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

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 841K values of HIMU-type alkali metabasites recovered from the shallow-sourced site (i.e., Fantangisn similar to a,-0.34 +/- 0.06%0, 2SD, n = 3) are slightly higher than those from the deep-sourced site (i.e., Asut Tesoru,-0.42 +/- 0.20%0, 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 841K and 811B suggest that K isotope fractionation most likely resulted from metamorphic dehydration. In contrast, no such correlation was observed for Li isotopes (87Li ranging from +3.7%0 to +6.6%0). 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%0 to-0.67%0) and heaviest Li isotope compositions (+8.1%0 to +8.3%0) in this study. These anomalous signatures were accompanied with the lowest Nb/Th, eNd(t) and eHf(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 841K, but MORB-like 87Li. Thus, they may transfer large amounts of fluid-mobile elements into the deep mantle that contributes to the K isotopic heterogeneity in the mantle.