Determination of Uranium and Thorium in Zircon, Apatite, and Fluorite: Application to Laser (U-Th)/He Thermochronology

Abstract

1 (U-Th)/He thermochronology is experiencing a revival ([1-6] and references therein) as potential applications and innovations to the existing technology broaden. As the number of laboratories around the world with an interest in developing (U-Th)/He capability increases, there is a growing appreciation of the importance of obtaining precise and accurate U and Th analysis on small samples (particularly with the advent of laser gas extraction [7]) and of the challenge this presents to geoscientists who may not necessarily have an expertise in analytical chemistry. In addition, there is a need for transparency in the reporting of measured data to facilitate interlaboratory comparison. Often, only concentration data in "ppm" are presented for the parent elements, and it is difficult to accurately recalculate the published ages from these data. This paper aims to present a summary of spike preparation, sample dissolution, and analytical techniques for U and Th on laser-gas extracted, single crystals of apatite, fluorite, rutile, titanite, and zircon. We also present full isotopic ratio datasets for replicate analyses of Durango, Yucca Mountain fluorite, and an Australian zircon that provide all the necessary information for age calculation.Theory relating to U and Th analysis for (UTh)/He thermochronology. There are a variety of methods of quantitative analysis available for induc-1 The text was submitted by the authors in English. tively coupled plasma mass-spectrometry (ICPMS). We have found that optimum results are obtained using an isotope dilution method in which the samples are spiked with suitable uranium and thorium isotopes ( 235 U and 230 Th) and the resulting 238 U/ 235 U and 232 Th/ 230 Th ratios are determined. These spikes were chosen because of their low or zero abundance in natural samples, but there are other options (e.g., 229 Th). The advantage of using isotope spiking is twofold: the potential problems associated with measurement drift and matrix effects (see below) are eliminated, and minor loss of U or Th after spike and sample isotopes have equilibrated does not affect the isotopic ratio or the accuracy of the determined age. Standardization is achieved by adding exactly the same amount of spike solution to a prepared standard solution of known uranium and thorium concentration. The U and Th isotope ratios for this spiked standard are measured by ICPMS in the same analytical run used for the samples.The precise amount of spike added and its abundance need not be well known, provided that the isotopic composition of the spike is known and exactly the same amount of spike is added to both the sample and standard. This is shown as follows. If a sample containing monoisotopic 232 Th at an abundance of c sample is spiked with monoisotopic 230 Th to give a spike isotope abundance of c spike , then, because count rate in ICPMS is directly proportional to isotope abundance, the iso-ARTICLES Abstract -We have developed a methodology for (U-Th)/He thermochronology on a variety of mineral species. With m...

Constraining exhumation pathway in an accretionary wedge by (U-Th)/He thermochronology—Case study on Meliatic nappes in the Western Carpathians

“…4 He was extracted under ultra-high vacuum from single-grain aliquots by heating to ∼960 • C (apatite) and ∼1250 • C (zircon) using a diode laser, and then measured by isotope dilution on a Pfeiffer Prisma QMS-200 mass spectrometer. Following He measurements, the apatites were spiked with 235 U and 230 Th and dissolved following the procedure of Evans et al (2005). The solutions were analysed by isotope dilution for U and Th, and by external calibration for Sm on a PerkinElmer SCIEX ELAN DRC II ICP-MS.…”

Thermochronological constraints on the long-term erosional history of the Karkonosze Mts., Central Europe

“…Following He measurements, the apatite was spiked with 233 U and 230 Th dissolved in nitric acid and analysed by isotope dilution inductively coupled mass spectrometry (ID ICP MS) for U, Th and Sm at the Geoscience Center Göttingen (Germany) on a Perkin Elmer (ELAN DRC II) ICP-MS. Zircon was loaded in Nb tubes, degassed at ∼ 1250°C and analysed for 4 He using the CSIRO Exploration and Mining extraction line in the John de Laeter Centre of Mass Spectrometry in Perth (Australia). Degassed zircon was dissolved following the procedure of Evans et al (2005) and analysed by ID ICP MS for U and Th at TSW Analytical Ltd in the University of Western Australia (Perth) on an Agilent 7500 mass spectrometer. For more details on analytical procedures, the reader is referred to Evans et al (2005) and Danišík et al (2008).…”

“…Degassed zircon was dissolved following the procedure of Evans et al (2005) and analysed by ID ICP MS for U and Th at TSW Analytical Ltd in the University of Western Australia (Perth) on an Agilent 7500 mass spectrometer. For more details on analytical procedures, the reader is referred to Evans et al (2005) and Danišík et al (2008).…”

Tracing metamorphism, exhumation and topographic evolution in orogenic belts by multiple thermochronology: a case study from the Nízke Tatry Mts., Western Carpathians

“…Following He measurements, the apatite was spiked with 233 U and 230 Th, dissolved in nitric acid and analyzed by isotope dilution inductively coupled mass spectrometry for U, Th and in one batch also for Sm on a Perkin Elmer (ELAN DRC II) ICP-MS. Zircon was loaded in Nb tubes, degassed at *1,250°C and analyzed for 4 He using the same facility as for apatite. Degassed zircon was dissolved following the procedure of Evans et al (2005) and analyzed by isotope dilution inductively coupled mass spectrometry for U and Th on an Agilent 7500 ICP-MS. For more details on analytical procedures, the reader is referred to Evans et al (2005) and Danišík et al (2008c). b Apatite fission track length data.…”