Genesis of distribution of strontium isotope in igneous rocks

The data obtained on the Earth’s hot heterogeneous accretion and fractionation of a global magma ocean can well explain specific isotope ages and Sr isotope ratios of igneous rocs. The oldest ages of the onset of formation of acid igneous rocks are due to generation of large volumes of acid residual melts upon low-pressure fractionation of the near-bottom parts of an early magma ocean. These parts crystallized under effect of the growing load of the upper parts formed during accretion. The long-term crystallization and fractionation of the deep layers of the magma ocean are responsible for the younging of ages of the onset of formation of various magmas in ancient platforms in the following sequence: acid - alkaline intermediate and basic - alkaline ultrabasic and carbonatitic - kimberlitic. The growing impact heat release in the process of accretion provided the existence of reverse geothermal gradient in the mantle and to the absence of present-day geodynamic setting in the Early Precambrian. They appeared in the Late Proterozoic following the mantle heating by the core. This is the reason for relatively young ages of oceans, oceanic basites and subduction-related igneous rocks.