HR: 1330h
AN: V42D-0385    [PDF]
TI: LA-ICPMS Measurements of Pb Isotopes and Th/U Ratios of HSDP Glasses from the Submarine Section of Mauna Kea Volcano
AU: * Jochum, K P
EM: kpj@mpch-mainz.mpg.de
AF: Max-Planck-Institut fuer Chemie, Postfach 3060, Mainz, 55020 Germany
AU: Amini, M A
AF: Max-Planck-Institut fuer Chemie, Postfach 3060, Mainz, 55020 Germany
AU: Stoll, B
AF: Max-Planck-Institut fuer Chemie, Postfach 3060, Mainz, 55020 Germany
AU: Herwig, K
AF: Max-Planck-Institut fuer Chemie, Postfach 3060, Mainz, 55020 Germany
AU: Hofmann, A W
AF: Max-Planck-Institut fuer Chemie, Postfach 3060, Mainz, 55020 Germany
AB: We have developed a LA-ICPMS technique to simultaneously determine Pb isotopes, Th, U and other trace element concentrations in small glass samples. The experiments were done with a New Wave UP-213 laser system and an Element 2 ICP mass spectrometer. The glasses were ablated for 60 s in single spots 120 $\mu$m in diameter, applying a laser energy density of 7mJ/cm$^{2}$ and a pulse repetition rate of 10 Hz. The geological MPI-DING reference glasses (Jochum et al., 2000), for which high precision isotope data are now available, were used to determine the mass bias of Pb isotopes and to calibrate the trace element data. Reproducibility (RSD) of the Pb isotope data for concentrations of about 0.5 - 2 ppm is about 0.2 - 0.7 % (208/206), 0.2 - 0.5 % (207/206) and better than 1.5 % (206/204, 207/204, 208/204), respectively. The quantity of Pb measured for one analysis is about 3 pg for a Pb concentration of 1 ppm. Th/U ratios are determined with a precision of about 1 - 3 %. Overall analytical uncertainties of the trace element concentration data are about 5 - 10 %. \\We have applied the technique for the direct analysis of carefully handpicked glass fragments from 19 samples of the submarine section of the Hawaii Scientific Drilling Project 2 (HSDP2). Some samples provided just a few glass chips. Samples cover the depth interval from 1314 to 3060 mbsl. Three to four fresh, 200 - 500 $\mu$m large, fragments of each sample were analyzed. In these fragments the results are indistinguishable at each spot implying a homogeneous distribution of trace elements. Our results show that $^{208}$Pb/$^{206}$Pb ratios vary significantly along the stratigraphic column of the HSDP2-core ranging between 2.070 and 2.121. The lowest values are found in samples coming from depths around 1400, 2800 and 3000 mbsl whereas the highest ratios are measured in samples of 2100 mbsl depth. These microanalytical data agree with the high-precision TIMS data of Eisele et al. (2003) who found temporal variations of Pb isotopes in the whole HSDP2-core. Attributing varying isotope ratios to source heterogeneities can be confirmed when considering trace element ratios. This is best demonstrated by the correlation of the element ratio of Th and U that are parent nuclides of $^{208}$Pb and $^{206}$Pb with the $^{208}$Pb/$^{206}$Pb ratios measured in the same glass fragments. \\Ref.: Jochum et al. (2000), Geostandards Newsletter 24, 87-133; Eisele et al. (2003), Geochem. Geophys. Geosyst. 4 (5), 8710.
DE: 1010 Chemical evolution
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
DE: 1094 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting