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C M Pieters - One of the best experts on this subject based on the ideXlab platform.
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the character of south pole aitken basin patterns of surface and subsurface composition
2018Co-Authors: D P Moriarty, C M PietersAbstract:Using Moon Mineralogy Mapper data, we characterize surface diversity across the enormous South Pole - Aitken Basin (SPA) by evaluating the abundance and composition of Pyroxenes, which are overwhelmingly the most abundant mafic mineral in the region. Although SPA exhibits significant complexity due to billions of years of geologic processes subsequent to formation, the basin has retained regular patterns of compositional heterogeneity across its structure. Four distinct, roughly concentric zones are defined: (1) a central SPA compositional anomaly (SPACA), which exhibits a pervasive elevated Ca,Fe-rich pyroxene abundance, (2) a Mg-Pyroxene Annulus, which is dominated by abundant Mg-rich Pyroxenes, (3) a Heterogeneous Annulus, which exhibits localized pyroxene-rich areas spatially mixed with feldspathic materials, and (4) the SPA Exterior, which is primarily feldspathic. Pyroxene compositions in the Heterogeneous Annulus are similar to those in the Mg-Pyroxene Annulus, and Mg-rich Pyroxenes also underlie the more Ca,Fe-rich pyroxene surface material across SPACA. The establishment of these four distinct compositional zones across SPA constrains future basin evolution models serves to guide potential sample return (and other science) targets.
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complexities in pyroxene compositions derived from absorption band centers examples from apollo samples hed meteorites synthetic pure Pyroxenes and remote sensing data
2016Co-Authors: D P Moriarty, C M PietersAbstract:We reexamine the relationship between pyroxene composition and near-infrared absorption bands, integrating measurements of diverse natural and synthetic samples. We test an algorithm (PLC) involving a two-part linear continuum removal and parabolic fits to the 1 and 2 μm bands—a computationally simple approach which can easily be automated and applied to remote sensing data. Employing a suite of synthetic pure Pyroxenes, the PLC technique is shown to derive similar band centers to the modified Gaussian model. PLC analyses are extended to natural pyroxene-bearing materials, including (1) bulk lunar basalts and pyroxene separates, (2) diverse lunar soils, and (3) HED meteorites. For natural Pyroxenes, the relationship between composition and absorption band center differs from that of synthetic Pyroxenes. These differences arise from complexities inherent in natural materials such as exsolution, zoning, mixing, and space weathering. For these reasons, band center measurements of natural pyroxene-bearing materials are compositionally nonunique and could represent three distinct scenarios (1) pyroxene with a narrow compositional range, (2) complexly zoned pyroxene grains, or (3) a mixture of multiple pyroxene (or nonpyroxene) components. Therefore, a universal quantitative relationship between band centers and pyroxene composition cannot be uniquely derived for natural pyroxene-bearing materials without additional geologic context. Nevertheless, useful relative relationships between composition and band center persist in most cases. These relationships are used to interpret M3 data from the Humboldtianum Basin. Four distinct compositional units are identified (1) Mare Humboldtianum basalts, (2) distinct outer basalts, (3) low-Ca pyroxene-bearing materials, and (4) feldspathic materials.
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new insights into lunar petrology distribution and composition of prominent low ca pyroxene exposures as observed by the moon mineralogy mapper m3
2011Co-Authors: Rachel L. Klima, C M Pieters, J Boardman, Robert O Green, John F Mustard, Jeff Nettles, N E Petro, P. Isaacson, James W. Head, M StaidAbstract:[1] Lunar geochemical groups such as Mg suite, ferroan anorthosite, and alkali suite rocks are difficult to distinguish from orbit because they are defined by both modal mineralogy and elemental composition of their constituent minerals. While modal mineralogy can be modeled, only specific minerals or elements can be directly detected. At near-infrared (NIR) wavelengths, Pyroxenes are among the most spectrally distinctive minerals, and their absorption bands are sensitive to structure and composition. Pyroxenes thus provide important clues to distinguish these geochemical groups and to understand lunar crustal evolution. Using Moon Mineralogy Mapper data, we search for lithologies dominated by strong low-calcium pyroxene (LCP) signatures. We compare the NIR absorptions of 20 LCPs to a suite of synthetic Pyroxenes to determine which lunar Pyroxenes appear magnesian enough to be candidate Mg suite norites. We detail three prominent regions of LCP (1) in South Pole–Aitken Basin (SPA), (2) south of Mare Frigoris, and (3) north of Mare Frigoris. The absorption band positions suggest that the LCPs north of Mare Frigoris and those in SPA are compositionally similar to one another and of ∼Mg50–75, implying that the mafic material excavated by the SPA impact was relatively iron-rich. Modified Gaussian modeling results suggest that the Apollo basin may have tapped different composition material than is exposed in much of SPA. The LCPs located in the highlands south of Mare Frigoris exhibit absorption bands at short wavelengths consistent with Mg > ∼80. The coincidence of these Mg-rich LCPs with the thorium measured by Lunar Prospector make them good candidates for KREEP-related Mg suite Pyroxenes.
