Stability of Titanian Clinohumite-olivine Intergrowths in HFSE-enriched Chlorite-harzburgite produced by High-pressure Dehydration of Antigorite-serpentinite and its Potential Implications for the Origin of Subduction Signatures

Depletion of high-field-strength trace elements (HFSE) relative to N-MORB is the most distinctive geochemical fingerprint of subduction magmatism. Proposed hypotheses advocate that this subduction signature is acquired during melting and/or fluid transfer either in the mantle wedge or in the crust of the subducting oceanic plate. Here we provide field-based and geochemical evidence from the Cerro del Almirez ultramafic massif (Sierra Nevada, Betic Cordillera, S.Spain) showing that high-pressure dehydration of antigorite-serpentinite produces chlorite-harzburgite relatively enriched in HFSE due to the stabilization of F-OH-Ti-clinohumite intergrowths with prograde olivine. F-OH-Ti-clinohumite occurs as porphyroblastic grains and within prograde olivine as irregular and lamellar, planar intergrowths at microscopic and submicroscopic scales. Chlorite-harzburgite is characterized by a brown pleochroic olivine with abundant oriented microscopic to submicroscopic oxide particles. The mean Ti content of bulk brown olivine is 144 ppm. Brown olivine preserves Ti-clinohumite lamellae that sometimes grade into ghost lamellae outlined by oxide trails. This observation suggests that some of the oxide inclusions in brown olivine are derived from the breakdown of Ti-clinohumite intergrowths. Thermodynamic modelling for selected Almirez bulk rock compositions indicates 695 oC and 1.7 to 2.0 GPa, as the PT stability conditions for the Almirez Ti-clinohumite -olivine intergrowths. The computed phase diagrams, in conjunction with geothermal conditions envisaged for different subduction settings, indicate that Ti-clinohumite is stable in the vicinity of the antigorite-serpentinite/chlorite-harzburgite phase boundary at subarc depths. In these settings, deserpentinization may act as a source of fluids leaching LILE, Pb, and Sr from the overlying crust and sediments on their way up to the mantle wedge. Stabilization of chlorite-harzburgites with clinohumite-olivine intergrowths in the mantle wedge may acts as a sink of HFSE by selectively fractionating them from other incompatible trace elements in fluids emanating from the slab. Resulting arc fluids in equilibrium with wedge chlorite-harzburgite are strongly depleted in HFSE and transfer this depletion to the overlying hot mantle wedge, where subduction magmas are generated.