Microscale Petrographic, Trace Element, and Isotopic Constraints on Glauconite Diagenesis in Altered Sedimentary Sequences: Implications for Glauconite Geochronology

Glauconite is an authigenic clay mineral that is common in marine sedimentary successions. Dating of glauconite to determine the depositional age of sedimentary sequences has a long history but has fallen into disfavor due to the difficulty of obtaining “pure” glauconite separates. Recent advances in sedimentary petrography and reaction cell mass spectrometry permit rapid in situ Rb‐Sr dating of carefully screened glauconite grains. However, glauconite remains susceptible to burial alteration so that successful application of in situ Rb‐Sr glauconite geochronology requires improved, microscale constraints on the impact of postdepositional alteration on glauconite Rb‐Sr systematics and articulation of robust criteria for identifying grains suitable for geochronology. Here, we address these questions by combining SEM‐EDS mineral mapping, geochemical characterization, and in situ Rb‐Sr dating of glauconite grains in partially altered lower Cambrian sedimentary sequences from the Arrowie and Amadeus basins in Australia. Our approach provides information at high spatial resolution, representing new insights into the interplay between source material, burial fluids, and diagenetic processes. Among the different glauconite classes, which we classify based on alteration and inclusion type, only the primary apatite‐bearing “pristine” glauconite returns an age within the error of the expected stratigraphic age. We attribute the preservation of a depositional Rb‐Sr age to the influence of Sr‐rich, alteration‐resistant apatite and the limited permeability of the clay‐rich strata hosting these grains. We conclude that our combined petrographic–geochemical screening approach holds considerable potential for identifying the best preserved glauconite grains for in situ Rb‐Sr geochronology.

Glauconite geochronology is a potential alternative where ash beds or black shales are absent (Bansal et al., 2019;P. E. Smith et al., 1998). Glauconite is an authigenic, green clay mineral that commonly forms mm-scale pellets and occurs in both siliciclastic and carbonate sequences in a wide range of marine and also continental depositional environments spanning the Precambrian to recent (Banerjee et al., 2016). Glauconite forms at or near the sediment-water interface, and is thought to evolve from K-poor but Fe 3+ -rich smectite (Charpentier et al., 2011;Gaudin et al., 2005) to mature K-and Fe 3+ -rich glauconite over a time-frame of 10 3 -10 6 years (Baldermann et al., 2013;López-Quirós et al., 2019. As glauconite pellets mature, they grow and equilibrate isotopically with seawater or seawater-derived fluids, facilitating their physical and chemical separation from the ambient sediment matrix, which in turn permits dating of marine sedimentary successions via glauconite geochronology (Clauer et al., 1992).
In situ Rb-Sr dating offers a promising new approach to glauconite geochronology. Unlike traditional thermal ionization mass spectrometric Rb-Sr dating which requires the dissolution of bulk sample powders followed by wet chemical separation of Rb and Sr (e.g., Gopalan, 2008), reaction cell mass spectrometry (ICP-MS/MS) resolves the spectral overlap of 87 Rb and 87 Sr via the addition of a reaction gas (e.g., N 2 O) to a reaction cell (Gorojovsky & Alard, 2020;Hogmalm et al., 2017;Redaa et al., 2021;Tillberg et al., 2017Tillberg et al., , 2020Zack & Hogmalm, 2016). Rb is measured on-mass while Sr isotopes are measured as mass-shifted oxides, allowing quantitative on-line separation of 87 Rb and 87 Sr. When combined with recent advances in sedimentary petrography (e.g., Han et al., 2022;Rafiei & Kennedy, 2019;Rafiei et al., 2020) this approach permits rapid in situ Rb-Sr dating of carefully screened glauconite grains (Farkas et al., 2018), potentially resolving the limitations of traditional glauconite geochronology and opening up a range of new applications (Scheiblhofer et al., 2022). However, despite the promise of this new approach, glauconite remains susceptible to burial alteration (Bansal et al., 2020;Guimaraes et al., 2000). Successful application of in situ Rb-Sr glauconite geochronology therefore requires improved, microscale constraints on the impact of postdepositional alteration on glauconite Rb-Sr systematics, as well as articulation of robust criteria for identifying grains suitable for geochronology applications.
Here we address this research gap by studying glauconite-bearing, partially altered marine sedimentary sequences of the lower Cambrian age of the Arrowie Basin (South Australia) and the Amadeus Basin (Central Australia). We combine detailed petrographic characterization of glauconite grains by SEM-EDS mineral mapping (Rafiei et al., 2020), in situ laser ablation Rb/Sr dating (Gorojovsky & Alard, 2020;Redaa et al., 2021Redaa et al., , 2022Zack & Hogmalm, 2016) and simultaneous trace element geochemical fingerprinting. The aim of this study is to assess the microscale impact of diagenesis on glauconite embedded in partially altered sedimentary sequences and implications for glauconite geochronology. Our microscale petrographic characterization approach promises to (a) reveal new insights into impact and mechanisms of glauconite diagenesis/alteration and (b) theoretically offers the choice to analyze the least altered glauconite grains. This novel approach yields new insights into the mechanisms and impact of postdepositional alteration on glauconite Rb-Sr systematics and Rb-Sr geochronology. We further consider whether targeting of the best-preserved glauconite grains in partially altered sedimentary sequences can produce stratigraphically meaningful dates, and the circumstances under which ages obtained on altered grains may record a postdepositional "diagenetic event." glauconite-rich intervals, preserving a record of one of the earliest complex animal ecosystems on Earth. Glauconites in these calcareous to mixed calcareous/siliciclastic rocks show a range of alteration states, as identified via variations in grain morphology and composition, with both heavily altered and more pristine grains preserved. Their age and postdepositional history make these intervals suitable analogs for common glauconite-bearing Proterozoic sequences, but with the advantage of precise chronostratigraphic constraints from bio-and chemostratigraphy (Betts et al., 2018), allowing us to assess the geological significance of in situ Rb-Sr ages.

