HR: 1340h
AN: DI33A-1132    [Abstracts]
TI: Volatile-rich komatiitic and picritic melt inclusions in Cr-spinel beach sand from Gorgona Island, Colombia
AU: * Shimizu, K
EM: shimmy@jamstec.go.jp
AF: IFREE, JAMSTEC, Natsushima, Yokosuka, 2370061, Japan
AU: Shimizu, N
EM: nshimizu@whoi.edu
AF: WHOI, Woods Hole, Woods Hole, MA 02543, United States
AU: Suzuki, K
EM: katz@jamstec.go.jp
AF: IFREE, JAMSTEC, Natsushima, Yokosuka, 2370061, Japan
AU: Tatsumi, Y
EM: tatsumi@jamstec.go.jp
AF: IFREE, JAMSTEC, Natsushima, Yokosuka, 2370061, Japan
AU: Komiya, T
EM: tkomiya@geo.titech.ac.jp
AF: Tokyo Inst. Tech., Meguro, Tokyo, 1528551, Japan
AU: Maruyama, S
EM: smaruyam@geo.titech.ac.jp
AF: Tokyo Inst. Tech., Meguro, Tokyo, 1528551, Japan
AB: Volatile content of komatiite is a key to constrain thermal evolution of the deep Earth. We report volatile contents with major and trace element compositions of melt inclusions (MIs) in chromian spinel (Cr-spinel) from beach sands of Gorgona Island, Colombia. Gorgona Island is ~90 Ma volcanic island, where picrites and the world-youngest komatiites occur. As Cr-spinel is dense and rigid oxide mineral that crystallizes only at early stages of crystallization, it is considered to be a superior container for retaining primitive melt, even including volatiles. Volatile (H₂O, CO₂, S, F and Cl) and trace element (K₂O, Sr, Y, Zr, Nb, Ba, La, Ce, Sm, Dy, Yb) compositions of ~80 MIs were analyzed by SIMS (Cameca-1280 and 3f, respectively) at WHOI. MIs in the Cr-spinel from Gorgona Is. are classified into three types by their host Cr-spinel compositions such as low-Ti (P-type), high-Ti with high-Cr# (BK-type) and high-Ti with low-Cr# (K-type). MIs of P-type, BK-type and K-type are mostly in compositional ranges of picrite, high TiO₂ komatiite (some basalt) and low TiO₂ komatiite in Gorgona Island, respectively. Water content of P-type MIs is variable, ranging from 0.05 to 0.9 wt%, whereas those of BK and K-type MIs are limited (< 0.1 wt%). On the other hand, CO₂ contents of BK-type and K- type MIs are highly scattered (40 to 4200 ppm), whereas that of P-type is relatively constant at ~200 ppm. All MIs with high CO₂ content (>500 ppm) do not contain (shrinkage) bubbles and many of them are low in K₂O. H₂O/K₂O, CO₂/K₂O, S/K₂O and F/K₂O ratios are positively correlated with Y/Sr ratios, indicating degassing trends of melt at crystallization, magma mixing and/or assimilation. Undegassed H₂O/K₂O, CO₂/K₂O, S/K₂O and F/K₂O ratios of komatiitic (picritic) melt are estimated to be ~10 (~40), ~80 (n.d.), ~7(~3) and ~1(~0.5), respectively, which are much higher than those estimated for the depleted source mantle of the MORB [1.6, 0.7, 1.6 and 0.2, respectively; Salters, V. & Stracke, A. (2004), Composition of the depleted mantle. Geochem. Geophys. Geosys. 5 (2003GC000597)]. The results suggest that Gorgona komatiite and picrite magmas were derived from volatile-rich sources. CO₂ degassing might also have contributed to eruption of high-density magmas to the surface. In addition, H₂O, S, F and Cl contents in MIs in olivine from a picrite were identical to those of P-type MIs in Cr-spinel, but CO₂ in olivine-hosted MIs were considerably lower (~50 ppm) than those in Cr-spinel. This indicates that entrapment pressure for MIs in Cr-spinel is likely to be greater than that for MIs in olivine. Therefore, in order to evaluate the volatile contents of undegassed magmas from oceanic islands, melt inclusions in Cr-spinel beach sand could be very useful.
DE: 1043 Fluid and melt inclusion geochemistry
DE: 8137 Hotspots, large igneous provinces, and flood basalt volcanism
DE: 9360 South America
DE: 9610 Cretaceous
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting