15:45 〜 16:00
[SCG52-08] The thickness of old oceanic lithosphere estimated from mantle xenoliths
キーワード：海洋リソスフェア、プチスポット火山、地質温度圧力計、マントル捕獲岩
The lithosphere, the rigid outer part covering the Earth’s surface, is bounded by the underlying asthenosphere with less viscous part of the mantle. The Earth’s lithosphere can be divided into more than 10 tectonic plates that vary in size, thickness, and principal types of crustal materials, and much of the Earth’s geological activity takes place at plate boundaries that are governed by their relative movements. One of the major unresolved problems in understanding the plate variations is what controls plate thickness. In the continental lithosphere, the depth of the lithosphere-asthenosphere boundary (LAB) reaches >200 km in the Archean craton (Pearson et al., 2021). In the oceanic lithosphere, the LAB depth beneath the northwestern Pacific (~130 Ma) is estimated to be ~82 ± 4.4 km (Kawakatsu et al., 2009). Although this contrasting thickness is apparently consistent with the theoretical prediction of a thermally controlled origin for the LAB, i.e., the boundary layer evolved with cooling age, it remains elusive whether the lithosphere can be thickened with cooling effects alone. This problem has long been debated because of the considerable gap between theoretical predictions and observations of seafloor depth and heat flow for old oceanic lithosphere (>70 Ma).
In this study, the LAB depth beneath 135 Ma seafloor was estimated from the xenolith geotherm beneath the petit-spot volcano, independently of theoretical modeling or seismic observations. The geotherm was reconstructed using the Brey and Köhler (1990) geothermobarometer for five garnet-bearing peridotites. As a result of careful observation of the compositional zoning recorded in two pyroxenes, we obtained a clear linear correlation between the derived temperatures and pressures for the five samples, ranging from ~800 to ~1300°C and 1.7 to 2.7 GPa. The regression line of the data (P/T = 0.002046 ± 0.000038) agreed within 2σ error with the geotherm predicted by the plate model GDH1 (Stein and Stein 1992). The LAB depth obtained from the intersection of the xenolith geotherm and the mantle adiabatic gradient estimated by Katsura (2022) is 88 ± 1.7 km, which is consistent with the estimate from the receiver function by Kawakatsu et al. (2009). These results support the the plate model prediction that the thickness of older oceanic lithosphere does not increase with age, but remains constant. Thus, the plate thickness cannot be controlled by thermal diffusion alone, and some mechanism is required to maintain a constant thickness.
In this study, the LAB depth beneath 135 Ma seafloor was estimated from the xenolith geotherm beneath the petit-spot volcano, independently of theoretical modeling or seismic observations. The geotherm was reconstructed using the Brey and Köhler (1990) geothermobarometer for five garnet-bearing peridotites. As a result of careful observation of the compositional zoning recorded in two pyroxenes, we obtained a clear linear correlation between the derived temperatures and pressures for the five samples, ranging from ~800 to ~1300°C and 1.7 to 2.7 GPa. The regression line of the data (P/T = 0.002046 ± 0.000038) agreed within 2σ error with the geotherm predicted by the plate model GDH1 (Stein and Stein 1992). The LAB depth obtained from the intersection of the xenolith geotherm and the mantle adiabatic gradient estimated by Katsura (2022) is 88 ± 1.7 km, which is consistent with the estimate from the receiver function by Kawakatsu et al. (2009). These results support the the plate model prediction that the thickness of older oceanic lithosphere does not increase with age, but remains constant. Thus, the plate thickness cannot be controlled by thermal diffusion alone, and some mechanism is required to maintain a constant thickness.