Volatile Components, Magmas, and Critical Fluids in Upwelling Mantle |
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| Authors: | WYLLIE PETER J; RYABCHIKOV IGOR D |
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| Institution: | 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 |
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| Abstract: | The phase diagram for lherzolite–CO2–H2O providesa framework for interpreting the distribution of phase assemblagesin the upper mantle with various thermal structures, in differenttectonic settings. Experiments show that at depths >80 km,the near-solidus partial melts from lherzolite–CO2–H2Oare dolomitic, changing through carbonate–silicate liquidswith rising temperatures to mafic liquids; vapor, if it coexists,is aqueous. Experimental data from simple systems suggest thata critical end-point (K) occurs on the mantle solidus at anundetermined depth. Isobaric (T–X) phase diagrams forvolatile-bearing systems with K elucidate the contrasting phaserelationships for lherzolite–CO2–H2O at depths belowand above a critical end-point, arbitrarily placed at 250 km.At levels deeper than K, lherzolite can exist with dolomiticmelt, aqueous vapor, or with critical fluids varying continuouslybetween these end-members. Analyses of fluids in microinclusionsof fibrous diamonds reveal this same range of compositions,supporting the occurrence of a critical end-point. Other evidencefrom diamonds indicates that the minimum depth for this end-pointis 125 km; maximum depth is not constrained. Constructed cross-sectionsshowing diagrammatically the phase fields intersected by upwellingmantle indicate how rising trace melts may influence trace elementconcentrations within a mantle plume. KEY WORDS: mantle solidus; critical end-point; dolomitic magma; diamond inclusions; critical fluids |
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