Ajax Limestone
Nine Ajax Limestone samples were collected from the AJX-M section (Gravestock, 1984), Mount Scott Range, Arrowie Basin, South Australia ( Figure 1). Two samples were selected for further study following initial petrographic screening (Ajax_332 and Ajax_356, positions marked on Figure 1). The lower Cambrian Ajax Limestone is part of the Hawker Group, which is predominantly comprised of carbonate successions interbedded with minor siliciclastic intervals. A range of depositional environments from shallow marine to carbonate platform, ramp, and slope have been suggested for the lower Cambrian successions of the Arrowie Basin (Jago et al., 2006), with the carbonate strata interpreted to be shallow water successions (James & Gravestock, 1990).
Although the deposition of the Arrowie sedimentary sequences was strongly affected by regional tectonic activity (Petermann Orogeny with a peak at ca. 550 Ma; Delamerian orogeny at ∼500 Ma) (Foden et al., 2006;Jago et al., 2006), petroleum systems maturation modeling indicates that the Cambrian Hawker Group succession is of relatively low-grade thermal maturity and immature for oil generation (vitrinite reflectance between 0.25 and 0.55%R 0 ) (Carr et al., 2012).
Geochronological data from tuff and shale intervals of the lower Cambrian successions in South Australia (for details see Betts et al. (2018)) and their bio/chemostratigraphic correlation with West Gondwana, Southern Britain, and South China indicate that the Ajax Limestone was deposited around ∼519 to ∼523 Ma. The two samples selected for further study have expected stratigraphic ages of ∼520 Ma, assuming a linear sedimentation rate with no major hiatuses.

Tempe Formation
Tempe Formation samples were obtained from the Hermannsburg_41 core ( Figure 1). After detailed petrographic examination of six samples, two were selected for further analysis (H41_307 and H41_320, positions marked on Figure 1). The H41 core was drilled in the Gardiner Range in the northern part of the Amadeus Basin (Central Australia). The Tempe Formation, which is part of the Pertaoorrta Group, was deposited in the intracratonic Amadeus Basin and is mainly comprised of clastic mud-and siltstones with interbedded limestones and cross-bedded glauconitic sandstones (Figure 1; P. M. Smith et al., 2015). Sedimentological examination of the Tempe Formation identified deposition in a range of sedimentary environments spanning shallow marine (lagoon, tidal flat and upper shoreface) to deeper marine (lower shoreface and offshore) settings (Bradshaw, 1991). Based on stratigraphic position and lithofacies, the two samples studied here are associated with the offshore deposits of the Tempe Formation.
The Tempe Formation was deposited during the Australian Ordian Stage, corresponding to Cambrian Series two, Stage four (P. M. Smith et al., 2015) spanning ∼514-509 Ma (Peng et al., 2020). An integrated geochemical, mineralogical and sequence stratigraphic study of the Tempe Formation and its correlation with sedimentary sequences across the Centralian Superbasin suggested a maximum age of 511 Ma due to the lack of sedimentation prior to this age (Schmid, 2017). Thermal maturity analyses on the organic-rich intervals of the Tempe Formation in the oil and gas fields across the northern parts of the Amadeus Basin revealed mature source rock units between oil and gas window (Jarrett et al., 2016). Several tectonic and orogenic events in this region, for example, the final stages of Peterman Orogeny, Larapinta Event (∼480-460 Ma; Buick et al., 2001;Maidment et al., 2006) and Alice Spring Orogeny (∼400-300 Ma; Shaw et al., 1984) have affected to variable degree the preservation and postdepositional history of the Tempe Formation.

SEM Imaging and Mineral Mapping
Samples from glauconite-rich intervals selected for petrographic examination were mounted in epoxy resin (30 mm diameter mounts), diamond polished and carbon coated. For imaging, we utilized a FEI Teneo LoVac field emission scanning electron microscope (SEM) equipped with dual Bruker XFlash Series 6 energy dispersive X-ray spectroscopy (EDS) detectors. The entire mount was scanned to obtain a backscattered electron (BSE) image tileset allowing preliminary sample screening (10 mm working distance, 15 kV accelerating voltage, 100 nm pixel resolution). Subsequently, regions of interest (10-80 in total) of potentially pristine or altered glauconites in each sample were selected for high-resolution BSE imaging (10 nm pixel resolution) and mineral mapping (EDS spectra: 1.5 μm step size, 8 ms acquisition time). Quantitative mineral mapping was carried out using FEI Maps Mineralogy software for automated data collection (both BSE imaging and EDS), whereas the FEI Nanomin software was utilized for the classification of the individual EDS spectra to identify minerals and calculate mineral abundances (for details see Abbott et al., 2019;Frank et al., 2020;Han et al., 2022;Rafiei et al., 2020).

Electron Probe Microanalysis (EPMA)
The chemical composition (Na, Mg, Al, Si, P, K, Ca, Ti, Cr, Mn, Fe, and Ni) of a number of grains for each alteration class was determined using a Cameca SX 100 Electron Microprobe at Macquarie University GeoAnalytical (MQGA) facilities, permitting assessment of glauconite maturity and validation of the petrographic classification of glauconite grain alteration. The operating conditions were 15 kV acceleration voltage with probe current of 20 nA and beam diameter of 1 μm (peak: 30 s and background counting: 15 s). Spots targeted for analysis on representative grains of each class were identified on the basis of BSE images and mineral maps. Glauconite maturity was primarily assessed via the K 2 O content (nascent: 1.7%-3.3%; slightly evolved: 3.3%-5%; evolved: 5%-6.6%; highly evolved: >8%; Odin & Matter, 1981).

