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David W. Mittlefehldt - One of the best experts on this subject based on the ideXlab platform.
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dating Howardite melt clasts evidence for an extended vestan bombardment
Lunar and Planetary Science Conference, 2016Co-Authors: J A Cartwright, K V Hodges, M Wadhwa, David W. MittlefehldtAbstract:Howardites are polymict breccias that, together with eucrites and diogenites (HED), likely originate from the vestan surface (regolith/ megaregolith), and display a heterogeneous distribution of eucritic and diogenitic material. Melt clasts are also present alongside other regolithic features within Howardites, and are noteworthy for their compositional variability and appearance. Melt clasts formed by impact events provide a snapshot of the timings and conditions of surface gardening and bombardment on the vestan surface. By dating such clasts, we aim to better constrain the timings of impact events on Vesta, and to establish whether the impact flux in the asteroid belt was similar to that on the Moon. As the Moon is used as the basis for characterising impact models of the inner solar system, it is necessary to verify that apparent wide-scale events are seen in other planetary bodies. In particular, the observed clustering of Apollo melt clast ages between 3.8-4.0 Ga has led to two hypotheses: 1) The Moon was subjected to a sudden event - 'Lunar Cataclysm' or period of 'Late Heavy Bombardment' (LHB), 2) The age cluster represents the end of an epoch of declining bombardment or 'Heavy Bombardment. No consensus has emerged regarding one or other hypothesis. We are testing these hypotheses by seeking evidence for such events in materials other than those derived from the Moon.
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Asteroid (4) Vesta: I. The Howardite-eucrite-diogenite (HED) clan of meteorites
Geochemistry, 2015Co-Authors: David W. MittlefehldtAbstract:Abstract The Howardite, eucrite and diogenite (HED) clan of meteorites are ultramafic and mafic igneous rocks and impact-engendered fragmental debris derived from a thoroughly differentiated asteroid. Earth-based telescopic observation and data returned from vestan orbit by the Dawn spacecraft make a compelling case that the asteroid (4) Vesta is the parent asteroid of HEDs, although this is not universally accepted. Diogenites are petrologically diverse and include dunitic, harzburgitic and noritic lithologic types in addition to the traditional orthopyroxenites. Diogenites form the lower crust of Vesta. Cumulate eucrites are gabbroic rocks formed by accumulation of pigeonite and plagioclase from a mafic magma at depth within the crust, while basaltic eucrites are melt compositions that likely represent shallow-level dikes and sills, and flows. Some basaltic eucrites are richer in incompatible trace elements compared to most eucrites, and these may represent mixed melts contaminated by partial melts of the mafic crust. Differentiation occurred within a few Myr of formation of the earliest solids in the Solar System. Evidence from oxygen isotope compositions and siderophile element contents favor a model of extensive melting of Vesta forming a global magma ocean that rapidly (period of a few Myr) segregated and crystallized to yield a metallic core, olivine-rich mantle, orthopyroxene-rich lower crust and basaltic upper crust. The igneous lithologies were subjected to post-crystallization thermal processing, and most eucrites show textural and mineral-compositional evidence for metamorphism. The cause of this common metamorphism is unclear, but may have resulted from rapid burial of early basalts by later flows caused by high effusion rates on Vesta. The observed surface of Vesta is covered by fragmental debris resulting from impacts, and most HEDs are brecciated. Many eucrites and diogenites are monomict breccias indicating a lack of mixing. However, many HEDs are polymict breccias. Howardites are the most thoroughly mixed polymict breccias, yet only some of them contain evidence for residence in the true regolith. Based on the numbers of meteorites, compositions of Howardites, and models of magma ocean solidification, cumulate eucrites and their residual ferroan mafic melts are minor components of the vestan crust.
