Reaction dating Dating deformation Analysis Monazite home
The following is a brief introduction into monazite geochronology from the Microanalysis Society Topical Conference (2016)
“Knowing the rate of disintegration of uranium, it would be possible to calculate the time required for the production of the proportions of lead found in the different minerals, or in other words the ages of the minerals.” – B.B. Boltwood, 1907 [1]. This remarkable realization, based on a suggestion by Ernest Rutherford, initiated the formal study of quantitative geochronology and establishment of absolute ages of minerals and rocks, which has also become a classic example of evolving modern metrology. The study of geochronology has led to a revolution in understanding planetary processes, and has benefitted from the continual, synergistic progression of technological innovation and scientific inquiry. Indeed, placing chronologic constraints on reaction and/or deformation histories at the finest scales is transforming the very way we think about both the processes and philosophy of geochronology.
The sensitivity and accuracy available for geochronologic analysis today are extraordinary, but not without numerous assumptions and caveats. Inconsistent or even conflicting results can lead to uncomfortable ad hoc explanations and creative speculation. EPMA offers insights with the potential to significantly improve our understanding of the observed phase interactions, and has proven in recent years to both illuminate existing geochronologic problems, and to reveal some problems unforeseen by any other methodology. Applications to multiply deformed and metamorphosed terranes, leading to the formation of polygenetic mineral grains down to the nano-scale, are at the exciting forefront of geochronological research [2][3]. The savvy researcher brings as many things as possible to bear on these complex problems, hoping to emerge with a coherent story, and high resolution EPMA is contributing substantially. The analyst must, however, always be wary of the possibility of thesis-driven results, convenient disregard of data that does not fit “what it should be”, and overlooking analytical details that can result in interesting data sets that are ultimately meaningless. It is most important to recognize that no geochronologic technique actually yields dates, but is actually measuring other values (e.g. isotopic mass ratios, characteristic X-ray intensities) that can be used to calculate dates using appropriate assumptions.
EPMA, of course, does not have a resolvable isotopic aspect, producing ratios of characteristic X-rays used to estimate elemental concentration based on “known” standards. EPMA is traditionally limited by aspects of sensitivity coupled with spatial resolution, governed primarily by evaluating signal/noise, and limiting electron scattering relative to critical excitation potentials in the specimen respectively. Because of the developmental synergy mentioned above, we look at these limitations today more as frontiers, to see what you can do, rather than strictly enforcing seeming barriers to say what you cannot do. The first step for the analyst is to remember that there is no correct answer, that the various tools used can yield remarkably different results, and that the same tools in different facilities can even sometimes yield quite different results. Today, EPMA labs are not trying to duplicate ID_TIMS, SIMS, or LA-ICPMS, nor are they trying to do a quick and dirty, relatively inexpensive assessment of ages. A massive amount of information is available by EPMA, including mapping and full quantitative analysis. Mapping in particular is changing the game as the full power of extensive image acquisition and processing are coming into their own for evaluating accessory phase locations and comparative compositional analysis. The locations of accessory minerals within a rock alone can yield some remarkable relationships heretofore unsuspected, and in a number of cases, EPMA can reveal mineral populations not seen by traditional separation techniques. In addition, all other geochronologic techniques have come to rely heavily on electron microscopy, and EPMA in particular, for essential phase location and imaging (BSE, X-ray mapping, CL), to elucidate mineral complexities not revealed any other way. EPMA remains the ultimate in-situ technique, allowing datable accessory phases to be kept in structural and petrologic context, and can access meaningful geochronologic information from domains substantially below 1 micrometer. With continued development of low kV techniques coupled with Schottky sources, this may yet further improve.
Minor and trace element micro-analysis are rapidly becoming very significant daily uses of EPMA in geoscience and other applications, and geochronology is but one application that is pushing development of both hardware and software to acquire better and more reliable results. Major element analysis, with large peak/background values, is more forgiving of minor background errors, whereas trace element analysis is fully dependent upon the proper characterization of background, including both shape and interferences, therefore great effort is spent on refining these aspects of analysis [2][4][5][6].
A basic strategy for the analysis of accessory minerals for geochronology encompasses both extensive X-ray mapping as well as detailed, highly sensitive quantitative analysis. It cannot be overemphasized that the entire quantitative strategy should be based on mapping, not just for efficiency, but for extracting the maximum information that is most pertinent to the questions being asked. The following general strategy is suggested:
- Generate full thin-section compositional map, including at least one major element as a base map (e.g. Mg, Ca, Al), and elements for accessory phase ID (e.g. Ce, Zr, Y).
- Process above and overlay accessory phase locations on base map.
- Select accessory phase grains, and map them individually at high magnification to reveal internal structure using appropriate elements (Y, Th, U, Ca, etc. for monazite).
- Process grain maps simultaneously to assess inter-grain domain compositions.
- Quantitative analysis of selected mineral grains based on individual domains defined in maps. Background is explicitly acquired for a single compositional domain and points are acquired until the requisite precision is reached.
Periodic analysis of a reference material is essential during quantitative sessions to monitor consistency. The protocol for quantitative analysis should stress background characterization, noting that complex matrices present many challenges. The best approach employs an adjustable multi-point method which allows high precision counting at a number of selected wavelength positions, identifying subtle interferences, and shape refinement to properly characterize background. As high spatial resolution and high count precision involves very high beam current density and lengthy count times, there is a potential for beam damage and X-ray emission time-dependence. Alternative coatings and advanced software procedures can help in both cases.
[1] BB Boltwood, Am. Jour. Sci. 23 (1907), p. 78.
[2] ML Williams, MJ Jercinovic, CJ Hetherington et al., A. Rev. Ear Plan. Sci. 35 (2007), p. 137.
[3] ML Williams and MJ Jercinovic, J. Metamorphic Geol. 30 (2012), p. 739.
[4] MJ Jercinovic and ML Williams, Am. Mineral. 90 (2005), p. 526.
[5] MJ Jercinovic, ML Williams, ED Lane, Chem. Geol. 254 (2008), p. 197.
[6] Allaz et al, EMAS 2011, Angers France (2011), p. 319.
