July 2023 LIP of the Month

Eruption history of the Columbia River Basalt Group constrained by high-precision U-Pb and 40Ar/39Ar geochronology

Jennifer Kasbohm1,2, Blair Schoene1, Darren F. Mark3,4, Joshua Murray1,5, Stephen Reidel6,*, Dawid Szymanowski1,7, Dan Barfod3, Tiffany Barry8

1 Department of Geosciences, Princeton University, Princeton, NJ 08544, USA

2 Department of Earth & Planetary Sciences, Yale University, New Haven, CT 06511, USA

3 Isotope Geosciences Unit, Scottish Universities Environmental Research Centre, University of Glasgow, Rankine Avenue, East Kilbride G12 8QQ, UK

4 Department of Earth and Environmental Science, University of St. Andrews, St Andrews KY16 9AJ, UK

5 Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

6 Pacific Northwest National Laboratory, Richland, Washington 99352, USA

7 Institute of Geochemistry and Petrology, ETH Zürich, 8092 Zürich, Switzerland

8 School of Geography, Geology and the Environment, University of Leicester, Leicester LE1 7RH, UK

* Retired

This is a summary and modification of: Kasbohm, J., Schoene B., Mark, D., Murray, J.*, Reidel, S., Szymanowski, D., Barford, D., Barry, T. (2023). Eruption history of the Columbia River Basalt Group constrained by high-precision U-Pb and 40Ar/39Ar geochronology. Earth & Planetary Science Letters 617, 118269. https://doi.org/10.1016/j.epsl.2023.118269.


Large igneous province volcanism of the Columbia River Basalt Group (CRBG) has been suggested to play a causal role in elevated global temperatures and atmospheric carbon dioxide levels of the Miocene Climate Optimum (MCO). However, assessing the connection between volcanism and warming is dependent upon an accurate and precise chronology for the timing and duration of CRBG emplacement. Building on our previous work (Kasbohm & Schoene, 2018), we present fifteen new high-precision ages, using CA-ID-TIMS U-Pb on zircon and multi-collector 40Ar/39Ar on basaltic groundmass, to provide a detailed dual-chronometer timeline for CRBG eruptions. We use both sets of new ages and precise stratigraphic positions of our samples in an integrated Markov Chain Monte Carlo model to calculate average long-term emplacement rates for main-phase CRBG volcanism of 0.2-0.9 km3/a, with a high likelihood of one prominent hiatus of 60-120 kyr duration occurring after main-phase emplacement. We analyzed trace elements and hafnium isotopes of each dated zircon from CRBG interbeds. The compositions are consistent with both Cascades subduction volcanism and evolved syn-CRBG volcanism proximal to the depositional area. Our age model also yields ages for all magnetic field reversals during the main phase of CRBG emplacement, which can be used to improve calibrations of Miocene paleoclimate records. We find that main-phase CRBG emplacement is coincident with the greatest sustained warmth of the MCO in astronomically-tuned records. Our work shows the power of using both U-Pb and 40Ar/39Ar geochronology in an integrated stratigraphic context to assess data reliability and develop the most robust age model possible for large igneous province emplacement.

1. Introduction

Large igneous provinces (LIPs), Earth’s most voluminous volcanic events, have punctuated Earth history, leaving evidence for mass extinctions and climatic perturbations in their wake (Ernst & Youbi, 2017). In the past decade, new radioisotope geochronology has yielded robust constraints on the timing, duration, and volumetric rates of eruption for a number of LIPs (Blackburn et al., 2013; Burgess et al., 2017; Schoene et al., 2019; Sprain et al., 2019). However, only a minority of LIPs possess the wide geographic exposure, well-defined stratigraphy, and datable material amenable to detailed geochronological studies (Kasbohm et al., 2021). Here we report new temporal constraints on the Columbia River Basalt Group (CRBG), the youngest and best-preserved LIP, which is a testbed for quantifying the tempo of LIP emplacement through radioisotope geochronology.