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compositional diversity of the vestan regolith derived from Howardite compositions and dawn vir spectra
2014 Goldschmidt Conference, 2014Co-Authors: David W. Mittlefehldt, E Ammannito, T Hiroi, S Deangeles, D P Moriarty, T Diiorio, C M Pieters, M C DesanctisAbstract:Brown University, Providence, RI, USA Howardite, eucrite and diogenite meteorites likely come from asteroid 4 Vesta [1]. Howardites - physical mixtures of eucrites and diogenites - are of two subtypes: regolithic Howardites were gardened in the true regolith; fragmental Howardites are simple polymict breccias [2]. The Dawn spacecraft imaged the howarditic surface of Vesta with the visible and infrared mapping spectrometer (VIR) resulting in qualitative maps of the distributions of distinct diogenite-rich and eucrite-rich terranes [3, 4]. We are developing a robust basis for quantitative mapping of the distribution of lithologic types using spectra acquired on splits of well-characterized Howardites [5, 6]. Spectra were measured on sample powders sieved to <75 µm in the laboratories of the Istituto di Astrofisica e Planetologia Spaziali and Brown University. Data reduction was done using the methods developed to process Dawn VIR spectra [4]. The band parameters for the ~1 and ~2 µm pyroxene absorption features (hereafter BI and BII) can be directly compared to Dawn VIR results. Regolithic Howardites have shallower BI and BII absorptions compared to fragmental Howardites with similar compositions. However, there are statistically significant correlations between Al or Ca contents and BI or BII center wavelengths regardless of Howardite subtype. Diogenites are poor in Al and Ca while eucrites are rich in these elements. The laboratory spectra can thus be directly correlated with the percentage of eucrite material contained in the Howardites. We are using these correlations to quantitatively map Al and Ca distributions, and thus the percentage of eucritic material, in the current regolith of Vesta. [1] McSween
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neutron absorption constraints on the composition of 4 vesta
Meteoritics & Planetary Science, 2013Co-Authors: T H Prettyman, Andrew W Beck, David W. Mittlefehldt, N Yamashita, W C Feldman, John S Hendricks, D J Lawrence, T J Mccoy, H Y Mcsween, P N PeplowskiAbstract:Global maps of the macroscopic thermal neutron absorption cross section of Vesta's regolith by the Gamma Ray and Neutron Detector (GRaND) on board the NASA Dawn spacecraft provide constraints on the abundance and distribution of Fe, Ca, Al, Mg, and other rock-forming elements. From a circular, polar low-altitude mapping orbit, GRaND sampled the regolith to decimeter depths with a spatial resolution of about 300 km. At this spatial scale, the variation in neutron absorption is about seven times lower than that of the Moon. The observed variation is consistent with the range of absorption for Howardite whole-rock compositions, which further supports the connection between Vesta and the Howardite, eucrite, and diogenite meteorites. We find a strong correlation between neutron absorption and the percentage of eucritic materials in Howardites and polymict breccias, which enables petrologic mapping of Vesta's surface. The distribution of basaltic eucrite and diogenite determined from neutron absorption measurements is qualitatively similar to that indicated by visible and near infrared spectroscopy. The Rheasilvia basin and ejecta blanket has relatively low absorption, consistent with Mg-rich orthopyroxene. Based on a combination of Fe and neutron absorption measurements, olivine-rich lithologies are not detected on the spatial scales sampled by GRaND. The sensitivity of GRaND to the presence of mantle material is described and implications for the absence of an olivine signature are discussed. High absorption values found in Vesta's “dark” hemisphere, where exogenic hydrogen has accumulated, indicate that this region is richer in basaltic eucrite, representative of Vesta's ancient upper crust.
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Neutron Absorption Measurements Constrain Eucrite-Diogenite Mixing in Vesta's Regolith
2013Co-Authors: T H Prettyman, Andrew W Beck, David W. Mittlefehldt, John S Hendricks, P N Peplowski, William C. Feldman, David J. Lawrence, Michael J. Toplis, Naoyuki Yamashita, Lucille LecorreAbstract:The NASA Dawn Mission s Gamma Ray and Neutron Detector (GRaND) [1] acquired mapping data during 5 months in a polar, low altitude mapping orbit (LAMO) with approx.460-km mean radius around main-belt asteroid Vesta (264-km mean radius) [2]. Neutrons and gamma rays are produced by galactic cosmic ray interactions and by the decay of natural radioelements (K, Th, U), providing information about the elemental composition of Vesta s regolith to depths of a few decimeters beneath the surface. From the data acquired in LAMO, maps of vestan neutron and gamma ray signatures were determined with a spatial resolution of approx.300 km full-width-at-half-maximum (FWHM), comparable in scale to the Rheasilvia impact basin (approx.500 km diameter). The data from Vesta encounter are available from the NASA Planetary Data System. Based on an analysis of gamma-ray spectra, Vesta s global-average regolith composition was found to be consistent with the Howardite, Eucrite, and Diogenite (HED) meteorites, reinforcing the HED-Vesta connection [2-7]. Further, an analysis of epithermal neutrons revealed variations in the abundance of hydrogen on Vesta s surface, reaching values up to 400 micro-g/g [2]. The association of high concentrations of hydrogen with equatorial, low-albedo surface regions indicated exogenic delivery of hydrogen by the infall of carbonaceous chondrite (CC) materials. This finding was buttressed by the presence of minimally-altered CC clasts in Howardites, with inferred bulk hydrogen abundances similar to that found by GRaND, and by studies using data from Dawn s Framing Camera (FC) and VIR instruments [8-10]. In addition, from an analysis of neutron absorption, spatial-variations in the abundance of elements other than hydrogen were detected [2].
Jean-alix Barrat - One of the best experts on this subject based on the ideXlab platform.
