CONCURRENT EVOLUTION OF 3HE/4HE RATIO IN THE EARTH'S MANTLE RESERVOIRS FOR THE FIRST 2 GA

Abstract

The differences in 3He/4He ratios between mid-oceanic ridge basalts (MORBs) and ocean island basalts (OIBs) require the existence of at least two distinct mantle reservoirs that have been preserved over long periods of the Earth's history. However, it is unclear how the 3He/4He ratios have evolved in the mantle over time, and when these reservoirs acquired their present-day signatures. Here we report the results of our model calculation for helium evolution in the mantle, and show that, from the time of the Earth's formation to the Late Archean (~2 Ga), the 3He/4He ratios in both the MORB and OIB mantle reservoirs evolved in a rather homogeneous fashion. In contrast to previous models, our model suggests that the distinction in 3He/4He ratios between the MORB-type source and the plume-type source is a relatively recent characteristic of the Earth's mantle.