Improving precision and accuracy of u-pb geochronology with application to the permian period
Creators: name: Huyskens, Magdalena Hedwig
Publisher: name: The Australian National University
Year: 2014

description: High-precision geochronology is a prerequisite for solving numerous geological problems, including deciphering the stratigraphic record to reconstruct climate and biosphere evolution. At present, Phanerozoic rocks can be dated with U-Pb geochronology with a precision of 0.05% or better on a weighted mean age using ID-TIMS. We established high-precision U-Pb geochronology at the Research School of Earth Sciences at The Australian National University, complementing existing high spatial resolution dating techniques. Our implementation of the method includes new developments aimed at improving precision and accuracy. The method was applied to two geological problems of the Permian Period. One essential requirement for U-Pb ID-TIMS is a mixed U-Pb spike. We prepared and calibrated a new 202Pb-205Pb-233U-236U spike, with a U/Pb ratio optimised for dating Phanerozoic samples. For a further increase in precision two possible approaches were tested aiming for improving counting statistics. A multi-ion counting array offers the possibility to substantially increase the total number of ion counts and therefore potentially increase the precision. This approach however was unsuccessful due to design limitations of the multiplier array of the Triton plus. The second approach was increasing the ionisation efficiency for Pb by preparing a more effective ion emitter. Several different commercially available silicagels were tested with differing particle sizes and concentrations. The highest ionisation efficiency of ~ 6%was found for a silicagel from Sigma-Aldrich in a concentration of 0.4%. Pb loss and inheritance are the major problem for accurate U-Pb geochronology of magmatic zircons. The problem of Pb loss was largely resolved with the chemical abrasion technique developed for multi-grain zircon geochronology. We determined optimal conditions for chemical abrasion of single zircon grains to ensure complete removal of zones that were affected by Pb loss. The testing was performed on three different zircon standards with various ages using a three step partial dissolution technique. It was found that annealing at a temperature of 900 degree C for 48 h followed by leaching at 190 degree C for 15 h is produces optimal results. We applied the newly established and improved U-Pb ID-TIMS technique to refine the chronology of two geological provinces, both of Permian age. First, we determined ages of zircons from volcanic ashes collected from the southern Sydney Basin sequences. These sequences were correlated with the type section of the Guadalupian-Lopingian-Boundary in South China through C-isotope stratigraphy. Using our U-Pb age data and the C-isotope correlation, we have determined a date of 257.9 +/- 0.5 Ma as the age of the Guadalupian-Lopingian-Boundary, and estimated that a major sea-level regression, which is possibly coinciding with the mid-Capitanian mass extinction, occurred after 258.9 Ma. In the second application we dated four sills of the Tarim large igneous province (LIP), which erupted in the early Permian, using the U-Pb systematics of baddeleyite. This LIP erupted in two major pulses, where the second pulse lasted at least 1.8 +/- 0.8 Myr from 283.1 +/- 0.2 to 281.3 +/- 0.6 Ma.