Metal in Ureilites: Petrologic Characterization

Introduction: Many of the petrologic characteristics of ureilites that are difficult to reconcile with normal igneous processes [1] can be explained by a smelting model, in which silicate mg (molar Mg/[Mg+Fe]) are controlled by carbon redox reactions [2-4]. However, their metal and siderophile element abundances appear to be inconsistent with such a model [5-7]. The smelting reaction (FeO + C → Fe + CO) predicts a correlation of mg with metal content, or (if the metal is removed) depletion of siderophile elements. In contrast, ureilites have uniformly low (a few %) metal contents, and relatively high (~0.01-1×CI) siderophile element abundances uncorrelated with mg. We are addressing this apparent contradiction through petrographic and trace element [8] characterization of metal in ureilites, and petrologic modelling [9]. Samples and Analytical: The initial samples studied were olivine + low-Ca pyroxene (lpyx) ureilites Kenna (Fo 79), PCA 82506 (Fo 78), ALHA77257 (Fo 85), EET 96328 (Fo 86) and EET 90019 (Fo 89), as well as augite-bearing ureilites META78008 (Fo 76), Hughes 009 (Fo 87) and ALH 82130 (Fo 95). Sections were characterized by SEM at KCC; metal, sulfides and oxidized metal were analyzed by EMPA at LPL. Types of Metal: Based on textures we distinguish 5 types of metal: 1) Metal lining grain boundaries (Fig. 1). In most ureilites this has been partially or completely oxidized, presumably terrestrially; 2) Reduction metal, which occurs in dense patches of sub-micron (occasionally up to 10 μm) grains in rims and veins in reduced olivine (Fig. 1); 3) Metal enclosed in graphite, either on grain boundaries or within silicates; 4) Sparsely distributed trails of submicron grains that occur in both lpyx and olivine and are not associated with reduced silicates (Fig. 1); 5) ~5-150 μm-sized carbide-metal-phosphide-sulfide spherules [10], which occur predominantly as inclusions in lpyx (rarely olivine). Sulfides show rounded boundaries with the carbide-metal-phosphide assemblages (Fig. 2). The latter consist of cohenite (Fe3C) intergrown with metal in a range of grain sizes, with phosphide as rims or dispersed grains (Fig. 2). Other than in the type 5 spherules, sulfides are very rare. In all samples, grain boundary metal (or oxide) is the most abundant type. We observed >50 spherules in ALHA77257,104 and >50 in one pigeonite grain alone in PCA 82506,24, but none in Kenna or EET90019 and only 1-2 in each of the other samples. A few grains of metal could not be classified. Fig. 1. BEI of PCA 82506,24 showing types 1, 2 and 4 metal (white). ol = olivine; pig = pigeonite.