Experimental study of the stability of cordierite and garnet in pelitic compositions at high pressures and temperatures |
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| Authors: | Dr B J Hensen D H Green |
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| Institution: | (1) Department of Geophysics and Geochemistry, Australian National University, Australia;(2) Geophysical Laboratory, 2801 Upton Str., 20008 N.W., Washington, D.C., USA |
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| Abstract: | The stability of cordierite and garnet has been studied experimentally in complex, silica oversaturated compositions (in the systems MgO-FeO-Al2O-CaO3-Na2O-K2OSiO2) in which the molecular ratio Al2O3/FeO+MgO<1. Compositions with 100 Mg/Mg+Fe2+ ratios (X) of 0, 30, 50, 70 and 100 have been used to investigate the role of this ratio in determining phase assemblages and P, T coordinates of reactions. The minimum pressure for appearance of garnet at a given temperature is strongly dependent on X
total rock.The X-values of co-existing phases (chiefly garnet, cordierite, hypersthene) in divariant equilibrium are a function of temperature and pressure and have been experimentally determined at 900° C, 1000° C and 1100° C. At high temperature (>1050° C) the phases sapphirine and spinel are stable with quartz in Mg-rich and Fe-rich compositions respectively. Experiments in the system MgO-FeO-Al2O3-SiO2 show that for a given X-value and temperature the pressure required to produce Ca-free garnet from hypersthene-cordierite assemblages is 1–2 kb greater than that required to produce garnet containing 6±2 mol percent grossular solid solution in the more complex Ca-bearing system. |
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