sphalerite geobarometry in the Balmat-Edwards district , New yorkl
Sphaleri te-pyri te-pyrrhoti te barometry was undertaken in the Balmat-Edwards distr ict in the NW Adirondacks to determine the pressures in the metamorphosed ore body. Metamorphic temperatures were estimated at 625 t 25"C using calcite-dolomite thermometry. Sphaleri tes with extensive exsolut ion of chalcopyrite are very inhomogeneous, even within ten microns of the po + py + sp tr iple point, and have iron contents of 16-12 mole percent FeS, corresponding to 3-7 kbar (Scott, 1973). This local variat ion in pressure is unreasonable; either the system did not equi l ibrate during metamorphism, errat ical ly reequil ibrated afterwards, or exsolved chalcopyrite has variably affected the barometer. A few sphaleri tes without microscopic chalcopyrite blebs yielded consistent FeS contents of 13-12 mole percent where touching pyrrhoti te-pyri te and correspond to pressures of 6.5-7.5 kbar. These data and the presence of si l l imanite, indicative of pressures less than 6-6.5 kbar at 600-650.C, in the adjacent paragneisses uggest hat the Balmat distr ict was subject to a regional metamorphism of 6.5 a 0.5 kbar. Incautious application of the sphaleri te barometer based on a few samples or on heterogeneous materials can lead to large errors in est imated pressures.
A recalibration of the garnet‐olivine geothermometer and a new geobarometer for garnet peridotites and garnet‐olivine‐plagioclase‐bearing granulites
The garnet‐olivine Fe‐Mg exchange geothermometer and the garnet‐olivine‐plagioclase geobarometer have been simultaneously calibrated using reversed experimental data based on the model reactions and between 900 and 1500 °C at 9.1–95.0 kbar and between 4.7 and 7.0 kbar at 750–1050 °C, respectively. The resulting garnet‐olivine thermometer reproduces experimental temperatures mostly within ±75 °C and the garnet‐olivine‐plagioclase barometer reproduces experimental pressures well within ±0.19 kbar. These new thermobarometers use the same garnet and olivine activity models and are thermodynamically consistent. Application of these thermobarometers to garnet peridotites from mantle xenoliths, orogenic garnet peridotites over the world and the Adirondack olivine‐bearing metagabbros yielded reasonable P–T results. The present garnet‐olivine thermometer can be used to measure medium‐high‐grade to ultrahigh‐grade, low‐pressure to ultrahigh–high‐pressure garnet peridotites and metagabbros, whereas the garnet‐olivine‐plagioclase barometer has limited application to garnet‐olivine‐plagioclase‐bearing granulites.
Comparison of different antenna configurations for probing of layered media
The paper presents the results of a comparative analysis of the different configurations of antenna systems (bistatic, monostatic, differential) in terms of the simplicity and accuracy of determining the thickness of the subsurface layers of stratified environment. To this end, corresponding diffraction problems are solved by the finite-difference time-domain forward solver for each measurement setup. The analysis has shown the advantages and prospects of using differential system with vertically spaced antennas in solving of practical problems of the layered medium GPR probing.