17:15 〜 19:15
[SCG51-P08] イタリア、カラブリア-シチリア島弧系の南ティレニア背弧海盆から掘削されたかんらん岩の岩石学的特徴
キーワード：国際深海科学掘削計画第402次研究航海、南ティレニア海、背弧海盆、かんらん岩、メルト-マントル相互反応
In island arc systems, plate subduction is a unique phenomenon of the earth that has a significant impact on magmatism, seismic activity, and material circulation. The subduction of the plate can cause the back-arc side to stretch, resulting in the expansion and formation of back-arc basins. Although magmatism from the volcanic front to the back-arc of island arc has been studied extensively, the formation and activity of back-arc basins remains largely unexplored due to the difficulty of collecting samples. However, research into back-arc basins is important for understanding the magmatism of island arc systems, including that on the back-arc side. Furthermore, mantle materials collected there is expected to provide information on primary magmatism.
The International Ocean Drilling Program Expedition 402 (IODP Exp. 402) was conducted in the Tyrrhenian Sea from February to April 2024, and core samples including peridotite were collected at the U1614 and U1616 boreholes. The southern Tyrrhenian back-arc basin is believed to have formed in relation to the subduction zone of the Calabria-Sicilia island arc system (Artemieva, 2023), and geophysical surveys from the region have inferred that mantle material is exposed on the seafloor (Prada et al., 2015). Although peridotite samples had been collected and studied during previous drilling ploject, the significant alteration suffered by the samples were a limiting fuctor in obtaing primary information. The objective of this study is to elucidate the petrological characteristics of 30 peridotite samples drilled at the U1616 borehole in the southern Tyrrhenian back-arc basin by mineral identification and petrographic analysis using a polarizing microscope, and major element composition measurement using an electron probe microanalyzer (EPMA).
Although the samples had undergone severe serpentinization, primary minerals were still observed. The upper part of the borehole section is mainly composed of harzburgitic peridotite, which consists of olivine, orthopyroxene, clinopyroxene, and brown to dark brown spinel. The lower part of the borehole section is also predominantly composed of harzburgitic peridotite, which contains olivine, orthopyroxene, clinopyroxene, black- opaque-fine-grained spinel, and plagioclase. The presence of dunite veins was observed in some samples obtained from the upper and lower parts of the borehole cross section in this study. The Fo contents (= atomic Mg/(Mg+Fe) ratio) of olivine and TiO2 content of clinopyroxene increase and decrease, respectively, from the lower to the upper part of the section. These observations suggest that the upper part section is depleted in melt components. The Cr# (atomic Cr/(Cr+Al) ratio) values of the spinels decrease the lower to the upper part of the section, suggesting that the upper part is enriched with melt components. In addition, the TiO2 contents of the spinels at the lower part of the section are very high (TiO2 wt% = ~1.9 wt%) and display a large variation. These results also suggest that the peridotites from the lower part are enriched with melt components compared to the peridotites of the upper part.
The mineralogical and petrological characteristics of the studied samples, when considered in conjunction with previous interpretations (Arai, 2005; Dick and Bullen, 1984; Ohara, 2003; Ohara, 2012), suggest that dunite veins with low spinel Cr# values were probably formed by the interaction between primary melts and wall rock peridotite. Additionally, the presence of plagioclase-bearing peridotite in the lower part of the section is likely the result of a reaction with basaltic melt that was saturated with plagioclase. The studied peridotite body exposed on the seafloor, is assumed to have been transported to the seafloor surface as an oceanic core complex (OCC) by the development of detachment faulting during the formation of the back-arc basin.
The International Ocean Drilling Program Expedition 402 (IODP Exp. 402) was conducted in the Tyrrhenian Sea from February to April 2024, and core samples including peridotite were collected at the U1614 and U1616 boreholes. The southern Tyrrhenian back-arc basin is believed to have formed in relation to the subduction zone of the Calabria-Sicilia island arc system (Artemieva, 2023), and geophysical surveys from the region have inferred that mantle material is exposed on the seafloor (Prada et al., 2015). Although peridotite samples had been collected and studied during previous drilling ploject, the significant alteration suffered by the samples were a limiting fuctor in obtaing primary information. The objective of this study is to elucidate the petrological characteristics of 30 peridotite samples drilled at the U1616 borehole in the southern Tyrrhenian back-arc basin by mineral identification and petrographic analysis using a polarizing microscope, and major element composition measurement using an electron probe microanalyzer (EPMA).
Although the samples had undergone severe serpentinization, primary minerals were still observed. The upper part of the borehole section is mainly composed of harzburgitic peridotite, which consists of olivine, orthopyroxene, clinopyroxene, and brown to dark brown spinel. The lower part of the borehole section is also predominantly composed of harzburgitic peridotite, which contains olivine, orthopyroxene, clinopyroxene, black- opaque-fine-grained spinel, and plagioclase. The presence of dunite veins was observed in some samples obtained from the upper and lower parts of the borehole cross section in this study. The Fo contents (= atomic Mg/(Mg+Fe) ratio) of olivine and TiO2 content of clinopyroxene increase and decrease, respectively, from the lower to the upper part of the section. These observations suggest that the upper part section is depleted in melt components. The Cr# (atomic Cr/(Cr+Al) ratio) values of the spinels decrease the lower to the upper part of the section, suggesting that the upper part is enriched with melt components. In addition, the TiO2 contents of the spinels at the lower part of the section are very high (TiO2 wt% = ~1.9 wt%) and display a large variation. These results also suggest that the peridotites from the lower part are enriched with melt components compared to the peridotites of the upper part.
The mineralogical and petrological characteristics of the studied samples, when considered in conjunction with previous interpretations (Arai, 2005; Dick and Bullen, 1984; Ohara, 2003; Ohara, 2012), suggest that dunite veins with low spinel Cr# values were probably formed by the interaction between primary melts and wall rock peridotite. Additionally, the presence of plagioclase-bearing peridotite in the lower part of the section is likely the result of a reaction with basaltic melt that was saturated with plagioclase. The studied peridotite body exposed on the seafloor, is assumed to have been transported to the seafloor surface as an oceanic core complex (OCC) by the development of detachment faulting during the formation of the back-arc basin.