The CRBG has been the subject of decades of effort to document its geochemistry, paleomagnetism, and mineralogy, leading to a well-defined volcanostratigraphy comprising five main formations and dozens of individual stratigraphic members, each with area and volume estimates obtained through mapping and coring campaigns (Reidel et al., 2013). The 210,000 km3 of the CRBG were emplaced in western North America during the Miocene, broadly coinciding with active regional volcanism of the Cascades subduction zone to the west, silicic volcanism of the Basin & Range to the south, and the time-progressive silicic eruptive activity of the High Lava Plains and Yellowstone-Snake River Plain hotspot tracks to the south and southeast (Figure 1A).

Figure 1. CRBG Maps. A) Overview map of the CRBG after Reidel et al. (2013) and Kasbohm & Schoene (2018), with areal extents and volume estimates for each formation, localities of regional volcanic activity labeled, and geochronology samples starred. Dashed lines bracket source dyke swarms, and the locations of geochronology samples are starred. In total, the Saddle Mountains Basalt, Picture Gorge Basalt (PGB) and Prineville Basalt (PVB) together comprise 2.6% of the total CRBG volume, and are not discussed further here, or included in any cumulative volume estimates. B) Geologic map (Washington State Department of Natural Resources, 2010) of CRBG exposure in southeastern Washington, with geochronology samples starred. We follow Reidel & Tolan (2013a) in using polarity chron abbreviations R1 (the first GRB reversed interval, chron C5Cn.2r), N1 (first normal interval, chron C5Cn.2n), R2 (second reversed interval, chron C5Cn.1r) and N2 (second normal interval, chron C5Cn.1n).

The CRBG is noteworthy for its temporal coincidence with the Miocene Climate Optimum (MCO), an episode of global warming at 17-15 Ma evidenced by paleoclimate proxies indicating a 4-6°C high-latitude sea surface temperature rise (Shevenell et al., 2004) and elevated atmospheric CO2 levels at or above 600 ppm (Rae et al., 2021). However, prior geochronology for the CRBG lacked the accuracy and precision required to assess in detail its connection to the MCO. Barry et al. (2013) reviewed K-Ar and 40Ar/39Ar geochronology obtained from CRBG basalts, and suggested that emplacement of the Steens, Imnaha, Grande Ronde, and Wanapum Basalts (formations; Reidel et al., 2013) occurred between 16.9-15.0 Ma. This age model, constructed from individual ages with low (up to ~±1 Myr, 2σ) precision and in some cases poor accuracy, yielded numerous inconsistencies between the magnetic polarity of the basalts and the Geomagnetic Polarity Timescale (GPTS; Hilgen et al., 2012) (Baksi, 2013).

More recent high-precision 40Ar/39Ar and U-Pb zircon geochronology has provided a clearer timeline for CRBG eruptions by dating sanidine or zircon extracted from volcanic ash material between lava flows (Kasbohm & Schoene, 2018; Mahood & Benson, 2017). The U-Pb zircon age model for the CRBG suggested that a portion of the Steens through the Wanapum Basalt was emplaced in ~750 kyr beginning at 16.65 Ma, with possible brief intervals of more rapid emplacement (Kasbohm & Schoene, 2018). Obtaining further high-precision geochronology through the CRBG basalt pile, in particular through the Grande Ronde Basalt (GRB), the largest CRBG formation comprising 72% of its total volume (Reidel et al., 2013), would address uncertainties remaining in the age model and corresponding CRBG eruption rates. The GRB is subdivided into 25 formal and informal stratigraphic members based on geochemistry, mineralogy, and magnetic polarity. Because the GRB was emplaced during two reversed and two normal polarity chrons, individual members are typically grouped into four different informal magnetostratigraphic units: R1 (the first reversed interval, chron C5Cn.2r), N1 (first normal interval, C5Cn.2n), R2 (second reversed interval, C5Cn.1r) and N2 (second normal interval, C5Cn.1n) (Reidel & Tolan, 2013a).

