Zircon (ZrSiO4) is an omnipresent mineral in the earth’s crust. The material is chemically inert and hard; therefore, it is highly resistant to weathering processes and can persist for a long time in a wide range of igneous, metamorphic, and sedimentary rocks. While forming zircon crystals from a melt, conditions can differ in due course, which impacts the growth of zircon.
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Such variations in conditions result in zonation in the zircon (such as zones of zircon composed of different amounts of trace elements). Akin to the growth rings in trees, these zones include chronological information related to the geological formation of the zircons.
Normally, the zonation is faint and does not, for example, result in large density differences, thus making it challenging to observe with ordinary scanning electron microscopy (SEM). Alternatively, cathodoluminescence (CL) imaging is highly sensitive to small concentration changes in trace elements — for example, rare-earth ions — because they have a spectrally distinct and strong CL response . Interestingly, zircons usually have high thorium and uranium content, which can be used for radiometric dating.
CL can be employed as a prescreening tool for high-resolution secondary-ion mass spectrometry (SIMS) to image the zonation pattern and identify the regions of interest in it for isotope analysis. Then, these can be cross-examined with the more expensive and time-consuming SIMS technique. Until now, such a combined analysis has been used to date zircons that are more than 4.3 billion years old, which is the oldest native crustal rock from earth ever to be dated .
Figure 1 illustrates a secondary electron image (a) and a panchromatic CL image obtained using a photomultiplier tube (PMT) in the SPARC CL system (b). In both the images, the zircon grains can be vividly differentiated from the background; however, the CL image shows significantly more structure within the grains. Another CL image is represented in Figure 1(c), where there is a strong CL contrast within a single grain and the zonation can be clearly seen. These results demonstrate that the SPARC CL system is an outstanding platform for rapidly revealing zonation patterns in such minerals and that it can also be used as a valuable screening tool for sophisticated geological characterization techniques.
Figure 1. (a) SEM image of some zircon grains. (b) Panchromatic PMT intensity image (grayscale) of the same area. (c) Close-up panchromatic PMT image of a single zircon grain. These measurements were taken at 10 kV acceleration voltage and 1 nA current with a 100 µs dwell time. CL images took ~1.5 minutes to collect. Samples courtesy of Prof. Jens Jahren (University of Oslo).
. Zircon, J.M. Hanchar, and P.W.O Hoskin, Rev. Mineral. Geochem. 53 (2003).
. S. A. Wilde et al., Nature 409 (2001) 175-178.
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