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Journal of Petrology Volume 41 Number 7 Pages 1195-1206 2000
© Oxford University Press 2000
Volatile Components, Magmas, and Critical Fluids in Upwelling Mantle
1DIVISION OF GEOLOGICAL AND PLANETARY SCIENCES, CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA, CA 91125, USA
2INSTITUTE FOR GEOLOGY OF ORE DEPOSITS, IGEM, RUSSIAN ACADEMY OF SCIENCES, STAROMONETNY, PEREULOK 35, MOSCOW 109017, RUSSIA
The phase diagram for lherzolite–CO2–H2O provides a framework for interpreting the distribution of phase assemblages in the upper mantle with various thermal structures, in different tectonic settings. Experiments show that at depths >80 km, the near-solidus partial melts from lherzolite–CO2–H2O are dolomitic, changing through carbonate–silicate liquids with rising temperatures to mafic liquids; vapor, if it coexists, is aqueous. Experimental data from simple systems suggest that a critical end-point (K) occurs on the mantle solidus at an undetermined depth. Isobaric (T–X) phase diagrams for volatile-bearing systems with K elucidate the contrasting phase relationships for lherzolite–CO2–H2O at depths below and above a critical end-point, arbitrarily placed at 250 km. At levels deeper than K, lherzolite can exist with dolomitic melt, aqueous vapor, or with critical fluids varying continuously between these end-members. Analyses of fluids in microinclusions of fibrous diamonds reveal this same range of compositions, supporting the occurrence of a critical end-point. Other evidence from diamonds indicates that the minimum depth for this end-point is 125 km; maximum depth is not constrained. Constructed cross-sections showing diagrammatically the phase fields intersected by upwelling mantle indicate how rising trace melts may influence trace element concentrations within a mantle plume.
KEY WORDS: mantle solidus; critical end-point; dolomitic magma; diamond inclusions; critical fluids
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