Genetic Relationship Between Ultramafic/Mafic-Hosted Massive Sulfides And Their Host Rocks At The Logatchev Hydrothermal Field At 14°45?N (MAR) Using PGE Geochemistry And Os Isotopes

The Logatchev hydrothermal field is situated on the east inner flank of the rift valley of the MAR at 14°45?N and is hosted by serpentinized peridotites and minor gabbronorites while basalts are subordinate. Hydrothermal precipitates recovered from the Logatchev field during a recent R/V Meteor cruise include massive chalcopyrite chimneys, massive pyrite crusts, silicified breccias, abundant secondary Cu-sulfides, red jaspers, abundant Fe-Mn-oxyhydroxides as well as atacamite and Mn-oxides (Kuhn et al., 2004). Previous results of hydrothermal fluid studies in the Logatchev field are somewhat at odds with theoretical models of ultramafic-seawater reaction. Butterfield et al. (2003) suggested that the Fe component of orthopyroxene plays the more important role compared to olivine in ultramafic-hosted hydrothermal systems. This consideration is supported by the recovery of large amounts of Opx-rich gabbronorites and orthopyroxenites in the Logatchev field (Kuhn et al., 2004). A possible tool to constrain the contribution of the different rock types (ultramafics, mafics, MORB) to a seafloor hydrothermal system is the systematic analysis of 187/188Os ratios of the host rocks as educts and the sulfides as products. For instance, abyssal peridotites mainly display low 187/188Os ratios (<0.127); in contrast MORB and gabbronorites have radiogenic ratios of >0.13 (Snow and Reisberg, 1995), orthopyroxenite has ratios of about 0.174 (Büchl et al., 2002) and seawater has an ever higher 187/188Os of about 1 (Sharma et al., 1997). Therefore, massive sulfides predominantly leached from ultramafic rocks should have Os isotopic ratios plotting on a mixing line between the ultramafic rocks and seawater (in a 187/188Os vs. 1/Os diagram), whereas those derived from MORB, gabbronorites and/or orthopyroxenites will be plotting on different mixing lines.