In Situ Rb/Sr Analysis
After petrographic classification of glauconites (see Section 3.1), we systematically targeted each class as well as cooccurring bioapatites, calcite, and dolomite. In situ Rb/Sr dating of selected glauconites was conducted over the course of five analytical sessions using an Agilent 8900 QQQ ICP-MS/MS coupled to a Teledyne Cetac G2-193 nm laser platform at Macquarie GeoAnalytical laboratories. In addition to Rb and Sr, the concentration of selected trace and major elements (  Yb 16 O, and 175 Lu 16 O) was simultaneously measured to search for any inclusions or alterations of glauconite grains. The operating parameters were 85 μm laser spot size, 2.5-5.5 J cm −2 fluency, and 5-10 Hz pulse repetition rate. Average depths for each spot analysis are approximately 25 μm in which, it is possible that the ablated volume contains microscale inclusions not identified petrographically on the exposed polished surface. A detailed description of instrument setup, analytical conditions and data reduction is provided in Gorojovsky and Alard (2020) and Table S1 in Supporting Information S1. Reference materials including the glass standard NIST SRM 610, the CRPG Mica-Mg nanopowder  and the USGS BHVO-2G were utilized as external standards to bracket between every 6-8 sample measurements, calibrate the data and monitor signal drift. To validate data accuracy in each analytical session we also analyzed a phlogopite megacryst (MSN), an igneous phlogopite mineral (MDC; see Redaa et al., 2021) and a GL-O grain mount (Derkowski et al., 2009) of known age (see Supporting Information S1). Data reduction was achieved through an in-house spreadsheet. Consistent with Gorojovsky and Alard's (2020) and Redaa et al. (2022) results, normalization to Mica-Mg and NIST 610 returned the most accurate and reproducible results for 87 Rb/ 86 Sr and 87 Sr/ 86 Sr of the standards, respectively, and is the preferred approach used throughout this study. Errorchons were constructed using the IsoplotR package (Vermeesch, 2018), utilizing the 87 Rb decay constant reported by Villa et al. (2015), λ Rb = 1.3972 ± 0.0045 × 10 −11 a −1 , the maximum likelihood model of York et al. (2004), and a 95% confidence limit. Analysis of the GL-O grain mount returned an age of 99.6 ± 9.6 Ma ( Figure  S1 in Supporting Information S1), which is in agreement with the expected stratigraphic age (99.6 Ma; Selby, 2009).
Trace element concentrations were obtained using the GLITTER software package (Griffin et al., 2008). To this end, average SiO 2 and CaO concentrations obtained by EMPA analysis were used as internal standard for glauconite versus bioapatite, calcite and dolomite, and NIST 610 (Jochum et al., 2011) was used as an external standard. Rb-poor bioapatites and calcite phases that were, petrographically, devoid of impurities were targeted to constrain the initial (seawater) 87 Sr/ 87 Sr. The majority of the spots from these phases revealed values close to expected Cambrian seawater (∼0.7091 for ∼520 Ma and 0.7090 for ∼509 Ma; Denison et al., 1998;Peng et al., 2020).
Samples collected from the Tempe Formation contain glauconite grains of various morphologies occurring in a carbonate or mixed carbonate to clastic matrix. Samples selected for detailed characterization host a combination of oval-to pellet-shaped glauconite grains, some of which are illitised ( Figure 2d) and/or contain a range of mineral inclusions such as dolomite, apatite and pyrite. The presence of abraded glauconite grains in glauconitic sandstone ( Figure 2e; sample H41_307) is indicative of transportation and intraformational reworking, consistent with cross-bedding of glauconitic sandstone intervals identified by P. M. Smith et al. (2015). Quantitative mineral mapping shows that sample H41_307 is comprised of glauconite (33 wt%, 200-1,500 μm), dolomite (29 wt%), quartz (13 wt%, 30-200 μm), feldspars (10 wt%, 10-200 μm), illite (5 wt%-<10 μm), apatite (5 wt%, micron-sized inclusions to bioclasts of few hundred microns size) chlorite (4 wt%, 10-200 μm), with trace amounts of pyrite and iron oxides. Quartz, feldspar, chlorite, some illite and dolomite grains show abraded and broken margins indicative of a detrital origin, however, some authigenic dolomite and illite grains are present as inter/intragranular pore fill (cement) (Figure 2e). Sample H41_320 is a dolomite-cemented glauconitic sandstone. It shows similar proportions of quartz, glauconite grains and feldspars; however, glauconite grains are homogenously illitised, apatite fossil remains are more abundant (8 wt%), and the intergranular space is entirely occupied by dolomite cement with no fine-grained matrix present ( Figure 2d). A further difference to H41_307 is the irregular shape/boundary of glauconite grains in H41_320, which appear to be impacted by the sharp faces of the dolomite crystals comprising the cement, suggesting that glauconite may have formed prior to dolomitization.

Glauconite Classification
Petrographic (Figures 3 and 4) and major element compositional data ( Figure S2 in Supporting Information S1) distinguishes seven classes of glauconite based on the presence of inclusions (primarily apatite and calcite) or the mode and extent of alteration (primarily illitisation). In addition to the above, trace impurities such as iron oxide, pyrite and dolomite were also observed but not considered for petrographic classification purposes as they were not abundant and, apart from dolomite, are not expected to impact on Rb/Sr systematics.

Pristine Glauconite (GL pristine )
Pristine grains (or pristine regions within partially altered grains), comprised of unaltered glauconite without detectable mineral inclusions (Figure 3a), were identified in three samples (Ajax_332, Ajax_356, and H41_307) and are here termed GL pristine . Pristine grains are mostly oval and well rounded (in the shape of fecal pellets) with smooth surfaces. They display a tightly packed ropy fabric, corresponding to mature glauconite (López-Quirós et al., 2019). Pristine grains are only rarely associated with fossil clasts, but embayment or cracks are filled by illite ( Figure 3a). GL pristine in all samples studied here is compositionally mature/highly evolved (∼10 wt% K 2 O), with elevated Al 2 O 3 contents (10-13 wt%) typical of Precambrian and Paleozoic glauconites (Banerjee et al., 2016). GL Pristine in both Ajax samples are similar in composition with average SiO 2 content of 49.46 ± 0.95 and 50.91 ± 0.98 wt%, MgO content of 3.38 ± 0.13 and 3.46 ± 0.07 wt% for samples 332 and 356, respectively. GL Pristine in H41_307 show a more homogeneous but distinctive compositional range compared to Ajax GL pristine . They are notably more enriched in SiO 2 (53.41 ± 0.64 wt%), MgO (4.14 ± 0.08 wt%) and Fe 2 O 3 , but relatively depleted in Al 2 O 3 (10.13 ± 0.47 wt%) and K 2 O (9.72 ± 0.10 wt%).

Glauconite With Apatite Inclusions (GL apatite )
The GL apatite classification refers to otherwise pristine glauconite grains containing apatite inclusions ( Figure 3b) and was only identified in sample H41_307. Accordingly, GL apatite has a similar chemical composition to GL Pristine in H41_307. Apatite inclusions are mostly of submicron size and spread sparsely through the grains. Given the commonly observed intergrowth of glauconite and apatite in recent to modern marine sediments (O'brien et al., 1990;Stille & Clauer, 1994;Tóth et al., 2010), and the common occurrence of glauconite in apatite bioclast cavities, we interpret apatite inclusions as cogenetic with glauconite.

