Innovative Tracers For Hydrous Melting In The Earth's Mantle

Veronique Le Roux, Geology & Geophysics
Nobumichi Shimizu, Geology & Geophysics



The subduction factory plays a major role in the cycling of volatiles in the deep Earth, because the subducting plate (slab) is enriched in fluids after millions of years of interaction with seawater.  Thus, fluids in the slab are dragged down to depths of several hundreds of kilometers.  Some of these fluids are released through dehydration reactions during the slab descent and percolate through the Earth’s mantle.  One of the major consequences of the presence of fluids in the mantle is that it facilitates melting and production of arc lavas.  This is one of the reasons why arcs (e.g.  Andes, Mariana) form at the surface of the planet.

However no constraints currently exist on the scale of hydrous melting (melting in the presence of water) in subduction environments because mantle rocks are impossible to access insitu.  That is why most subduction-related studies focus on the melt product (arc lavas) to indirectly assess the composition of the Earth’s mantle at depth in these environments.  However, ophiolites - tectonically thrusted pieces of oceanic crust and mantle - are considered to be close analogs to subduction-related rocks.  Here we propose to use a novel geochemical approach to trace hydrous melting directly, in natural mantle rocks recovered from arcs and ophiolites, using the NENIMF facility (secondary ion mass spectrometer) at WHOI.  Particularly, we will test whether halogen elements Fluorine (F) and Chlorine (Cl) can differentiate between hydrous melting and dry melting, as suggested in the recent experimental study of Dalou et al.  (2013).  If we can show that experimental results in the laboratory readily apply to natural mantle rocks, F and Cl could be one of the most useful tracers of fluid cycling in the Earth’s mantle.