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 |
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| Authors: | MYSEN BJ?RN O; BOETTCHER A L |
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| Institution: | Department of Ceosciences, The Pennsylvania State University University Park, Pa. 16802, U.S. |
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| Abstract: | The system peridotite-H2O–CO2 serves as a simplified modelfor the phase relations of mantle peridotite involving morethan one volatile component. Run products obtained in a studyof phase relations of four mantle peridotites in the presenceof H2O- and (H2O+CO2)- bearing vapors and with controlled hydrogenfugacity (fH2) at high pressures and temperatures have beensubjected to a detailed chemical investigation, principallyby the electron microprobe. Mg/(Mg+ Fe) of all phases generally increases with increasingtemperature and with increasing Mg/(Mg+Fe) of the starting material.This ratio appears to decrease with increasing pressure forolivine, and for amphibole coexisting with garnet. DecreasingfH2 from that of IW buffer to that of MH buffer decreases Mg/(Mg+Fe)of the partial melt from approximately 0-85 to approximately0.50, whereas the Fo content of coexisting olivine increasesslightly less than 3 per cent and the Mg/(Mg+Fe) of clinopyroxeneincreases about 4 per cent. However, the variations in Fo contentof olivines are within those observed in olivines from naturalmantle peridotite. The chemistry of other silicate mineralsdoes not significantly reflect variations of fH2. Consequently,the peridotite mineralogy and/or chemistry is not a good indicatorfor the fH2 conditions during crystallization. All crystalline phases, except amphibole, and to some extentgarnet, show increasing Cr content with increasing temperatureand increasing Cr content of the starting material, resultingin a positive correlation with Mg/(Mg+Fe). Partial melts aredepleted in Cr2O3 relative to the crystalline phases. High Mg/Mg+Fe)and Cr2O3 are thus expected in crystal residues after partialmelting. The absolute values depend on degree of melting andthe composition of the parent peridotite. Liquids formed by anatexis of mantle peridotite are andesiticunder conditions of XH2Ov > 0.6 to at least 25 kb total pressureand to more than 200?C above the peridotite solidus. This observationsupports numerous suggestions that andesite genesis in islandarcs may result from partial melting of underlying peridotitemantle. In contrast to basaltic rocks, the absence of amphibole(paragasitic hornblende) does not affect the silica-saturatednature of the liquids. Increasing K2O content of the startingmaterial (up to 1 wt. per cent K2O) results in increasing potassiumcontent of the amphibole ( 1 wt. per cent K2O) as well as theappearance of phlogopite. The liquid under these conditionsis relatively K20-poor (less than 1 wt. per cent K2O). Partial melts are olivine normative with XH2O ![{els]](http://petrology.oxfordjournals.org/math/els.gif) 0.5, and initialliquids contain normative ol and ne at XH2O 0.4. The alkalinityof these liquids increases with decreasing XH2O below valuesof 0.5. The (ol+opx)-normative liquids resemble oceanic basaltswhereas (ol+ne)-normative liquids resemble olivine nepheliniteand melilite basalt. Low aHlo and high aCo2 conditions may bethose under which kimberlites and related rocks are formed inthe mantle. |
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