The alkali element and boron geochemistry of the Escanaba Trough sediment-hosted hydrothermal system

A suite of conjugate pore fluid and sediment samples were collected during Leg 169 of the ODP from within the elastic sedimentary sequences which host massive sulphides at Central Hill, Escanaba Trough (ODP Site 1038). We rep ort the alkali element and boron, and Li and B isotope data for these sampl es. Relative to a reference site (Site 1037) located outside the zone of hi gh heat flow, pore fluids from Site 1038; show a wide variation in Cl (300- 800 mM), and have far higher concentrations of Li (up to 6.2 mM), B (up to 9.7 mM), Ca (up to 5.0 mu M), and Rb (up to 97 mu M). We show that the pore fluids are derived from hydrothermal circulation that has extended into th e basement oceanic crust, with input of the alkali elements and B as the ri sing hydrothermal fluids interact geochemically with the overlying elastic sediments. There is, however, no marked depletion of these elements in the conjugate sediments, suggesting that. there has been advective transport of fluids away from the primary hydrothermal reaction site. This is supported by modelling of the Li and B isotope systematics of the pore fluids, which shows that they record extensive formation of secondary minerals during co oling of the fluids from similar to 350 to similar to 20 degrees C. Precipi tation of metal-rich sulphides would have occurred prior to the formation o f these minerals, thus, the pore fluid Li and B isotope data can place impo rtant constraints on the locus of sulphide deposition beneath the seafloor at Escanaba. (C) 1999 Elsevier Science B.V. All rights reserved.