Investigating Mantle Recycling and the Origin of the HIMU Component with Stable Thallium Isotopes

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

Our planet has a concentric structure with a metallic core, a magnesio-silicate rich mantle and a thin, highly differentiated crust. The crust is subdivided into rigid plates that move relative to each other driven by large convection cells in the plastically deforming mantle. This convection is the dominant mechanism by which heat is released from the mantle, and it drives - directly or indirectly - virtually all geologic processes related to plate tectonics. New crust is constantly formed at mid-ocean ridges from melting uppermost mantle materials, and it is recycled back into the mantle at subduction zones. It has, therefore, been concluded that the Earth's mantle has to contain fragments of ancient crust that was once at the surface, but was driven into the deep mantle at subduction zones. There is a chance to identify domains in the mantle that contain recycled crustal materials through the precise analysis of the chemical and isotopic composition of volcanic rocks that are derived from melts sourced in the mantle. The most important source of this type are volcanic ocean islands that are located far away from plate boundaries; they erupt a magma type called ocean island basalts. They are scattered all around the globe and are known to exhibit large chemical and isotopic heterogeneity compared with the more homogenous reservoir that is tapped at mid ocean ridges. Numerous studies have focused on characterizing the nature and origin of these mantle domains, but most of the geochemical tools employed to date are only capable of providing ambiguous information. In this project we use a novel tracer that allows us to determine the exact origin of the material in the mantle beneath ocean island basalts. In this project we, we have identified the element thallium and its isotopes (masses 203 and 205) as the ideal geochemical tracer to identify recycled crust in ocean island basalts. Sediments and weathered basalts of the oceanic crust have characteristic thallium isotopic compositions that are distinctly different from the composition of normal mantle rocks. In addition, these crustal materials have very high thallium concentrations compared to the mantle, which enables us to detect even minor contamination of the mantle with recycled crust. Our proposed detailed study of the processes controlling deep recycling of thallium and its isotopes will include: (A) Thallium concentrations and isotopic compositions of crustal rocks that were once subducted and exhumed back to the surface ('eclogites'). These will be used to unravel the transport mechanism for thallium deep inside subduction zones. (B) Thallium isotope and concentration data for basalt samples from the Austral-Cook islands and St. Helena will be determined. These are two important ocean-island basalt localities, and the data will be compared to the eclogite data in order to evaluate the connection of subducted oceanic crust and ocean island basalts.