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Frank E Brenker - One of the best experts on this subject based on the ideXlab platform.
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Asteroidal processes recorded by polyphase deformation in a harzburgitic Diogenite NWA 5480
Journal of Structural Geology, 2015Co-Authors: Beverley J Tkalcec, Frank E BrenkerAbstract:Abstract Expectations regarding structural deformation of asteroidal meteorites have typically revolved around impact-induced shock metamorphism or the gravity-driven axial compression of cumulates at the base of magma chambers. Recent structural analyses, however, of several olivine-rich Diogenites (harzburgites) reveal solid-state plastic deformation not attributable to either scenario and propose dynamic mantle movements in the parent body, assumed to be Vesta. In this study we examine the microstructures of pyroxene and olivine in the olivine-rich Diogenite NWA 5480. Coarse-grained, poikilitic texture, exsolution lamellae and plastic deformation attest to polyphase deformation and a re-heating event, followed by relatively slow cooling. Observations suggest that impact events alone are insufficient to generate and sustain the thermal and deformation conditions required to achieve all of the observed features. The proposed dynamic mantle movements in the Vestan interior may offer a means of heat transport to the system to provide a thermal environment inducive to slow cooling as well as generate the incremental stress fields required for the polyphase plastic deformation observed in the olivine.
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plastic deformation of olivine rich Diogenites and implications for mantle processes on the Diogenite parent body
Meteoritics & Planetary Science, 2014Co-Authors: Beverley J Tkalcec, Frank E BrenkerAbstract:Numerous petrologic and geochemical studies so far on the howardite, eucrite, and Diogenite (HED) meteorites have produced various crystallization scenarios for their parent body, believed to be the differentiated asteroid 4 Vesta. Structural analyses of Diogenites can reveal important insights into postcrystallization deformation on the parent body. Recently published results (Tkalcec et al. 2013) of structural analysis on the olivine-rich Diogenite NWA 5480 reveal that it underwent solid-state plastic deformation, although not at the base of a magma chamber. Dynamic mantle downwelling has been proposed as a plausible deformation mechanism (Tkalcec et al. 2013). The purpose of this study is to investigate whether the plastic deformation found in NWA 5480 is an isolated case. We expand the structural analysis on NWA 5480 and extend it to NWA 5784 and MIL 07001,6, two other samples of rare olivine-rich Diogenites, using electron-backscattered-diffraction (EBSD) techniques. Our EBSD results show that the Diogenites analyzed in this study underwent solid-state plastic deformation, confirming that the observed deformation of NWA 5480 was not an isolated case on the Diogenite parent body. The lattice-preferred orientations (LPOs) of olivine in NWA 5784 and NWA 5480 are clearly distinct from that typical for cumulate rocks at the base of magma chambers, indicating a different stress environment and a different deformation mechanism. The LPO of olivine in MIL 07001 is less conclusive. The structural results of this study suggest that plastic deformation occurred on the Diogenite parent body at high temperatures (1273 < T ≤ 1573 K) in the solid state, i.e., after crystallization of the Diogenites themselves, in a dynamic environment with active stress fields.
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Plastic deformation of olivine-rich Diogenites and implications for mantle processes on the Diogenite parent body
Meteoritics & Planetary Science, 2014Co-Authors: Beverley J Tkalcec, Frank E BrenkerAbstract:Numerous petrologic and geochemical studies so far on the howardite, eucrite, and Diogenite (HED) meteorites have produced various crystallization scenarios for their parent body, believed to be the differentiated asteroid 4 Vesta. Structural analyses of Diogenites can reveal important insights into postcrystallization deformation on the parent body. Recently published results (Tkalcec et al. 2013) of structural analysis on the olivine-rich Diogenite NWA 5480 reveal that it underwent solid-state plastic deformation, although not at the base of a magma chamber. Dynamic mantle downwelling has been proposed as a plausible deformation mechanism (Tkalcec et al. 2013). The purpose of this study is to investigate whether the plastic deformation found in NWA 5480 is an isolated case. We expand the structural analysis on NWA 5480 and extend it to NWA 5784 and MIL 07001,6, two other samples of rare olivine-rich Diogenites, using electron-backscattered-diffraction (EBSD) techniques. Our EBSD results show that the Diogenites analyzed in this study underwent solid-state plastic deformation, confirming that the observed deformation of NWA 5480 was not an isolated case on the Diogenite parent body. The lattice-preferred orientations (LPOs) of olivine in NWA 5784 and NWA 5480 are clearly distinct from that typical for cumulate rocks at the base of magma chambers, indicating a different stress environment and a different deformation mechanism. The LPO of olivine in MIL 07001 is less conclusive. The structural results of this study suggest that plastic deformation occurred on the Diogenite parent body at high temperatures (1273
James J. Papike - One of the best experts on this subject based on the ideXlab platform.
