Low-Pressure Experimental Constraints on the Evolution of Komatiites |
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Authors: | THY P |
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Institution: | DEPARTMENT OF GEOLOGY, UNIVERSITY OF CALIFORNIA DAVIS, CA 95616, USA |
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Abstract: | Melting experiments were performed on a komatiitic basalt with17 wt% MgO from Munro Township, Ontario, at I-atm pressure andan oxygen fugacity controlled approximately to the fayalite-magnetite-quartzbuffer. The experiments showed that olivine appears at 1344±5°C,spinel at 1334±6°C plagioclase at 1185±5°C,augite at 1176±5°C and pigeonite at 1154±6°C.Compositionally, olivine varies from Fo90 to Fo74 and displaysan average KFe/MgD (ol/liq) of 032. The spinels are chromitesand chromian spinels with Mg/(Mg + Fe2+) ratios between 066and 0;32, which show a marked correlation with meltingtemperature. The pyroxenes show an average KFe/MgD (px/liq)of 026, identical for augite and pigeonite. Plagiodaseranges compositionally between An82 and An72 Plotted in thepseudo-quaternary basalt phase diagram, the liquid line of descentis similar to that observed for quartz tholeiitic magmas. Therefore,the low-pressure, late-stage evolution products of komatiiteand basaltic komatiite parental magmas will chemically and mineralogicallybe ferrobasaltic quartz tholeiites. High-temperature and high-pressuremodeling suggests that the main observed compositional variationof Munro komatiites can be explained by low-pressure crystalfractionation and accumulation of olivine into komatiite liquidswith below 215235 wt% MgO and eruptive temperaturesbelow 14351465°C for oxygen fugacities between thefayalite-magnetite quartz (FMQ) and iron-wiistite (IW) buffers.The maximum magnesium content of liquid komatiites, assumingequilibrium Fo94 olivine, is 2729 wt% MgO and eruptivetemperatures are between 1515 and 1540°C. KEY WORDS: komatiites; experimental petrology; Munro Township; Ontario |
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