Metal in Ureilites: Siderophile Elements from La-icp-ms

Introduction: Bulk ureilites have near chondritic abundances of siderophile elements [1-7], showing no correlation with Fe contents (~5-16 wt.%) or mg (molar Mg/[Mg+Fe]). These observations are difficult to reconcile with any igneous model for their silicates [8]. We address this problem through detailed characterization of the metal phases in ureilites [9], and petrologic modelling [10]. Here we present siderophile element data from LA-ICP-MS for metal, oxidized metal and silicates in initial samples. Samples and Analytical: Samples were olivinepigeonite ureilites Kenna (Fo 79), ALHA77257 (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). Petrographic descriptions and compositions from EMPA are given in [9]. LAICP-MS analyses were perfomed at the Univ. of Md. using a ThermoFinnigan Element 2 single collector magnetic sector ICP coupled to a New Wave UP213 laser. Laser power was between 2-3mJcm, with a repetition rate of 5-7Hz. Spotsizes were determined largely by mineral grain size and ranged from 15μm80μm. Ablated material was entrained in a stream of He to transport to the mass spectrometer. Coahuila, NIST610 and Babb’s Mill (Troost) were used as standards, normalizing to Ni or Fe [9]. Analyzed elements were Si, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Mo, Ru, Rh, Pd, Sb, W, Re, Os, Ir, Pt and Au. A subset of these are presented for comparison to bulk rock data (plots CI-normalized). Results: Unsurprisingly, in all cases silicate cores show lower siderophile abundances (orders of magnitude) than grain boundary and included metal. Kenna (Fig. 1a): Metal in our section of Kenna is almost all oxidized, resulting in large apparent variations in abundances of siderophile elements. Nevertheless, the average of these analyses provides a reasonable fit to the bulk rock at the 8% level. We observed one preserved metal grain in graphite. Both the metal and the graphite itself (due to tiny metal inclusions?) show a siderophile element pattern similar to that of the bulk rock. One olivine reduction rim has higher abundances than cores. Au data (not shown) are erratic; we suspect the section was once Au-coated. ALHA77257,104 (Fig. 1b): Most of the metal in this sample is preserved. Analyses of grain boundary metal show a consistent pattern very similar to that of the bulk rock, and 1.7% metal could account for the bulk siderophile abundances. Cohenite-metalphosphide-sulfide spherules in pigeonite show siderophile abundances essentially identical to those of the grain boundary metal. One spherule in olivine has a more fractionated pattern. One olivine reduction rim has higher abundances than cores.