Glauconite With Patchy Illite Alteration (GL illitisedP )
Glauconite grains hosting illite in pore spaces within their flaky and loosely packed fabric or in the elongate pores/cracks located between bundles of tightly packed bundles of ropy glauconite in otherwise pristine grains (Figure 3d), termed GL illitisedP (illitised patchy), were identified in both Ajax samples as well as in H41_307. Illite, occurring as patchy pore fill exhibits acicular and intergrown habits with pristine glauconite bundles, is interpreted to have formed via coupled dissolution-precipitation from glauconite or to have precipitated from pore fluids passing through the porous, loosely packed glauconite fabric (Lanson et al., 2002;Pevear, 1999). EMP analysis shows that GL illitisedP has a wider compositional range than GL Pristine and GL apatite , reflecting highly localized alteration of the glauconite by diagenetic illite. Illite altered grains are relatively Fe 2 O 3 and MgO depleted but K 2 O and Al 2 O 3 enriched (median concentrations) compared to pristine grains ( Figure S2 in Supporting Information S1). We attribute individual analyses showing Fe 2 O 3 concentrations greater than observed in pristine samples, together with low concentrations in other major elements ( Figure S2 in Supporting Information S1), to the presence of nm-scale iron oxide inclusions, as also evident in BSE images (Figure 4b).

Glauconite With Homogenous Illite Alteration (GL illitisedH )
Alteration of glauconite grains resulting in homogenous (as opposed to patchy or localized) replacement of glauconite by illite is here termed GL illitisedH , and was identified only in samples Ajax_356 and H41_320. Our petrographic observations suggest that although the initial shape of the pristine glauconite grain is preserved in this class, their original platy, loosely packed and more porous fabric allowed a homogenous alteration to proceed (Figure 3c). Fe 2 O 3 and K 2 O are depleted whilst Al 2 O 3 and MgO are increased and SiO 2 remains unchanged in this class compared to GL pristine , GL apatite , and GL illitisedP .

Illitised Glauconite With Calcite Inclusion (GL I+C )
GL illitisedP grains hosting calcite inclusions are here classified as GL I+C . This class was only identified in Ajax samples, which are limestone hosted (Figure 3f). Calcite inclusions are ∼5-10 microns in size and are only identified in illitised regions of GL illitisedP grains, suggesting that calcite inclusions formed contemporaneously with illitisation. Where calcite inclusions are present, we find decreased concentrations of major glauconite and illite elements (K 2 O, Al 2 O 3 , Fe 2 O 3 , etc.) with corresponding increases in CaO. Apart from this, no compositional differences to GL illitisedP are recognized. We distinguish this class from GL illitisedP because calcite inclusions are expected to influence Sr systematics due to the relatively higher content of Sr in carbonates.

Illitised Glauconite With Apatite Inclusion (GL I+A )
Where GL illitisedP or GL illitisedH (only in sample H41_320) grains also contain apatite inclusions, we classified them as GL I+A (Figure 3e). This is the only class that is identified in all four samples.

Illitised Glauconite With Calcite and Apatite Inclusions (GL I+A+C )
Patchily illitised grains containing both primary (apatite) and secondary (calcite) inclusions are classed as GL I+A_C (Figure 3g), and were only observed in sample Ajax_332.  (Wiman, 1903); (d) Common occurrence of iron oxides between glauconite plates and throughout the limestone matrix in Ajax samples marked by back arrows (sample Ajax_332).

87 Sr/ 86 Sr, Trace, and Major Elements
Up to 12 representative spots per sample for each petrographically defined glauconite class were analyzed by LA-ICP-MS/MS to determine 87 Rb/ 86 Sr, 87 Sr/ 86 Sr as well as selected major and trace element abundances. The results identify compositional ranges largely consistent with those expected from petrographic and EMPA data, but also identify multiple instances of potential impurities, inclusions or alteration in the laser-sampled volume (subsurface) not evident in mineral mapping data (surface; see Figures 5 and 6). The average trace element concentrations and 87 Sr/ 86 Sr values of the most pristine grains (after geochemical screening to remove all spots with inclusions) in samples with GL pristine grains (GL illitisedH in sample H41_320), pure bioapatite, pure calcite and pure dolomite are shown in Table 1. These values along with the ideal composition of each mineral (Table  S3 in Supporting Information S1) were used to define the endmembers to calculate the mixing lines shown in Figures 5 and 6. Mixing lines are based on linear mixing between these endmembers. The average composition of geochemically screened GL-O grains is also shown for comparison.

Ajax Limestone
In comparison to the average elemental concentration of GL-O grains measured in this study (Table 1), Ajax Limestone GL pristine grains are enriched in trace elements, including Mn, Ni, and Cu, but depleted in Cr. Although GL illitisedP and GL pristine grains in BSE imaging and EDS mineral mapping are petrographically readily distinguished, their compositional ranges largely overlap and are characterized by high Rb, low Sr, low Ca/P and low Ce. The presence of calcite inclusions is characterized by increased Ca and Sr but decreased Rb, relative to GL pristine and GL illitisedP , resulting in high Ca/P, low Rb/Sr and moderately increased Ce (REEs). The presence of apatite inclusions, by contrast, increases Sr, P and REEs (incl. Ce) and decreases Rb, resulting in low Ca/P and Rb/Sr but elevated Ce relative to glauconite and illite. On this basis, a substantial proportion of both GL pristine and GL illitisedP contain previously unrecognized calcite inclusions ( Figure 5), whereas unidentified apatite inclusions are rare, possibly indicating that calcite inclusions are smaller and more difficult to recognize than apatite inclusions. A few analyzed spots in GL-O grains show the presence of apatite inclusions, with mixing toward the apatite endmember, in agreement with previous findings of Boulesteix et al. (2020). The same criteria confirm the petrographically based classification of GL I+C , GL I+A and GL I+A+C (see two and three endmember mixing arrays between glauconite/illite, calcite and apatite in Ca/P vs. Sr and Ce plots; Figure 5). It is notable that carbonate-inclusion containing spots define a mixing array reaching Ce concentrations that are close to an order of magnitude greater than expected based on mixing between glauconite/illite and pure primary carbonate endmembers. This suggests that carbonate inclusions are compositionally distinct to the calcite comprising the limestone matrix, and potentially of diagenetic origin (Ca/P vs. Ce plot, Figure 5), consistent with the petrographic association between carbonate inclusions and illitised domains noted previously. Finally, several spots with Rb/Sr ∼>30 and Sr <80 ppm show significantly more radiogenic 87 Sr/ 86 Sr than can be accounted for by mixing between glauconite, illite and apatite/calcite endmembers, suggesting diagenetic incorporation of an unidentified component characterized by low Sr content but highly radiogenic Sr composition possibly sourced from the decomposition of detrital feldspar or mica (see discussion).