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Potassium isotopic compositions of Howardite-eucrite-diogenite meteorites
Geochimica et Cosmochimica Acta, 2019Co-Authors: Zhen Tian, Jean-alix Barrat, Heng Chen, Bruce Fegley, Katharina Lodders, James M.d. Day, Kun WangAbstract:Abstract We report new high-precision stable K isotope data for three martian meteorites, one lunar meteorite, one ordinary chondrite, four terrestrial igneous United States Geological Survey (USGS) reference materials, and twenty Howardite–eucrite–diogenite [HED] meteorites. The three martian meteorites define a relatively narrow δ41K range with an average of −0.36 ± 0.12‰ (2 SD) that is slightly heavier than the Bulk Silicate Earth (BSE) K isotopic composition (−0.48 ± 0.03‰). Except for the four Northwest Africa samples which were terrestrially contaminated, all HED meteorites reveal substantial 41K enrichment compared to BSE, lunar samples, martian meteorites, and chondrites. We propose that the average δ41K (+0.36 ± 0.16‰) obtained from HED meteorites is representative of Bulk Silicate 4-Vesta. The coupled volatile depletion and heavy K isotope enrichment in 4-Vesta could be attributed to both nebula-scale processes and parent-body events. The asteroid 4-Vesta is likely to have accreted from planetary feedstocks that have been significantly volatile-depleted prior to the major phases of planetary accretion in the early Solar System, with secondary effects of K loss during accretionary growth and magma ocean degassing.
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Geochemistry and oxygen isotope composition of main-group pallasites and olivine-rich clasts in mesosiderites: Implications for the “Great Dunite Shortage” and HED-mesosiderite connection
Geochimica et Cosmochimica Acta Supplement, 2015Co-Authors: Richard C. Greenwood, Ian A. Franchi, Jean-alix Barrat, Henning Haack, P. C. Buchanan, Akira Yamaguchi, Diane Johnson, A. W. R. Bevan, Edward R.d. Scott, T H BurbineAbstract:Evidence from iron meteorites indicates that a large number of differentiated planetesimals formed early in Solar System history. These bodies should have had well-developed olivine-rich mantles and consequentially such materials ought to be abundant both as asteroids and meteorites, which they are not. To investigate this "Great Dunite Shortage" we have undertaken a geochemical and oxygen isotope study of main-group pallasites and dunitic rocks from mesosiderites. Oxygen isotope analysis of 24 main-group pallasites (103 replicates) yielded a mean Delta O-17 value of -0.187 +/- 0.016 parts per thousand (2 sigma), which is fully resolved from the HED Delta O-17 value of -0.246 +/- 0.014 (2 sigma) obtained in our earlier study and demonstrates that both groups represent distinct populations and were derived from separate parent bodies. Our results show no evidence for Delta O-17 bimodality within the main-group pallasites, as suggested by a number of previous studies. Olivine-rich materials from the Vaca Muerta, Mount Padbury and Lamont mesosiderites, and from two related dunites (NWA 2968 and NWA 3329), have Delta O-17 values within error of the mesosiderite average. This indicates that these olivine-rich materials are co-genetic with other mesosiderite clasts and are not fragments from an isotopically distinct pallasite-like impactor. Despite its extreme lithologic diversity the mesosiderite parent body was essentially homogeneous with respect to Delta O-17, a feature best explained by an early phase of large-scale melting (magma ocean), followed by prolonged igneous differentiation. Based on the results of magma ocean modeling studies, we infer that Mg-rich olivines in mesosiderites formed as cumulates in high-level chambers and do not represent samples of the underlying mantle. By analogy, recently documented Mg-rich olivines in Howardites may have a similar origin. Although the Dawn mission did not detect mesosiderite-like material on Vesta, evidence linking the mesosiderites and HEDs includes: (i) their nearly identical oxygen isotope compositions; (ii) the presence in both of coarse-grained Mg-rich olivines; (iii) both have synchronous Lu-Hf and Mn-Cr ages; (iv) there are compositional similarities between the metal in both; and (v) mesosiderite-like material has been identified in a Howardite breccia. The source of the mesosiderites remains an outstanding question in meteorite science. The underrepresentation of olivine-rich materials amongst both asteroids and meteorites results from a range of factors. However, evidence from pallasites and mesosiderites indicates that the most important reason for this olivine shortage lies in the early, catastrophic destruction of planetesimals in the terrestrial planet-forming region and the subsequent preferential loss of their olivine-rich mantles
