INTRODUCTION

Fluid production and transport affect the thermal regime of convergent margins, metamorphism in the suprasubduction zone region, diagenesis in forearc sediments, biological activity, and, ultimately, the composition of arc and backarc magmas.

Cold (~2°C) springs of fluid with salinity lower than seawater have been found in several mud volcanoes in the Mariana intraoceanic subduction complex between the northwestward subducting Pacific plate and the overriding Philippine plate. Volatiles released from the downgoing Pacific plate hydrate the overlying mantle wedge and convert depleted harzburgite to low-density serpentinite. The resulting serpentinite mud, containing variably serpentinized harzburgite clasts, ascends buoyantly along fractures and extrudes at the seafloor, where it forms large (30 km diameter, 2 km high) mud volcanoes along the outer Mariana forearc in a band that extends from 50 to 120 km behind the trench axis (Fryer, 1992; Fryer et al. 1985, 1995, 2000).

Relative to seawater, the deep upwelling fluids in South Chamorro Seamount (Leg 195) and Conical Seamount (Leg 125) have higher to much higher sulfate, alkalinity, K, Rb, B, light hydrocarbons, and ammonium concentrations; higher pH, Na/Cl, ¹⁸O, and D values; and lower to much lower chloride, Mg, Ca, Sr, Li, Si, and phosphate concentrations and Sr isotopic ratios (Benton, 1997; Benton et al., 2001; Mottl, 1992; Mottl et al., 2003). By contrast, pore fluids from inactive Torishima Forearc Seamount (Leg 125) show different behaviors for these geochemical tracers, purportedly produced by reaction of cold seawater with harzburgite (Mottl, 1992). Therefore, the deeply sourced upwelling fluids within the two active serpentine mud volcanoes clearly cannot have originated by simple reaction of seawater with peridotite, but rather by dehydration of sediments and altered basalt at the top of the subducting Pacific plate (Fryer et al., 1999; Mottl et al., 2003).

The South Chamorro Seamount F, Cl, Br, and B concentrations and B isotopes are discussed below. They add new insights to previous studies of the Mariana forearc region (Benton, 1997; Benton et al., 2001; Savov et al., 2004) on the geochemical cycling, fluid production and transport, and origin of probably the most pristine slab-derived fluids recovered from a subduction zone.

Halogens are excellent tracers of fluids during subduction because their geochemical behavior is dominated by strong partitioning into the fluid phase. Chloride, Br, and F substitutions in minerals typically involve sites occupied chiefly by hydroxyl ions and, as such, are at least partly covalently bonded in the crystal structure. Therefore, it has been predicted that Cl, Br, and F exchange will be a sensitive function of fluid pH (Zhu, 1993). Temperature is also influencing the halogen exchange dynamics. The large size differences between the hydroxyl ions, fluoride, chloride, and bromide suggest that their relative exchange should also be sensitively affected by pressure (Vanko, 1986). B isotope fractionation between solids and fluids is thought to relate to the preferential incorporation of ¹⁰B-rich B(OH)₄ into silicate minerals. Thus, it is strongly influenced by pH and temperature as well (Spivack et al., 1987; You et al., 1996; Palmer et al., 1998; Sanyal et al., 2000). B isotope ratios and concentrations may provide evidence for serpentine formation and clay alteration at depth (Spivack et al., 1987; Palmer, 1996; Deyhle et al., 2001; Deyhle and Kopf, 2001, 2002; Benton et al., 2001). When serpentine forms, B uptake is high and isotope ratios (¹¹B/¹⁰B) in the residual solution increase (Bonatti et al., 1984; Spivack and Edmond, 1987; Benton et al., 2001) because of preferential uptake of ¹⁰B by solids. The adsorption of B also occurs at low temperature, and the adsorbed species is predominantly the light isotope ¹⁰B as well (Spivack et al., 1987; Palmer and Swihart, 1996). In general, at greater depths and thus increasing temperature and pressure, B enriches in fluids by processes such as release of adsorbed B from clays, mineral dehydration reactions, possible release of structurally bound B, and alteration of volcanic rocks (Seyfried et al., 1984; Spivack et al., 1987; You et al., 1996; Palmer and Swihart, 1996, Deyhle and Kopf, 2001, 2002).