Noble gas tracers at the Mid-Cayman Spreading Center

Mark Kurz, Marine Chemistry & Geochemistry



The Mid-Cayman Spreading Center is unique among mid-ocean ridges due to its great water depth, slow spreading rate, and tectonic setting.  This proposal seeks seed funds to include noble gas research in a funded cruise to the Mid-Cayman Spreading Center, in the Fall of 2009.  Helium is an ideal tracer of hydrothermal activity; the less abundant isotope 3He is rare in the hydrosphere and enriched in mantle melts, making it a sensitive and unambiguous indicator of mantle inputs to the deep ocean.  Measurement of 3He in the hydrothermal fluids collected on the cruise will therefore serve as an indicator of mantle helium inputs, and the ratio of helium to the other gases (e.g. CO2/3He) will be used to quantify mantle fluxes and place the hydrothermal fluids into a global framework.

An additional goal of this proposal is to make preliminary measurements on the Mid- Cayman Spreading Center rocks that were collected in the 1970s, which will provide a context for the new samples to be collected.  We propose to make new helium, neon, and argon measurements in a subset of the basalt glasses, gabbros, and peridotites in the WHOI collection.  The combination of helium, neon, and argon provides an important indicator of degassing processes because solubility decreases in the order He>Ne>Ar, and the isotopes can be used to exclude atmospheric contamination effects for all three gases.  The Mid- Cayman Spreading Center samples are among the deepest erupted volcanic glasses, and the goal will be to examine the influence of pressure on the degassing characteristics of the glasses. The measurements on the gabbros and peridotites will test the new hypothesis that deformation is a control on noble gas abundances in the upper mantle and crust. We will also make preliminary measurements on the rock samples that will be collected on the 2009 cruise.