Tempe Formation
Petrographically and geochemically, H41_307 includes the most pristine grains out of the samples studied, with only a few GL pristine spots unambiguously containing subsurface apatite inclusions (falling along glauconite-apatite mixing line on Ce vs. Ca/P plot; Figure 6). GL pristine and GL illitisedP show a similar compositional range, confirming the difficulty of distinguishing illite alteration geochemically, whereas consistently low Ca contents and the absence of a mixing array toward calcite or dolomite endmembers confirm the absence of carbonate inclusions across all petrographically defined glauconite classes. The trace element composition of H41_307 GL pristine is notably distinct to Ajax GL pristine , with elevated Sr (∼1 order of magnitude), V, Cr and Cu concentrations but lower Mn, Ni and REEs. This agrees with previous observations made on the basis of major elements contents.
Relative to GL-O standard, GL pristine is enriched in V, Mn, Cu and Sr but depleted in Ni and Cr (Table 1). No anomalously radiogenic 87 Sr/ 86 Sr values were measured in sample H41_307.
The average concentrations of most trace elements in sample H41_320, including Ti, V, Mn, Co, Cu, and Sr in the GL illitisedH spots, are higher than in GL pristine in other samples (Table 1). Both GL illitisedH and GL I+A show a wide compositional range with mixing between three endmembers, that is, glauconite, dolomite and apatite. In addition, two distinct groupings of GL illitisedH are recognized, one with elevated Sr concentrations (seen in pellet-shaped grains) similar to those in H41_307 GL pristine grains (see Figure 6, Table 1), and one more similar to low-Sr GL pristine of Ajax and GL-O glauconites (seen in various grain shapes and fabrics), resulting in two distinct mixing arrays in Ca/P versus Sr plots. Finally, more than 50% of the GL illitisedH spots and a small proportion of GL I+A define a clear mixing array toward significantly more radiogenic 87 Sr/ 86 Sr than can be explained by mixing between glauconite, illite and apatite/dolomite endmembers, suggesting diagenetic incorporation of low Sr content but highly radiogenic Sr ( 87 Sr/ 86 Sr ≥3) endmember (Figures 6f and 6g), likely sourced from detrital feldspar or mica. A highly radiogenic influence is evident in both dolomite-inclusion containing and dolomite-free GL illitisedH , as indicated by Ca/P versus 87 Sr/ 86 Sr plots.  Table S3 of Supporting Information S1. Cut-offs are based on the reclassified GL pristine grains and measured GL-O grains. Gray, blue, and pink shaded areas indicate the approximate compositional ranges for pristine glauconite, calcite inclusion-bearing spots and apatite inclusion-bearing spots, respectively. Axes are a mix of linear and log.

In Situ Rb-Sr Geochronology
Here, we use the trace element results outlined in the previous section as an additional screen to select the cleanest analyzed volumes/spots for each class to avoid the spots containing unidentified inclusions in constructing Rb/Sr errorchrons. To this end, where anomalous spots closely resemble the composition of another class, we reclassify them as that class, and where they are entirely anomalous, we remove them from the errorchrons. For instance, Ce values of >1 ppm are considered to indicate the presence of apatite or carbonate and Ca/P of >∼30 (based on  Table S3 of Supporting Information S1 as presented in Figure 5 with one exception: Here, the carbonate phase is dolomite and the grains with potential dolomite inclusions are highlighted by teal shaded areas. Gray, blue and pink shaded areas indicate the approximate compositional range for pristine glauconite, dolomite inclusion-bearing spots and apatite inclusion-bearing spots, respectively. Green circles in Sr versus 87 Sr/ 86 Sr graph show the majority of analyzed data points are divided into two groups with low and high Sr content. Axes are a mix of linear and log.

Note.
These values are LA-ICPMS measurements obtained after geochemical screening and reclassification of the anomalous data points as described in the text.
the range of Ca/P in GL-O standards) is considered to indicate the presence of carbonate inclusions. The compositional thresholds used for reclassification are shown as shaded areas in Figures 5 and 6.
We then construct a "total" errorchron for all spots analyzed in each sample (Figure 7), including all the altered or unaltered spots, as well as a separate errorchron for each alteration class (calculated ages plotted in Figure 8). Both are anchored by the use of the mean Rb/Sr and 87 Sr/ 86 Sr measured on pure apatite or carbonate in each sample (Table 1). All the total errorchrons show scattered data with mean square of the weighted deviation (MSWD) of >1, which is expected given the variety of diagenetically altered grains. The number of spots used for errorchrons before and after reclassification as well as the errorchron age, the estimated uncertainty (1σ), 95% confidence interval (CI), initial 87 Sr/ 86 Sr and MSWD of each errorchron are given in Table 2.

Ajax Limestone
Inclusion of all the analyzed spots on a total errorchron returns an age of 459.1 ± 3.2 Ma, which is ∼61 Ma younger than the expected stratigraphic age for Ajax_332 (∼520 Ma), whereas the calculated 87 Sr/ 86 Sr initial of 0.7330 ± 0.0015 is significantly greater than the lower Cambrian seawater ratio of ∼0.709 (Veizer et al., 1999). Individual errorchrons for GL pristine , GL illiteP , GL I+A , and GL I+C classes return similar ages (e.g., ≈450-485 Ma, Table 2). GL I+A+C returns a comparatively older age of 509.3 ± 16.7 Ma (Figure 8) within the error of the estimated stratigraphic age.
The total errorchron for Ajax_356 returns an age of 483.2 ± 1.9 Ma, which is ∼37 Ma younger than the estimated stratigraphic age of this sample (∼520 Ma), and an 87 Sr/ 86 Sr initial of 0.7100 ± 0.0005, that is within the error of the lower Cambrian seawater composition (∼0.709; Veizer et al., 1999). The errorchrons for GL pristine , GL illitisedP , and GL illitisedH return ages of 468.9 ± 3.1, 485.9 ± 3.5 Ma and 473.5 ± 6.5 Ma, respectively (Table 2, Figure 8). Apatite-bearing glauconites (GL I+A ) return an older age (506.1 ± 6.3 Ma) that is closer but does not overlap the estimated stratigraphic age. Finally, illitised grains containing carbonate inclusions (GL I+C ) yield an age of 516.0 ± 6.1, which is thus the closest age to the expected stratigraphic age.

Tempe Formation
A total errorchron for H41_307 returns an age of 484.4 ± 2.2 Ma (Figure 8), which is ∼25 Ma younger than the estimated stratigraphic age (∼509 Ma), and an initial 87  A total errorchron for H41_320 yields an age of 500.6 ± 4.1 Ma, thus falling well within the error of the estimated stratigraphic age for the Tempe formation (∼509 Ma), as well as a radiogenic (nonmarine) 87 Sr/ 86 Sr initial of 0.7194 ± 0.0015. An individual errorchron for GL I+A returns a much older age (GL I+A : 563.3 ± 13.6 Ma). GL illitisedH returns an age of 487.3 ± 5.1 Ma (Figure 8). The newly added GL I+C class, incorporating all spots with compositional evidence for carbonate inclusions, returns an age of 533.5 ± 10.2 Ma.

