Thermal Ionization Mass Spectrometry

The Thermal Ionization Mass Spectrometry (TIMS) Laboratory is under the supervision of Dr J.D. Walker. The KU TIMS laboratory in Nichols Hall houses a 1987 version, full automated VG Sector variable 6-collector TIMS with a 10-sample turret, Daly multiplier, optical pyrometer, etc. The Sector was purchased with substantial funding from NSF and is still a state-of-the-art instrument: renovations during the summer of 1995 overhauled the collector cups and replaced the analog Daly system with an ion-counting system for an order of magnitude improvement in sensitivity.

High-precision Sr and Nd analyses are routine; typical external and internal previsions for these elements are +/- 20 ppm. Pb analysis on rocks and minerals is also routine: we analyze as little as 25 pg total Pb with blank levels of 0.5 to 3 pg. Replicate analyses of standards give reproducibility of fractionation better than 0.0025%/amu. The main emphasis of TIMS at KU is for geochronology (especially U-Th bearing accessory minderal such as zircon, monazite, rutile, and titanite) and petrologic studies. The low Pb blanks and high sensitivity of our VG Sector allow for routine high-precision geochronological studies. In addition, 235U and 230Th spiked samples can be analyzed by isotope dilution by TIMS as well as ICP-MS.

TIMS details

We have one TIMS machine in our laboratory. It is a 1987 version, fully-automated VG Sector variable 6-collector system, 10-sample turret, Daly multiplier, optical pyrometer, etc. The Sector was purchased with substantial funding from NSF and is still a state-of-the-art instrument: renovations during the summer of 1995 overhauled the collector cups and replaced the analog Daly system with an ion-counting system for an order of magnitude improvement in sensitivity. High-precision Sr and Nd analyses are routine; typical external and internal precisions for these elements are +/- 20ppm. Pb analysis on rocks and minerals is also routine: we analyze as little as 25 pg total Pb with blank levels of 0.5 to 3 pg. In addition, replicate analyses of standards give reproducibility of fractionation better than 0.025%/amu.

The main emphasis of TIMS at KU is for geochronology (especially U-Th-bearing accessory minerals such as zircon, monazite, rutile, and sphene) and petrologic studies. The low Pb blanks and high sensitivity of our VG Sector allow for routine and high-precision geochronological studies. In addition, 235U and 230Th spiked samples can be analyzed by isotope dilution by TIMS as well as ICP-MS.

Chemistry

Our U-Pb procedures generally follow the techniques developed by Krogh (1973, 1982) with modifications proposed by J. Mattinson (pers. comm.) and R. Parrish (1987) for smaller dissolution capsules. We can routinely analyse zircon fractions of 50 micrograms or less, including single crystals for coarser-grained separates. Pb blanks for these procedures are routinely less than 50 picograms, with blanks of less than 5 picograms for ultra-micro chemistry. Although further improvements in blanks are in progress, the current levels are negligible for the work in progress.

Rb-Sr chemistry follows traditional procedures, with dissolution using HF-HNO3 acid in sealed teflon dissolution vessels and elemental separations using HCl elution on cation exchange columns. In many instances the Rb-Sr whole-rock samples are aliquots from Sm-Nd whole-rock dissolutions.

Our Sm-Nd chemical procedures follow those of J. Patchett (e.g., Patchett and Ruiz, 1987), which use the teflon powder-HDEHP column procedure (Richard and others, 1976). We routinely use high-temperature dissolution in Krogh-type bombs (Krogh, 1973) to guarantee decomposition of refractory accessory phases.

References Cited

Krogh, T. E., 1973, A low contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determinations. Geochim. Cosmochim. Acta, v. 37, p. 485-494.

Krogh, T.E., 1982, Improved accuracy of U-Pb ages by the creation of more concordant systems using an air abrasion technique. Geochimica et Cosmochimica Acta, v. 46, p. 637-649.

Parrish, R.R., 1987, An improved micro-capsule for zircon dissolution in U-Pb geochronology. Isotope Geoscience, v. 66, p. 99-102.

Patchett, P.J., and Ruiz, J., 1987, Nd isotopic ages of crust formation and metamorphism in the Precambrian of eastern and southern Mexico. Contributions to Mineralogy and Petrology, v. 96, p. 523-528.

Richard, P., Shimizu, N., and Allegre, C.J., 1976, 143Nd/146Nd, a natural tracer: an application to oceanic basalts. Earth and Planetary Science Letters, v. 31, p. 269-278.