U–Pb dating of calcite in ancient carbonates for age estimates of syn- to post-depositional processes: a case study from the upper Ediacaran strata of Finnmark, Arctic Norway

Abstract Results of in situ U–Pb dating of calcite spherulites, cone-in-cone (CIC) calcite and calcite fibres from a calcareous concretion of the upper Ediacaran of Finnmark, Arctic Norway, are reported. Calcite spherulites from the innermost layers of the concretion yielded a lower intercept age of 563 ± 70 Ma, which, although imprecise, is within uncertainty of the age of sedimentation based on fossil assemblages. Non-deformed CIC calcite from the bottom part of the concretion yielded an age of 475 ± 25 Ma, which is interpreted as the age of CIC calcite formation during a period of fluid overpressure induced during burial of the sediments. Deformed CIC calcite from the top part of the concretion yielded an age of 418 ± 23 Ma, which overlaps with a known Caledonian tectono-metamorphic event, and indicates a potential post-depositional overprint at this time. Calcite fibres that grew in small fissures along spherulite rims, which are interpreted as a recrystallization feature during deformation and formation of a cleavage, gave an imprecise age of 486 ± 161 Ma. Our results show that U–Pb dating of calcite can provide age constraints for ancient carbonates and syn- to post-depositional processes that operated during burial and metamorphic overprinting.


Introduction
Calcite U-Pb geochronology has attracted increasing interest in recent years within the Earth Sciences community. The method provides constraints on the ages of sediment deposition and diagenesis (e.g. Israelson et al. 1996;Rasbury & Cole, 2009;Hill et al. 2016;Godeau et al. 2018;Pisapia et al. 2018;Drost et al. 2019), fossils (e.g. Rasbury & Cole, 2009;Yokoyama et al. 2018;Drost et al. 2019) and mineralization along fracture and fault planes (e.g. Roberts & Walker, 2016;Goodfellow et al. 2017;Nuriel et al. 2017;Parrish et al. 2018;Holdsworth et al. 2019), among others. Regardless of the successful application of calcite U-Pb geochronology in recent years, the method has its challenges. Calcite is typically low in U and rich in initial Pb; it is also susceptible to alteration or recrystallization at low temperature in the presence of fluids, and allows Pb diffusion above moderate temperatures (Cherniak, 1997). Carbonate formation can be complex and long-lived (Rasbury & Cole, 2009); the question therefore arises as to which geological 'event' is actually being dated (Rasbury & Cole, 2009;Drost et al. 2019;Roberts et al. 2020). The in situ technique, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), helps overcome some of these challenges, namely by allowing for the measurement of discrete zones of uranium enrichment that are typical of diagenetic and hydrothermal calcite (Roberts et al. 2020), and by allowing a combination of U-Pb analysis with other in situ petrographic and analytical techniques.
In the present case study, we focus on calcite from carbonates of the upper Ediacaranlower Cambrian Manndrapselva Member of the Stáhpogieddi Formation (Vestertana Group, Gaissa Nappe Complex) of the Digermulen Peninsula in eastern Finnmark, Arctic Norway (Fig. 1). The study area has attracted renewed research interest because of new findings of Ediacaran-aged fossils (e.g. Högström et al. 2013;Jensen et al. 2018a, b). The upper Ediacaran succession comprises siliciclastic sedimentary rocks with recently described carbonates, some with calcite spherulites and cone-in-cone (CIC) calcite (Meinhold et al. 2019a). Although the age of the carbonate-bearing part of the succession is well established as late Ediacaran based on fossils, the timing of the formation of the various types of calcite is poorly constrained. We therefore applied in situ U-Pb dating of calcite using LA-ICP-MS to address this question. Having different types of calcite in close proximity to each other, and having approximate age constraints of sedimentation based on biostratigraphy (Högström et al. 2013;McIlroy & Brasier, 2017;Jensen et al. 2018a, b) and of the low-grade metamorphic overprint (see discussion in Meinhold et al. 2019b), allows for the testing of the applicability of U-Pb calcite geochronology on a thin-section scale, and whether ages this far back into deep time can be related to a geological 'event' in a meaningful manner.