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Metamorphic Diogenite Grosvenor Mountains 95555: Mineral chemistry of orthopyroxene and spinel and comparisons to the Diogenite suite
Meteoritics & Planetary Science, 2000Co-Authors: James J. Papike, C. K. Shearer, Michael Spilde, J. M. KarnerAbstract:Abstract-Grosvenor Mountains (GRO) 95555 is a relatively newly discovered and unique metamorphic Diogenite. It does not show the usual brecciated appearance of other Diogenites or wide compositional variability of orthopyroxene or spinel. Electron microprobe analysis (EMPA) and secondary ion mass spectrometer (SIMS) analysis of orthopyroxene and EMPA of spinel show limited compositional variability. Compositions for orthopyroxene for Fe/(Fe + Mg) atomic, Al, Zr, Y, and Yb fall in the middle of the compositional ranges of the Diogenite suite. Apparently, GRO 95555 formed at sufficient depth or location to escape brecciation by meteorite bombardment and in a thermal regime that led to high-grade metamorphism resulting in homogeneous mineral compositions. The believed INTRODUCTION howardite, eucrite, and Diogenite (HED) lithologies are to have originated on asteroid 4 Vesta during an extensive - - melting event at -4.6 Ga (Consolmagno and Drake,
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Diogenites as asteroidal cumulates; insights from spinel chemistry
American Mineralogist, 1999Co-Authors: Laurie E. Bowman, James J. Papike, Michael SpildeAbstract:The chemical composition of spinel was determined for a suite of 19 Diogenites (orthopyroxenites) thought to be from asteroid 4 Vesta. Previous studies (Fowler et al. 1994, 1995) of orthopyroxene demonstrated that these Diogenites are linked genetically, perhaps through fractional crystallization, in one or more crustal intrusions. The present study focuses on spinel to see if it also retains some chemical signatures of an igneous history. The chemical compositions across spinel grains reveal flat concentration profiles, indicating major subsolidus exchange with orthopyroxene. Nevertheless, significant chemical differences exist among the average spinel compositions from individual Diogenites in the suite. A chemical continuum exists from high-Cr, low-Al spinel in Diogenite LAP 91900 (Cr 203 60.7 wt%; Al203 6.1 wt%) to low-Cr, high-Al spinel in Diogenite ALHA 77256 (Cr 203 44.7 wt%; Al203 21.8 wt%), which may represent one or more fractionation series. In these trends, Al and Ti behave as incompatible elements whose abundances increase with crystallization. These systematics differ from those in spinel in terrestrial or lunar basaltic systems because of the extremely Al-depleted nature of the Diogenite parental melts. In terrestrial and lunar basalts, the increase in Al concentration in spinel is interrupted when plagioclase crystallizes. In Diogenite parental melts, plagioclase does not come onto the liquidus until the very end of spinel crystallization.
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Automated energy dispersive spectrometer modal analysis applied to the Diogenites
Meteoritics & Planetary Science, 1997Co-Authors: L. E. Bowman, Michael Spilde, James J. PapikeAbstract:— We have analyzed the modal abundances of 23 of the known 24 Diogenites in 31 thin sections using an energy dispersive spectrometer (EDS) and automated phase distribution analysis software. Orthopyroxene is predictably the most abundant phase, ranging from 27.7 vol% to 99.8 vol% in these samples. The grand average mode of all the analyzed Diogenites includes the “olivine Diogenites” but not ALH 85015, a probable howardite, and ALHA81208, a sample with an abundant silica phase. The grand average of these 21 Diogenites is: orthopyroxene 92.2 vol%, olivine 4.2 vol%, clinopyroxene 1.2 vol%, chromite 0.9 vol%, plagioclase 0.4 vol%, FeNi metal 0.1 vol%, troilite 0.6 vol%, and silica phase 0.4 vol%. Plagioclase feldspar is extremely depleted in all samples, with modal abundance from none detected to 4.6 vol% in range. Such a low volume of plagioclase may indicate that the Diogenite parental melts originated in a source region depleted in Al (Warren, 1985; Stolper, 1975), which is consistent with crystallization from a melt derived from material that had previously experienced extraction of a eucrite-type melt.