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near infrared spectra of clinoPyroxenes effects of calcium content and crystal structure
2011Co-Authors: Rachel L. Klima, Darby M Dyar, C M PietersAbstract:Pyroxenes are among the most common minerals in the solar system and are ideally suited for remote geochemical analysis because of the sensitivity of their distinctive spectra to mineral composition. Fe 2+ is responsible for the dominant pyroxene absorptions in the visible and near-infrared, but substitutions of other cations such as Ca 2+ change the crystal structure and site geometries and thus the crystal field splitting energies of the Fe cations. To define spectral systematics resulting from major pyroxene cations (Ca 2+ ,M g 2+ , and Fe 2+ ), we focus on a suite of Pyroxenes synthesized with only Ca 2+ ,M g 2+ , and Fe 2+ in the two octahedral sites, specifically examining the effect of Ca 2+ on pyroxene absorption bands. The modified Gaussian model is used to deconvolve pyroxene spectra into component bands that can then be linked directly to crystal field absorptions. In orthoPyroxenes and low-Ca clinoPyroxenes, Ca 2+ -content has a strong and predictable effect on the positions of the absorption bands. At a threshold of Wo30, the crystal field environment stagnates and the M2 bands cease to change significantly as more Ca 2+ is added. At Wo50, when most of the M2 sites are filled by Ca 2+ , band positions do not change drastically, although the presence and strengths of the 1 and 2 lm bands are affected by even trace amounts of Fe 2+ in the M2 site. It is thus apparent that next-nearest neighbors and the distortions they impose on the pyroxene lattice affect the electronic states around the Fe 2+ cations and control absorption band properties.
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characterization of the 1 2 μm m1 pyroxene band extracting cooling history from near ir spectra of Pyroxenes and pyroxene dominated rocks
2008Co-Authors: Rachel L. Klima, C M Pieters, Darby M DyarAbstract:The 1.2 m band in near-infrared spectra of Pyroxenes results from Fe2+ in the M1 crystallographic site. The distribution of Fe and Mg between the M1 and M2 sites is in part a function of the cooling rate and thermal history of a pyroxene. Combining near-infrared and Mssbauer spectra for a series of compositionally controlled synthetic Mg, Fe, Ca Pyroxenes, we quantify the strength of the 1.2 m band as a function of Fe2+ in the M1 site. Near-infrared spectra are deconvolved into component absorptions that can be assigned to the M1 and M2 sites using the modified Gaussian model. The relative strength of the 1.2 μm band is shown to be directly related to the amount of Fe2+ in the M1 site measured by Mossbauer spectroscopy. The strength of the 1.2 μm band relative to the combined strengths of the 1.2 and 2 μm bands, or the M1 intensity ratio, is calculated for 51 howardite, eucrite, and diogenite (HED) meteorites. Diogenites and cumulate eucrites exhibit the lowest M1 intensity ratios, consistent with their formation as slowly cooled cumulates. Basaltic eucrites exhibit a large range of M1 intensity ratios, all of which are consistently higher than the diogenites and cumulate eucrites. This example illustrates how the M1 intensity ratio can be a used as a tool for characterizing the cooling history of remotely detected pyroxene-dominated rocks.
Mark S Ghiorso - One of the best experts on this subject based on the ideXlab platform.
