13:45 〜 14:00
[SEM13-01] Revised magnetostratigraphy and eruption timing of Afro-Arabian Large Igneous Province using new 40Ar/39Ar ages
キーワード：アフリカ-アラビア巨大火成岩岩石区、磁気層序、アルゴン-アルゴン年代測定、地磁気極性タイムスケール
We report four new 40Ar/39Ar ages on fresh groundmass grains of basalts from the 2 km thick Lima-Limo section in Ethiopia to construct magnetostratigraphy of the Afro-Arabian Large Igneous Province (AALIP). A succession of seven magnetozones (R4-N3-R3-N2-R2-N1-R1, from top to base) were identified by Ahn et al. (in press), which were tentatively correlated to Chron C11n.1r (29.477-29.527 Ma) (R4) - C11n.2n (29.527-29.970 Ma) (N3 to N2) - C11r (29.970-30.591 Ma) (R2 to R1) of the geomagnetic polarity time scale 2016 (GPTS 2016; Ogg et al., 2016).
New ages were obtained from the R4, N3, and N1 magnetozones. They have ~40 % higher precision than the previously reported ages (Coulie et al., 2003; Hofmann et al., 1997), and they are concordant with stratigraphy. We calibrated our ages using the FCs standard from Kuiper et al. (2008) and the 40K decay constant of Min et al. (2000), which were used in GPTS 2016. We compared our calibrated ages and their uncertainties with GPTS 2016 and the revised GPTS model proposed by Sahy et al. (2017). The difference of the two ages obtained from the R4 magnetozone is 0.39 Myr. This supports that the R4 magnetozone belongs to Chron C11r rather than Chron C11n.1r; the former duration is 0.621 Myr and the latter 0.05 Myr (Ogg et al., 2016). The magnetozones from N1 to N3 are correlated to Chron C12n (31.23 to 30.41 Ma), which constrain that the R1 magnetozone is correlated to Chron C12r (33.09 to 31.23 Ma) (ages are based on Sahy et al., 2017).
From the duration of 0.5 Myr for C11r according to the GPTS model of Sahy et al. (2017) and the area of ~600,000 km2 (Mohr, 1983), the average eruption rate within the R4 magnetozone is estimated to be 1.1 km3/yr, which is about one third of the rate of the Siberian LIP (Pavlov et al., 2019). On the other hand, AALIP has large successive silicic components in the N3 magnetozone. Touchard et al. (2003) correlated them with tephra layers in ODP sediment cores of the Indian Ocean using magnetostratigraphy and geochemistry of volcanic glass, and suggested a linkage with the Oi-2 cooling event. However, based on our age determination, the tephra layers are not coeval with the AALIP silicic eruptions, and there was no tephra layer in the cores that could be correlated to the silicic eruptions (Touchard et al., 2003). This mismatch suggests that the AALIP silicic eruptions would not have caused the climate change.
New ages were obtained from the R4, N3, and N1 magnetozones. They have ~40 % higher precision than the previously reported ages (Coulie et al., 2003; Hofmann et al., 1997), and they are concordant with stratigraphy. We calibrated our ages using the FCs standard from Kuiper et al. (2008) and the 40K decay constant of Min et al. (2000), which were used in GPTS 2016. We compared our calibrated ages and their uncertainties with GPTS 2016 and the revised GPTS model proposed by Sahy et al. (2017). The difference of the two ages obtained from the R4 magnetozone is 0.39 Myr. This supports that the R4 magnetozone belongs to Chron C11r rather than Chron C11n.1r; the former duration is 0.621 Myr and the latter 0.05 Myr (Ogg et al., 2016). The magnetozones from N1 to N3 are correlated to Chron C12n (31.23 to 30.41 Ma), which constrain that the R1 magnetozone is correlated to Chron C12r (33.09 to 31.23 Ma) (ages are based on Sahy et al., 2017).
From the duration of 0.5 Myr for C11r according to the GPTS model of Sahy et al. (2017) and the area of ~600,000 km2 (Mohr, 1983), the average eruption rate within the R4 magnetozone is estimated to be 1.1 km3/yr, which is about one third of the rate of the Siberian LIP (Pavlov et al., 2019). On the other hand, AALIP has large successive silicic components in the N3 magnetozone. Touchard et al. (2003) correlated them with tephra layers in ODP sediment cores of the Indian Ocean using magnetostratigraphy and geochemistry of volcanic glass, and suggested a linkage with the Oi-2 cooling event. However, based on our age determination, the tephra layers are not coeval with the AALIP silicic eruptions, and there was no tephra layer in the cores that could be correlated to the silicic eruptions (Touchard et al., 2003). This mismatch suggests that the AALIP silicic eruptions would not have caused the climate change.