Project IV.2: Interlaboratory biases and first principles calibration of 40Ar/39Ar standards (Leah Morgan supervised by Jan Wijbrans, Igor Villa and Klaudia Kuiper )

Objective: Here we aim to improve the absolute uncertainty in the 40Ar/39Ar method independent of other methods by focusing on new primary standard calibrations. In addition, interlaboratory bias will be assessed to confirm claimed precision levels of 0.1% or better that are required for providing tight constraints for pre-Neogene astronomical tuning.

Background: Primary calibrations of K/Ar standards are a necessary step in determining the absolute concentration of 40Ar in a mineral due to radioactive decay of 40K. Previous absolute calibrations in the 1960's and 70's were carried out on low-temperature K-bearing minerals with an open structure (e.g. biotite) whose ages cannot be determined to better than 0.5%. At that time it was difficult to quantitatively remove argon from sanidine using existing vacuum furnace techniques, due to its high melting point (>1700 °C) and viscous melts. All sanidine standards used in argon geochronology today have been calibrated using intercalibration techniques that can be traced to a limited number of primarily calibrated biotite standards (Renne et al., 1998). To independently improve the 40Ar/39Ar method we should amongst others improve the accuracy of primary standard calibrations. In addition a systematic and detailed assessment of its interlaboratory bias is necessary to achieve the claimed precision levels of 0.1% or better.

Research strategy and methodology: We plan to use sanidine exploiting modern laser fusion and furnace technologies, as the most promising method of calibrating a gas pipette system against known quantities of air argon. This will be the first time that sanidine standards will be calibrated absolutely against a known quantity of air argon. Suitable candidates for such an exercise are the sanidine of the Fish Canyon Tuff and tephras from the Melilla peninsula in NE Morrocco. Using a modern furnace technique we can achieve temperatures of ca 2100 °C which is sufficient to remove radiogenic argon from sanidine crystals. In addition standard laser fusion techniques allow us to remove argon quantitatively from sanidine melts at temperatures > 2500 oC. The challenge today lies in the absolute calibration of reference gas aliquots released via a gas pipette system, against absolutely measured quantities of 40Ar isolated from air. We intend to follow a modernized version of the technique used in the 1960's by the ANU argon laboratory, as the most promising method of calibrating a gas pipette system against known quantities of air argon.
Further, the intercalibration effort of argon laboratories contributing to the GTSnext will focus on standardizing and streamlining chemical-analytical and mathematical procedures and protocols in order to improve interlaboratory data comparison. Questions related to culling of mass spectrometry data, the complexity of natural samples and related assumptions on a normal distribution will be addressed. To assess different data reduction software packages, a real and/or synthetic raw dataset should be developed and distributed to test the output. The last EARTHTIME experiment will be reviewed and new guidelines be formulated for the next experiment, now attempting to explore single variables. This European led 40Ar/39Ar interlaboratory experiment under highly specified conditions will also feed into an ESF Research Networking Program.

Feasibility: The primary calibration of a sanidine standard will be challenging and requires a more advanced researcher (ER). Collaboration is crucial for the intercalibration experiment, but the community wide consensus to tackle intercalibration issues within GTSnext, EARTHTIME and an ESF Research Networking Program enhances its success rate significantly.

Innovative aspects and relevance: This will be the first time that a sanidine standard will be prepared on first principles basis, and when combined with the alternative 40K intercalibration (against astronomical tuning, see I.2) it will provide a comprehensive dataset for assessing the accuracy and precision of the 40K decay constant.

Link with other projects: The ER will be responsible for coordinating all interlaboratory experiments related to task I.1, II.2 and III.3 and for linking obtained data with project IV.3 to extend the dataset of U/Pb and 40Ar/39Ar pairs.