Subduction recycling of sediments and altered oceanic crust in marine systems is intimately linked to the systematics of both fluid-mobile and volatile elements, including iodine and boron. The relative abundance and isotopic signature of these two elements in marine sediments are distinct from those of igneous rocks, and each has been used as a tracer of the dehydration processes in the forearc (e.g., You et al., 1993; Deyhle and Kopf, 2002; Kopf et al., 2000; Deyhle et al., 2001; Muramatsu et al., 2001) and of subduction recycling (e.g., Leeman and Carr, 1995; Ryan et al., 1996; Noll et al., 1996; Snyder et al., 2003, 2002; Snyder and Fehn, 2002). Until recently, forearc studies have focused mostly on fluid chemistry along the accretionary portions of forearcs, where dewatering and early diagenetic releases of iodine and boron have been confined to relatively low temperatures along the décollement. Active fluid-rock interactions at greater depths have largely eluded researchers, even though they are important in determining the budget of fluid-mobile elements that are transported to subarc depths.
We present recent data comparing the relative mobility of iodine and boron in submarine mud volcanoes recovered from the Mariana forearc region during Ocean Drilling Program (ODP) Legs 125 and 195 (Fig. F1). Serpentinized peridotites, serpentinite muds, and pore fluids recovered from these mud volcanoes provide a unique window into the processes occurring at depths of ~30 km at the top of the subducted Pacific plate (Benton et al., 2001; Mottl et al., 2003) and, therefore, can bridge the spatial gap between early forearc processes and island arc devolatilization. Earlier investigations on the production of metamorphic fluids associated with serpentinization in the western United States noted that the waters were alkaline and contained significant concentrations of boron (as high as 28,000 µM) (Barnes and O'Neil, 1969; Barnes et al., 1972). In the Mariana marine environment, Benton et al. (2001) determined 
11B values in mud and clasts from Conical Seamount and found them to be roughly 30
lighter than seawater and 5 heavier than most arc volcanics. They hypothesized that a progressive release of fluids enriched in 11B as sediments and altered oceanic crust is transported to depth. The work of Wei et al. (this volume) as well as Savov et al. (2004) confirms these trends by looking at the 11B of pore fluids, clasts, and mud from both the Conical and South Chamorro seamounts.
Despite a growing body of evidence regarding how boron behaves during its migration from trench to forearc to arc, relatively little is known regarding how the boron system is coupled with other elemental systems. In this investigation, we focus on the relationship between iodine, which is concentrated in marine organic matter, and the presence of boron in both mineral and fluid phases. Recent investigations of iodine in the same boron-rich springs hosted in serpentinites that were studied by Barnes et al. (1972) suggest a general correlation between the two elements (Hurwitz et al., 2004). High iodine concentrations are also accompanied by high boron concentrations in a variety of springs and fumaroles (Goff and McMurtry, 2000), suggesting that both elements are a significant component of island arc volatiles. By measuring iodine and bromine concentrations in both the fluid and the solid phases of the Mariana mud volcanoes we can infer how dehydration reactions influence the distribution of these elements in the early stages of subduction, before they are introduced into the subarc mantle.
The large ionic radius of iodine makes it highly incompatible in most igneous mineral phases (McDonough and Sun, 1995; Déruelle et al., 1992; Muramatsu and Wedepohl, 1998). Because it is biophilic (Elderfield and Truesdale, 1980), iodine is an important tracer of the fate of organic matter at convergent plate margins. Bulk iodine determinations for a broad variety of rock types by Muramatsu and Wedepohl (1998), yielded fairly uniform concentrations of 0.05–0.07 µmol/kg for igneous rocks, consistently less than the average concentration for sedimentary rocks of 11 µmol/kg. An earlier investigation (Becker et al., 1968) yielded similar results for a variety of ultramafic rocks (0.07–5 µmol/kg) but also suggested that carbonatites in the lower crust may be slightly enriched in iodine (3.5–7.8 µmol/kg). Similar work by Déruelle et al. (1992) concluded that because iodine is enriched in the crust and depleted in the mantle, then the overall iodine cycle must be similar to that of the noble gases, insomuch as these elements are released before they can be introduced into the upper mantle (Staudacher and Allégre, 1988). Despite this, it has been hypothesized that subducted organics contribute significantly to the volatile budget of carbon and nitrogen in island arc systems (e.g., Sano and Marty, 1995; Sano et al., 1998; Fischer et al., 2002) and studies of forearc metasedimentary rocks suggest that at least a portion of the subducted volatile elements are delivered to the subarc mantle (Bebout, 1995). Thus, it is likely that a portion of the marine iodine reservoir is also subducted along with the other volatile components. Subsequent studies have demonstrated that the marine-cosmogenic signature of subducted 129I is preserved in island arc fumaroles and geothermal systems (Snyder et al., 2002; Snyder and Fehn, 2002; Fehn and Snyder, 2003). The mineral phases that might enhance the transfer of iodine to depths beneath the mantle wedge are unknown; however, previous work has focused on quantifying both the marine input (Muramatsu and Wedepohl, 1998) and the volcanic output (Snyder and Fehn, 2002) without specifying a mechanism for iodine transfer.
