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doi:10.2204/iodp.proc.301.203.2008

Results

Lead isotope data on sediment samples were 18.913–19.172 for ²⁰⁶Pb/²⁰⁴Pb, 15.632–15.675 for ²⁰⁷Pb/²⁰⁴Pb, and 38.743–39.092 for ²⁰⁸Pb/²⁰⁴Pb (Table T1). Three samples from Sections 301-U1301C-4H-5, 11H-1, and 301-U1301D-5H-1 are described as slightly more radiogenic (²⁰⁶Pb/²⁰⁴Pb > 19.1) than other samples (²⁰⁶Pb/²⁰⁴Pb < 19.1) (Fig. F2).

The Pb isotopic variation with depth profile of sediment cores is shown in Figure F3. With the exception of the three radiogenic samples (²⁰⁶Pb/²⁰⁴Pb > 19.1), the sediment samples display a slight decline in ²⁰⁶Pb/²⁰⁴Pb with increasing depth, whereas ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb are almost unchanged.

The field defined by these sediment samples departs from the Juan de Fuca and Gorda Ridge basalt field (Fig. F2). The sediment data plot close to the field of surface sediments from the northern Juan de Fuca Ridge (Cousens et al., 2002) but demonstrate a wider range of variation. They are distinct from altered turbidites from the northern Juan de Fuca Ridge, although the trends defined by sediment samples are subparallel to the turbidites field. An interesting point is that the trends defined by these sediment samples extend at the lower ²⁰⁶Pb/²⁰⁴Pb end toward the Pacific seawater field represented by Mn crust (von Blanckenburg et al., 1996). Conversely, massive sulfides from the northern Juan de Fuca Ridge (Cousens et al., 2002) plot between the basalts field and sediments with steeper trend; Pb isotopic composition of the most radiogenic massive sulfides overlap our sediment samples with the lowest ²⁰⁶Pb/²⁰⁴Pb.

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