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Tomohiro Usui - One of the best experts on this subject based on the ideXlab platform.
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geochemistry and sm nd chronology of a stannern group Eucrite northwest africa 7188
Meteoritics & Planetary Science, 2019Co-Authors: Saya Kagami, Makiko K. Haba, Tomohiro Usui, Tetsuya Yokoyama, R C GreenwoodAbstract:We report the results of a detailed study of the basaltic Eucrite Northwest Africa (NWA) 7188, including its mineralogical and bulk geochemical characteristics, oxygen isotopic composition, and 147,146Sm‐143,142Nd mineral isochron ages. The texture and chemical composition of pyroxene and plagioclase demonstrate that NWA 7188 is a monomict Eucrite with a metamorphic grade of type 4. The oxygen isotopic composition and the Fe/Mn ratios of pyroxene confirmed that NWA 7188 belongs to the howardite–Eucrite–diogenite meteorite suite, generally considered to originate from asteroid 4 Vesta. Whole‐rock TiO2, La, and Hf concentrations and a CI chondrite‐normalized rare earth element pattern are in good agreement with those of representative Stannern‐group Eucrites. The 147,146Sm‐143,142Nd isochrons for NWA 7188 yielded ages of 4582 ± 190 and 4554 +17/−19 Ma, respectively. The closure temperature of the Sm‐Nd system for different fractions of NWA 7188 was estimated to be >865 °C, suggesting that the Sm‐Nd decay system has either been resistant to reheating at ~800 °C during the global metamorphism or only partially reset. Therefore, the 146Sm‐142Nd age of NWA 7188 corresponds to the period of initial crystallization of basaltic magmas and/or global metamorphism on the parent body, and is unlikely to reflect Sm‐Nd disturbance by late reheating and impact events. In either case, NWA 7188 is a rare Stannern‐group Eucrite that preserves the chronological information regarding the initial crustal evolution of Vesta.
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mixing relations of the howardite Eucrite diogenite suite a new statistical approach of independent component analysis for the dawn mission
Meteoritics & Planetary Science, 2013Co-Authors: Tomohiro Usui, Hikaru IwamoriAbstract:Dawn has recently revealed that the surface of Vesta is heterogeneously covered by polymictic regoliths represented by mixtures of howardite, Eucrite, and diogenite (HED) meteorites. Mixing relations of the HED suite are examined here using a new computational statistical approach of independent component analysis (ICA). We performed eight-component ICA (Si, Ti, Al, Cr, Fe, Mn, Mg, and Ca) for 209 HED bulk-rock compositions. The ICA results indicate that the HED bulk-rock compositions can be reduced into three independent components (IC) and these IC vectors can reasonably explain compositional variation, petrographic observations, and the mixing relations of the HED suite. The IC-1 vector represents a Eucrite variation that extends from cumulate Eucrite toward main-group (MG) and incompatible-element enriched Eucrites. The IC-2 vector represents a compositional variation of howardites that extends from diogenites to MG-Eucrites, indicating the well-known two-component mixing trend of diogenite and Eucrite. The IC-3 vector represents a compositional variation defined by diogenites and olivine-bearing diogenites, suggesting mixing of olivine and orthopyroxene. Among the three ICs, the diogenite-Eucrite mixing trend IC-2 is most statistically robust and dominates the compositional variations of the HED suite. Our ICA study further indicates that the combination of only three elements (Mg, Si, and Fe) approximates the eight-component ICA model, and that the limited number of resolvable γ-ray spectra obtained by the Dawn mission possibly discriminates olivine lithologies from the olivine-free regolith breccias on the surface of Vesta.
