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Chlorine-rich metasomatic H2O-CO2 fluids in amphibole-bearing peridotites from Injibara (Lake Tana region, Ethiopian plateau): Nature and evolution of volatiles in the mantle of a region of continental flood basalts |
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| Authors: | Maria Luce Frezzotti Simona Ferrando Maurizio Petrelli Andrea Perucchi |
| Institution: | a Department of Earth Sciences, University of Siena, Via Laterina 8, 53100 Siena, Italy b Department of Mineralogical and Petrological Sciences, University of Torino, Via V. Caluso 35, 10125 Torino, Italy c Department of Earth Sciences, University of Perugia, P.zza Università 1, 06100 Perugia, Italy d IGAG - CNR, c/o Department of Earth Sciences, University Roma 1 - La Sapienza, P.za A. Moro 5, 00185 Roma, Italy e Sincrotrone ELETTRA, Trieste, 34012 Basovizza (Trieste), Italy |
| Abstract: | Petrological and geochemical study of volatile bearing phases (fluid inclusions, amphibole, and nominally anhydrous minerals) in a spinel lherzolite xenolith suite from Quaternary lavas at Injibara (Lake Tana region, Ethiopian plateau) shows compelling evidence for metasomatism in the lithospheric mantle in a region of mantle upwelling and continental flood basalts. The xenolith suite consists of deformed (i.e., protogranular to porphyroclastic texture) Cl-rich pargasite lherzolites, metasomatized (LILE and Pb enrichment in clinopyroxene and amphibole) at T ? 1000 °C. Lherzolites contain chlorine-rich H2O-CO2 fluid inclusions, but no melt inclusions. Fluid inclusions are preserved only in orthopyroxene, while in olivine, they underwent extensive interaction with the host mineral. The metasomatic fluid composition is estimated: XCO2 = 0.64, XH2O = 0.33, XNa = 0.006, XMg = 0.006, XCl = 0.018, (salinity = 14-10 NaCl eq. wt.%, aH2O = 0.2, Cl = 4-5 mol.%). Fluid isochores correspond to trapping pressures of 1.4-1.5 GPa or 50-54 km depth (at T = 950 °C). Synchrotron sourced micro-infrared mapping (ELECTRA, Trieste) shows gradients for H2O-distribution in nominally anhydrous minerals, with considerable enrichment at grain boundaries, along intragranular microfractures, and around fluid inclusions. Total water amounts in lherzolites are variable from about 150 up to 400 ppm. Calculated trace-element pattern of metasomatic fluid phases, combined with distribution and amount of H2O in nominally anhydrous minerals, delineate a metasomatic Cl- and LILE-rich fluid phase heterogeneously distributed in the continental lithosphere. Present data suggest that Cl-rich aqueous fluids were important metasomatic agents beneath the Ethiopian plateau, locally forming a source of high water content in the peridotite, which may be easily melted. High Cl, LILE, and Pb in metasomatic fluid phases suggest the contribution of recycled altered oceanic lithosphere component in their source. |
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