Geodynamics, Flows of Volatile Components, and Their Exchange between the Mantle and the Earth's Upper Shells

摘要

The distribution of flows of volatile components between the mantle and the upper shells of the Earth—including the crust, hydrosphere, and atmosphere—has been analyzed. The analysis is based on the idea that the masses of volatile components, together with magmatic melts, removed to the Earth's surface, their masses submerged along with the subducted lithosphere, and the masses of volatiles contained in the upper shells are mutually balanced. The leading role of plate tectonics and mantle plumes in formation of the Earth's crust was accepted as a geodynamic background of such balance. It was also assumed that the rates of geologic processes varied insignificantly through time. By summarizing the water, chlorine, fluorine, and sulfur contents in melt inclusions and quenched glasses that mimic compositions of natural magmas, we estimated the average volatile contents in the magmatic melts related to the major geodynamic settings, including mid-ocean ridges, oceanic islands and lava plateaus, island arcs and active continental margins, and intracontinental igneous provinces. Based on these estimates, the masses of volatiles, along with melts, removed to the Earth's surface have been assessed, beginning from the planet's origination. A scope of the descending flows of volatiles was determined from information on abundances of volatile components in the crustal rocks and from masses of oceanic and continental crust subducted during geological history. The obtained balance unequivocally indicates that the Earth had a thick primary exosphere mainly consisting of water and, to a lesser extent, chlorine, whereas contributions of sulfur and fluorine were insignificant. The water mass in the primary exosphere was more than 1.5 times larger than in the present-day hydrosphere and crust. Subduction played the crucial role in the fate of protoocean. The amount of water that was taken into the subduction zones is comparable with its mass in all of the present-day outer shells. The balance calculations show that practically all chlorine of the primary Earth's exosphere, as well as the prevalent mass of sulfur and about 60% of fluorine released along with melts over geological history, were recycled through the subduction zones. The origin of enriched mantle sources that feed intraplate igneous provinces is also related to the recycled lithosphere. In particular, this is indicated by enrichment of magmas from oceanic islands and intraconlinental igneous provinces in H_2O, Cl, and F relative to the melts from mid-ocean ridges by factors 1.5, 2.5, and 4, respectively. Relationships between the thick primary hydrous exosphere of the Earth and its geodynamic evolution through geological history are discussed.