likely produced in neighbouring antigorite-serpentinites, may have shifted bulk tremolite ophicarbonate rock compositions towards higher SiO2/MgO ratios along with an increase in B contents and depletion in fluid mobile elements such as As, Sb, and Sr.The massive, contact metamorphic CO2 mobilisation documented here occurred at P-T conditions that are similar to those that can be achieved in collisional orogenic settings, whose clockwise P-T-t paths are often characterised by further heating upon initial decompression. They thus evolve at large angle to devolatilisation reactions and silicate-carbonate rock-buffered fluid XCO2 isopleths, thus fostering carbon mobilisation towards metamorphic peak temperatures at moderate to low pressures (in the region of between >700 °C – 1.0 GPa and >600 °C – 0.3 GPa). Barrovian metamorphism, too, can reach T-P conditions of 800 °C – 1 GPa. Amagmatic CO2 mobilised this way will eventually reach the atmosphere via dispersion in the groundwater table and diffuse outgassing. Despite the huge uncertainties associated with quantification of such metamorphic carbon fluxes, geological time scale global carbon cycling models should more rigorously explore the significance of these contributions.

Trace Element Composition of Silicate Minerals in the Chondrules and Matrix of the Buschhof Meteorite

The paper presents results of SIMS and SEM-EDS studies of the silicate minerals from one radial and one granular chondrule of the Buschhof ordinary equilibrated L6 chondrite and discusses geochemical features of the olivine, pyroxene, and plagioclase from the chondrules and matrix of this meteorite. Results show that pyroxene and plagioclase from the radial chondrule did not preserve the chondritic Ca/Al and Y/Ho ratios and positive correlation between La and Yb. Olivine from the granular chondrule has no traces of cosmochemical fractionation. The fact that chondritic element ratios preserve in the silicates of the granular chondrule indicates chondrule formation by the melting of refractory chondrule precursors. Both chondrules show evidence of interaction with the nebular gas and thermal metamorphism. The differences in the concentrations of trace elements in minerals of the chondrules and matrix reflect the rapid cooling of the silicate phases from the central part of the chondrule toward the meteorite matrix.

Formation of the Flatreef deposit, northern Bushveld, by hydrodynamic and hydromagmatic processes

New lithological and whole rock compositional data show that the main platinum-group element (PGE) horizons of the Flatreef succession show strong compositional similarities to the Merensky and Bastard reefs of the western Bushveld Complex (WBC), notably in terms of many immobile and incompatible minor and trace elements such as TiO2, Zr, Y, and REE. However, Al2O3, CaO, and Na2O contents are markedly lower in the Flatreef, whereas MgO contents are higher. In view of broadly similar silicate mineral compositions in the Flatreef and the WBC reefs, we suggest that the major element compositional differences between the rocks are largely due to higher modal proportions of orthopyroxene and olivine and lower proportions of plagioclase in the Flatreef. The thickness of the mineralised interval is much greater in the Flatreef than in the WBC (several 10 s of m vs ~ 1 m) and the abundance of sulfides in the Flatreef is typically somewhat higher (on average ~ 1.5% vs ~ 1%). These data, complemented by textural observations, are interpreted to reflect enhanced hydrodynamic crystal sorting accompanied by percolation of sulfide melt through incompletely solidified cumulates. Further genetic constraints are provided by metal data: The concentration of Ni (~ 3000 ppm) in the Flatreef is broadly similar to that in the Merensky Reef of the WBC, but Cu contents are markedly higher (average ~ 1500 ppm vs 700 ppm in the WBC). The concentrations of most PGE are slightly lower (Flatreef ~ 1.5–2 ppm Pt, ~ 100–150 ppb Rh; WBC MR 3.7 ppm Pt, 240 ppb Rh), but Pd has broadly similar contents (Flatreef ~ 1.2–2 ppm; WBC MR 1.54 ppm). The relatively high Cu content of the Flatreef is interpreted as a result of assimilation of Cu sulfides from the sedimentary floor rocks. The reason for the enrichment of Pd relative to Pt, especially in the basal rocks, remains unclear. It could reflect mobilisation of Pd via a fluid phase from the country rocks or the interior of the intrusion, relatively enhanced partitioning of Pd into the sulfides, or relative Pt depletion of the earliest magma pulses in response to Pt alloy fractionation triggered by contamination with reducing country rocks.

