EARTHTIME NEEDS AND PATHS FORWARD IN HIGH PRECISION-HIGH ACCURACY ARGON GEOCHRONOLOGY

From the dawn of the Earthtime initiative a primary challenge of the argon geochronology community has been to obtain near per mil interlaboratory comparability. This was necessary so that multiple laboratories could forge ahead on multiple synergetic lines of research towards a unified goal of achieving an accurately calibrated geological time scale. Several labs have now achieved this interlaboratory comparability and have identified new challenges exposed by ultra high precision multicollector mass spectrometry analyses. Excess grain-to-grain age scatter abounds in nearly all ultra-high precision sanidine analyses and within single grain age spectrum analyses. Likely no single mechanism accounts for the scatter and thus determining what part, if any, of a complex age distribution is accurate poses a major hurdle in Earthtime research. In general obtaining an inaccurately old age seems more probable than a young age related to consequences of argon loss. Apparently, radiogenic argon can be retained in anticryts despite residing at elevated temperature relative to argon diffusion. Also, highly radiogenic magmatic argon can be housed in melt inclusions and is finitely soluble within the crystal lattice. The latter can be impossible to detect without other constraints. Another major need in Earthtime research that will promote detrital sanidine (DS) geochronology is development of a well-populated database that is calibrated between labs. DS geochronology is proving to be a powerful chronometer and source indicator for sedimentary rocks. DS is complementary to DZ but excels in precision for Cenozoic rocks making it more capable of determining accurate maximum deposition ages with high precision correlation to volcanic sources. Additional ways to improve the value of DS is to more fully utilize a suite of noble gases produced during irradiation. Ne can track Na, Kr may track Sr and Xe may track Ba. These are all important chemical tracers for sanidine provenance. Imaging analysis of post fusion sanidine beads can also provide K concentration without weighing each grain. With our current technology advances, the argon community is well positioned for the next generation of Earthtime research and we can expect breakthroughs in argon systematics, timescale research and ultra precise rate processes.