Ureilite Vein Metal – Indigeneous or Impact Material?

Introduction: Ureilites are ultramafic, igneous achondrites, which contain olivine (ol), pyroxene, metal and carbon. About ~10 vol% dark interstitial, fine-grained matter is usually referred as vein material [1]. Ureilites have lost ~25% basalt [2] and most of their metallic iron. Presently, many authors assume that the majority of ureilites formed as asteroidal melting residues [3, 4]. Although heterogeneous in oxygen isotopes [5], ureilites are suggested to originate from a single parent body that has been disrupted by impact. Variations in fayalite content are related to simultaneous smelting and melting at various depths [6]. The origin of the vein metal is not yet clear. While most authors assume it to be indigeneous [2, 6], we propose a metal rich impactor as metal source. P have been measured in the vein metal of the monomictic ureilites ALH 84136, ALHA 77257, EET 87517, EET 96042, LEW 85440 and GRA 95205. The ureilites represent the whole range of fayalite (fa) concentrations ranging from ~2 wt% FeO to ~20 wt% FeO in ol. Ni concentrations in vein metal range from 2.5 wt% to 6 wt%. The majority of the data lie between 3.5 and 5.5 wt% Ni. The concentrations of Co range from 0.25 wt% to 0.65 wt%, with most data lying between 0.35 wt% and 0.45 wt%. There is no correlation of Ni-or Co-concentrations with ol FeO. The Ni/Co-ratio of the vein metal is ~10 and lower than the chondritic Ni/Co-ratio of ~22 [7]. Thermodynamic calculations: Calculations of equilibration temperatures between ureilite ol and vein metal gave different temperatures for Fe-Ni and Fe-Co exchange reactions [8]. While equilibration temperatures for Fe-Co exchange generally give 900°C to 1250°C, Fe-Ni equilibration temperatures are unplaus-ibly high ranging from 1300°C to 1900°C. Ureilite vein metal is not in equilibrium with ureilite ol at any temperature. The same calculations done for model metals for ureilite parent bodies (UPB) with CI1-, L-, Hand nd CV3-chondritic composition give similar and plausible temperatures of about 1150°C for both exchange reactions [8]. Mass balance calculations: In order to understand if the vein metal could have been produced from the UPB or if an external source is needed, mass balance calculations were performed. Most authors prefer a CI1-or CV3-like initial bulk UPB composition ([6],[9]). The amount and composition of metal available in the UPB depends strongly on the grade of oxidation of the UPB. The more oxidized