3:45 PM - 4:00 PM
[SGC38-02] Molybdenum isotopic composition in igneous rocks from NE Japan
Keywords:Tohoku, Subduction zone, Molybdenum isotopic composition, Izu islands
Subduction zones play a key role in the recycling of Earth’s surface materials, including seawater and sediments. In particular, dehydration of the subducting slab not only transfers fluid-mobile elements to the mantle wedge, but also causes volcanism and intra-slab earthquakes, resulting in significant impacts on the Earth’s surface environment. There have been various geochemical studies of slab dehydration and primary melt generation in subduction zones (e.g., Tollstrup et al., 2010). Recently, Molybdenum (Mo) isotope ratios (δ98/95Mo = (98/95Mosample / 98/95MoNIST3134 - 1) * 1000) in volcanic arc lavas have become an important geochemical tracer for understanding element cycling in subduction zones (e.g., Freymuth et al., 2015, König et al., 2016). We have previously measured Mo isotope ratios in basaltic samples from the Izu Islands by the double spike method using TIMS and showed that δ98/95Mo values decrease from volcanic front (VF) to rear arc (RA) samples (Tamura et al., JpGU2022).
The Northeast (NE) Japan Arc is a volcanic arc adjacent to the Izu Islands consisting of the Pacific and Eurasian plates. In addition to the VF, the NE Japan Arc has a volcanic arc on the RA side, making it a suitable region for comparing geochemical compositional changes from the VF to the RA. In this study, we measured Mo isotope ratios of basaltic to andesitic samples from the NE Japan Arc (Mt. Iwaki, Mt. Akita-Komagatake, Mt. Iwate, Mt. Chokaisan, and Mt. Zao) and compared the results with previous studies.
In the NE Japan Arc, Mt. Iwaki (–0.24‰, N = 1) and Mt. Chokai (–1.17 to –1.05‰, N = 2) in the RA show lower δ98/95Mo values compared to Mt. Akita-Komagatake (0.12 to 1.03‰, N = 3) and Mt. Iwate (0.02 to 0.15‰, N = 2) in the VF. This observation is consistent with the general trend of δ98/95Mo variation in the Izu Islands (Tamura et al., JpGU2022). An exceptionally low δ98/95Mo value was observed in a VF sample from Mt. Zao (–0.78‰, N = 1). In the NE Japan arc, the δ98/95Mo value tends to decrease with increasing SiO2.
Voegelin et al. (2014) measured Mo isotopic ratios in the Kos Plateau Tuff, Aegean Arc, and found that the δ98/95Mo value increased by 0.2‰ as SiO2 increased from 51 to 68%. The authors argued that crystal differentiation of hornblende and biotite resulted in Mo isotopic fractionation such that the melt was enriched in heavier Mo isotopes. On the other hand, at Akita-Komagatake, δ98/95Mo decreases from 1.03 to 0.12‰ as SiO2 increases from 50.66 to 55.68%. Similarly, at Izu Oshima, δ98/95Mo decreases from 0.26 to –0.46‰ when SiO2 increases from 51.53 to 56.69%. Therefore, in contrast to the Aegean Arc samples, the δ98/95Mo of basaltic andesites and andesites in the NE Japan Arc may be lower than those of basalts. The sample in Mt. Zao has exceptionally low δ98/95Mo (–0.78‰). However, the SiO2 of this sample is 57.79%, suggesting that the basaltic magma may have had a higher δ98/95Mo value. The three samples from the RA have SiO2 above 56%, which may not reflect the δ98/95Mo of the primary magma. However, one Chokai sample (SiO2 = 56.46%, δ98/95Mo = –1.17‰) has a δ98/95Mo value significantly lower than that of the Akita-Komagatake sample with a similar SiO2 abundance (SiO2 = 55.68%, δ98/95Mo = 0.12‰). Therefore, as in the Izu Islands, the δ98/95Mo of basaltic magma in NE Japan is expected to decrease with the depth of the Pacific plate.
The Northeast (NE) Japan Arc is a volcanic arc adjacent to the Izu Islands consisting of the Pacific and Eurasian plates. In addition to the VF, the NE Japan Arc has a volcanic arc on the RA side, making it a suitable region for comparing geochemical compositional changes from the VF to the RA. In this study, we measured Mo isotope ratios of basaltic to andesitic samples from the NE Japan Arc (Mt. Iwaki, Mt. Akita-Komagatake, Mt. Iwate, Mt. Chokaisan, and Mt. Zao) and compared the results with previous studies.
In the NE Japan Arc, Mt. Iwaki (–0.24‰, N = 1) and Mt. Chokai (–1.17 to –1.05‰, N = 2) in the RA show lower δ98/95Mo values compared to Mt. Akita-Komagatake (0.12 to 1.03‰, N = 3) and Mt. Iwate (0.02 to 0.15‰, N = 2) in the VF. This observation is consistent with the general trend of δ98/95Mo variation in the Izu Islands (Tamura et al., JpGU2022). An exceptionally low δ98/95Mo value was observed in a VF sample from Mt. Zao (–0.78‰, N = 1). In the NE Japan arc, the δ98/95Mo value tends to decrease with increasing SiO2.
Voegelin et al. (2014) measured Mo isotopic ratios in the Kos Plateau Tuff, Aegean Arc, and found that the δ98/95Mo value increased by 0.2‰ as SiO2 increased from 51 to 68%. The authors argued that crystal differentiation of hornblende and biotite resulted in Mo isotopic fractionation such that the melt was enriched in heavier Mo isotopes. On the other hand, at Akita-Komagatake, δ98/95Mo decreases from 1.03 to 0.12‰ as SiO2 increases from 50.66 to 55.68%. Similarly, at Izu Oshima, δ98/95Mo decreases from 0.26 to –0.46‰ when SiO2 increases from 51.53 to 56.69%. Therefore, in contrast to the Aegean Arc samples, the δ98/95Mo of basaltic andesites and andesites in the NE Japan Arc may be lower than those of basalts. The sample in Mt. Zao has exceptionally low δ98/95Mo (–0.78‰). However, the SiO2 of this sample is 57.79%, suggesting that the basaltic magma may have had a higher δ98/95Mo value. The three samples from the RA have SiO2 above 56%, which may not reflect the δ98/95Mo of the primary magma. However, one Chokai sample (SiO2 = 56.46%, δ98/95Mo = –1.17‰) has a δ98/95Mo value significantly lower than that of the Akita-Komagatake sample with a similar SiO2 abundance (SiO2 = 55.68%, δ98/95Mo = 0.12‰). Therefore, as in the Izu Islands, the δ98/95Mo of basaltic magma in NE Japan is expected to decrease with the depth of the Pacific plate.