Electron Microprobe Petrochronology

Abstract

The term petrochronology has increasingly appeared in publications and presentations over the past decade. The term has been defined in a somewhat narrow sense as “the interpretation of isotopic dates in the light of complementary elemental or isotopic information from the same mineral(s)” (Kylander-Clark et al. 2013). Although complementary isotopic and elementary information are certainly a central and critical part of most, if not all, petrochronology studies, the range of recent studies that might use the term covers a much broader scope. The term “petrochronology” might alternatively be defined as the detailed incorporation of chronometer phases into the petrologic (and tectonic) evolution of their host rocks, in order to place direct age constraints on petrologic and structural processes. As noted by Kylander-Clark et al. (2013), the linkage between geochronology and petrology can involve a variety of data including mineral textures and fabrics, the distribution of mineral modes or volume proportions, compositional zoning, mineral inclusion relationships, and certainly major element, trace element, and isotopic composition of the chronometer and all other phases. Electron probe micro-analysis (EPMA) has a central and critical role to play in establishing the linkage between chronometer phases and their host assemblage. The basic instrument is an electron microscope which can be used in either scanning or fixed beam modes, with integrated wavelength dispersive spectrometers (WDS), energy dispersive spectrometers (EDS), electron detectors (to image secondary and backscattered signals) a light optical system, and optionally cathodoluminescence (CL) detection. The electron microprobe is used to investigate the distribution, composition, and compositional zonation of all mineral phases, the data that underpin thermobarometric analysis and modeling of P–T histories. The microprobe, with μm-scale spatial resolution, can also characterize compositional zonation in very small accessory phases including monazite, xenotime, zircon, allanite, titanite, apatite, and others. This, as discussed below, can be a …