Experimental Pt Phase Relations of Silicic, Alkaline, Aluminous Glasses Trapped in Mantle Xenoliths

David S. Draper and Trevor H. Green, Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC), School of Earth Sciences, Macquarie University, Sydney NSW 2109 Australia

Many mantle xenoliths contain silicate glasses, trapped as discrete phases, whose compositions would at first glance seem difficult to account for via trapping of host basaltic liquid, melting of mantle materials (with or without a volatile flux), or melting of typical hydrated mantle (i.e., amphibole- or phlogopite-bearing). These

glasses have the following ranges of major-element composition (wt. %): SiO2, 55-65; TiO2, 0.2-2.0; Al2O3, 18-24; FeO*, 0.5-1.5; MgO, 0.75-1.5; CaO, 2-5; Na2O, 3-6; and K2O, 4.5-7.0. The (very few) published trace element data on these glasses show enrichments in incompatible trace elements such as light rare earths and high field-strength elements. Schiano and coworkers (e.g. Nature 368:621-624) have also identified glasses having very similar bulk compositions but occurring as melt inclusions in xenolith minerals. These two separate investigations are consistent with the view that these compositions represent a type of metasomatic agent (in addition to CO2- and H2O-rich fluids and carbonate liquids). Accordingly, we have performed a series of experiments on three such liquid compositions in order to identify the mineral assemblage(s) with which they could coexist. It is our goal to place major-element, phase-equilibrium constraints on this type of mantle metasomatism. Experiments were run at pressures ranging from 1.0 to 3.0 GPa under both anhydrous and COH-fluid-saturated

conditions; in the latter, fluid compositions were either XH2O = 1.0 or XH2O = 0.5. Under anhydrous conditions, two of our three studied liquid compositions coexist with mantle-like phases (spinel, Fo-rich olivine, En-rich orthopyroxene, Di-rich clinopyroxene, Py-rich

garnet), and one of them shows near-liquidus saturation with olivine, orthopyroxene, and clinopyroxene at P = 1.0 to 1.2 GPa. Under water-saturated conditions, phlogopitic mica is the liquidus phase for all three compositions at all pressures investigated. At fluid saturation with XH2O = 0.5, near-liquidus phase relations are similar to the anhydrous case; however, further beneath the liquidus initially-crystallizing mafic phases react with liquid to form phlogopite at pressures up to 2.0 GPa. The near-liquidus mineralogy persists over a very large temperature range--approximately 900 to 1100 C--and overlaps the range of temperatures thought to prevail in the upper mantle. At 3.0 GPa, CO2 solubility in the melt greatly depresses the liquidus surface--liquidus temperatures at this pressure are ~1000-1050 C compared to 1100-1125 C at 2.0 GPa--and near-liquidus mafic phases (garnet, orthopyroxene) give way to carbonates (magnesite-siderite solid solutions and ferroan dolomite) and kyanite rather than to phlogopite as at lower pressures.

These results are somewhat surprising; most workers would not consider liquids so rich in silica, alumina, and the alkalies to be saturated in mantle minerals. The near-liquidus saturation, however, implies that the studied liquids can coexist with the saturating minerals. Because these minerals are like those typical of depleted mantle, it appears clear that these liquids could easily migrate through such mantle material without undergoing large-scale, bulk compositional change via wallrock reaction. Therefore, they could serve as effective metasomatic agents because they could survive to convey whatever trace elements they might carry from one mantle region to another, rather than be forced to react with mantle minerals or to freeze into immobility. Another implication of the near-liquidus phase relations is that the protolith(s) for these liquids could be either hydrated mantle material, or possibly an eclogitic assemblage (garnet and aluminosilicate at 3.0 GPa), but this interpretation is subject to clarification from additional experimentation. Finally, the appearance of carbonates near the high-pressure liquidii for these compositions suggests that it may be possible to find conditions where there is overlap between silicate and carbonate agents of metasomatism.