Estimation of P-T entrapment conditions of a subduction fluid using elastic thermobarometry: A case study from Cabo Ortegal Complex, Spain

Fluid and mineral inclusions in metamorphic rocks allow the understanding of fluid-involved processes in subduction-zones providing essential contributions to the nature of geochemical processes and element cycling in present day subduction zones. In this work, we studied ultramafic granulite from the high-pressure (HP) and high-temperature (HT) metamorphic series of the Cabo Ortegal Complex, Spain, combining quartz-in-garnet and zircon-in-garnet Raman spectroscopy-based elastic geothermobarometry with Ti-in-quartz trace element thermometry. The studied quartz and zircon inclusions occur within garnet, together with rutile and multiphase fluid inclusions (MFI). Textural evidence, like occurrence in the same 3D cluster and common intergrowth of mineral inclusions, shows that both crystal inclusions and MFI were likely entrapped simultaneously. Hence, the application of elastic thermobarometry to quartz and zircon inclusions in these rocks provides excellent opportunity to define P-T environment of entrapment. Results from Raman spectroscopy on multiple quartz and zircon inclusions showed that the remnant elastic inclusion pressure (Pinc) at room conditions for both (on average 0.51 ± 0.04 GPa and 0.72 ± 0.05 GPa for the quartz and zircon inclusions, respectively) fall within the range of 2σ uncertainty confirming the crystallization within the same growth-stage of garnet. Intersection of the entrapment isomekes is at a P-T of 1.8 ± 0.2 GPa and 880 ± 70 °C. Electron microprobe measurements on quartz inclusions from the same garnet zone show uniform Ti concentrations (45–59 ppm). Isopleths calculated from Ti-in-quartz thermometer intersect the average quartz-in-garnet isomeke within the P-T range indicated by the intersection of quartz and zircon entrapment isomekes, which is P = 1.8 ± 0.2 GPa and T = 860 ± 70 °C. Besides, we made a comparison of different reference materials applied for zircon-in-garnet elastic thermobarometry verified by independent Ti-in-quartz trace element thermometry. Our findings indicate that elastic thermobarometry on mineral inclusions provide a reliable constraint on the entrapment P-T conditions of coexisting fluid inclusions.