What process(s) is responsible for the geochemical difference between PREMA, FOZO and HIMU?

Isotopic variation in ocean island basalts (OIB) is commonly explained by mixing of distinct and isolated reservoirs in the Earth’s interior (White, 1985; Zindler and Hart, 1986; Hofmann, 1997; Stracke, 2012). In early research on mantle reservoirs, the isotopic composition of OIBs was mainly explained by the mixing of depleted Mid Ocean Ridge Basalt (MORB) mantle (DMM) and three enriched reservoirs: high-µ (HIMU; µ = 238U/204Pb), Enriched Mantle 1 (EM1) and Enriched Mantle 2 (EM2) with isotopic compositions that are enriched extremes. In addition to these enriched/depleted reservoirs, the importance of reservoirs with isotopic compositions that are common and intermediate has been pointed out; these are the Focal Zone (FOZO; Hart et al., 1992), the Common component (C; Hanan and Graham, 1996), Prevalent Mantle (PREMA; Zindler and Hart, 1986) and Primitive Helium Mantle (PHEM; Farley et al., 1992). As these intermediate reservoirs could be characterized by high 3He/4He ratios, which might be suggestive of their lower mantle origin, origin(s) of these reservoirs should the important for better understanding of chemical evolution of Earth.

In this study, geochemical modeling of the origin of PREMA, FOZO and HIMU was conducted from the perspective of chemical fractionation at mid-ocean ridges and subduction zones. Chemical fractionation at a mid-ocean ridge, i.e., crystal fractionation, was estimated by comparing the chemical composition of common and differentiated MORBs, which are determined based on the magnesium number of MORBs from the Mid-Atlantic ridge and East Pacific Rise. The results suggest that the common MORBs could evolve to have isotopic signatures of the PREMA after 1-2 Ga since they have recycled back into the mantle. In contrast, high degree of dehydration of common MORBs at a subduction zone can produce FOZO isotopic signatures. Thus, the FOZO-PREMA isotopic array can be explained by the recycling of common MORBs that release various amount of water during subduction, which is consistent with the ubiquitous occurrence of OIBs on the PREMA-FOZO array. The differentiated MORBs can only produce HIMU if the amount of dehydrated water is large (~4 %). As the frequency of differentiated MORBs is much less than that of common MORBs, the rare occurrence of HIMU is reasonably well understood.