Composition and structure of minerals from Paleoproterozoic sedimentary rocks in the section of the Kola Superdeep Borehole and their homologs in outcrops

Abstract

Geochemical and structural features of microcline, quartz, and carbonates from the sedimentary Luchlompolo Formation (III SF) of the Pechenga volcano sedimentary complex were compared in outcrops at a depth of 20–400 m in the section of satellite Borehole IX and of 4.8 km in the Kola Superdeep Borehole (SD-3). The rocks occupying an identical position in the section of this formation are homologous and characterized by similar composition and spectroscopic properties of rock-forming minerals. A certain variance in defect structure is revealed in minerals from shallow and deep levels. The crystal structure of microcline and dolomite from deep levels of the Kola Superdeep is distinguished by a larger amount of defects. The isomorphic replacement of Ca with Mn ions in dolomite increases with depth, probably owing to more intense metamorphism and further long-term standing at elevated pressure and temperature. The concentration of Al admixture in quartz decreases with depth, and isomorphic substitution of Al for Si shifts toward compensation with the alkali element in response to the same factors. The decrease in the number of natural paramagnetic Al centers in quartz and decrease in oxygen vacancies with depth is regarded as an effect of the current PT conditions.