Geological setting
The study area is located in eastern Finnmark, Arctic Norway, and is part of the Gaissa Nappe Complex (Fig. 1a). The Stáhpogieddi Formation of the Vestertana Group has received much attention in recent years as it contains the only Ediacara-type fossils in Scandinavia as well as its most complete Ediacaran-Cambrian transition (Farmer et al. 1992;Högström et al. 2013;Jensen et al. 2018a, b) (Fig. 1b). The Manndrapselva Member of the Stáhpogieddi Formation consists of a basal sandstone-dominated The carbonates crop out along a coastal section at the eastern part of the Digermulen Peninsula (geographic coordinates: 70°35' 31.0″ N, 28°11 0 30.3″ E) (Fig. 1c). They occur as beds, lenses and concretions. Some consist of calcite spherulites and CIC structures made of calcite (see Meinhold et al. 2019a for details) (Fig. 2a, b). The upper Ediacaran sedimentary succession was deposited along the western margin of Baltica (in present-day coordinates) in a marine basinal environment (Fig. 1d). The rocks were metamorphosed during the Scandinavian Caledonian orogeny (Meinhold et al. 2019b).

Methodology
Bedrock sample material was cut with a rock saw perpendicular to the bedding to obtain a rock slice for thick-section preparation and in situ U-Pb dating of calcite ( Fig. 2c-g). U-Pb geochronology of calcite was conducted at the Geochronology & Tracers Facility, British Geological Survey (Nottingham, UK), following the procedures described in Roberts & Walker (2016) and Roberts et al. (2017). The analyses were performed using a New Wave Research 193UC excimer laser ablation system, coupled to a Nu Instruments Attom single-collector sector-field ICP-MS. The method involves standard-sample bracketing with normalization to NIST 614 silicate glass (Woodhead & Hergt 2001) for Pb-Pb ratios and WC1 carbonate reference material (Roberts et al. 2017) for U-Pb ratios. The laser parameters comprised a 80 μm static spot, fired at 10 Hz, with a c. 3 J cm -2 fluence, for 20 s of ablation. The material was pre-ablated to clean the sample site with 150 μm spots for 2 s. Data are plotted on a Tera-Wasserburg concordia diagram ( 207 Pb/ 206 Pb versus 238 U/ 206 Pb). The ages are determined by linear regression between common and radiogenic lead compositions and as lower intercepts on a Tera-Wasserburg concordia using the Microsoft Excel add-in Isoplot 4.15 (Ludwig, 2012 (Horstwood et al. 2016). Full analytical data are provided in online Supplementary Table S1 and Figure S1 (available at http://journals.cambridge.org/geo).

Geochronological results
Four types of calcite from a carbonate concretion of the second cycle of the Manndrapselva Member of the Stáhpogieddi Formation were studied (Fig. 3). The domains include calcite spherulites, both undeformed and deformed CIC calcite, and calcite fibres grown in fissures along the spherulite rims ( Fig. 2c-g). The majority of the ablated spots yielded low U and Pb contents ranging from less than 0.1 to 2.1 ppm (average, 0.17 ppm; median, 0.06 ppm; n = 305) and  Meinhold et al. (2019a). Each data-point ellipse denotes Pb/U ratios with error in 2σ uncertainty including propagation of systematic uncertainties for each laser-ablation spot. The lower intercept of the regression line through the majority of data indicates the age of calcite crystallization.
over the range 0.47-20.6 ppm (average, 2.5 ppm; median, 1.5 ppm; n = 305), respectively. The analyses of all samples were dominated by common lead, with only a small abundance of radiogenic lead, leading to large uncertainties on the regressed ages. The proportion of radiogenic lead varied in each sample, with the undeformed CIC calcite yielding the greatest abundance of radiogenic lead and, subsequently, the most precise age (Fig. 3b).

