Tracing island arc petrogenesis using potassium isotopes

Arc lavas display significant chemical and isotopic heterogeneity mainly due to recycled materials from subducting slabs. However, the extent to which different types of subducted sediments and oceanic crust contribute to the petrogenesis of arc magmas, as well as the roles of the mantle wedge and overlying crust, remain debated. Potassium (K) isotopes have the potential to provide new insights into the processes and sources of arc magmatism because sediments and altered oceanic crust are highly enriched in K and have distinct δ⁴¹K values compared with the mid-ocean ridge basalts and upper mantle (-0.42 ± 0.08‰, 2SD). Here we report K isotopic compositions of 32 well-characterized arc lavas from the circum-Pacific margins. We find low δ⁴¹K values (-0.86‰ ∼ -0.38‰) in the Setouchi arc samples, which we interpret as the result of incorporation of isotopically light sediments into the subarc mantle. The Kurile and Panama arc lavas have high δ⁴¹K values (-0.36‰ ∼ 0.02‰) and their δ⁴¹K values correlate positively with the Ba/Th ratios, indicating ∼0.5–2% fluid additions from dehydrated altered crust. Adakites have variable but overall heavy K isotope compositions (-0.44‰ ∼ -0.01‰) , which correlate with the diagnostic trace-element indicators of Sr/Y, La/Yb and K/U. These results indicate limited K isotope fractionation during metamorphic dehydration of hot slab subduction, and/or fluid metasomatism before the slab melting. Our study suggests that fluids and melts originating from subducting sediments and slabs exerted distinct influences on the origins of different types of arc magmas, demonstrating the significance of K isotopes in studying the petrogenesis of arc magmas.