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Mixing relations of the howardite‐Eucrite‐diogenite suite: A new statistical approach of independent component analysis for the Dawn mission
Meteoritics & Planetary Science, 2013Co-Authors: Tomohiro Usui, Hikaru IwamoriAbstract:Dawn has recently revealed that the surface of Vesta is heterogeneously covered by polymictic regoliths represented by mixtures of howardite, Eucrite, and diogenite (HED) meteorites. Mixing relations of the HED suite are examined here using a new computational statistical approach of independent component analysis (ICA). We performed eight-component ICA (Si, Ti, Al, Cr, Fe, Mn, Mg, and Ca) for 209 HED bulk-rock compositions. The ICA results indicate that the HED bulk-rock compositions can be reduced into three independent components (IC) and these IC vectors can reasonably explain compositional variation, petrographic observations, and the mixing relations of the HED suite. The IC-1 vector represents a Eucrite variation that extends from cumulate Eucrite toward main-group (MG) and incompatible-element enriched Eucrites. The IC-2 vector represents a compositional variation of howardites that extends from diogenites to MG-Eucrites, indicating the well-known two-component mixing trend of diogenite and Eucrite. The IC-3 vector represents a compositional variation defined by diogenites and olivine-bearing diogenites, suggesting mixing of olivine and orthopyroxene. Among the three ICs, the diogenite-Eucrite mixing trend IC-2 is most statistically robust and dominates the compositional variations of the HED suite. Our ICA study further indicates that the combination of only three elements (Mg, Si, and Fe) approximates the eight-component ICA model, and that the limited number of resolvable γ-ray spectra obtained by the Dawn mission possibly discriminates olivine lithologies from the olivine-free regolith breccias on the surface of Vesta.
Paul H Warren - One of the best experts on this subject based on the ideXlab platform.
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Siderophile and other geochemical constraints on mixing relationships among HED-meteoritic breccias
Geochimica et Cosmochimica Acta, 2009Co-Authors: Paul H Warren, Gregory W. Kallemeyn, Finn Ulff-møller, Heinz Huber, Won-hie ChoeAbstract:Abstract We have used neutron activation and electron-probe fused-bead techniques to analyze the bulk major and trace-element compositions of 104 named HED meteorites (about 100–102 distinct meteorites, depending upon pairings), including 32 polymict Eucrites, 30 howardites and six diogenites. Most were not previously analyzed for siderophile trace elements; many not even for major elements. Our typical sample was ∼350 mg, and in some cases two separate chips were analyzed as a test of meteorite heterogeneity. Meteorites with extraordinary compositions include Bluewing 001, an unequilibrated Eucrite that is rich in Ti, Sm and other incompatible elements; Y-791192, a cumulate-dominated polymict Eucrite; and LEW 87002, an oddly Sm-rich howardite dominated by a ferroan variety of diogenite. The Eucrite:diogenite mixing ratio is the single most important factor determining the compositions of polymict HEDs, but wide ranges in Eucrite incompatible element contents, in diogenite Cr and V contents, and in Sc contents of both Eucrites and diogenites, make for diversity among the polymict HEDs. As our new siderophile data help to show, the common practice of describing the entire class of howardites as regolith breccias is erroneous. Most howardites are fragmental breccias showing no sign of origin from true (in the lunar sense, i.e., soil-like) near-surface regolith. Howardites are highly diverse in Ni content, often remarkably Ni-poor, compared to lunar regolith breccias. However, the few (8) howardites with between 300 and 1200 μg/g Ni consistently show some combination of other traits suggestive of regolith origin. Most importantly, all four cases (or five if we include Malvern, which appears to have been altered by annealing) of howardites known to have enrichments in solar-wind noble gases belong to the >300 μg/g Ni group. In many cases, an abundance of glasses, particularly in spheroidal or turbid-brown form, provides additional evidence for regolith origin. We propose that the important subset of howardites that are regolith breccias be formally distinguished by the designation regolithic howardite. Apart from high siderophile levels, the regolithic howardites are compositionally distinctive in having Al2O3 consistently near 8–9 wt%; corresponding to a Eucrite:diogenite mixing ratio of precisely 2:1. Assuming the HEDs are reasonably representative of the ancient (i.e., pre-vestoid-launch) surface of Vesta, this clustering of regolith composition is difficult to explain unless most of the ancient diogenite component was brought to the surface in a single early episode (i.e., probably a single great impact), after which smaller-scale cratering (with no further major excavations of diogenite until the vestoid-forming event), efficiently homogenized the surface. Such a single-excavation model may also help to explain why diogenites, in marked contrast with Eucrites, are seldom polymict; and why Al2O3-poor (diogenite-dominated) howardites consistently lack major siderophile enrichments. The low siderophile contents of polymict Eucrites are most enigmatic. Possibly in the HED-asteroidal context (low collision velocities, etc.), only materials blended by multiple impacts consistently acquire major enrichments in siderophile elements.
