Hydrothermal mineralization at a submarine island arc volcano, Kick 'em Jenny, Lesser Antilles island arc.

摘要

Vigorous hydrothermal venting was observed in the crater of Kick 'em Jenny (KeJ) submarine volcano by a remotely operated vehicle (ROV) during cruise RB-03-03 of the R/V Ronald Brown in 2003. Bubbles and shimmering water ascended through the water column, but shimmering water was also seen flowing down gradient from a fissure at the top of a fine-grained sediment mound structure. These observations support the phase-separation hypothesis proposed by Koschinsky et al. (2007) based on a water column survey conducted on the flanks of KeJ in 2001. This study presents petrographic, lithologic, and geochemical analyses of samples collected during the 2003 cruise and provides further support for the hypothesis of phase-separation at KeJ based on experimental, fluid inclusion, and thermodynamic studies that suggest a correlation between specific trace elements and either the vapor or brine phase of a fluid that has undergone boiling. Sediments collected from areas of hydrothermal activity (hydrothermal mound sediment) are enriched, by a factor ≥2, in Cu, As, and Sb relative to sediment collected from areas void of hydrothermal activity (primary volcanic sediment). Copper, arsenic, and antimony have been suggested by numerous workers to partition into the vapor phase of a phase-separated fluid. A hydrothermal breccia collected during 2003 also supports the hypothesis of phase-separation, but its geochemical characteristics represent a metal-depleted brine phase. The altered exterior of the hydrothermal breccia is depleted, relative to the fresh interior, in trace elements, most notably Cu and As, which are associated with the vapor phase and are known to substitute for Fe in pyrite. The lack of relative trace element enrichments suggests the pyrite precipitated from a barren hydrothermal fluid that had deposited most of its trace elements, including the majority of metals, subseafloor, at the boiling horizon, as proposed by Koschinsky et al. (2007). In addition to geochemical analyses, sediment samples collected from the KeJ crater in areas of fine-grained mounds with hydrothermal venting and areas of coarser sediment without venting (primary volcanic sediment) were analyzed for grain size. The hydrothermal mound sediment was found to contain a larger percentage of clay-size grains than the primary volcanic sediment. The presence of clay minerals commonly associated with hydrothermal alteration, such as illite and mixed layer clays were confirmed by XRD analysis. A mass balance model was developed to determine if the hydrothermal mound sediment could be an alteration product of the primary volcanic sediment. A percent change calculation for major oxides was used to compare model sediment compositions with the composition of hydrothermal mound sample RB-03-03-18. The best-fit model is composed of 71% primary volcanic sediment, 4% I/S mixed layer clays, 3% smectite, 17% talc, 4% illite, and 1 pyrite. Clay minerals represent 28% of the total model sediment. This result is higher than the value calculated in the grain size analysis, 19% clay-sized grains and clay minerals for RB-03-03-18, but less than the XRD value of 42 wt% clay minerals. Nonetheless, the model composition is within the range of values calculated by two independent methods, which suggests the hydrothermal mound sediment is likely an alteration product of the primary volcanic sediment.