| Techniques for precise determination of the isotopic compositions of very small amounts (nanogram level) of Pb and Sr with thermal ionization mass spectrometry are developed and refined. They are then applied to address questions where isotopic relationships for Sr and Pb can potentially make important contributions. Three main ones are briefly described below.; The ages and diagenesis of Paleozoic limestones from three locations are studied by examining U-Pb systematics and Sr isotopic relationships. For the Columbus Limestone of Ohio, from near Columbus and Kelleys Island of Lake Erie, 87Sr/86Sr ratios show heterogeneities but are consistent with the depositional age (∼385 Ma). In contrast, U-Pb dates are 60 m.y. younger. Both isotopic systems reflect a pervasive diagenetic event, probably regional in extent, long after lithification. For the San Jorge Formation of Argentina, U-Pb isochrons and Sr isotopes agree with an early Ordovician depositional age. These results carry important implications for U-Pb dating of limestones.; Small (∼1 mg), single crystals of phlogopite isolated whole from slowly cooled marbles of the Adirondack Lowlands (New York) are analyzed to obtain precise Rb-Sr ages. These ages correlate very well with 40Ar/ 39Ar ages on like crystals; those from one marble are uniform while those from another vary significantly. Ages are not correlated with physical or chemical parameters and suggest that closure of the phlogopite Ar and Sr isotopic systems was not controlled by volume diffusion. The results have important implications for the meaning of isotopic ages in slowly cooled rocks.; The level and sources of Pb and Sr in the Columbus (Ohio) water system are studied. Both Pb and Sr isotopic compositions vary significantly in waters collected from different residences. The 87Sr/86Sr ratios reflect sources and are controlled by the geology of the drainage systems of the different reservoirs. The Pb isotopic compositions vary widely and are not related to those of Pb pipes from the distribution system. Water Pb concentrations and isotopes appear to reflect residential plumbing point sources rather than source water or the supply system. The results demonstrate that isotopes are able to address the sources of Pb in municipal systems. |
