Multistage aqueous alteration in CeC 022 and other nakhlites

Abstract

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.