Diffusion-driven chromium isotope fractionation in ultramafic cumulate minerals: Elemental and isotopic evidence from the Stillwater Complex

Abstract To investigate chromium diffusion kinetics in ultramafic cumulate minerals, we analyzed the Cr elemental and isotopic compositions of olivine, orthopyroxene, and chromite from the Stillwater layered intrusion. Core-to-rim compositional profiles reveal that Cr elemental concentrations decrease from 60 to 20 ppm in olivine and from 5000–4600 to 2700–2400 ppm in orthopyroxene. These zoned Cr distributions in olivine and orthopyroxene suggest that Cr was lost by diffusion into the melt. Olivine and orthopyroxene have δ53Cr values ranging from –0.09 to 0.25‰ and from –0.11 to 0.07‰, respectively, higher than the values of coexisting chromite (–0.23 to –0.07‰). This isotopic disequilibrium can be explained by diffusion-driven kinetic fractionation during cooling. The preferential diffusion of light Cr isotopes from silicate minerals to the melt resulted in isotopically heavier olivine and orthopyroxene, but the kinetic diffusion between chromite and melt negligibly affected the isotopic compositions of chromite grains due to their high Cr concentrations. Modeling results based on the observed Cr contents and isotopic compositions of silicate minerals constrain the cooling time of the Peridotite Zone in the Stillwater magmatic system to have been 10–100 kyr.

A laboratory investigation on the performance of South African acid producing gold mine tailings and its possible use in mine reclamation

This paper presents the results of laboratory investigations conducted on gold mine tailings (GMT) to assess their chemical, mineralogical and geotechnical characteristics in view of assessing its suitability as an alternative backfilling solution in mine reclamation. Chemical characterization revealed that GMT is dominated by Si, Al, and Fe with notable amounts of Cr, Zr, Zn, Pb, Ce, As, Ba, Ni, V, Sr, Nd, Cu, U, and Co. Mineralogical characterization revealed a composition of silicate minerals with secondary minerals such as jarosite, goethite and hematite. GMT composites showed improved strength characteristics. The particle sizes of the tailings are capable of producing a good paste fill that will require lower water–cement ratio. Moreover, the plasticity of the tailings provide for a likelihood for shear resistance to sliding in fluvial conditions. Curing and addition of cement showed positive effects on the compressive strength and shear strength of the tailings. However, the effect of curing and cement addition on the compaction characteristics and permeability of the tailings were negligible. GMT showed favorable characteristics for use in mine backfilling; it would be interesting to evaluate higher cement ratios to improve the characteristics of the tailings.

Evolution of the North West Arm and the Central Sector of Mashava Igneous Complex in south central Zimbabwe from an investigation of its silicate minerals compositions

