UMass Probe-Monazite

Brief summary from Williams et al. (2007). The most powerful and unambiguous approach to monazite geochronology and geochemistry involves integrating monazite into chemical reactions with the major silicate phases. Monazite can be used to specifically date the metamorphic reactions and the silicate phases produced in those reactions. Although monazite may originate via allanite breakdown in some pelitic rocks, many metamorphic rocks have a number of monazite generations and thus a number of monazite-in reactions (see Spear and Pyle, 2002; Wing et al., 2003). The partitioning of Y and HREE between garnet, monazite, and xenotime may be the most commonly used and successful link between monazite and chemical reactions. After xenotime, garnet is the major repository of HREE and Y in metamorphic rocks, and the relative timing of garnet formation or breakdown strongly influences the HREE and Y distribution in metamorphic monazite. In pelitic rocks with xenotime present, both garnet and monazite can grow during the progressive breakdown of xenotime. Once xenotime has been depleted, monazite and garnet are generally on opposite sides of reactions because they represent the two dominant sinks for Y in pelitic rocks. Monazite growth reactions correspond to garnet breakdown and vice versa (see Pyle and Spear, 1999; Gibson et al., 2004). One particular reaction that has received attention in pelitic rocks is the staurolite-in reaction at which garnet is commonly consumed (Pyle and Spear, 2003). The loss of garnet may be partly or even dominantly responsible for the appearance of monazite that has been observed at the staurolite-in isograd. A second monazite growth event may occur in migmatitic pelites if garnet is consumed during melt crystallization. The second broad group of garnet-monazite connections involves decompression of a wide variety of garnet-bearing rocks, where cordierite, orthopyroxene, or plagioclase are produced at the expense of garnet. These decompression reactions often involve loss of a significant amount of garnet, and perhaps an associated generation of Y-rich monazite. Monazite growth may originate during plagioclase breakdown, particularly at high pressure. Besides the release of P and LREE (Kohn & Malloy 2004), plagioclase breakdown has also been correlated with significant SrO concentrations (up to 1.85 wt.%) (Krenn & Finger 2004) and a distinct Eu anomaly in monazite. Bingen (1996) concluded that a reaction consuming hornblende, allanite, and apatite (±titanite) and producing monazite and plagioclase coincided with the amphibolite to granulite transition in southwestern Norway. Although the REEs probably mainly come from allanite, the proposed reaction also involves a contribution of medium and heavy REEs from hornblende and titanite. Therefore, monazite is linked to the silicate reaction and a specific point on the P-T path.

	Upper: X-ray compositional map (yttrium) of monazite from felsic granulite, Legs Lake shear zone, Snowbird tectonic zone, Saskatchewan, Canada. High-Y rims are ca 1860 Ma, overgrowing polygenetic Archean (ca 2520 Ma) core domains. Age histograms (below map), derived via EPMA, represent 2σ standard error of the mean of analysis populations corresponding to domains shown on the compositional map. For details, see Mahan et al. (2006a).

Summary of EPMA geochronologic data for monazite in dike 04G-019B, Athabasca granulite terrane. All histograms are scaled relative to the consistency standard (Moacyr monazite). The white bar is scaled to show the range of dates for the thirteen zircon fractions analyzed by ID-TIMS. Dates are quoted at the 2σ level of uncertainty after Williams et al. (2006), including 1% uncertainty on the modeled background intensity. For details see Dumond et al. (2008).

Brief summary from Williams et al. (2007). Providing absolute timing constraints on deformation events is one of the major challenges of tectonic analysis. Although cross-cutting relationships of plutons, dikes, and other igneous rocks remain some of the strongest constraints, these tend to place rather coarse limits, and ambiguities can be introduced because of differing rheological properties between igneous and metamorphic rocks. In multiply deformed rocks, deformation histories commonly depend on interpretations of fabric relationships involving porphyroblasts, foliations, lineations, etc. Monazite can be a component of the fabric of deformed rocks and thus can offer a more direct means of constraining the timing of deformation events. Monazite can be a fabric component in deformed rocks, and therefore offers a direct means of contraining the timing of deformation events. Timing constraints on deformation events can be obtained via metamorphic reactions associated with monazite growth (e.g. garnet replacement by cordierite, Mahan et al., 2006). Inclusion trails within monazite porphyroclasts can also constrain deformation (see Dahl et al., 2005). Alignment of elongate or platy monazite grains aligned with fabric can also provide timing constraints. Syntectonic monazite can grow in lineation directions or extensional quadrants (see figures right and below). Some monazite crystals are actually offset by fractures related to larger scale fracture systems (Shaw et al., 2001). Locally, such fractures may be filled with later monazite bracketing the time of deformation (right). The specific domains with fill such fractures are volumetrically minor components of larger monazites, illustrating the need for in-situ microanalysis combined with high spatial resolution compositional mapping.
	Monazite from Athabasca granulite terrane, western Canadian Shield, showing partially resorbed igneous core (ca. 1930 Ma), and outer ignous core (ca. 1920 Ma). Tip (left) is separated by ca. 1850 Ma syn-kinematic low-Ca monazite, dating motion of the Grease River shear zone, refer to lower figure in "Reaction dating" page. For details, see Dumond et al. (2008).

Summary of EPMA monazite data for garnet-bearing S1 felsic granulite migmatites. Neoarchean lower crustal flow results in syn-kinematic monazite growth, elongated with strain. Paleoproterozoic overgrowths (ca. 1900 Ma) respond to steep fabric development in a stronger lower crust with discrete shear zones. For details see Dumond et al. (2010).

Step	Procedure	Explanation
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). 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 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 μm. Generally YLα, ThMα, UMβ, and PbMα, (and / or other geochemically important elements). 200nA, 100msec, focused beam.
4	Generate age maps (optional)	Estimate pixel U, Th, Pb, and U concentrations in high-magnification maps by producing k-ratios (subtract background intensities, reference to standard intensities), then applying matrix correction factors. Calculate age values by iteration of age equation, then re-assemble pixels into “age map”.
5	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).
6	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 .
7	Background acquisition	Method 1: Acquire wavelength scans of regions around YLα, ThMα, UMβ, and PbMα (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. Method 2: Acquire backgrounds via Probe for EPMA multi-point method. Backgrounds can be automatically regressed from high precision background measurements in selected domains.
8	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.
9	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), YLα, ThMα, UMβ, and PbMα, 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.
10	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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