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Abstract from Hetherington et al. (2008).

Xenotime (YPO4) is iso-structural with zircon-group minerals and lies at one end of a compositional solidsolution with monazite. As with monazite and zircon, naturally occurring xenotime may accommodate significant concentrations of rare earth elements, as well as Th and U, making it an important repository for these valuable geochemical marker elements and a potential geochronometer. Xenotime grows in sediments during diagenesis, in low- to granulite-grade metamorphic rocks, in migmatites, and is common in peraluminous igneous rocks. Xenotime can be a complement, or indeed an alternative, to monazite and/or zircon in geochemical and geochronological studies. To maximise the petrographic potential of xenotime, a complete understanding of its composition and relationship to rock-forming assemblages is essential. Electron beam techniques provide micron-scale resolution of compositional and textural variation and yield precise dates for individual micro-volumes in multi-domain grains. Methods for collecting trace element data by electron probe microanalysis differ from those routinely employed to collect major element concentrations. Careful characterisation of background curvature and background interferences around the Th, U and Pb peaks is required. To avoid large overlap correction for Y–Lγ2,3 on Pb–Mα, analysis of Pb is made on the Mβ-peak. A broad array of geochemical and geochronological applications can take advantage of the links between xenotime composition and texture and the evolution of silicate–mineral assemblages in a diversity of environments.

 

Xenotime growth during diagenesis, Potsdam sandstone, Adirondack region (NY). Upper, BSE image of zircon overgrown by younger xenotime (bright rims). Maps of Th and Dy (L and R respectively, below BSE image) of red-outlined area in BSE image reveal compositional zoning. Xenotime is ca. 350-380Ma, overgrowing precambrian zircon.
Right: Dy La compositional map of xenotime (bright) overgrowing zircon. Potsdam sandstone, Adirondack region (NY). Note Dy zoning in zircon.
 
Step
Procedure Explanation  
1
Full section map Carbon coat thin section (vacuum evaporation to ~250Å) and collect map of Y (can be included in monazite search map), along with base-map reference element (Mg, Al, Ca, etc.). Typically 1024X512 pixels, 35 μm pixel step size, defocused beam (~ 35 μm). 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 Y spots and import adjusted maps as layers into Adobe Illustrator or equivalent. Mark spots from Y maps on separate layer with circles, dots, etc. Overlay marked layer on base map (Mg, Al, Ca, etc.) 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 μm. Generally ThMα, UMβ, PbMβ, and some HREE (and / or other geochemically important elements). 200nA, 100msec, focused beam. Example maps to right are Dy Lα and Th Mα.
4
Collect major element analyses (if non-integrated analysis 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 metallic coat Lightly polish section (≤0.3 μm 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 wavelength scans of regions around ThMα, UMβ, and PbMβ (8-sinθ steps over 8400 sinθ 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. See Hetherington et al. (2008).

Method 2: Acquire backgrounds via Probe for EPMA multi-point method. Backgrounds can be automatically regressed from high precision background measurements in selected domains.

7
Background analysis For Method 1, 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. Enter background intensities into analysis definitions (along with appropriate major element concentrations if full analysis is not to be performed). Calibrate (at 15nA, 15kV, focused beam), ThMα, UMβ, and PbMβ, 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).  
 
Co

Dumond, G., Goncalves, P., Williams, M.L., and Jercinovic, M.J. (2010) Subhorizontal fabric in exhumed continental lower crust and implications for lower crustal flow: Athabasca granulite terrane, western Canadian Shield. Tectonics, 29, doi:10.1029/2009TC002514. PDF

Mahan, K.H., Wernicke, B.P., and Jercinovic, M.J. (2010) Th-U-total Pb geochronology of authigenic monazite in the Adelaide rift complex, South Autstralia, and implications for the age of the type Sturtian and Marinoan glacial deposits. Earth and Plantetary Science Letters, 289, 76-86. PDF

Jercinovic, M.J., Williams, M.L., and Lane, E.D. (2008) In-situ trace element analysis of monazite and other fine-grained accessory minerals by EPMA. Chemical Geology 254, 197-215. PDF

Dumond, G., McLean, N., Williams, M.L., Jercinovic, M.J., and Bowring, S.A. (2008) High-resolution dating of granite petrogenesis and deformation in a lower crustal shear zone, Athabasca granulite terrane, western Canadian Shield. Chemical Geology 254, 175-196. PDF

Hetherington, C.J., Williams, M.L., Jercinovic, M.J., and Mahan, K. (2008) Application of electron-probe microanalysis to composition, chronology, and occurrence of xenotime for understanding geologic processes. Chemical Geology 254, 123-147. PDF

Budzyn, B., Hetherington, C.J., Williams, M.L., Jercinovic, M.J., Dumond, G., and Michalik, M. (2008) Application of electron probe micro-analysis total Th-U-Pb geochronology to provenance studies of sedimentary rocks: An example from the Carpathian Flysch. Chemical Geology 254, 148-163. PDF

