The isotopic footprint of a submarine mineral system: Cr and Sr isotopes in the Iheya North hydrothermal field, Okinawa Trough (IODP expedition 331)

Lucy McGee1, Thomas Burke1, Juraj Farkas1, Bradley Cave1, Chris Yeats2

1Department of Earth Sciences, University of Adelaide, Adelaide, Australia 2Geological Survey of New South Wales, Department of Regional NSW, Maitland, Australia

‘Black smokers’, or submarine hydrothermal vents, represent sites of intense hydrothermalism. Such chimneys transport metals into the ocean where they precipitate as Fe, Pb, Zn and Cu sulfide minerals. However, the spatial footprint of these high-temperature processes transporting metals in marine environments is not well constrained. Legacy material from the 2011 IODP expedition 331 to the Iheya North hydrothermal field in the Okinawa Trough provide a unique opportunity to investigate the isotopic footprint of key metals in the hydrothermal system. The sample set is based on the analysis of four cores: one situated at the foot of an actively forming massive sulfide mound, one drilled at a background site located 1km away from the hydrothermal vents and two cores sampled between these sites, representing a high temperature vent and a lower temperature vent. Samples taken at various depths from the four cores represent a wide range of material, from unaltered distal marine clays, through hydrothermally altered clays and volcanic material, to massive sulfides.

Bulk digestions on 30 samples give a suite of trace element analyses which show large variations, particularly in transition metal element concentrations. SEM-MLA mapping of key samples show important interactions between sulfide phases and the presence of gypsum/anhydrite shows oxidation and reduction in the same sample. The large variation between magmatic and seawater endmembers in 87Sr/86Sr isotopic ratios provide an excellent opportunity to investigate the magmatic ‘footprint’ of the metalliferous hydrothermal system in marine settings and how far into the background this can be detected. Stable Cr isotopes have the potential to show important redox interactions during anticipated oxidation of hydrothermal Cr(III) species in marine settings into oxidised Cr(VI), and possible back-reduction to Cr(III).


Biography

Lucy is a high temperature geochemist with a background in volcanology and igneous geology. She enjoys using isotopes to constrain Earth System processes related to magmas and is currently interested in the processes of metal transport in active systems.

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