Bulk and triple oxygen isotope geochemistry of marine ferromanganese crusts, nodules, and deposits
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Ferromanganese (Fe-Mn) crusts, nodules, and deposits are chemical precipitates that are ubiquitous in the global ocean and contain valuable metals and metalloids. The chemical and mineralogical composition of Fe-Mn precipitates from the southern Mariana arc were used to decipher their genetic assemblage, assess their potential as a source of trace metals, and place them into context, globally. Discrimination diagrams indicate that 10 samples are predominantly of hydrothermal origin, nine are mixed-origin (hydrogenetic-hydrothermal), and three are hydrogenetic. Together, the samples form a continuum from the hydrogenetic to hydrothermal endmembers. Average metal contents of Fe-Mn precipitates from the southern Mariana arc are low relative to hydrogenetic Fe-Mn crusts and hydrothermal Fe-Mn deposits from the northern Mariana arc and elsewhere, globally, and are therefore unlikely to be viable exploration targets. Ferromanganese precipitates can have considerably large fractions of carbonate minerals and non-stoichiometric water which, if unaccounted for, can interfere with oxygen isotope measurements of the oxide-silicate fraction. To ensure consistent results between studies, Fe-Mn precipitates should be acidified using 20 % HCl acid for 3 hours, rinsed, dried completely, and then pre-fluorinated using BrF5 at ambient temperature for 90 minutes, prior to conventional BrF5 extraction of oxygen. Reference materials NOD-A-1 and NOD-P-1 have treated δ18O values of 11.0 ± 1.0 ‰ and 11.1 ± 0.7 ‰, respectively, and each have a complex mineralogy typical of a Mn nodule. Ferromanganese precipitates have triple oxygen isotope ratios that reflect the various reservoirs of this element, formation conditions, and isotope fractionating processes. The Δ’17O values of layered Fe-Mn precipitates do not appear to reflect equilibrium isotopic fractionation processes. Oxygen isotope ratios were measured in the silicate mineral fraction of select layers, to constrain the isotopic composition of this endmember, and through mass-balance calculation, those of the oxide-oxyhydroxide endmember. Individual layers are characterized mineralogically and geochemically, and although mineralogy and ambient formation conditions appear to control the element contents in Fe-Mn precipitates, these factors exert minimal influence on 17O compared to tropospheric dissolved O2 incorporated during precipitation.
URI for this recordhttp://hdl.handle.net/1974/28718
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