HR: 0800h
AN: V31C-1444    [Abstracts]
TI: Immiscible Transition from Carbonate-rich to Silicate-rich Melts in Eclogite+CO$_{2}$ and Genesis of Ocean Island Melilitite
AU: * Dasgupta, R
EM: dasg0007@umn.edu
AF: Department of Geology & Geophysics, University of Minnesota, 310 Pillsbury Dr SE, Minneapolis, MN 55455 United States
AU: Stalker, K
EM: Stalkerk@carleton.edu
AF: Department of Geology & Geophysics, University of Minnesota, 310 Pillsbury Dr SE, Minneapolis, MN 55455 United States
AU: Hirschmann, M M
EM: hirsc022@umn.edu
AF: Department of Geology & Geophysics, University of Minnesota, 310 Pillsbury Dr SE, Minneapolis, MN 55455 United States
AB: Derivation of highly silica-undersaturated lavas such as olivine melilitites and melilite nephelinites from the mantle has been attributed to the effects of CO$_{2}$. However, experimental studies have so far failed to demonstrate equilibrium of melilititic melts with a four-phase peridotite assemblage. Instead, the liquidus mineralogy of these silica-undersaturated magmas at high-pressures appears to be dominated by cpx$^{1}$. Although, experimental partial melts from natural peridotite+CO$_{2}$ span a continuum from carbonatite to alkali-basalts$^{2}$, ocean-island melilitites have distinctly higher TiO$_{2}$, FeO*, and CaO/(CaO+MgO)$^{3,4}$ than compositions derived thus far from a carbonated lherzolite source. Partial melting experiments of a nominally anhydrous, natural eclogite with a small amount of added carbonate (SLEC1; 5 wt.% bulk CO$_{2}$) were performed to investigate the transition between carbonate and silicate melts with increasing temperature. Experiments were conducted in a piston cylinder at 3 GPa from 1050 to 1400 $\deg$C. Garnet and cpx appear in all the experiments and ilmenite is observed from 1075 to $\sim$1200 $\deg$C. An Fe-bearing calcio-dolomitic melt is present from the solidus (1050-1075 $\deg$C) up to 1375 $\deg$C. Beginning at 1275 $\deg$C, it coexists with a silica-poor silicate melt. Textural criteria indicate only a single CO$_{2}$-rich silicate melt phase at 1400 $\deg$C, coexisting with garnet and minor cpx. The liquidus temperature is estimated to be $\sim$1415 $\deg$C from the melt fraction-temperature trend. With increasing temperature, the carbonate melt becomes richer in SiO$_{2}$ ($\sim$2 to 5 wt.%) and Al$_{2}$O$_{3}$ ($\sim$0.75 to 2.25 wt.%) and poorer in CaO ($\sim$30 to 25 wt.% from $\sim$1200 to 1375 $\deg$C). Compositions of silicate partial melts change systematically with increasing temperature, increasing in SiO$_{2}$ ($\sim$36 to 41 wt.%), Al$_{2}$O$_{3}$ ($\sim$4.5 to 9.5 wt.%), MgO ($\sim$9.5 to 13 wt.%), CaO ($\sim$8 to 14 wt.%) and decreasing in TiO$_{2}$ ($\sim$14 to 2.5 wt.%), FeO ($\sim$20 to 13 wt.%), Na$_{2}$O ($\sim$3.3 to 1.7 wt.%). A wide temperature interval of coexisting carbonate and silicate partial melts of carbonated eclogite is distinct from the continuous transition from carbonate to silicate melts observed in carbonated peridotite systems$^{2,5}$. At high-temperature, the silicate melts generated from SLEC1 are comparable to strongly silica-undersaturated, alkalic OIB lavas and closely resembles ocean island melilitite and nepheline melilitite$^{3,4}$ in its SiO$_{2}$, FeO*, MgO, CaO, TiO$_{2}$, and Na$_{2}$O content. They are also similar to melilite bearing lavas of continental affinity, though the match is not as close. Although the SLEC1 derived immiscible silicate melts are lower in Al$_{2}$O$_{3}$ than primitive alkalic OIB lavas, liquids richer in Al$_{2}$O$_{3}$ may be produced at slightly lower pressures. Geochemical and geodynamical investigations of carbonated eclogite sources for melilitic volcanic series thus merit consideration. 1. Brey, G and Green, D. H. 1977, CMP 61, 141-162. 2. Hirose, K. 1997, GRL 24, 2837-2840. 3. Clague, D. A. and Frey, F. A. 1982, JP 23, 447-504. 4. Hoernle, K. and Schmincke, H.-U. 1993, JP 34, 573-597. 5. Moore, K. R. and Wood, B. J. 1998, JP 39, 1943-1951.
DE: 4806 Carbon cycling
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting