DOI: 10.1594/pangaea.779191
title: (Table 1) Sr, Nd, and Pb isotopic composition of ODP Leg 129 basalts

description: Basaltic rocks recovered from three drill sites in the western Pacific during Ocean Drilling Program Leg 129 have fairly distinct Sr, Nd, and Pb isotopic compositions. The Cretaceous alkali olivine dolerites from Site 800 in the northern part of Pigafetta Basin have fairly low 87Sr/86Sri (0.70292-0.70320) and 143Nd/144Ndi (0.51277-0.51281) and high present-day Pb isotopic ratios (206Pb/204Pb = 20.53-21.45; 207Pb/204Pb = 15.70-15.77; 208Pb/204Pb = 40.02-40.68). The Middle Jurassic tholeiites from Site 801 in the southern part of the basin have low 87Sr/86Sri (0.70237-0.70248), high 143Nd/144Ndi (0.51298-0.51322), and moderate present-day Pb isotopic ratios (206Pb/204Pb = 18.20-19.12; 207Pb/204Pb = 15.47-15.60; 208Pb/204Pb = 37.56-38.18); isotopic compositions of the alkali olivine basalts overlying the tholeiites fall between those of the tholeiites and Site 800 dolerites. The Cretaceous tholeiites from Site 802 in the East Mariana Basin have high 87Sr/86Sri (0.70360-0.70372), fairly low 143Nd/144Ndi (0.51277-0.51280), and fairly low and homogeneous present-day Pb isotopic ratios (206Pb/204Pb = 18.37-18.39; 207Pb/204Pb = 15.49-15.51; 208Pb/204Pb = 38.34-38.39). Isotopic compositions of Site 801 tholeiites are indistinguishable from those of modern mid-ocean ridge basalts, consistent with the proposal that these tholeiites are a part of the oldest Pacific crust. The diverse isotopic compositions of the younger basalts appear to be the result of Jurassic Pacific plate migration over the geologically anomalous south-central Pacific region, wherein they acquired their distinct isotopic compositions. The anomalous region was volcanically more active during the Cretaceous than at present.
description: Analytical uncertainty on 87Sr/86Sr measurements is ± 0.00002 but in-run precisions are a lot better than this; uncertainty on 143Nd/144Nd is 0.000023 (errors listed are in-run precisions and refer to the last significant digits of the Nd isotopic compositions; uncertanties on Pb isotopic compositions are approximately 206Pb/204Pb ± 0.015, 207Pb/204Pb ± 0.019 and 208Pb/204pb ± 0.06. Sr isotopes were fractionation corrected to 87Sr/86Sr = 0.1194 and normalized to 87Sr/86Sr = 0.71025 for NBS 987; Nd isotopic ratios were measured in oxide form and fractionation corrected to 146NdO/144NdO = 0.72225 (146Nd/144Nd = 0.7219) and are reported relative to 143Nd/l44Nd = 0.511860 for the La Jolla Standard; isotopic compositions of Pb were corrected for mass fractionation based on average measured values for NBS 981 using the values given in Catanzaro et al. (1968). Initial isotopic compositions (with subscript i) were calculated for an assumed age of 173 m.y. for Site 801 tholeiites, 157 m.y. for Site 801 alkali basalts, 126 m.y. for Site 800 dolerites, and 115 m.y. for Site 802 tholeiites. The Sr isotopic compositions of Site 801 tholeiites were not corrected for age because these are incompatible element-depleted tholeiites with very low Rb/Sr (&lt;0.01) ratios, and thus corrections are negligible. Rb, Sr, Sm, and Nd concentrations of whole-rock samples were determined by ICP-AES and AA except those of Samples 129-800A-58R-2,52-60 cm, and 129-801C-1R-1,109-114 cm, and the mineral separates which were analyzed by the isotope dilution method. The uncertainties of trace element abundance determinations by ICP-AES and AA are generally better than 5%, whereas those by isotope dilution are better than 1%.
description: Supplement to: Castillo, Paterno R; Floyd, Peter A; France-Lanord, Christian (1992): Isotope geochemistry of Leg 129 basalts: implications for the origin of the widespread cretaceous volcanic event in the Pacific. In: Larson, RL; Lancelot, Y; et al. (eds.), Proceedings of the Ocean Drilling Program, Scientific Results, College Station, TX (Ocean Drilling Program), 129, 405-413