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thermodynamics of multicomponent Pyroxenes iii calibration of fe 2 mg 1 tial 2 mgsi 2 1 tife 2 3 mgsi 2 1 alfe 3 mgsi 1 naal camg 1 al 2 mgsi 1 and ca mg 1 exchange reactions between Pyroxenes and silicate melts
1994Co-Authors: Richard O Sack, Mark S GhiorsoAbstract:A thermodynamic model for the Gibbs free energy of igneous Pyroxenes with the general formula [Na, Ca, Fe2+, Mg]M2[Fe2+, Mg, Ti, Al, Fe3+]M1[Al, Fe3+, Si]TetSiO6 is calibrated from experimentally determined compositions of coexisting pyroxene and silicate melt. The model is based upon the general formulation, and relies upon the calibration of the “quadrilateral” subsystem, previously published by the present authors. The calibration database of pyroxene-liquid equilibria spans a broad spectrum of temperature, pressure and oxygen fugacity conditions, ranging from 1000°–1600°C, 0.001–30 kbar and iron-wustite to air. Chemical potentials of endmember pyroxene components as well as exchange potentials between Pyroxenes and coexisting liquids are defined utilizing the present authors' thermodynamic melt model. Model parameters are extracted from these relations by regression analysis. The resulting model and derivative endmember properties are internally consistent with an existing standard state thermodynamic database. The success of the model and its applicability to igneous petrogenesis are demonstrated by comparing calculated and experimentally determined liquidus compositions, temperatures and symmetry states for Pyroxenes crystallizing from a variety of silicate melts, ranging in composition from tholeiites and angrites through rhyolites to potash ankaratrites.
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thermodynamics of multicomponent Pyroxenes ii phase relations in the quadrilateral
1994Co-Authors: Richard O Sack, Mark S GhiorsoAbstract:The model for the thermodynamic properties of multicomponent Pyroxenes (Part I) is calibrated for ortho- and clinoPyroxenes in the quadrilateral subsystem defined by the end-member components Mg2Si2O6, CaMgSi2O6, CaFeSi2O6, and Fe2Si2O6. This calibration accounts for: (1) Fe-Mg partitioning relations between orthoPyroxenes and augites, and between pigeonites and augites, (2) miscibility gap features along the constituent binary joins CaMgSi2O6-Mg2Si2O6 and CaFeSi2O6-Fe2Si2O6, (3) calorimetric data for CaMgSi2O6-Mg2Si2O6 Pyroxenes, and (4) the P-T-X systematics of both the reaction pigeonite=orthopyroxene+augite, and miscibility gap featurs, over the temperature and pressure ranges 800–1500°C and 0–30 kbar. The calibration is achieved with the simplifying assumption that all regular-solution-type parameters are constants independent of temperature. It is predicated on the assumptions that: (1) the Ca-Mg substitution is more nonideal in Pbca Pyroxenes than in C2/c Pyroxenes, and (2) entropies of about 3 and 6.5 J/K-mol are associated with the change of Ca from 6- to 8-fold coordination in the M2 site in magnesian and iron C2/c Pyroxenes, respectively. The model predicts that Fe2+-Mg2+ M1-M2 site preferences in C2/c Pyroxenes are highly dependent on Ca and Mg contents, with Fe2+ more strongly preferring M2 sites both in Ca-rich C2/c Pyroxenes with a given Fe/(Fe+Mg) ratio, and in magnesian C2/c Pyroxenes with intermediate Ca/(Ca+Fe+Mg) ratios. The proposed model is internally consistent with our previous analyses of the solution properties of spinels, rhombohedral oxides, and Fe-Mg olivines and orthPyroxenes. Results of our calibration extend an existing database to include estimates for the thermodynamic properties of the C2/c and Pbca pyroxene end-members clinoenstatite, clinoferrosilite, hedenbergite, orthodiopside, and orthohedenbergite. Phase relations within the quadrilateral and its constitutent subsystems are calculated for temperatures and pressures over the range 800–1700°C and 0–50 kbar and compare favorably with experimental constraints.
Darby M Dyar - One of the best experts on this subject based on the ideXlab platform.