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Siderophile and other geochemical constraints on mixing relationships among HED-meteoritic breccias
Geochimica et Cosmochimica Acta, 2009Co-Authors: Paul H Warren, Gregory W. Kallemeyn, Finn Ulff-møller, Heinz Huber, Won-hie ChoeAbstract:We have used neutron activation and electron-probe fused-bead techniques to analyze the bulk major and trace-element compositions of 104 named HED meteorites (about 100–102 distinct meteorites, depending upon pairings), including 32 polymict Eucrites, 30 howardites and six diogenites. Most were not previously analyzed for siderophile trace elements; many not even for major elements. Our typical sample was 350 mg, and in some cases two separate chips were analyzed as a test of meteorite heterogeneity. Meteorites with extraordinary compositions include Bluewing 001, an unequilibrated Eucrite that is rich in Ti, Sm and other incompatible elements; Y-791192, a cumulate-dominated polymict Eucrite; and LEW 87002, an oddly Sm-rich howardite dominated by a ferroan variety of diogenite. The Eucrite:diogenite mixing ratio is the single most important factor determining the compositions of polymict HEDs, but wide ranges in Eucrite incompatible element contents, in diogenite Cr and V contents, and in Sc contents of both Eucrites and diogenites, make for diversity among the polymict HEDs. As our new siderophile data help to show, the common practice of describing the entire class of howardites as regolith breccias is erroneous. Most howardites are fragmental breccias showing no sign of origin from true (in the lunar sense, i.e., soil-like) near-surface regolith. Howardites are highly diverse in Ni content, often remarkably Ni-poor, compared to lunar regolith breccias. However, the few (8) howardites with between 300 and 1200 lg/g Ni consistently show some combination of other traits suggestive of regolith origin. Most importantly, all four cases (or five if we include Malvern, which appears to have been altered by annealing) of howardites known to have enrichments in solar-wind noble gases belong to the >300 lg/g Ni group. In many cases, an abundance of glasses, particularly in spheroidal or turbid-brown form, provides additional evidence for regolith origin. We propose that the important subset of howardites that are regolith breccias be formally distinguished by the designation regolithic howardite. Apart from high siderophile levels, the regolithic howardites are compositionally distinctive in having Al2O3 consistently near 8–9 wt%; corresponding to a Eucrite:diogenite mixing ratio of precisely 2:1. Assuming the HEDs are reasonably representative of the ancient (i.e., pre-vestoid-launch) surface of Vesta, this clustering of regolith composition is difficult to explain unless most of the ancient diogenite component was brought to the surface in a single early episode (i.e., probably a single great impact), after which smaller-scale cratering (with no further major excavations of diogenite until the vestoid-forming event), efficiently homogenized the surface. Such a single-excavation model may also help to explain why diogenites, in marked contrast with Eucrites, are seldom polymict; and why Al2O3-poor (diogenite-dominated) howardites consistently lack major siderophile enrichments. The low siderophile contents of polymict Eucrites are most enigmatic. Possibly in the HEDasteroidal context (low collision velocities, etc.), only materials blended by multiple impacts consistently acquire major enrichments in siderophile elements. 2009 Elsevier Ltd. All rights reserved.
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Magnesium oxide-iron oxide mass balance constraints and a more detailed model for the relationship between Eucrites and diogenites
Meteoritics & Planetary Science, 1997Co-Authors: Paul H WarrenAbstract:— According to a currently popular model for petrogenesis on the howardite, Eucrite, and diogenite (HED) parent asteroid, the diogenites are not comagmatic with most Eucrites but instead formed in separate orthopyroxenite-dominated plutons. This model can be tested for consistency with mass balance for MgO and FeO, assuming the overall diogenite/(diogenite + Eucrite) ratio, d, of the parent asteroid is at least comparable to the average d for the Eucrite + diogenite dominated howardite regolith breccias. Average mg# (=MgO/[MgO + FeO]) is much lower for Eucrites, especially noncumulate Eucrites, than for diogenites. Unless the diogenite parent magmas eventually produced a large proportion of low-mg# residual basalt and gabbro (RBG), the implied initial magma's mg# is vastly higher than that of any noncumulate Eucrite. Starting from a source previously depleted by putative primary Eucrite genesis, melt mg# can be estimated as a function of the exchange reaction KD and degree of melting. Using several very conservative assumptions (e.g., assuming that the total [MgO + FeO] concentration is nearly the same in the nascent melt as in the residual solids), the degree of melting required to yield a melt with mg# high enough to satisfy mass balance, without implying an RBG component that accounts for >50% of all Eucrites, is an implausibly high 60–80 wt%. The separate orthopyroxenitic plutons (SOP) model also seems inconsistent with the uniform density of melts across the diogenite-Eucrite compositional spectrum (2.77–2.82 