U-Pb sphene dating of metamorphism: the importance of sphene growth in the contact aureole of the Red Mountain pluton, Laramie Mountains, Wyoming

Article Abstract:

Uranium-lead sphene analysis is a powerful tool for dating metamorphic events. Geochronological study of amphibolites and granite gneisses within the contact aureole of the Red Mountain pluton, Laramie anorthosite complex, Wyoming, reveals a 300 million year spread in 207Pb/206Pb ages. This range of ages resulted from two periods of sphene growth, corresponding to phases of regional and contact metamorphism, respectively. The ages were reset by thermal diffusion at high temperatures and the U-Pb ages recorded the new growth of sphene. Textural study distinguished the two phases of sphene.

Geological time, Geochronology, Isotope geology

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Prograde and retrograde fluid flow during contact metamorphism of siliceous carbonate rocks from the Ballachulish aureole, Scotland

Article Abstract:

The mineralogical and stable isotope study of siliceous carbonate rocks from the Ballachulish aureole, Scotland, provides information about retrograde and prograde fluid flow during contact metamorphism. A model describes the infiltration of dolomites and limestones by CO2-H2O fluid and the subsequent alteration modes, reaction textures, and incompatibility between different minerals. Most of the univariant assemblages formed during retrograde metamorphism. The lack of stable isotope alteration of the carbonates explains the small values of prograde and retrograde time-integrated flux.

Scotland, Natural history, Rocks, Carbonate, Carbonate rocks

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Geobarometers involving clinopyroxene, garnet, plagioclase, ilmenite, rutile, sphene and quartz: estimation of pressure in quartz-absent assemblages

Article Abstract:

Andratite's and hedenbergite's entropy and enthalpy of formation were determined using stability relations and experimental data. The findings for hedenbergite was used together with thermodynamic data from related phases to construct geobarometers based on three reactions. The pressure values from two geobarometers, one containing rutile, he other, sphene, corresponds well with data from a geobarometer involving quartz. This implies that geobarometers containing rutile and sphene can be employed in pressure measurement in assemblages without quartz.

Measurement, Environmental aspects, Crystals, Pressure, Rutile, Minerals (Geology), Rocks, Pressure measurement, Rock-forming minerals

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