ROOSEVELT COUNTY 075 - A PETROLOGIC, CHEMICAL AND ISOTOPIC STUDY OF THE MOST UNEQUILIBRATED KNOWN H CHONDRITE

Roosevelt County (RC) 075 was recovered in 1990 as a single 258-gram s tone. Classification of this meteorite is complicated by its highly un equilibrated nature and its severe terrestrial weathering, but we favo r H classification. This is supported by O isotopes and estimates of t he original Fe,Ni metal content. The O isotopic composition is similar to that of a number of reduced ordinary chondrites (eg., Cerro los Ca lvos, Willaroy), although RC 075 exhibits no evidence of reduced miner al compositions. Chondrule diameters are consistent with classificatio n as an L chondrite, but large uncertainties in chondrule diameters of RC 075 and poorly constrained means of H, L and LL chondrites prevent use of this parameter for reliable classification. Other parameters a re compromised by severe weathering (eg., siderophile element abundanc es) or unsuitable for discrimination between unequilibrated H, L and L L chondrites (eg., Co in kamacite, deltaC-13). Petrologic subtype 3.2 +/- 0.1 is suggested by the degree of olivine heterogeneity, the compo sitions of chondrule olivines, the thermoluminescence sensitivity, the abundances and types of chondrules mapped on cathodolumincscence mosa ics, and the amount of presolar SiC. The meteorite is very weakly shoc ked (S2), with some chondrules essentially unshocked and, thus, is cla ssified as an H3.2(S2) chondrite. Weathering is evident by a LREE enri chment due to clay contamination, reduced levels of many siderophile e lements, the almost total loss of Fe,Ni metal and troilite, and the re duced concentrations of noble gases. Some components of the meteorite (eg., type IA chondrules, SiC) appear to preserve their nebular states , with little modification from thermal metamorphism. We conclude that RC 075 is the most unequilibrated H chondrite yet recovered and may p rovide additional insights into the origin of primitive materials in t he solar nebula.