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near infrared spectra of clinoPyroxenes effects of calcium content and crystal structure
2011Co-Authors: Rachel L. Klima, Darby M Dyar, C M PietersAbstract:Pyroxenes are among the most common minerals in the solar system and are ideally suited for remote geochemical analysis because of the sensitivity of their distinctive spectra to mineral composition. Fe 2+ is responsible for the dominant pyroxene absorptions in the visible and near-infrared, but substitutions of other cations such as Ca 2+ change the crystal structure and site geometries and thus the crystal field splitting energies of the Fe cations. To define spectral systematics resulting from major pyroxene cations (Ca 2+ ,M g 2+ , and Fe 2+ ), we focus on a suite of Pyroxenes synthesized with only Ca 2+ ,M g 2+ , and Fe 2+ in the two octahedral sites, specifically examining the effect of Ca 2+ on pyroxene absorption bands. The modified Gaussian model is used to deconvolve pyroxene spectra into component bands that can then be linked directly to crystal field absorptions. In orthoPyroxenes and low-Ca clinoPyroxenes, Ca 2+ -content has a strong and predictable effect on the positions of the absorption bands. At a threshold of Wo30, the crystal field environment stagnates and the M2 bands cease to change significantly as more Ca 2+ is added. At Wo50, when most of the M2 sites are filled by Ca 2+ , band positions do not change drastically, although the presence and strengths of the 1 and 2 lm bands are affected by even trace amounts of Fe 2+ in the M2 site. It is thus apparent that next-nearest neighbors and the distortions they impose on the pyroxene lattice affect the electronic states around the Fe 2+ cations and control absorption band properties.
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characterization of the 1 2 μm m1 pyroxene band extracting cooling history from near ir spectra of Pyroxenes and pyroxene dominated rocks
2008Co-Authors: Rachel L. Klima, C M Pieters, Darby M DyarAbstract:The 1.2 m band in near-infrared spectra of Pyroxenes results from Fe2+ in the M1 crystallographic site. The distribution of Fe and Mg between the M1 and M2 sites is in part a function of the cooling rate and thermal history of a pyroxene. Combining near-infrared and Mssbauer spectra for a series of compositionally controlled synthetic Mg, Fe, Ca Pyroxenes, we quantify the strength of the 1.2 m band as a function of Fe2+ in the M1 site. Near-infrared spectra are deconvolved into component absorptions that can be assigned to the M1 and M2 sites using the modified Gaussian model. The relative strength of the 1.2 μm band is shown to be directly related to the amount of Fe2+ in the M1 site measured by Mossbauer spectroscopy. The strength of the 1.2 μm band relative to the combined strengths of the 1.2 and 2 μm bands, or the M1 intensity ratio, is calculated for 51 howardite, eucrite, and diogenite (HED) meteorites. Diogenites and cumulate eucrites exhibit the lowest M1 intensity ratios, consistent with their formation as slowly cooled cumulates. Basaltic eucrites exhibit a large range of M1 intensity ratios, all of which are consistently higher than the diogenites and cumulate eucrites. This example illustrates how the M1 intensity ratio can be a used as a tool for characterizing the cooling history of remotely detected pyroxene-dominated rocks.
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spectroscopy of synthetic mg fe Pyroxenes i spin allowed and spin forbidden crystal field bands in the visible and near infrared
2007Co-Authors: R L Klima, C M Pieters, Darby M DyarAbstract:Understanding the fundamental crystal chemical controls on visible and near-infrared reflectance spectra of Pyroxenes is critical to quantitatively assessing the mineral chemistry of Pyroxenes viewed by remote sensing. This study focuses on the analysis ofspectroscopic measurements of a comprehensive set of synthetic Mg-Fe Pyroxenes from the visible through the near-infrared (0.3-2.6 μm) to address the constraints of crystal structure and Fe^2+ content on spin-forbidden and spin-allowed crystal field absorptions in Ca-freeorthoPyroxenes. The chemistry and oxidation state of the synthetic Pyroxenes are characterized. Coordinated Mssbauer spectroscopy is used to determine site occupancy of Fe^2+ in the M1 and M2 crystallographic sites. Properties of visible and near-infrared absorption bands of the synthetic Pyroxenes are quantified using the modified Gaussian model. The 1 and 2 μm spin-allowed crystal field absorption bands move regularly with increasing iron content, defining a much tighter trend than observed previously. A spin-allowed crystal field absorption band at 1.2 μm is explicitly verified, even at low total iron contents, indicating that some portion of Fe^2+ resides in the M1 site. The 1.2 μm band intensifies and shifts to longer wavelengths with increasing iron content. At visible wavelengths, spin-forbidden crystal field absorptions are observed in all iron-bearing samples. The most prominent absorption near 506 nm, attributed to iron in the M2 site, shifts to slightly longer wavelengths with iron content. The purity and extent of this pyroxene series allows visible wavelength absorption bands to be directly assigned to specific transitions of Fe^2+ in the M1 and M2 sites.
