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Peridotite, kimberlite, and carbonatite explained in the system CaO-MgO-SiO_2-CO_2

Wyllie, Peter J. and Huang, Wuu-Liang (1975) Peridotite, kimberlite, and carbonatite explained in the system CaO-MgO-SiO_2-CO_2. Geology, 3 (11). pp. 621-624. ISSN 0091-7613. doi:10.1130/0091-7613(1975)3<621:PKACEI>2.0.CO;2.

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The key to the origin of carbonatite and kimberlite lies in the system CaO-MgO-SiO_2-CO_2. Increase in pressure causes a carbonation reaction in the peridotite assemblage as follows: forsterite + clinopyroxene + CO_2 ⇌ orthopyroxene + carbonate (Ca:Mg::70:30). This reaction passes through 15 kb–960°C with slope 45 b/°C and terminates at an invariant point near 25 kb-1200°C, where melting begins. This intersection of the carbonation reaction with the solidus introduces primary carbonate minerals alongside peridotite minerals on the liquidus surface. At 20 kb the melting temperature of the peridotite assemblage Fo + Opx + Cpx is lowered 75°C by solution of about 5 wt percent CO_2. The liquid corresponds to undersilicated basic magma. Stabilization of carbonate increases CO_2 solubility in the liquid, and above 25 kb the liquidus reaction involving Fo + Opx + Cpx + CO_2 sweeps down through 400°C via a pressure maximum at 32 kb to meet the invariant point at 25 kb. The peridotite solidus curve at higher pressures involves fusion of silicates and carbonates, producing a carbonatitic liquid with more than 45 wt percent CO_2. Progressive fusion produces a kimberlitic liquid. There is an intricate series of reactions between 25 kb and 35 kb involving changes in silicate and carbonate phase fields on the CO_2-saturated liquidus surface. Fractional crystallization of CO_2-bearing under-silicated basic magmas at most pressures yields residual kimberlite and carbonatite. Kimberlite and carbonatite magmas rising from the asthenosphere evolve CO_2 as they reach a reaction boundary at a depth of about 100 to 80 km. This contributes to their explosive eruption. Free CO_2 cannot coexist with subsolidus mantle peridotite with normal temperature distributions. CO_2 appears to be as effective as H_2O in causing incipient melting in the asthenosphere.

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Alternate Title:Peridotite, kimberlite, and carbonatite explained in the system CaO-MgO-SiO2-CO2
Additional Information:© 1975 Geological Society of America. Manuscript received July 3, 1975; Manuscript accepted Aug. 12, 1975. Reviewed by A. T. Anderson and J. R. Goldsmith. Supported by National Science Foundation Grant GA-41730, and by the Materials Research Laboratory of the National Science Foundation.
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Issue or Number:11
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Official Citation:Peridotite, kimberlite, and carbonatite explained in the system CaO-MgO-SiO2-CO2 Geology, November, 1975, v. 3, p. 621-624, doi:10.1130/0091-7613(1975)3<621:PKACEI>2.0.CO;2
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:65579
Deposited By: Tony Diaz
Deposited On:25 Mar 2016 03:08
Last Modified:10 Nov 2021 23:47

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