12:00 〜 12:15
[SVC36-12] 東北日本弧地殻深部におけるマグマの酸化還元状態: 一の目潟マール産沈積岩捕獲岩からの制約
キーワード：硫化鉱物、硫酸塩鉱物、一の目潟マール、苦鉄質捕獲岩
Significant number of previous studies suggest that arc magmas have relatively higher fO2 than that of mid ocean ridge basalt (MORB) or oceanic island basalt. It has been a matter of debate whether the oxidized nature of subduction zone magmas is derived from the mantle source or acquired through their differentiation in the crust. Behavior of sulfide and sulfate is sensitive to and thus can be a tracer of the redox state (or oxygen fugacity, fO2) of magmas, because the valence state of sulfur in the magma changes from -2 to +6 through oxidation. The arc's lower crust in the subduction zones has been recently attracting many attentions as a region where magmas may change their chemical compositions significantly. The Ichinomegata maar, located in the back arc of NE Japan, is known for the rare occurrence of deep-crustal xenoliths in subduction zones. A recent petrographic study revealed that mafic xenoliths are fractionated cumulates of the calc-alkaline host magmas (Yanagida et al., 2018). Those cumulates are, therefore, the best samples to investigate the differentiation conditions of arc magmas.
In this study, we found both pyrrhotite and anhydrite from the deep crustal mafic xenoliths from the Ichinomegata maar. Almost all the pyrrhotite grains occur as small rounded inclusions whereas some as relatively large irregular shaped interstitial phases. Pyrrhotite inclusions are mainly in hornblende and some inclusions are in clinopyroxene and plagioclase. Magmatic pyrrhotite shows approximately constant non-stoichiometoric compositions (Fe+Ni+Cu/S) and a trend from Ni-rich to Cu-rich metal composition. Anhydrite occurs as inclusions in plagioclase. The occurrence of anhydrite and the fO2 calculated from the pyrrhotite non-stoichiometry show fO2 of +1 log unit above Ni-NiO buffer, which is almost the same as the average value of arc magmas and clearly higher than that of MORB. This study shows that the oxidized nature of the magmas of the Ichinomegata-maar was already achieved in the deep crust and the shallow oxidation model should be discarded.
In this study, we found both pyrrhotite and anhydrite from the deep crustal mafic xenoliths from the Ichinomegata maar. Almost all the pyrrhotite grains occur as small rounded inclusions whereas some as relatively large irregular shaped interstitial phases. Pyrrhotite inclusions are mainly in hornblende and some inclusions are in clinopyroxene and plagioclase. Magmatic pyrrhotite shows approximately constant non-stoichiometoric compositions (Fe+Ni+Cu/S) and a trend from Ni-rich to Cu-rich metal composition. Anhydrite occurs as inclusions in plagioclase. The occurrence of anhydrite and the fO2 calculated from the pyrrhotite non-stoichiometry show fO2 of +1 log unit above Ni-NiO buffer, which is almost the same as the average value of arc magmas and clearly higher than that of MORB. This study shows that the oxidized nature of the magmas of the Ichinomegata-maar was already achieved in the deep crust and the shallow oxidation model should be discarded.