Northwest Africa 1109 and Camel Donga: Metal-bearing Brecciated Eucrites

Introduction: HED (howardites, eucrites, diogenites) meteorites are the largest group of achondrites. HED meteorites are products of earliest magmatic activity in a large asteroid (4Vesta) 4.56 Ga ago. The textures and bulk compositions suggest that almost all HED meteorites experienced impact events shortly after the formation. Some polymict breccias of HED meteorites (e.g., polymict eucrites, howardites) contain foreign materials derived from impactors such as fragments of carbonaceous chondrites, FeNi metals, metal-sulfide-rich clasts. Mesosiderites are believed to have formed by large impact onto a large planetesimal (HED parent body?) by molten FeNi metals. We studied two brecciated eucrites that contain certain amounts of Fe-metals. They are Camel Donga and NWA1109. Camel Donga is a monomict eucrite that contains about 2% of metallic Fe [1]. NWA1109 is a polymict breccia that contains a small amount of Fe-metal. NWA1109 has not been neither petrologically characterized nor chemically analyzed in detail. In order to better understand the origin of the Femetals, we performed a petrological and geochemical study of these meteorites, focusing on the occurrences of Fe-metals and platinum group elements (PGEs) in NWA1109. Samples and analytical techniques: Surfaces of Camel Donga show a matrix with many clasts, fragments of pyroxene and plagioclase, and brown stains (Fe-FeS-rich portions) (< 2-3 mm in size). For chemical analyses, we made no attempt to separate brown stains from silicate portions. The broken surface of sample for chemical analyses is a relatively homogeneous matrix (or deformed clast?) with many brown stains. The samples include a (relatively) large fragment (1.759g; ,22) and a fraction of mixed fine grains (0.136g; ,132) Cut surfaces of NWA1109 show a homogeneous gray matrix with embedded angular dark-clasts (<5 mm), basaltic clasts (< a few mm), and irregular brown stains (metal-rich portions) up to 3-4 mm in size. The metal-rich portions occur not evenly in the matrix. We separate a portion rich in brown stains (31) and four portions of gray matrix (32, 33, 34, and 35) for chemical analyses. The weight of these samples ranges from 0.332 to 0.622 gram. Bulk chemical compositions were determined for Camel Donga and NWA1109 by using prompt gamma-ray analysis (PGA), instrumental neutron activation analysis (INAA), instrumental photon activation analysis (IPAA) and isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS). All the samples of Camel Donga and NWA1109 were analyzed by PGA, INAA and IPAA for major, minor and trace elements. Only NWA1109,31 was analyzed by ID-ICP-MS for PGEs. We made polished thin and thick sections (PTSs) of adjacent chips for the chemical analyses. The PTSs were examined optically, and with a SEM and EPMA. Bulk chemical composition: All five samples of NWA1109 have identical chemical compositions within analytical uncertainties for major and minor elements. Abundances of trace lithophile elements including rare earth elements (REEs) are 10-20 times CI values and within the range for basaltic eucrites. The abundances of Ti and Sm show a good correlation for eucrites (Fig. 1). NWA1109 is plotted in the area for basaltic eucrites. This is also supported by the Mg number (=molar Mg/(Mg+Fe) x 100) (33 – 39), Al2O3/Na2O ratios (22.2 26.4) and Al2O3/CaO ratios (1.17 1.22). These facts are consistent with the observation that most of the clast components in NWA1109 are basaltic eucrites. Only one sample (,31) yielded a positive Ir value (6.4 ppb) with large analytical uncertainties of 34% (1δ).