HR: 14:10h
AN: DI43A-03    [Abstracts]
TI: CO₂-Brine Mantle Metasomatic Fluids in the Lithosphere Beneath the Ethiopian Plateau
AU: * Frezzotti, M
EM: frezzottiml@unisi.it
AF: Dip. Scienze Terra, Via Laterina 8, Siena, 53100, Italy
AU: Ferrando, S
EM: simona.ferrando@unito.it
AF: Dip. Scienze Mineralogiche e petrologiche, Via V. Caluso 35, Torino, 10125, Italy
AU: Peccerillo, A
EM: pecceang@unipg.it
AF: Dip. Scienze Terra, P.za Universita' 1, Perugia, 06100, Italy
AU: Tecce, F
EM: f.tecce@cnr.igag.it
AF: IGAG - CNR, P.za A. Moro 5, 00195, Roma, Italy
AU: Perucchi, A
EM: andrea.perucchi@elettra.trieste.it
AF: CNR-INFM Coherentia, Sincrotrone Trieste, Strada Statale 14, Basovizza (Trieste), 34012, Italy
AB: Fluid inclusions in amphibole-bearing spinel lherzolite xenoliths from Quaternary volcanic necks at Injibara (western part of the Ethiopian plateau) show compelling evidence for brines in the lithospheric mantle in a region of continental flood basalts. The petrography of the xenoliths indicates a transition from deformed (protogranular to porphyroclastic texture) Cl-rich pargasite metasomatized lherzolites (LREE, LILE, and Pb enrichment in clinopyroxene) at 950 - 1015°C, to cryptically metasomatized equigranular lherzolites (LREE flat to depleted in clinopyroxene), at higher temperatures (1050 - 1150°C). All pargasite-bearing lherzolites contain CO₂-brine fluid inclusions, but no melt inclusions. Fluid inclusions are preserved only in orthopyroxene, while in clinopyroxene and olivine, inclusions underwent extensive post- entrapment interaction with host mineral. The fluid inclusion composition is: X_CO2 = 0.64, X_H2O = 0.33, X_Na = 0.006, X_Mg = 0.006, X_Cl = 0.018, (salinity = 14 - 10 NaCl eq. wt. %), although water contents in the originally trapped fluids could have been higher. Trapping pressures are of about 1.4 GPa (at T = 1000°C). Synchrotron sourced micro-infrared mapping (ELECTRA, Trieste, Italy) in nominally anhydrous minerals shows gradients for H distribution, with considerable enrichment at grain boundaries, and along intragranular microfractures. The petrography, major and trace element data, together with constraints from fluid inclusions and microinfrared analyses, indicate two consecutive metasomatic events: a first event of interaction between lherzolite and a C-O-H fluid phase which is Cl-rich, followed by lherzolite interaction with high fractions of basaltic melts, during heating. Present work points towards solute-rich aqueous fluids being important metasomatic agents beneath the Ethiopian plateau, locally forming hydrous regions, which may be easily melted. Presence of high-chlorine in fluids suggests a contribution of recycled altered oceanic lithosphere in their source.
DE: 1043 Fluid and melt inclusion geochemistry
DE: 1065 Major and trace element geochemistry
DE: 3615 Intra-plate processes (1033, 8415)
DE: 3621 Mantle processes (1038)
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting