Petrochronology by Laser-Ablation Inductively Coupled Plasma Mass Spectrometry

Abstract

Petrochronology is a field of Earth science in which the isotopic and / or elemental composition of a mineral chronometer is interpreted in combination with its age, thus yielding a more synergistic combination of petrology and chronology that can be used to interpret geologic processes. It has recently attracted renewed interest as technologies for mineral analysis have improved. Examples are many, and continue to grow, from the early adoption of U / Th ratios in zircon as an indicator for magmatic vs. igneous crystallization (e.g., Ahrens 1965), to using the Nd isotopic composition in titanite to track source contribution over time (see Applications ; B. R. Hacker, personal communication). Age and chemical information can be obtained by a variety of techniques: electron microprobe (age; major and minor elements; see Williams et al. 2017), secondary ion mass spectrometry (SIMS; age; trace elements; isotopic ratios; see Schmitt and Vazquez 2017), and laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS; age; trace elements; isotopic ratios). Laser-ablation ICPMS instrumentation and techniques, the focus of this chapter, have been employed as a petrochronologic tool for decades, starting with separate analyses of ages and elemental and / or isotopic compositions, which were then combined and interpreted. For example, Zheng et al. (2009) employed LA-ICPMS to analyze the trace-element (TE) chemistry, Hf isotopic composition, and age of zircons from kimberlites by using three spots on each zircon grain, one for each type of analysis. This work was relatively time consuming and expensive, given the required number of analytical sessions, but yielded far better confidence in the conclusions, because of the link between physical conditions (petrology) and time (chronology). Instrumentation and techniques which employ LA-ICPMS have continued to improve, particularly in the ease with which petrochronologic data can be obtained. A single LA-ICPMS instrument can now measure both the …