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Geochemistry and oxygen isotope composition of main-group pallasites and olivine-rich clasts in mesosiderites: Implications for the “Great Dunite Shortage” and HED-mesosiderite connection
Geochimica et Cosmochimica Acta, 2015Co-Authors: Richard C. Greenwood, Ian A. Franchi, Jean-alix Barrat, Edward Scott, Henning Haack, P. C. Buchanan, Akira Yamaguchi, Diane Johnson, A. W. R. Bevan, T H BurbineAbstract:Evidence from iron meteorites indicates that a large number of differentiated planetesimals formed early in Solar System history. These bodies should have had welldeveloped olivine-rich mantles and consequentially such materials ought to be abundant both as asteroids and meteorites, which they are not. To investigate this “Great Dunite Shortage” we have undertaken a geochemical and oxygen isotope study of main-group pallasites and dunitic rocks from mesosiderites. Oxygen isotope analysis of 24 main-group pallasites (103 replicates) yielded a mean 17O value of -0.187±0.016‰ (2 ), which is fully resolved from the HED 17O value of -0.246 ± 0.014 (2 ) obtained in our earlier study and demonstrates that both groups represent distinct populations and were derived from separate parent bodies. Our results show no evidence for 17O bimodality within the main-group pallasites, as suggested by a number of previous studies. Olivine-rich materials from the Vaca Muerta, Mount Padbury and Lamont mesosiderites, and from two related dunites (NWA 2968 and NWA 3329), have 17O values within error of the mesosiderite average. This indicates that these olivine-rich materials are co-genetic with other mesosiderite clasts and are not fragments from an isotopically distinct pallasite-like impactor. Despite its extreme lithologic diversity the mesosiderite parent body was essentially homogeneous with respect to 17O, a feature best explained by an early phase of large-scale melting (magma ocean), followed by prolonged igneous differentiation. Based on the results of magma ocean modeling studies, we infer that Mg-rich olivines in mesosiderites formed as cumulates in high-level chambers and do not represent samples of the underlying mantle. By analogy, recently documented Mg-rich olivines in Howardites may have a similar origin. Although the Dawn mission did not detect mesosiderite-like material on Vesta, evidence linking the mesosiderites and HEDs includes: i) their nearly identical oxygen isotope compositions; ii) the presence in both of coarse-grained Mg-rich olivines; iii) both have synchronous Lu-Hf and Mn-Cr ages; iv) there are compositional similarities between the metal in both; and v) mesosiderite-like material has been identified in a Howardite breccia. The source of the mesosiderites remains an outstanding question in meteorite science. The underrepresentation of olivine-rich materials amongst both asteroids and meteorites results from a range of factors. However, evidence from pallasites and mesosiderites indicates that the most important reason for this olivine shortage lies in the early, catastrophic destruction of planetesimals in the terrestrial planet-forming region and the subsequent preferential loss of their olivine-rich mantles.
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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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oxygen isotope variation in stony iron meteorites
Science, 2006Co-Authors: R C Greenwood, Albert Jambon, Ian A. Franchi, Jean-alix Barrat, T H BurbineAbstract:Asteroidal material, delivered to Earth as meteorites, preserves a record of the earliest stages of planetary formation. High-precision oxygen isotope analyses for the two major groups of stony-iron meteorites (main-group pallasites and mesosiderites) demonstrate that each group is from a distinct asteroidal source. Mesosiderites are isotopically identical to the Howardite-eucrite-diogenite clan and, like them, are probably derived from the asteroid 4 Vesta. Main-group pallasites represent intermixed core-mantle material from a single disrupted asteroid and have no known equivalents among the basaltic meteorites. The stony-iron meteorites demonstrate that intense asteroidal deformation accompanied planetary accretion in the early Solar System.
T H Burbine - One of the best experts on this subject based on the ideXlab platform.
