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Melts in the mantle modeled in the system CaO-MgO-SiO_2-CO_2 at 2.7 GPa

Lee, W. J. and Huang, W. L. and Wyllie, P. (2000) Melts in the mantle modeled in the system CaO-MgO-SiO_2-CO_2 at 2.7 GPa. Contributions to Mineralogy and Petrology, 138 (3). pp. 199-213. ISSN 0010-7999. http://resolver.caltech.edu/CaltechAUTHORS:20160112-142051569

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Abstract

The effect of CO_2 on mantle peridotites is modeled by experimental data for the system CaO-MgO-SiO_2-CO_2 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). CO_2-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, CO_2 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% CaCO_3) and a dolomitic carbonatite melt [57% CaCO_3 (CC), 33% MgCO_3 (MC), 10% CaMgSi_2O_6 (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 CO_2 (enhancing crack propagation), and metasomatizing lherzolite toward wehrlite.


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http://dx.doi.org/10.1007/s004100050557DOIArticle
http://link.springer.com/article/10.1007/s004100050557PublisherArticle
Alternate Title:Melts in the mantle modeled in the system CaO-MgO-SiO2-CO2 at 2.7 GPa
Additional Information:© 2000 Springer-Verlag. Received: 20 March 1998; Accepted: 7 July 1999. We thank J. Dalton for review of the manuscript. This research was supported by the Earth Science section of the United States National Science Foundation, grant EAR-9218806. This is contribution no. 8638 of the Division of Geological and Planetary Sciences, California Institute of Technology.
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NSFEAR-9218806
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Caltech Division of Geological and Planetary Sciences8638
Record Number:CaltechAUTHORS:20160112-142051569
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20160112-142051569
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:63602
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:12 Jan 2016 22:32
Last Modified:09 Jan 2017 01:19

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