Nature of the chemical heterogeneity of the continental lithospheric mantle

Abstract

The paper reports data on the chemical composition of mantle peridotite xenoliths from kimberlites and alkaline basalts that represent the continental lithospheric mantle (CLM) beneath Early Precambrian and Late Proterozoic-Cenozoic structures, respectively. In order to identify compositional trends during the melting of primitive material and propose the most reliable criteria for constraining the conditions of this process and its degree, we analyzed literature data on the melting of spinel and garnet peridotites within broad temperature and pressure ranges. It was determined that the degree of melting (F%) of pristine peridotite of composition close to that of the primitive mantle (PM) can be deduced from the Mg/Si and Al/Si ratios in the residue; an equation was proposed for evaluating F from the Mg/Si ratio. The Ca/Al ratio of residues at low (1–1.5 GPa) pressures and degrees of melting from 2–3 to 20–25% increases several times but decreases with increasing F at pressures higher than 3 GPa. The Na partition coefficient between melt and residue decreases at increasing pressure and approaches one at a pressure close to 20 GPa. Residues after low-degree melting are strongly depleted in Ti, Zr, Y, and Nb but are enriched in Cr. The application of these criteria to the composition of xenoliths brought to the surface from the mantle occurring beneath tectonic structures of various age led us to conclude that compositional heterogeneities of CLM (particularly the variations in the concentrations of major and certain siderophile elements) are controlled, first of all, by the melting of the mantle source material. These processes occurred under various thermodynamic conditions (T, P, and \( f_{O_2 } \)) and differed in their intensity, and this predetermined the compositional diversity of the residual mantle material (its concentrations of Mg, Al, Si, Ca, Na, K, Ni, Co, V, and Cr). Our results are principally consistent with the hypothesis of the global magmatic ocean. It is thought that the early phases of its consolidation were variably controlled by the fractionation of minerals, for example, majorite. Moreover, heterogeneities in the distribution of siderophile elements could be partly predetermined by changes in the properties of these elements at ultrahigh temperatures and pressures. The processes of partial melting were the most intense during the early evolution of the mantle (perhaps, in the Early Precambrian), and hence, the mantle has different chemical composition beneath Archean cratons and Phanerozoic foldbelts.