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Geochemistry and oxygen isotope composition of main-group pallasites and olivine-rich clasts in mesosiderites: Implications for the “Great Dunite Shortage” and HED-mesosiderite connection
Geochimica et Cosmochimica Acta Supplement, 2015Co-Authors: Richard C. Greenwood, Ian A. Franchi, Jean-alix Barrat, Henning Haack, P. C. Buchanan, Akira Yamaguchi, Diane Johnson, A. W. R. Bevan, Edward R.d. Scott, T H BurbineAbstract:Evidence from iron meteorites indicates that a large number of differentiated planetesimals formed early in Solar System history. These bodies should have had well-developed olivine-rich mantles and consequentially such materials ought to be abundant both as asteroids and meteorites, which they are not. To investigate this "Great Dunite Shortage" we have undertaken a geochemical and oxygen isotope study of main-group pallasites and dunitic rocks from mesosiderites. Oxygen isotope analysis of 24 main-group pallasites (103 replicates) yielded a mean Delta O-17 value of -0.187 +/- 0.016 parts per thousand (2 sigma), which is fully resolved from the HED Delta O-17 value of -0.246 +/- 0.014 (2 sigma) obtained in our earlier study and demonstrates that both groups represent distinct populations and were derived from separate parent bodies. Our results show no evidence for Delta O-17 bimodality within the main-group pallasites, as suggested by a number of previous studies. Olivine-rich materials from the Vaca Muerta, Mount Padbury and Lamont mesosiderites, and from two related dunites (NWA 2968 and NWA 3329), have Delta O-17 values within error of the mesosiderite average. This indicates that these olivine-rich materials are co-genetic with other mesosiderite clasts and are not fragments from an isotopically distinct pallasite-like impactor. Despite its extreme lithologic diversity the mesosiderite parent body was essentially homogeneous with respect to Delta O-17, a feature best explained by an early phase of large-scale melting (magma ocean), followed by prolonged igneous differentiation. Based on the results of magma ocean modeling studies, we infer that Mg-rich olivines in mesosiderites formed as cumulates in high-level chambers and do not represent samples of the underlying mantle. By analogy, recently documented Mg-rich olivines in Howardites may have a similar origin. Although the Dawn mission did not detect mesosiderite-like material on Vesta, evidence linking the mesosiderites and HEDs includes: (i) their nearly identical oxygen isotope compositions; (ii) the presence in both of coarse-grained Mg-rich olivines; (iii) both have synchronous Lu-Hf and Mn-Cr ages; (iv) there are compositional similarities between the metal in both; and (v) mesosiderite-like material has been identified in a Howardite breccia. The source of the mesosiderites remains an outstanding question in meteorite science. The underrepresentation of olivine-rich materials amongst both asteroids and meteorites results from a range of factors. However, evidence from pallasites and mesosiderites indicates that the most important reason for this olivine shortage lies in the early, catastrophic destruction of planetesimals in the terrestrial planet-forming region and the subsequent preferential loss of their olivine-rich mantles
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Geochemistry and oxygen isotope composition of main-group pallasites and olivine-rich clasts in mesosiderites: Implications for the “Great Dunite Shortage” and HED-mesosiderite connection
Geochimica et Cosmochimica Acta, 2015Co-Authors: Richard C. Greenwood, Ian A. Franchi, Jean-alix Barrat, Edward Scott, Henning Haack, P. C. Buchanan, Akira Yamaguchi, Diane Johnson, A. W. R. Bevan, T H BurbineAbstract:Evidence from iron meteorites indicates that a large number of differentiated planetesimals formed early in Solar System history. These bodies should have had welldeveloped olivine-rich mantles and consequentially such materials ought to be abundant both as asteroids and meteorites, which they are not. To investigate this “Great Dunite Shortage” we have undertaken a geochemical and oxygen isotope study of main-group pallasites and dunitic rocks from mesosiderites. Oxygen isotope analysis of 24 main-group pallasites (103 replicates) yielded a mean 17O value of -0.187±0.016‰ (2 ), which is fully resolved from the HED 17O value of -0.246 ± 0.014 (2 ) obtained in our earlier study and demonstrates that both groups represent distinct populations and were derived from separate parent bodies. Our results show no evidence for 17O bimodality within the main-group pallasites, as suggested by a number of previous studies. Olivine-rich materials from the Vaca Muerta, Mount Padbury and Lamont mesosiderites, and from two related dunites (NWA 2968 and NWA 3329), have 17O values within error of the mesosiderite average. This indicates that these olivine-rich materials are co-genetic with other mesosiderite clasts and are not fragments from an isotopically distinct pallasite-like impactor. Despite its extreme lithologic diversity the mesosiderite parent body was essentially homogeneous with respect to 17O, a feature best explained by an early phase of large-scale melting (magma ocean), followed by prolonged igneous differentiation. Based on the results of magma ocean modeling studies, we infer that Mg-rich olivines in mesosiderites formed as cumulates in high-level chambers and do not represent samples of the underlying mantle. By analogy, recently documented Mg-rich olivines in Howardites may have a similar origin. Although the Dawn mission did not detect mesosiderite-like material on Vesta, evidence linking the mesosiderites and HEDs includes: i) their nearly identical oxygen isotope compositions; ii) the presence in both of coarse-grained Mg-rich olivines; iii) both have synchronous Lu-Hf and Mn-Cr ages; iv) there are compositional similarities between the metal in both; and v) mesosiderite-like material has been identified in a Howardite breccia. The source of the mesosiderites remains an outstanding question in meteorite science. The underrepresentation of olivine-rich materials amongst both asteroids and meteorites results from a range of factors. However, evidence from pallasites and mesosiderites indicates that the most important reason for this olivine shortage lies in the early, catastrophic destruction of planetesimals in the terrestrial planet-forming region and the subsequent preferential loss of their olivine-rich mantles.
