Difference in dehydration condition and variation in amount of dehydration water: a possible origin of HIMU-FOZO-PREMA reservoirs

Abstract:

Isotopic composition of the ocean island basalts (OIB) are commonly explained by mixing of mantle reservoirs, such as depleted MORB mantle (DMM), HIMU (high-u: u = ²³⁸U/²⁰⁴Pb) EM1 (Enriched Mantle 1) and EM2. In addition, reservoirs whose isotopic compositions are common and intermediate has been pointed out, such as, FOZO (Focal Zone) and PREMA (Prevalent Mantle). Although these intermediate reservoirs have been used to describe the isotopic distribution of OIB, geochemical relationship between the intermediate and conventional reservoirs are still ambiguous. To elucidate the relationship, geochemical modeling has been conducted that focuses on origin of HIMU, FOZO, and PREMA. Our model includes magma differentiation process at mid-ocean ridges and variable dehydration conditions at subduction zones. The effect of magma differentiation process at a mid-ocean ridge was estimated by comparing the chemical composition of common and differentiated MORBs, which are determined based on the magnesium number of MORB. The variation in dehydration conditions are examined with different amount of water of dehydration with partition coefficients of aqueous and supercritical fluids reported by Kessel et al. (2005). The results suggest that the common MORBs without subduction modification could evolve to have isotopic signatures of PREMA after ca. 1-2 Ga since they are recycled back into the mantle, suggesting the importance of recycling of “dry” MORBs. In contrast, intensively dehydrated common MORBs can produce the FOZO isotopic signatures. Thus, variation in amount of dehydration water could be responsible for the FOZO-PREMA isotopic array. For the production of HIMU isotopic signature, additional conditions may be required, i.e., prolonged dehydration from sub-arc to sub-backarc. Difference in dehydration pressure could induce chemical fractionation between Pb, Th and U. We also found that only the strongly dehydrated differentiated MORBs can produce extreme HIMU isotopic signature. Magma differentiation at a mid-ocean ridge and a variable amount of water of dehydration at different pressure during subduction can explain the geochemical connection between HIMU, FOZO, and PREMA.