HED Meteorite und Vesta

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Neue Denkansätze und eine Arbeitshypothese:

Vesta and extensively melted asteroids: Why HED meteorites are probably not from Vesta

John T. Wasson

Earth and Planetary Science Letters, Volume 381, 1 November 2013, Pages 138–146

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Highlights

• Compositional data on iron meteorites require >26 extensively melted asteroids.
• Now, after Dawn, the link between Vesta and HEDs still based on spectral data.
• O- and Cr-isotopic data show a strong link between HEDs and IIIAB irons.
• The working model should be that the HEDs are from the IIIAB asteroid.

Abstract

Most researchers hold that the HED clan of differentiated meteorites originated on Vesta largely based on the assumption that nearly all V-type asteroids with basaltic reflection spectra are fragments spalled off Vesta. Although it is a reasonable working hypothesis that most of the V-type asteroids in the Vesta family originated on Vesta, the spectra are not unique enough to confirm this; a sizable fraction may have been produced during the destruction of a differentiated asteroid in the same large region of dynamic space. Observations of asteroids in the inner Asteroid Belt show that more than half of the V-type asteroids do not belong to the Vesta dynamic family.

Iron-meteorite evidence shows that at least 26 asteroids experienced extensive melting and would have generated basalts and other differentiated stony meteorites. Most iron meteorites show high degrees of elemental fractionations that lead to the conclusion that they experienced fractional crystallization; it is probable that all these bodies generated basalts. There are 9 of these “magmatic” iron-meteorite groups and test criteria mainly based on extreme fractionations indicate that an additional 17 disrupted asteroids hosted fractionally crystallized cores and thus that ⩾26 asteroids experienced extensive melting; this estimate is much lower than previous estimates that included nonmagmatic irons.

Within expected planetary heterogeneities the O-isotopic composition of HEDs is the same as that in oxides from IIIAB irons, the largest magmatic group of iron meteorites. View the MathML source values are also very similar in IIIABs and HEDs. The O- and Cr-isotopic ties are much stronger than the spectral tie thus the working hypothesis should be that HEDs are from the IIIAB parent asteroid.

Remote elemental analysis could confirm that HEDs are not from Vesta. If future remote analysis measures K contents ⩾0.6 mg/g this will indicate that HEDs did not originate on Vesta.

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Olivine in an unexpected location on Vesta’s surface

    E. Ammannito, M. C. De Sanctis, E. Palomba, A. Longobardo,D. W. Mittlefehldt,   
    H. Y. McSween, S. Marchi, M. T. Capria, F. Capaccioni, A. Frigeri,   
    C. M. Pieters, O. Ruesch, F. Tosi, F. Zambon, F. Carraro,  S. Fonte, H. Hiesinger,   
    G. Magni, L. A. McFadden, C. A. Raymond, C. T. Russell & J. M. Sunshine

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    Nature (2013) doi:10.1038/nature12665, Published online 06 November 2013

Olivine is a major component of the mantle of differentiated bodies, including Earth. Howardite, eucrite and diogenite (HED) meteorites represent regolith, basaltic-crust, lower-crust and possibly ultramafic-mantle samples of asteroid Vesta, which is the lone surviving, large, differentiated, basaltic rocky protoplanet in the Solar System1. Only a few of these meteorites, the orthopyroxene-rich diogenites, contain olivine, typically with a concentration of less than 25 per cent by volume2. Olivine was tentatively identified on Vesta3, 4, on the basis of spectral and colour data, but other observations did not confirm its presence5. Here we report that olivine is indeed present locally on Vesta’s surface but that, unexpectedly, it has not been found within the deep, south-pole basins, which are thought to be excavated mantle rocks6, 7, 8. Instead, it occurs as near-surface materials in the northern hemisphere. Unlike the meteorites, the olivine-rich (more than 50 per cent by volume) material is not associated with diogenite but seems to be mixed with howardite, the most common7, 9 surface material. Olivine is exposed in crater walls and in ejecta scattered diffusely over a broad area. The size of the olivine exposures and the absence of associated diogenite favour a mantle source, but the exposures are located far from the deep impact basins. The amount and distribution of observed olivine-rich material suggest a complex evolutionary history for Vesta.

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Passend zum Wasson-Artikel oben

The oxygen isotope composition of diogenites: Evidence for early global melting on a single, compositionally diverse, HED parent body.

Greenwood RC, Barrat J-A, Yamaguchi A, Franchi IA, Scott ERD, Bottke WF and Gibson JM (2014)

Earth and Planetary Science Letters 390:165–174.

Highlights

•New oxygen isotope data shows that diogenites are homogeneous in terms of Δ17O.
•HED Δ17O homogeneity was the result of early global, parent body, melting.
•Our data is consistent with a single parent body source for the HEDs.
•Oxygen isotope variation is consistent with 4 Vesta as the HED source asteroid.
•Models for a common HED–IIIAB–pallasite source are not supported by our data.

Oxygen isotope measurements of a suite of 22 diogenites demonstrate that they have a restricted range of Δ17O values: View the MathML source. 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 View the MathML source 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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Composition and mineralogy of dark material units on Vesta

Ernesto Palomba, Andrea Longobardo, Maria Cristina De Sanctis, Francesca Zambon,
Federico Tosi, Eleonora Ammannito, Fabrizio Capaccioni, Alessandro Frigeri,
Maria Teresa Capria, Edward A. Cloutis, Ralf Jaumann, Jean-Philippe Combe, Carol A. Raymond,Christopher T. Russell

Icarus (2.5.2014)
http://dx.doi.org/10.1016/j.icarus.2014.04.040

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Highlights

• We detected dark material units in the Vesta surface, building a catalogue.
• Composition similar to the typical non dark vestan material with the addition of a darkening agent.
• The darkening agent is confirmed to be Carbonacoues Chondrite (CC) material because it is hydrated.
• The darker units on Vesta contain 40% of CC.
• The regolith grain size is homogenous all around Vesta and is less than 45 micron.

