¹Laurette Piani, ¹François Robert, ¹Laurent Remusat
¹Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, Sorbonne Universités, UMR CNRS 7590, Université Pierre et Marie Curie, IRD, Muséum National d’Histoire Naturelle, 57 rue Cuvier, Case 52, 75231 Paris Cedex 5, France

Organic matter and hydrous silicates are intimately mixed in the matrix of chondrites and in-situ determination of their individual D/H ratios is therefore challenging. Nevertheless, the D/H ratio of each pure component in this mixture should yield a comprehensible signature of the origin and evolution of water and organic matter in our solar system.
We measured hydrogen isotope ratios of organic and hydrous silicates in the matrices of two carbonaceous chondrites (Orgueil CI1 and Renazzo CR2) and one unequilibrated ordinary chondrite (Semarkona, LL3.0). A novel protocol was adopted, involving NanoSIMS imaging of H isotopes of monoatomatic (H−) and molecular (OH−) secondary ions collected at the same location. This allowed the most enriched component with respect to D to be identified in the mixture. Using this protocol, we found that in carbonaceous chondrites the isotopically homogeneous hydrous silicates are mixed with D-rich organic matter. The opposite was observed in Semarkona. Hydrous silicates in Semarkona display highly heterogeneous D/H ratios, ranging from 150 to 1800×10−61800×10−6 (δDSMOW=−40δDSMOW=−40 to 10 600‰). Organic matter in Semarkona does not show such large isotopic variations. This suggests limited isotopic exchange between the two phases during aqueous alteration. Our study greatly expands the range of water isotopic values measured so far in solar system objects. This D-rich water reservoir was sampled by the LL ordinary chondrite parent body and an estimate (≤9%) of its relative contribution to the D/H ratio of water in Oort cloud family comets is proposed.

Reference
Piani L, Robert F, Remusat L (2015) Micron-scale D/H heterogeneity in chondrite matrices: A signature of the pristine solar system water?
Earth and Planetary Acience Letters, 415, 154–164
Link to Article [doi:10.1016/j.epsl.2015.01.039]

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¹Mathieu Touboul, ²Peter Sprung, ^1,3Sarah A. Aciego, ^1,4Bernard Bourdon, ^1,2Thorsten Kleine
¹Institute for Geochemistry and Petrology, ETH Zürich, Clausiusstrasse 25, 8092 Zurich, Switzerland.
²Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm Klemm-Str. 10, 48149 Münster, Germany
³Department of Earth and Environmental Sciences, 2534 C.C. Little Building, 1100 N. University, Ann Arbor, MI, 48109, USA
⁴Laboratoire de Géologie des Lyon, Ecole Normale Supérieure de Lyon, CNRS and UCBL, 46, Allée d’Italie, 69364 Lyon cedex 7, France

The 182Hf-182W systematics of 12 whole-rock eucrites, including basaltic and cumulate samples, have been investigated. The Hf-W isotope systematics of both basaltic and cumulate eucrites are consistent with derivation from a single mantle source characterized by a strongly suprachondritic Hf/W ratio (180Hf/184W of ∼19). The elevated Hf/W of this mantle source was established by core formation within ∼1 Ma after CAI formation or, alternatively, represents that of the residual melt of a magma ocean from which the eucrites ultimately formed. In the latter case the time of core formation is more uncertain and core formation may have occurred slightly later than ∼1 Ma. The investigated basaltic eucrites fall into three distinct age groups with Hf-W ages of ∼4 Ma (Stannern), ∼11 Ma (Bereba, Bouvante) and ∼22 Ma (Camel Donga, Juvinas) after CAI formation and provide evidence for a protracted history of magmatism and crustal metamorphism on the eucrite parent body, lasting for at least ∼20 Ma. Evidence for even later activity is provided by the cumulate eucrites, which exhibit only small if any variations in 182W/184W in spite of variable 180Hf/184W, indicating cooling below the Hf-W closure temperature when 182Hf was nearly extinct. On the basis of three cumulate eucrites, a Hf-W age of 38±21 Ma is inferred, but even younger ages would also be consistent with the Hf-W data.

