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.