Alexander N Krot - One of the best experts on this subject based on the ideXlab platform.
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oxygen isotope compositions of chondrule phenocrysts and matrix grains in kakangari k grouplet chondrite implication to a chondrule matrix genetic relationship
2015Co-Authors: Kazuhide Nagashima, Alexander N Krot, G R HussAbstract:Abstract To investigate a possible relationship between chondrules and matrix, we studied mineralogy, mineral chemistry, and in situ O-isotope compositions of chondrules, clastic matrix grains, and amoeboid olivine aggregates (AOAs) in the Kakangari K-grouplet chondrite. Most olivines and low-Ca Pyroxenes in the Kakangari chondrules, matrix, and AOAs have similar magnesium-rich compositions, Fo ∼95–97 (∼0.3–0.5 wt% MnO) and En ∼90–96 , respectively. These rather uniform chemical compositions of the different chondritic components are likely due to partial Fe–Mg–Mn equilibration during thermal metamorphism experienced by the host meteorite. Oxygen-isotope compositions of olivine and low-Ca pyroxene grains in chondrules and matrix plot along a slope-1 line on a three O-isotope diagram and show a range from 16 O-enriched composition similar to that of the Sun to 16 O-depleted composition similar to the terrestrial O-isotope composition. Most olivines and low-Ca Pyroxenes in chondrules are 16 O-poor and plot on or close to the terrestrial mass-fractionation line (mean Δ 17 O values ±2 standard deviations: 0.0 ± 0.8‰ and +0.2 ± 0.9‰ for olivine and pyroxene, respectively), consistent with the previously reported compositions of bulk chondrules (Δ 17 O = −0.16 ± 0.70‰). In addition to these 16 O-poor grains, a coarse-grained igneous rim surrounding a porphyritic chondrule contains abundant 16 O-rich relict olivines (Δ 17 O ∼ −24‰). Oxygen-isotope compositions of olivines and low-Ca Pyroxenes in matrix show a bimodal distribution: 12 out of 13 olivine and 4 out of 17 pyroxene grains measured are similarly 16 O-rich (Δ 17 O ∼ −23.5 ± 2.9‰), others are similarly 16 O-poor (Δ 17 O ∼ −0.1 ± 1.7‰). Due to slow oxygen self-diffusion, olivines and low-Ca Pyroxenes largely retained their original oxygen-isotope compositions. The nearly identical O-isotope compositions between the chondrule phenocrysts and the 16 O-poor matrix grains suggest both chondrules and matrix of Kakangari sampled isotopically the same reservoirs. In addition, the presence of abundant 16 O-rich grains in matrix and the chondrule igneous rim suggests both components acquired similar precursor inventories. These observations imply that chondrules and matrix in Kakangari are genetically related in the sense that material that formed matrix was one of the precursors of chondrules and chondrules and some fraction of matrix experienced the same thermal processing event. The 16 O-enriched bulk matrix value compared to the bulk chondrules reported previously is likely due to presence of abundant 16 O-rich grains in the Kakangari matrix.
