HR: 14:25h
AN: DI43A-04    [Abstracts]
TI: Melting of Hydrous, Carbonate-bearing Mantle Peridotite
AU: * Gudfinnsson, G H
EM: g.gudfinnsson@uni-bayreuth.de
AF: Bayerisches Geoinstitut, University of Bayreuth, Bayreuth, D-95440, Germany
AU: Keshav, S
EM: keshav@uni-bayreuth.de
AF: Bayerisches Geoinstitut, University of Bayreuth, Bayreuth, D-95440, Germany
AU: Presnall, D C
EM: dpresnall@ciw.edu
AF: Department of Geosciences, The University of Texas at Dallas, P.O. Box 830688, Richardson, TX 75083-0688, United States
AB: At least some carbonatites contain significant amounts of H₂O. This could have important implications for phase relations at the generation of primary carbonatite melts and greatly reduce the solidus temperature of the source. The trace element systematics of carbonatites and the metasomatism associated with their emplacement in the lithosphere could also be affected. There are strong indications that primary carbonatitic melts are produced by very small amount of melting in the asthenosphere, which is experimentally most easily studied in simplified systems. Dalton and Presnall (1998, Contrib. Mineral. Petrol. 131, 123-135) investigated the problem of carbonatite generation by determining the isobarically invariant solidus of carbonate-bearing (dolomite (dmt) at pressures up to 4.8 GPa and magnesite (mst) at higher pressures) garnet lherzolite in the system CaO-MgO-Al₂O₃-SiO₂-CO₂ (CMAS-CO₂) at 3-7 GPa. In this pressure range, the melts are magnesiocarbonatitic with CaO/(CaO+MgO) of 0.67 and 0.53 at 3 and 7 GPa, respectively. We have initiated an experimental study in this same pressure range to determine the phase equilibria involving the same phase assemblage in the system CMAS-CO₂-H₂O. In these experiments, the phase equilibria are isobarically univariant. The resulting surface in P-T space, along which CO₂- and H₂O-bearing melts coexist with the crystalline phase assemblage, fo+en+di+gt+mst/dmt, is bounded at the high-temperature side by the carbonate-bearing garnet lherzolite solidus in the CMAS-CO₂ system. As temperatures become lower along the surface, the amount of H₂O in the melt steadily increases. So far, we have not reached the low- temperature limits of the surface, and with H₂O content of the melts up to >10 wt%, saturation of a water-rich phase has still not occurred nor is liquid immiscibility seen. The quenched melts produced in the experiments are composed of an intergrowth of carbonate and silicate. Because all indications are that H₂O exsolves to form a fluid phase during quenching, we have to estimate the H₂O content in the melt on the assumption that all the H₂O in the starting composition is retained in the sample capsule during experiments and that the amount of H₂O incorporated in the crystalline phases is minor compared to its amount in the melt phase. In an experiment at 3.5 GPa, 1100°C, which is 170°C lower than the solidus in the CMAS-CO₂ system at the same pressure, the melt contains about 10 wt% H₂O. The composition of the melt has shifted toward more calcic composition than anhydrous melts at the same pressure, and with CaO/(CaO+MgO) of 0.70 the composition is starting to approach that of a calciocarbonatite (CaO/(CaO+MgO) >0.8). The amount of SiO₂ is also very low, only 1-2 wt%. Hence, it is possible that at lower pressures and with even larger amounts of H₂O, calciocarbonatites could be produced by melting of carbonated mantle peridotite. The few data available from higher pressures (up to 7 GPa) indicate that the effect of H₂O on melt composition and solidus depression is smaller as pressure increases.
DE: 3619 Magma genesis and partial melting (1037)
DE: 3630 Experimental mineralogy and petrology
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting