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Proc Natl Acad Sci U S A ; 107(24): 10810-4, 2010 Jun 15.
Artigo em Inglês | MEDLINE | ID: mdl-20534492


Recent high precision (142)Nd isotope measurements showed that global silicate differentiation may have occurred as early as 30-75 Myr after the Solar System formation [Bennett V, et al. (2007) Science 318:1907-1910]. This time scale is almost contemporaneous with Earth's core formation at approximately 30 Myr [Yin Q, et al. (2002) Nature 418:949-952]. The (182)Hf-(182)W system provides a powerful complement to the (142)Nd results for early silicate differentiation, because both core formation and silicate differentiation fractionate Hf from W. Here we show that eleven terrestrial samples from diverse tectonic settings, including five early Archean samples from Isua, Greenland, of which three have been previously shown with (142)Nd anomalies, all have a homogeneous W isotopic composition, which is approximately 2epsilon-unit more radiogenic than the chondritic value. By using a 3-stage model calculation that describes the isotopic evolution in chondritic reservoir and core segregation, as well as silicate differentiation, we show that the W isotopic composition of terrestrial samples provides the most stringent time constraint for early core formation (27.5-38 Myr) followed by early terrestrial silicate differentiation (38-75 Myr) that is consistent with the terrestrial (142)Nd anomalies.

Anal Chem ; 80(23): 9213-9, 2008 Dec 01.
Artigo em Inglês | MEDLINE | ID: mdl-19551942


It is widely recognized that the natural isotopic variation of Mo can provide crucial information about the geochemical circulation of Mo, and the ocean is an important reservoir of Mo. To obtain precise isotopic data on Mo in seawater samples using multiple collector-inductively coupled plasma mass spectrometry (MC-ICPMS), we have developed a preconcentration technique using 8-hydroxyquinoline bonded covalently to a vinyl polymer resin (TSK-8HQ). By optimizing the procedure, Mo in seawater could be effectively separated from matrix elements such as alkali, alkaline earth, and transition metals. With this technique, even with a 50-fold enrichment factor, the changes in the 98Mo/95Mo ratio during preconcentration were smaller than twice the standard deviation (SD) in this study. Mass discrimination of Mo isotopes during the measurement was externally corrected for by normalizing 86Sr/88Sr to 0.1194 using an exponential law. We evaluated delta98/95Mo to a precision of +/- 0.08 per thousand (+/-2 SD); this value was found to be less than one-third of previous reported values. Moreover, we were able to determine an accurate ratio for every pair of stable Mo isotopes, which was impossible with previous methods owing to the isobaric interference from the external elements (Zr and Ru). In this study, delta92/98Mo in seawater was first determined so that it had the smallest relative error. We applied the proposed method to four kinds of seawater samples. The Mo compositions were constant among them, with average delta98/95Mo and delta92/98Mo values of 2.45 +/- 0.11 and -4.94 +/- 0.09 per thousand (+/-2 SD), respectively. Our data indicate that seawater is enriched in heavy Mo isotopes than previously reported.