Sandra Kamo gave a review talk on the application of U-Pb geochronology to understanding the causes of mass extinctions. Kamo provided two examples: the K-T where U-Pb dating of shocked zircons in ejecta layers has allowed a link to be made to the target rocks of the Chicxilub impact site and the P-Tr where the issue of a link between the Siberian Traps and the P/Tr extinction is still on-going. Of course, not only is the exact timing of the Siberian Traps relative to the extinction event of interest are but also whether a protracted magmatic event could have delayed the early Triassic biotic recovery or not. It is clear that ultra-high resolution geochronology will be required to resolve this debate and that both the Ar-Ar and U-Pb systems will need to be utilized.
In the other review talk, Klaudia Kuiper presented a synthesis of ongoing work from the Neogene of the Mediterranean that has culminated in the demonstration of the potential for using astronomical tuning of cyclic sedimentary rocks to determine ages of volcanic rocks. Klaudia demonstrated how this approach has the potential to provide more accurate ages for Ar standards (such as FCT). The absolute age of the Ar standards such as FCT is crucial for resolving any systematic differences between Ar-Ar and U-Pb.
It is clear that the largest impediments to further progress in establishing a high-precision calibration of the geological time scale are the persistent systematic differences between ages derived using the Ar-Ar and U-Pb techniques. One of the major goals of Phase I of EARTHTIME is to minimize interlaboratory bias. The first step in the process is to establish exactly what the biases are and towards that end experiments were designed for both the Ar-Ar and U-Pb communities.
Progress Report on Calibration Experiments
Two talks were given summarizing recent results of the EARTHTIME interlaboratory comparisons for U-Pb and Ar-Ar standards (Condon et al., 2005 and Heizler et al., 2005). Data were reported from seven labs for U-Pb and as many as 10 labs for Ar-Ar.
Two zircon standards, Temora and R33, were distributed to participants. The results of various analyses in the different laboratories are shown in the following two figures. The agreement between labs is in general quite good with most labs getting ages of about 417 Ma for Temora and 419.4 Ma for R33.
The variation in precision is most likely related to blank levels, whereas differences in calculated weighted mean dates are most probably controlled by spike calibrations. Nevertheless, the comparisons show promise, but it is obvious that more data and greater lab participation are needed. As part of the EARTHTIME initiative we will be distributing to interested labs, a calibration solutions as well as a new spike, that should improve inter-laboratory comparisons dramatically.
If you have new or additional U-Pb data for Temora or R33 zircons please send it to Dan Condon.
For all of the Ar-Ar standards tested (GA1550 Biotite; TCR Sanidine, AC Sanidine, PP-20 Hornblende) the interlaboratory results scatter more than the individual data populations suggesting minor inaccuracies between individual laboratories. Scatter is likely related to variations in data reduction methods and neutron fluence monitoring approaches. A follow up phase of this type of experiment will be to minimize procedural differences amongst participating laboratories. Any facility wishing to participate should contact Matt Heizler at NM Tech. All new Ar-data should be sent to Matt Heizler.
Apparent ages for GA1550 biotite determined relative to Fish Canyon Sanidine at 28.02 Ma. Data given in black reveal moderate scatter (MSWD=3.63) between individual laboratories. Data from VU were known to be inaccurate due to instrument problems. S&M (2003) and R (1998) are from previous studies by Spell and McDougall (2003) and Renne et al. (1998).
Apparent ages for Taylor Creek Rhyolite sanidine (TCR) determined relative to Fish Canyon Sanidine at 28.02 Ma. Data reveal moderate scatter (MSWD=3.12) between individual laboratories. Considering that TCR is similar in composition and age compared to FC it is somewhat surprising to observe a nonuniform distribution. Studies like this indicate additional effort is required to achieve the ultimate goals of the EARTHTIME initiative.