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The partial melting of a phlogopite-bearing garnet lherzolite in the model mantle system SiO₂-Al₂O₃-MgO-FeO-CaO-K₂O-H₂O

J.-P. Li¹, D.H. Green and H.StC. O’Neill
¹ Visiting fellow (Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, China)

Phlogopite is a potential host for K₂O and H₂O in the metasomatized portions of the upper mantle. Many petrological and geochemical studies suggested that the phlogopite-bearing mantle lherzolite might be the source material of high-K basalts. For example, the shoshonitic rocks on the Tibetan plateau revealed that K₂O appears to be buffered at about 4% over a wide range of SiO₂ (46-60%). A possible explanation for this apparent buffering behaviour is that the shoshonites were derived from a low degree partial melting, leaving a phlogopite-bearing garnet lherzolite as residual material. To test this possible hypothesis, it is necessary to determine precisely the melt composition in equilibrium with Ph-Ol-Gt-Opx-Cpx. For this purpose, experiments have been carried out in the system SiO₂-Al₂O₃-MgO-FeO-CaO-K₂O-H₂O at a pressure of 28 kb in the piston-cylinder apparatus.

Experiments were carried out using Au₈₀Pd₂₀ as capsule, which avoided Fe loss from the charge. The sandwich method was used, in which an upper and lower phlogopite-bearing garnet lherzolite layers sandwich a middle layer of glass of the composition estimated to be in equilibrium with phlogopite-bearing gt-lherzolite. This geometrical arrangement results in pool of melt large enough to be analyzed directly by electron microprobe. The composition of the initial melt is adjusted by successive approximations until the appropriate composition is found. Starting phlogopite was synthesized at 28 kb and 850°C and the garnet lherzolite (the assemblage ol+opx+cpx+gt with mg#=90) at 28 kb and 1250°C. A phlogopite-bearing garnet lherzolite was then obtained by adding 12 wt% phlogopite. Glass was made by melting oxide mixture at 1550°C under argon in an atmospheric furnace. To ensure a molar K₂O/H₂O ratio of unity as in phlogopite, necessary H₂O was added as Mg(OH)₂.

The preliminary results show that the melt in equilibrium with a phlogopite-bearing garnet lherzolite at 1220°C and 28 kb is very rich in SiO₂(50%), K₂O(18%) and Al₂O₃(17%), and poor in MgO(3.5%), FeO(3.6%) and CaO(2.1%). Phlogopite appears to be stable to within about 30°C above the solidus. Elimination of the phlogopite with temperature initially results in only a small change in melt composition, i.e. K₂O decreases to about 17%, Al₂O₃ remains almost the same (17%), and SiO₂, MgO, FeO and CaO increase to 50.5%, 4.5%, 4.0% and 2.8%, respectively, at 1240°C. The melt should be rich in H₂O, but the precise amount needs to be obtained by IR spectroscopy.

The above results imply that mantle phlogopite-bearing garnet lherzolite would not produce shoshonitic magma by low degree partial; rather, such melts would be extremely rich in Al₂O₃, K₂O and H₂O. For confirming the suggestions above, the experiments in Na₂O-containing system are being conducted.