Discussion
Improved understanding of postdepositional alteration and criteria for identifying grains suitable for geochronology applications are essential prerequisites for the broader application of in situ Rb-Sr glauconite geochronology. We discuss new microscale insights into the mechanisms and impact of postdepositional alteration on glauconite Rb-Sr systematics and Rb-Sr geochronology, enabled by our novel combination of detailed petrographic characterization of glauconite grains with in situ laser ablation Rb/Sr dating and trace element geochemical fingerprinting. We further discuss whether targeting of the best-preserved glauconite grains in partially altered sedimentary sequences can produce stratigraphically meaningful dates, and the circumstances under which ages obtained on altered grains may record a postdepositional "diagenetic event."

A Fabric/Textural Control on the Cooccurrence of Pristine and Altered Grains?
The common cooccurrence of altered and seemingly well-preserved glauconite grains in a given sample or stratigraphic level prompts the question: why are some glauconite grains altered whilst others remain seemingly unaffected? The main type of glauconite alteration recognized in our sample set is patchy or homogenous illitisation, commonly accompanied by the presence of calcite or dolomite inclusions. Systematic differences in the fabric of pristine versus altered glauconite are indicative of a fabric control on the style and extent of alteration. (a) "Pristine" glauconite grains (GL pristine ) show tightly packed, low porosity fabric and only contain minor patches of illite or calcite inclusions (Figure 3a). We suggest that the tightly packed fabric of pristine glauconite may have limited ingress of burial diagenetic fluids, preserving these grains from extensive alteration. We note, however, that the lower-than expected measured 87 Sr/ 86 Sr for many GL pristine spots (Table S2 in Supporting Information S1, see also Section 4.3.1) suggest that minor fluid alteration impacted a substantial proportion of even the petrographically "pristine" grains. (b) Patchy illitisation (GL illitisedP ) is associated with glauconite grains exhibiting distinctly bimodal porosity and ropy fabric, with illite and calcite occurring mainly within elongate pore spaces present between bundles of more tightly packed, "non-porous" glauconite. (c) Homogenous illitisation (GL illitisedH ), by contrast, is associated with grains showing a more uniformly porous and loosely packed fabric, which is likely to have facilitated even fluid ingress and homogenous rather than patchy illitisation. Furthermore, observation of fabrics associated with each alteration style in samples containing only unaltered mature glauconite (e.g., Figure 4a) demonstrates that the style and extent of alteration are more likely controlled by fabrics, rather than being the product of differing degrees of alteration. We therefore conclude that the cooccurrence of altered and unaltered glauconite is primarily a function of the range of prealteration grain fabrics contained within a particular sample, as well as the magnitude of alteration experienced by a sample.

Controls on Illitisation of Glauconite and the Significance of Other Inclusions
Postdepositional alteration of glauconites into illite or chlorite is thought to be controlled by the intensity of leaching, as well as pH and redox state of the diagenetic fluids (e.g., Bansal et al., 2020;Guimaraes et al., 2000), in addition to the influence of grain fabric outlined previously. The presence of Fe-(oxy)hydroxides versus pyrite in association with altered glauconite can be used to estimate the local paleo-redox state of the altering fluids (Baldermann et al., 2017). Ajax samples showed ample evidence of Fe-oxides but no pyrite (Figure 4d), which we attribute to alteration under relatively oxic conditions due to a presumably shallower burial setting, which did not generate reduced, organic-acid rich fluids, consistent with the modeled low-grade thermal maturity of the source rocks in Hawker Group succession (Carr et al., 2012). Samples from the Tempe Formation, by contrast, contain pyrite but no iron oxides, indicating alteration under reducing or anoxic conditions. In sample H41_320, this is associated with intensely illitised glauconites carrying elevated amounts of trace elements, as well as highly radiogenic Sr in some spots (Figure 7), likely sourced from the dissolution of Rb-rich detrital phases such as mica and K-feldspar. We attribute these properties to (a) the presence of acidic, reducing fluids formed by thermal maturation of organic-rich source rocks in the region (Jarrett et al., 2016) and (b) the relatively high permeability of the fossiliferous dolostone. In contrast, the intensity of illitisation in sample H41_307 is moderate with only patchy illitisation observed, likely due to the lower permeability of the clay-rich host lithology and the densely packed glauconite texture. Although an abundance of pyrite within partially altered glauconite (Figure 2e vs. Figure 2d) documents the presence of a reducing fluid, the absence of excessively radiogenic Sr (Figure 7) and lower trace element concentrations in sample H41_307 (Table 1) is consistent with less fluid passage and minimal detrital silicate dissolution. As noted previously, criteria for identifying well-preserved grains are essential before in situ Rb-Sr glauconite geochronology can be applied more broadly. Different styles or extents of illitisation of glauconite are readily discriminated petrographically. However, most of the GL illitisedP spots show overlapping chemical compositions (both in major and trace elements) with GL pristine grains, consistent with the similar mineral chemistry of illite and glauconite, which is largely distinguished by the low Fe content in illite compared to the higher Fe content in glauconite. Given that our results suggest that Fe liberated during illitisation of glauconite is commonly retained at the microscale, as distinct Fe-bearing microinclusions, this limits the utility of geochemical screening (for potentially petrographically unidentified subsurface alteration) and further suggests that both GL pristine and GL illitisedP grains contain complex intergrowths of altered and unaltered regions. On the other hand, geochemical screening is able to identify the presence of cryptic or subsurface carbonate and apatite inclusions in petrographically pristine and illitised grains. Ca is barely incorporated into the glauconite structure (Stille & Clauer, 1994); therefore, high levels of Ca and Sr can be attributed to the presence of cryptic Ca-rich carbonate/calcite inclusions. The clear petrographic association between calcite inclusions and illitised domains identifies a late diagenetic origin ∼ penecontemporaneous with illitisation for a majority of the carbonate inclusions, as also supported by the mixing array toward anomalously radiogenic Sr measured in calcite-inclusion-bearing grains ( Figure S3 in Supporting Information S1). The Ca concentration can therefore be used to screen for the presence of cryptic illitisation and carbonation in GL pristine grains, which is helpful given the otherwise similar chemical composition of GL pristine versus GL illitised , although this relationship may not apply in other settings. Apatite inclusions, by contrast, are here interpreted as cogenetic phases with glauconite. Not only are intergrowths of glauconite and apatite in recent to modern marine sediments well documented (e.g., O'Brien et al., 1990;Stille & Clauer, 1994;Tóth et al., 2010), the growth of glauconite in phosphatic bioclast cavities ( Figure 2) and the presence of apatite inclusions in both GL illitisedP and GL pristine grains argue for an earliest diagenetic origin, precipitating from seawater and/or seawater-derived pore fluids, or possibly representing the remnants of bioapatite substrates. This is further supported by 87 Sr/ 86 Sr initial that is close to seawater composition for apatite-bearing grains (Table 2) and the absence of anomalously radiogenic Sr in apatite-inclusion-bearing grains (see Section 4.3).

