A Sample Characterization Toolkit for Carbonate U-Pb
Geochronology

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 domains that are favorably interrogated when 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 toolsets on three natural samples with the dual goal of enhancing the study of carbonates and in establishing a new set of precisely characterized natural standards 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 fluids. 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 XRF mapping and spectroscopy to better understand U in these natural samples, we have analyzed multiple aliquots of each of them for 87Sr/86Sr. The Sr isotope compositions are reasonably homogeneous in all three samples, so that these could be used as Sr isotope standards as well. This combination could streamline split stream analyses of 87Sr/86Sr and U/Pb geochronology.


Synchrotron X-ray fluorescence (SXRF) imaging and X-ray absorption spectroscopy (XAS) techniques have the ability to provide information about both the distribution of major and trace elements, including U and Pb, in samples as well as their valence state and speciation. Using XAS methods it is possible to quantify U valence state at concentrations of a few ppm. Synchrotron X-ray beamlines are available through a competitive process of general user proposals (Sutton et al., 2002).
These brighter sources offer micron sized resolution for low ppm levels of the elements of interest. Emerging tender energy spectroscopy (TES) techniques allow mapping of elements such as Mg and S that are important in carbonates (Northrup, 2019). 95 In addition to element maps which show element distribution and the relationships among elements, spots can be chosen for spectroscopy, providing mineralogy (Mg, Ca) and redox (U, Fe, Mn etc).
Bench-top microscale XRF mapping (µXRF) of major, minor, and trace elements by energy-dispersive spectroscopy permits geochemical characterization of samples at the micron to decimeter scale. X-ray focusing optics, rather than collomation, allow high spatial resolutions (ca. 15-25 µm) with laboratory X-ray sources. µXRF has been used for chemical imagining in diverse 100 fields, including materials characterization, archaeology, and earth sciences (de Winter et al., 2017;Allwood et al., 2018;Katsuta et al., 2019;Haschke , last;de Winter and Claeys, 2017), and related instruments will be deployed on interplanetary spacecraft (Williford et al., 2018). Qualitative and semi-quantitative imaging and standardless analysis is rapid and versatile, but quantitative composition determination of carbonate rocks is challenging due to the X-ray-transparent carbonate matrix (de Winter et al., 2017). As shown here, in some samples with high U concentrations, U abundance can be mapped with 105 appropriate analytical conditions, such as incident radiation filtering, multiple beam dwell passes, acquisition under vacuum, and preparation of thick samples that permit excitation of all available U atoms through the sample depth. Data presented here was acquired using an M4 Tornado (Bruker) operating with a 30 W Rh x-ray tube excited at 50 kV and focused to a 17 µm spot (1 σ of incident energy at Mo-Kσ) on flat, polished, thick (> 2 mm) samples under 20 mbar vacuum. Fluorescence energy was detected with two 60 mm 2 XFlash silicon drift detectors (Bruker), energy spectra were deconvolved with Bruker software, and 110 total counts in each emission line region were exported for plotting and visualization using MATLAB (Mathworks).

Case Studies
We use three natural carbonates that have potential as LAcarb standards to illustrate imaging and spectroscopy techniques. We also discuss what is known of the geology and geochemistry to provide context for other samples that might be dated.

