Volatile components, magmas, and critical fluids in upwelling mantle

The phase diagram for lherzolite CO2-H2O provides a framework for interpret ing the distributions 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 a re dolomitic, changing through carbonate-silicate liquids with rising tempe ratures to mafic liquids; vapor, if it coexists, is aqueous. Experimental d ata 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 relationsh ips 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 contin uously between these end-members. Analyses of fluids in microinclusions of fibrous diamonds reveal this same range of compositions, supporting the occ urrence of a critical end-point. Other evidence from diamonds indicates tha t the minimum depth for this end-point is 125 km; maximum depth is not cons trained. Constructed cross-sections showing diagrammatically the phase fiel ds intersected by upwelling mantle indicate how rising trace melts may infl uence trace element concentrations within a mantle plume.