Mantle convection and diamonds

Abstract

Research subject. The present evolutionary stage of geodynamic theory is associated with the idea of thermochemical convection of various levels in the Earth's mantle, where the centrifugal branches are represented by plumes, and the centripetal - by subduction zones. Aim. The study of diamonds contributes to an understanding of when, at what level in mantle, under what P-T conditions and geochemical environment particular diamonds originated, which were then transported by centrifugal convection flows to the Earth's surface, thereby permitting characterization of this flow. Materials and methods. Generalization of published materials and characterization of mineral inclusions in diamonds allow the general structure of mantle convection to be clarified in different epochs and different regions. Results. The data obtained on mineral inclusions in diamonds, along with the experimental data on the P-T conditions of their mineral parageneses and geophysical data on mantle properties, indicate that the depth of diamond formation varies from the lower lithosphere in the upper part of the upper mantle (≈150-250 km) to the bottom of the lower mantle. At the same time, the diamonds containing mineral inclusions, characteristic of the lower mantle, account for only the first percents of the general number of diamonds. Conclusions. The transport of diamonds from different depths of their origin is a reliable indication of convection processes (as a plume activity) in the mantle. This information provides evidence to the real existence of plumes, which is important in the context of ongoing discussions on the depth of their origin. However, the study of mineral inclusions in diamonds, particularly in superdeep diamonds, is a challenging task due to the retrograde changes, resorption and sometimes complete dissolution on their way to the surface. These circumstances minimize the probability of occurrence of superdeep diamonds and require consideration when making judgements about the reality of existence of superdeep diamonds.