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Roberts et al. (2017) offered WC-1 as a primary standard for LAcarb. This late Permian marine cement sample is reasonably homogeneous with high enough U/Pb and radiogenic enough Pb isotopes to make it a suitable reference material. A brief description of the field relationships and U-Pb characterization were presented in Roberts et al. (2017), but here we expand on this to offer additional insight into the standard as well as to highlight characterization techniques that are relevant to any carbonate study. Similar buildups and cements are described from important oil plays in Kazakhstan (Dickson and Kenter, 120 this type offer good potential for giving ages of penecontemporaneous marine diagenesis. The Permian Reef Complex, spectacularly exposed in the Guadalupe Mountains National Park of West Texas, stands in relief primarily due to the type of cements that make up WC-1. Neptunian dikes are found parallel to the reef track, filling fractures that resulted from tensional stresses as the reef built out into the Permian Basin (Hunt et al., 2003;Budd et al., 2013;125 Frost et al., 2013). Early marine cements in the reef and within these Neptunian dikes are primarily botryoidal aragonites and are typically a dark brown color. The dark brown color is from organic matter that is occluded in the carbonate such that when broken or sawn these cements smell like hydrocarbons (anecdotally this strong smell is a good criterion for selecting carbonates for dating). The cements have mostly been altered to calcite, though Chafetz et al. (2008) found original aragonite in similar cements. Calcite with the greatest alteration is light brown and has less favorable U/Pb (Jones et al., 1995). Petrograpically these 130 cements preserve details of the original fibrous aragonite as inclusion trails. WC-1 (Walnut Canyon) comes from a Neptunian dike in the Tansil Member equivalent of the of the Permian Reef Complex. This Neptunian dike is exposed in Walnut Canyon just outside the Guadalupe Mountains National Park entrance. The botryoids that fill the Neptunian dike were made of radiating aragonite needles that bundle into cm scale packages. The botryoids grow atop one another from both sides of the dike ( Fig.   2). At the hand specimen scale, cross-cutting white veins are obvious ( (Fig 3). Other veins are smaller but can be detected with 135 petrography and element mapping (Fig. 4). The Permian Reef Complex has seen many episodes of diagenetic fluids, including meteoric fluids accompanying the Basin and Range extension that exhumed the reef in the Neogene (Bishop et al., 2014;Loyd et al., 2013;Scholle et al., 1992;Budd et al., 2013).
Cathodoluminescence (CL) has been a go-to test for diagenesis, and when an activator such as Mn is present, this often gives phenomenal images that illuminate alteration. However, it is also understood that Fe quenches luminescence, so the Mn/Fe ratio 140 matters for understanding what luminescence or non-luminescence means (Barnaby and Rimstidt, 1989). In the case of Walnut Canyon, the well preserved botryoids are non-luminescent and cements that line the botryoids are brightly luminescent (Fig.   4). Even through the botryoids are non-luminescent with excellent textural preservation, the wide range of Sr concentrations across the botryoids shows that they have been variably altered (Fig.4, Fig. 5, Fig. 6). Element imaging is complimentary to petrography, and while petrographic techniques like CL can provide qualitative information on Mn and Fe, XRF and LA 145 imaging techniques give quantitative information on the Fe/Mn, Sr/Ca and other ratios and element concentrations, providing far more insight into the carbonate diagenesis. µXRF scanning is reasonably accessible, and we suggest that depending on the original mineralogy, elements like Sr (Fig. 4, Fig. 5) could be mapped and registered so that only the best preserved parts of a rock slab could be targeted for a LAcarb standard. This should greatly reduce the scatter that is currently seen in the WC-1 standard.

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The Walnut Canyon sample has incredible preservation of the original fibrous aragonite texture even though it is almost entirely converted to calcite. This neomorphic replacement may facilitate retention of U (Kelly et al., 2003), or perhaps the organic matter (that gives the sample its color and pungent smell when broken) complexed U and retained it in the calcite.
Remarkably, Sr is lost well before U in this system as evidenced by the rather homogeneous U concentration at about 4.5 ppm (which makes this a good U/Pb standard) and the highly variable Sr (Fig. 4). Isotope dilution of this sample doesn't give much 155 5 https://doi.org/10.5194/gchron-2020-20 Preprint. Discussion started: 3 July 2020 c Author(s) 2020. CC BY 4.0 License. of a spread in U/Pb, so to maximize the spread a range of aliquots from dark brown to light brown were utilized (Roberts et al., 2017). Like the result reported in Jones et al. (1995), this produced an age that is nominally younger than the age of the reef based on dating of ash beds (Wu et al., 2020) (254 instead of 257 Ma) , but not outside the uncertainty. The cements postdate reef cementation because they fill fractures, but contain internal sediments and likely cemented penecontemporaneously with reef deposition. The 87 Sr/ 86 Sr ratios range from 0.7069-0.7072 (Fig. 7), with an average of 0.706930(69) for samples that High resolution, on the fly element mapping in the tender energy range (TES) allows us to examine U, Mg, Sr and S in the Walnut Canyon sample. Comparing U, S, and Sr in a RGB map (Fig. 8) illustrates the spotty retention of Sr and S relative to U.
As seen in the µXRF images (Fig. 5), Mg is elevated in veins and is thus introduced by later fluids. U(M5) XANES shows that the botryoidal cements have entirely oxidized U (Fig. 9). Uranium concentrations in this samples are less than 5 ppm, and the 175 ability to explore U oxidation states at this concentration is a major advance. An important factor to be considered for exploring for favorable U/Pb in carbonates is how U is incorporated. In the Walnut Canyon case we hypothesize the uranyl was brought to the site of precipitation by an oxidizing fluid (seawater) and structurally incorporated in aragonite which does not exclude U (Reeder et al., 2001;Kelly et al., 2003). Through neomorphism to calcite, U was left behind because active functional groups such as carboxyl have a high affinity for uranyl. Perhaps calcite grew around these complexed U ions preserving near original 180 U concentrations. In contrast, Sr, which is also strongly partitioned into aragonite but has a lower affinity for calcite, was lost to the fluid because it does not have an affinity for organic matter.

