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...
A computer algorithm is described which allows urine to be modelled as a saturated equilibrium solution with respect to any combination of the solids calcium oxalate, calcium hydrogen phosphate (brushite), amorphous calcium phosphate, uric acid, sodium hydrogen urate and ammonium hydrogen urate. It is demonstrated that this model of urine, unlike the widely accepted metastable supersaturated solution model, explains the long-known calcium salt crystalluria versus pH curves of both non-stone-forming and stone-forming urine. Further, the saturation model accounts for why most "infection" stones do not contain calcium oxalate and why most "urate" stones are composed solely of uric acid and not admixed with alkali metal hydrogen urate salts. The supersaturation model of urine cannot explain satisfactorily these well-known phenomena. For example, the supersaturation model predicts that virtually all "infection" stones should contain calcium oxalate along with calcium phosphate and, perhaps, struvite.
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