An investigation of the economically important Mashava Igneous Complex (MIC) in Zimbabwe was conducted using petrographic and silicate mineral compositions to gain some insights into its origin. The MIC, an ultramafic-mafic complex located in the western extremity of the Masvingo greenstone belt (MGSB), is composed of amphibole, serpentine, epidote, chlorite, clinopyroxene, orthopyroxene, plagioclase, and olivine. Spinel, sulfide minerals, and minor quartz as well as other opaque minerals are also present.The dominant mineral is serpentine, which displays a wide range in both FeO and SiO2 compositions, and the majority of this mineral belongs to the lizardite, chrysotile, and antigorite varieties. Amphiboles display a wide range in composition from tremolite and actinolite which are both consistent with greenschist facies metamorphism, through magnesiohornblende to tschermakite which are both consistent with amphibolite facies metamorphism. Clinozoisite is the only epidote type occurring in the MIC, whereas chlorite varies widely in composition which reflects the varying concentrations in iron and silica in this mineral. Chlorite types range from talc chlorite through penninite, diabantite, pycnochlorite, brunsvigite, ripidolite, and pseudothuringite.Plagioclase shows a wide compositional range from high albite to bytownite, with only two samples plotting in the alkali feldspar anorthoclase field. Pyroxene ranges in composition from enstatite through pigeonite and augite to diopside whereas olivine is rich in MgO with forsterite contents ranging from 80 to 89. Application of the chlorite geothermometry equation of Kranidiotis and MacLean (1987) yields temperatures ranging from ∼200–320 °C whereas temperatures ranging from 130 to 445 °C are obtained using equations of Lanari et al. (2014) for the formation of this mineral, generally consistent with low- to medium-grade metamorphic conditions.Chlorites in the North West Arm were formed at relatively lower temperatures (97–234 °C) than those at which chlorites in the Central Sector were formed (204–445 °C). Pressure estimates for the metamorphism of the MIC range from 3.8 to 7.3 kbars, which corresponds to depths of ∼13–25 km, respectively. Based on the different silicate mineral compositions of the North West Arm and the Central Sector, it is inferred that these two blocks underwent different petrogenetic and evolutionary histories but they were later juxtaposed, possibly during thrusting of the former onto its present position.

Characterization of matrix material in Northwest Africa 5343: Weathering and thermal metamorphism of the least equilibrated CK chondrite

AbstractBased on the chemical heterogeneity of chondrule and matrix olivine, Northwest Africa (NWA) 5343 is the least metamorphosed CK chondrite reported so far. To better constrain the lower limit of metamorphism in the CK chondrites, we performed a detailed analysis of matrix material in NWA 5343, including characterization of the texture and bulk composition and analyses of individual silicate minerals. Results suggest that NWA 5343 is petrologic type 3.6 or 3.7. Although silicate minerals in the matrix seem to be equilibrated to roughly the same extent throughout the sample, there are recognizable differences in grain size and shape. These textural differences may be the result of transient heating events during impacts, which would be likely on the CK chondrite parent body. The difference between the extent of chemical equilibration and texture may also suggest that grain size and shape are still sensitive to metamorphism at petrologic subtypes where silicate mineral equilibration is nearly complete (e.g., &gt;3.7). Carbonate material present in NWA 5343 is a product of terrestrial weathering; however, infiltration of a Ca‐bearing fluid did not influence the composition of silicate minerals in the matrix. To evaluate the possibility of a continuous metamorphic sequence between the CV and CK chondrites, the bulk matrix composition of NWA 5343 is compared to the CVred chondrite, Vigarano. Although the matrix composition of NWA 5343 could be derived by secondary processing of a Vigarano‐like precursor, porosity and texture of matrix olivine in NWA 5343 are hard to reconcile with a continuous metamorphic sequence.

Geochemistry of the Topuk Pluton associated with the Kozbudaklar W-skarn deposit (Western Anatolia, Turkey): Implication for crystallization conditions