Here, we provide new U-Pb and 40Ar/39Ar age constraints on 15 samples from interbeds and basaltic lavas of the CRBG, intercalated with 8 samples dated by Kasbohm & Schoene (2018). These datasets show excellent agreement between chronometers at the ~10 kyr level, permitting integration of these datasets into a single Markov Chain Monte Carlo age model. Our model constrains the eruptive tempo of the CRBG and places tighter age estimates on magnetic reversals observed in the basalt pile, which are critical for building a robust GPTS and subsequent age models for climate proxy records across the MCO. To discern the provenance of the ashbed zircons sampled for U-Pb geochronology, we performed zircon trace element and Hf isotopic analyses that suggest a likely continental arc origin for all CRBG zircons. We show that advances in 40Ar/39Ar mass spectrometry, application of Bayesian age models, and a continued progress in assessing systematic uncertainties have the potential to yield a highly resolved record of the timing and tempo of LIP emplacement.

2. Methods

2.1 U-Pb Geochronology

We use U-Pb zircon geochronology by chemical abrasion – isotope dilution – thermal ionization mass spectrometry (CA-ID-TIMS) to provide accurate and precise age constraints on CRBG interbeds, as we did in our previous work (Kasbohm & Schoene, 2018). Interbeds were sampled from known stratigraphic positions (Figure 1), and manifest as either ash layers or red weathered lava flow top horizons containing ash material – the latter we identify as “red boles” as they are known in the Deccan Traps (Ghosh et al., 2006; Inamdar & Kumar, 1994). Individual zircons were separated from each sample and dated through CA-ID-TIMS at Princeton University.

2.2 40Ar/39Ar Geochronology

The CRBG lavas are mostly aphyric with notable exceptions in the Steens, Imnaha, Picture Gorge, Wanapum, and Saddle Mountains Basalts. Aphyric lavas require a 40Ar/39Ar dating approach that targets groundmass. Nine samples taken from the Steens through Wanapum Basalts were dated using the 40Ar/39Ar CO2 laser step-heating approach. Samples include materials collected during field seasons in 2013 and 2015 (Figure 1), as well as materials previously 40Ar/39Ar dated by Barry et al. (2010). Fresh groundmass was prepared for 40Ar/39Ar dating from all samples. 40Ar/39Ar ages were calculated using the optimisation model of Renne et al. (2010) and the parameters of Renne et al. (2011). All ages are reported at the 2σ confidence interval.

2.3 Zircon trace element geochemistry and Hf isotopes

To explore the origin of CRBG interbed zircons, we performed zircon trace element and Hf isotopic analyses for all zircons dated in Kasbohm & Schoene (2018) and this work. During U-Pb ion exchange chromatography ‘wash’ fractions were obtained following the ‘trace element analysis’ (TEA) protocol (Schoene et al., 2010). These fractions consisted of zircon matrix components, including Zr, Hf, minor, and trace elements. Concentrations of these elements in aliquots of ~1/3 of the wash fraction volume were analyzed at Princeton University on a quadrupole inductively coupled plasma mass spectrometer. Hf separations from the remaining solution followed Eddy et al. (2017) and Hf isotopic analyses were performed at Princeton University on a multi-collector inductively coupled plasma mass spectrometer. Methods are detailed in O’Connor et al. (2022).

3. Results

The sampling of materials from a well-characterized stratigraphic context allows CRBG lava flows dated through 40Ar/39Ar geochronology to be placed within stratigraphic order relative to the volcaniclastic units that were sampled for zircon U-Pb geochronology. As such, a composite section with samples taken at known values of cumulative volume can be utilized for age modeling.

3.1 U-Pb geochronology results

Individual zircon 206Pb/238U dates from fourteen CRBG interbed horizons are presented in Figure 2 with 95% confidence intervals; six new GRB samples are shown alongside our prior dataset (Kasbohm & Schoene, 2018). Dates within each sample spread beyond analytical uncertainty as a result of prolonged pre-eruptive crystallization in the magma, or inheritance of older grains from the host rock or volcanic edifice (Cooper, 2015; Miller et al., 2007; Simon et al., 2008). To robustly calculate an eruption age from dispersed zircon dates, in this study each sample was subjected to a Bayesian Markov Chain Monte Carlo (MCMC) model, which makes a probabilistic estimate of eruption age based on all of the individual zircon dates and their analytical uncertainties, and uses informed priors about the true distribution of zircon crystallization before eruption (Keller et al., 2018) (Figure 2). Finally, eruption ages were input into an additional MCMC simulation (Schoene et al., 2019), which imposes stratigraphic order as a constraint to further refine eruption ages, and calculates volumetric eruption rates (Figure 3).

