UREILITES: TRACE ELEMENT CLUES TO THEIR ORIGIN

Abstract

Carbonaceous vein separates from Kenna and Haverö, as well as bulk Kenna, were analyzed by RNAA for Ag, Au, Bi, Br, Cd, Cs, Ge, In, Ir, Ni, Pd, Os, Rb, Re, Sb, Se, Te, Tl. U, and Zn. The data are reviewed together with four earlier Chicago analyses of bulk ureilites. Linear regressions confirm the presence of two metal components, with the following Cl-normalized ratios: Ir/Ni = 14.6, ≤ 1; Ge/Ni = 5.4, 2.4; Au/Ni = 2.3, 0.9. The high-Ir component is enriched in vein separates and hence belongs to veins; the lowIr component belongs to the ultramafic rock. Vein material is enriched in all elements analyzed by us except Zn, and accounts for most of the C, noble gases, and presumably siderophiles in the meteorite. Most of the properties of ureilites apparently can be explained by the cumulate model of Berkley et al. (1980), with certain modifications. Comparison of ureilites with three other ultramafic rocks from different planets (Earth's mantle, lunar dunite, and Chassigny) suggests that the ureilite parent body had a primitive chondritic composition, similar to C3V chondrites but richer in metal and carbon. It melted, causing depletion of incompatibles to a mean abundance of ~0.02 × Cl and incomplete segregation of metal, FeS, and C. Fractional crystallization or melting of metal in the presence of S and C apparently can explain the fractionations of Ir, Re, Ni, Au, and perhaps Ge, obviating the need for extraneous sources of vein metal or unusual parent-body compositions. Noble gases from the parent material may have been retrapped in carbon during magmatism, provided the system was closed.