^1,2Swarna Prava Das,³Alessandro Maturilli,³Aurélie Van den Neucker,⁴Markus Patzek,⁵Dipak Kumar Panda,⁶Gopal K. Pradhan,^1,2Guneshwar Thangjam,^1,2,7Surya Snata Rout
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.70183]
¹School of Earth and Planetary Sciences, National Institute of Science Education and Research (NISER), Khordha, Odisha,752050, India
²Homi Bhabha National Institute, Training School Complex, Mumbai, 400094, India
³Institute of Space Research, German Aerospace Centre (DLR), Berlin, 12489, Germany
⁴Institut für Planetologie (IfP), Universität Münster, Münster, 48149, Germany
⁵Planetary Science Division, Physical Research Laboratory, Ahmedabad, Gujarat, 380009, India
⁶Department of Physics, School of Applied Sciences, KIIT Deemed to be University, Bhubaneswar, 751024, India
⁷Center for Interdisciplinary Sciences (CIS), NISER Bhubaneswar, Khordha, Odisha, 752050, India
Published by arrangement with John Wiley & Sons

Volatile-rich xenolithic clasts in different types of brecciated meteorites represent unique pristine solar system material. This study investigates the maturity and thermal history of organic matter using Raman spectroscopy and aqueous alteration effects using infrared spectroscopy in the matrix of 15 volatile-rich clasts (C1 and CM-like) present in a polymict eucrite (NWA 7542) and a howardite (Sarıçiçek). Most of the studied C1 and CM-like clasts show similar maturity of organic matter as CI chondrites and CM chondrites, respectively. One CM-like clast from the polymict eucrite NWA 7542 shows Raman spectral signatures of heating after aqueous alteration, and another C1 and a CM-like clast from the howardite Sarıçiçek exhibit unique Raman spectral properties probably related to differences in accreted precursor organics compared to CI and CM-chondrites. One olivine-rich, unclassified clast has a high concentration of fayalitic olivine in its matrix, similar to oxidized CV chondrites and other features similar to CM- or C2 chondrites. Various evidence shows that this clast was heated up to 700–800 °C post aqueous alteration followed by the formation of fayalitic olivine during a metasomatic alteration process. Peak metamorphic temperature (PMT) estimated using different thermometric approaches does not provide reliable data for clasts altered at low temperatures (<200 °C).

^1,2Olga Ageeva,³Cyril Lorenz,¹Gerlinde Habler,⁴Lutz Nasdala,²Leonid Aranovich,²Olga Zhilicheva,²Sergey Borisovsky,¹Rainer Abart
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.70185]
¹Department of Lithospheric Research, University of Vienna, Josef-Holaubek-Platz 2, Wien, 1090, Austria
²Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences(IGEM RAS), Staromonetny Per. 35, Moscow, 119017, Russia
³Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences (GEOHI RAS),Kosygin Street 19, Moscow, 119991, Russia
⁴Department of Mineralogy and Crystallography, University of Vienna, Josef-Holaubek-Platz 2, Wien, 1090, Austria
Published by arrangement with John Wiley & Sons

Two samples of the meteorite Northwest Africa 8741 (NWA 8741) were investigated using petrographic, mineral chemical, and crystal orientation analysis to reconstruct its evolution. NWA 8741 is an A4 mesosiderite composed of lithic clasts of pyroxenite and single-grain porphyroclasts of olivine and orthopyroxene as well as aggregates of Ni-rich metallic iron embedded in a medium-grained matrix of plagioclase, orthopyroxene, cristobalite, tridymite, minor chromite, clinopyroxene, and small grains of metallic iron with low Ni-contents. The mesosiderite NWA 8741 formed by a collision event, which led to the ejection of silicate and metal melts and of solid fragments from a differentiated parent body and the projectile. The matrix minerals crystallized from the silicate melt, while the metallic melt forming the Ni-rich metallic iron aggregates, and the silicate clasts were incorporated by mechanical mixing. The crystallization of the matrix phases proceeded at low oxygen fugacity, ensuring the stability of metallic iron. Interaction between the metallic and silicate melt caused partial oxidation of phosphorus and chromium originally dissolved in the metallic melt, leading to the formation of merrillite and Cr-rich spinel. The melt was out of equilibrium with the inherited olivine and orthopyroxene clasts, and a series of mineral-melt reactions led to the partial replacement of the inherited olivine by aggregates of orthopyroxene and Cr-spinel and to the partial replacement of the inherited orthopyroxene by aggregates of cristobalite, Cr-spinel, and plagioclase. During the subsequent sub-solidus evolution, the oxygen fugacity was still low, allowing the formation of Ni-poor iron grains and silica by the partial reduction of ferrous iron from the Fe-Mg silicates, and the partial replacement of olivine by symplectic orthopyroxene-metallic iron intergrowth. Finally, the replacement of olivine by troilite-orthopyroxene and of orthopyroxene by troilite-tridymite aggregates and the partial transformation of Ni-poor metallic iron to troilite indicate an elevated sulfur fugacity during the late sub-solidus evolutionary stages of mesosiderite NWA 8741. Overall, NWA 8741 records a multistage history of impact-induced mixing, melt-rock interaction, and subsequent sub-solidus metamorphism during the evolution of the mesosiderite parent body.