Figure 2. U-Pb zircon CA-ID-TIMS geochronological data. Rank order plot of U-Pb geochronological data presented in this study, with each bar representing an individual zircon analysis, and analyses from each sample are grouped by color. Samples are arranged in stratigraphic order (younging bottom to top), and the height of each bar shows 2σ uncertainty. The result of our Bayesian zircon eruptive age modeling after Keller et al. (2018) (median and 95% credible interval) is highlighted (the top bar for each sample) and represents the input used in our Markov Chain Monte Carlo superposition age model. Samples with an asterisk are from Kasbohm & Schoene (2018). The columns on the left indicate the stratigraphic position and cumulative volume estimate for each sample, as well as CRBG magnetostratigraphy. The position of the Vantage sedimentary interbed is labelled with “V”.

Figure 3. CRBG U-Pb and 40Ar/39Ar age models. A) U-Pb (red) and 40Ar/39Ar (blue) age models for cumulative volume erupted through the CRBG. Input data for the models are U-Pb eruptive ages after Keller et al. (2018) and 40Ar/39Ar dates with 2σ analytical uncertainty. Shading indicates 95% credible interval of the models, with internal uncertainties only. The position of the Vantage sedimentary interbed is labelled with “V”. B) Results from (A) were used in further MCMC simulation to calculate average long-term emplacement rates through the eruptive history of the CRBG, averaged over 20 kyr windows, shown with contours up to the 68% credible interval (CI).

3.2 40Ar/39Ar geochronology results

Forty-eight aliquots of 9 samples were 40Ar/39Ar dated (Figure 4). All samples yield plateau ages consistent with the criteria of Schaen et al. (2020). All 48 plateaus define inverse isochrons with initial 40Ar/36Ar trapped components indistinguishable from atmospheric argon (Mark et al., 2011) and ages indistinguishable from the plateau ages. Up to 9 aliquots of each sample were analysed to maximize analytical precision. Systematic uncertainties were propagated into the weighted average age for each individual sample. All sample ages adhere to stratigraphic younging.

Figure 4. 40Ar/39Ar data.40Ar/39Ar age spectra showing multiple aliquots for each sample. Data and results are shown at the 2σ uncertainty, weighted average age ± analytical/systematic uncertainty.

The analytical precision achieved by this study shows an order of magnitude improvement relative to data reported by Barry et al. (2010) and represents a significant improvement in precision relative to other 40Ar/39Ar studies of the CRBG (reviewed in Baksi, 2013; Barry et al., 2013) due to the meticulous sample preparation detailed above and our experimental approach. While Barry et al. (2010) utilized a single-collector MAP 215-50 noble gas mass spectrometer, we utilized a multi-collector noble gas mass spectrometer (ARGUS V) (Mark et al., 2009). Analysis of 20 mg aliquots of material allowed for 15 step experiments to tease apart the variable argon reservoirs within the samples to recover relatively high radiogenic 40Ar plateau steps. Importantly, the multi-aliquot approach has also allowed us to drive down internal uncertainties through replication and reproducibility of the data. As such, the 2σ systemic uncertainty for each sample is of the same order of magnitude as the 2σ systematic uncertainty for U-Pb ages.

3.3 CRBG Eruption Rates

Our geochronological data yield a high-resolution age model for the vast majority of the CRBG that agrees well with our previous U-Pb age estimates. We show excellent agreement between U-Pb and 40Ar/39Ar age estimates of the timing of each eruptive member of the CRBG, allowing us to leverage our results into an integrated dual-chronometer age model (Figure 5) that modifies the modelling approach of Schoene et al. (2019) to account for systematic uncertainties in each method. We use the outputs of this dual-chronometer model to calculate all ages and durations (with full systematic, 2σ uncertainties), quoted here as our preferred interpretation.