Reference
Touboul M, Sprung P, Aciego SA, Bourdon B, Kleine T (2015) Hf-W chronology of the eucrite parent Body. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.02.018]

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^1,2Seann J. McKibbin, ¹Trevor R. Ireland, ¹Yuri Amelin, ¹Peter Holden
¹Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia
²Analytical, Environmental and Geo-Chemistry (AMGC), Vrije Universiteit Brussel, Brussels, Belgium

Angrite meteorites are suitable for Mn-Cr relative dating (53Mn decays to 53Cr with a half life of 3.7 Myr) using secondary ion mass spectrometry (SIMS) because they contain olivine and kirschsteinite with very high 55Mn/52Cr ratios arising from very low Cr concentrations. Discrepant Mn-Cr and U-Pb time intervals between the extrusive or ‘quenched’ angrite D’Orbigny and some slowly cooled or ‘plutonic’ angrites suggests that some have been affected by secondary disturbances, but this seems to have occurred in quenched rather than in slow-cooled plutonic angrites, where such disturbance or delay of isotopic closure might be expected. Using SIMS, we investigate the Mn-Cr systematics of quenched angrites to higher precision than previously achieved by this method and extend our investigation to non-quenched (plutonic or sub-volcanic) angrites. High values of 3.54 (±0.18) × 10-6 and 3.40 (±0.19) × 10-6 (2-sigma) are found for the initial 53Mn/55Mn of the quenched angrites D’Orbigny and Sahara 99555, which are preserved by Cr-poor olivine and kirschsteinite. The previously reported initial 53Mn/55Mn value of D’Orbigny obtained from bulk-rock and mineral separates is slightly lower and was probably controlled by Cr-rich olivine. Results can be interpreted in terms of the diffusivity of Cr in this mineral. Very low Cr concentrations in Ca-rich olivine and kirschsteinite are probably charge balanced by Al; this substitutes for Si and likely diffuses at a very slow rate because Si is the slowest-diffusing cation in olivine. Diffusion in Cr-rich Mg-Fe olivine is probably controlled by cation vacancies because of deficiency in charge-balancing Al and is therefore more prone to disturbance. The higher initial 53Mn/55Mn found by SIMS for extrusive angrites is more likely to reflect closure of Cr in kirschsteinite at the time of crystallisation, simultaneous with closure of U-Pb and Hf-W isotope systematics for these meteorites obtained from pyroxenes. For the younger angrites Northwest Africa (NWA) 4590 and 4801 we have found initial 53Mn/55Mn values which are consistent with more precise work, at 0.90 (±0.4) × 10-6 and 0.13 (±1.1) × 10-6 respectively. Our work shows that SIMS can usefully constrain and distinguish the ages of angrites of different petrologic groups. In reviewing the petrology of angrites, we suggest that NWA 2999, 4590, and 4801 underwent a secondary partial melting and Cr (+/-Pb) disturbance event that the sub-volcanic Lewis Cliff 86010, and perhaps the plutonic Angra dos Reis, did not. With our higher initial 53Mn/55Mn for D’Orbigny and Sahara 99555 as well as previous data, a combined quenched angrite initial 53Mn/55Mn of 3.47 (±0.12) × 10-6 (2-sigma, MSWD 1.00) yields consistent Mn-Cr and U-Pb intervals between these angrites and Lewis Cliff 86010. Discrepant Mn-Cr timescales for other plutonic and sub-volcanic angrites represents resetting during the secondary partial melting event at ∼4557.2 Ma and indicates a relative order of disturbance of isotope systems: Mn-Cr in olivine before U-Pb in pyroxene, with Hf-W in pyroxene being the most resistant.

Reference
McKibbin SJ, Ireland TR, Amelin Y, Holden P (2015) Mn-Cr dating of Fe- and Ca-rich olivine from ‘quenched’ and ‘plutonic’ angrite meteorites using Secondary Ion Mass Spectrometry. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.02.019]

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