The Kozbudaklar scheelite-bearing skarn deposit in the Tavşanlı Zone, western Turkey, occurs at the contact between Eocene Topuk pluton and Triassic İnönü marble of calcic character. The Topuk pluton is medium-coarse grained, granodiorite in composition and has a hypidiomorphic equigranular texture. The host rock contains mafic microgranular enclaves (MME) of monzodiorite–monzogabbro composition and is interrupted by porphyritic granodiorite and granite-aplite vein rocks. The pluton is calk-alkaline, metaluminous and composed of I-type melt character. δ18O and δD compositions of silicate minerals from granodioritic host rock are 5.9–10.6‰ and −77.0 to −71.4‰ and conformable with the range of unaltered I-type granites. Trace element contents indicate that pluton is crystallized from mantle-derived magma interacted with continental crust in a volcanic arc or subduction related setting. Major and trace element concentrations of Topuk pluton are quite consistent with geochemical patterns of Cu-skarn granitoids.Results of mineral chemistry analysis of the pluton yield that plagioclases are of oligoclase–andesine, amphiboles are of magnesio-hornblende and biotites are of ferro-magnesian composition. Amphiboles and biotites of granodioritic host rock are represented by calc-alkaline, I-type melt composition evolved in a subduction environment. Based on the results of plagioclase–Al in hornblende and amphibole chemistry data from the pluton, two different stages are proposed for the magma crystallization. The first stage was developed in a relatively deeper environment (>15 km) under high pressure (>4 kbar) and low log ƒO2 (>−17.6) conditions which reflect fractional crystallization and magma-mixing depth of basaltic magma and these conditions are not correlated with scheelite mineralization. The second crystallization stage of magma which proceeded at shallow depths (<6 km) was also developed in two separate phases with respect to P-T conditions. The first phase associated with scheelite mineralization is characterized by high temperature (788–854 °C), relatively high pressure (1.20–1.62 kbar), shallow depth (5–6 km) and high log ƒO2 (>−12.9 to −11.0) values are accompanied by high H2O contents (5.39–6.88 wt.%). High water content of melt gave rise to magma to ascend to shallower depths (4–3 km) and crystallization to proceed under low pressure (∼1.00 kbar), high temperature (751–859 °C) and log ƒO2 (−13.3 to −11.0) conditions with lower water contents (4.55–5.50 wt.%).

What can we learn from the trace element composition of silicate minerals in the Bushveld Complex?

What can we learn from the trace element composition of silicate minerals in the Bushveld Complex?

Two refractory Wild 2 terminal particles from a carrot‐shaped track characterized combining MIR/FIR/Raman microspectroscopy and FE‐SEM/EDS analyses

AbstractWe present the analyses results of two bulk Terminal Particles, C2112,7,171,0,0 and C2112,9,171,0,0, derived from the Jupiter‐family comet 81P/Wild 2 returned by the Stardust mission. Each particle embedded in a slab of silica aerogel was pressed in a diamond cell. This preparation, as expected, made it difficult to identify the minerals and organic materials present in these particles. This problem was overcome using a combination of three different analytical techniques, viz. FE‐SEM/EDS, IR, and Raman microspectroscopy that allowed identifying the minerals and small amounts of amorphous carbon present in both particles. TP2 and TP3 were dominated by Ca‐free and low‐Ca, Mg‐rich, Mg,Fe‐olivine. The presence of melilite in both particles is supported by IR microspectroscopy, but is not confirmed by Raman microspectroscopy, possibly because the amounts are too small to be detected. TP2 and TP3 show similar silicate mineral compositions, but Ni‐free and low‐Ni, subsulfur (Fe,Ni)S grains are present in TP2 only. TP2 contains indigenous amorphous carbon hot spots; no indigenous carbon was identified in TP3. These nonchondritic particles probably originated in a differentiated body. This work found an unanticipated carbon contamination following the FE‐SEM/EDS analyses. It is suggested that organic materials in the embedding silica aerogel are irradiated during FE‐SEM/EDS analyses creating a carbon gas that develops a strong fluorescence continuum. The combination of the selected analytical techniques can be used to characterize bulk Wild 2 particles without the need of extraction and removal of the encapsulating aerogel. This approach offers a relatively fast sample preparation procedure, but compressing the samples can cause spurious artifacts, viz. silica contamination. Because of the combination of techniques, we account for these artifacts.

