[B-PT23_30PM2] Decoding the history of Earth: From Hadean to Modern

[BPT23-P03_PG] Major element composition and forming condotion of the hidden reservoir

3-min talk in an oral session

*Nozomi KONDO¹, Tetsu KOGISO¹ (1.Graduate School of Human and Environmental Studies, kyoto University)

Keywords:hidden reservoir, proto-crust, ¹⁴²Nd/¹⁴⁴Nd

Solidification of the magma-ocean and subsequent mantle-crust differentiation could have significant influence on the evolution of the solid Earth and hydrosphere, but its detail is still unclear. Previous studies have suggested that the difference in ¹⁴²Nd/¹⁴⁴Nd between chondrites and bulk silicate Earth (BSE) resulted from the formation of an incompatible element-rich reservoir that had formed in the early Earth and then got hidden into the Earth’s interior or lost outside the Earth. Although various models for the composition and the origin of such a “hidden reservoir” have been proposed, they have not focused on the major element composition of the hidden reservoir. However, the major element composition is crucial to know the density of the hidden reservoir and to examine whether the hidden reservoir rose to form the proto-crust or sunk in the early mantle. In order to determine the major element composition of the hidden reservoir, we estimated the melting condition for the formation of the hidden reservoir with constraints of ¹⁴²Nd/¹⁴⁴Nd and ¹⁴³Nd/¹⁴⁴Nd systematics in the ancient and modern mantle. This study assumed that the hidden reservoir had formed at pressures less than 10 GPa, on the basis of previous studies that estimated the initial depth of melt segregation to be at this pressure range in the solidifying magma ocean. Then we calculated the Sm/Nd ratio that is conformable to the difference in ¹⁴²Nd/¹⁴⁴Nd between chondrites and BSE, and estimated the melt fraction that satisfies this Sm/Nd ratio. From this calculation, the melt fraction was estimated to be <5.2% at 1 GPa, <3.2% at 3 GPa and <1.4% at 7 GPa. From these calculated melt fractions and previous experimental data, we estimated that the major element compositions of the hidden reservoir were incompatible element-rich tholeiite, picrite, and komatiite, respectively.Ancient hotter mantle should have melted at higher pressure, but on the other hand, the melt fraction was estimated to be small. In order to satisfy the small melt fraction at deep melting, the lithosphere must be thick, as suggested by Korenaga (2009) who showed the possibility of thick lithosphere in the hotter mantle. From these results, a likely composition of the hidden reservoir is incompatible element-rich picrite-komatiite. Solomatov and Stevenson(1993),Journal of Geophysical Research, 98, 5407-5418 Korenaga(2009), Geophysical Journal International, 179, 154-170