SPEED DATING!: SAMPLING AND APPLICATIONS FOR U-TH-PB GEOCHRONOLOGY

U-Th-Pb geochronology is the most versatile and widely used geochronometer It is applied to a wide range of rock types spanning the age of the Solar System to the Pleistocene. U-Th-Pb geochronology usually focuses on measuring dates in single minerals or domains within minerals at the tens of microns scale. Versatility in spatial resolution derives from the multiple techniques used to measure U-Th-Pb isotopes. Thermal ionization mass spectrometry (TIMS) yields precision <±0.1% on fragments of minerals tens of µm large. In situ techniques such as laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) and secondary ion mass spectrometry (SIMS) can measure <20 μm spots in single minerals in grain mount or in thin section at ~±2% precision. Which technique to use depends on the material available and the questions being asked.

A key goal of U-Th-Pb geochronology is to tie dates to geologic processes. This workflow starts with wise sample collection, including full characterization of field relations and careful consideration of what a date from a given rock will tell you. Ultimately, initial care during fieldwork reduces the number of samples needed and the cost of the project. Additionally, both in situ techniques and TIMS have improved at tying dates to processes by utilizing textural and geochemical information. In every case, a better understanding of how minerals grow and are transported in a variety of environments is crucial to linking date to process. Therefore, the best geochronology projects are those in which geochronologists work together with other geologists from planning fieldwork to interpreting age data.

International initiatives, such as EARTHTIME, have focused on cooperation and sharing between labs and outreach to a broader community with a goal to broaden the accessibility of geochronology and to better understand dates generated. U-Th-Pb geochronologists have adopted this approach and geochronology in general has benefited from these efforts through improved precision and accuracy of dates, better knowledge of decay constants, and a better understanding of the limitations and benefits of different dating methods. Ultimately, these efforts will lead to a better understanding of the history and workings of Earth.