[S-IT22] [EE] 核-マントルの相互作用と共進化

[SIT22-01] Origin of geochemical mantle components: Role of spreading ridges and thermal evolution of mantle

*Jun-Ichi Kimura¹、James B Gill²、Peter E van Keken³、Hiroshi Kawabata⁴、Susanne Skora⁵ (1.Department of Solid Earth Geochemistry, Japan Agency for Marine-Earth Scienc and Technology、2.Department of Earth and Planetary Sciences, University of California Santa Cruz、3.Department of Terrestrial Magnetism, Carnegie Institution for Science、4.Research and Education Faculty, Multidisciplinary Science Cluster, Interdisciplinary Science Unit, Kochi University、5.Institute of Geochemistry and Petrology, Earth Sciences, ETH Zürich)

キーワード：Depleted mantle, Thermal evolution, Chemical evolution

We explore the element redistribution at mid-ocean ridges (MOR) using a numerical model to evaluate the role of the decompression mantle melting process in the Earth’s geochemical cycle, particularly in the formation of the depleted mantle component. Our model uses a trace element mass balance based on an internally consistent thermodynamic-petrologic computation to explain the composition of MOR basalt (MORB) and residual peridotite. Model results for MORB-like basalts from 3.5 to 0 Ga indicate a high mantle potential temperature (T_p) of 1650–1500°C during 3.5–1.5 Ga before decreasing gradually to 1320°C today. The source mantle composition changed from primitive (PM) to depleted as T_p decreased, but this source mantle is variable with an early depleted reservoir (EDR) mantle periodically present. We examine two-stage Sr-Nd-Hf-Pb isotopic evolution of the mantle residues from melting of PM or EDR at MOR that formed ancient MORB-like basalts. Formation of depleted MORB source mantle (DMM) is also examined using modern MORBs. At high-T_p (3.5–1.5 Ga), the MOR process formed extremely depleted DMM. This coincided with formation of the majority of the continental crust, the sub-continental lithospheric mantle, and the enriched mantle components formed at subduction zones. During cooler-T_p mantle conditions (1.5–0 Ga), the MOR process formed most of the modern ocean basin DMM. Changes in the mode of mantle convection from vigorous deep mantle recharge before ~1.5 Ga to less vigorous afterwards is suggested to explain the thermochemical mantle evolution.