Transport and storage of chlorine in the Earth‘s upper mantle: an assessment of mechanisms based on high pressure – high temperature experiments with implications for the global chlorine cycle and subduction zone magmatism

Chlorine is the dominant anion in virtually all geological fluids and can play a fundamental role in magmatic and postmagmatic processes including metal transport and deposition, (trace) element partitioning between solids and fluids/melts and partial melting of rocks. In spite of the importance of Cl, little is known about (1) the crust-mantle transfer mechanisms of Cl in subduction zones, (2) the high P-T stability fields of Cl-rich phases such as chlorapatite and the Cl-storage capacities of potential Cl-hosts in the Earth‘s mantle, (3) the crystal chemistry of Cl under high P-T conditions and (4) the influence of Cl on the melting behaviour of common rock types such as basalts. In order to address some of these questions we propose a high P-T experimental study in the range 1-15 GPa and 700-1800°C based on multi-anvil and piston cylinder experiments in combination with electron microprobe-, ion microprobe analysis, laser Raman spectroscopy and crystal structure analysis. The project will focus (1) on the stability, phase relations and crystal chemistry of Cl-rich apatite and trioctahedral mica, (2) on the Cl-storage capacities of these phases under high P and T and (3) on the partial melting behaviour of MORBs under fluid-present and fluid-absent conditions in the presence of Cl. The results will provide crucial information (1) on the crust-mantle transfer mechanisms of Cl and the Cl storage capacity of potentially important Cl-hosts in the mantle, (2) on the crystal chemistry of Cl in silicates and phosphates under upper mantle P-T conditions, and (3) on the influence of Cl on phase relations and partial melting of MORBs with implications for Cl-transport as solid and/or fluid inclusions in subduction zones and the P-T conditions that allow slab melting and breakdown of hydrous phases. Together, these results will help to devise more accurate models for the global Cl-cycle, for the fluid budget in subduction zones and the generation of certain subduction zone magmas parental to adakites or tonalite-trondhjemite-granodiorite series rocks.

Leitung: Konzett, Jürgen

Finanziert durch: Fonds zur Förderung der wissenschaftlichen Forschung (FWF)

Laufzeit: 2001 - 2005

Kooperationspartner: Australian National University; Bayrisches Geoinstitut/Universität Bayreuth; ETH-Zürich