Genetic Role of Calcium Content in Olivine Crystals of Ultramafic and Mafic Rocks

Abstract

The results of genetic systematization of olivine compositions formed both under experimental conditions and its natural differences from rocks of basic and ultramafic compositions of various depth facies and formed in various geodynamic settings are presented. Similar work [53], performed in the 70s of the last century and has been actively cited to this day. Generalization of the subsequent accumulated data showed the error of one of the main conclusions in this work – the dependence of the Cao content in olivine on the hydrostatic pressure during its formation from the melt. According to experimental studies conducted in recent years, up to a pressure of 29 GPA, the content of calcium in olivine, which has grown from the main-ultrabasic melts, does not depend on the pressure and is not lower than 0.1 wt.% CaO. Experiments in solidus conditions involving fluid and natural data have demonstrated that one of the leading factors affecting the calcium content in olivine are metasomatic processes that lead to the removal of calcium from olivine crystals. The role of metasomatic transformations of olivine in terms of calcium content, despite the apparent insignificance of secondary changes in it, is clearly visible in the examples of basalts and gabbro of the modern oceanic crust and the same facies differences in ophiolite complexes. The wide development of metasomatic transformations of igneous rocks of various facies and ages indicates the need to take into account the calcium content in olivine as an equilibrium criterion when calculating temperatures and pressures for paragenesis involving olivine. Olivines, which are part of ultrabasic xenoliths carried out by sub-alkaline magmas, including those carried out by kimberlites, are overwhelmingly represented by low-calcium differences. Low levels of calcium in olivines from these mantle fragments suggest that magmatic melts of the main-ultramafic compositions are not in equilibrium with the mantle substance to depths of about ~ 200 km, and possibly more. Among the compositions of olivines from fresh effusive rocks, its inclusions and microlites are mostly represented by calcium-containing differences. Only in kimberlites, almost all the differences in its crystals (inclusions, microlites) in the rock are represented by low-calcium differences. Olivines included in diamonds are also overwhelmingly represented by low calcium differences. This suggests that the composition of kimberlite olivine is associated with metasomatic transformations, and the growth of diamonds from kimberlites is due to the fluid. Inside the natural single crystals of diamond, there are polymetallic films buried in the body of crystals. Similar films were formed on the faces of diamond crystals formed in the Lav pores (that is, almost on the surface of the day) in 2012-13 of the Tolbachinsky Fissure eruption. These data allow us to create artificial "soft" conditions (CVD, solution, etc.) for the growth of single-crystal diamond films on similar polymetallic or single-element films. For these purposes, elements such as zirconium, dysprosium, erbium, and others can be used.