自然銅と炭化水素を包有する斜長石巨晶 マグマ混合の最盛期を凍結か （２） 炭化水素を包有する斜長石巨晶とその地球化学的意義

Abstract

Part 2. Hydrocarbons, verified by high-temperature Micro-FTIR, are found to be included by the same anorthite and labradorite megacrysts containing native coppers, as described in Part 1, in basaltic rocks, from Japanese island arc and in the western continental margin of North America, respectively. A finding of Muntz metal (Cu60Zn40) within the anorthite megacryst offerred the establishment of Hume-Rothery's rule in nature. Concerning the genesis of native coppers coexistent with the hydrocarbon, the exsolution model fortified by their purity and oriented distribution has been suggested and can account for crystal growth of native coppers based on the theory of Ostwald ripening. While continental margins and Island arcs have geological and tectonical backgrounds common to each other, remarkable difference in morphology between fractured labradorites and euhedral anorthites seems to flare up in the power of the volcanic eruption. Moreover it proved that the corroded olivines within anorthite megacrysts hereupon have also a trace amount of hydrocarbons, detection of which discloses that the magmas crystallizing these anorthite and olivines contained the same hydrocarbons. Fe/Mg ratio of the corroded olivines, Sr-content and Sr-isotope ratio of anorthite megacrysts typical of arc-volcanism in Japan afford the lateral variations from the Japan trench to the Back arc side, and their geochemical behaviour witnesses the rapid crystallization of anorthite megacrysts and their corroded olivines in close association with the crust components and the slab-sediments subducted beneath the island arc. Review (2) must be highly speculative because of the paucity of experimental and natural evidences on the megacrysts and their inclusions. Nevertheless it is upbeat, and the key point is that island arc magmatism should be reconstructed as the petrochemical system containing transition metals (e.g. Cu and Zn), hydrocarbons and chlorines. This reconstruction must elicit the far-reaching effect of mixing the basaltic magma with the sea water and slab-sediments rich in metals. Advances in understanding the geochemical system that formed anorthite megacrysts have come from the mineralogical study of their active inclusions providing more questions than answers.