13 research outputs found

    Recent volcanism in the Siqueiros transform fault: picritic basalts and implications for MORB magma genesis

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    Small constructional volcanic landforms and very fresh-looking lava flows are present along one of the inferred active strike-slip faults that connect two small spreading centers (A and B) in the western portion of the Siqueiros transform domain. The most primitive lavas (picritic and olivine-phyric basalts), exclusively recovered from the young-looking flows within the A-B strike-slip fault, contain millimeter-sized olivine phenocrysts (up to 20 modal%) that have a limited compositional range (Fo91.5-Fo89.5) and complexly zoned CrAl spinels. High-MgO (9.5–10.6 wt%) glasses sampled from the young lava flows contain 1–7% olivine phenocrysts (Fo90.5-Fo89) that could have formed by equilibrium crystallization from basaltic melts with Mg# values between 71 and 74. These high MgO (and high Al2O3) glasses may be near-primary melts from incompatible-element depleted oceanic mantle and little modified by crustal mixing and/or fractionation processes. Phase chemistry and major element systematics indicate that the picritic basalts are not primary liquids and formed by the accumulation of olivine and minor spinel from high-MgO melts (10% \u3c MgO \u3c 14%). Compared to typical N-MORB from the East Pacific Rise, the Siqueiros lavas are more primitive and depleted in incompatible elements. Phase equilibria calculations and comparisons with experimental data and trace element modeling support this hypothesis. They indicate such primary mid-ocean ridge basalt magmas formed by 10–18% accumulative decompression melting in the spinel peridotite field (but small amounts of melting in the garnet peridotite field are not precluded). The compositional variations of the primitive magmas may result from the accumulation of different small batch melt fractions from a polybaric melting column

    Prokaryotic diversity, distribution, and insights into their role in biogeochemical cycling in marine basalts

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    We used molecular techniques to analyze basalts of varying ages that were collected from the East Pacific Rise, 9 oN, from the rift axis of the Juan de Fuca Ridge, and from neighboring seamounts. Cluster analysis of 16S rDNA Terminal Restriction Fragment Polymorphism data revealed that basalt endoliths are distinct from seawater and that communities clustered, to some degree, based on the age of the host rock. This age-based clustering suggests that alteration processes may affect community structure. Cloning and sequencing of bacterial and archaeal 16S rRNA genes revealed twelve different phyla and sub-phyla associated with basalts. These include the Gemmatimonadetes, Nitrospirae, the candidate phylum SBR1093 in the c, andin the Archaea Marine Benthic Group B, none of which have been previously reported in basalts. We delineated novel ocean crust clades in the gamma-Proteobacteria, Planctomycetes, and Actinobacteria that are composed entirely of basalt associated microflora, and may represent basalt ecotypes. Finally, microarray analysis of functional genes in basalt revealed that genes coding for previously unreported processes such as carbon fixation, methane-oxidation, methanogenesis, and nitrogen fixation are present, suggesting that basalts harbor previously unrecognized metabolic diversity. These novel processes could exert a profound influence on ocean chemistry
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