^1,2L. Krämer Ruggiu,²B. Devouard,²J. Gattacceca,³L. Bonal,⁴L. Piani,⁵H. Leroux,⁶O. Grauby
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14295]
¹Archaeology, Environmental Changes and Geo-Chemistry, Vrije Universiteit Brussel, Brussels, Belgium
²CNRS, IRD, INRA, CEREGE, Aix Marseille Univ, Aix-en-Provence, France
³CNRS, IPAG, Univ. Grenoble Alpes, Grenoble, France
⁴CRPG, CNRS, Université de Lorraine, Vandoeuvre-les-Nancy, France
⁵CNRS, INRAE, Centrale Lille, UMR 8207 – UMET, Univ. Lille, Lille, France
⁶CNRS, CINaM, Aix-Marseille Université, Marseille, France
Published by arrangement with John Wiley & Sons

We studied Caleta el Cobre 022, a nakhlite showing a high abundance of aqueous alteration products, commonly called “iddingsite” and compared it to eight other nakhlites, in order to constrain the composition and the history of the aqueous alteration of nakhlites. Olivine grains in nakhlites display planes of secondary fluid inclusions, composed of pyroxene, magnetite, and a void potentially filled by a fluid. They were formed by a first fluid alteration event, previous to the iddingsite alteration event, probably from a late magmatic fluid circulation. We observed magnetite–pyroxene symplectites in olivine grains in most nakhlites, related to the same fluid-assisted tardi-magmatic event as the crystallization of the secondary inclusion planes. Those secondary inclusions and symplectites can be observed at the center of iddingsite veins, inside the most altered nakhlites, and are thus interpreted as being weakness planes, easing the circulation of the fluid forming the iddingsite inside the olivine grains. In every nakhlite, the alteration veins show at least two types of iddingsite: a coarse iddingsite with crystals around 50 nm, up to 200 nm, and a fine iddingsite with a nanocrystalline to amorphous texture with crystalline domains <10 nm. Both iddingsite types are composed mainly of Si, Mg, and Fe, with anticorrelated Si and Fe contents. The coarse iddingsite is composed of a mixture of phyllosilicates, with Fe-oxyhydroxides and minor siderite, and the fine iddingsite has a composition close to saponite. Organic matter located in coarse iddingsite is detected by Raman spectroscopy in the iddingsite of many nakhlites and was confirmed by the TEM study of NWA 10153. In addition, the TEM study of NWA 10153 displays complex chemical zoning in the fine iddingsite of Mg, Ca, Mn, S, P, and Al, suggesting at least two stages of circulations. Both the compositions and textures of the two types of iddingsite are suggestive of a progressive evolution of the alteration fluid, enriched in elements from basaltic mineral dissolution, with crystallization mainly by filling of existing fractures, and selective dissolution of host olivine. We also observe pyrrhotite–magnetite veinlets at the center of iddingsite veins and cross-cutting iddingsite veins and silicates, which are interpreted as the result of another later fluid circulation.

¹Daniel Sheikh,²Alex M. Ruzicka,³Melinda L. Hutson
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14298]
Cascadia Meteorite Laboratory, Department of Geology, Portland State University, Portland, Oregon, USA
Published by arrangement with John Wiley & Sons

Pink spinel anorthosite (PSA), a distinctive plagioclase and spinel-rich lithology (spinel >20%) observed on the lunar surface by the Moon Mineralogy Mapper (M3) imaging spectrometer, has sparked considerable interest in understanding magmatic processes on the Moon that cannot be explained by the well-established lunar magma ocean paradigm. Competing ideas on the PSA-forming mechanisms have invoked either (1) impact melting of troctolitic source rocks on the lunar surface or (2) magma–wallrock interactions between anorthositic crust and Mg-suite parental melts, but have been difficult to evaluate given the lack of ground truth samples. Here, we investigate the textures and mineral compositions of seven PSA clasts in lunar meteorite Northwest Africa (NWA) 15500, and the bulk trace element compositions of a PSA clast separate and NWA 15500 host lithologies A and B. Our findings suggest derivation of PSA from an incompatible-element-poor source and are consistent with PSA representing an Mg-suite lithology genetically related to pink spinel troctolites that reflects increased degrees of crustal assimilation during magma–wallrock interactions, and a sourcing of PSA far from the Procellarum KREEP Terrane. Excavation of PSA material was followed by multiple, subsequent localized impact events, resulting in the formation of Lithologies A and B.