Experimental study of Fe-Mg- and Ca-distribution between coexisting ortho- and clinopyroxenes at P=294 MPa,T=750 and 800° C |
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Authors: | V I Fonarev A A Graphchikov |
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Institution: | (1) Institute of Experimental Mineralogy, USSR Acad. Sci., 142432 Chernogolovka, Moscow District, USSR |
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Abstract: | The Fe-Mg-Ca-distribution was investigated in synthesis experiments and with the mineral assemblage orthopyroxene+clinopyroxene+quartz. The phase compositions were identified by X-ray diffraction and, where possible, by electron microprobe. The attainment of equilibrium in the run products was signalled by the compositions from control runs (different solutions) becoming closely similar, by recycling runs, by the attainment of equilibrium from different directions (depending on the composition of starting phases), and by special kinetic experiments.The study produced the following results: (1) the Ca content of the clinopyroxenes decreases with increasing Fe (mol%) from 48.4 at X
Cpx
Fe
=5 to 39.8 at X
Cpx
Fe
=45 (800° C); from 47.6 at X
Cpx
Fe
= 10 to 41.7 at X
Cpx
Fe
=45 (750° C); increasing temperature expands the stability field of the less calcic clinopyroxenes. (2) The Ca content of orthopyroxenes increases slightly with Fe content from 1.8 at X
Opx
Fe
=20.5 to 3.2 at X
Opx
Fe
=75; the temperature effect on the Ca content under the T, P conditions of the experiment was not large. (3) Fe and Mg distribution between the coexisting ortho-and clinopyroxenes is largely temperature-dependent, particularly in the compositional range X
Opx
Fe
=15–75 mol%; as T increases, Fe redistributes from the rhombic to monoclinic mineral.Preliminary estimates of rock formation temperatures using the obtained data show that most of the known two-pyroxene geothermometers overstate the actual values by 50–150° C. |
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Keywords: | |
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