13 - Mesosiderites

Summary

Introduction

Historically, mesosiderites were classified as one of the two major divisions of stony-iron meteorites, the other being the pallasites (Chapter 14). Meteorites within the two divisions have very different compositions, textures and petrogeneses, and it is now thought to be more appropriate to consider mesosiderites and pallasites (as well as iron meteorites) as metal-rich achondrites [13.1]. As of June 2014, and not accounting for pairing, there are currently 202 mesosiderites, only seven of which are observed falls [see Meteoritical Bulletin Database, www.lpi.usra.edu/meteor/metbull.php].

There are several reviews of mesosiderites, from which much of the following information has been drawn [13.2– 13.5]. Mesosiderites are polymict breccias composed of metal and a variety of silicate clasts (up to several cm across) set in a matrix of silicate plus metal. The silicate clasts are both lithic and mineral; the former (up to ∼10 cm across) are igneous in nature, and include basalts, gabbros and pyroxenites. Dunites are less abundant and anorthosites are rare. Mineral clasts are almost all monomineralic, either coarsegrained low-calcium pyroxene or olivine; plagioclase clasts are less abundant [13.6, 13.7]. Metal is unevenly distributed as cm-sized slugs, veins and smaller (submillimetre to millimetre) grains mixed with the silicate clasts.

There have been several schemes for the classification of mesosiderites, the most successful of which were based on the texture of silicate matrix [13.8, 13.9]. Texture, however, is a secondary classification parameter, related to parentbody processing rather than formation environment. The classification scheme now generally adopted is based on silicate mineralogy [13.10]. Mesosiderites are subdivided into three classes (A, B and C) according to plagioclase content and the ratio of orthopyroxene to plagioclase (Figure 13.1). Silicates in class A mesosiderites have a large eucritic component (higher plagioclase and clinopyroxene abundances); those in class B are more diogenitic (containing a greater amount of orthopyroxene), whilst silicates in the only member of class C (RKP A79015) are composed almost entirely of orthopyroxene [13.11]. The three classes are also recognized on the basis of the siderophile composition of the metal component (Figure 13.4 below) [13.12–13.14]. Note, though, that a recent analysis of the metal in RKP A79015 does not distinguish it from class A mesosiderites [13.15]. Modal mineralogies are sufficiently distinct that mineralogical class is readily established; the same parameter allows mesosiderites to be distinguished from howardites and eucrites.