FY 2007

Hydrothermal fluid fluxes calculated from the isotopic mass balance of thallium in the ocean crust

Nielsen, S.G., M. Rehkämper, D.A.H. Teagle, D.A. Butterfield, J.C. Alt, and A.N. Halliday

Earth Planet. Sci. Lett., 251(1–2), 120–133, doi: 10.1016/j.epsl.2006.09.002 (2006)

Hydrothermal fluids expelled from the seafloor at high and low temperatures play pivotal roles in controlling seawater chemistry. However, the magnitude of the high temperature water flux of mid-ocean ridge axes remains widely disputed and the volume of low temperature vent fluids at ridge flanks is virtually unconstrained. Here, we determine both high and low temperature hydrothermal fluid fluxes using the chemical and isotopic mass balance of the element thallium (Tl) in the ocean crust. Thallium is a unique tracer of ocean floor hydrothermal exchange because of its contrasting behavior during seafloor alteration at low and high temperatures and the distinctive isotopic signatures of fresh and altered MORB and seawater.

The calculated high temperature hydrothermal water flux is (0.17-2.93) × 10¹³ kg/yr with a best estimate of 0.72 × 10¹³ kg/yr. This result suggests that only about 5 to 80% of the heat available at mid-ocean ridge axes from the crystallization and cooling of the freshly formed ocean crust, is released by high temperature black smoker fluids.The residual thermal energy ismost likely lost via conduction and/ or through the circulation of intermediate temperature hydrothermal fluids that do not alter the chemical budgets of Tl in the ocean crust.

The Tl-based calculations indicate that the low temperature hydrothermal water flux at ridge flanks is (0.2-5.4) × 10¹⁷ kg/yr. This implies that the fluids have an average temperature anomaly of only about 0.1 to 3.6°C relative to ambient seawater. If these low temperatures are correct then both Sr and Mg are expected to be relatively unreactive in ridge-flank hydrothermal systems and this may explain why the extent of basalt alteration that is observed for altered ocean crust appears insufficient to balance the oceanic budgets of ⁸⁷Sr/⁸⁶Sr and Mg.