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Carbonation reactions for mantle lherzolite and harzburgite

Huang, Wuu-Liang and Wyllie, Peter J. (1984) Carbonation reactions for mantle lherzolite and harzburgite. In: Petrology: Igneous and metamorphic rocks. Proceedings of the 27th International Geological Congress. No.9. VNU Science Press , Utrecht, The Netherlands, pp. 455-473. ISBN 9067640093.

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The synthetic system CaO-MgO-SiO_2-CO_2 contains assemblages corresponding to lherzolite and harzburgite: forsterite(Fo)+ orthopyroxene(Opx)+clinopyroxene (Cpx) and Fo+enstatite(En). Fully carbonated, these are transformed into siliceous limestones: dolomite(Do)+magnesite(Mc)+quartz/coesite(Qz) and MC+Qz, respectively, which can coexist with free CO_2. Partly carbonated, they form dolomite-lherzolite (Do+Fo+Opx+Cpx) and magnesite-harzburgite (MC+Fo+En) which cannot coexist with CO_2, but can coexist with (H_2O+CO_2), buffered to high H_2O/CO_2. Review of experimental and theoretical data for decarbonation reactions and pyroxene-carbonate exchange reactions in CaO-MgO-SiO_2-CO_2 shows one invariant point Q_1 for Opx+Cpx+Do+Mc+ Qz+CO_2 near 1090°C and 34 kb; and another Q_2 for Cc+Do+Cpx+Opx+Fo+CO_2 between the 7 kb and 13 kb, 800°C and 950°C. Dolomites in the high-pressure lherzolite reaction (6): Do/Cc+Opx= Fo+Cpx+CO_2 contain more than 70% CaCO_3, at 1200°C and 27 kb; below the crest of the calcite-dolomite solvus, the reaction for carbonate-lherzolite is represented by (6A) with magnesian calcite down to Q_2, and by (6B) with near-stoichiometric dolomite below Q_2. Within limited depth intervals in the upper mantle with H_2O present, solid magnesite-harzburgite can coexist with partially melted lherzolite. Eruption of kimberlite could transport xenoliths of lherzolite and magnesite-harzburgite, with subsequent disruptive dissociation of magnesite and dispersal of the low-calcium garnet characteristic of depleted harzburgites from the deep roots of cratons. This result is consistent with the explanation of Boyd and Gurney for the correlation of such garnets with diamond-bearing kimberlites.

Item Type:Book Section
Additional Information:© 1984 VNU Science Press. This research was supported by the Earth Sciences Section of the National Science Foundation, EAR-8311758. We thank R.C. Newton for consultations, and E. Woermann for discussions.
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Series Name:Proceedings of the 27th International Geological Congress
Issue or Number:9
Record Number:CaltechAUTHORS:20160406-154807836
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:65975
Deposited By: Tony Diaz
Deposited On:06 Apr 2016 23:11
Last Modified:03 Oct 2019 09:52

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