Melting of a Hydrous Mantle: II. Geochemistry of Crystals and Liquids Formed by Anatexis of Mantle Peridotite at High Pressures and High Temperatures as a Function of Controlled Activities of Water, Hydrogen, and Carbon Dioxide
The system peridotite-H 2 O–CO 2 serves as a simplified model for the phase relations of mantle peridotite involving more than one volatile component. Run products obtained in a study of phase relations of four mantle peridotites in the presence of H 2 O- and (H 2 O+CO 2 )- bearing vapors and with controlled hydrogen fugacity ( f H 2 ) at high pressures and temperatures have been subjected to a detailed chemical investigation, principally by the electron microprobe. Mg/(Mg+∑Fe) of all phases generally increases with increasing temperature and with increasing Mg/(Mg+∑Fe) of the starting material. This ratio appears to decrease with increasing pressure for olivine, and for amphibole coexisting with garnet. Decreasing f H 2 from that of IW buffer to that of MH buffer decreases Mg/(Mg+∑Fe) of the partial melt from approximately 0-85 to approximately 0.50, whereas the Fo content of coexisting olivine increases slightly less than 3 per cent and the Mg/(Mg+∑Fe) of clinopyroxene increases about 4 per cent. However, the variations in Fo content of olivines are within those observed in olivines from natural mantle peridotite. The chemistry of other silicate minerals does not significantly reflect variations of f H 2 . Consequently, the peridotite mineralogy and/or chemistry is not a good indicator for the f H 2 conditions during crystallization. All crystalline phases, except amphibole, and to some extent garnet, show increasing Cr content with increasing temperature and increasing Cr content of the starting material, resulting in a positive correlation with Mg/(Mg+∑Fe). Partial melts are depleted in Cr 2 O 3 relative to the crystalline phases. High Mg/Mg+∑Fe) and Cr 2 O 3 are thus expected in crystal residues after partial melting. The absolute values depend on degree of melting and the composition of the parent peridotite. Liquids formed by anatexis of mantle peridotite are andesitic under conditions of X H 2 O v > 0.6 to at least 25 kb total pressure and to more than 200°C above the peridotite solidus. This observation supports numerous suggestions that andesite genesis in island arcs may result from partial melting of underlying peridotite mantle. In contrast to basaltic rocks, the absence of amphibole (paragasitic hornblende) does not affect the silica-saturated nature of the liquids. Increasing K 2 O content of the starting material (up to 1 wt. per cent K 2 O) results in increasing potassium content of the amphibole (∼1 wt. per cent K 2 O) as well as the appearance of phlogopite. The liquid under these conditions is relatively K 2 0-poor (less than 1 wt. per cent K 2 O). Partial melts are olivine normative with X H 2 O ⪕ 0.5, and initial liquids contain normative ol and ne at X H 2 O ⪕ 0.4. The alkalinity of these liquids increases with decreasing X H 2 O below values of 0.5. The (ol+opx)-normative liquids resemble oceanic basalts whereas (ol+ne)-normative liquids resemble olivine nephelinite and melilite basalt. Low aHlo and high a Co 2 conditions may be those under which kimberlites and related rocks are formed in the mantle.