Trace element proxies of seafloor hydrothermal fluids based on secondary ion mass spectrometry (SIMS) of black smoker chimney linings

Abstract

Sampling of paired black smoker chimney linings and seafloor hydrothermal vent fluids supports the development of trace element proxies for sulfide mineral deposition environments by facilitating analyses of trace element partitioning between mineral and fluid phases under well-constrained physiochemical conditions. Here, concentrations of Co, Ni, Ga, Ag, and In in chalcopyrite lining 22 black smoker chimneys (29 for Co, Ag, and In) are measured using secondary ion mass spectrometry (SIMS) calibrated against inductively coupled plasma mass spectrometry (ICP-MS) and NIST-traceable reference solutions. To provide additional data on the trace element concentrations of vent fluid pairs for 19 of the 29 black smoker chimney linings investigated, this paper also presents new ICP-MS data for 33 hydrothermal vent fluids collected from the Tahi Moana-1, ABE, Tu’i Malila, and Mariner vent fields on the Eastern Lau Spreading Center and Valu Fa Ridge.The chalcopyrite black smoker chimney linings investigated represent a variety of temperature (269–395 °C), chemical (e.g., pH (at 25 °C) = 2.3–4.4), and geologic conditions. Electron microprobe results indicate that mineral stoichiometry ranges from stoichiometric chalcopyrite to mol Cu : mol Fe = 0.65. Trace element concentrations obtained by SIMS are: Co (<2 ng/g–760 μg/g), Ni (<17 ng/g–454 μg/g), Ga (<0.9 ng/g–48 μg/g), Ag (60 μg/g–3800 μg/g), In (<0.5 ng/g–270 μg/g). Concentrations of Ag in chalcopyrite strongly correlate with the free ion activity ratio of {Ag+}:{Cu+} in paired vent fluids, with high Ag concentrations in chalcopyrite indicating formation from near neutral vent fluids containing low Cu concentrations or low-pH vent fluids with high Ag concentrations attributable to subsurface Ag remobilization. Chalcopyrite with low Ag precipitates from low-pH Cu-rich fluids unaffected by extensive Ag remobilization. Concentrations of Ga and In in chalcopyrite exhibit a negative trend with vent fluid pH, possibly reflecting the strength of Ga and In OH− complexes. Thus, Ga and In concentrations differentiate Ag-rich chalcopyrite formed from near-neutral Cu-poor vent fluids or that formed from Ag-rich low-pH vent fluids. In contrast, Co and Ni exhibit no trend with fluid data, but correlate with mineral Cu:Fe ratios, possibly reflecting the greater availability of Fe(II) lattice sites or paired substitution of 2+ ions.Overall, this study demonstrates the potential of paired vent fluid and black smoker chimney samples to provide insight into the partitioning of trace elements in sulfide mineral deposition environments and related proxies of important fluid parameters such as pH and metal concentrations. This study also demonstrates the utility of SIMS to precisely analyze trace elements in chalcopyrite at high spatial resolutions and low detection limits.