Abiotic passive nitrogen and methane enrichment during exhumation of subducted rocks: Primary multiphase fluid inclusions in high-pressure rocks from the Cabo Ortegal Complex, NW Spain

Abstract

Primary multiphase fluid inclusions (MFI) were studied in one eclogite and two granulites from the Cabo Ortegal Complex (COC, NW Spain) by means of Raman imaging, Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS) and Focused Ion Beam - Scanning Electron Microscopy (FIB)-SEM. Complementary, secondary MFI in pyroxenites from COC were also investigated. MFI hosted in eclogite and granulites occur along growth zones or in 3D clusters in garnet porphyroblasts suggesting a primary origin at high-pressure (HP) metamorphic conditions. The mineral assemblage of MFI is mainly composed of Fe-Mg-Ca-carbonates and phyllosilicates +/- graphite +/- quartz +/- corundum +/- pyrite +/- apatite +/- rutile and a fluid phase composed of nitrogen +/- methane +/- carbon-dioxide. The mineral proportions vary among the lithologies. Dominant carbonates and hydrous silicates are interpreted as step-daughter minerals (crystals formed in the MFI after entrapment as a result of fluid-host interaction), whereas apatite, quartz and rutile are considered in part as accidentally trapped minerals since they also occur as crystal inclusions together with MFI in each rock type. Quartz and corundum occur together in MFI in ultramafic granulite and are regarded as step-daughter minerals in this lithology. These observations suggest that the MFI are products of post-entrapment reactions of a homogeneous COHN fluid system with the host mineral. Thermodynamic calculations in the CaFMAS-COHN system confirmed that bulk composition of the MFI in eclogite is similar to the host garnet+COHN composition except for a potential lost of H2O. Carbonation and hydration reaction between the host (i.e. garnet or pyroxene) and the fluid inclusion results in the consumption of all CO2 and part of the H2O from the fluid phase producing Ca-Fe-Mg-carbonates and hydrous step-daughter minerals, mostly pyrophyllite and chlorite. Nitrogen content of the originally trapped COHN fluid in eclogite was estimated to have a maximum value of 10 mol% at peak HP conditions and 30-40 mol% at retrograde conditions that is within the range of the observed MFI in the residual fluid (13-68 mol%). Pseudosection modelling confirmed the stability of the phase assemblage in the MFI in a specific low-pressure, low-temperature stability field (between 300 degrees C and 400 degrees C at pressures < 1 GPa), caused by H2O- and CO2-consuming reactions possibly in a single step. Our findings indicate that such processes in the exhuming HP units may play a role in global nitrogen and carbon cycling as well as potentially contributing to nitrogen and methane supply to subsurface-surface environments during devolatilization in the forearc regions of convergent plate margins.