g/cm3), which implies that diogenitic magmas should have been as capable as Eucrites of extruding to form lavas. This difficulty cannot be reduced by simply assuming that later-formed magmas were systematically both more plutonic and more MgO-rich than earlier ones, because the plutonic cumulate Eucrites equilibrated with melts systematically lower in mg# than noncumulate Eucrites. Conceivably, the bulk mg# of the asteroid's silicate system was increased between primary-melt Eucrite genesis and SOP diogenite genesis by graphite-fueled reduction of FeO. However, the graphite oxidation process generates a huge proportion of gas, which would have enhanced the buoyancy of the nascent diogenite-parent magmas, thus exacerbating the difficulty of achieving the implied high degrees of partial melting. To avoid these difficulties but still form most Eucrites as rapidly cooled extrusives, I propose the NERD (noncumulate Eucrites as extruded residua of diogenites) model. In this model, the diogenites form as early cumulates from a large magma system (probably a global “magma ocean”) that yields a large proportion of eucritic melt as residuum. This residual melt zone undergoes relatively little crystallization during a period when it is episodically tapped to produce extrusions, dikes and sills of rapidly cooled noncumulate Eucrites. Slight (∼5–10%) porosity in the nascent eucritic crust keeps it marginally buoyant over the residual melt zone. The common thermal metamorphism of noncumulate Eucrites results from baking by superjacent flows during the episodic venting of the melt zone. The NERD model's greatest advantage is that it does not require implausibly high degrees of localized melting in the mature stages of igneous evolution of the HED asteroid.
R C Greenwood - One of the best experts on this subject based on the ideXlab platform.
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geochemistry and sm nd chronology of a stannern group Eucrite northwest africa 7188
Meteoritics & Planetary Science, 2019Co-Authors: Saya Kagami, Makiko K. Haba, Tomohiro Usui, Tetsuya Yokoyama, R C GreenwoodAbstract:We report the results of a detailed study of the basaltic Eucrite Northwest Africa (NWA) 7188, including its mineralogical and bulk geochemical characteristics, oxygen isotopic composition, and 147,146Sm‐143,142Nd mineral isochron ages. The texture and chemical composition of pyroxene and plagioclase demonstrate that NWA 7188 is a monomict Eucrite with a metamorphic grade of type 4. The oxygen isotopic composition and the Fe/Mn ratios of pyroxene confirmed that NWA 7188 belongs to the howardite–Eucrite–diogenite meteorite suite, generally considered to originate from asteroid 4 Vesta. Whole‐rock TiO2, La, and Hf concentrations and a CI chondrite‐normalized rare earth element pattern are in good agreement with those of representative Stannern‐group Eucrites. The 147,146Sm‐143,142Nd isochrons for NWA 7188 yielded ages of 4582 ± 190 and 4554 +17/−19 Ma, respectively. The closure temperature of the Sm‐Nd system for different fractions of NWA 7188 was estimated to be >865 °C, suggesting that the Sm‐Nd decay system has either been resistant to reheating at ~800 °C during the global metamorphism or only partially reset. Therefore, the 146Sm‐142Nd age of NWA 7188 corresponds to the period of initial crystallization of basaltic magmas and/or global metamorphism on the parent body, and is unlikely to reflect Sm‐Nd disturbance by late reheating and impact events. In either case, NWA 7188 is a rare Stannern‐group Eucrite that preserves the chronological information regarding the initial crustal evolution of Vesta.
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experimental insights into stannern trend Eucrite petrogenesis
Meteoritics & Planetary Science, 2018Co-Authors: R. G. Mayne, S. D. Crossley, N. G. Lunning, S. Yang, Munir Humayun, Richard D. Ash, J. M. Sunshine, T J Mccoy, R C GreenwoodAbstract:The incompatible trace element‐enriched Stannern‐trend Eucrites have long been recognized as requiring a distinct petrogenesis from the Main Group‐Nuevo Laredo (MGNL) Eucrites. Barrat et al. (2007) proposed that Stannern‐trend Eucrites formed via assimilation of crustal partial melts by a MGNL‐trend magma. Previous experimental studies of low‐degree partial melting of Eucrites did not produce sufficiently large melt pools for both major and trace element analyses. Low‐degree partial melts produced near the solidus are potentially the best analog to the assimilated crustal melts. We partially melted the unbrecciated, unequilibrated MGNL‐trend Eucrite NWA 8562 in a 1 atm gas‐mixing furnace, at IW‐0.5, and at temperatures between 1050 and 1200 °C. We found that low‐degree partial melts formed at 1050 °C are incompatible trace element enriched, although the experimental melts did not reach equilibrium at all temperatures. Using our experimental melt compositions and binary mixing modeling, the FeO/MgO trend of the resultant magmas coincides with the range of known Stannern‐trend Eucrites when a primary magma is contaminated by crustal partial melts. When experimental major element compositions for eucritic crustal partial melts are combined with trace element concentrations determined by previous modeling (Barrat et al. 2007), the Stannern‐trend can be replicated with respect to both major, minor, and trace element concentrations.
