Research School of Earth Sciences
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Current ResearchAge of the Solar System’s first solids (joint with A.N. Krot, Uni. Hawaii, M. Wadhwa, Arizona State Uni, Q.-Z. Yin, Uni. California Davis, and A.J. Irving, Uni. Washington) The questions that we are trying to decipher are: • When materials with various nucleosynthetic histories were mixed together, and how completely were they mixed, Precise dating of the early Solar System materials is extremely demanding analytically, because of low concentrations of the parent and daughter elements, the need to analyse individual mineral grains or their parts to get interpretable results from complex and heterogeneous meteorites, and limited quantities of the best preserved meteorites available for research. Time intervals between the ages of CAIs, chondrules, angrites, and eucrite Asuka 881394, relative to the age of the angrite D’Orbigny (U-Pb – Amelin 2008, Mn-Cr – Glavin et al. 2004, Al-Mg – Wadhwa et al., submitted to GCA, Hf-W – Markowski et al. 2007). Diagonal lines indicate consistent age intervals measured with each pair of chronometers. In many instances, the dates are discrepant between chronometers well outside of the 2σ error limits. For building an accurate timescale of the early solar system, these discrepancies must be understood and resolved. Preliminary results presented at conferences:
Timing of fractionation between volatile and refractory elements in the protoplanetary diskFor some parent-daughter nuclide pairs, there is a substantial difference in volatility (expressed as 50%-condensation temperatures from the gas of the bulk Solar System composition) between the parent and daughter elements: about 300 °C between Al and Mg, 660 °C between Rb and Sr, and about 900 °C between Pb and U or Th. This means that, in many cases, we date the processes that separate refractory and volatile elements, i.e. heating and cooling, or evaporation and condensation. Furthermore, different parent-daughter pairs cover different parts of the temperature scale, and provide complementary information about different stages of heating and cooling. Despite the early success, the initial Sr chronometer fell into oblivion since the early 1990’s. The main reason why this method was discontinued was the insufficient time resolution that was limited by the precision of Sr isotopic analysis. The difference of 0.005% in 87Sr/86Sr (similar to the typical analytical precision in the early studies) corresponds to a ca. 2 Ma difference in the age, assuming the Rb/Sr ratio in the solar photosphere or in CI chondrites. Modern thermal ionization mass spectrometry can yield 5-10 times better precision, thus making the initial Sr dates as precise as the 53Mn-53Cr and U-Pb dates. Precision of initial Sr dates is mainly controlled by precision of 87Sr/86Sr ratios. In this case, analytical advancements directly propagate into improved precision of the ages.
Refining the decay constant of Rb-87 (Primary investigator - E. Rotenberg, University of Toronto). Preliminary results presented at conferences:
The nature of ionic emission from molten silica gel and related decoupled fractionation of odd- and eisotopes (joint with D.W. Davis, University of Toronto, and W.J. Davis, Geological Survey of Canada) The 207Pb/206Pb isotopic ratio of Pb evaporated from molten silica gel, normalized to 208Pb/206Pb, drifts as the sample gets exhausted. The 205Pb/206Pb ratio behaves similarly, whereas the ratios of even-mass isotopes 202Pb, 204Pb, 206Pb and 208Pb remain unchanged. The magnitude of the odd-even mass bias varies between silica gels prepared using different methods. It is possible that the observed mass bias is due to nuclear field shift. Preliminary results presented at conferences:
Detecting small-scale natural uranium isotope variations (joint with Claudine Stirling, University of Otago) |
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