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 degrees C). Group I samples are fresh basalts with chemical compositions identical to the mid-ocean ridge basalts (MORBs). Their higher 841K 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 841K 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 841K 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 841K 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 841K 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.