PRIMARY RESULTS

In addition to the Leg 205 pore fluids analyzed for F, Br, Ba, Rb, and Cs concentrations and ⁸⁷Sr/⁸⁶Sr and ⁷Li values and bulk sediments analyzed for Ba, Rb, and Cs concentrations, Leg 170 pore fluids were also analyzed for F and Br concentrations.

Fluoride concentrations and ⁸⁷Sr/⁸⁶Sr and ⁷Li/⁶Li ratios in the basal sections of Sites 1039 and 1253 (Figs. F1, F3, F9, F10, F11) show the same reversals in concentrations and isotope ratios that approach modern seawater values as observed during Leg 170. Site 1253 is ~1.5 km closer to the trench than Site 1039, suggesting that the observed trends are regional. These chemical and isotopic trends are driven by a fluid flow system in the upper igneous complex and/or basement that governs the transport of heat in this section of the downgoing slab. A preliminary model for this fluid flow system was provided by Silver et al. (2000). Similar to Cl concentrations, Br concentrations do not show a reversal in the basal sediments. Because of the high concentration of Br, the Br/Cl ratios at Sites 1039 and 1253 (Fig. F2) are conservative throughout the sediment section. The apparent increase in F concentration in the pore fluids of the transition zone between the hemipelagic and pelagic calcareous sections at Sites 1039 and 1253 (Fig. F1A) is not seen when the data are normalized to Cl concentrations (Fig. F1D).

The most striking observations at the décollement and fracture zones at Sites 1040 and 1254 are

1. The sharp maxima in F and Br concentrations that when normalized to Cl concentrations remain distinct (Figs. F1C, F1F, F2C, F2F),
2. The minima in Rb and K concentrations (Fig. F5),
3. The less radiogenic Sr isotope ratios at these same horizons (Figs. F9, F11), and
4. The lower ⁷Li values (Fig. F10).

These data, together with shipboard and Leg 170 geochemical data, constrain the source depth of the fluid transporting solutes to the ocean along the décollement and fracture zones to a source at ~150°C. As discussed in Silver et al. (2000), this temperature corresponds to10–15 km depth. These signals are subdued at Sites 1043 and 1255 because of their proximity to the trench; therefore, they are more dilute.

1. The origin of the positive relationship between F and Ca concentrations both at the oceanic reference Sites 1039 and 1053 and particularly in the décollement and fracture zones observed at Sites 1040 and 1254 (Fig. F3) is being investigated. The observation that the deep-sourced fluid is enriched in F, an important metal-complexing agent, may help to explain some of the poorly understood observations of trace metal concentrations in arc volcanic rocks.
2. Organic matter in the prism sediments at Sites 1043 and 1255 and 1040 and 1254 is responsible for the somewhat elevated Br concentrations and Br/Cl ratios (Fig. F2B, F2C, F2E, F2F).
3. Barium concentrations are slightly enriched in the hemipelagic and transition zone pore fluids (Site 1039). The organic C content of these sediments is higher than in the calcareous section. The extremely high pore fluid Ba concentrations at Site 1040 below the décollement, in the zone where the sulfate is depleted to zero concentration, is most remarkable (Fig. F8), indicating that some of the sediment Ba is being mobilized and returned to the ocean, as discussed by Solomon et al. (this volume). The corresponding observed decrease in the sediment Ba concentrations in the hemipelagic section as the plate moves from Sites 1039 and 1253 to Sites 1040 and 1254 is shown in Figures F6 and F7 and discussed in Solomon et al. (this volume).
4. The concentrations increase with depth at Sites 1039 and 1253, and the behavior of Rb is almost identical to that of K (Figs. F4, F5). This is especially distinct in the décollement and fracture zones at Sites 1040 and 1254, as discussed above. The higher than seawater Rb and K concentrations in the hemipelagic section at Sites 1039 and 1253 are most likely a result of ion exchange reactions between Rb (K) and NH₄. Cesium concen-trations are approximately seawater value in the hemipelagic section and increase with depth toward the basal portion of the pelagic sediments. Unlike Ca, Sr, and Li, Cs concentrations increase in the basal section and may reflect communication with a slightly altered fluid system in uppermost basement.

Sediment Rb and Cs concentrations vary with depth at Sites 1039 and 1253 and 1040 and 1254 (Figs. F6, F7), reflecting the relative dilution by biogenic material with distance from the trench. In the hemipelagic sediments Rb and Cs concentrations are >28 ppm and >1 ppm, respectively. At relative plate velocity of 88 km/m.y., Site 1039 was ~440 km from the Middle America Trench at ~5 Ma, thus receiving a mixture of terrigenous and biogenic sediments. In the pelagic carbonates section (with intermittent ash) that was deposited when the plate was closer to the ridgecrest, Rb and Cs concentrations are low (<0 ppm and <0.5 ppm, respectively) and extremely depleted in the basal "baked sediments," most likely because of diagenetically enhanced mobility at moderate to high temperatures (W. Wei, pers. comm., 2005). In the prism section of Sites 1040 and 1254, Rb and Cs concentrations are similar to those in the hemipelagic sediments at the reference Sites 1039 and 1253; however, the average Rb/Cs ratio in the hemipelagic sediments is ~15, and in the prism sediments it is ~25, which may indicate different sources.