pages: Goldschmidt Conference Abstracts 
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  High-pressure microbiology in the  synchrotron light  I. <sub> </sub> D <sub>ANIEL</sub> <sup>1</sup> , <sub> </sub> A. <sub> </sub> P <sub>ICARD</sub> <sup>2,3</sup> , <sub> </sub> D. <sub> </sub> T <sub>ESTEMALE</sub> <sup>4</sup> <sub> AND </sub>   J.-L. <sub> </sub> H <sub>AZEMANN</sub> <sup>3</sup>  
  Subduction factory unroofed:  Modern submarine magmatism in the  North Fiji Basin, Southwest Pacifc  L <sub>EONID </sub> D <sub>ANYUSHEVSKY AND </sub> T <sub>REVOR </sub> F <sub>ALLOON</sub>  
  CODES CeO and School of Earth Sciences, University of  Tasmania, PB 79, Hoibart, TAS 7001, Australia  (*correspondence l.dan@utas.edu.au,  Trevor.Falloon@utas.edu.au)    An extensional setting at the southern termination of the  North Fiji Backarc Basin (southwest Pacific), which occurs in  response to westward roll-back of the Vanuatu trench and  eastward roll-back of the Tonga trench, allows subduction- related magmas to reach the surface (seafloor) with only a  minimal  extent  of  both  fractionation  and  crustal  contamination, thus providing a unique insight into magma  generation processes with the subduction factory.  Volcanic rocks collected within this are during the  SS10/2004, SS08/2006 and SS03/2009 voyages of the R/V  “Southern Surveyor” reveal a large spectrum of subduction- related magma compositions from backarc basin basalts to  boninites, calc-alkaline basalts and high-Mg adakites. All  magma series have very primitive, high MgO endmembers,  which contain abundant high-magnesian olivine phenocrysts  (Fo 92-94) formed during the earliest stages of melt evolution.  The data reveal that melts produced due to adiabatic  decompression of the mantle wedge play an important role in  magma genesis. Within the studied area, a number of such  melts can be identified which differ in both the extent of the  contribution of the subduction-derived components, and the  extent of depletion of the mantle source. Also abundant are  melts produced by melting of the basaltic component of the  subducted oceanic crust, which extensively re-equilibrate with  the surrounding mantle during their accent to the surface.  These melts are also characterised by a range of compositions,  range from low-Si, high-Mg to high-Si, high-Mg adakites.  There is clear petrographic and geochemical evidence for  extensive mixing between high-Mg adakitic and backarc basin  magmas in this area, which results in formation of primitive  magmas with typical calc-alkaline and boninitic affinities. Our  results suggest that in less extensional settings, where  unfractionated magmas rarely reach the surface, the role of  this mixing in the genesis of typical calc-alkaline magmas may  be more difficult to identify, however it may play an important  role in subduction-related magma genesis in general. 
  <sub>1</sub> Université Lyon 1, ENS de Lyon, CNRS, UMR 5276,  Laboratoire de Géologie de Lyon, France  (isabelle.daniel@univ-lyon1.fr)  <sub>2</sub> Max Planck Insitute for Marine Microbiology,  Biogeochemistry Department, Bremen, Germany  <sub>3</sub> MARUM, Center for Marine Environmental Sciences,  Bremen, Germany  <sub>4</sub> Institut Néel, Département MCMF, Grenoble, France    The Earth’s subsurface is characterized by hostile  conditions for life in terms of temperature, pressure and  nutrient availability. Although our current view of the  biosphere extension is restricted to shallow geological depths,  deep life may enconter pressures of hundreds MPa. As an  important microbial energetic process, dissimilatory metal  reduction needed to be investigated as a function of pressure.  We measured the effects of pressure on the reduction of  Se(IV) and Fe(III) to Se(0) and Fe(II), respectively by the  bacterial model Shewanella oneidensis MR-1. This strain is a  mesophilic and piezosensitive counterpart of psychrophilic  and piezophilic Shewanella representatives that have been  frequently isolated from deep-sea environments. Kinetics and  yields of Se(IV) and Fe(III) reduction were monitored  in situ  by X-ray Absorption Spectroscopy (XAS) in an autoclave  optimized for  in situ  XAS measurements [1]. Most  measurements were performed at the BM30B beamline of the  European Synchrotron Radiation Facility (ESRF, Grenoble,  France). Early measurements on the reduction of Se(IV) were  also performed in diamond anvil cell at the ID22 beamline of  the ESRF.   Metal reduction occurs in cultures of MR-1 at pressures in  excess of 100 MPa. This shows that the metabolic activity of a  microbe, despite being piezosensitive, extends far beyond its  pressure limits for growth here at 50 MPa. Consequently,  considering only the ability to grow in the conditions of the  deep subsurface as a proof of metabolic activity may lead to  an underestimation of the impact of the biosphere in deep  environments. Although the exact experimental conditions do  not mimic complex subsurface environments, we show here  that the metabolic activity of a surface microbe potentially  brought to the deep subsurface can affect significantly  biogeochemical cycles as those of selenium, but more  importantly those of iron and carbon.  [1] Picard  et al . (2011)  Geobiology ,  9 , 196-204.  
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