Step heating of Ar/Ar standard mineral mixtures: Investigation of a fine-grained bulk sediment provenance tool |
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| Authors: | Sam VanLaningham Darren F Mark |
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| Institution: | a University of Alaska Fairbanks, School of Fisheries and Ocean Sciences, Fairbanks, AK 99775, USA
b NERC Argon Isotope Facility, Scottish Universities Environmental Research Centre (SUERC), East Kilbride, G75 0QF Scotland, UK |
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| Abstract: | 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 40Ar/39Ar ages from silt-sized sediment show potential, but many questions remain about the significance of a bulk sediment 40Ar/39Ar age. We interrogate bulk sediment 40Ar/39Ar ages by step heating mixtures of well-constrained 40Ar/39Ar 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 40Ar/39Ar age of each monitor mineral. Thus, bulk sediment 40Ar/39Ar 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. |
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