Trace elements in anorthite megacrysts from the Kurile Island Arc: a window to across-arc geochemical variations in magma compositions

摘要

Anorthite megacrysts from twelve volcanic centers of the Kurile Island Arc were analyzed by the ion microprobe for concentrations of: Li, Be, B, F, Mg, P, Cl, K, Ti, Fe, Co, Rb, Sr, Y, Zr, Nb, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, and Pb. These 1-2-cm megacrysts are mainly found as early phenocrysts in basalts, but are also present in andesites, rhyolites and hybrid rocks from almost every studied volcanic center, both subaerial and submarine. Electron and ion microprobe studies show that the cores of these megacrysts are homogeneous and unzoned, whereas rims often result from overgrowth of sodium-richer plagioclase in equilibrium with its host, silica-richer melt. The cores of these megacrysts can be used to see back through fractional crystallization and magma mixing and serve as a 'window' into an earlier basaltic stage of magmatic evolution. Petrological observations and numerical crystallization modeling indicate that highly anorthitic plagioclase is the first or second (after olivine) liquids phase in high-Al basalts regardless of varying K_2O contents across the arc. Given the narrow compositional range of anorthite megacrysts, the across-arc chemical variations are well displayed by direct comparison of trace element concentrations in the megacrysts across the arc. The concentration of K in megacrysts increases linearly by a factor of 11; Ba, 13; Sr, Eu, 4; Be, Ti, P, 3; Li, 2, REE, 5-2; whereas concentrations of Pb, B and halogens and their ratios of HFSE either remain constant or decrease with deepening of the Benioff Zone from 110 to 220 km. We used ion microprobe-derived plagioclase-basalt partition coefficients to convert trace element concentrations in megacrysts to parental magmatic values. The reconstructed concentrations of several key trace elements and their correlation with Benioff Zone depth are consistent with whole-rock basalt data, but all give systematically more primitive compositions. Therefore, anorthite megacrysts are helpful for retrieving the chemistry of parental basalts which have subsequently suffered variable differentiation and mixing. They can be used to assess delicate variations in primary basaltic magma chemistry as a function of Benioff Zone depth. We also report new results for Sr, Nd and Pb isotopes for anorthite-bearing rocks and other rocks from the same volcanoes. Across-arc zoning with respect to trace element concentrations, ratios and isotopic values is largely linear and requires superposition of several petrogenetic processes: trace element transfer by fluids, variation in degree of partial melting and progressive change of the melting source.