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Petrogenetic models for magmatism on the eucrite parent body: Evidence from orthopyroxene in Diogenites
Meteoritics & Planetary Science, 1997Co-Authors: C. K. Shearer, G. W. Fowler, James J. PapikeAbstract:Diogenites are recognized as a major constituent of the howardite, eucrite and Diogenite (HED) meteorite group. Recently, several papers (Mittlefehldt, 1994; Fowler et al., 1994, 1995) have identified trace- element systematics in Diogenites that appeared to mimic simple magmatic processes that involved large degrees of crystallization (up to 95% orthopyroxene) of basalt with extremely high normative hypersthene. Such a crystallization scenario linking all the Diogenites is highly unlikely. The purpose of this study is to explore other possible models relating the Diogenites. Computational major-element melting models of a variety of different potential bulk compositions for the eucrite parent body (EPB) mantle indicate that these compositions show a similar sequence in residuum min- eral assemblage with increasing degrees of partial melting. Numerous bulk compositions would produce melts with Mg# appropriate for diogenitic parent magmas at low to moderate degrees of partial melting (1 5% to 30%). These calculations also show that melts with similar Mg# and variable incompatible element con- centrations may be produced during small to moderate degrees of EPB mantle melting. The trace-element characteristic of the orthopyroxene in Diogenites does not support a model for large amounts of fractional crystallization of a single "hypersthene normative" basaltic magma following either small-scale or large-scale EPB mantle melting. Small degrees of fractional crystallization of a series of dis- tinct basaltic magmas are much more likely. Only two melting models that we considered hold any promise for producing different batches of "diogenitic magmas." The first model involves the fractional melting of a homogeneous source that produces parental magmas to Diogenites with an extensive range of incompatible elements and limited variations in Mg#. There are several requirements for this model to work. The first requirement of this model is that the DorthoPYroxene/melt must change during melting or crystallization to compress the range of incompatible elements in the calculated diogenitic magmas. The second prerequisite is that either some of the calculated diogenitic magmas are parental to eucrites or the Mg# in diogenitic magmas are influenced by slight changes in oxygen fugacity during partial melting. The second model in- volves batch melting of a source that reflects accretional heterogeneities capable of generating diogenitic magmas with the calculated Mg# and incompatible element contents. Both of these models require small to moderate degrees of partial melting that may limit the efficiency of core separation.
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Diogenites as asteroidal cumulates: insights from orthopyroxene trace element chemistry
Geochimica et Cosmochimica Acta, 1995Co-Authors: G. W. Fowler, C. K. Shearer, James J. Papike, G. D. LayneAbstract:Eucrite, howardite, and Diogenite members of the achondrite meteorites are considered by many to be genetically related. Therefore, each provides a piece of the puzzle for reconstructing magmatic processes on the eucrite parent body (EPB). The interpretation of the magmatic history of the Diogenites (orthopyroxenites) is compromised to a great extent because the magmatic major element signature of orthopyroxene has been reset and some minor elements such as Al have been compromised by coupled substitution mechanisms. As a further test of the models for the origin of Diogenites, we have analyzed a suite of twenty-one Diogenites (≈160 individual analyses) for minor and trace elements using ion microprobe techniques. The concentrations of incompatible elements are low in the orthopyroxenes analyzed, while their variability in the orthopyroxenes is both extensive and consistent. The range of averages in Yb varies by a factor of 16 from Ellemeet to LEW 88679. Over this suite of Diogenites, Zr varies by a factor of 117 and Y varies by a factor of 151. This variability exceeds the range noted by previous INAA studies of orthopyroxene separates. These incompatible trace elements exhibit a strong positive correlation with Ti. The consistent incompatible element variability among Diogenites, limited textural evidence for subsolidus exsolution modification, and the expected slower diffusion rates of the REE, Ti, and Y relative to Fe-Mg indicate that the trace elements in the diogenitic orthopyroxene may reliably preserve the magmatic history