Unraveling a hydrous alkaline metasomatic agent beneath the Styrian Basin: an inclusion study from mantle xenolith

Aradi L. et al. (2019) ECROFI2019 konferencián bemutatott absztraktj

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

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, SEM-EDS and 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 300–400 °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

Fluid infiltration in the lithospheric mantle beneath the Rio Grande Rift, USA: a fluid-inclusion study

Spinel-peridotite xenoliths, hosted in alkali basalts (similar to 15 Ma), were collected from Adam's Diggings in the western margin of the Rio Grande Rift (RGR), New Mexico, USA. We selected five representative spinel-peridotite xenoliths, showing abundant fluid inclusions (FIs). Petrographic observations allowed the distinction of two generations of fluid-inclusion assemblages, both hosted in orthopyroxenes, namely Type-1 (earlier) and Type-2 (later). Both types of fluid inclusions were characterized combining microthermometry, high-resolution Raman micro-spectroscopy, and focused ion beam-scanning electron microscopy. The results of this study indicate that the timing and depth of entrapment, as well as the composition of trapped fluid were different between Type-1 and Type-2 FIs. The earlier fluid infiltration (C-O-N-S) happened before or during formation of exsolution lamellae and was trapped as Type-1 FIs in the cores of orthopyroxenes, whereas the later fluid infiltration (C-O-H-S) was trapped as Type-2 FIs after the formation of the orthopyroxene porphyroclasts with exsolution lamellae. The two fluid percolation events in the Adam's Diggings peridotites indicate the complexity of mantle fluids around the RGR. During ascent of the xenoliths within a basaltic lava, post-entrapment reactions produced magnesite and quartz in Type-1 FIs and magnesite and talc in Type-2 FIs as reaction products of the fluid and its host mineral (orthopyroxene)

Reconstruction of magma chamber processes preserved in olivine-phlogopite micro-ijolites from the Oldoinyo Lengai, Tanzania

A detailed petrographic and mineralogical investigation of olivine-phlogopite micro-ijolite xenoliths from Oldoinyo Lengai, Tanzania indicates a complex evolutional history. These xenoliths consist of diverse textural subdomains characterized by minerals ranging from early-formed olivine, through diopside-hosted perovskite and phlogopite, to evolved aegirine-augite and titanite. Thermometry and mineral compositions in the subdomains suggest crystallization temperatures from 1070–970 °C to 850–700 °C at plutonic pressures and SiO2-activities controlled by perovskite-titanite equilibria. Double coronas are a characteristic textural feature of the olivine-phlogopite micro-ijolite, consisting of olivine cores surrounded by an inner clinopyroxene corona and an outer phlogopite corona. These double coronas might have formed during early magma chamber processes, including magma movement to a subsequent chamber resulting in dissolution of olivine with subsequent crystallization and accumulation of diopside and phlogopite. Diopside−aegirine-augite compositional zonation indicates several magma injections followed by cooling periods, during the formation of micro-ijolite groundmass. Mg# (80–83) and Ca (0.1–0.3 in wt%) contents of olivine together with the presence of primary melt inclusions in clinopyroxene, phlogopite, and nepheline indicate a magmatic origin from a possible parental olivine-nephelinite melt. There is evidence for subsolidus, or near-solidus, re-equilibration processes as indicated by the reaction of olivine with titanite forming symplectitic textures of ilmenite and diopside with minor zirconolite. Ti-exchange between phlogopite phenocrysts and other Ti-bearing minerals (perovskite, titanite, magnetite) resulted in ∼750 °C equilibrium temperatures for phlogopite, which are much lower than mafic magmatic (>900 °C) conditions. Calculated subsolidus temperatures suggest crystallization of olivine-phlogopite micro-ijolites over a 10–20 km depth interval

Amphibole-hosted fluid inclusion in mantle xenolith from the Eastern Transylvanian Basin

Szabó Ábel et al. 2019 ECROFI2019 absztrakt kötetben megjelent absztraktj