Step-heating of 40Ar/39Ar standard mineral mixtures: Investigation of a fine-grained bulk sediment provenance tool

VanLanigham, S. and Mark, D.F. (2011) Step-heating of 40Ar/39Ar standard mineral mixtures: Investigation of a fine-grained bulk sediment provenance tool. Geochimica et Cosmochimica Acta, 75(9), pp. 2324-2335. (doi: 10.1016/j.gca.2011.01.038)

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Quantitative techniques that link sediments to their sources are needed to understand a range of tectonic, climate, and anthropogenic driven Earth surface processes. Many provenance techniques exist for sand-sized material but fewer are available for fine-grained sediment archives. In this respect, bulk <sup>40</sup>Ar/<sup>39</sup>Ar ages from silt-sized sediment show potential, but many questions remain about the significance of a bulk sediment <sup>40</sup>Ar/<sup>39</sup>Ar age. We interrogate bulk sediment <sup>40</sup>Ar/<sup>39</sup>Ar ages by step heating mixtures of well-constrained <sup>40</sup>Ar/<sup>39</sup>Ar mineral standards crushed to silt-sized. Silt-sized end member components Alder Creek Sanidine, Taylor Creek Sanidine and Heidelberg Biotite all yield plateau ages within uncertainty of their coarse-grained counterparts. High-resolution step heating (as many as 43 steps) of the mineral mixtures shows that biotite degasses first at lower temperatures compared to the two sanidines that degas generally in concert. Concordant age steps develop at both low and high temperatures and the transition from the isotope signal being dominated by one mineral phase to another is clearly observed. We show that age spectra for the mineral standard mixtures can be modeled and predicted for all mixtures by assuming a (simplistic) Gaussian distributed release of Ar, and by using the degassing maxima, variance, K concentration, and <sup>40</sup>Ar/<sup>39</sup>Ar age of each monitor mineral. Thus, bulk sediment <sup>40</sup>Ar/<sup>39</sup>Ar ages can be robust indicators of the average cooling/crystallization age of all contributing K-bearing minerals to a depositional center. Furthermore, we discuss the potential to deconvolve individual mineral age populations by model inversion. The application of this bulk sediment provenance technique should not be considered a replacement for single grain analyses. It should be applied to environments that do not provide sand-sized sediment archives (e.g., distal terrigenous sedimentary archives) when information about source changes through time are more important than precise sediment source identification.

Item Type:Articles
Glasgow Author(s) Enlighten ID:Mark, Professor Darren
Authors: VanLanigham, S., and Mark, D.F.
College/School:College of Science and Engineering > Scottish Universities Environmental Research Centre
Journal Name:Geochimica et Cosmochimica Acta
ISSN (Online):1872-9533
Published Online:02 February 2011

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