8 - Winonaite–IAB–IIICD Clan

Summary

Introduction

Winonaites are a group of primitive achondrites with chondritic chemistry and reduced mineralogy. According to the Meteoritical Bulletin (www.lpi.usra.edu/meteor/metbull. php), not accounting for pairing, there are currently (June 2014) 26 winonaites, only one of which (Pontlyfni) was observed to fall. The type specimen, Winona, was found in 1928 and originally classified as a mesosiderite [8.1]. The group was also known as the forsterite chondrites [8.2] and IAB chondrites [8.3]. The last name was suggested because of the similarities with silicate inclusions in IAB irons, which are so strong that Mount Morris (Wisconsin) was considered to be part of the silicate fraction of the Pine River IAB iron meteorite [8.4]. It was proposed that winonaites and IAB irons had a common parent body [8.5], and also that the IAB and IIICD iron meteorites formed a single group (the IAB complex) [8.6]. Winonaites are now classified as part of the Win–IAB complex (or Win–IAB–IIICD) clan of primitive achondrites (Figure 1.1) [8.7]. Note, however, that the link between IIICD meteorites and winonaites is weaker than that between IAB meteorites and winonaites [8.5].

Three main criteria are used to classify winonaites: their reduced mineralogy (Fa_<7), oxygen isotope composition and the abundance and distribution of metal and troilite [8.8]. Zag (b) is listed as a winonaite in the Meteoritical Bulletin database (and has an oxygen isotopic composition typical of winonaites), but has a mineralogy (olivine Fa19.4; pyroxene Fs25.7) that suggests a relationship with the ungrouped achondrite Divnoe [8.9].

Mineralogy

The modal mineralogy of winonaites is generally similar to that of chondrites, with a mineral chemistry between ordinary and enstatite chondrites [8.5, 8.8]. Figure 8.1 shows the composition of olivine and pyroxene in winonaites (and IAB silicates); modal abundances of minerals are given in Table 8.1. Low-Ca pyroxene is the main mafic silicate phase. Other silicates occur in lesser abundances: olivine, calcic pyroxene (mainly diopside) and plagioclase [8.8, 8.10–8.13].

There is a wide range of plagioclase compositions among winonaites, but only slight variation within an individual meteorite: plagioclase in Pontlyfni is An_7–9, in Tierra Blanca it is An_12–15 and in Y-75300 it is An25 [8.5, 8.8, 8.10]. Fe-Ni metal (0.1–27 vol.%) and troilite (0.2–28 vol.%) have wide abundance ranges, although this might be the influence of terrestrial weathering [8.5, 8.8]. Accessory phases include chromite, daubréelite, alabandite, schreibersite, graphite, K-feldspar, apatite and whitlockite [8.11, 8.12].