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the abundance and isotopic composition of water in Eucrites
Meteoritics & Planetary Science, 2016Co-Authors: T J Barrett, R C Greenwood, I A Franchi, J J Barnes, Romain Tartese, M Anand, B L A Charlier, M M GradyAbstract:Volatile elements play a key role in the dynamics of planetary evolution. Extensive work has been carried out to determine the abundance, distribution, and source(s) of volatiles in planetary bodies such as the Earth, Moon, and Mars. A recent study showed that the water in apatite from Eucrites has similar hydrogen isotopic compositions compared to water in terrestrial rocks and carbonaceous chondrites, suggesting that water accreted very early in the inner solar system given the ancient crystallization ages (~4.5 Ga) of Eucrites. Here, the measurements of water (reported as equivalent H2O abundances) and the hydrogen isotopic composition (δD) of apatite from five basaltic Eucrites and one cumulate Eucrite are reported. Apatite H2O abundances range from ~30 to ~3500 ppm and are associated with a weighted average δD value of −34 ± 67‰. No systematic variations or correlations are observed in H2O abundance or δD value with Eucrite geochemical trend or metamorphic grade. These results extend the range of previously published hydrogen isotope data for Eucrites and confirm the striking homogeneity in the H-isotopic composition of water in Eucrites, which is consistent with a common source for water in the inner solar system.
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the stannern trend Eucrites contamination of main group eucritic magmas by crustal partial melts
Geochimica et Cosmochimica Acta, 2007Co-Authors: Jean-alix Barrat, Akira Yamaguchi, R C Greenwood, Marcel Bohn, Joseph Cotten, M Benoit, I A FranchiAbstract:We report on the petrology of a new Eucrite belonging to the Stannern trend and discuss the origin of this trend. The Eucrite Northwest Africa 4523 (NWA 4523) is an equilibrated Eucrite consisting of dark clasts embedded in a fine-grained crystallized matrix. Two types of clasts have been observed: medium-grained ophitic/subophitic clasts, and very fine-grained clasts. Despite textural differences, the clasts display the same mineralogy, in particular the same kind of pyroxenes with pigeonitic cores containing sparse exsolution lamellae, and augitic rims, zoned plagioclases and the occurrence of K-feldspar. The major and trace element abundances of a large medium-grained clast are very similar to Stannern or Bouvante. The Stannern trend Eucrites are characterized by high incompatible trace element abundances. Their trace element patterns normalized to a representative Main Group Eucrite, exhibit significant Eu, Sr and Be negative anomalies. In this paper, we show that contamination of Main Group eucritic magmas by melts derived by partial melting of the asteroid’s crust can successfully explain both the high incompatible trace elements concentrations and the distinctive Eu, Sr, Be anomalies shown by the Stannern trend Eucrites. This model is in agreement with the view that Stannern and some Main Group-Nuevo Laredo trend Eucrites have been contemporaneously erupted, and with the probable assumption that Stannern trend Eucrites formed rather late in the history of the 4-Vesta’s crust.
Jean-alix Barrat - One of the best experts on this subject based on the ideXlab platform.