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pyroxene mineralogies of near earth vestoids
Meteoritics & Planetary Science, 2009Co-Authors: T H Burbine, P. C. Buchanan, Tenzin Dolkar, Richard P BinzelAbstract:We have calculated pyroxene mineralogies of seven near-Earth asteroids (NEAs) with reflectance spectra similar to HEDs (Howardites, eucrites, and diogenites). Two different sets of formulas (Gaffey et al. 2002; Burbine et al. 2007) are used to calculate the pyroxene mineralogies of the NEAs from their Band I and II centers. The band centers have been adjusted to compensate for the low temperatures on the asteroid surfaces. All of the derived mineralogies from the Gaffey et al. (2002) formulas and the Burbine et al. (2007) formulas overlap. The derived wollastonite (Wo) contents are very similar with differences being only approximately 1 mol%. The derived ferrosilite (Fs) contents differ by only 3 to 8 mol%. The determined pyroxene mineralogies for all seven near- Earth vestoids are consistent with eucrites or Howardites. None of the objects have pyroxene mineralogies consistent with diogenites. The absence of near-Earth vestoids with pyroxene mineralogies similar to diogenites may indicate that it is difficult to produce sizeable (km-sized or larger) bodies that are predominantly composed of diogenitic material, suggesting these objects are rubble piles of mixed ejecta.
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oxygen isotope variation in stony iron meteorites
Science, 2006Co-Authors: R C Greenwood, Albert Jambon, Ian A. Franchi, Jean-alix Barrat, T H BurbineAbstract:Asteroidal material, delivered to Earth as meteorites, preserves a record of the earliest stages of planetary formation. High-precision oxygen isotope analyses for the two major groups of stony-iron meteorites (main-group pallasites and mesosiderites) demonstrate that each group is from a distinct asteroidal source. Mesosiderites are isotopically identical to the Howardite-eucrite-diogenite clan and, like them, are probably derived from the asteroid 4 Vesta. Main-group pallasites represent intermixed core-mantle material from a single disrupted asteroid and have no known equivalents among the basaltic meteorites. The stony-iron meteorites demonstrate that intense asteroidal deformation accompanied planetary accretion in the early Solar System.
D Rumble - One of the best experts on this subject based on the ideXlab platform.
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oxygen isotope evidence for rapid mixing of the hed meteorite parent body
Earth and Planetary Science Letters, 2004Co-Authors: Uwe Wiechert, Alex N Halliday, H Palme, D RumbleAbstract:Abstract The 16 O, 17 O and 18 O abundances of Howardites, eucrites, and diogenites have been used to assign them to a single ‘HED’ parent body, thought to be asteroid 4 Vesta. We report the first evidence of oxygen isotopic heterogeneity among HED meteorites indicating incompletely mixed sources. New high-precision oxygen isotope measurements of 34 HED meteorites reveal that most have the same Δ 17 O′, consistent with a very rapid early history of large-scale mixing on Vesta. However, Howardites are on average very slightly enriched in 16 O, whereas Ibitira, Caldera, Pasamonte, and ALHA78132 are 16 O-depleted compared to other investigated eucrites. The Δ 17 O′ of Ibitira is completely different from all other HEDs measured. Some of the results for eucrites and diogenites can be explained by partial melting and rapid mixing of the interior of Vesta. Others require a separate parent body or indicate that parts of the outer layer of Vesta retained some primary isotopic heterogeneity. The oxygen isotopic composition of Howardites provides an upper limit for the amount of admixed carbonaceous chondritic material into the HED parent body regolith.
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oxygen isotope evidence for rapid mixing of the hed meteorite parent body
Earth and Planetary Science Letters, 2004Co-Authors: Uwe Wiechert, Alex N Halliday, H Palme, D RumbleAbstract:Abstract The 16 O, 17 O and 18 O abundances of Howardites, eucrites, and diogenites have been used to assign them to a single ‘HED’ parent body, thought to be asteroid 4 Vesta. We report the first evidence of oxygen isotopic heterogeneity among HED meteorites indicating incompletely mixed sources. New high-precision oxygen isotope measurements of 34 HED meteorites reveal that most have the same Δ 17 O′, consistent with a very rapid early history of large-scale mixing on Vesta. However, Howardites are on average very slightly enriched in 16 O, whereas Ibitira, Caldera, Pasamonte, and ALHA78132 are 16 O-depleted compared to other investigated eucrites. The Δ 17 O′ of Ibitira is completely different from all other HEDs measured. Some of the results for eucrites and diogenites can be explained by partial melting and rapid mixing of the interior of Vesta. Others require a separate parent body or indicate that parts of the outer layer of Vesta retained some primary isotopic heterogeneity. The oxygen isotopic composition of Howardites provides an upper limit for the amount of admixed carbonaceous chondritic material into the HED parent body regolith.