Figure 5. CRBG dual-chronometer age model. A) Dual chronometer age model for cumulative volume erupted through the CRBG. Input data for the models are U-Pb eruptive ages after Keller et al. (2018) and 40Ar/39Ar dates with 2σ analytical uncertainty. Shading indicates 95% credible interval of the models, incorporating full systematic uncertainties. Indigo bars on the left indicate the likelihood of the longest CRBG hiatus (most likely 50-90 kyr) occurring at each stratigraphic position. The age model also yields magnetic field reversal and formation boundary ages. The position of the Vantage sedimentary interbed is labelled with “V”. B) Results from (A) for average long-term emplacement rates through the eruptive history of the CRBG, averaged over 20 kyr windows, shown with contours up to 68% credible intervals (CI).

Our age model allows for a more thorough interrogation of the GRB. Interbeds immediately below the first, and above the last, GRB members – both of which were dated through 40Ar/39Ar geochronology – robustly constrain the duration of the GRB to 458+38/-42 kyr. The age model yields estimates for the timing of the magnetic field reversals bracketing the four magnetostratigraphic units of the GRB as well as the preceding and subsequent chrons. Ages for the bottom, and near the top, of the Vantage sedimentary interbed (labeled in purple as V in Figures 2, 3, and 5), the most prominent though not necessarily the longest-lived interbed in the CRBG (Barry et al., 2010), yield a duration of 57+84/-44 kyr.

Our dual-chronometer age model (Figure 5) calculates an average long-term emplacement rate of ~0.3 km3/a for the Steens, Imnaha, and the first half of the GRB. More rapid emplacement of up to ~0.9 km3/a occurs during the Wapshilla Ridge Member, in chron C5Cn.1r (R2). Fluxes slow drastically during the emplacement of the Wanapum Basalt, with rates slowing to ≤0.1 km3/a. Because our ages indicate relatively continuous eruptions, we were interested in determining the timing and duration of the longest possible hiatus within the CRBG, prior to the Saddle Mountains Basalt. We queried each run of our dual-chronometer model, and we find that the longest hiatus was most likely 60-120 kyr duration during early stages of Wanapum Basalt emplacement, and likely coincident with deposition of the Vantage interbed. Since the Vantage interbed is underlain by the Basalt of Museum (Sentinel Bluffs Member, GRB) and overlain by the Basalt of Ginkgo (Frenchman Springs Member, Wanapum Basalt), Vantage deposition was likely coeval with early Wanapum Basalt members (e.g., Eckler Mountain). This result is consistent with our calculated low eruption rate for the Wanapum Basalt.

3.4 Zircon geochemistry and Hf isotopes of CRBG interbeds

At the sample level, CRBG interbed zircons exhibit somewhat variable Hf isotope and trace element compositions but show no clear stratigraphic or age-based trends (Figure 6). For the most part, within individual samples, zircons tend to exhibit similar εHf and rare earth element (REE) ratios (e.g., Lu/Gd). There is no obvious trend of increasing or decreasing εΗf or REE ratios through time or by sample. The εHf and REE ratios of most of the ash samples also overlap with many red bole samples, with no discernible offset based on the observed lithology of these samples. Other zircon trace element concentrations are plotted in Figures 7 after Grimes et al. (2015) to discern the origin of these zircons, and are interpreted in section 4.2.

Figure 6. CRBG zircon geochemistry. Results from TIMS-TEA and hafnium isotope analysis of all interbed zircons dated in the CRBG. Lu/Gd was chosen as a representative rare earth element ratio. A) εHf versus zircon age; depleted mantle εHf value is ~+16, which is outside the range of the plot. B) Lu/Gd versus zircon age. C) Lu/Gd versus εHf. D) Box and whisker plots visualizing the data displayed in A-C, with values grouped by sample, lithology (red bole versus ash layer), and CRBG formation.

Figure 7. CRBG zircon provenance. Tectono-magmatic zircon provenance diagram after Grimes et al. (2015), outlining fields for mid-ocean ridge (MOR type) ocean island and other plume-influenced (OI type) and continental arc type zircons. CRBG zircons analyzed are color-coded by sample, with lithology identified by shape, and are compared to ~100 ka zircons analyzed from Yellowstone volcanics by Stelten et al. (2013).

