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Solid earth sciences
201608201608
Major element composition of an Early Enriched Reservoir: Constraints from 142Nd/144Nd isotope systematics in the early Earth and high pressure melting experiments of a primitive peridotite
Kondo N, Yoshino T, Matsukage K N, Kogiso T
142Nd anomaly, Early Enriched Reservoir, Hadean, Major element composition, Near-solidus melt, High-pressure melting experiment
Schematic illustrations of EER formation. (Top) The 142Nd/144Nd evolution considered in this study. A source region differentiates into the EER and EDR. The EER is hidden within or has been lost from the Earth. The EDR and intact region were mixed to form the ASE. (Bottom) A box model of the EER formation, where X is the fraction of the source region relative to the whole mantle. ASE: Accessible Silicate Earth (part of the silicate Earth from which we can obtain materials for analysis); EER: Early Enriched Reservoir (low Sm/Nd reservoir which can compensate for the 142Nd/144Nd difference between ASE and chondrites); EDR: Early Depleted Reservoir (residual solid produced by the EER formation); CHUR: Chondritic Uniform Reservoir (putative primitive mantle having a chondritic composition)
The Accessible Silicate Earth (ASE) has a higher 142Nd/144Nd ratio than most chondrites. Thus, if the Earth is assumed to have formed from these chondrites, a complement low-142Nd/144Nd reservoir is needed. Such a low-142Nd/144Nd reservoir is believed to have been derived from a melt in the early Earth and is called the Early Enriched Reservoir (EER). Although the major element composition of the EER is crucial for estimating its chemical and physical properties (e.g., density) and is also essential for understanding the origin and fate of the EER, which are both major factors that determine the present composition of the Earth, it has not yet been robustly established. In order to determine the major element composition of the EER, we estimated the age and pressure–temperature conditions to form the EER that would best explain its Nd isotopic characteristics, based on Sm–Nd partitioning and its dependence on pressure, temperature, and melting phase relations. Our estimate indicates that the EER formed within 33.5 Myr of Solar System formation and at near-solidus temperatures and shallow upper-mantle pressures. We then performed high-pressure melting experiments on primitive peridotite to determine the major element composition of the EER at estimated temperature at 7 GPa and calculated the density of the EER. The result of our experiments indicates that the near-solidus melt is iron-rich komatiite. The estimated density of the near-solidus melt is lower than that of the primitive peridotite, suggesting that the EER melt would have ascended in the mantle to form an early crust. Given that high mantle potential temperatures are assumed to have existed in the Hadean, it follows that the EER melt was generated at high pressure and, therefore, its composition would have been picritic to komatiitic. As the formation age of the EER estimated in our study precedes the last giant, lunar-forming impact, the picritic to komatiitic crust (EER) would most likely have been ejected from the Earth by the last giant impact or preceding impacts. Thus, the EER has been lost, leaving the Earth more depleted than its original composition.