Andrew W Beck - One of the best experts on this subject based on the ideXlab platform.
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grosvenor mountains 95 Howardite pairing group insights into the surface regolith of asteroid 4 vesta
Meteoritics & Planetary Science, 2016Co-Authors: Marc W Caffee, K C Welten, N G Lunning, H Y Mcsween, Andrew W BeckAbstract:Regolithic Howardites are analogs for the surface materials of asteroid 4 Vesta, recently mapped by the Dawn spacecraft. Rigorously evaluating pairing of Howardites recovered in 1995 in the Grosvenor Mountains (GRO 95), Antarctica, enables an examination of a larger, more representative regolith sample. Previous work on two of the Howardites studied here concluded that GRO 95602 and GRO 95535 are solar wind-rich surface regolith samples and that they are not paired with each other, leading to uncertainty regarding pairing relationships between the other GRO 95 Howardites. Based on petrology, cosmic-ray exposure history, and terrestrial age, four GRO 95 Howardites are paired. The paired Howardites (GRO 95534, 95535, 95574, 95581) were from a meteoroid with radius of 10–15 cm, a preatmospheric size comparable to that of Kapoeta, the largest known regolithic Howardite. The paired GRO 95 Howardites contain clasts of at least 18 separate HED lithologies, providing evidence they were assembled from diverse source materials. The total eucrite:diogenite mixing ratio (ratio of all eucrite lithologies to all diogenite lithologies) in the paired GRO 95 Howardites is ~2:1. Petrographically determined basaltic eucrite:cumulate eucrite ratios in regolithic Howardites, studied here and previously, vary more widely than total eucrite:diogenite ratios. Relative to eucritic pyroxene, plagioclase is depleted in these Howardites, which provides evidence that plagioclase is preferentially comminuted in the vestan regolith. The extent of plagioclase depletion could be an indicator of regolith maturity.
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neutron absorption constraints on the composition of 4 vesta
Meteoritics & Planetary Science, 2013Co-Authors: T H Prettyman, Andrew W Beck, David W. Mittlefehldt, N Yamashita, W C Feldman, John S Hendricks, D J Lawrence, T J Mccoy, H Y Mcsween, P N PeplowskiAbstract:Global maps of the macroscopic thermal neutron absorption cross section of Vesta's regolith by the Gamma Ray and Neutron Detector (GRaND) on board the NASA Dawn spacecraft provide constraints on the abundance and distribution of Fe, Ca, Al, Mg, and other rock-forming elements. From a circular, polar low-altitude mapping orbit, GRaND sampled the regolith to decimeter depths with a spatial resolution of about 300 km. At this spatial scale, the variation in neutron absorption is about seven times lower than that of the Moon. The observed variation is consistent with the range of absorption for Howardite whole-rock compositions, which further supports the connection between Vesta and the Howardite, eucrite, and diogenite meteorites. We find a strong correlation between neutron absorption and the percentage of eucritic materials in Howardites and polymict breccias, which enables petrologic mapping of Vesta's surface. The distribution of basaltic eucrite and diogenite determined from neutron absorption measurements is qualitatively similar to that indicated by visible and near infrared spectroscopy. The Rheasilvia basin and ejecta blanket has relatively low absorption, consistent with Mg-rich orthopyroxene. Based on a combination of Fe and neutron absorption measurements, olivine-rich lithologies are not detected on the spatial scales sampled by GRaND. The sensitivity of GRaND to the presence of mantle material is described and implications for the absence of an olivine signature are discussed. High absorption values found in Vesta's “dark” hemisphere, where exogenic hydrogen has accumulated, indicate that this region is richer in basaltic eucrite, representative of Vesta's ancient upper crust.
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Neutron Absorption Measurements Constrain Eucrite-Diogenite Mixing in Vesta's Regolith
2013Co-Authors: T H Prettyman, Andrew W Beck, David W. Mittlefehldt, John S Hendricks, P N Peplowski, William C. Feldman, David J. Lawrence, Michael J. Toplis, Naoyuki Yamashita, Lucille LecorreAbstract:The NASA Dawn Mission s Gamma Ray and Neutron Detector (GRaND) [1] acquired mapping data during 5 months in a polar, low altitude mapping orbit (LAMO) with approx.460-km mean radius around main-belt asteroid Vesta (264-km mean radius) [2]. Neutrons and gamma rays are produced by galactic cosmic ray interactions and by the decay of natural radioelements (K, Th, U), providing information about the elemental composition of Vesta s regolith to depths of a few decimeters beneath the surface. From the data acquired in LAMO, maps of vestan neutron and gamma ray signatures were determined with a spatial resolution of approx.300 km full-width-at-half-maximum (FWHM), comparable in scale to the Rheasilvia impact basin (approx.500 km diameter). The data from Vesta encounter are available from the NASA Planetary Data System. Based on an analysis of gamma-ray spectra, Vesta s global-average regolith composition was found to be consistent with the Howardite, Eucrite, and Diogenite (HED) meteorites, reinforcing the HED-Vesta connection [2-7]. Further, an analysis of epithermal neutrons revealed variations in the abundance of hydrogen on Vesta s surface, reaching values up to 400 micro-g/g [2]. The association of high concentrations of hydrogen with equatorial, low-albedo surface regions indicated exogenic delivery of hydrogen by the infall of carbonaceous chondrite (CC) materials. This finding was buttressed by the presence of minimally-altered CC clasts in Howardites, with inferred bulk hydrogen abundances similar to that found by GRaND, and by studies using data from Dawn s Framing Camera (FC) and VIR instruments [8-10]. In addition, from an analysis of neutron absorption, spatial-variations in the abundance of elements other than hydrogen were detected [2].
