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Kimberlite Magmas from the System Peridotite-CO_2-H_2O

Wyllie, Peter J. (1979) Kimberlite Magmas from the System Peridotite-CO_2-H_2O. In: Kimberlites, diatremes, and diamonds: their geology, petrology, and geochemistry. Vol.1. American Geophysical Union , Washington, DC, pp. 319-329. http://resolver.caltech.edu/CaltechAUTHORS:20160308-092317207

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Abstract

Phlogopite-dolomite-peridotite is the most promising source rock for kimberlites and related magmas. At pressures above about 30 kb, very little CO_2 (low CO_2/H_2O in vapor) is required to produce dolomite in mantle peridotite. If oxygen fugacity is too low, however, CO_2 and carbonate are reduced to carbon, and dolomite is unable to exert its distinctive influence on magma compositions. The oxygen fugacity at various depths in the mantle is a critical factor. Rare diamonds and even rarer carbonates occur in peridotite nodules from kimberlite, and CO_2 is brought to the surface in mantle-derived minerals and lavas. Phase relationships in peridotite-CO_2-H_2O provide a first step for evaluation of the behavior of components C-H-O at depth. Experimental and theoretical data from various sources have been combined for analysis of the near-solidus phase relationships in peridotite-CO_2-H_2O. The divariant solidus surface is traversed by a series of univariant lines where the vapor phase is buffered by amphibole, dolomite (magnesite at higher press ures), phlogopite, or combinations of these. The lines limit the range of vapor-phase compositions that can coexist with peridotite at various pressures. The buffering capacity of dolomite is far greater than that of the hydrous minerals. The buffered curves for partly carbonated peridotite, with and without phlogopite, extend to lower temperatures and higher pressures from an invariant point near 26 kb and 1200°C. Near this line there is a temperature maximum (a ridge) on the solidus surface, separating the low-pressure surface, where CO_2/H_2O in vapor is higher than in liquid, from the high-pressure surface, where CO_2/H_2O in vapor is lower than in liquid. Enrichment of the high-pressure liquids in CO_2 is associated with the generation of dolomite and low- SiO_2 liquids. Because of this maximum on the solidus, near-solidus magmas rising along an adiabat would evolve volatile components in the depth interval 100-80 km, which could contribute to the explosive eruption of kimberlites. The subcontinental upper mantle is probably heterogeneous with respect to incompatible elements, because local melting due to sparsely distributed CO_2 and H_2O (dolomite and phlogopite) is followed by magmatic flushes, as the melt migrates upwards.


Item Type:Book Section
Alternate Title:Kimberlite Magmas from the System Peridotite-CO2-H2O
Additional Information:© 1979 American Geophysical Union. This research was supported by the Earth Sciences Section, National Science Foundation Grant EAR 76-20410.
Funders:
Funding AgencyGrant Number
NSFEAR 76-20410
Record Number:CaltechAUTHORS:20160308-092317207
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20160308-092317207
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:65183
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:14 Mar 2016 23:57
Last Modified:14 Mar 2016 23:57

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