In situ physical and chemical characterisation of the Eyjafjallajökull aerosol plume in the free troposphere over Italy

Abstract. Continuous measurements of physical and chemical properties at the Mt. Cimone (Italy) GAW-WMO (Global Atmosphere Watch, World Meteorological Organization) Global Station (2165 m a.s.l.) have allowed the detection of the volcanic aerosol plume resulting from the Eyjafjallajökull (Iceland) eruption of spring 2010. The event affected the Mt. Cimone site after a transport over a distance of more than 3000 km. Two main transport episodes were detected during the eruption period, showing a volcanic fingerprint discernible against the free tropospheric background conditions typical of the site, the first from April 19 to 21 and the second from 18 to 20 May 2010. This paper reports the modification of aerosol characteristics observed during the two episodes, both characterised by an abrupt increase in fine and, especially, coarse mode particle number. Analysis of major, minor and trace elements by different analytical techniques (ionic chromatography, particle induced X-ray emission–particle induced gamma-ray emission (PIXE–PIGE) and inductively coupled plasma mass spectrometry (ICP-MS)) were performed on aerosols collected by ground-level discrete sampling. The resulting database allows the characterisation of aerosol chemical composition during the volcanic plume transport and in background conditions. During the passage of the volcanic plume, the fine fraction was dominated by sulphates, denoting the secondary origin of this mode, mainly resulting from in-plume oxidation of volcanic SO2. By contrast, the coarse fraction was characterised by increased concentration of numerous elements of crustal origin, such as Fe, Ti, Mn, Ca, Na, and Mg, which enter the composition of silicate minerals. Data analysis of selected elements (Ti, Al, Fe, Mn) allowed the estimation of the volcanic plume's contribution to total PM10, resulting in a local enhancement of up to 9.5 μg m−3, i.e. 40% of total PM10 on 18 May, which was the most intense of the two episodes. These results appear significant, especially in light of the huge distance of Mt. Cimone from the source, confirming the widespread diffusion of the Eyjafjallajökull ashes over Europe.

A modified iterative sandwich method for determination of near-solidus partial melt compositions. II. Application to determination of near-solidus melt compositions of carbonated peridotite

We performed modified iterative sandwich experiments (MISE) to determine the composition of carbonatitic melt generated near the solidus of natural, fertile peridotite + CO2 at 1,200–1,245°C and 6.6 GPa. Six iterations were performed with natural peridotite (MixKLB-1: Mg# = 89.7) and ∼10 wt% added carbonate to achieve the equilibrium carbonatite composition. Compositions of melts and coexisting minerals converged to a constant composition after the fourth iteration, with the silicate mineral compositions matching those expected at the solidus of carbonated peridotite at 6.6 GPa and 1,230°C, as determined from a sub-solidus experiment with MixKLB-1 peridotite. Partial melts expected from a carbonated lherzolite at a melt fraction of 0.01–0.05% at 6.6 GPa have the composition of sodic iron-bearing dolomitic carbonatite, with molar Ca/(Ca + Mg) of 0.413 ± 0.001, Ca# [100 × molar Ca/(Ca + Mg + Fe*)] of 37.1 ± 0.1, and Mg# of 83.7 ± 0.6. SiO2, TiO2 and Al2O3 concentrations are 4.1 ± 0.1, 1.0 ± 0.1, and 0.30 ± 0.02 wt%, whereas the Na2O concentration is 4.0 ± 0.2 wt%. Comparison of our results with other iterative sandwich experiments at lower pressures indicate that near-solidus carbonatite derived from mantle lherzolite become less calcic with increasing pressure. Thus carbonatitic melt percolating through the deep mantle must dissolve cpx from surrounding peridotite and precipitate opx. Significant FeO* and Na2O concentrations in near solidus carbonatitic partial melt likely account for the ∼150°C lower solidus temperature of natural carbonated peridotite compared to the solidus of synthetic peridotite in the system CMAS + CO2. The experiments demonstrate that the MISE method can determine the composition of partial melts at very low melt fraction after a small number of iterations.

