Different Types of CaSiO3 in the Earth’s Mantle and Its Geochemical Heterogeneity: the Juina Area in Brazil as an Example

Abstract

CaSiO₃ inclusions in diamonds from the Juina area in Brazil have low Fe (0.08–0.53 wt % FeO) and Al (0–1.52 wt % Al₂O₃) concentrations; they belong to the ultramafic association. Two different types exist among CaSiO₃ grains. Type I has a normal REE_n pattern, while type II has a sinusoidal REE_n pattern. Type I CaSiO₃ associates with high-Mg–high-Ni protogenetic ferropericlase, and type II associates with high-Fe–low-Ni syngenetic ferropericlase. Thus, type I CaSiO₃ grains are protogenetic, formed, like high-Mg–high-Ni ferropericlase, in the upper part of the lower mantle as davemaoite (CaSi-perovskite), and type II CaSiO₃ were formed in the transition zone as breyite. The enrichment of CaSiO₃ in REE, particularly in LREE, corresponds to high values of their partition coefficient CaSiO₃/melt and shows the CaSiO₃’s origin from a mantle material under high pressures. The isotope characteristics of the studied CaSiO₃ demonstrate strong geochemical heterogeneity in the inclusions. The ⁸⁷Rb/⁸⁶Sr ratios in type II CaSiO₃ (0.127–3.23) are 3–4 orders higher than in type I (0.0008). Even within a single diamond, different CaSiO₃ grains have ⁸⁷Rb/⁸⁶Sr ratios varying from 0.014 to 3.23. The same is true for U/Pb isotope systematics (e.g., ²³⁸U/²⁰⁶Pb varies in one sample in an order of magnitude from 0.031 to 0.312) and, to some extent, for Sm/Nd ratios. This implies the geochemical heterogeneity in Deep Earth on a very small scale.