Assessing the Geological Significance of Glauconite Rb-Sr Ages
Evolved glauconites that equilibrated isotopically with seawater and were subsequently preserved in a closed system, without being significantly impacted by late stage or burial diagenesis, are ideal candidates for sediment dating applications. This condition, however, may be rare in sedimentary rocks of Proterozoic and even Paleozoic to Mesozoic age, where the Rb-Sr isotopic system is commonly disrupted and partially reset, yielding erroneous glauconite Rb-Sr ages. The Rb-Sr system of mica-type minerals, especially those with interlayer-associated cations such as K + (and by inference Rb + ) in glauconite, has been suggested to be susceptible to ion exchange reactions with circulating fluids during diagenesis (Keppens & Pasteels, 1982). Compositional changes accompanying structural modifications during diagenesis and the presence of residual detrital clay materials containing highly radiogenic Sr have been proposed as possible explanations for the less radiogenic Sr (younger ages) and presence of excessively radiogenic Sr (older ages), respectively (Hurley et al., 1960(Hurley et al., , 1961Keppens & Pasteels, 1982). Additionally, the dissolution of detrital clays in the host sediment matrix could result in the introduction of more radiogenic Sr into the pore water (Clauer et al., 1982), which can subsequently be incorporated by diagenetic phases precipitating from that radiogenic and nonmarine fluid. Diagenetic processes impacting on the Rb-Sr system fall into these possible scenarios: (a) (partial) equilibration of glauconites and diagenetic alteration products with burial fluids, resulting in a nonmarine (usually radiogenic) 87

Origin and Significance of Excessively Young Glauconite Rb-Sr Ages
Inclusion-free but partly illitised glauconite (GL pristine , GL illitisedP , and GL illitisedH ) within a given sample show similar concentrations of Rb and Sr ( Figures S4 and S5 in Supporting Information S1), arguing against significant Rb gain or Sr loss. Thus, the young ages are unlikely to be due to postdepositional perturbation of the Rb/Sr. Alternatively, young ages can be attributed to isotopic exchange with burial fluids. Sr is present at low concentrations in glauconite (typically <10 ppm; Table 1) and is thought to act as an exchange ion where glauconite retains expandable smectite layers (Hower, 1961). Therefore, Sr in glauconite (with potential expandable layers) is likely easily isotopically equilibrated with burial fluids, which typically have high Sr concentrations and are most commonly less radiogenic than the glauconite-hosted Sr (Chaudhuri & Clauer, 1993) due to the ingrowth of radiogenic 87 Sr over time in high Rb/Sr glauconites.
The impact of isotopic exchange during diagenesis can be evaluated by considering 87 Sr/ 86 Sr initial values calculated for both total errorchrons and alteration-class specific errorchrons and by assessing Δ 87 Sr values, the latter being a measure of deviation between the measured 87 Sr/ 86 Sr and the theoretical closed system/unaltered 87 Sr/ 86 Sr. While the uncertainties in our measurements are to the same precision, our results ( Figure 7 and Table 2) show that 87 Sr/ 86 Sr initial from both total errorchrons and alteration-class specific errorchrons are more radiogenic than the early Cambrian marine 87 Sr/ 86 Sr (∼0.709), consistent with postdepositional Sr exchange. Furthermore, Δ 87 Sr values for GL illitisedP , GL illitisedH and GL pristine classes are almost uniformly ≤0% (Figure 9), notwithstanding considerable measurement uncertainty (Figure 10), identifying excessively unradiogenic Sr as a common characteristic of classes returning younger Rb-Sr ages. The observation of Δ 87 Sr that is commonly <0% in GL pristine spots (mainly Ajax Limestone samples; Figure 9), and with similar Δ 87 Sr range as GL illitised spots, further implies (a) the occurrence of finely intergrown pristine glauconite and illite at the sub-µm scale and in the subsurface of analytical volumes, which was not recognized petrographically and could not be screened for geochemically (overlapping compositions; see Section 4.2), and (b) that exchange or partial equilibration with diagenetic fluids did not necessarily result in detectable mineral alteration. We therefore recommend that glauconites with porous and loosely packed fabrics should be avoided even in the absence of evidence for alteration; glauconites with tightly packed fabrics are predicted to be the best targets for in situ Rb-Sr dating.
Whether targeting of altered grains permits dating of postdepositional events is of significant interest as this would broaden the utility of glauconite geochronology. The younger ages obtained for illitised glauconites (between ∼475 and ∼485 Ma) in H41 samples coincide with time intervals suggested for increased subsidence rates in the Amadeus Basin (around 470 Ma; Shaw et al., 1991) and a tectonothermal event centered on the Arunta Inlier, located northeast of the Amadeus Basin (467 ± 8 Ma; Hand et al., 1999). However, in the broader geological context, the "young ages" measured in several samples are more likely to reflect isotopic mixing due to partial exchange with nonmarine diagenetic fluids rather than an age recording later diagenetic and/or postdepositional tectonic events. Full resetting of the Rb-Sr geochronometer in glauconite that would allow dating of postdepositional or late diagenetic events requires that all analyzed grains fully equilibrate with the diagenetic fluid. We expect that this would be expressed as a nonmarine 87 Sr/ 86 Sr initial in such altered glauconites and ∼uniform 87 Sr/ 86 Sr composition for a given Rb/Sr ratio. Analyzed glauconites from both Arrowie and Amadeus basins, however, show considerable spreads in Δ 87 Sr for spots with similar Rb/Sr ratios (e.g., Ajax 356; Figure 9), arguing for partial exchange with a relatively unradiogenic fluid. We therefore consider it unlikely that our "young ages" are geologically meaningful. However, full resetting or equilibration of the Rb-Sr isotope system in glauconite grains and their alteration products is likely to occur in other settings with more intense fluid-rock interaction. The interpretative framework established here will assist in the evaluation of circumstances where this is potentially the case, facilitating dating of specific postdepositional or tectonic events. Further and more systematic work is needed to test these scenarios.

