Eleventh Annual V. M. Goldschmidt Conference (2001) 3886.pdf

THE AND ISOTOPIC COMPOSITION OF CARBONADO : AN IN SITU STUDY. E. P. Vicenzi1 and P. J. Heaney2, 1Smithsonian Institution (Department of Mineral Sciences, National Museum of Natural History, Washington, D.C. 20560-0119, [email protected]), 2Penn State University (De- partment of Geosciences, 309 Deike Building, University Park, PA. 16802, [email protected]).

Introduction: Carbonado are polycrystalline with, or unable to exclude contributions from sub- diamond aggregates found in alluvial deposits of Bra- ducted organic material. Because the vast majority of zil and the Central African Republic (CAR). These our d15N data are negative values a subducted organic nodules represent one of the most unusual varieties of component in the source region of carbonado appears diamond [1]. Their high porosity, unique inclusion unlikely. assemblages, high concentration of radiogenic noble References: gases, and limited range of isotopically light carbon [1] Haggerty S. E. Science: 285: 851-860. have given rise to a several proposed mechanisms of [2] Hauri E. H. et al. EPSL (submitted). [3] De S. et al. formation, ranging from: impact, exposure (2001) EPSL 185:315-330. [4] Shelkov D. et al. to U/Th-rich regions of the crust during diamond (1997) Proceedings of the Sixth Intl. growth, and subduction of crustal organic matter into Conf. in Russian Geology and Geophysics 38: 332- the mantle followed by volcanic extrusion. We have 340. [5] Yokochi R. et al. (2000) Goldschmidt 5: measured d13C, d15N, and N concentrations in 4 1114. specimens from Brazil and the CAR with an aim to- ward testing the subduction hypothesis. Method: A CAMECA ims 6f ion microprobe was used in high mass resolution mode to determine d15N, extreme energy filtering mode for d13C, and the 12C 14N molecular peak was used to measure N concentra- tions [2]. Secondary ions were extracted from an area ~20-25 microns in diameter within a larger sputter crater. Results: Carbon . Carbonado display a restricted range in carbon values relative to periodotitic, eclogitic, and impactite [3]. The range of d13C values measured in our Brazilian sample is simi- larly narrow at –24.7 to –31.2 (+/- 0.3) per mil, while the CAR values are clustered between –26.6 to -32.0 with a single measurement of –21.1 per mil. Nitrogen isotopes. In contrast to the carbon sys- tem the range in d15N values from Brazilian carbonado is quite large +7.4 to –16.1 (+/- ~2) per mil, and is encompassed within a single nodule. Nitrogen iso- topes in CAR diamond are lighter and are more lim- ited in range at –1.9 to –8.0 per mil. Nitrogen concentrations. No systematic correla- tion exists between elemental nitrogen abundances and isotopic compositions. Brazilian carbonado have somewhat lower nitrogen concentrations (39-295 ppm) relative to those from the CAR (87-1091 ppm). Discussion: The nitrogen isotopic composition of carbonado is heterogeneous on the scale of 10’s to 100’s of micrometers, and as such, precipitation from a fluid rather than a melt is more consistent with this result. Bulk nitrogen isotope data determined in pre- vious investigations. [4,5] were reported as consistent