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Iron isotope fractionation in planetary crusts
Geochimica et Cosmochimica Acta, 2012Co-Authors: Kun Wang, Frederic Moynier, Jean-alix Barrat, Nicolas Dauphas, Paul Craddock, Corliss SioAbstract:We present new high precision iron isotope data (δ56Fe vs. IRMM-014 in per mil) for four groups of achondrites: one lunar meteorite, 11 martian meteorites, 32 howardite-Eucrite-diogenite meteorites (HEDs), and eight angrites. Angrite meteorites are the only planetary materials, other than Earth/Moon system, significantly enriched in the heavy isotopes of Fe compared to chondrites (by an average of +0.12‰ in δ56Fe). While the reason for such fractionation is not completely understood, it might be related to isotopic fractionation by volatilization during accretion or more likely magmatic differentiation in the angrite parent-body. We also report precise data on martian and HED meteorites, yielding an average δ56Fe of 0.00 ± 0.01‰. Stannern-trend Eucrites are isotopically heavier by +0.05‰ in δ56Fe than other Eucrites. We show that this difference can be ascribed to the enrichment of heavy iron isotopes in ilmenite during igneous differentiation. Preferential dissolution of isotopically heavy ilmenite during remelting of eucritic crust could have generated the heavy iron isotope composition of Stannern-trend Eucrites. This supports the view that Stannern-trend Eucrites are derived from main-group Eucrite source magma by assimilation of previously formed asteroidal crust. These new results show that iron isotopes are not only fractionated in terrestrial and lunar basalts, but also in two other differentiated planetary crusts. We suggest that igneous processes might be responsible for the iron isotope variations documented in planetary crusts.
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Zinc isotopes in HEDs: clues to the formation of 4-Vesta, and the unique composition of Pecora Escarpment 82502
Geochimica et Cosmochimica Acta, 2012Co-Authors: Randal Paniello, Frederic Moynier, Jean-alix Barrat, Pierre Beck, Frank Podosek, Sylvain PichatAbstract:The δ66Zn (permil deviation of the 66Zn/64Zn ratio from a terrestrial standard) values for a suite of 20 non-Antarctic HED (howardite - Eucrite - diogenite) meteorites and one mesosiderite, and for 8 Antarctic Eucrites and diogenites, were measured in order to determine the role of volatization in the formation of their presumed parent body, the asteroid 4-Vesta. The 20 non-Antarctic HEDs had δ66Zn values that ranged from -2.0 ‰ to +1.67 ‰, with a mean value of -0.01 ± 0.39 ‰ (2se); this range likely represents a small-scale heterogeneity due to brecciation induced by multiple impacts. The non-Antarctic Eucrites (δ66Zn = +0.00 ± 0.58 ‰ (2se), n=12) were isotopically the same as the diogenites (δ66Zn = -0.31 ± 0.80 ‰ (2se), n=4), and the howardites (δ66Zn = +0.26 ± 0.37 ‰ (2se), n=4). On average, non-Antarctic Eucrite falls were isotopically heavier (+0.50 ‰) than non-Antarctic finds (-1.00 ‰). The Antarctic finds studied were all unbrecciated samples, and they were significantly heavier than the non-Antarctic samples with a δ66Zn range of +1.63 to +6.22‰ for 4 Eucrites (mean, +4.32‰) and +0.94 to +1.60‰ for 3 diogenites (mean +1.23‰), excluding one anomalous sample, while their Zn concentration is significantly lower than the brecciated samples. These data suggest that the unbrecciated Eucrites probably represent the eucritic crust shortly after differentiation and cooling of the parent asteroid, at which time volatization of lighter zinc isotopes led to an isotopically heavy crust. Early impact events caused the ejection of these unbrecciated meteorites, which were subsequently spared from brecciation caused by multiple additional impacts on the much larger Vesta. The range of δ66Zn values and Zn concentration for the brecciated HEDs in this study supports a major contribution to the Vestan surface by chondritic impactors (-1.30 < δ66Zn < +0.76 ‰ for ordinary and carbonaceous chondrites). The anomalous Eucrite PCA 82502 (δ66Zn= -7.75 ‰) is significantly isotopically lighter than the other HEDs and is the natural sample with the lightest Zn isotopic composition reported in the solar system to date. This meteorite most likely originated from a distinct parent body.
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the stannern trend Eucrites contamination of main group eucritic magmas by crustal partial melts
Geochimica et Cosmochimica Acta, 2007Co-Authors: Jean-alix Barrat, Akira Yamaguchi, R C Greenwood, Marcel Bohn, Joseph Cotten, M Benoit, I A FranchiAbstract:We report on the petrology of a new Eucrite belonging to the Stannern trend and discuss the origin of this trend. The Eucrite Northwest Africa 4523 (NWA 4523) is an equilibrated Eucrite consisting of dark clasts embedded in a fine-grained crystallized matrix. Two types of clasts have been observed: medium-grained ophitic/subophitic clasts, and very fine-grained clasts. Despite textural differences, the clasts display the same mineralogy, in particular the same kind of pyroxenes with pigeonitic cores containing sparse exsolution lamellae, and augitic rims, zoned plagioclases and the occurrence of K-feldspar. The major and trace element abundances of a large medium-grained clast are very similar to Stannern or Bouvante. The Stannern trend Eucrites are characterized by high incompatible trace element abundances. Their trace element patterns normalized to a representative Main Group Eucrite, exhibit significant Eu, Sr and Be negative anomalies. In this paper, we show that contamination of Main Group eucritic magmas by melts derived by partial melting of the asteroid’s crust can successfully explain both the high incompatible trace elements concentrations and the distinctive Eu, Sr, Be anomalies shown by the Stannern trend Eucrites. This model is in agreement with the view that Stannern and some Main Group-Nuevo Laredo trend Eucrites have been contemporaneously erupted, and with the probable assumption that Stannern trend Eucrites formed rather late in the history of the 4-Vesta’s crust.
