In-situ U-Pb age dating and the oxygen isotopic composition of accessory minerals in jadeitites: investigating the timing of fluid release and fluid-rock interactions in subduction zones

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When crustal rocks return into Earth’s mantle at subduction zones, fluids released from subducted material initiate melting in the mantle wedge which ultimately leads to the build-up of new continental crust and venting of volcanic gases into the atmosphere. Since fluids act as major agents of mass and energy transfer in such a setting, understanding how and when fluids are released is of paramount importance for constructing a coherent picture of this ongoing global material recycling at subduction zones. Jadeitites – rocks composed mainly of the sodic clinopyroxene jadeite - either directly witnessed such subduction-zone fluids or represent crystallized portions of these fluids themselves. The timing of fluid release(s) could then be constrained by determining the formation ages of jadeitites by dating certain accessory minerals such as zircon. However, zircon has the ability to survive partial melting and high-grade metamorphism which leads to some ambiguity concerning its primary origin in jadeitites. In this study it is therefore proposed to perform in-situ U-Pb dating by secondary ion mass spectrometry (SIMS) of other common accessories in jadeitites including titanite, rutile, and apatite which can often be directly linked to the effect of fluids. In combination with SIMS oxygen isotope analysis of accessory phases, the proposed investigations can provide valuable information about jadeitite formation and, ultimately, fluid production in subduction zones.