Potassium isotopic fractionation during multistage alteration of oceanic crust in the southern Mariana Trench

Abstract

To further constrain the migration trajectory of K isotopes during oceanic crust alteration, we present detailed mineral, elemental, and K isotopic data of oceanic crust samples from the southern Mariana Trench. These samples are divided into four groups with various colors and alteration intensities, in which secondary mineral assemblages indicate a progressive alteration of oceanic crust at low temperatures (<100 °C). Group I samples are fresh basalts with chemical compositions identical to the mid-ocean ridge basalts (MORBs). Their higher δ41K values (0.08‰ to 0.11‰) than MORBs (−0.44 ± 0.17‰) might point to a 41K-enriched magmatic source. Group II samples are slightly altered basalts at the initial stage of low-temperature alteration and have lower δ41K values (−0.31‰ to 0.05‰) than Group I, which are interpreted as a result of chemical exchange with seawater via equilibrium and kinetic isotopic fractionations. Group III samples are celadonite phase-dominated rocks at the second stage. They have higher K2O contents (3.53–8.39 wt%) and lower δ41K values (−0.71‰ to −0.45‰) than Group II, reflecting the preferential incorporation of isotopically light K. In contrast, the clay mineral phase at the late stage dominates Group IV samples with lower K2O contents (1.79–4.20 wt%) and higher δ41K values (−0.22‰ to −0.07‰) than Group III, indicating a loss of isotopically light K during the transformation of the high-K celadonite phase to the low-K clay mineral phase. Overall, systemic variations in secondary minerals, K2O contents, and δ41K values from Group I to Group IV samples suggest a multistage K isotopic fractionation and shed new light on a key role of the high-K celadonite phase in K isotopic behavior during oceanic crust alteration.