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ARE STANNERN-TREND EucriteS ORDINARY EucriteS CONTAMINATED BY CRUSTAL PARTIAL MELTS?
2007Co-Authors: Jean-alix Barrat, Akira Yamaguchi, Marcel Bohn, Joseph Cotten, Richard C. Greenwood, Mathieu Benoit, Ian A. FranchiAbstract:Stannern trend Eucrites are not only richer in incompatible trace element than the other Eucrites, but their trace element patterns are clearly distinctive, with pronounced nega-tive Be, Sr and Eu anomalies. Many authors have proposed that the diversity of the eucritic melts could simply reflect part of the diversity of the primary melts from the parent body mantle [e.g., 1- 3]. Alternatively, to explain the decoupling of major elements from incompatible trace elements requires a complex scenario involving interaction between eucritic and highly residual melts [4-5]. As an alternative to previous hypotheses for the origin of Stannern trend Eucrites, we propose a model of contamination of ordinary Eucrites by melts produced by melting of the asteroidal crust. The composition of melts generated by the partial melting of eucritic crust can be calculated theoretically. The first partial melts would certainly not be acidic, but rather intermediate to basic in composition. We have calculated the trace element abundances of melts produced by partial melting of an equili-brated Eucrite using literature partition coefficients, and the com-position of our reference Eucrite Juvinas The calculated magmas produced by partial melting of a eucritic crust are rich in incom-patible trace elements and display pronounced negative anoma-lies in K, Ba, Be, Sr, Eu and Ti relative to ordinary Eucrites. Con-tamination of a normal Eucrite by a crustal partial melt should have little effect on its major element concentration, but a huge impact on the incompatible trace element abundance, which is exactly the variation displayed by Stannern trend Eucrites. We have calculated the melts obtained by simple mixing between a Eucrite and a crustal partial melt produced by 10 % melting. The trace element abundances are strikingly well reproduced by this model. The trace element concentrations of Stannern, and Bou-vante are explained by a combination of about 85 % Juvinas and 15 % crustal partial melt. The calculated proportions are obvi-ously dependent to the compositions of the end members. For example, very different proportions are obtained using the partial melts calculated for 5 % or 15 % of melting of the eucritic crust.
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Petrology and geochemistry of the unbrecciated achondrite Northwest Africa 1240 (NWA 1240): An HED parent body
Geochimica et Cosmochimica Acta, 2002Co-Authors: Jean-alix Barrat, Janne Blichert-toft, Marcel Bohn, Albert Jambon, V. Sautter, C. Göpel, Philippe Gillet, Omar Boudouma, F. KellerAbstract:NWA 1240 is an unusual Eucrite recently recovered in Morocco as a single stone of 98 g. It is an unbrecciated greenish-brown rock nearly devoid of fusion crust. It displays porphyritic texture consisting of skeletal hollow low-Ca pyroxene phenocrysts set in a variolitic (fan-spherulitic) mesostasis of fine elongate pyroxene and plagioclase crystals. Minor phases are skeletal chromite, iron, silica, troilite, ilmenite and minute amounts of phosphate and fayalite. Pyroxenes are unequilibrated and show one of the widest ranges of composition so far described for a Eucrite, from En76.0Wo1.9Fs22.1 to compositions nearly devoid of Mg (unusual ferrosilite and Fe-augite symplectites and possibly pyroxferroite). Plagioclase crystals contain significant amounts of Fe and Mg, which are possibly controlled by the Ca(Mg,Fe2!)Si3O8 plagioclase component. To discuss the potential effects of hot-desert weathering on NWA 1240, we have analyzed a series of Saharan Eucrites (Agoult, Aoufous, Igdi, Smara, NWA 047 and NWA 049) and large aliquots (0.39 to 2.8 g) of Eucrite falls (Bereba, Bouvante, Jonzac, Juvinas and Serra de Mage´). These results indicate that among the elements we have determined, Pb, Ba and Sr are the most sensitive indicators of Saharan weathering. The bulk composition of NWA 1240 has been determined for 45 elements by ICP-AES and ICP-MS. The data show that the meteorite is not significantly weathered: its Pb concentration is very low; Ba and Sr concentrations are not anomalously high; the Th/U and Hf/Sm ratios are chondritic (Th/U " 3.65, Hf/Sm " 0.74). NWA 1240 is rich in MgO (10.4 wt%) and Cr2O3 (0.71 wt%), and displays striking similarities with cumulate Eucrites, such as having similar incompatible trace element patterns and a significant positive Eu anomaly (Eu/Eu* " 1.37). The combination of fast cooling and cumulate Eucrite-dominated composition suggests that NWA 1240 is not an igneous rock but rather an impact melt.
