Figure 25. Model evolutionary trends for 4He/3He ratio in the upper mantle. All models outlined below envisage: the upper mantle as severely degassing reservoir; the parent elements are accumulating into the continental crust growing through time as in [Kramers and Tolstikhin, 1998]. 4He/3He ratios in the upper mantle were similar to those trapped by the UACC ~3 Ga ago or earlier, in contrast to the age of mantle metasomatism derived from Rb-Sr systematics, 0.43 - 0.72 Ga, depending on accepted Rb/Sr, 87Sr/86Srinitial and models of Sr isotope compositions in the upper mantle, i.e., at most 350 Ma before the formatioin age of the Complexes, 370 Ma (Section 2). Models: A - a completely isolated (during whole history of the earth) lower mantle having the bulk silicate earth concentrations of U and Th [Azbel and Tolstikhin, 1992]; B - a moderately degassed lower mantle having ignorantly low abundances of the parent elements [O'Nions and Tolstikhin, in preparation]; the model is similar to that suggested by Albarede [1998]); C - a moderately degassed substantially isolated lower mantle having the bulk earth abundances of the parent elements; this model satisfies the Pu-U-I-Xe systematics [O'Nions and Tolstikhin, in preparation]; D - same as C but satisfies to U-Th-K-He-Ne-Ar systematics [Tolstikhin and Marty, 1998].