The oceanic crust alteration is a key geochemical process that affects the K isotopic compositions of oceans and the oceanic crust, thus playing a vital role on global K cycle. However, little is known about the migration trajectory of K isotopes during a progressive alteration of oceanic crust. In this study, we report 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, and 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.

Potassium isotopes,Mariana trench,Oceanic basalts,Multistage alteration