NaSi⇌CaAl exchange equilibrium between plagioclase and amphibole |
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Authors: | Frank S Spear |
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Institution: | (1) Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, 02139 Cambridge, Massachusetts, USA |
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Abstract: | The exchange equilibrium between plagioclase and amphibole, 2 albite+tschermakite=2 anorthite+glaucophane, has been calibrated empirically using data from natural amphibolites. The partition coefficient, K
D, for the exchange reaction is (X
an/X
ab)plag ·(Na, M4/Ca, M4)amph.. Partitioning is systematic between plagioclase and amphibole in suites collected from single exposures, but the solid solutions are highly non-ideal: values of In K
D range from –3.0 at X
an=0.30 to –1.0 at X
an=0.90 in samples from a single roadcut. Changes in both K
D and the topology of the ternary reciprocal exchange diagram occur with increasing metamorphic grade. Temperature dependence of In K
D is moderate with ¯H35 to 47 kcal at X
an=0.25; pressure dependence is small with ¯V –0.24 cal/bar. Usefulness of this exchange equilibrium as a geothermometer is restricted by uncertainties in the calculation of the amphibole formula from a microprobe analysis, especially with regard to Na, M4 in amphibole, to approximately ±50 ° C. |
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Keywords: | |
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