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Calciocarbonatite and magnesiocarbonatite rocks and magmas represented in the system CaO-MgO-CO_2-H_2O at 0.2 GPa

Lee, W.-J. and Fanelli, M. F. and Cava, N. and Wyllie, P. J. (2000) Calciocarbonatite and magnesiocarbonatite rocks and magmas represented in the system CaO-MgO-CO_2-H_2O at 0.2 GPa. Mineralogy and Petrology, 68 (4). pp. 225-256. ISSN 0930-0708. doi:10.1007/s007100050011.

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The low-pressure eutectic for the coprecipitation of calcite, portlandite, and periclase/brucite (with H_2O-rich vapor) has served as a model for the existence and crystallization of carbonatite magmas. Attempts to determine conditions for the appearance of dolomite at this eutectic have been unsuccessful. We have discovered a second low-temperature eutectic for more magnesian liquids which excludes portlandite and includes dolomite (all results are vapor-saturated). Addition of Ca(OH)_2-Mg(OH)_2 to CaCO_3-MgCO_3 at 0.2 GPa depresses the liquidus to temperatures below the crest of the calcite-dolomite solvus; the vapor-saturated liquidus surface falls steeply, and the field boundary for liquids coexisting with calcite and periclase reaches a peritectic at 880 °C, where a narrow field for liquidus dolomite begins, extending down to the eutectic at 659 °C for the coprecipitation of calcite, dolomite and periclase (brucite should replace periclase at slightly higher pressures). The calcite liquidus is very large. The field boundary for coexistence of calcite and dolomite extends approximately in the direction from CaMg(CO_3)_2 towards Mg(OH)_2. The results illustrate conditions for the formation of mineral-specific cumulates from variable magma compositions. Hydrous (or sodic) carbonate-rich liquids with compositions from CaCO_3 to CaMg(CO_3)_2 will precipitate calcite-carbonatites first, followed by calcite-dolomite-carbonatites, with the prospect of precipitating dolomite-carbonatite alone through a limited temperature interval, and with periclase joining the assemblage in the closing stages. Periclase in the Fe-free system may represent the ubiquitous occurrence of magnetite in natural carbonatites. The restricted range for the precipitation of dolomite-carbonatites adds credibility to the evidence for primary magnesiocarbonatite (near-dolomite composition) magmas. Magnesiocarbonatite magmas can precipitate much calcite-carbonatite rock.

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Alternate Title:Calciocarbonatite and magnesiocarbonatite rocks and magmas represented in the system CaO-MgO-CO2-H2O at 0.2 GPa
Additional Information:© 2000 Springer-Verlag. Received July 20, 1998; revised version accepted August 18, 1999. We thank J. A. Dalton and R. Luth for constructive reviews. This research was supported by US National Science Foundation Grant EAR-9218806.
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Issue or Number:4
Record Number:CaltechAUTHORS:20160112-152036575
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:63609
Deposited By: Tony Diaz
Deposited On:12 Jan 2016 23:28
Last Modified:10 Nov 2021 23:18

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