Williams, M.L., Jercinovic, M.J., and Hetherington (2007), C.J. Microprobe Monazite Geochronology: understanding geologic processes through integration of composition and chronology. Annual Review of Earth and Planetary Sciences 37, 137-175. PDF

Heumann, M.J., Bickford, M.E., Hill, B.M., McLelland, J.M., Selleck, B.W., and Jercinovic, M.J. (2007) Timing of anatexis in metapelites from the Adirondack lowlands and southern highlands, a manifestation of the Shawinigan Orogeny and subsequent AMCG magmatism. Geological Society of America Bulletin 118, 1283-1298.

Cruz, Francisco, W., Burns, S.J., Jercinovic, M.J., Karmann, I., Sharp, W.D., and Vuille, M. (2007) Evidence of rainfall variations in southern Brazil from trace element ratios (Mg/Ca and Sr/Ca) in a Late Pleistocene stalagmite. Geochimica et Cosmochimica Acta 71, 2250-2263. PDF

Mahan, K.H., Williams, M.L., Flowers, R.M., Jercinovic, M.J., Baldwin, J.A., and Bowring, S.A. (2006) Geochronological constraints on the Legs Lake shear zone with implications for regional exhumation of lower continental crust, western Churchill Province, Canadian Shield. Contributions to Mineralogy and Petrology 152, 223-242. PDF

Mahan, K.H., Goncalves, P., Williams, M.L., and Jercinovic, M.J. (2006) Dating metamorphic reactions and fluid flow: Application to exhumation of high-P granulites in a crustal-scale shear zone, western Canadian Shield. Journal of Metamorphic Geology 24, 193-217. PDF

Boyce, J. W., K. V. Hodges, W. J. Olszewski, M. J. Jercinovic, Carpenter, B.D., and Reiners, P.W. (2006), Laser microprobe (U-Th)/He geochronology, Geochem. Cosmochim Acta,70, 3031-3039. PDF

Baldwin, J.A., Bowring, S.A., Williams, M.L., & Mahan, K.H., (2006) Geochronological constraints on the evolution of high-pressure felsic granulites from an integrated electron microprobe and ID-TIMS geochemical study. Lithos 88, 173-200.

Williams, M.L., Jercinovic, M.J., Goncalves, P., and Mahan, K. (2006) Format and philosophy for collecting, compiling, and reporting microprobe monazite ages. Chemical Geology225 , 1-15. PDF

 

 

Jercinovic, M.J., and Williams, M.L. (2005) Analytical perils (and progress) in electron microprobe trace element analysis applied to geochronology: Background acquisition, interferences, and beam irradiation effects. American Mineralogist90 , 526-546. PDF

Dahl, P.S., Terry, M.P., Jercinovic, M.J., Williams, M.L., Hamilton, M.A., Foland, K.A., Clement, S.M., and Friberg, L.M. (2005) Electron probe (Ultrachron) microchronometry of metamorphic monazite: Unraveling the timing of polyphase thermotectonism in the easternmost Wyoming Craton (Black Hills, South Dakota). American Mineralogist90 , 1712-1728. PDF

Dahl, P.S., Hamilton, M.A.., Jercinovic, M.J., Terry, M.P., Williams, M.L., and Frei, R. (2005) Comparative isotopic and chemical geochronometry of monazite in metamorphic rocks from the eastern Wyoming province (USA), with implications for U-Th-Pb dating by electron microprobe. American Mineralogist 90 , 619-638. PDF

Goncalves, P., Williams, M.L., and Jercinovic, M.J. (2005) Electron microprobe age mapping. American Mineralogist 90 , 578-585. PDF

Williams, M.L., and Jercinovic, M.J. (2002) Microprobe monazite geochronology: Putting absolute time into microstructural analysis. Journal of Structural Geology24 , 1013-1028.

Shaw, C.A., Karlstrom, K.E., Williams, M.L., Jercinovic, M.J., and McCoy, A.M. (2001) Electron microprobe monazite dating of ca. 1.71 – 1.63 Ga and ca. 1.45-1.38 deformation in the Homestake shear zone, Colorado: Origin and early evolution of a persistent intracontinental tectonic zone. Geology29 , 739-742.

Terry, M.T., Robinson, P., Hamilton, M.A., and Jercinovic, M.J. (2000) Monazite geochronology of UHP and HP metamorphism, deformation, and exhumation, Nordoyane, Western Gneiss Region, Norway. American Mineralogist 85, 1651-1664.

Williams, M.L., Jercinovic, M.J., and Terry, M.P. (1999) Age mapping and dating of monazite on the electron microprobe: Deconvoluting multistage tectonic histories. Geology27, 1023-1026.

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