Thallium Isotopes as Tracers of the Origin Serpentinizing Fluids in the Mariana Fore Arc


DOEI Funded Research: 2011


This proposal seeks funds to conduct a pilot study of thallium (Tl) isotopes in serpentinized peridotites from mud volcanoes located at the Mariana fore arc recovered during Ocean Drilling Program (ODP) Legs 125 (Conical Seamount) and 195 (South Chamorro Seamount). This novel stable isotope system will provide unprecedented new insights into the origin and pathways of fluids that buoyantly rise from the décollement along deep-seated faults thereby hydrating the supra subduction zone fore arc mantle of the Philippine Plate. These fluids are sweated off during subduction of the incoming Pacific plate as it is pressurized and heated on its descent; however, it remains unclear whether they are expelled during compaction/devolatilization of sediments overlying the basaltic crust or whether they are expelled by devolatilization of the altered basaltic crust or perhaps a combination of both.

The Tl isotope system is uniquely suited to investigate fore arc fluid sources because the pristine mantle is isotopically homogeneous and depleted in Tl, while pelagic sediments and altered oceanic crust of the subducting plate are enriched in Tl by several orders of magnitude and characterized by highly fractionated Tl isotope compositions. Fluids entering the fore arc mantle will thus be characterized by distinct Tl isotopic fingerprints and this property will enable us to distinguish between a sedimentary and a crustal source of the fluids that drive serpentinization of the fore arc mantle. Two preliminary measurements of serpentinized harzburgites from South Chamorro show Tl isotopic compositions that are similar to pelagic sediments, which may suggest that the fore arc mantle is serpentinized by fluids expelled from sediments overlying the basaltic crust. 

Constraining the origin of fluids responsible for serpentinization reactions is imperative not only to better understand the metasomatic reactions at the subducting plate-fore arc mantle interface and their mineralogical and geophysical consequences, it will also provide new insights into the water budget in the shallow parts of subduction zones. Therefore, verification of Tl isotopes as a tracer of the origin of serpentinization fluids will open up for additional studies of Tl isotopes in serpentinites in other arcs.