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mixing relations of the howardite Eucrite diogenite suite a new statistical approach of independent component analysis for the dawn mission
Meteoritics & Planetary Science, 2013Co-Authors: Tomohiro Usui, Hikaru IwamoriAbstract:Dawn has recently revealed that the surface of Vesta is heterogeneously covered by polymictic regoliths represented by mixtures of howardite, Eucrite, and diogenite (HED) meteorites. Mixing relations of the HED suite are examined here using a new computational statistical approach of independent component analysis (ICA). We performed eight-component ICA (Si, Ti, Al, Cr, Fe, Mn, Mg, and Ca) for 209 HED bulk-rock compositions. The ICA results indicate that the HED bulk-rock compositions can be reduced into three independent components (IC) and these IC vectors can reasonably explain compositional variation, petrographic observations, and the mixing relations of the HED suite. The IC-1 vector represents a Eucrite variation that extends from cumulate Eucrite toward main-group (MG) and incompatible-element enriched Eucrites. The IC-2 vector represents a compositional variation of howardites that extends from diogenites to MG-Eucrites, indicating the well-known two-component mixing trend of diogenite and Eucrite. The IC-3 vector represents a compositional variation defined by diogenites and olivine-bearing diogenites, suggesting mixing of olivine and orthopyroxene. Among the three ICs, the diogenite-Eucrite mixing trend IC-2 is most statistically robust and dominates the compositional variations of the HED suite. Our ICA study further indicates that the combination of only three elements (Mg, Si, and Fe) approximates the eight-component ICA model, and that the limited number of resolvable γ-ray spectra obtained by the Dawn mission possibly discriminates olivine lithologies from the olivine-free regolith breccias on the surface of Vesta.
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Mixing relations of the howardite‐Eucrite‐diogenite suite: A new statistical approach of independent component analysis for the Dawn mission
Meteoritics & Planetary Science, 2013Co-Authors: Tomohiro Usui, Hikaru IwamoriAbstract:Dawn has recently revealed that the surface of Vesta is heterogeneously covered by polymictic regoliths represented by mixtures of howardite, Eucrite, and diogenite (HED) meteorites. Mixing relations of the HED suite are examined here using a new computational statistical approach of independent component analysis (ICA). We performed eight-component ICA (Si, Ti, Al, Cr, Fe, Mn, Mg, and Ca) for 209 HED bulk-rock compositions. The ICA results indicate that the HED bulk-rock compositions can be reduced into three independent components (IC) and these IC vectors can reasonably explain compositional variation, petrographic observations, and the mixing relations of the HED suite. The IC-1 vector represents a Eucrite variation that extends from cumulate Eucrite toward main-group (MG) and incompatible-element enriched Eucrites. The IC-2 vector represents a compositional variation of howardites that extends from diogenites to MG-Eucrites, indicating the well-known two-component mixing trend of diogenite and Eucrite. The IC-3 vector represents a compositional variation defined by diogenites and olivine-bearing diogenites, suggesting mixing of olivine and orthopyroxene. Among the three ICs, the diogenite-Eucrite mixing trend IC-2 is most statistically robust and dominates the compositional variations of the HED suite. Our ICA study further indicates that the combination of only three elements (Mg, Si, and Fe) approximates the eight-component ICA model, and that the limited number of resolvable γ-ray spectra obtained by the Dawn mission possibly discriminates olivine lithologies from the olivine-free regolith breccias on the surface of Vesta.