14 - Pallasites

Summary

Introduction

Historically, pallasites were classified as one of the two major divisions of stony-iron meteorites, the other being the mesosiderites (Chapter 13). 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 [14.1]. As of June 2014, and not accounting for pairing, there are currently 98 pallasites, only four of which are observed falls (see Meteoritical Bulletin Database, www.lpi.usra.edu/meteor/metbull.php).

There are several reviews of pallasites, from which much of the following information has been drawn [14.2–14.5]. Pallasites are mainly composed of subequal proportions of olivine and Fe-Ni metal, with minor amounts of troilite, chromite, Ca-phosphates and schreibersite [14.2]. They are divided into three groups on the basis of mineralogy and oxygen isotope composition [14.2, 14.3, 14.6–14.12]. Most pallasites belong to the Main Group (PMG). The Eagle Station pallasites (PES; Eagle Station, Cold Bay and Itzawisis) and Pyroxene pallasites (Px-PAL; NWA 1911, Vermillion, Yamato 8451 and Zinder) are much smaller grouplets [14.9–14.12]. Milton is an ungrouped pallasite [14.13].

Mineralogy

The major mineral assemblage of most PMG is of olivine and Fe-Ni metal (kamacite and taenite), with minor troilite and schreibersite plus chromite [14.8, 14.12]. Olivine is ubiquitous, although its abundance varies both within pallasite groups and within individual meteorites; pyroxene is almost absent from PMG (Table 14.1). Olivine in most PMG has a narrow compositional range from Fa_11.2 to Fa_13.5 (Figure 14.1).

Four PMG (Phillips County, Rawlinna 001, Springwater, Zaisho) have slightly more ferroan olivine compositions than the rest (Fa_15.9–18.3), and are designated PMG-as (for anomalous silicates) (Figure 14.1) [14.12]; olivines from the ungrouped pallasite Milton (Fa17.2) are also in this range [14.13]. Three of the PMG-as contain phosphorous-rich olivine, with up to 5 wt.% P₂O₅ [14.2, 14.15]. A further six PMG (Argonia, Brenham, Glorieta Mountain, Huckitta, Krasnojarsk, Pavlodar) have anomalous metal compositions; they are designated PMG-am [14.12]. The PMG-am have olivine compositions that fall at the more fayalitic end of the PMG range, at Fa_12.2–13.5 [14.12].