Marian B. Holness

Temperature and pressure dependence of quartz-aqueous fluid dihedral angles: the control of adsorbed H2O on the permeability of quartzites

Abstract The dihedral angles of H2O CO2 fluids in quartz aggregates have been determined experimentally at 450–1080°C and 1–9.5 kbar. At 4 kbar, the quartz-quartz-H2O dihedral angle increases linearly from values close to 60° at 450°C to about 81° at 600°C. Further increase in temperature has little effect on the angle until 900°C, whereupon the dihedral angle decreases at an increasing rate until values below 60° are reached close to the melting point at 1098°C. Pure CO2 fluids have a constant dihedral angle of 98° in the temperature range 600–1080°C. An equimolar H2O and CO2 fluid has a constant dihedral angle of 92° in the temperature range 600–900°C which decreases with increasing temperature at approximately the same rate as the quartz-quartz-H2O angle until 70° is attained at 1080°C. The pressure at which the quartz-quartz-H2O dihedral angle maximum occurs shows a positive correlation with temperature. It is shown that the observed temperature and pressure dependence of quartz-aqueous fluid dihedral angles may be due to the presence of an adsorbed layer of H2O on the quartz-fluid interface with an adsorption density of about 8.5 molecules/nm2, and a thicker layer on the quartz grain boundary with an adsorption density of 0.04 moles/cm3. The quartz-quartz-H2O dihedral angle is contoured as a function of P and T, showing the metamorphic regimes for which quartz-rich rocks will be permeable to pervasive grain-edge flow of H2O. Windows of permeability exist at temperatures near the melting point and at much lower temperatures such as those at which infiltration-driven hydration reactions occur during retrogression.

Equilibrium dihedral angles in the system H2O−CO2−NaCl-calcite, and implications for fluid flow during metamorphism

AbstractFluid-calcite-calcite dihedral angles have been measured for fluids in the system H2O−CO2−NaCl, between 1 and 2 kbar, and 550–750° C. It is found that the calcite-calcite-H2O dihedral angle decreases steadily with addition of NaCl from a value of about 80° (pure water) to 44° (60 wt% NaCl). The CO2−H2O system displays a well-defined minimum at

Reaction-induced microcracking: An experimental investigation of a mechanism for enhancing anatectic melt extraction

X_{CO_2 } = 0.5

Ion microprobe study of marble from Naxos, Greece: Grain-scale fluid pathways and stable isotope equilibration during metamorphism

, with a dihedral angle of 50°, in contrast to those of pure CO2 and H2O which are 90° and 80° respectively. Experiments containing fluids which are immiscible at run conditions showed a bimodal distribution of dihedral angles in the CO2−H2O−NaCl system, which can be approximately correlated with the compositions of the two fluid phases. Such bimodality was only observed for immiscible fluids in the H2O−NaCl system if the quench rate exceeded about 200°C per min. This is probably due to the extremely rapid establishment of the single phase dihedral angle on quenching. The fluid phase topology in devolatilising marbles will only be a connected network for very saline brines and fluids with

The structure of the halite-brine interface inferred from pressure and temperature variations of equilibrium dihedral angles in the halite-H2OCO2 system

X_{CO_2 }

Possible Effects of Spreading Rate on Morb Isotopic and Rare Earth Composition Arising from Melting of a Heterogeneous Source

close to 0.5. Fluids trapped in fluid inclusions in calcite grains in marbles may be predominantly H2O-rich or CO2-rich, and of low salinity. All other fluid compositions in the H2O−CO2−NaCl-calcite system will occupy isolated pores, the largest of which will grow at the expense of the smallest. Escape of fluid produced during devolatilisation reactions under such conditions will occur by fluid overpressuring and hydrofracture. In contrast, previous experimental studies of quartz-fluid dihedral angles between 950° and 1100° C (Watson and Brenan 1987) predict that quartz-dominated lithologies will permit pervasive flow of H2O−NaCl fluids, but not of H2O−CO2 fluids. Documented geological examples of differences in permeability and fluid flow mechanism between metamorphic argillites, psammites and limestones which support the results of the experimental studies are discussed.