Day: September 26, 2013

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

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John T. Wasson

Institute of Geophysics and Planetary Physics, Department of Earth and Space Sciences, Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095-1567, USA

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. ε54Cr 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.

Reference
Wasson JT (2013) Vesta and extensively melted asteroids: Why HED meteorites are probably not from Vesta. Earth and Planetary Science Letters 381:138–146
[doi:10.1016/j.epsl.2013.09.002]
Copyright Elsevier

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The classification of CM and CR chondrites using bulk H, C and N abundances and isotopic compositions

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Conel M.O’D. Alexandera,*, Kieren T. Howardb, Roxane Bowdenc and Marilyn L. FogelcaDepartment of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington, DC 20015, USA
bKingsborough Community College of the City University of New York (CUNY), 2001 Oriental Blvd., Brooklyn, NY 11235, USA
cGeophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, N.W., Washington, DC 20015, USA

Here we show that bulk H, C and N elemental and isotopic analyses can be used to classify CM and CR chondrites. These meteorites in both groups form well-defined trends in plots of H content vs. δD and C/H vs. δD, and these trends appear to primarily reflect varying degrees of aqueous alteration. The subset of samples with evidence for thermal alteration plot well away from these trends. In CMs, both bulk H and N isotopic compositions, in particular, strongly correlate with petrologic indicators of the degree of alteration and have been used to classify 54 unheated or weakly heated meteorites on a scale of 2–3. However, extrapolation of the trends based on this scale to type 3.0 predicts relatively high water contents, and the schemes cannot be used to classify altered meteorite belonging to other chondrite groups. Here we propose a different classification scheme based on the degree of hydration (wt.% H in water and OH) of a meteorite that can be determined straightforwardly from a meteorite’s bulk H and C contents. Our estimates of the extent of hydration in CMs correlate well with petrologic estimates of the extent of hydration and with the previously determined phyllosilicate abundances. This is not the case for the CRs, which we suggest is due to cryptic alteration of some CRs at low temperatures.

Reference
Alexander CMO’D, Howard KT, Bowden R and Fogel ML (in press) The classification of CM and CR chondrites using bulk H, C and N abundances and isotopic compositions. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.05.019]
Copyright Elsevier

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Grove Mountains 020090 enriched lherzolitic shergottite: A two-stage formation model

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Yangting Lin1*, Sen Hu1, Bingkui Miao2, Lin Xu3, Yu Liu1, Liewen Xie1, Lu Feng1, and Jing Yang1

1Key Laboratory of the Earth’s Deep Interior, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
2Department of Resources & Environmental Engineering, Guilin University of Technology, Guilin 541004, China
3National Astronomical Observatories, Chinese Academy of Sciences, Beijing, China

Grove Mountains (GRV) 020090 is an enriched lherzolitic shergottite, distinct from other lherzolitic shergottites, except RBT 04262/1. Its characteristics include high abundance of plagioclase (24.2 vol% in the nonpoikilitic area), presence of K-feldspar, common occurrence of baddeleyite, high FeO contents of olivine (bimodal peaks at Fa 33 mol% and Fa 41 mol%) and low-Ca pyroxenes (bimodal peaks at Fs 23.8–31.7 mol% and Fs 25.7–33.9 mol%), and significant LREE enrichment of phosphates (500–610 × CI). The bulk composition of GRV 020090 suggests derivation from partial melting of an enriched reservoir. However, the REE patterns of the cores of pigeonite oikocrysts and the olivine chadacrysts are indistinguishable from those of GRV 99027 and other moderately depleted lherzolitic shergottites, and reveal a LREE-depleted pattern of the primordial parent magma. We propose that the primordial parent magma of GRV 020090 was derived from a moderately depleted Martian upper mantle reservoir, and later the residual melt was contaminated by oxidized and enriched Martian crustal materials as it ascended up to the subsurface. GRV 020090 and RBT 04262/1 may have sampled an igneous unit different from other lherzolitic shergottites.

