Metamorphism and Melting in an “unmetamorphosed” Mesosiderite

Introduction: The mesosiderites are amongst the most enigmatic of all the meteorite groups and their formation is not fully understood. Evidence taken from both the metal and silicate portions indicates that they have a multi-stage formation history [1,2]: 1. Initial crystallization and cooling of silicates, followed by brecciation on the surface of the parent body 2. Metal-silicate mixing, some melting, and recrystallization, followed by rapid cooling, as evidenced by the formation of overgrowths on the silicates. 3. Deep burial and slow cooling at low temperature. Mesosiderites are classified according to both their petrology and degree of metamorphism [3,4]. The metamorphic scale is based purely on the texture of the matrix and, therefore, does not give any information on the history of the individual clasts or the metal-silicate mixing prior to cooling. It is these first two stages that are the least understood and, therefore, samples that preserve evidence of them, i.e. the least metamophosed, will be the key to understanding the formation of the mesosiderites. Here we present SEM and EMP analyses from two silicate clasts within Crab Orchard specifically to compare the degree of metamorphism of these clasts to similar clasts in eucrites and to understand the igneous processes that occurred upon mixing with metal. Eucrite-like clast:. This clast consists of a core of inverted pigeonite which poikilitically encloses a finely-exsolved augite (Fig. 1). The inverted pigeonite contains both fine-grained exsolution lamellae and blebs (Fig. 1b). This core is overgrown by a Mg-rich orthopyroxene. Pyroxene compositions in this clast (excluding the orthopyroxene rims) are similar to cumulate eucrites (e.g., Moore County) for both major quadrilateral and minor elements (Al, Ti, Cr) (Fig. 2). However, the exsolution within this grain is of a much finer scale than that of the cumulate eucrites, likely reflecting a different cooling history. Diogenite-like clast: The core of this clast has a relatively constant orthopyroxene composition similar to that of diogenites, but exhibits a complex overgrowth resulting in an overall normal zonation (Fig. 3). Unlike the overgrowths described by [6] for similar clasts, in which FeO zoning is the prominent feature, this clast displays a thin inner rim of augite between the magnesian orthopyroxene core and the FeO-rich orthopyroxene rim. Discussion: While Crab Orchard has been described as a type 1 mesosiderite [3], the clasts exhibit evidence of significant metamorphic equilibration and exsolution in their cores. We can compare the degree of equilibration to that observed in eucrites using using calculated two-pyroxene exsolution temperatures and equilibration of elements (both major and minor) within the pyroxenes, as was done for unbrecciated eucrites [5]. We calculate an exsolution temperature for the Crab Orchard eucrite-like clast of 704 ±54 °C [7], lower than that of Moore County (934 ±54 °C) but comparable to the cumulate Serra de Magé (794 ±24 °C). Basaltic eucrite GRO 95533 has an almost identical calculated exsolution temperature of 706 ±30 °C. It is also texturally similar, containing fine-scale exso-