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

Abstract CaSiO 3 inclusions in diamonds from the Juina area in Brazil have low Fe (0.08–0.53 wt % FeO) and Al (0–1.52 wt % Al 2 O 3 ) concentrations; they belong to the ultramafic association. Two different types exist among CaSiO 3 grains. Type I has a normal REE n pattern, while type II has a sinusoidal REE n pattern. Type I CaSiO 3 associates with high-Mg–high-Ni protogenetic ferropericlase, and type II associates with high-Fe–low-Ni syngenetic ferropericlase. Thus, type I CaSiO 3 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 CaSiO 3 were formed in the transition zone as breyite. The enrichment of CaSiO 3 in REE, particularly in LREE, corresponds to high values of their partition coefficient CaSiO 3 /melt and shows the CaSiO 3 ’s origin from a mantle material under high pressures. The isotope characteristics of the studied CaSiO 3 demonstrate strong geochemical heterogeneity in the inclusions. The 87 Rb/ 86 Sr ratios in type II CaSiO 3 (0.127–3.23) are 3–4 orders higher than in type I (0.0008). Even within a single diamond, different CaSiO 3 grains have 87 Rb/ 86 Sr ratios varying from 0.014 to 3.23. The same is true for U/Pb isotope systematics (e.g., 238 U/ 206 Pb 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.