of the Diogenites. Based on the incompatible trace element systematics of Y and Yb, over 90% crystallization is necessary to explain the variation in concentrations from Peckelsheim (most depleted) to LEW 88679 (most enriched) assuming constant D's. Over 70% crystallization of orthopyroxene is required if DY and DYb increase by a factor of three over the same suite of Diogenites. Based on terrestrial analogs, it appears highly unlikely that a single basaltic magma will produce such a mono-mineralic orthopyroxene cumulate horizon with 70–90% crystallization of the parental melt. Two models that potentially explain this extensive incompatible element variability are: (1) the melts from which the Diogenites formed are normative orthopyroxene enriched and normative plagioclase depleted or; (2) the suite of Diogenites represent multiple basaltic melts with distinctly different incompatible element enrichments. Melt compositions that were back-calculated from the orthopyroxene data indicate that the Diogenites crystallized from melts that had a wider range in incompatible elements than that exhibited by the main group eucrites. If the assumptions made in the calculation of these melts compositions are correct, this may be interpreted to mean that either many of the Diogenites are not fractional crystallization products of eucritic melts or that the eucritic melts that were parental to the incompatible element enriched Diogenites have not yet been sampled.
Jean-alix Barrat - One of the best experts on this subject based on the ideXlab platform.
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the oxygen isotope composition of Diogenites evidence for early global melting on a single compositionally diverse hed parent body
Earth and Planetary Science Letters, 2014Co-Authors: R.c. Greenwood, Jean-alix Barrat, I.a. Franchi, Akira Yamaguchi, Edward R D Scott, W F Bottke, J M GibsonAbstract:Oxygen isotope measurements of a suite of 22 Diogenites demonstrate that they have a restricted range of Δ17O values: −0.246±0.014(2σ). These results indicate that the Diogenites form a single population consistent with a single parent body source, rather than multiple sources as has recently been suggested. Our previously published analysis of eucrites and cumulate eucrites (n=34) give very similar results to the Diogenites, with ΔO17=−0.241±0.016‰(2σ) and confirm that Diogenites and eucrites are from the same parent asteroid. The isotopic homogeneity displayed by Diogenites, eucrites and cumulate eucrites, provides strong evidence for an early large-scale melting event on the HED parent body, possibly resulting in the formation of a magma ocean. The paradox, whereby Diogenites show isotopic evidence in favor of global melting, but also geochemical features indicative of late stage interaction with eucritic crust, may reflect a rapid transition from global to serial magmatism on their parent body. The fact that all the lithologically varied HED units have an isotopically homogeneous composition supports the proposal that they are derived from a single, large, diverse asteroid, most likely 4 Vesta. The recent suggestion that the HEDs are not from Vesta, but instead represent material from the same asteroidal source as the main-group pallasites and IIIAB irons can be excluded by our oxygen isotope data.
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Comment on “The origin of eucrites, Diogenites, and olivine Diogenites: Magma ocean crystallization and shallow magma processes on Vesta” by B. E. Mandler and L. T. Elkins-Tanton
Meteoritics & Planetary Science, 2014Co-Authors: Jean-alix Barrat, Akira YamaguchiAbstract:Mandler and Elkins-Tanton (2013) recently proposed an upgraded magma ocean model for the differentiation history of the giant asteroid 4 Vesta. They show that a combination of both equilibrium crystallization and fractional crystallization processes can reproduce the major element compositions of eucritic melts and broadly the range of mineral compositions observed in Diogenites. They assert that their model accounts for all the howardites, eucrites, and Diogenites (HEDs), and use it to predict the crustal thickness and the proportions of the various lithologies. Here, we show that their model fails to explain the trace element diversity of the Diogenites, contrary to their claim. The diversity of the heavy REE enrichment exhibited by the orthopyroxenes in Diogenites is inconsistent with crystallization of these cumulates in either shallow magma chambers replenished by melts from a magma ocean or in a magma ocean. Thus, proportions of the various HED lithologies and the crustal thickness predicted from this model are not necessarily valid.
