In the Andean Central Southern volcanic zone, basalts from small eruptive centers near the large composite center Volcan Villarrica are poor in fluid mobile elements, such as B, Cs, Rb, K, Pb, Ba, and U, compared with concurrently erupted Villarrica basalts. New 10Be and U-series isotopic data for these centers show that fluid mobile element–poor small eruptive center basalts have small 10Be/9Be ratios (1.6–1.9 × 10−11) and (238U/230Th) activity ratios near 1.0, whereas basalts from Villarrica show 238U enrichment and larger 10Be/9Be ratios (4.0–6.4 × 10−11). These results suggest that small eruptive center basalts include materials derived from the subducted lithosphere that were stored in the mantle wedge for 350 k.y. to 3 m.y. That these materials are poor in fluid mobile elements may reflect fluid expulsion during solidification or their formation in an initially hotter subduction setting. In contrast, the composite center basalts sample materials rich in fluid mobile elements that were recently transferred into the mantle wedge from the subducted lithosphere. The results confirm that mantle wedges in subduction zones include subducted materials added to the wedge over both long and short time scales.

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