Using apatite to dispel the “trapped liquid” concept and to understand the loss of interstitial liquid by compaction in mafic cumulates: an example from the Stillwater Complex, Montana

The size, paragenetic setting, distribution, and composition of apatite grains in 16 thin sections from one sample of an olivine gabbro from the Stillwater Complex, Montana were documented to determine whether they grew from trapped interstitial liquid or in an open system. The grains show extreme variability in cross-sectional area, ranging from 300,000 μm2. The apatite grains are not associated with quartz, Fe–Ti oxides, or evolved overgrowths on cumulus grains as expected for the crystallization of a trapped liquid, which suggests that they grew in an open system. The grains are very irregularly distributed, with a single cluster comprising almost 25% of the cumulative apatite cross-sectional area from all 16 slides. The spatial patterns of the apatite indicate that the interstitial liquid moved in tubular channels during the final stages of compaction. The trace-element composition of liquids calculated from apatite and silicate mineral compositions are similar, indicating that the apatite crystallized from liquids that were not dramatically enriched in incompatible elements. We conclude that the interstitial liquid never actually became trapped, that liquid was lost in an open system to essentially zero porosity, and that the model of cumulates as a combination of cumulus grains and trapped liquid should be discarded.

Graves Nunataks 95209: A snapshot of metal segregation and core formation

GRA 95209 may provide our best opportunity to date to understand the earliest stages of core formation in asteroidal bodies. This lodranite preserves a physically, chemically, and mineralogically complex set of metal–sulfide veins. High-resolution X-ray computed tomography revealed three distinct lithologies. The dominant mixed metal–silicate–sulfide matrix is cut by metal-rich, graphite-bearing veins exceeding 1 cm in width and grades into a volumetrically minor metal-poor region. Silicate compositions and modal abundances are typical for lodranites, while the mineralogy of the metal–sulfide component is complex and differs among the three lithologies. Kamacite and troilite occur with chromite, tetrataenite, schreibersite, graphite, and a range of phosphates. An 39Ar– 40Ar age of 4.521 ± 0.006 Ga measures the time of closure of the K–Ar system. Carbon rosettes within the metal-rich vein are nitrogen-poor, well crystallized, include kamacite sub-grains of composition comparable to the host metal, and are essentially isotopically homogeneous (δ 13C ∼ −33‰). In contrast, carbon rosettes within metal of the metal-poor lithology are N-poor, poorly crystallized, include kamacite grains that are Ni-poor compared to their host metal, and are isotopically heterogeneous (δ 13C ranging from −50 to +80‰) even within a single metal grain. The silicate portion of GRA 95209 is similar to the lodranite EET 84302, sharing a common texture, silicate mineral compositions, and Ar–Ar age. GRA 95209 and EET 84302 are intermediate between acapulcoites and lodranites. Both experienced Fe,Ni-FeS melting with extensive melt migration, but record only the onset of silicate partial melting with limited migration of silicate melt. The complex metal–sulfide veins in GRA 95209 resulted from low-degree partial melting and melt migration and intruded the matrix lithology. Reactions between solid minerals and melt, including oxidation–reduction reactions, produced the array of phosphates, schreibersite, and tetrataenite. Extensive reduction in the metal-rich vein resulted from its origin in a hotter portion of the asteroid. This difference in thermal history is supported by the graphite structures and isotopic compositions. The graphite rosettes in the metal-rich vein are consistent with high-temperature igneous processing. In contrast, the carbon in the metal-poor lithology appears to preserve a record of formation in the nebula prior to parent-body formation. Carbon incorporated from the solar nebula into a differentiating asteroid is preferentially incorporated in metal–sulfide melts that form a core, but does not achieve isotopic homogeneity until extensive thermal processing occurs.