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The structure of the asteroid 4 Vesta as revealed by models of planet-scale collisions
Nature, 2013Co-Authors: M. Jutzi, Jean-alix Barrat, E. Asphaug, P. Gillet, W. BenzAbstract:Asteroid 4 Vesta seems to be a major intact protoplanet, with a surface composition similar to that of the HED (howardite–eucrite–Diogenite) meteorites^ 1 , 2 , 3 , 4 . The southern hemisphere is dominated by a giant impact scar^ 5 , but previous impact models^ 6 , 7 , 8 have failed to reproduce the observed topography. The recent discovery that Vesta’s southern hemisphere is dominated by two overlapping basins^ 9 provides an opportunity to model Vesta’s topography more accurately. Here we report three-dimensional simulations of Vesta’s global evolution under two overlapping planet-scale collisions. We closely reproduce its observed shape, and provide maps of impact excavation and ejecta deposition. Spiral patterns observed in the younger basin Rheasilvia^ 9 , about one billion years old^ 10 , are attributed to Coriolis forces during crater collapse. Surface materials exposed in the north come from a depth of about 20 kilometres, according to our models, whereas materials exposed inside the southern double-excavation come from depths of about 60–100 kilometres. If Vesta began as a layered, completely differentiated protoplanet, then our model predicts large areas of pure Diogenites and olivine-rich rocks. These are not seen^ 11 , 12 , 13 , possibly implying that the outer 100 kilometres or so of Vesta is composed mainly of a basaltic crust (eucrites) with ultramafic intrusions (Diogenites). NASA's Dawn mission to the asteroid 4 Vesta revealed some unexpected surface features, including a huge crater, named Veneneia, underlying the even larger crater, Rheasilvia, that was formed about a billion years ago. The juxtaposition of these two impact basins provides an opportunity to develop an accurate model of Vesta's topography. Martin Jutzi et al . report three-dimensional simulations of Vesta's global evolution through two successive planet-scale collisions. Their model closely reproduces Vesta's observed shape and provides a firm basis for the interpretation of the surface mineralogy and geology of Vesta, and by extension, other small Solar System bodies. Three-dimensional simulations of the global evolution of asteroid 4 Vesta under two overlapping planet-scale collisions closely reproduce its observed shape; but expected large areas of olivine-rich rocks and pure Diogenites are not observed on the surface, possibly implying that the outer ∼100 km is composed mainly of a basaltic crust (eucrites) with ultramafic intrusions (Diogenites).
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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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nir spectral trends of hed meteorites can we discriminate between the magmatic evolution mechanical mixing and observation geometry effects
Icarus, 2011Co-Authors: P Beck, F Grisolle, Frederic Moynier, E Quirico, Jean-alix Barrat, B Schmitt, Philippe Gillet, Christian BeckAbstract:The Howardite-Eucrite-Diogenite (HED) suite is a family of differentiated meteorites that provide a unique opportunity to study the differentiation of small bodies. The likely parent-body of this meteorite group, (4) Vesta is presently under study by the Dawn mission, scrutinizing its surface in the visible and NIR infrared range. Here, we discuss how well the magmatic trends observed in HED might be retrieved from NIR spectroscopy, by studying laboratory spectra of 10 HED meteorites together with spectra from the RELAB database. We show that although an exsolution process did occur for most eucrites (i.e. decomposition of a primary calcic pyroxene into a high-Ca and low-Ca pyroxene), it does not affect the "bulk pyroxene" trend retrieved from the location of the pyroxene crystal field bands (Band I with a maximum of absorption around at about 1 μm and Band II around 2 μm). Absolute values of the chemical composition appears however to deviate from the expected chemical composition. We show that mechanical mixture (i.e. impact gardening) will produce a linear mixing in the pyroxenes band position diagram (Band I position vs Band II position). This diagram also reveals that howardite are not pure mixtures of an average eucrite and average Diogenite. Because asteroid surfaces are expected to show topography, we also study the effect of observation geometry on the NIR spectra of an eucrite and a Diogenite by measuring the bi-directional reflectance spectra from 0.4 to 4.6 μm. Results show that these meteorites tend to act as forward scatterers, leading to a decrease of integrated band area (relative to the continuum) at high phase angles. The position of the two strong crystal field bands shows only small variability with observation geometry. Retrieval of the magmatic trends from the Band I vs Band II diagram should not be affected by observation geometry effects. Finally we performed NIR reflectance measurement on olivine Diogenites. The presence of olivine can be suggested by using the Band Area Ratio vs Band I diagram, but this phase might affect the retrieval of pyroxene composition from the position of Band I and Band II.