Reference
Lin Y, Hu S, Miao B, Xu L, Liu Y, Xie L, Feng L and Yang J (in press) Grove Mountains 020090 enriched lherzolitic shergottite: A two-stage formation model. Meteoritics & Planetary Science
[doi:10.1111/maps.12183]
Published by arrangement with John Wiley & Sons

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Silicon isotope variations in the inner solar system: Implications for planetary formation, differentiation and composition

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Thomas Zambardia,b,*, Franck Poitrassona, Alexandre Corgnec,d, Merlin Méheuta, Ghylaine Quittée, Mahesh Anandf,g

aGéosciences Environnement Toulouse, CNRS Université de Toulouse – IRD, 14 avenue Edouard Belin, 31400 Toulouse, France
bDepartment of Geology – Natural History Building, University of Illinois at Urbana-Champaign, 1301 W. Green Street, 61801 Urbana, IL, USA
cInstitut de Recherche en Astrophysique et Planétologie, CNRS – Université de Toulouse, 14 avenue Edouard Belin, 31400 Toulouse, France
dInstituto de Geociencias, Universidad Austral de Chile, Casilla 567, Valdivia, Chile
eLaboratoire de Géologie de Lyon: Terre, Planètes, Environnement, CNRS, ENS de Lyon, Université Lyon 1, 46 allée d’Italie, 69364 Lyon, France
fDepartment of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK
gDepartment of Mineralogy, The Natural History Museum, London SW7 5BD, UK

Accurate and precise Si isotope measurements were obtained using magnesium doping and high-resolution plasma source mass spectrometry for samples representative of the Earth, as well as lunar samples, meteorites from Mars (SNC), eucrites, a howardite, carbonaceous chondrites (CC), ordinary chondrites (OC) and enstatite chondrites (EC). Our data confirm that significant Si isotope fractionations exist among the inner solar system planetary bodies. They show that the Earth and the Moon share the same Si isotopic composition, which is heavier than all other measured bodies, in agreement with most of previous studies. At the other end of the spectrum, enstatite chondrites have the lightest Si isotope compositions. In order to precisely estimate the amount of Si that may have entered the Earth’s core, we developed a refined model of Si partitioning based on continuous planetary accretion that takes into account the likely variations in T, P and fO2 during the Earth’s accretion, as well as isotopic constraints involving metal–silicate partitioning derived from both experimental and natural sample data sets.
Assuming that the difference between the isotopic signature of the bulk silicate Earth (BSE) and chondrites solely results from Si isotope fractionation during core formation, our model implies that at least ~12 wt% Si has entered the Earth’s core, which is greater than most of the estimates based on physical constraints on core density or geochemical mass balance calculations.
This result leads us to propose two hypotheses to explain this apparent contradiction: (1) At least part of the Earth’s building blocks had a Si isotope composition heavier than that observed in chondrites (i.e., δ30Si > -0.39‰). (2) If on the contrary the Earth accreted only from material having chondritic δ30Si, then an additional process besides mantle–core differentiation is required to generate a stronger isotope fractionation and lead to the observed heavy isotope composition of the bulk silicate Earth. It may be the loss of light Si isotopes during partial planetary vaporization in the aftermath of the Moon-forming giant impact. This process, which may have affected metallic cores, required a thorough isotopic re-equilibration between core and silicate to explain the similar heavy isotope composition of the silicate portions of the Earth and the Moon.

Reference
Zambardi T, Poitrasson F, Corgne A, Méheut M, Quitté G and Anand M (2013) Silicon isotope variations in the inner solar system: Implications for planetary formation, differentiation and composition. Geochimica et Cosmochimica Acta 121:67–83.
[doi:10.1016/j.gca.2013.06.040]
Copyright Elsevier

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Small meteoroids’ major contribution to Mercury’s exosphere

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E B Grotheera,b,∗, S A Liviba

aUniversity of Texas at San Antonio, San Antonio, TX 78249, United States
bSouthwest Research Institute, San Antonio, TX 78238, United States

The contribution of the meteoroid population to the generation of Mercury’s exosphere is analyzed to determine which segment contributes most greatly to exospheric refilling via the process of meteoritic impact vaporization. For the meteoroid data, a differential mass distribution based on work by Grün et al. [1985] and a differential velocity distribution based on the work of Zook [1975] is used. These distributions are then evaluated using the method employed by Cintala [1992] to determine impact rates for selected mass and velocity segments of the meteoroid population.
The amount of vapor created by a single meteor impact is determined by using the framework created by Berezhnoy & Klumov [2008]. By combining the impact rate of meteoroids with the amount of vapor a single such impact creates, we derive the total vapor production rate which that meteoroid mass segment contributes to the Herman exosphere. It is shown that meteoroids with a mass of 2.1 × 10−4 g release the largest amount of vapor into Mercury’s exosphere. For meteoroids in the mass range of 10−18 g to 10 g, 90% of all the vapor produced is due to impacts by meteoroids in the mass range 4.2 × 10−7 g ≤ m ≤ 8.3×10−2 g.

Reference
Grotheer EB and Livib SA (2013) Small meteoroids’ major contribution to Mercury’s exosphere. Icarus (in press).
[doi:10.1016/j.icarus.2013.07.032]
Copyright Elsevier

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