Origin and Significance of Excessively Old Glauconite Rb-Sr Ages
Interestingly, some altered grains in classes containing carbonate inclusions return ages that are older than noninclusion-bearing GL illitised and GL pristine grains (Ajax samples) and, in the case of H41_320, also older than the expected stratigraphic age (Figure 8). Indeed, carbonate-inclusion-bearing grains are commonly associated with somewhat more radiogenic 87 Sr/ 86 Sr values than predicted based on their measured Rb/Sr, stratigraphic age and assuming isotopic equilibration with coeval seawater (Figures 9 and 10; Figure S3 in Supporting Information S1). Such excessively "old" ages with more radiogenic Sr signatures are most commonly attributed to incomplete maturation leading to the (partial) retention of the inherited and thus older detrital substrate phases (Derkowski et al., 2009;Hower et al., 1963;Hurley et al., 1959). However, we find (a) no petrographic evidence for detrital inclusions in these "older" spots; (b) excessively radiogenic glauconite cooccurs with "younger" and 87 Sr depleted glauconite; and (c) the shape, size and fabric of these "older" glauconite pellets to be entirely consistent with authigenic clay rather than purported detrital phases; a different mechanism(s) must therefore be responsible for the observed more radiogenic Sr data and associated "old" ages.
Unlike glauconite, carbonates can incorporate relatively great amounts of Sr, which is of diagenetic origin, non-marine and highly radiogenic Sr. Petrographic and geochemical observations demonstrate a burial diagenetic origin for the carbon inclusions (see Section 4.2) so that the ages obtained for carbonate-bearing samples must be considered to represent isotopic mixing ages. We propose that the precipitation of secondary carbonate inclusions from diagenetic fluids shifted the Rb-Sr to lower values, potentially resulting in "older" ages. The observation of a mixing array toward a highly radiogenic Sr endmember (see Figures 6 and 7; Figure S3 in Supporting Information S1) suggests that at least some of these carbonate inclusions also captured highly radiogenic Sr sourced from the dissolution of ambient Rb-rich and highly radiogenic mineral phases such as detrital mica and/or K-feldspar, resulting in anomalously high 87 Sr/ 86 Sr values in affected glauconites. Consistent with this, data points classified as GL I+C and GL I+A+C show Δ 87 Sr values that are almost uniformly ≥0% (Figures 9 and 10). Perhaps future studies can compare the results of in situ laser ablation Rb-Sr technique with the conventional technique where samples can be treated by acid to eliminate the effect of carbonates.

True Depositional Age From Apatite-Bearing, Petrographically Unaltered Glauconite?
Petrographic considerations and their common cooccurrence in recent sediments suggest that apatite is cogenetic with glauconite in our sample set (see Section 4.3). This is also supported by the distribution of Δ 87 Sr for apatite-bearing samples that is centered around 0% and is therefore quite distinct from that of calcite inclusion-bearing grains ( Figure S6 in Supporting Information S1). Apatite-bearing but otherwise pristine glauconite grains in sample H41_307 return an age that overlaps with the expected stratigraphic age of the Tempe Fm ( Figure 9) as well as an 87 Sr/ 86 Sr initial that is very similar to the early Cambrian seawater value of ∼0.709 (Table 2). Overall, apatite-bearing glauconite, except for H41-320, yield ages closer to the stratigraphic ages ( Figure 9). We suggest that the high concentration of Sr in relatively alteration-resistant apatite inclusions limits isotopic exchange of Sr in apatite-containing glauconite grains, preserving 87 Sr/ 86 Sr through diagenesis. Although our results suggest that apatite-bearing glauconite is a prospective target for obtaining Rb-Sr depositional ages in partially altered sedimentary sequences, these results are based on a limited number of samples. Perhaps further work on phosphatic glauconite in hardgrounds, which are typically well-preserved, could be very helpful to test to what extent apatite-bearing glauconite preserves a primary age in altered sequences.

Conclusions
The combined petrographic and in situ geochemical screening approach applied in this study allows for the differentiation of variable degrees and styles of glauconite alteration in diagenetically impacted Cambrian sedimentary rocks. Our study shows that prealteration grain fabric and the magnitude of alteration are the controlling factors in the cooccurrence of variably preserved glauconite. Illitisation is identified as the primary alteration phase, but geochemical fingerprinting shows that diagenetic carbonate (calcite, dolomite) inclusions are a common feature even in petrographically seemingly "pristine" glauconite grains. It is shown that petrographically distinct "pristine" and illitised grains have overlapping compositions, potentially reflecting cryptic alteration and diagenetic Sr exchange with relatively unradiogenic burial. Both illitised and pristine glauconite therefore return Rb-Sr ages that are younger than the expected stratigraphic age. The presence of carbonate inclusions, in contrast, is associated with relatively older Rb-Sr ages and, in Tempe Fm sample H41_320, with an Rb-Sr age that is older than the stratigraphic age. We attribute this to diagenetic Sr incorporation in carbonates, decreasing the Rb/Sr ratio of the inclusion-bearing grains and increasing their 87 Sr/ 86 Sr ratios, where Sr is sourced from the dissolution of detrital components in the ambient sediment matrix. Both "young" and "old" ages are considered isotopic mixing ages. Glauconite containing cogenetic apatite, by contrast, returns an age consistent with the expected stratigraphic age for the Tempe Fm (∼505.1 ± 5.5 Ma in sample H41_307). We attribute this to the high Sr content and low reactivity of apatite relative to glauconite, although further work on a larger number of samples is required to examine this conclusion.
Our study shows that both petrographic and geochemical screening should be considered for in situ Rb-Sr dating of glauconite. Internal texture/fabric and relative porosity/permeability of glauconite grains are also important factors to consider, especially in light of our sampling laser spot size of 85 microns of glauconite with micron-sized inclusions. While, further systematic work on an extensive number of samples is required to assess the impact of host lithology, grain fabric and permeability, as well as the impacted alteration fluid and burial on the fidelity of glauconite Rb-Sr geochronology, the ubiquity of glauconites, new technology and approaches allowing rapid petrographic, trace elemental and isotopic screening of each grain opens new possibilities to address these issues and gain new insights on the depositional and burial history of sedimentary rocks.

Data Availability Statement
The data used in this research are available in Supporting Information S1 and are also available in Figshare at https://doi.org/10.25949/21644864.