Hf-Nd isotope decoupling in bulk abyssal peridotites due to serpentinization

- Improved method for REE, HFSE extraction from depleted rocks
- Hf isotopes are not affected by serpentinization.
- Hf concentrations are a reliable tracer of magmatic processes in peridotites.

Serpentinized abyssal peridotites are the product of the chemical interaction between the Earth's oceans and the peridotitic upper mantle. This reaction is dominated by the addition of water and fluid mobile elements to the peridotite, with implications on the cycling of water and fluid mobile elements in subduction zones. Rare earth elements (REE) were generally thought as immobile during serpentinization. However the identification of Nd (and Sr) isotopic disequilibrium between fresh clinopyroxene and their host bulk serpentinized peridotite suggest addition of Nd and other light REE to the peridotite during serpentinization (Frisby et al. (2016)). As with the REE, the high-field strength elements (HFSE) have also been considered immobile and non-reactive during serpentinization. However, this has yet to be explicitly tested. Here we present a modified dissolution and extraction method that improves on the yields of REE and HFSE from highly depleted ultramafic rocks (peridotites) through the use of B(OH)3 during digestion dry-down steps, Fe-precipitation for REE-HFSE sequestration, and liquid-liquid exchange for Fe removal. Using this method we report on the Nd and Hf isotope compositions of bulk rock serpentinized abyssal peridotites from the South-West Indian Ridge (SWIR), Mid-Atlantic Ridge (MIR) and Mid-Caymen Rise (MCR) and on samples where their clinopyroxene Hf, Nd isotopes were previously reported. The bulk rock Hf isotopes are equal to clinopyroxene to within 1 εHf, while Nd isotopes in the bulk peridotite are up to 8 εNd units less radiogenic than their clinopyroxene. Melt-rock reaction between the depleted peridotite endmember and local enriched lavas cannot generate the observed decoupling between Hf and Nd isotopes. Our data is instead consistent with seawater - peridotite interaction as this results in significant shift in Nd isotopes with little to no change in Hf isotopes until extremely high water/rock mass ratios of > 100,000. Our data shows that Hf isotopes are not affected by serpentinization and that Hf isotopes in bulk peridotites better record the magmatic composition of peridotites than Nd or Sr. The largest Nd isotopic difference between clinopyroxene and bulk peridotite is observed in the most LREE-depleted samples, while refertilized samples show little change. Based on the bulk rock (serpentinized) and clinopyroxene (magmatic value) Nd isotope compositions we estimate that 1-34% of the bulk rock Nd budget is seawater derived, but only 1-6% of Sm and < 2% of Lu and Hf in these samples. Therefore serpentinization will lower the Sm/Nd relative to Lu/Hf ratio in a peridotite and this may lead to decoupling between Hf and Nd isotopes upon recycling and aging, assuming no additional fractionation of these elements during the subduction process. Alternatively, if Nd becomes fluid-mobile upon dehydration during subduction then the excess Nd in the serpentinized peridotites may become an additional reservoir for Nd in arc volcanism.