Magnesium Isotope Fractionation during Hydrothermal Alteration of the Oceanic Crust

Zhengrong Wang, Geology & Geophysics
Jeffrey Seewald, Marine Chemistry & Geochemistry
Bernhard Peucker-Ehrenbrink, Marine Chemistry & Geochemistry
Stanley Hart, Geology & Geophysics

DOEI Project Funded: 2006

Mg2+ is the second most abundant cation in seawater. Analyses of high-temperature hydrothermal vent fluids indicate that Mg is quantitatively lost from seawater to the hydrothermally-altered wall rock. It is less clear how effectively Mg is removed from seawater in low- or moderate-temperature hydrothermal systems. This uncertainty arises from the difficulty of distinguishing late-stage admixture of un-reacted seawater into Mg-free hydrothermal fluids from low-temperature hydrothermal fluids that still contain some Mg. As low-temperature hydrothermal fluids dominate the fluid flux through oceanic crust, variations in the Mg isotope composition may serve as a tracer of low-temperature fluid flow if 1) Mg isotopes are fractionated during low-temperature reactions with the wall rock and 2) Mg is not quantitatively stripped from the fluids. Our proposed research includes two different, but intellectually coherent parts: 1) a Mg isotope study of hydrothermal fluids and minerals, and 2) an experimental determination of equilibrium Mg isotope fractionation between Mg-rich hydrothermal minerals (caminite and brucite) and seawater at controlled conditions in Jeff Seewald’s experimental hydrothermal fluid laboratory. We will combine our excellent analytical facilities (clean laboratories and multicollector ICP-MS) with expertise in conducting hydrothermal cell experiments. The primary objective of this work is to determine whether formation of caminite and brucite under hydrothermal conditions is accompanied by measurable fractionation of Mg isotopes. If so, we will experimentally establish a quantitative relationship of Mg isotopic and elemental fractionation between seawater and Mg-rich hydrothermal minerals (caminite and brucite) at various conditions to further understand mechanisms of water-rock interaction and mechanisms of fluid transport and mixing. More generally, this study will be a prototype project for constraining Mg isotopic and elemental distribution among different hydrothermal minerals, fluids and rocks in oceanic crust.