Petrochronology and TIMS

Abstract

Thermal ionization mass spectrometers, or TIMS, were developed by the pioneers of mass spectrometry in the mid-20th century, and have since been workhorses for generating isotopic data for a wide range of elements. Later-developed mass spectrometric techniques have many advantages over TIMS, including higher spatial resolution with in situ techniques, such as secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), and greater versatility in terms the elements that can be easily-and well-measured. The reason TIMS persists as an important method for geochronology is that for some key parent-daughter systems (e.g., U–Pb, Sm–Nd), it can produce isotopic data and resultant dates with 10–100 times higher precision and more quantifiable accuracy than in situ techniques, even when sample sizes are very small (such as those that might result from single crystals, or even small portions of zoned crystals). For many questions in the geosciences, the highest achievable precision and accuracy are required to resolve the timescales of processes and/or correlate events globally. As an example, modern TIMS U–Pb geochronology is capable of producing dates with precision and accuracy better than 0.1% of the age for single crystals with only a few picograms (pg) of Pb. Therefore, it is possible to constrain the durations of single zircon crystal growth in magmatic systems over tens to hundreds of kyr in Mesozoic and younger rocks. If these dates and rates can be connected with other igneous processes such as magma transfer, emplacement and crystallization, then it becomes possible to calibrate thermal and mass budgets in magmatic systems and evaluate competing models for pluton assembly and subvolcanic magma storage. As another example, Sm–Nd geochronology of garnet permits dates with precision better than ±1 million years for garnets of any age, including multiple concentric growth zones in single crystals. Such …