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neutron absorption measurements constrain eucrite diogenite mixing in
2013Co-Authors: T H Prettyman, Andrew W Beck, V Reddy, N Yamashita, H Mizzon, Max PlanckAbstract:The NASA Dawn Mission’s Gam-ma Ray and Neutron Detector (GRaND) [1] acquired mapping data during 5 months in a polar, low altitude mapping orbit (LAMO) with ~460-km mean radius around main-belt asteroid Vesta (264-km mean radius) [2]. Neutrons and gamma rays are produced by galac-tic cosmic ray interactions and by the decay of natural radioelements (K, Th, U), providing information about the elemental composition of Vesta’s regolith to depths of a few decimeters beneath the surface. From the data acquired in LAMO, maps of vestan neutron and gam-ma ray signatures were determined with a spatial reso-lution of ~300 km full-width-at-half-maximum (FWHM), comparable in scale to the Rheasilvia impact basin (~500 km diameter). The data from Vesta en-counter are available from the NASA Planetary Data System. Based on an analysis of gamma-ray spectra, Vesta’s global-average regolith composition was found to be consistent with the Howardite, Eucrite, and Diogenite (HED) meteorites, reinforcing the HED-Vesta connec-tion [2-7]. Further, an analysis of epithermal neutrons revealed variations in the abundance of hydrogen on Vesta’s surface, reaching values up to 400 g/g [2]. The association of high concentrations of hydrogen with equatorial, low-albedo surface regions indicated exogenic delivery of hydrogen by the infall of carbona-ceous chondrite (CC) materials. This finding was but-tressed by the presence of minimally-altered CC clasts in Howardites, with inferred bulk hydrogen abundances similar to that found by GRaND, and by studies using data from Dawn’s Framing Camera (FC) and VIR in-struments [8-10]. In addition, from an analysis of neu-tron absorption, spatial-variations in the abundance of elements other than hydrogen were detected [2]. Ongoing mapping studies with GRaND data in-clude the determination Fe abundances from the 7.6 MeV
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petrologic and textural diversity among the pca 02 Howardite group one of the largest pieces of the vestan surface
Meteoritics & Planetary Science, 2012Co-Authors: Andrew W Beck, K C Welten, H Y Mcsween, C E Viviano, Marc W CaffeeAbstract:Abstract— Nine Howardites and two diogenites were recovered from the Pecora Escarpment Icefield (PCA) in 2002. Cosmogenic radionuclide abundances indicate that the samples are paired and that they constituted an approximately 1 m (diameter) meteoroid prior to atmospheric entry. At about 1 m in diameter, the PCA 02 HED group represents one of the largest single pre-atmospheric pieces of the Vestan surface yet described. Mineral and textural variations were measured in six of the PCA 02 Howardites to investigate meter-scale diversity of the Vestan surface. Mineral compositions span the range of known eucrite and diogenite compositions. Additional non-diogenitic groups of Mg- and Fe-rich olivine are observed, and are interpreted to have been formed by exogenic contamination and impact melting, respectively. These Howardites contain olivine-rich impact melts that likely formed from dunite- and harzburgite-rich target rocks. Containing the first recognized olivine-rich HED impact melts, these samples provide meteoritic evidence that olivine-rich lithologies have been exposed on the surface of Vesta. Finally, we present a new method for mapping distributions of lithologies in Howardites using 8 elemental X-ray maps. Proportions of diogenite and eucrite vary considerably among the PCA 02 Howardites, suggesting they originated from a heterogeneous portion of the Vestan surface. While whole sample modes are dominated by diogenite, the finer grain size fractions are consistently more eucritic. This discrepancy has implications for near-infrared spectral observations of portions of Vesta’s surface that are similar to the PCA 02 Howardites, as the finer grained eucritic material will disproportionately dominate the spectra.