Zinc isotopic systematics of Kamchatka-Aleutian arc magmas controlled by mantle melting.

Geochemical characteristics of arc magmas reflect incorporation of subducted materials to their mantle wedge sources in subduction zones. Subduction component addition has been proposed to modify the Zn isotopic budget of arc magmas. However, the lack of a systematic study on Zn isotopic compositions of arc magmas hampers a better understanding of Zn isotope behavior in subduction zones. To address this issue, we have determined Zn isotopic compositions of 37 well-characterized arc rocks from the Kamchatka and Central-Eastern Aleutian arcs. These rocks record contributions of fluids and melts derived from altered oceanic crust (AOC) without overprints of sediment melts and thus allow focus on the potential effects of AOC-derived fluids and melts on the Zn isotopic budget of arc magmas. For comparison, nine basalts from the Gakkel, Mid-Atlantic and Southeast Indian Ridges, and the Lau Basin and nine adakites from Central America were also analyzed. Rocks from the Kamchatka-Aleutian arcs have δ 66 Zn from 0.16 to 0.31‰ that are mostly similar to those of mid-ocean ridge basalts (MORBs), back-arc basin basalts (BABBs), and adakites (δ 66 Zn = 0.23–0.33‰), but a significant number of arc samples also display δ 66 Zn higher than that of the depleted MORB-type mantle (DMM), indicating Zn isotope fractionation during magmatic processes and/or modifications of the mantle wedge Zn isotopic budget by incorporation of AOC-derived fluids and melts. The lack of correlations of δ 66 Zn with geochemical indicators of magma differentiation (e.g., MgO, SiO 2 , and Zn/Fe T ) indicate that fractionation of olivine, pyroxene, and magnetite has a limited effect on the Zn isotopic compositions of arc magmas. Even though the mantle sources of arc rocks investigated here are strongly affected by AOC-derived fluids and melts that have higher δ 66 Zn compared to the DMM, we observe no systematic variations of δ 66 Zn with indicators of subduction components (e.g., Ba/La, Ba/Th, Sr/Y, Hf/Lu and 87 Sr/ 86 Sr). This suggests that insignificant transport of Zn from the subducting Pacific slab to the Kamchatka and Central-Eastern Aleutian mantle wedge. Our model calculations suggest that the observed offset of δ 66 Zn between the mantle and arc magmas can be attributed to isotope fractionation during partial melting with no need for contributions from subduction components. [ABSTRACT FROM AUTHOR]