4. Discussion

4.1 A new dual-chronometer high-precision age model for the CRBG

Using nine 40Ar/39Ar and fourteen U-Pb ages, we construct a dual-chronometer age model for the CRBG, from the Upper Steens through the Wanapum Basalts (Figure 5). Samples young upward stratigraphically in our integrated dataset when compared with both analytical (X) and systematic inter-chronometer (Z) uncertainties, representing a marked improvement in the accuracy and precision of CRBG age data over that obtained in prior work (Baksi, 2013; Barry et al., 2013).

Considered separately (Figure 3), our 40Ar/39Ar and U-Pb age models show excellent agreement in the average long-term CRBG emplacement rate of ~0.3 km3/a through the Steens, Imnaha, and early GRB. In the latter GRB and Wanapum Basalt, the shapes of the eruptive curves differ slightly, though they still overlap. The apparent difference in rates can be explained by the fact that the 40Ar/39Ar age model has only two samples in the interior of the GRB to calibrate effusion rates (in N1 and R2); the other two GRB samples in the model are at the top and bottom contacts of the formation. However, the overlap between our age models in Figure 3 also leaves open the possibility for nearly complete agreement between our techniques, a significant advance in the application of geochronology to LIPs, which we take one step further by integrating both datasets into our dual-chronometer model (Figure 5). Our preferred integrated model captures both the Wapshilla Ridge pulse and the Vantage hiatus, robustly constraining the emplacement history of the CRBG. Ideally, future studies of LIPs undertaking a dual-chronometer approach could be designed from the start for sample coverage evenly spaced throughout the stratigraphy using both techniques, which also provides a check on the reliability of both datasets.

Although our age model suggests an average long-term emplacement rate for the CRBG of 0.2-0.9 km3/a, we recognize the crucial difference between this rate calculation and the timescales of the emplacement of individual lava flows. Studies suggesting that individual CRBG lava flows may have been emplaced in as little as ~1 month (Reidel et al., 2018 & references therein) indicate that much of the eruptive history of the CRBG may have been characterized by pauses in volcanism, that we cannot resolve with the ~10-50 kyr precision of geochronology. Thermal models (Petcovic & Dufek, 2005) combined with thermochronologic data (Karlstrom et al., 2019) have been used to show that feeder dyke segments were active for at most 1-10 years with fluxes of 1-3 km3/yr. Using a magnetic geothermometer, Biasi & Karlstrom (2021) have also shown that dyke segments were active for months to years, with emplacement rates as high as 1-8 km3/day sustained over several years. We agree with Biasi & Karlstrom (2021) that the rapid emplacement of individual eruptions would have had a severe environmental effect that is likely underestimated in climate models thus far.

Our model for average long-term volumetric emplacement rates throughout the duration of the CRBG may be compared to other well-dated LIPs. U-Pb ID-TIMS geochronology shows that the CAMP, Deccan Traps, and Siberian Traps were also all emplaced in less than a million years, with age models suggesting 10-100 kyr-long periods of high eruptive flux interspersed with intervals of reduced effusion (Kasbohm et al., 2021). The Deccan Traps emplacement is shown to occur in four pulses of ≤20 km3/a (Schoene et al., 2019); the Siberian Traps have been modeled to be emplaced as a few pulses lasting 10-100 kyr (Pavlov et al., 2019). In the CAMP, some U-Pb dates from dykes and sills fall during apparent eruption hiatuses (Blackburn et al., 2013; Davies et al., 2017), perhaps indicating that in all LIPs, lulls in eruptions need not be lulls in magmatism. More detailed geochronology is needed to assess whether apparent eruption hiatuses in some LIPs are simply a transition to intrusion-dominated magmatism or regional phenomena and an artifact of incomplete sampling. In contrast to other LIPs with multiple potential hiatuses of ~100 kyr, we model one eruption hiatus during CRBG emplacement that was unlikely to be longer than 60-120 kyr. The gradual, rather than pulsed, emplacement rate of the CRBG, as well as its smaller total volume – less than or equal to the volume of just one Deccan Traps pulse – may partly explain why the CRBG is not associated with the more catastrophic environmental and ecological effects of other LIPs.

