Melting phase relations of a mica–clinopyroxenite

Authors: Sean P Funk Robert W Luth

Publish Date: 2013/05/08

Volume: 166, Issue: 2, Pages: 393-409

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Abstract

Melting experiments were conducted on a mica–clinopyroxenite xenolith brought up in a minette dyke in southern Alberta Canada near Milk River Both the minettes and mica–clinopyroxenite xenoliths were studied by Buhlmann et al Can J Earth Sci 371629–1650 2000 who hypothesized that the minettes formed by partial melting of a mantle source containing clinopyroxene + phlogopite ± olivine at pressures ≥17 GPa In liquidus experiments performed on the most primitive minette in our previous study Funk and Luth in Contrib Mineral Petrol 164999–1009 2012 we found a multiple saturation point where olivine and orthopyroxene coexisted with liquid at 177 GPa and 1350 °C We argued that the minette originally formed by partial melting of clinopyroxene + phlogopite but had reequilibrated with a harzburgite during ascent In the current study we wanted to test both the source region hypothesis of Buhlmann et al and our reequilibration hypothesis by studying the nearsolidus phase equilibria of a mica + clinopyroxene assemblage We found the solidus for our xenolith has a steep slope in P–T space and lies at temperatures above those of a normal cratonic geotherm implying that this mica–clinopyroxenite is stable in the cratonic mantle Melting could occur at greater depths where the solidus is extrapolated to cross the geotherm or must be induced either by raising the temperatures of the surrounding rocks or by introducing hydrous fluids into the source Our melts are in equilibrium with clinopyroxene and olivine The compositions of the liquids derived from melting this xenolith are similar to madupitic lamproites from the Leucite Hills Wyoming studied by Carmichael Contrib Mineral Petrol 1524–66 1967 and Barton and Hamilton Contrib Mineral Petrol 6641–49 1978 Contrib Mineral Petrol 69133–142 1979 Barton and Hamilton Contrib Mineral Petrol 69133–142 1979 proposed that the madupitic lamproites may have come from a source containing mica and pyroxene This study supports their hypothesis The composition of the most primitive minette from southern Alberta lies between our experimental melt and a population of representative mantle orthopyroxenes We conclude from our study that the Milk River minettes were likely derived from a source containing phlogopite clinopyroxene and trace amounts of apatite which formed olivine upon melting During ascent the melts changed composition by reacting with orthopyroxeneThis research was funded by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada NSERC to R Luth and by a NSERC Postgraduate Scholarship fund to S Funk We would also like to thank S Matveev for his extensive work and expertise on the microprobe analyses and to T Chacko and C Currie for their helpful discussions We gratefully acknowledge reviews by Sebastien Pilet and an anonymous reviewer which helped us improve the manuscript

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