Liquidus equilibria of some primary lunar and terrestrial melts in the garnet stability field

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Melting experiments conducted at 28 and 30 kbar on synthetic starting materials with compositions appropriate to terrestrial (depleted) and lunar mantles (undifferentiated and cumulate) show that (1) low-degree melts of mantles with lherzolitic mineral assemblages (olivine + orthopyroxene + clinopyroxene + garnet) will be (picritic) tholeftic to transitional in character; (2) the melting equilibrium is o1 + gar + CPX = opx + liq over a wide range of MgO/FeO; (3) the clinopyroxene coexisting with low-degree melts is low in CaO (~7 wt%), as predicted by Bertka and Holloway (1993); (4) the solubility of aluminous components in pyroxene is much greater than at low pressure; and (5) the CaO concentration in clinopyroxenes coexisting with olivine and garnet in various runs ranges continuously from 7 to 16 wt%. The data place the following constraints on magma genesis: (1) polybaric fractional fusion of lunar and depleted terrestrial sources with olivine, two pyroxenes, and an aluminous phase will produce pooled melts that crystallize olivine, augite, and plagioclase before low-Ca pyroxene at low-pressure (i.e., tholeiites); (2) the absence of tholeftic lavas or cumulates on the Moon indicates high-degree melts and/or no residual aluminous phase in lunar source regions; and (3) the low-Ca clinopyroxene on the high-pressure lherzolite solidus is a super-critical clinopyroxene that appears on the anhydrous lherzolite solidus with increasing pressure when the of + pig + aug liquidus boundary curve develops a critical end point. This critical end point terminates the high-Al 2 O 3 end of the of + pig + aug liquidus curve. As the critical end point migrates to the Opx-Wo join, it progressively eliminates the of + pig + aug liquidus curve, which disappears entirely by about 20 kbar.[/a]

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