Reaction dating Dating deformation Analysis Monazite home
1 Full section map
Carbon coat thin section (vacuum evaporation to ~250Å) and collect map of Ce (and / or La), along with base-map reference element (Mg, Al, Ca, etc.). Typically 1024X512 pixels (adjust to cover section), 35 ?m pixel step size, defocused beam (~ 35 ?m). 350 nA, 15kV, 10 msec count time / pixel.
2 Process maps for accessory mineral selection
Import raw maps into image analysis program (Adobe Photoshop or equivalent). Adjust I/O levels to highlight Ce (or La) spots and import adjusted maps as layers into Adobe Illustrator or equivalent. Mark spots from REE maps on separate layer with circles, dots, etc. Overlay marked layer on base map (Mg or Al) to identify accessory phases in textural context.
3 Map minerals at high magnification
Map selected grains, usually beam rastering at resolution giving step size <1 ?m. Generally YL?, ThM?, UM?, and CaK?, (and / or other geochemically important elements). 200nA, 100msec, focused beam.
4 Collect major element analyses (if non-integrated analysis). Preferably major elements collected during trace analysis (skip to step 5)
Run major element analyses, 15nA, 15kV, focused beam, of monazite, xenotime, etc. Should collect analyses from all domains identified in maps (step 3 above).
5 Remove carbon coat, then apply aluminum coat
Lightly polish section (?0.3 ?m polishing compound) to remove C-coat. Apply Al+C coat to thin section(s) and standards by vacuum evaporation. Should be ~ 200Å aluminum followed by ~80Å carbon.
6 Background acquisition
Method 1: Acquire backgrounds via Probe for EPMA multi-point method. Backgrounds can be automatically regressed from high precision background measurements in selected domains.
Method 2: Acquire wavelength scans of regions around YL?, ThM?, UM?, and PbM? (8-sin? steps over 8400 sin? range, 1500msec/step, 200nA, 15kV, focused beam, differential mode PHA). Collected counts are converted to dead-time corrected cps/nA. Backgrounds should be acquired for each identified compositional domain, particularly guided by thorium variation.
7 Background analysis
For Method 1 above, open multipoint dialog to examine regression fit, edit as necessary. Method 2, above, apply digital noise filter to scan data, select appropriate background regions (avoiding interferences), and regress included data (exponential or polynomial best-fit). Apply regressed line to peak position to calculate intensity of background.
8 Obtain trace element analyses or full-integrated analysis.
For background Method 1, background intensities are automatically entered into database for analysis. For Background method 2, enter background intensities into analysis definitions (along with appropriate major element concentrations if full analysis is not to be performed). Calibrate as necessary, then analyze unknowns (200nA, 15kV (max), focused beam, 600 sec per point (min)). Identified compositional domains should be individually analyzed. Multiple analyses within a domain increases the precision on the age estimate as evaluated via the standard error of the mean. Spectral overlaps are critical, and accurate corrections must be employed. High precision overlap calibrations are obtained from standards with none of the element of interest, and significant amounts of the interfering elements.
9 Calculate ages
Calculate Pb as a function of age, Th, and U concentrations. By iteration of age, converge to the measured Pb concentration. Age calculations are constrained compositionally. Mapped domains are sampled repeatedly to obtain the statistics necessary to address the geochronologic problem. See Williams et al. (2006).
