A. L. Boettcher

Experimental investigation and application of garnet granulite equilibria

Two mineralogic geobarometers based on the assemblages olivine-plagioclase-garnet and orthopyroxeneplagioclase-garnet-quartz have been calibrated from the reaction (1) fayalite+anorthite⇋garnet (Gr1Alm2). The reaction boundary has been determined to within 0.2 kbar using piston-cylinder apparatus. It is located at 4.7, 5.1, 5.5, 5.8, 6.2, 6.6, and 7.0 kbar at 750, 800, 850, 900, 950, 1,000, and 1,050° C, respectively. Summation of ΔG for reaction (1) and fayalite +quartz⇋ferrosilite locates to within 0.3 kbar the following model garnet-forming reaction for quartz-saturated granulites: (2) ferrosilite+ anorthite⇋garnet(Gr1 Alm2) + quartz. Geobarometers based on (1) and (2) are widely applicable in granulite terranes and yield precise pressures that are in agreement with other well-calibrated barometers. Pressures of 7–10 kbar are inferred for many granulite terranes requiring the widespread development of 60–70 km thick continental crust by mid-Proterozoic.

Reinvestigation and application of olivine-quartz-orthopyroxene barometry

Experiments in a piston-cylinder apparatus have been carried out at 700–1050°C, 10–16 kbar to determine the stability of ferrosilite (FeSiO3) relative to fayalite + quartz. Reaction reversals within 0.1-kbar intervals locate the equilibrium at 10.5, 11.0, 11.5, 12.0, 12.6, 13.3, 14.1 and 14.8 kbar at 700, 750, 800, 850, 900, 950, 1000, and 1050°C, respectively, reflecting the intercept with the α-β quartz transition at about 880°C. The tight reversals severely constrain the reaction slope, providing a basis for limited extrapolation and calculations. However, the lack of accurate activity and cation-distribution data for orthopyroxene and olivine generates substantial uncertainties when considering the effects of large proportions of additional components such as MgO. Experiments and calculations indicate that additional components dramatically extend the pyroxene stability field and that pressures that have been inferred from ferrosilite-rich pyroxenes in natural assemblages are 1–3 kbar too high.

Stability of phlogopite-quartz and sanidine-quartz: A model for melting in the lower crust

AbstractThe melting of phlogopite-quartz and sanidine-quartz under vapor-absent conditions and in the presence of H2O-CO2 vapor have been determined from 5–20 kbar. In the lower crust (P=6–10 kbar), phlogopite + quartz melts incongruently to enstatite + liquid at temperatures as low as 710° C in the presence of H2O. When the activity of water is sufficiently reduced by addition of CO2, phlogopite + quartz undergoes a dehydration reaction to enstatite + sanidine + vapor, for example at 790±10° C, 5 kbar, with

Low‐friction, anhydrous, low‐ to high‐temperature furnace sample assembly for piston‐cylinder apparatus

X_{H_2 O}^V

Mantle Metasomatism and the Kimberlite-Lamprophyre Association: Evidence from an Eclogite Nodule from Roberts Victor Mine, South Africa

=0.35. In the absence of vapor, phlogopite + quartz is stable up to a maximum temperature of 900° C in the crust; at higher temperatures this assemblage melts incongruently to enstatite + sanidine + liquid. The melting of sanidine-quartz in the presence of H2O-CO2 vapor shows marked topological differences from melting in the system albite-H2O-CO2, and as a result, apparent activity coefficients for water calculated from sanidine-quartz H2O-CO2 are less than those calculated from albite-H2O-CO2 by up to a factor of five.These data shed light on anatexis in the lower crust, but uncertainties related to ordering of Al and Si in natural and synthetic micas forestall a more rigorous analysis. Nevertheless, maximum temperatures for some granulite terranes can be established.

The quartz ⇆ coesite transformation: A precise determination and the effects of other components

It is apparent from the initial attempts at quantitative tectonic modeling of regional metamorphism (England and Richardson, 1977; Wells, 1980) that a detailed knowledge of metamorphic pressure is essential to the accurate formulation of any tectonic model attempting to incorporate metamorphic pressure—temperature—time relationships. There is, therefore, a need for pressure data of greater accuracy and precision than has been available. Of greatest importance is knowledge not only of the so-called peak metamorphic pressures but also regional variations in the inferred maximum pressures, as well as the prograde and retrograde pressure path of rocks, especially of those that were once buried to the deepest levels of the crust. Unfortunately, at least some of the information necessary to infer the prograde metamorphic path is destroyed by the metamorphic process itself, although in a few terranes, limited data on the prograde pressure-temperature path can be inferred (e.g., Tracy et al., 1976; Phillips and Wall, 1981). Ability to infer such data depends in part on availability of numerous well-calibrated geobarometers applicable in a variety of bulk compositions.

Experimental investigations and geological applications of equilibria in the system FeO-TiO 2 -Al 2 O 3 -SiO 2 -H 2 O

A newly designed furnace assembly for piston‐cylinder, high‐pressure apparatus uses NaCl or KBr rather than hydrous materials such as talc or pyrophyllite. Unlike previous designs, our cell can be used hundreds of degrees above the melting temperatures of the salts. This cell requires little or no pressure correction and provides conditions that are free of H2 and H2O. The electrical power requirements are only about 60% of that for talc furnace assemblies, resulting in cooler operation and longer life for the steel and carbide in the pistons and pressure vessels.

Experimental investigations and geological applications of orthopyroxene geobarometry

• • . opx-oliv microprobe analyses of run products indicate a small preference of Mn for pyroxene over olivine with K~ Mn-Fe = 1.2-1.5, similar to values observed for natural pairs. M6ssbauer spectra are consistent with a random distribution of Mn between the M1 and M2 sites in the orthopyroxene. These experimental data allow downward revision of pressure estimates based on the orthopyroxene barometer in areas where Mn is a significant component in orthopyroxene.