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Rutile U-Pb Geochronology

U-Pb analyses on SHRIMP-RG are complicated because the uranium concentrations are typically low (<15 ppm), common Pb contamination can be high, and there are well-documented orientation effects that can bias calculated age (which can correlate with the orientation of the crystal) (Taylor et al., 2012; Schmitt and Zack 2012). We implement several procedures that help combat these analytical issues. We use a large O- primary beam (~50-90 nA) to minimize the orientation-induced sputter rate bias as compared to O2-; a large primary beam also increases the sputter yield of secondary ions (i.e. Pb and U). To decrease molecular interferences at Pb peaks and increase the signal:noise ratio, we also use the energy filter to only accept high-energy ions into the collector. Although moving the energy window minimizes the background, it also cuts the total secondary ion transmission by a factor of ~4. Because common lead and the amount of molecular interferences vary by sample, energy filtering is not always used. 

The slope of ln(Pb+/U+) v. ln(UO+/U+) is traditionally used to calibrate U-Pb analyses (i.e. for minerals such as zircon). However, Schmitt and Zack (2012) show that using a U-Pb standard calibration based on the slope of ln(Pb+/U+) vs ln(UO2+/UO+) results in a good linear fit and decreases the age scatter caused by rutile grain orientation (Taylor et al., 2012).

Figure showing reproducibility and precision of U-Pb analyses of rutile using ln(UO+/U+) versus ln(UO2+/UO+) to correct data for instrument mass fractionation. (a) Unknown rutile analyses reduced two different ways: data corrected using ln(UO+/U+) in blue and ln(UO2+/UO+) in pink. Note that the latter reduces scatter due to ‘orientation effects’ and reduced the analytical error for individual calculated U-Pb ages. Blue and pink shaded regions are weighted mean with 1 sigma error for UO and UO2 correction, respectively. (b) Primary rutile standard, R10b, analyzed during the course of a SHRIMP-RG run. Using ln(UO2+/UO+) increases standard analytical precision and reduces scatter. Grey shaded area outlined in black represents published age of standard measured by TIMS (Luvizotto et al., 2009) and 1 sigma error of data.

Rutile U-Pb analyses and age uncertainties

Individual rutile U-Pb analyses typically take 25-30 minutes per spot. The spot size is approximately 45x45 microns. The run table for rutile includes the following: HfO, TaO, 204Pb, a background measured at 0.045 mass units above the 204Pb peak, Ti3O4 (guide peak), 206Pb, 207Pb, 208Pb, U, Th, ThO, UO, and UO2.

Analyses of trace elements in rutile is relatively straight-forward on SHRIMP-RG, including Co, Cu, Zn, REE, Hf, Ta, U, and Th. It is not practical to measure V, Cr, Nb, and Zr because they occur in high concentrations, and should be measured by electron microprobe or laser ablation.

Analytical depends heavily on U ppm and common Pb ppm. If the grain is young, more time is spent counting on U, UO2, 207Pb, and 206Pb. Age uncertainty decreases as rutile age and U concentration increases. 500 Ma rutile standard containing ~10 ppm U has a reproducibility of 2.1%. 1.0 Ga rutile standard containing ~30 ppm U has a reproducibility of 1.9%. U concentrations less than 1 ppm are not feasible on SHRIMP-RG.

Other rutile dating methods

Rutile can be analyzed via TIMS and LA-ICPMS. TIMS provides very precise ages within ~0.5%. However, TIMS destroys the grain and cannot resolve internal age zonation. LA-ICPMS rutile analyses have an error of 1% (Zack et al., 2011) and take much less time (only a few minutes) than SHRIMP-RG. Also, U-Pb and trace element concentrations can be performed in the same run. For both TIMS and LA-ICPMS, rutile grain orientation effects are not an issue.

Analyzing rutile via SHRIMP-RG has several benefits. Small analytical spot size allows for high spatial resolution. This is advantage because it permits as assessment of zoning in rutile and dating of rutile as small inclusions.


References 

Luvizotto G.L., Zack T., Meyer H.P., Ludwig T., Triebold S., Kronz A., Münker C., Stockli D.F., Prowatke S., Klemme S., Jacob D.E., von Eynatten H. 2009. Rutile crystals as potential trace element and isotope mineral standards for microanalysis. Chemical Geology 261, 346–369.

Schmitt A.K. and Zack T. 2012. High-sensitivity U–Pb rutile dating by secondary ion mass spectrometry (SIMS) with an O2+ primary beam. Chemical Geology 332, 65-73.

Taylor R., Clark C., Reddy S.M. 2012. The effect of grain orientation on secondary ion mass spectrometry (SIMS) analysis of rutile. Chemical Geology 300, 81–87.

Zack T., Stockli D.F., Luvizotto G.L., Barth M.G., Belousova E., Wolfe M.R., Hinton R.W. 2011. In situ U–Pb rutile dating by LA-ICP-MS: 208Pb correction and prospects for geological applications. Contributions to Mineralogy and Petrology 162, 515-530.