Extended time scales of carbonaceous chondrite aqueous alteration evidenced by a xenolith in LaPaz Icefield 02239 (CM2)

1Martin R. LEE,1Cameron FLOYD,1Pierre-Etienne MARTIN,2Xuchao 2 A. FRANCHI,1Laura JENKINS,1Sammy GRIFFIN
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.13978]
1School of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK2School of Physical
2Sciences, Open University, Milton Keynes MK7 6AA, UK*Corresponding author.Martin R. Lee, School of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK
Published by arrangement with John Wiley & Sons

LaPaz Icefield (LAP) 02239 is a mildly aqueously altered CM2 carbonaceouschondrite that hosts a xenolith from a primitive chondritic parent body. The xenolithcontains chondrules and calcium- and aluminum-rich inclusions (CAIs) in a very fine-grained matrix. The chondrules are comparable in mineralogy and oxygen isotopiccomposition with those in the CMs, and its CAIs are also mineralogically similar to the CMpopulation apart for being unusually small and abundant. The presence of serpentinedemonstrates that the xenolith has been aqueously altered, and its phyllosilicate-rich matrixhas a comparable oxygen isotopic composition to the matrices of CM meteorites. Thexenolith’s chondrules lack fine-grained rims, whereas the xenolith itself has a fine-grainedrim that is petrographically and chemically comparable with the rims on coarse grainedobjects in LAP 02239 and other CM meteorites. These properties show that the xenolith’sparent body was formed from similar materials to the CM parent body(ies). Following itslithification by aqueous alteration, a piece of the xenolith’s parent body was impact-ejected,acquired a fine-grained rim while free-floating in the protoplanetary disc, then was accretedalong with rimmed chondrules and other materials to make the LAP 02239 parent body.Subsequent aqueous processing of the LAP 02239 parent body altered the fine-grained rimson the xenolith, chondrules, and CAIs. The xenolith shows that the timespan of geologicalevolution of carbonaceous chondrite parent bodies was sufficiently long for some of them tohave been aqueously altered before others had formed.INTRODUCTIONThe Mighei-like (CM) carbonaceous chondrite (CC)meteorites have close spectroscopic affinities to C-complexasteroids (Burbine,2016) and so likely sample one or moreof them. The parent body(ies) of the CMs were formed bythe accretion of relatively coarse-grained objects (i.e.,chondrules and calcium- and aluminum-rich inclusions[CAIs]) together with fine-grained material that formsthe enclosing matrix (Barber,1981; McSween Jr. &Richardson,1977). The chondrules and other coarse-grainedobjects typically have fine-grained rims that they acquiredwhile free-floating in the protoplanetary disc (Hanna &Ketcham,2018; Metzler et al.,1992). The constituents of thecoarse-grained objects, fine-grained rims and matrix(anhydrous silicates, metal, sulfides, oxides, and amorphousmaterials) were partially to completely altered by parentbody aqueous activity at~4563 Ma (Bunch & Chang,1980;Fujiya et al.,2012; McSween Jr.,1979a,1979b). Theresultant secondary minerals are volumetrically dominatedby phyllosilicates that are intergrown with carbonates,oxides, and sulfides (Barber,1981; Bunch & Chang,1980;Fuchs et al.,1973;Howardetal.,2009,2011,2015;Leeet al.,2014; Tomeoka & Buseck,1985;Trigo-Rodrıguezet al.,2019; Zolensky et al.,1993).TheCMsareclassifiedby petrologic type/subtype using various properties thatMeteoritics & Planetary Science1–16 (2023)doi: 10.1111/maps.139781Ó2023 The Authors.Meteoritics & Planetary Sciencepublished by Wiley Periodicals LLC on behalf of The Meteoritical Society.This is an open access article under the terms of theCreative Commons AttributionLicense, which permits use,distribution and reproduction in any medium, provided the original work is properly cited.


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