4.2 Provenance of CRBG interbed zircons

We obtained hafnium isotopes and trace element concentrations from all CRBG interbed zircons dated for U-Pb geochronology with the goals of discerning zircon provenance and better understanding how these interbeds formed. While zircon trace elements may vary within a single eruptive center (e.g., Claiborne et al., 2018), hafnium isotopes are considered more indicative of source magma composition (e.g., Vervoort & Blichert-Toft, 1999). The cross-plot of Lu/Gd and εHf in Figure 6C shows some grouping of samples of similar or varying ages, which could indicate that each group is sourced from airfall from distinct eruptive centers that may have erupted multiple times. We would expect more uniformly scattering distributions if all red bole zircons were detrital. The offset in εHf values for Vantage zircons in samples CRB1531 and CRB1533 clearly indicate a different source magma from the other CRBG zircons. The geographic position of Vantage west of the rest of the samples, and the lithological interpretation of the interbed as a Cascades volcaniclastic lahar (Reidel & Tolan, 2013b; Tolan et al., 2009) support this interpretation. Apart from these offsets, there is no systematic difference in the chemistry of zircons sourced from red boles or ash layers - zircons from both lithologies overlap, even when erupted at different times (such as CRB1586 and CRB1634). We thereby interpret all zircons from both red bole and ash layer lithologies to be of magmatic rather than detrital origin and suggest that red boles incorporate volcaniclastic airfall sourced from regional volcanism. This interpretation is bolstered by the excellent agreement between our U-Pb ages for interbeds and 40Ar/39Ar ages on adjacent lava flows.

Grimes et al. (2015) interpreted trace element ratios in zircon to aid in the determination of the tectono-magmatic setting that allowed for zircon crystallization. The most diagnostic of these, U/Yb vs Nb/Yb (Figure 7), shows that nearly all CRBG zircons fall in the continental arc-type field. The most parsimonious interpretation for most, if not all U-Pb CRBG interbeds sampled is that they are composed wholly or in part of volcaniclastic material sourced from the Cascades arc, with positive εHf values of these zircons indicative of a source dominated by juvenile, radiogenic mantle-derived material with modest involvement of old lithospheric sources.

4.3 Geologic Time Scale and paleoenvironmental implications

Using our highly resolved CRBG age model, we can probe the connection of the CRBG with the MCO. One obstacle to comparing the timing of the CRBG with the MCO has been the disputed calibration of the Miocene GPTS, for which there have been several proposals. Our new CRBG U-Pb age model gives ages for the onsets of polarity chrons C5Cn.3n through C5Br, with an average uncertainty of ±40 kyr. The new CRBG age model confirms our previous observation (Kasbohm & Schoene, 2018) that magnetic field reversal ages in the CRBG show greater concordance with the astronomically-tuned age model for magnetic field reversals in IODP site U1336 (Kochhann et al., 2016) than with either the prior calibration of the Neogene GPTS in Geologic Timescale 2012 (Hilgen et al., 2012) or other astronomically-tuned GPTS calibrations from Sites 1090 and 154 (Billups et al., 2004; Pälike et al., 2006). This evaluation shows the importance of utilizing both tuning and geochronology to assess the timing of magnetic field reversals and gain a full understanding of sedimentation in tuned sections (e.g., Sahy et al., 2017). An accurate GPTS is essential to anchor the paleoclimate proxy records that document critical events in Earth’s climate history, like the MCO.

