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## Accuracy?

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**Possibilities:**1) Geochemical 2) Analytical - beam damage What are we doing to these things? - background Unexpected interferences? Details of background shape? - other factors**Let’s assess some of the details of X-ray counting and**analysis...**Voltage**Issues: spatial resolution absorption corrections Excitation potentials and interferences**X-ray counting follows Poisson statistics, at high count**rates:**High Current Beam Effects**Charging Slight deflection off normal incidence • Reduction of electron potential at specimen Distortion of excitation volume and (Z) shapes**High Current Beam Effects**Specimen damage? Bright spots after analysis • Potential thermal effects • T = 4.8Eoi / kd • Eo =accelerating potential • i = beam current • k = thermal conductivity • d = beam diameter • at 200nA, 1 um beam diameter, 15 kV • k ~ 6-10 E –3 W cm-1 K-1 T = 1500 – 3000 K uncoated**200 nA, 40 min.**Electron Dose Damage effects? [LGG246-5m1 C-coat]**Changes in major element composition of monazite due to**electron exposure (200 nA, 40 min.)**Changes in major element composition of monazite due to**electron exposure (200 nA, 40 min.)**Changes in major element composition of monazite due to**electron exposure (200 nA, 40 min.)**200 nA, 40 min.**Electron Dose Damage effects? [LGG246-5m1 Au-coat]**Changes in major element composition of monazite due to**electron exposure (200 nA, 40 min.)**Backgrounds and Interferences**Some we know well from the tables: Th Mb on U Ma Y Lg on Pb Ma Choose analytical lines and background points based on tables and WDS scans**A look at potential overlaps and background interferences**- LGG 246-5 m1 Scan pt.**Choosing background points**Background interferences very difficult - must go to differential mode!**REE**Choosing background points Ce La1,2 (2) La La1 (2) La Lb1 (2)**Choosing background points**Th Mg U Mb Monazite from pegmatite associated with ~ 1400 Ma reactivation event in NM No interference correction for Th on U Mb = 1342 +/- 5 Ma Interference corrected = 1358 +/- 5 Ma**Back to LGG 246-5 (Grand Canyon)**Ce La1,2 (2) La La1 (2) La Lb1 (2) Isotopic Age ~ 1685 Ma Integral mode: 1581 +/- 3 Charging??**Au coat**Ce La1,2 (2) La La1 (2) La Lb1 (2)**Au coat**Isotopic Age ~ 1685 Ma Integral (C) Differential (Au) 1586 +/- 3 Ma 1642 +/- 5 Ma ppm bkg (c/s/nA) ppm bkg (c/s/nA) Th 74848 +/- 2324 0.443 70862 +/- 193 0.379 U 2844 +/- 52 1.100 3259 +/- 51 1.0109 Pb 6252 +/- 176 0.285 6146 +/- 196 0.232 With U corrected for Th overlap: U 2976 +/- 39 ppm Age 1658 +/- 5 Ma Th Mg U Mb**Background curvature….**We know the Brehmsstrahlung will have a natural curvature - expressed by modified Kramers’ Law: NEE = kEZ[(Eo -E)/E]E NE = # photons from E to E+ E Eo = acc. potential Z = ave. atomic number kE = Kramers’ constant The actual background intensity as a function of wavelength in WDS is, however, highly dependent on spectrometer efficiency...**Use WDS scans to obtain background**Regress selected background regions using either Polynomial or exponential models**LGG 246-5m1**With U corrected for Th overlap: U 2587 +/- 141 ppm Age 1692 +/- 2 Ma (high Th core) Average for entire grain: 1686 +/- 3 Ma Isotopic Age ~ 1685 Ma Integral (C) Scanned - Differential (Au) 1586 +/- 3 Ma 1672 +/- 2 Ma ppm bkg (c/s/nA) ppm bkg (c/s/nA) Th 74848 +/- 2324 0.443 74678 +/- 238 0.365 U 2844 +/- 52 1.100 2885 +/- 142 1.006 Pb 6252 +/- 176 0.285 6606 +/- 229 0.206**LGG 245 m3 - what if we go back to one of the ones that**initially looked good? Old system (2 pt. backgrounds): Age = 1683 +/- 7 Ma New system (scanned backgrounds - overlap correction): Age 1688 +/- 4 Ma**Summary for Wards Monazite**Scanned backgrounds and Th corrected: 1399 +/- 4 Ma Th Mg U Mb Monazite from pegmatite associated with ~ 1400 Ma reactivation event in NM No interference correction for Th on U Mb = 1342 +/- 5 Ma Interference corrected = 1358 +/- 5 Ma**Black Hills - PR-1**SHRIMP II 1761 +/- 11 Ma 1716 +/- 12 Ma (overgrowths) EMPA - three grains 1774 +/- 5 Ma 1760 +/- 16 Ma 1787 +/- 9 Ma 1691 +/- 6 Ma (overgrowths)**Overlaps…continued**Th Mg K Ka1 REE L-energies = 4.7-7.9 KeV Excitation potential K (K-shell) = 3.6 KeV**Procedure**• Full thin-section map to find monazite (C-coated) • Select grains for detailed mapping based on texture, etc. • Map individual monazite grains for Y, Th, Pb, and U • Identify compositional domains and obtain major element compositions • Apply Au-coat • Run background scans in each domain (200nA, 15 kV) • Pick background regions in each scan and regress - usually exponential • Enter background intensities into trace program with appropriate major element compositions • Analyze trace elements (Y, Th, Pb, U - 200nA, 15kV, 600-900 sec.) A number of analyses should be obtained from each domain • Calculate ages for points in a domain and apply standard error of the mean to estimate the precision**Other considerations:**• Effect of major element compositional variation • Choice of Matrix corrections and physical constants • Assessment of Pb diffusion - closure temperature • What role do fluids play in monazite reactions?**Sources of Error**• Counting statistics • C = counts • R = count rate • T = count time • For the count rate:**Calibration….**If the count times are equal for peak and background… And the combined relative error for unknown and standard becomes… Calibrate on high concentration standards.**Other sources of error:**• Compositional domain boundary effects • Grain edge difficulties (including fluorescence and polishing artifacts!) • Distortion of excitation volume due to induced potentials • Accuracy of background fluorescence corrections • Detector dead time constants • Discrepancies between high and low current measurement • Counter tube charge build-up • Internal counter pressure changes**Standard error of the mean**Quantifying error: • Propagation of count statistics through age equation • Monte - Carlo error estimation