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キーワード:
Iceland; mantle plume; oxygen isotopes; trace elements; mineral inclusions; ion microprobe
要旨:
Oxygen-isotope compositions are reported for glass inclusions, their host olivines (Fo(87-90)) and tholeiitic pillow rim glasses from the neovolcanic rift zone of Iceland. The inclusion compositions range from enriched ([La/Sm](n) = 1.4- 4.8) to very depleted ([La/Sm](n) = 0.08-0.35). The whole rocks and matrix glasses display little variation ([La/Sm], = 0.410.51). Glass inclusions vary somewhat stronger in O-18/O-16 ratios (delta(18)O=4.0-6.2 +/- 0.6parts per thousand) than the matrix glasses (delta(18)O = 4.6-5.6 +/- 0.6 parts per thousand). The range of olivine phenocryst composition is even wider (delta(18)O = 2.4-6.9 +/- 0.5 parts per thousand), and in some cases the variations within individual crystals reach 1.0- 2.5 parts per thousand. Most glass inclusions are in O-isotopic equilibrium with the surrounding host olivine but not with the central parts of the host crystals; similarly, most olivines are out of equilibrium with their host matrix glasses. This implies that the glass inclusions and olivine phenocrysts can preserve O-isotopic compositions inherited from contrasting parental mantle-derived magmas, which might have mixed in a common volcanic system. The isotopic heterogeneity of individual olivine crystals allows one to estimate their residence times in the magma reservoir using the rate of 180 diffusion in olivine. The time required to achieve isotopic homogeneity over 90-520 mum ranges from 25 to 1030 yr. The delta(18)O values found in depleted glass inclusions and their host olivines are unlikely to have resulted from magma contamination by upper crustal rocks. We suggest two possibilities, implying either an O-isotope heterogeneity of the Icelandic mantle or a mixing of two end-members: a refractory component with 'normal-mantle' or somewhat elevated delta(18)O values, high Ca/Na and strong light rare earth element depletion mixed with a component having lower delta(18)O and Ca/Na but also depleted in incompatible elements. Based on delta(18)O-La/Sm and delta(18)O-delta(11)B relationships, the variations of delta(18)O between 4.6 and 6.2 parts per thousand in enriched and depleted glass inclusions are interpreted to reflect those of primary magmas and, consequently, of their source region. (C) 2002 Elsevier Science B.V. All rights reserved.