Discussion
The calcite spherulites are interpreted as a primary feature, forming in the sedimentary environment (Meinhold et al. 2019a). Within the large uncertainty (563 ± 70 Ma), the obtained age overlaps with the estimated timing of sedimentation (late Ediacaran; c. 545 Ma) based on body and trace fossil assemblages (Högström et al. 2013;McIlroy & Brasier, 2017;Jensen et al. 2018b) (Fig. 1b).
U-Pb ages from the CIC calcite from the bottom and the top of the concretion are surprisingly different. Undeformed CIC calcite (bottom of concretion) gave an age of 475 ± 25 Ma, whereas deformed CIC calcite (top of concretion) is younger, that is, 418 ± 23 Ma. The age of the undeformed CIC calcite is interpreted as the age of CIC calcite formation during a period of fluid overpressure as the sediments were buried. The age fits well with age estimates based on the required overburden to obtain the fluid overpressure needed to form CIC structures (see discussion in Meinhold et al. 2019a). In the case of the undeformed CIC calcite, the robust isochron (in terms of MSWD) implies that the isotopic system has remained a closed system (no loss or gain of U or Pb). The Early-Middle Ordovician age is interpreted as being meaningful and representing an approximate age estimate of the CIC formation. On the contrary, the apparent Silurian-Devonian age of the deformed CIC calcite is within the age range of the post-depositional overprint related to a late Caledonian tectono-metamorphic event in the Gaissa Nappe Complex of the Caledonides of Finnmark (see discussion in Meinhold et al. 2019b). This age is also robust in terms of MSWD (0.67), but lacks any measurement with abundant radiogenic lead. At face value, the regressed ages of the deformed and undeformed CIC are different, implying that the deformation of the CIC structures in the top part of the concretion may have occurred during the formation of the cleavage, and providing implications for the resetting of the U-Pb system in calcite.
Based on the colour of organic-walled microfossils from the Manndrapselva Member of the Stáhpogieddi Formation, the sedimentary rocks show a post-mature level indicating a thermal overprint of 200-250°C (T. Palacios, unpublished data, 2019). The maximum metamorphic overprint is given as low epizonal and reached around 300°C (see Meinhold et al. 2019b). If we assume that the CIC calcite from the top part of the concretion was originally undeformed and formed contemporaneously with the CIC calcite from the bottom part of the concretion (see Meinhold et al. 2019a), the data suggest that the original age of CIC calcite from the top part of the concretion has been reset during the Caledonian metamorphic overprint. Volume diffusion of Pb at temperatures of 250-300°C is a possibility, based on the experimental study of Pb diffusion in calcite (Cherniak, 1997), but mobility may also have been enhanced by grain deformation. Fluid infiltration is another possibility for resetting of the U-Pb system, although it is more likely to mobilize uranium since U(VI), in the form of uranyl ion (UO 2 2þ ), is highly soluble in oxidized waters (Langmuir, 1978); however, it may be expected that fluid-assisted alteration would have obliterated or at least affected the calcite growth structures, and this is not the case. We emphasize that although the data imply a resetting of the U-Pb system in the deformed CIC calcite, the lack of measured radiogenic lead, and hence precise age constraints, leads us to apply caution to this interpretation. Nevertheless, the data provide compelling results that suggest the U-Pb calcite dating method has the potential for examining the timing of depositional, diagenetic and low-grade metamorphic events in sedimentary carbonates.
The calcite fibres grown in small fissures along the spherulite rims ( Fig. 2c) gave an age of 486 ± 161 Ma, which is within uncertainty of both the sedimentation age and the age of the metamorphic overprint and deformation. Formation of the calcite fibres is interpreted as being caused by dissolution and precipitation, during fracture and vein formation upon burial and compaction of the sediments. However, the uranium concentrations are extremely low, with the majority of analyses yielding < 0.0012 ppm, leading to a lack of measurable radiogenic lead and a large age uncertainty. We are therefore unable to refine the interpretation of the calcite fibres any further than that defined by the petrographic analysis (Meinhold et al. 2019a).

Conclusions
U-Pb calcite dates from an upper Ediacaran carbonate concretion provide timing constraints for depositional, diagenetic and potentially metamorphic processes, overlapping and confirming previous estimates based on relative bracketing of events. Our data show that LA-ICP-MS U-Pb dating of calcite can be a suitable complementary method to approximate the age of syn-to postdepositional processes that operate during burial and metamorphic overprinting, and can be applied to 'ancient' carbonates. Of note, Precambrian sedimentary sequences often lack tight age constraints, particularly in settings where more robust geochronometers such as zircon (requiring cross-cutting intrusive or interbedded extrusive rocks) may be absent.