Seung-gu Lee - One of the best experts on this subject based on the ideXlab platform.
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176Lu–176Hf and 87Rb–87Sr Systematics and Rare Earth Element Abundances of Nine Diogenite Meteorites: Evidence for Their Crystallization from Partial Melts of the Vestan Mantle
The Astrophysical Journal, 2019Co-Authors: Takaharu Saito, Hiroshi Hidaka, Seung-gu LeeAbstract:Howardite–eucrite–Diogenite meteorites are believed to originate in the crust of the asteroid 4 Vesta, whose differentiation processes are still controversial. In this study, the first 176Lu–176Hf isotopic data of nine Diogenites are presented together with their 87Rb–87Sr isotopic compositions and rare earth element (REE) abundances to investigate the differentiation process of Diogenites. The 176Lu–176Hf data sets of nine Diogenites revealed the significantly higher initial 176Hf/177Hf ratio of Diogenites than that of eucrites, while there are no resolvable differences between their ages. Based on the high initial ratio and the early formation of Diogenites, their source material is estimated to be the Vestan mantle. The 87Rb–87Sr systematics of nine Diogenites are entirely disturbed probably due to impact events on Vesta. The significant variation observed in the REE abundances of nine Diogenites suggests their crystallization from compositionally diverse melts. Based on the mantle origin and compositional diversity of Diogenites, we propose the crystallization of Diogenites from partial melts of the Vestan mantle. The variation of the trace element abundances of Diogenites can be explained by the variation of the degree of the partial melting. The timescale between the crystallization and partial melting of the Vestan mantle is estimated to be ~100–600 Ma from the 176Lu–176Hf isotopic data of nine Diogenites, while a heat source for the partial melting is uncertain.
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176lu 176hf and 87rb 87sr systematics and rare earth element abundances of nine Diogenite meteorites evidence for their crystallization from partial melts of the vestan mantle
The Astrophysical Journal, 2019Co-Authors: Takaharu Saito, Hiroshi Hidaka, Seung-gu LeeAbstract:Howardite–eucrite–Diogenite meteorites are believed to originate in the crust of the asteroid 4 Vesta, whose differentiation processes are still controversial. In this study, the first 176Lu–176Hf isotopic data of nine Diogenites are presented together with their 87Rb–87Sr isotopic compositions and rare earth element (REE) abundances to investigate the differentiation process of Diogenites. The 176Lu–176Hf data sets of nine Diogenites revealed the significantly higher initial 176Hf/177Hf ratio of Diogenites than that of eucrites, while there are no resolvable differences between their ages. Based on the high initial ratio and the early formation of Diogenites, their source material is estimated to be the Vestan mantle. The 87Rb–87Sr systematics of nine Diogenites are entirely disturbed probably due to impact events on Vesta. The significant variation observed in the REE abundances of nine Diogenites suggests their crystallization from compositionally diverse melts. Based on the mantle origin and compositional diversity of Diogenites, we propose the crystallization of Diogenites from partial melts of the Vestan mantle. The variation of the trace element abundances of Diogenites can be explained by the variation of the degree of the partial melting. The timescale between the crystallization and partial melting of the Vestan mantle is estimated to be ~100–600 Ma from the 176Lu–176Hf isotopic data of nine Diogenites, while a heat source for the partial melting is uncertain.
Beverley J Tkalcec - One of the best experts on this subject based on the ideXlab platform.
