Zoned and Unzoned Metal Grains in the Ch Chondrites Alh

Introduction: CH chondrites are metal rich, fine-grained meteorites that have undergone relatively little thermal metamorphism [1]. Meibom et al. [2] proposed that the radially zoned metal grains found in CH and also in some CB chondrites recorded the formation of these grains by condensation of metal in the solar neb-ula. This conclusion was based on measured profiles of the minor elements Ni, Co, Cr, and P. However, redox processes could have the same effect on these elements as condensation processes, creating uncertainty in interpretation of the profiles. Campbell et al. [3] used trace element microanalysis of key siderophile elements (e.g., Pd, Ir) to verify that the zoned metal in the CB b chondrites [1] were nebular condensates. In this report we present analogous measurements on metal in two CH chondrites, extending earlier work [4], and show that the Ni-zoned metal in these meteorites also contain nebular condensate metal, similar to the results of the CBb chondrites [3]. We further examine the Ni-unzoned metal in CH chondrites, and show that it is commonly zoned in Cu, a volatile element. Experimental: Sections of two CH chondrites, Allan Hills (ALH) 85085 and Pecora Escarpment (PCA) 91467, were examined by SEM and optical mi-croscopy, and metal grains were selected for laser ab-lation ICP-MS analysis based on their size and Ni zoning. The zoned and unzoned grains studied were 50-200 µm across, and included some of the largest in the section. Laser ablation ICP-MS analysis followed procedures similar to those of [3]. Because of the relatively small size of metal grains, the analyses were optimized for high spatial resolution by reducing the laser spot size to 13 µm diameter (~10 µm depth) and restricting the isotopes analyzed to only 57 Fe, and Ru were chosen to represent a range of volatilities relative to that of Fe (Cu>Pd≈Fe>Ru). Errors at each point were dominated by counting statistics and were typically 5-15%. In addition to the profile measurements on zoned and unzoned grains, several smaller grains were also selected for "bulk" analyses using laser spots of 50 µm. Results: The metal grains in PCA 91467 and ALH 85085 that were zoned in Ni were found also to be zoned in Co and Ru, sometimes in Cu, but not in Pd (Figure 1). Refractory siderophile compositions at the grain rims tended to be chondritic in the relative abundances of the siderophiles, while the grain cores were