Mineralogy of a Martian meteorite as determined by Raman spectroscopy

AbstractUsing the Raman point‐count procedure that we anticipate using in planetary surface exploration, we have identified and characterized the major, minor and trace mineral phases in rock chips of a Martian meteorite, EETA 79001. Raman spectra are shown for pyroxene, olivine, maskelynite (shocked, isotropized feldspar), chromite, magnetite, ilmenite, ulvöspinel, pyroxferroite, merrillite, apatite, anatase, an Fe sulfide, calcite and hematite. Raman spectra provide information on compositional variations of pyroxene, olivine and FeTiCr oxides and modal proportions of the rock. The maskelynite‐free mineral mode obtained on rough rock surfaces is consistent with modes obtained by optical microscopy on a thin section of lithology A of this meteorite. Compositions and variations in compositions of major silicate minerals (pyroxene and olivine) on the basis of Raman peak positions are consistent with those obtained in previous studies by electron probe microanalysis. The variations in composition of the silicate minerals represent different stages of crystallization during the formation of this rock near the Martian surface. Copyright © 2004 John Wiley &amp; Sons, Ltd.

The Worden meteorite: A new ordinary chondrite fall from Michigan, USA

Abstract— An ordinary chondrite fall in southeast Michigan, USA (near the crossroads hamlet of Worden in northeast Washtenaw County) penetrated the garage roof of a private home on 1997 September 1. The Worden chondrite comprises silicate matrix, mineral fragments, chondrules, chondrule fragments, and opaque primary phases. Electron microprobe analyses (olivine, Fa23.9; orthopyroxene, Fs20.1, En78.8, Wo1) indicate diagnostic L‐chondrite silicate mineral compositions. Recognizable chondrules and chondrule fragments constitute up to 42 vol%. Chondrule boundaries are readily discernable (especially where chondrules have rims of opaque material) but not sharp, and discrete plagioclase crystals are visible in the devitrified mesostasis of barred olivine chondrules; together, these characteristics suggest petrologic type 5. The spatially averaged ensemble of shock‐related features (optical extinction of olivine grains, minor localized shock melt) suggests assignment of a shock stage of S3.The 26Al and 22Na activities are typical for an L chondrite. Worden fell near the most recent solar minimum in 1997, and the 22Na production rate should have been at a level approaching the maximum levels due to solar modulation of the galactic cosmic‐ray flux. The low value for the Worden 22Na activity relative to those observed in chondrite falls associated with the 1969 solar maximum and 1976 solar minimum suggests a relatively small preatmospheric size. The 60Co activity confirms the indication of a small body. The 56Co activity was essentially zero, indicating that none of the recovered meteorite contained material exposed to solar cosmic rays.The Worden chondrite is the fourth documented fall in Michigan, and the fourth stony meteorite recovered in Michigan; all other Michigan meteorites are finds, not observed falls, and are iron meteorites. All Michigan falls to date are ordinary chondrites. The three falls prior to Worden were Allegan (H5) fell 1899; Rose City (H5, brecciated, black (dark matrix)) fell 1921; and Coleman (L6, veined (shocked); Osborn et al., 1997) fell 1994. Worden has a much lighter matrix than Rose City, and is less extensively thermally and shock metamorphosed than either Rose City (Stöffler et al., 1991) or Coleman.

A primary platinum occurrence in the Freetown Layered Intrusion, Sierra Leone

The primary source of the alluvial platinum-group minerals (PGM) of the Freetown Layered Intrusion, Sierra Leone, has been unknown since their discovery 70 years ago. This is a report on work in progress to establish a primary occurrence related to the igneous layering. New whole-rock major and minor element analyses and precious metal assays show that at key horizons there is a significant change in the silicate mineral compositions and in Cu, Cr and Ni, as well as in Pt and Pd, comparable to those encountered in other PGM-bearing layered intrusions. Anomalous (0.1 to 0.4 g/t) Pt levels occur in pyroxene troctolites, which maintain these levels along strike.