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Lead isotopic compositions and U, Th, and Pb concentrations have been determined in 11 basalt and 3 sediment samples from Leg 34 of the Deep Sea Drilling Project. The "Th/U ratios and the Pb isotopic compositions of the basalts suggest that they are typical oceanic tholeiites originating from the extinct Galapagos Rise. The similarity in the Pb isotopic composition, when corrected for in situ U decay, for all basalt samples analyzed suggests that they were extruded from a portion of the mantle in which the Pb isotopic composition is homogeneous, or that mantle convection and/or mixing proportions of partial melts between the mantle plume and the low velocity zone were constant for the interval 15-40 m.y. ago. The Pb concentration in the Pliocene sediment analyzed is about average for deep-sea sediments, but is about 10 to 20 times higher than those of the middle and lower Miocene sediments analyzed. The low Pb concentration in the Miocene sediments may be attributed to carbonate accumulation. The Pb isotopic composition of the lower Miocene sediment is similar to those of the basalt samples analyzed in this study, but the Pb isotopic compositions of the two overlying sediments are different. INTRODUCTION By drilling into the Nazca plate we may be able to determine the origin of the metalliferous sediments of the Bauer Deep (Boström and Peterson, 1966; Dasch et al., 1971) and whether Andean magmas and related ore bodies can be related chemically to subducted metalliferous sediments. We have analyzed the Pb isotopic compositions and Pb, U, and Th concentrations of three sediment and six basalt samples from Site 319 (Holes 319 and 319A) and five basalt samples from Site 321 of Leg 34 of the Deep Sea Drilling Project. This is a preliminary report of whole rock data. A complete report will be published upon completion of analyses of mineral separates. EXPERIMENTAL Samples Samples were collected by the participants of Leg 34 of the Deep Sea Drilling Project aboard Glomar Challenger (Hart and Yeats,pers. comm.; Scientific Staff of Leg 34, 1974). Three sediments, representing the upper, middle, and bottom sediment units, and one basalt from just below the sediment-basalt boundary from Hole 319, five basalt samples from Hole 319A, and five basalt samples from Site 321 were analyzed for U, Th, and Pb concentrations and Pb isotopic compositions. Site 319 is in the Bauer Deep, on the west side of the extinct Galapagos Rise (13°01.04'S, 101°13.46'W). Site 321 is on the eastern edge of the Nazca plate west of the Peru-Chile Trench (12°01.29'S, 81°54.24'W). Sample depths below the sea floor and analytical results are shown in Table 1. Analytical Procedures Rock samples were cut into 3/4-in. cubes with a diamond saw, then washed with doubly distilled 2.5 N HC1 for about 30 sec and rinsed four times with quadruply distilled water with the aid of an ultrasonic vibrator. The cubes used for whole rock analysis were crushed in a stainless steel mortar and powdered in a carbide-lined stainless steel container by a vibrating mill. Chemical procedures were similar to those reported by Nunes et al. (in press) with modifications to accommodate larger sample sizes. Lead, uranium, and thorium concentrations were determined by isotope dilution on an aliquot of a dissolved sample. Samples were decomposed using HC1O4 as well as HF and HNO3 in a Teflon bomb to insure total dissolution of the sample, and thus, complete equilibrium between thorium in the spike and sample. The resulting solution was evaporated to dryness and then fumed with HCIO4 to convert all fluorides to more soluble perchlorates. The residue was then taken up with dilute HNO3 and divided into two parts, one of which was spiked (designated by an A next to the concentration data in Table 1). The concentrations of some samples were checked by total spiking a powder split before decomposition (designated by a TS in Table 1). Lead was separated by a double anion exchange resin method, first in HBr medium and then in HC1 medium. Lead blanks ranged from 0.7 to 2.0 ng for 0.1to 0.3-gsized samples (TS) and from 2.0 to 4.1 ng for 0.5to 1.0-

Lead Isotopic Composition and Uranium, Thorium, and Lead Concentrations in Sediments and Basalts from the Nazca Plate