Sediment incorporation in island-arc magmas: Inferences from 10Be

The radioisotope ¹⁰Be has been used as a tracer to evaluate subduction and recycling of sediments in island arcs. As a cosmogenic isotope strongly enriched in oceanic sediments, it is especially suitable for monitoring sediment subduction.We report here ¹⁰Be results for 106 arc volcanic rocks and 33 basalts from mid-ocean ridges, oceanic islands, continental rifts, and continental flood basalt provinces. The 33 basalts from non-arc environments all contain < 1 × 10⁶ atom/gram (g⁻¹) and form a reference or “control group” against which the arc data may be compared. Lavas from the Sunda, Mariana and Halmahera arcs, each with 12 samples, are indistinguishable from the control group. Nine samples from Japan range from 0.1 × 10⁶ to 13.5 × 10⁶ g⁻¹ of ¹⁰Be. Lavas from the Aleutians and Central America (excluding Costa Rica) consistently have ¹⁰Be contents greater than those of the control group, extending to 15.3 × 10⁶ g⁻¹ for the Aleutians and 24 × 10⁶ g⁻¹ for Central America. Both the Japan and Central American arcs show a geographic variation in ¹⁰Be abundances; the Aleutian arc does not.Conceptually, four models may be used to explain the incorporation of ¹⁰Be in island arcs. Model 1 is the limiting case calculation presented in Brownet al. (1982). Physically, it corresponds to an assumption that all ¹⁰Be in the uppermost sediment layers is carried to depth, the ¹⁰Be is mechanically decoupled from the sediment pile and is added to the arc magma source region. This model results in the highest calculated ¹⁰Be contents in arc lavas. In the more physically reasonable Model 2, the ¹⁰Be-rich upper sediments are assumed to mix with the deeper, ¹⁰Be-poor, sediments and the sediments are subsequently incorporated into the source region. Model 3 assumes ¹⁰Be to be incorporated from sedimentary layers encountered during magma ascent. In Model 4 only sediments contained within grabens in the downgoing slab may be subducted. As yet insufficient data exist to permit conclusive evaluation of these models, but correlations between ¹⁰Be contents in arc volcanic rocks and recent sedimentation rates and sediment thickness, measured outboard of the trench, suggest that Model 2 may be the most important mechanism for ¹⁰Be incorporation in many island arcs.