New experimental data for the system CaO-MgO-SiO2-H2O and a synthesis of inferred phase relations |
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| Authors: | Richard D Warner |
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| Institution: | Institute of Meteoritics, The University of New Mexico, Albuquerque, New Mexico 87131, U.S.A. |
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| Abstract: | Subsolidus and vapor-saturated liquidus phase relations for a portion of the system CaO-MgO-SiO2-H2O, as inferred from experimental data for the composition regions CaMgSi2O6-Mg2SiO4-SiO2-H2O and CaMgSi2O6-Mg2SiO4-Ca3MgSi2O8 (merwinite)-H2O, are presented in pressure-temperature projection. Sixteen invariant points and 39 univariant reactions are defined on the basis of the 1 atm and 10 kbar (vapor-saturated) liquidus diagrams. Lack of experimental control over many of the reactions makes the depicted relations schematic in part.An invariant point involving orthoenstatite, protoenstatite, pigeonite, and diopside (all solid solutions) occurs at low pressure (probably between 1 and 2 kbar). At pressures below this invariant point, orthoenstatite breaks down at high temperature to the assemblage diopside + protoenstatite; with increasing temperature, the latter assemblage reacts to form pigeonite. At pressures above the invariant point, pigeonite forms according to the reaction diopside + orthoenstatite = pigeonite, and the assemblage diopside + protoenstatite is not stable. At 1 atm, both pigeonite and protoenstatite occur as primary liquidus phases, but at pressures above 6–7 kbar orthoenstatite is the only Ca-poor pyroxene polymorph which appears on the vapor-saturated liquidus surface.At pressures above approximately 10.8 kbar, only diopside, forsterite, and merwinite occur as primary liquidus phases in the system CaMgSi2O6-Mg2SiO4-Ca3MgSi2O8-H2O, in the presence of an aqueous vapor phase. At pressures between 1 atm and 10.2 kbar, both akermanite and monticellite also occur as primary liquidus phases. Comparison of the 1 atm and 10 kbar vapor-saturated liquidus diagrams suggests that melilite basalt bears a low pressure, or shallow depth, relationship to monticellite-bearing ultrabasites. |
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