Melts in the mantle modeled in the system CaO-MgO-SiO2-CO2 at 2.7 GPa |
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Authors: | W J Lee W L Huang P Wyllie |
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Institution: | (1) Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA e-mail: wjl@alumni.caltech.edu Tel.: +1-626-282-2000; Fax: +1-626-282-7569, US;(2) Department of Geology, National Taiwan University, Taipei, Taiwan, ROC, TW;(3) Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA, US |
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Abstract: | The effect of CO2 on mantle peridotites is modeled by experimental data for the system CaO-MgO-SiO2-CO2 at 2.7 GPa. The experiments provide isotherms for the vapor-saturated liquidus surface, bracket piercing points for field
boundaries on the surface, and define the positions and compositions of isobaric invariant liquids on the boundaries (eutectics
and peritectics). CO2-saturated carbonatitic liquids (>80% carbonate) exist through approximately 200 °C above the solidus, with a transition to
silicate liquids (>80% silicate) within ∼75 °C across a plateau on the liquidus. Carbonate-rich magmas cannot cross the silicate-carbonate
liquidus field boundary, so the carbonate liquidus field is therefore a forbidden volume for liquid magmas. This confirms
the fact that rounded, pure carbonates in mantle xenoliths cannot represent original liquids. A P-T diagram is constructed
for the carbonation and melting reactions for mineral assemblages corresponding to lherzolite, harzburgite, websterite and
wehrlite, with carbonate, CO2 vapor (V), or both. The changing compositions of liquids in solidus reactions on the P-T diagram are illustrated by the changing
compositions of eutectic and peritectic liquids on the liquidus surface. At an invariant point Q (∼2.8 GPa/1230 °C), all peridotite
assemblages coexist with a calcite-dolomite solid solution (75 ± 5% CaCO3) and a dolomitic carbonatite melt 57% CaCO3 (CC), 33% MgCO3 (MC), 10% CaMgSi2O6 (Di)], with 63% CC in the carbonate component. At higher pressures, dolomite-lherzolite, dolomite-harzburgite-V, and dolomite-websterite-V
melt to yield similar liquids. Magnesian calcite-wehrlite is the only peridotite melting to carbonatitic liquids (more calcic)
at pressures below Q (∼70 km). Dolomitic carbonatite magma rising through mantle to the near-isobaric solidus ledge near Q
will begin to crystallize, releasing CO2 (enhancing crack propagation), and metasomatizing lherzolite toward wehrlite.
Received: 20 March 1998 / Accepted: 7 July 1999 |
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