HR: 15:25h
AN: V33D-08    [Abstracts]
TI: High flux of (diamond CO2-H2O-H2S) fluids in the mantle beneath Hawaii: evidence from fluid inclusions in garnet pyroxenites from Salt Lake Crater (Oahu, Hawaii)
AU: * Frezzotti, M
EM: frezzottiml@unisi.it
AF: Dipartimento Scienze della Terra, Via Laterina 8, Siena, 53100 Italy
AU: Peccerillo, A
EM: pecceang@unipg.it
AF: Dipartimento Scienze della Terra, P.za Universit… 1, Perugia, 06100 Italy
AB: Fluid inclusions in garnet pyroxenite xenoliths from Salt lake Crater (Oahu) preserve evidence for a high flux of deep (diamond C-O-H-S) fluids (or melts) in the mantle beneath Hawaii. Garnet pyroxenites are dry and consist of clinopyroxene, orthopyroxene, olivine and garnet; they are interpreted as magmatic segregations (cumulates) from alkali-basaltic magmas within upper mantle lherzolites, reequibrated within the Hawaiian lithosphere at 1000-1150°C and 1.6-2.5 GPa (~50-80 km). In all mineral phases, CO₂-rich fluid inclusions are extremely abundant, forming intergranular and intragranular trails. In ortho- and clinopyroxene, a few isolated early fluid inclusions are preserved (3 -50 μ m), which were trapped at an earlier stage and before cooling of host rocks in the mantle. Early inclusions contain exceptionally high-density CO₂-rich fluids (superdense; d = 1.21 g/cm³), corresponding to pressures of 1.8-2 GPa, at the inferred mantle temperatures (Frezzotti et. al., 1992). Raman analyses in selected high-density and superdense early CO₂ inclusions reveals vibrational bands (3638 cm^-1 and 2609 cm^-1), which are characteristic of OH^- stretching vibrations in isolated H₂O molecules, and of H₂S, respectively. A few among inclusions additionally show the typical vibration for diamond at 1332 cm^-1. Fluids contained within inclusions are CO₂-rich, and contain minor amounts of H₂O and H₂S ± diamond, defining a complex fluid mixture in the C-O-H-S system in the mantle beneath Hawaii, at P ~ 2 GPa. The non-systematic presence of diamonds within fluid inclusions indicates that diamonds were already present in mantle fluids at the time of trapping as inclusions, and constrains fluid origin at pressures above 5 GPa (~160 km). At such high pressures, the fluid phase may have been a CO₂-rich melt (i.e. carbonatitic) containing water and H₂S, or a CH₄-H₂O-H₂S fluid, depending on local fO₂ conditions (Green and Falloon, 1998; Gudfinnsson and Presnall, 2005). Present data suggest that significant amounts of (diamond) CO₂-H₂S-H₂O fluids are present in the mantle beneath Hawaii. A high (C-O-H-S) volatile flux rising from the asthenosphere represents a strong metasomatic agent able to carry very high concentrations of incompatible trace elements, and to strongly increase mantle fusibility (Gudfinnsson and Presnall, 2005). Such rising fluids or melts could induce partial melting at the base of the lithosphere at normal mantle temperatures, obviating the need for concentrated hot jets localized under ``hotspot'' volcanoes. Preservation of diamonds within fluid inclusions further suggests a fluid phase evolution in a ``cold'' mantle environment. References. Frezzotti, M.L., Burke, E.A.J., De Vivo B., Stefanini B. & Villa I.M. (1992) Eur. J. Mineral., 4, 1137-1153. Green, D. H. and Falloon, T. J. (1998) Pyrolite: A Ringwood concept and its current expression. pp 311-380 in The Earth's Mantle; Composition, Structure, and Evolution, ed I.N.S. Jackson, Cambridge, Cambridge University Press, 566 pp. Gudfinnsson, G.H. and Presnall, D. C. (2005). J. Petrology, 46, 1645-1659.
DE: 3619 Magma genesis and partial melting (1037)
DE: 3621 Mantle processes (1038)
DE: 8415 Intra-plate processes (1033, 3615)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005