¹Robbin Visser,¹Timm John,²Martin J.Whitehouse,³Markus Patzek,³Addi Bischoff
Earth and Planetary Science Letters 547, 116440 Link to Article [https://doi.org/10.1016/j.epsl.2020.116440]
¹Freie Universität Berlin, Institut für Geologische Wissenschaften, Berlin, Germany
²Swedish Museum of Natural History, Stockholm, Sweden
³Institut für Planetologie, University of Münster, Münster, Germany
Copyright Elsevier

A key process in the early solar system that significantly affects the further evolution and transport of highly volatile elements throughout the solar system hydrothermal parent body alteration. To determine whether hydrothermal alteration in outer solar system parent bodies occurred more or less simultaneously or due to a sequence of multiple different events, we investigated low-temperature hydrothermally altered CM and CI chondrites along with volatile-rich CM-like clasts and C1 clasts with abundant mineral phases that contain volatiles. In this respect, C1 clasts are particularly important as they closely resemble the CI chondrites but originate from isotopically different parent bodies. Specifically, we applied the SIMS-based Mn/Cr in situ dating technique to carbonates, a common hydrothermally formed phase in low-temperature hydrothermally altered meteorites. The Mn/Cr ages of dolomites in CI chondrites and C1 clasts as well as calcites in CM chondrites and CM-like clasts reveal that nearly all carbonates in low-temperature hydrothermally altered clasts and chondrites were formed within a brief period between 2-6 Ma after CAI formation. Given this sharp separation, and that hardly any material contains carbonates formed later than ∼6 Ma after CAI formation, hydrothermal alteration likely occurred near-contemporaneously among different parent bodies in the outer solar system. Further, the timing of hydrothermal alteration matches peak heating of ²⁶Al decay that ceased at ∼5 Ma after CAI formation. Hereby, these results are consistent with a model in which the carbonates in low-temperature hydrothermally altered parent bodies precipitated from the fluid produced by melting ice. The results also show that other potential heating events (e.g., impacts) only negligibly contributed to creating environments where fluid-mediated dissolution and precipitation of carbonates was possible. Additionally, the isotopic (H, O, Cr, and S) differences between C1 clasts and CI chondrites are most likely not caused by differences in timing of hydrothermal aqueous alteration and, thus, are best explained by spatially different isotopic reservoirs.

^1,2M.D.Suttle,^3,4Z.Dionnet,⁵I.Franchi,^1,6L.Folco,⁵J.Gibson,⁵R.C.Greenwood,³A.Rotundi,⁷A.King,²S.S.Russell
Earth and Planetary Science Letters 546, 116444 Link to Article [https://doi.org/10.1016/j.epsl.2020.116444]
¹Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italy
²Planetary Materials Group, Department of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, UK
³DIST-Università di Napoli “Parthenope”, Centro Direzionale Isola C4, 80143 Naples, Italy
⁴INAF-IAPS, via Fosso del Cavaliere 100, 00133 Rome, Italy
⁵School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
⁶CISUP, Centro per l’Integrazione della Strumentazione dell’Università di Pisa, Lungarno Pacinotti 43, 56126 Pisa, Italy
⁷Psiche beamline, Synchrotron SOLEIL, Orne des Meurisiers, France
Copyright Elsevier

Bulk oxygen isotope data has the potential to match extraterrestrial samples to parent body sources based on distinctive
O and
O ratios. We analysed 10 giant (>500 μm) micrometeorites using combined micro-Computer Tomography (μCT) and O-isotope analysis to pair internal textures to inferred parent body groups. We identify three ordinary chondrite particles (L and LL groups), four from CR chondrites and the first micrometeorite from the enstatite chondrite (EH4) group. In addition, two micrometeorites are from hydrated carbonaceous chondrite parent bodies with 16O-poor isotopic compositions and plot above the terrestrial fractionation line. They experienced intense aqueous alteration, contain pseudomorphic chondrules and are petrographically similar to the CM1/CR1 chondrites. These micrometeorites may be members of the newly established CY chondrites and/or derived from the enigmatic “Group 4” micrometeorite population, previously identified by Yada et al., 2005 [GCA, 69:5789-5804], Suavet et al., 2010 [EPSL, 293:313-320] (and others). One of our 16O-poor micrometeorite plots on the same isotopic trendline as the CO, CM and CY chondrites – “the CM mixing line” (with a slope of ∼0.7 and a
O intercept of -4.23‰), this implies a close relationship and potentially a genetic link to these hydrated chondrites. If position along the CM mixing line reflects the amount of 16O-poor (heavy) water-ice accreted onto the parent body at formation, then the CY chondrites and these 16O-poor micrometeorites must have accreted at least as much water-ice as CM chondrites but potentially more. In addition, thermal metamorphism could have played a role in further raising the bulk O-isotope compositions through the preferential loss of isotopically light water during phyllosilicate dehydration. The study of micrometeorites provides insights into asteroid belt diversity through the discovery of material not currently sampled by larger meteorites, perhaps as a result of atmospheric entry biases preventing the survival of large blocks of friable hydrated material.