Abstract:
The precision of Tl isotopic measurements by thermal ionization mass spectrometry (TIMS) is severely limited by the fact that Tl possesses only two naturally occurring isotopes, such that there is no invariant isotope ratio that can be used to correct for instrumental mass discrimination. In this paper we describe new chemical and mass spectrometric techniques for the determination of Tl isotopic compositions at a level of precision hitherto unattained. Thallium is first separated from the geological matrix using a two-stage anion-exchange procedure. Thallium isotopic compositions are then determined by multiple-collector inductively coupled plasma-mass spectrometry with correction for mass discrimination using the known isotopic composition of Pb that is admixed to the sample solutions. With these procedures we achieve a precision of 0.01-0.02% for Tl isotope ratio measurements in geological samples and this is a factor of =<3-4 better than the best published results by tims. however, without adequate precautions, experimental artifacts can be generated that result in apparent tl isotopic fractionations of up to one per mil.analysis five terrestrial samples indicate existence variations related natural fractionation processes on earth. two three igneous rocks analyzed this study display compositions indistinguishable from our laboratory standard, reference material nist-997 tl. a third sample, is characterized #Tl ~ 2.5 +/- 1.5, where #Tl represents the deviation of the 205Tl/203Tl ratio of the sample relative to NIST-997 Tl in parts per 104. Even larger deviations were identified for two ferromanganese crusts from the Pacific Ocean, which display #Tl-values of +5.0 +/- 1.5 and +11.7 +/- 1.3. We suggest that the large variability of Tl isotopic compositions in the latter samples are caused by low-temperature processes related to the formation of the Fe-Mn crusts by precipitation and scavenging from seawater. Thallium is the heaviest element for which such a natural isotopic fractionation has been verified.For a bulk sample of the C3V chondrite Allende we have determined a Tl isotopic composition of #Tl = -2.1 +/- 1.4, the lowest value measured in this study. By combining our data for Allende with a previously published Tl isotopic composition for the L3 chondrite Mezo-Madaras we obtain an upper limit of 9.2 x 10-5 for the 205Pb/204Pb abundance ratio of the bulk solar system at the time of volatile element depletion. If the chemical depletion of volatile elements occurred early, during the condensation of the solar nebula, our result would indicate that the initial Solar System abundance of 205Pb is significantly lower than some current astrophysical estimates.