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amoeboid olivine aggregates with low ca Pyroxenes a genetic link between refractory inclusions and chondrules
2004Co-Authors: Alexander N Krot, Michail I Petaev, Hisayoshi YurimotoAbstract:Amoeboid olivine aggregates (AOAs) in primitive (unmetamorphosed and unaltered) carbona- ceous chondrites are uniformly 16 O-enriched ( 17 O 20‰) and consist of forsterite (Fa2), FeNi-metal, and a refractory component (individual CAIs and fine-grained minerals interspersed with forsterite grains) composed of Al-diopside, anorthite, spinel, and exceptionally rare melilite (A u k15); some CAIs in AOAs have compact, igneous textures. Melilite in AOAs is replaced by a fine-grained mixture of spinel, Al-diopside, and anorthite. Spinel is corroded by anorthite or by Al-diopside. In 10% of 500 AOAs studied in the CR, CV, CM, CO, CH, CB, and ungrouped carbonaceous chondrites Acfer 094, Adelaide, and LEW85332, forsterite is replaced to a various degree by low-Ca pyroxene. There are three major textural occurrences of low-Ca pyroxene in AOAs: (i) thin (10 m) discontinuous layers around forsterite grains or along forsterite grain boundaries in AOA peripheries; (ii) haloes and subhedral grains around FeNi-metal nodules in AOA peripheries, and (iii) thick (up to 70 m) continuous layers with abundant tiny inclusions of FeNi-metal grains around AOAs. AOAs with low-Ca pyroxene appear to have experienced melting of various degrees. In the most extensively melted AOA in the CV chondrite Leoville, only spinel grains are relict; forsterite, anorthite and Al-diopside were melted. This AOA has an igneous rim of low-Ca pyroxene with abundant FeNi-metal nodules and is texturally similar to Type I chondrules. Based on these observations and thermodynamic analysis, we conclude that AOAs are aggregates of relatively low temperature solar nebular condensates originated in 16 O-rich gaseous reservoir(s), probably CAI-forming region(s). Some of the CAIs were melted before aggregation into AOAs. Many AOAs must have also experienced melting, but of a much smaller degree than chondrules. Before and possibly after aggregation, melilite and spinel reacted with the gaseous SiO and Mg to form Ca-Tschermakite (CaAl2SiO6)-diopside (CaMgSi2O6) solid solution and anorthite. Solid or incipiently melted olivine in some AOAs reacted with gaseous SiO in the CAI- or chondrule-forming regions to form low-Ca pyroxene: Mg2SiO4 SiO(g) H2O(g) Mg2Si2O6 H2(g). Some low-Ca Pyroxenes in AOAs may have formed by oxidation of Si-bearing FeNi-metal: Mg2SiO4 Si(in FeNi) 2H2O(g) Mg2Si2O6 2H2(g) and by direct gas-solid condensation: Mg(g) SiO(g) H2O(g) Mg2Si2O6(s) H2(g) from fractionated (Mg/Si ratio solar) nebular gas. Although bulk compositions of AOAs are rather similar to those of Type I chondrules, on the projection from spinel onto the plane Ca2SiO4-Mg2SiO4-Al2O3, these objects plot on different sides of the anorthite-forsterite thermal divide, suggesting that Type I chondrules cannot be produced from AOAs by an igneous fractionation. Formation of low-Ca pyroxene by reaction of AOAs with gaseous SiO and by melting of silica-rich dust accreted around AOAs moves bulk compositions of the AOAs towards chondrules, and provide possible mechanisms of transformation of refractory materials into chondrules or chondrule precursors. The rare occurrences of low-Ca pyroxene in AOAs may indicate that either AOAs were isolated from the hot nebular gas before condensation of low-Ca pyroxene or that condensation of low-Ca pyroxene by reaction between forsterite and gaseous SiO was kinetically inhibited. If the latter is correct, then the common occurrences of pyroxene-rich Type I chondrules may require either direct condensation of low-Ca Pyroxenes or SiO2 from fractionated nebular gas or condensation of gaseous SiO into chondrule melts. Copyright © 2004 Elsevier Ltd
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secondary calcium iron rich minerals in the bali like and allende like oxidized cv3 chondrites and allende dark inclusions
1998Co-Authors: Alexander N Krot, Michael I Petaev, M E Zolensky, K Keil, E Scott, Keiko NakamuraAbstract:— We have characterized Ca-Fe-rich silicates (salite-hedenbergite Pyroxenes (Fs10–50Wo45–50), andradite (Ca3Fe2Si3O12), kirschsteinite (CaFeSiO4), and wollastonite (Ca3Si3O9)) in the type I chondrules and matrices in the Bali-like and Allende-like oxidized CV3 chondrites and Allende dark inclusions. In type I chondrules in the Bali-like CV3 chondrites, metal is oxidized to magnetite; magnetite-sulfide nodules are replaced by Ca-Fe-rich Pyroxenes with minor andradite and pure fayalite. We infer that Ca-Fe-rich Pyroxenes, andradite, fayalite, magnetite, and phyllosilicates (which occur in mesostases) formed at relatively low temperatures (<300 °C) in the presence of aqueous solutions. Thermodynamic analysis of phase relations in the Si-Fe-Ca-O-H system and large O isotopic fractionation of the coexisting magnetite and fayalite (∼20%) (Krot et al., 1998) are consistent with this interpretation. In type I chondrules in the Allende-like CV3 chondrites and dark inclusions, magnetite-sulfide nodules are replaced by Ca-Fe-rich Pyroxenes and ferrous olivine; low-Ca pyroxene and forsterite phenocrysts are rimmed and veined by ferrous olivine. It appear that the Ca-Fe-rich Pyroxenes predate formation of ferrous olivine; the latter postdates formation of talc and biopyriboles (Brearley, 1997). The Allende dark inclusions are crosscut by Ca-Fe-pyroxene-andradite veins and surrounded by Ca-rich rims that consist of Ca-Fe-rich Pyroxenes, andradite, wollastonite, and kirschsteinite. Calcium-rich veins and rims formed after aggregation and lithification of the dark inclusions. The rimmed dark inclusions show zoned depletion in Ca, which is due to a lower abundance of Ca-Fe-rich Pyroxenes close to the rim. Calcium was probably leached from the inclusions and redeposited along their edges. We infer that the Allende-like chondrites and dark inclusions experienced similar aqueous alteration to the Bali-like chondrites and were metamorphosed subsequently, which resulted in loss of aqueous solutions and dehydration of phyllosilicates. We conclude that Ca-Fe-rich silicates in the oxidized CV3 chondrites and Allende dark inclusions are secondary and resulted from aqueous fluid-rock interactions during progressive metamorphism of a heterogeneous mixture of hydrous (ices?) and anhydrous materials; the latter were possibly mineralogically similar to the reduced CV3 chondrites.
