Abstract
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 87Sr/86Sr 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 87Sr/86Sr and U–Pb geochronology.
Highlights
Carbonates are ubiquitous in the Earth’s crust and form from a variety of fluids, often with characteristic isotopic and element ratios (Fig. 1)
Field relationships and petrography allow us to consider the relative timing of carbonate precipitation and alteration, and the tools for demonstrating these relationships are an important first step to any study
X-ray fluorescence (XRF) mapping and laser ablation mapping allows us to establish domains within rocks that are equivalent in time and to work out details of the fluids that were responsible for carbonate precipitation
Summary
Carbonates are ubiquitous in the Earth’s crust and form from a variety of fluids, often with characteristic isotopic and element ratios (Fig. 1). This approach uses relative relationships of cements from the thin-section scale and correlation of these throughout a basin or region (Meyers, 1991, 1974) This field to petrographic approach has been used to select samples for geochemistry, providing fundamental insights into the fluid evolution of basins and degree of rock–water interaction (e.g., Banner and Hanson, 1990), as well as establishing trace and minor element incorporation trends (e.g., Mn/Fe) useful for tracking burial histories (Barnaby and Rimstidt, 1989). Chemical imaging techniques; carbonate staining; and polarized light, fluorescence, electron, and cathodoluminescence microscopy provide spatial context for quantitative microscale analyses of fluid inclusion, ion microprobe, electron microprobe, and laser ablation measurements Integrated data from such a toolkit enables collection of LA-U– Pb isochrons that reliably dates a single paragenetic event and permits geologically meaningful interpretation of U–Pb ages. These tools build on the more commonly used petrographic approaches, cathodoluminescence (CL) imaging, such as will be demonstrated in the three case studies presented here
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