Middle Miocene Tufa-Barstow Formation
The Middle Miocene Barstow Formation crops out in the Rainbow Basin near Barstow California. Tufa mounds are found near the top of the Owl Conglomerate (Cole et al., 2004(Cole et al., , 2005 and are assumed to have formed where springs entered a lake similar 185 to Mono and Pyramid Lake tufa towers. These tufa deposits have more than 100 ppm U, several ppm Pb and favorable U/Pb systematics (Cole et al., 2005). Here we build on the work of Cole et al. (2004Cole et al. ( , 2005 with additional synchrotron imaging, µXRF imaging and spectroscopy techniques. The sample that we are using to illustrate these tools for characterization is large enough to be suitable for distribution as a secondary standard for LA carbonate dating (Fig. 10). Most of the samples used in the (Cole et al., 2005) study were small slabs from Vicki Pedone (Cal State Northridge) are too small to to distribute beyond our lab, so we are characterizing a new sample that is large enough to distribute widely. The tufa samples ranged in age from 14.8-17 Ma (Cole et al., 2005). Isotope dilution on this sample is under way and will be completed when Covid quarantine is lifted. However, based on the published ages the sample is approximately 15 Ma, and has 238 U/ 206 Pb between 150-450. Becker et al. (2001) showed that layers in the Barstow tufa deposits have a pattern of low to high U concentrations across laminae revealed by fission track mapping. Uranium is lower in the beginning of a laminae where larger crystals of calcite 195 formed, and is higher in micritic calcite which forms the caps of laminae. Becker et al. (2001) reasoned that the laminae reflect seasonal increases in fresh water supply which delivers Ca to a body of water with high alkalinity similar to the Great Basin Lakes. Phosphor imaging shows that in addition to these fine scale trends observed with fission tracks, there are mm to cm scale layers of higher and lower U concentrations (Cole et al., 2004).
Synchrotron and µXRF imaging of Sr and Mn faithfully matches the primary layering, demonstrating that later alteration 200 has been minor (Fig. 11, Fig. 12). A comparison of U-L α maps with and without an incident radiation filter shows that the filter eliminates artifacts that result from scatter in pores, and returns an accurate map of the U (Fig. 13). The filtered U µXRF map shows that U is elevated in the micritic areas of the tufa. µXRF maps of Al and Si show that large pores that are typical of the tufa deposits have clay minerals lining them (Fig. 12. Fe, Cu and Zn are also elevated in the pores. A map of Ba closely matches the S map suggesting that barite may be present. Strontianite is found in economic abundance in the Barstow 205 Formation (Knopf, 1917). The extremely high Sr concentrations in the tufa calcite (10's of thousands of ppm), suggests these fluids were contributing Sr during deposition. The 87 Sr/ 86 Sr isotope ratios are similar throughout the sample, with 3 aliquots ranging from 0.719877 to 0.721038 (Table 1). Combining the criteria for selection of aliquots for U/Pb, with high Sr content, is likely to greatly narrow this range, making this a good Sr isotope standard as well as a secondary U/Pb carbonate standard.
The U in these tufas is in the reduced state U(IV) as shown by measurements at the L3 edge (Cole et al., 2004), and new data 210 from the M5 edge (Fig. 14). With the high U concentration, the Barstow tufa could be a standard for U XANES analyses. The Barstow tufa calcite is luminescent throughout (Fig. 15), consistent with the occurrence of reduced U in this sample. While it is easy to conceive of a stratified lake with reducing bottom waters, reduced U is insoluble in most solutions, begging the question of how it is available to go into the calcite. We hypothesize that U(IV) is complexed with some oxyanion in the lake water (phosphate, bicarbonate, etc) that keeps it in solution and perhaps is also incorporated into the calcite lattice. Elevated 215 actinide concentrations are found in the Great Basin lakes, and it is thought that the carbonate alkalinity is responsible for this elevation (Anderson et al., 1982;Simpson et al., 1982). The oxidation state(s) of U in the Mono Lake carbonates is not known, but we imagine waters with similar chemistry might be responsible for elevated U in the Barstow tufa. While we do not have temperatures of formation, the morphology of the tufa is similar to that at Pyramid Lake where warm springs enter the lake (Cole et al., 2004). Perhaps thermal fluids are important for U(IV) mobility.

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One of the biggest advances in LAcarb dating is a contribution by Drost et al. (2018) which used laser ablation imaging and inspection for concentrations or ratios that reflect something that could be considered to be related (like Sr concentrations).
Pooling of pixels based on this criteria from across the mapped region and binning based on some additional criteria like 235 U/ 207 Pb gives isochron plots with range of values that is typically greater than, and certainly more filled in than, would be     Figure 14. Uranium M5 edge spectroscopy showing that uranium is in the reduced state; for reference, the U(VI) speleothem of (Kelly et al., 2003). Both measured at the TES beamline as described in Figure 9.