H. Y. Mcsween - One of the best experts on this subject based on the ideXlab platform.
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crystallization kinetics of olivine phyric shergottites
2014Co-Authors: Megan Elizabeth Ennis, H. Y. McsweenAbstract:Crystal size distribution (CSD) and spatial distribution pattern (SDP) analyses are applied to the early crystallizing phases, olivine and pyroxene, in olivine-phyric shergottites (Elephant moraine [EET] 79001A, Dar al Gani [DaG] 476, and dhofar [Dho] 019) from each sampling locality inferred from Mars ejection ages. Trace element zonation patterns (P and Cr) in olivine are also used to characterize the crystallization history of these Martian basalts. Previously reported 2-D CSDs for these meteorites are re-evaluated using a newer stereographically corrected methodology. Kinks in the olivine CSD plots suggest several populations that crystallized under different conditions. CSDs for pyroxene in DaG 476 and EET 79001A reveal single populations that grew under steady-state conditions; Pyroxenes in Dho 019 were too intergrown for CSD analysis. Magma chamber residence times of several days for small grains to several months for olivine megacrysts are calculated using the CSD slopes and growth rates inferred from previous experimental data. Phosphorus imaging in olivines in DaG 476 and Dho 019 indicate rapid growth of skeletal, sector-zoned, or patchy cores, probably in response to delayed nucleation, followed by slow growth, and finally rapid dendritic growth with back-filling to form oscillatory zoning in rims. SPD analyses indicate that olivine and pyroxene crystals grew or accumulated in clusters rather than as randomly distributed grains. These data reveal complex solidification histories for Martian basalts, and are generally consistent with the formation at depth of olivine megacryst cores, which were entrained in ascending magmas that crystallized Pyroxenes, small olivines, and oscillatory rims on megacrysts.
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water in martian magmas clues from light lithophile elements in shergottite and nakhlite Pyroxenes
2001Co-Authors: R C F Lentz, H. Y. Mcsween, Jeffrey G Ryan, Lee R RiciputiAbstract:Abstract There is abundant geomorphic evidence that Mars once had potentially significant amounts of water on its surface. Bulk martian meteorites are curiously dry, and hydrated minerals found in some of these rocks are also surprisingly low in water content. We look for evidence of pre-eruptive magmatic water by analyzing the abundances of Li, Be, and B, light lithophile elements that have proven useful in tracking water-magma interactions in terrestrial studies because of their solubility differences. We performed secondary ionization mass spectrometer (SIMS) analysis of these incompatible elements in Pyroxenes of two nakhlites and two basaltic shergottites, with quite different results. In Nakhla and Lafayette, all three elements behave as incompatible elements, with increasing abundance with magma evolution from pyroxene cores to rims. In Shergotty and Zagami, Be increases, but both B and Li decrease from pyroxene cores to rims. From terrestrial studies, it is known that Be is virtually insoluble in aqueous hydrothermal fluids, whereas B and Li are quite soluble. We suggest, therefore, that the elemental decreases in the shergottite Pyroxenes reflect dissolution and loss of B and Li in a hot, aqueous fluid exsolved from the magma. Consistent with our results, recent experimental work proposes that the shergottite parent magmas contained 1.8 wt% water (Dann et al., 2001) . We suggest that the pyroxene cores grew at depth (>4 km) where the water would remain dissolved in the magma. Once the magma began to ascend, the volatile component could gradually exsolve, removing the soluble species from the melt in the process. Upon eruption, the volatile component might then be lost through degassing, leaving a B- and Li-depleted magma to crystallize pyroxene rims and plagioclase. This magmatic water might have derived from the martian mantle or resulted from deep crustal contamination. If the water contents proposed for the shergottite magmas, and implied by our results, are typical of basaltic magmas on Mars, this mechanism could provide an efficient method of delivering substantial amounts of water to the martian surface at later times in martian history.
