Figure 3. A. Contrasting Pb isotopic composition of Marianas (open circle) and Izu-Bonin (solid circle) arc volcanics. Marianas volcanics form a mixing trend (arrow), almost perfectly coincident with mixtures of ODP Hole 801C sediment (open boxes) and basalt (solid boxes) averages. Drilling at Site BON-8A will test whether the Izu-Bonin Arc trend (arrow) is consistent with different subducted material than for the Marianas. Modern Indian MORB, Pacific MORB, and Honshu Arc data are shown for reference. Data sources: Elliott, et al., 1997; Gill, et al., 1994; Plank and Langmuir, 1998; Castillo et al., 1992a; and Gust et al., 1997. B. Correlation between Ba flux in subducted sediment and Ba enrichment of arc basalts for various arcs (Ant = Northern Antilles, Mar = Marianas, T= Tonga, Mex = Mexico, J = Java, Al = Aleutians, and G = Guatemala) around the world (after Plank and Langmuir, 1993). Open circles = three different sediment flux estimates for the Marianas, based on the three ODP Sites drilled during Leg 129 (800-802) (Plank and Langmuir, 1998). Although there are variations from site to site, the average sediment input to the Marianas is fairly well constrained (+/- 20%). Note Izu volcanics are lower in Ba/Na than Marianas volcanics by a factor of two. Drilling at Site BON-8A will help to test if the low Ba/Na of the Izu volcanics is related to a lower Ba sediment flux. Shown for reference are the average Mariana Ba sediment flux and the flux for a 600-m section of chert (with 125 ppm Ba, similar to the upper radiolarites in Hole 801C; Karl, et al., 1992).
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