CO2-rich fluid inclusions in greenschists, migmatites, granulites, and hydrated granulites

Data was discussed from several different terrains in which CO2-rich fluid inclusions occur despite parageneses that predict the presence of H2O-rich fluids. CO2-rich fluid inclusions, some having densities appropriate for peak-metamorphic conditions, were found in greenschists, amphibolites, migmatites, and hydrated granulites. The author suggested that there may be a common process that leads to CO2-rich secondary inclusions in metamorphic rocks

Oxygen-isotope and trace element constraints on the origins of silica-rich melts in the subarc mantle

Peridotitic xenoliths in basaltic andesites from Batan island in the Luzon arc contain silica-rich (broadly dacitic) hydrous melt inclusions that were likely trapped when these rocks were within the upper mantle wedge underlying the arc. These melt inclusions have been previously interpreted to be slab-derived melts. We tested this hypothesis by analyzing the oxygen isotope compositions of these inclusions with an ion microprobe. The melt inclusions from Batan xenoliths have δ 18OVSMOW values of 6.45 ± 0.51‰. These values are consistent with the melts having been in oxygen isotope exchange equilibrium with average mantle peridotite at temperatures of ≥875°C. We suggest the δ 18O values of Batan inclusions, as well as their major and trace element compositions, can be explained if they are low-degree melts (or differentiation products of such melts) of peridotites in the mantle wedge that had previously undergone extensive melt extraction followed by metasomatism by small amounts (several percent or less) of slab-derived components. A model based on the trace element contents of Batan inclusions suggests that this metasomatic agent was an aqueous fluid extracted from subducted basalts and had many characteristics similar to slab-derived components of the sources of arc-related basalts at Batan and elsewhere. Batan inclusions bear similarities to “adakites,” a class of arc-related lava widely considered to be slab-derived melts. Our results suggest the alternative interpretation that at least some adakite-like liquids might be generated from low-degree melting of metasomatized peridotites

Modification of fluid inclusions in quartz by deviatoric stress. II: experimentally induced changes in inclusion volume and composition

Fluid inclusions in quartz are known to modify their densities during shear deformation. Modifications of chemical composition are also suspected. However, such changes have not been experimentally demonstrated, their mechanisms remain unexplained, and no criteria are available to assess whether deformed inclusions preserve information on paleofluid properties. To address these issues, quartz crystals containing natural CO2-H2O-NaCl fluid inclusions have been experimentally subjected to compressive deviatoric stresses of 90-250MPa at 700°C and ~600MPa confining pressure. The resulting microcracking of the inclusions leads to expansion by up to 20%, producing low fluid densities that bear no relation to physical conditions outside the sample. Nevertheless, the chemical composition of the precursor inclusions is preserved. With time the microcracks heal and form swarms of tiny satellite inclusions with a wide range of densities, the highest reflecting the value of the maximum principle stress, σ 1. These new inclusions lose H2O via diffusion, thereby passively increasing their salt and gas contents, and triggering plastic deformation of the surrounding quartz via H2O-weakening. Using microstructural criteria to identify the characteristic types of modified inclusions, both the pre-deformation fluid composition and syn-deformation maximum stress on the host mineral can be derived from microthermometric analysis and thermodynamic modellin

A new method of reconstructing the P-T conditions of fluid circulation in an accretionary prism (Shimanto, Japan) from microthermometry of methane-bearing aqueous inclusions

International audienceIn paleo-accretionary prisms and the shallow metamorphic domains of orogens, circulating fluids trapped in inclusions are commonly composed of a mixture of salt water and methane, producing two types of fluid inclusions: methane-bearing aqueous and methane-rich gaseous fluid inclusions. In such geological settings, where multiple stages of deformation, veining and fluid influx are prevalent, textural relationships between aqueous and gaseous inclusions are often ambiguous, preventing the microthermometric determination of fluid trapping pressure and temperature conditions. To assess the P-T conditions of deep circulating fluids from the Hyuga unit of the Shimanto paleo-accretionary prism on Kyushu, Japan, we have developed a new computational code, applicable to the H2O-CH4-NaCl system, which allows the characterization of CH4-bearing aqueous inclusions using only the temperatures of their phase transitions estimated by microthermometry: Tmi, the melting temperature of ice; Thyd, the melting temperature of gas hydrate and Th,aq, homogenization temperature. This thermodynamic modeling calculates the bulk density and composition of aqueous inclusions, as well as their P-T isochoric paths in a P-T diagram with an estimated precision of approximatively 10 %. We use this computational tool to reconstruct the entrapment P-T conditions of aqueous inclusions in the Hyuga unit, and we show that these aqueous inclusions cannot be cogenetic with methane gaseous inclusions present in the same rocks. As a result, we propose that pulses of a high-pressure, methane-rich fluid transiently percolated through a rock wetted by a lower-pressure aqueous fluid. By coupling microthermometric results with petrological data, we infer that the exhumation of the Hyuga unit from the peak metamorphic conditions was nearly isothermal and ended up under a very hot geothermal gradient. In subduction or collision zones, modeling aqueous fluid inclusions in the ternary H2O-CH4-NaCl system and not simply in the binary H2O-NaCl is necessary, as the addition of even a small amount of methane to the water raises significantly the isochores to higher pressures. Our new code provides therefore the possibility to estimate precisely the pressure conditions of fluids circulating at depth

A study on inclusion formation mechanism in alpha-LiIO sub 3 crystals

The spatial distribution of inclusions in alpha-LiIO3 crystals by means of an argon laser beam scanning technique is studied. The effects of crystal dimensions and solution fluid flow on the inclusion formation in the alpha-LiIO3 crystals were observed. It was further shown that the fluid flow plays an important role in the formation of inclusions. The results obtained were further applied and verified by growing a perfect alpha-LiIO3 single crystal. An experimental foundation for further theoretical studies on the causes of inclusions may be provided