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geochemical evidence for magmatic water within mars from Pyroxenes in the shergotty meteorite
2001Co-Authors: H. Y. Mcsween, R C F Lentz, Lee R Riciputi, T L Grove, J C Dann, Astrid Holzheid, Jeffrey G RyanAbstract:Observations of martian surface morphology have been used to argue that an ancient ocean once existed on Mars1. It has been thought that significant quantities of such water could have been supplied to the martian surface through volcanic outgassing, but this suggestion is contradicted by the low magmatic water content that is generally inferred from chemical analyses of igneous martian meteorites2. Here, however, we report the distributions of trace elements within Pyroxenes of the Shergotty meteorite—a basalt body ejected 175 million years ago from Mars3—as well as hydrous and anhydrous crystallization experiments that, together, imply that water contents of pre-eruptive magma on Mars could have been up to 1.8%. We found that in the Shergotty meteorite, the inner cores of pyroxene minerals (which formed at depth in the martian crust) are enriched in soluble trace elements when compared to the outer rims (which crystallized on or near to the martian surface). This implies that water was present in Pyroxenes at depth but was largely lost as Pyroxenes were carried to the surface during magma ascent. We conclude that ascending magmas possibly delivered significant quantities of water to the martian surface in recent times, reconciling geologic and petrologic constraints on the outgassing history of Mars.
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crystallization of the basaltic shergottites insights from crystal size distribution csd analysis of Pyroxenes
2000Co-Authors: Rachel C F Lentze, H. Y. McsweenAbstract:— Quantitative petrographic analysis, using the crystal size distribution (CSD) method, provides a novel approach for examining the crystallization histories of basaltic shergottites. Grain number densities at different sizes are plotted against grain size, and the resulting curve relates to the geologic processes involved with the crystallization of the grain population. Most basaltic shergottites are dominated by pigeonite and augite; and because plagioclase is primarily interstitial, and therefore constrained in its growth by the surrounding Pyroxenes, we limited our size measurements to the pyroxene phases. The groundmasses of Elephant Moraine (EET) A79001 lithology A and Dar al Gani (DaG) 476 are fine grained with cumulus pyroxene and interstitial plagioclase glass. Their simple linear CSD plots record a single stage of pyroxene crystallization under steady-state conditions of continuous nucleation and growth. The textures of Queen Alexandra Range (QUE) 94201 and EETA79001 lithology B are quite different from the other shergottites, with intergrown pyroxene and plagioclase. Likewise, their CSD plots are also distinct, with curved trends that suggest a lack of large grains, most likely because of interference between simultaneously growing silicate phases. However, the CSD plot shapes are smooth, also implying a single stage of growth. Shergotty and Zagami, with coarser cumulus textures, display CSD plots that are generally linear over most grain sizes. This implies that conditions of nucleation and growth were dominant during formation of the pyroxene populations. Both plots, however, also display kinks, implying multiple stages of growth. A similar kink is also visible in a CSD plot of only the Mg-rich cores of Shergotty Pyroxenes, which suggests the feature represents changes in conditions during core crystallization, rather than an event coincident with the change in composition to the Fe-rich rims. The plot may be interpreted as representing two stages of core growth with an intervening short hiatus of nucleation, with continued crystallization associated with ascent of the magma. Eruption onto the surface probably triggered the compositional change to Fe-rich rims. The CSD analysis of products from a controlled crystallization study agree with experimental and petrologic estimates that cooling rates for Zagami were on the order of a few tenths of a degree per hour. Growth rates derived from these cooling rates suggest crystallization of Shergotty and Zagami Pyroxenes occurred over a period of a few weeks to months.
