Age Dating from Electron Microprobe Analyses of U, Th, and Pb: Geological Advantages and Analytical Difficulties

Abstract

Electron microprobe analysis of U, Th, and Pb in naturally occurring minerals can indicate their age. Where the Pb is entirely due to the radioactive decay of U and Th, the time since mineral formation or equilibration can be calculated. Uraninite (UO2), monazite (REE PO4), zircon (ZrSiO4), and xenotime (YPO4) have been used, the latter containing U and/or Th in minor proportions. Any stable U- or Th-bearing phase can be considered. Careful analysis is required with attention to interferences, background measurement, detection limits, and Pb-free sample preparation. Extended counting times (600 s) at a probe current >200 nA are recommended. Ages can be determined from uraninite older than 2 Ma for a Pb detection limit of 0.02% and up to 700-1,000 Ma, after which Pb can be lost from the structure. The youngest monazite ages permitted by the Pb detection limit are 50-100 Ma and ages greater than 3,000 Ma have been determined. The method does not provide the detail of isotopic methods, but results can be obtained more readily. Examples show dating of cheralite ((Ca,Ce)(Th,Ce)(PO4)2), a rock containing primary and secondary UO2, and a suite of detrital uraninite grains that formed a part of a mineral exploration program.

Relative Abundance and Compositional Variation of Silicates, Oxides and Phosphates in the W-Sn-Rich Lodes of the Panasqueira Mine (Portugal): Implications for the Ore-Forming Process

“…Accordingly, the compositional variation of the analyzed uraninite grains is fully described by the usual substitution mechanism U 4+ = (Th, Pb) 4+ . In addition, and considering the iterative calculation method reported in literature [179,180], average ages of ≈298 ± 2 Ma and ≈283 ± 2 Ma can be estimated for these two groups of uraninite, respectively.…”

A Mineralisation Age for the Sediment-Hosted Blackbush Uranium Prospect, North-Eastern Eyre Peninsula, South Australia

“…Ages were calculated iteratively using the general, widely applied age formula as defined and given by Montel et al [28]. Although initially used to date monazite, in which U is at comparatively low levels, the formula is widely used in chemical U-Th-Pb dating studies via EPMA, and is identical to that given for minerals containing essential uranium (e.g., [11,12,24,29]) and can be used for any other U-bearing mineral that retains radiogenic Pb, irrespective of concentrations.…”

“…The objective of the present work is to obtain an accurate age for coffinite-dominant uranium mineralisation in the Blackbush resource, using the chemical (microprobe) age method of Bowles [11,12]. Geochronology is focussed on coffinite within the Kanaka Beds and forming the major part of the mineralisation but also in saprolite at the unconformity, as well as uraninite within granite-hosted veins immediately below.…”

Electron Probe Microanalysis: A Review of the Past, Present, and Future

“…The idea of trace element analysis, which has been emerging more and more frequently as a favorite issue, is now considered almost common place with the new generation of electron probes with both WDS and EDS spectrometers. The paper on age dating through EPMA of U, Th, and Pb by John Bowles in this issue (Bowles, 2015) is a good example of the possible applications of reliable low and trace element concentration analysis by EPMA. Ultimately, the capability of trace element analysis by electron microprobe has become a reality with present-day instruments and data processing software, reaching well below the 0.1 wt% level and into the parts per million (ppm) range (Jercinovic et al, 2012).…”