Support for Quantifying Partitioning of Trace Elements into Seafloor Hydrothermal Deposits

Meg Tivey, Marine Chemistry & Geochemistry

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Abstract

An exciting aspect of the discovery of “black smoker” chimneys on the seafloor in 1979 was the ability to sample “ore-forming” vent fluids and the corresponding mineral deposits, and the possibility of directly linking their compositions.  The deposits result from mineral precipitation as the hot (up to ~400°C) fluids are emitted and mix with seawater.   The vent fluids vary in temperature and composition depending on the path they have taken through the seafloor.   Important factors along the fluid path include temperature, pressure (or depth), the composition of the rock, and possible addition of gases from deeper magmas.  Thus the concentrations of different elements in vent fluids can be used as indicators, or proxies, for conditions deeper in the crust where vent fluids form.  Similarly, vent deposit compositions are sensitive to differences in vent fluid compositions, and can act as more permanent recorders, and as proxies, for vent fluid compositions.  Element concentrations of both fluids and deposits can be used to investigate exchanges of heat and material between the seafloor and the ocean, and how these exchanges are affected by seafloor spreading rate, seafloor composition, and volcanic/tectonic activity.

This proposal is focused on use of trace metals in deposits (e.g., Co, Ag, Mn) as proxies for vent fluid compositions, specifically on quantifying the partitioning of these metals between vent fluids and the sulfide minerals that precipitate along the linings of the chimney conduits.  Analyses will be done on an already collected set of fluid-chimney pairs (fluid and chimney sampled at the same time).  Secondary ion mass spectrometry (SIMS) analyses will be done of the minerals to determine which elements are homogeneous and thus may be partitioning in a characteristic manner from the fluid into the mineral.  Then quantification of the trace element concentrations will be done by micro-drilling or picking lining minerals and analyzing the subsamples by high resolution inductively coupled plasma-mass spectrometry (HR-ICPMS).  Data will be combined with fluid data and thermodynamic calculations to provide new constraints on trace element partitioning into sulfide minerals at temperatures from 200 to 400°C and pH from 2.3 to 4.  Results will be used to test two hypotheses: 1) Minerals precipitating from vent fluids along linings of chimneys incorporate some trace elements in a characteristic manner, with partitioning a function of element concentrations, pH, and temperature; and 2) Seafloor sulfide deposits record critical information about the trace element compositions of the fluids from which they formed, and environmental conditions (e.g., temperature, pH) at the time of precipitation; thus trace element distributions could be used as proxies for hydrothermal fluid properties and composition.  If successful, this project will lead to future projects, allowing retrieval of vent fluid trace element data from archived chimney samples; measurements across thick chimney linings to examine constancy of vent fluid composition over time; and/or analysis of inactive chimneys to infer likely vent fluid compositions from which they formed.  Because seafloor deposits record changes in fluid chemistry over time and persist long after hydrothermal fluids have ceased to flow, the development of these proxies could add a new dimension to our understanding of deep-sea hydrothermal systems.