The effect of material parameters on the shape of the phase separation surfaces within the earth's mantle

Details The effect of material parameters on the shape of the phase separation surfaces within the earth's mantle The effect of material parameters on the shape of the phase separation surfaces within the earth's mantle

Jun 1980

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1980pepi...22..238l&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 22, Issue 3-4, p. 238-243.

Physics

Scientific paper

The effects of compressibility, thermal expansion and changes in entropy of transformations on the shape of a phase equilibrium curve (the Clausius-Clapeyron curve) have been studied. The curve has been found to take a dimensionless polynomial form and to conform to the following relations y=Cx(1 + ax + bx2), x=(T-T0)/T0, y=(P-P0)/P0 where P0 and T0 denote reference point coordinates, and coefficients C, a, b are constants dependent on material factors. Based on data from the literature, the phase equilibrium equation appears to take the following forms for the selected mineral transitions (P in GPa, T in kelvins):
(a)|for Mg2SiO4: olivine --> spinel P=7.82 + 4.57 X 10-13T-0.70X10-6T2+0.007X10-9T3,700 K(b)|for Mg2SiO4 --> SiO2 + 2 MgO (spinel --> oxides) P=25.95 - 2.81 X 10-3T+0.89 X 10-6T2+0.059 X 10-9T3,700 K(c)|for quartz --> coesite P=1.42 + 1.27 X 10-3T-0.69 X 10-6T2+0.78 X 10-9T3,600 KIt is concluded on the basis of calculations of mineral phase transitions taking place in the Earth's mantle that the influence of compressibility and thermal expansion should not be neglected. It is insignificant in the olivine --> spinel transition, but in the quartz --> coesite transition or spinel decomposition it is essential. Moreover, compressibility and thermal expansion shift the minimum of the Clausius-Clapeyron curve, e.g., in the spinel --> oxides decomposition (Fig. 3b).
Based on the equations for the phase equilibrium curves, the different shapes possibly taken by the phase separation surfaces in the Earth's mantle within the descending slab and its surroundings have been discussed. Of special interest are the olivine --> spinel transition in which the surface is shaped as an upward convexity about 60 km high, and the surface of the spinel --> oxides transition which is almost unaffected by the descending slab.

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