Experimental Petrology of a Highly Potassic Magma |
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| Authors: | EDGAR A D; GREEN D H; HIBBERSON W O |
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| Institution: | Research School of Earth Sciences, Australian National University Canberra |
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| Abstract: | The melting behaviour of a highly potassic biotite mafuriteof the Central African olivine leucitite kindred has been studiedexperimentally as a function of pressure (to 30kb) temperature,and water content (0%, 5%, 15%, 25%, and 40% H2O). Olivine isthe liquidus phase up to 30 kb for all water contents studiedexcept for anhydrous (clinopyroxene on the liquidus) and 15%H2O (phlogopite on the liquidus) conditions. Analyses of phasescrystallizing from the biotite mafurite show that pressure hasvery little effect on the composition of clinopyroxene whichis extremely calcium-rich, and low in Al2O3 and TiO2 for allconditions investigated. Phlogopite has low TiO2 content andtitanphlogopite cannot be a refractory phase in the upper mantlecausing Ti-depletion in partial melts in equilibrium with titanphlogopite.There are apparently no conditions where the extremely potassicbiotite mafurite could be a partial melt from pyrolite but derivationfrom an olivine+clinopyroxene+phlogopite+ilmenite assemblageoccurring as ‘enriched’ patches in the upper mantle,is possible. Liquids in equilibrium with phlogopite as a residualphase at  30 kb would be olivine nephelinites with approximately5% K2O, Na2O/K2O  1 and TiO2 > 5+. Crystal elutriation withtransported residual phlogopite reacting (phlogopite+liquid1  olivine+liquid 2) at lower pressures provides a mechanismfor K-enrichment and generating Na2O/K2O < 1. |
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