Isotopic composition of the earth's helium and the problem of the motive forces of tectogenesis

Abstract

The isotopic composition of helium ( 3 He 4 He = R ) contained in underground fluids was studied in different geotectonic provinces. Provincial mean values ( R ) were estimated by a routine statistic procedure. The R- values were found to depend on the age of tectono-magnetic activity ( t): as t increases, R decreases. R min of ∼2 · 10 −8 is observed in the ancient (pre-Baikalian) structures of continents. Much higher R-values (≈1.2 · 10 −5) are characteristic for the oceanic rift zones. R max of ∼2 · 10 −5 is established in Iceland which is considered as a “hot spot”. The source of these types of He isotopic composition appear to be continental crust, depleted mantle reservoir and undepleted mantle. Regional characteristics of the R distribution are similar to those of a terrestrial heat flow ( q). Correlation of the values of R and q observed at each station of He sampling was revealed at the 95% probability level. Quasi-synchronous changes of q and R with geologic time reflect two (geophysical and geochemical) aspects of one and the same general geologic process — tectogenesis. The triple relationship of q , R and t demonstrates that this process is related to the influx of thermal energy and silicate material by the heat-mass flow from the mantle into the lithosphere. The silicate composition of this flow is confirmed by the correlation between R and 87 Sr 86 Sr . In the course of geologic time these evidences of the mantle heat-mass flow are gradually reduced because of cooling of the newly formed blocks of the lithosphere, the dissipation of the mantle He into space, and the generation of the radiogenic isotopes.