Day: January 22, 2024

The fate of primary iron sulfides in the CM1 carbonaceous chondrites: Effects of advanced aqueous alteration on primary components

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1,2S. A. Singerling,3C. M. Corrigan,4A. J. Brearley
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14132]
1Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA
2Department of Geosciences, Goethe University Frankfurt, Altenhoeferallee 1, 60438 Frankfurt am Main, Germany.
3Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC, USA
4Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA
Published by arrangement with John Wiley & Sons

We have carried out a SEM-EPMA-TEM study to determine the textures and compositions of relict primary iron sulfides and their alteration products in a suite of moderately to heavily altered CM1 carbonaceous chondrites. We observed four textural groups of altered primary iron sulfides: (1) pentlandite+phyllosilicate (2P) grains, characterized by pentlandite with submicron lenses of phyllosilicates; (2) pyrrhotite+pentlandite+magnetite (PPM) grains, characterized by pyrrhotite–pentlandite exsolution textures with magnetite veining and secondary pentlandite; (3) pentlandite+serpentine (PS) grains, characterized by relict pentlandite exsolution, serpentine, and secondary pentlandite; and (4) pyrrhotite+pentlandite+magnetite+serpentine (PPMS) grains, characterized by features of both the PPM and PS grains. We have determined that all four groups were initially primary iron sulfides, which formed from crystallization of immiscible sulfide melts within silicate chondrules in the solar nebula. The fact that such different alteration products could result from the same precursor sulfides within even the same meteorite sample further underscores the complexity of the aqueous alteration environment for the CM chondrites. The different alteration reactions for each textural group place constraints on the mechanisms and conditions of alteration with evidence for acidic environments, oxidizing environments, and changing fluid compositions (Ni-bearing and Si-Mg-bearing).

Nucleosynthetic isotope variations in chondritic meteorites and their relationship to bulk chemistry

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1Herbert Palme,2,3Klaus Mezger
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14127]
1Forschungsinstitut und Naturmuseum Senckenberg, Frankfurt am Main, Germany
2Institut für Geologie, Universität Bern, Bern, Switzerland
3Center for Space and Habitability, Universität Bern, Bern, Switzerland
Published by arrangement with John Wiley & Sons

The relationship of mass-independent stable isotope anomalies with the chemistry of chondritic meteorites provides constraints on mixing and fractionation processes in the early solar nebula. The present study emphasizes the strong correlation of nucleosynthetic isotope variations among ordinary chondrites (OC), enstatite chondrites (EC), Earth, CI-chondrites, and Ca, Al-rich inclusions (CAI) in ε50Ti versus ε54Cr space. This correlation indicates variable contamination of chondritic reservoirs with material from a single source providing neutron-rich nuclei such as 50Ti, 54Cr, and 62Ni. The well-defined linear relationship of ε50Ti versus ε54Cr indicates that all reservoirs on the correlation line (“chondrite reference line”) started with a CI-chondritic (solar) Cr/Ti ratio, irrespective of the present Cr/Ti ratio of the samples falling on the chondrite reference line. The isotope compositions of carbonaceous chondrites (CC) do not fit the chondrite reference line. Their isotope composition is consistent with a mixture of chondritic meteorites originally falling on the chondrite reference line and volatile element depleted CAIs. However, CC cannot result from addition of CAIs to OC or EC. Neither can OC and EC be produced by loss of refractory components from CI-meteorites. Also, stable isotopes are inconsistent with OC being derived from EC, and vice versa, by a chemical fractionation process. The enrichment of the Earth in refractory lithophile elements is not the result of addition of a refractory component to a chondritic reservoir. It is rather the result of internal fractionation of a chondritic reservoir.