Abstract

Vesta is the asteroid with the largest albedo variation among the known rocky Solar System objects and shows a widespread occurrence of dark material (DM) and bright material (BM) units. In the first observation phases by the Dawn spacecraft, two main extensions of low albedo areas were identified on Vesta and found to be closely correlated with carbonaceous, OH-rich, material. In this work we use the hyperspectral data provided by the VIR-Dawn imaging spectrometer onboard Dawn to detect and analyze individual, well-defined, dark material units. We define DM units assuming a relative criterion, i.e. reflectance lower than the surroundings. By coupling visible and infrared images of the same area we are able to select real dark material units, discarding false detections created by shadowing effects. A detailed final catalogue of 123 dark units is presented, containing the geographical parameters and the
main spectral characteristics for each unit. Independently of the geological context of the dark units, all DMs show similar spectral properties, dominated by the pyroxene absorption features, as is the average spectrum of Vesta. This finding suggests a similar composition, with the presence of darkening agents that also weaken pyroxene band depths. The majority (90%) of the DM units shows a positive correlation between low albedo and an OH band centered at 2.8 μm, confirming the hypothesis that the darkening agents are carbonaceous chondrites, probably delivered by low-velocity impacts of primitive asteroids. A comparison with laboratory spectra allows us to better constrain the size and the composition of the darkening agents. These DM areas seem to be made of eucritic material. The regolith grain size seems to be nearly constant around an average value of 25 μm, and is quite homogenous at least in the first hundreds of meters beneath the Vesta surface, suggesting similar processing mechanisms for both DM and BM.

The Geological Nature of Dark Material on Vesta and Implicatons for the Subsurface Structure

R. Jaumann, A. Nass, K. Otto, K. Krohn, K. Stephan, T.B. McCord, D.A. Williams, C.A. Raymond, D.T. Blewett, H. Hiesinger, R.A. Yingst, M.C. De Sanctis, E. Palomba, T. Roatsch, K.-D. Matz, F. Preusker, F. Scholten, C.T. Russell

Icarus (2.5.2014)
http://dx.doi.org/10.1016/j.icarus.2014.04.035

LINK

Highlights

•Dark material deposits on Vesta are almost correlated with impact features.
•Dark material deposits are isolated patches of less than 100 m thickness within the first 2000 m of the subsurface.
•Impacts multiply reworked the dark material subsurface layers and produced the inhomogeneous complex mixing with the regolith.
•Dark material deposits are closely associated with the impact structure of the Veneneia basin indicating an exogenic origin.

Abstract

Deposits of dark material appear on Vesta’s surface as features of relatively low-albedo in the visible wavelength range of Dawn’s camera and spectrometer. Mixed with the regolith and partially excavated by younger impacts, the material is exposed as individual layered outcrops in crater walls or ejecta patches, having been uncovered and broken up by the impact. Dark fans on crater walls and dark deposits on crater floors are the result of gravity-driven mass wasting triggered by steep slopes and impact seismicity. The fact that dark material is mixed with impact ejecta indicates that it has been processed together with the ejected material. Some small craters display continuous dark ejecta similar to lunar dark-halo impact craters, indicating that the impact excavated the material from beneath a higher-albedo surface. The asymmetric distribution of dark material in impact craters and ejecta suggests non-continuous distribution in the local subsurface. Some positive-relief dark edifices appear to be impact-sculpted hills with dark material distributed over the hill slopes. Dark features inside and outside of craters are in some places arranged as linear outcrops along scarps or as dark streaks perpendicular to the local topography. The spectral characteristics of the dark material resemble that of Vesta’s regolith. Dark material is distributed unevenly across Vesta’s surface with clusters of all types of dark material exposures. On a local scale, some craters expose or are associated with dark material, while others in the immediate vicinity do not show evidence for dark material. While the variety of surface exposures of dark material and their different geological correlations with surface features, as well as their uneven distribution, indicate a globally inhomogeneous distribution in the subsurface, the dark material seems to be correlated with the rim and ejecta of the older Veneneia south polar basin structure. The origin of the dark material is still being debated, however, the geological analysis suggests that it is exogenic, from carbon-rich low-velocity impactors, rather than endogenic, from freshly exposed mafic material or melt, exposed or created by impacts.

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Neues zum Thema:

Mineralogy of V-type asteroids as a constraining tool of their past history

S.F.A. Batista, T.M. Seixas, M.A. Salgueiro da Silva, R.M.G. de Albuquerque

Planetary and Space Science
DOI: 10.1016/j.pss.2014.10.012
available online 4 November 2014

LINK

Our results strength the relationship between HED meteorites and V-type asteroids, as also suggested by the results from the Dawn mission (15, 16, 30 and 31). The latter is contradicting the results from Schiller et al. (2011) and Wasson (2013), which questioned this linkage. We also report the best meteoritic kind for the studied V-type asteroids, by comparing their spectra and mineralogies. We also report the pyroxenes mineralogical distributions of the HED meteorites, inferred through the Hapke radiative transfer model, which can be suitable for comparison with future mineralogical studies of these objects.