Slab dehydration and potassium-lithium recycling in the forearc indicated by potassium and lithium isotope compositions of exhumed metabasites

Abstract

Slab dehydration at subduction zones has broad implications for arc magmatism, mantle heterogeneity and elemental cycling. We present the petrography, mineral chemistry and whole-rock potassium (K) and lithium (Li) isotopic compositions of subducted metabasites exhumed from serpentinite mud volcanoes from the Mariana forearc that were recovered during International Ocean Discovery Program Expedition 366, to evaluate the capability of K and Li isotopes for tracking slab dehydration and the evolution of the slab-derived fluids at shallow subduction zones. The results show that the δ41K values of HIMU-type alkali metabasites recovered from the shallow-sourced site (i.e., Fantangisña, −0.34 ± 0.06‰, 2SD, n = 3) are slightly higher than those from the deep-sourced site (i.e., Asùt Tesoru, −0.42 ± 0.20‰, 2SD, n = 8). In addition, all the HIMU-type metabasites have homogeneous radiogenic isotope compositions (e.g., Sr, Nd and Hf), and the clinopyroxenes therein show similar trace elements patterns, indicating they have a common mantle source. These observations together with the positive correlation between δ41K and δ11B suggest that K isotope fractionation most likely resulted from metamorphic dehydration. In contrast, no such correlation was observed for Li isotopes (δ7Li ranging from +3.7‰ to +6.6‰). The Li isotope compositions can be modeled by low-temperature fluid/rock interactions with varying water/rock ratios, implying that the Li isotopic system was mainly controlled by fluid metasomatism within the subduction channel. Notably, EM-II-type metabasites have the lightest K isotope compositions (−0.72‰ to −0.67‰) and heaviest Li isotope compositions (+8.1‰ to +8.3‰) in this study. These anomalous signatures were accompanied with the lowest Nb/Th, εNd(t) and εHf(t), indicating that the K and Li isotope data of EM-II-type metabasites were indicative of their mantle source (e.g., EM-II mantle reservoir). Rayleigh fractionation modeling illustrates that the deeply subducted metabasites are characterized by high K and Li concentrations, and low δ41K, but MORB-like δ7Li. Thus, they may transfer large amounts of fluid-mobile elements into the deep mantle that contributes to the K isotopic heterogeneity in the mantle.