Tools for uranium characterization in carbonate samples: case studies of natural U–Pb geochronology reference materials
Geochronology(2021)
摘要
Abstract. Laser ablation U–Pb analyses of carbonate (LAcarb) samples has
greatly expanded the potential for U–Pb dating to a variety of carbonate-producing settings. Carbonates that were previously considered impossible to
date using isotope dilution methods may preserve radiogenic domains that can
be dated using spatially resolved laser ablation geochronology techniques.
Work is ongoing to identify reference materials and to consider best
practices for LAcarb. In this study we apply standard and emerging
characterization tool sets on three natural samples with the dual goal of enhancing the study of carbonates and establishing a new set of well-characterized natural reference materials for LAcarb studies. We start with
the existing carbonate reference material WC-1 from the Permian Reef Complex
of Texas, building on the published description to offer a deeper look at U
and associated trace elements. We consider a tufa sample from the Miocene
Barstow Formation of the Mojave Block, California, as a possible secondary
calcite reference material due to its well-behaved U–Pb systematics. There
are currently no natural dolomite standards. We present an unusual dolomite
sample with very well-behaved U–Pb systematics from the Miocene of the
Turkana Basin of Kenya as a possible dolomite reference material for LAcarb
dating. In addition to using X-ray fluorescence (XRF) mapping and spectroscopy to better
understand U in these natural samples, we have analyzed multiple aliquots of
each of them for 87 Sr / 86 Sr by thermal ionization mass spectrometry (TIMS). The Sr isotope compositions
are analytically homogeneous within petrographically homogeneous regions of
all three samples, and thus these materials could be used as Sr isotope standards
as well. While not part of the current contribution, this combination could
streamline simultaneous LA analyses of 87 Sr / 86 Sr and U–Pb
geochronology.
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关键词
uranium characterization,carbonate samples
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