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Asteroidal processes recorded by polyphase deformation in a harzburgitic Diogenite NWA 5480
Journal of Structural Geology, 2015Co-Authors: Beverley J Tkalcec, Frank E BrenkerAbstract:Abstract Expectations regarding structural deformation of asteroidal meteorites have typically revolved around impact-induced shock metamorphism or the gravity-driven axial compression of cumulates at the base of magma chambers. Recent structural analyses, however, of several olivine-rich Diogenites (harzburgites) reveal solid-state plastic deformation not attributable to either scenario and propose dynamic mantle movements in the parent body, assumed to be Vesta. In this study we examine the microstructures of pyroxene and olivine in the olivine-rich Diogenite NWA 5480. Coarse-grained, poikilitic texture, exsolution lamellae and plastic deformation attest to polyphase deformation and a re-heating event, followed by relatively slow cooling. Observations suggest that impact events alone are insufficient to generate and sustain the thermal and deformation conditions required to achieve all of the observed features. The proposed dynamic mantle movements in the Vestan interior may offer a means of heat transport to the system to provide a thermal environment inducive to slow cooling as well as generate the incremental stress fields required for the polyphase plastic deformation observed in the olivine.
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plastic deformation of olivine rich Diogenites and implications for mantle processes on the Diogenite parent body
Meteoritics & Planetary Science, 2014Co-Authors: Beverley J Tkalcec, Frank E BrenkerAbstract:Numerous petrologic and geochemical studies so far on the howardite, eucrite, and Diogenite (HED) meteorites have produced various crystallization scenarios for their parent body, believed to be the differentiated asteroid 4 Vesta. Structural analyses of Diogenites can reveal important insights into postcrystallization deformation on the parent body. Recently published results (Tkalcec et al. 2013) of structural analysis on the olivine-rich Diogenite NWA 5480 reveal that it underwent solid-state plastic deformation, although not at the base of a magma chamber. Dynamic mantle downwelling has been proposed as a plausible deformation mechanism (Tkalcec et al. 2013). The purpose of this study is to investigate whether the plastic deformation found in NWA 5480 is an isolated case. We expand the structural analysis on NWA 5480 and extend it to NWA 5784 and MIL 07001,6, two other samples of rare olivine-rich Diogenites, using electron-backscattered-diffraction (EBSD) techniques. Our EBSD results show that the Diogenites analyzed in this study underwent solid-state plastic deformation, confirming that the observed deformation of NWA 5480 was not an isolated case on the Diogenite parent body. The lattice-preferred orientations (LPOs) of olivine in NWA 5784 and NWA 5480 are clearly distinct from that typical for cumulate rocks at the base of magma chambers, indicating a different stress environment and a different deformation mechanism. The LPO of olivine in MIL 07001 is less conclusive. The structural results of this study suggest that plastic deformation occurred on the Diogenite parent body at high temperatures (1273 < T ≤ 1573 K) in the solid state, i.e., after crystallization of the Diogenites themselves, in a dynamic environment with active stress fields.
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Plastic deformation of olivine-rich Diogenites and implications for mantle processes on the Diogenite parent body
Meteoritics & Planetary Science, 2014Co-Authors: Beverley J Tkalcec, Frank E BrenkerAbstract:Numerous petrologic and geochemical studies so far on the howardite, eucrite, and Diogenite (HED) meteorites have produced various crystallization scenarios for their parent body, believed to be the differentiated asteroid 4 Vesta. Structural analyses of Diogenites can reveal important insights into postcrystallization deformation on the parent body. Recently published results (Tkalcec et al. 2013) of structural analysis on the olivine-rich Diogenite NWA 5480 reveal that it underwent solid-state plastic deformation, although not at the base of a magma chamber. Dynamic mantle downwelling has been proposed as a plausible deformation mechanism (Tkalcec et al. 2013). The purpose of this study is to investigate whether the plastic deformation found in NWA 5480 is an isolated case. We expand the structural analysis on NWA 5480 and extend it to NWA 5784 and MIL 07001,6, two other samples of rare olivine-rich Diogenites, using electron-backscattered-diffraction (EBSD) techniques. Our EBSD results show that the Diogenites analyzed in this study underwent solid-state plastic deformation, confirming that the observed deformation of NWA 5480 was not an isolated case on the Diogenite parent body. The lattice-preferred orientations (LPOs) of olivine in NWA 5784 and NWA 5480 are clearly distinct from that typical for cumulate rocks at the base of magma chambers, indicating a different stress environment and a different deformation mechanism. The LPO of olivine in MIL 07001 is less conclusive. The structural results of this study suggest that plastic deformation occurred on the Diogenite parent body at high temperatures (1273