Effect of Serpentinite Dehydration in Subducting Slabs on Isotopic Diversity in Recycled Oceanic Crust and Its Role in Isotopic Heterogeneity of the Mantle

We conducted geochemical modeling of the isotopic evolution of subducted oceanic crust that takes into account the chemical variation produced at mid-ocean ridges and in subduction zones, and examined the suitability of our model for generating the high-mu (HIMU), focal zone (FOZO), and prevalent mantle (PREMA) mantle components. Chemical variation produced at mid-ocean ridges was represented by the chemical compositions of two groups of mid-ocean ridge basalts (depleted and enriched). Chemical variation produced in subduction zones was investigated with pressure-temperature paths of slabs of different ages (governing the physicochemical conditions of element exchanges), as determined using representative subduction zones involving young (hot), intermediate, and old (cold) slabs. The results suggest that dehydration of oceanic crust cannot alone produce isotopic variation beyond the bounds of PREMA compositions. Producing the wider range of isotopic diversity from PREMA to FOZO requires various degrees of element partitioning between subducted oceanic crust and fluids (aqueous or supercritical) released by dehydration of slab serpentinite. The extremely radiogenic Pb isotopic signature of HIMU can only be produced by extensive reaction between subducted oceanic crust and fluids derived from slab serpentinite along the specific geothermal gradient resulting from the relatively slow descent of moderately old slabs. The rarity of such tectonic conditions explains the scarcity of HIMU.