A motivating question for our work has been to assess the temporal correlation between the CRBG and the MCO; our new results bolster the correlation documented by Kasbohm & Schoene (2018). Though we lack a sample in the Lower Steens Basalt, given the fairly constant average emplacement rate we present here, it seems unlikely that Steens Basalt eruptions began earlier than 16.9-16.75 Ma (assuming constant emplacement rates of 0.1-0.2 km3/yr). With samples in and above the Roza Member, we show that Wanapum Basalt eruptions mostly concluded by ~15.9 Ma. Meanwhile, a compilation of astronomically tuned records of the Cenozoic suggests that the onset of MCO warming, signified by the decline in benthic foraminiferal δ18O, occurred 17.0-16.9 Ma, and that low δ18O values persisted until ~14.5 Ma (Westerhold et al., 2020). We note that in Sites U1337 (Holbourn et al., 2015) and U1336 (Kochhann et al., 2016), the longest sustained interval of the lowest δ18O values occurs from ~16.9-16.0 Ma, coincident with the main phase of CRBG volcanism. Similarly, a foraminiferal δ11B isotope proxy for MCO atmospheric CO2 concentrations show a brief increase at ~16.7 Ma, followed by sustained elevated levels from ~16.5-15.8 Ma (Sosdian et al., 2018). If the age models for these records are accurate, these patterns permit a connection between an increase in global temperatures, pCO2 and main-phase CRBG magmatism.

A key question remaining in discerning the role of the CRBG in the MCO is the relative order of warming and surface volcanism. Our prior work (Kasbohm & Schoene, 2018) suggested that Steens Basalt emplacement began ~16.7 Ma, 200 kyr after astronomically-tuned calibrations for MCO warming (Westerhold et al., 2020). Our new age model suggests an earlier onset for Steens volcanism, either beginning synchronously with the MCO at 16.9 Ma, or postdating the MCO by ~125 kyr. While a potential offset may seem to suggest another more important driving factor for the MCO than LIP volcanism, a new study modeling subsurface sill emplacement of the CRBG shows that intrusive emplacement would have released enormous quantities of CO2 ~200 kyr prior to surface volcanism, with modeled temperatures during sill emplacement matching those observed during the MCO (Tian & Buck, 2022). Such a mechanism may have allowed for the CRBG to play a causal role in the MCO if the onset of warming occurred prior to volcanism. Further studies that temporally constrain climatic fluctuations through the duration of the MCO, the onset of CRBG volcanism, and initial sill emplacement will allow testing of this possibility.

Our geochronology conclusively shows that all but ~1% of CRBG eruptions had ceased by ~15.9 Ma, and therefore, the CRBG cannot be invoked as a source of CO2 through the entire duration of the MCO. In a carbon cycle modeling study undertaken to discern the connection between the MCO, CRBG, and the concurrent Monterey Carbon Isotope Excursion, Sosdian et al. (2020) suggest that elevated surface ocean dissolved inorganic carbon (DIC) throughout the MCO was caused by continuous eruption of the CRBG over millions of years, citing an outdated age model of CRBG emplacement rates (Hooper et al., 2002). Since our new CRBG age model constrains the timing of the main phase of volcanism to ~16.8-15.9 Ma, with eruptive fluxes at least 10 times that of Hooper et al. (2002), we suggest that another mechanism of sustaining elevated DIC and seawater temperatures through this interval must be invoked after 15.9 Ma. One scenario is that elevated crustal geotherms sustained decarbonation reactions of carbon-rich sediments long after CRBG magmatism ceased, although such sediments are minor relative to the mostly cratonic and volcanic country rock in the eruptive area of the CRBG (Reidel et al., 2013). Another possible mechanism is a potentially slower silicate weathering feedback allowing for sustained warm conditions following the cessation of main phase CRBG volcanism, although a slight increase in δ18O values and decrease in pCO2 estimates after ~15.8 Ma is consistent with a responsive weathering feedback. Both explanations require better data on the magnitude of each effect, especially since they may explain carbon cycle observations during other examples of LIP emplacement and environmental change, such as mass extinction events (e.g., Ruhl & Kürschner, 2011).

4.4 Benefits of a dual-chronometer approach and remaining systematic uncertainties

Our work represents the first time that 40Ar/39Ar ages for CRBG lavas have shown excellent agreement with U-Pb ages, and the first time that these techniques have been not only juxtaposed (as in Baksi, 2022) but fully integrated in a single study to produce an age model for a LIP. After decades of effort to reduce systematic uncertainties in the U-Pb and 40Ar/39Ar geochronometers along with improvements in measurement technologies, we can now obtain high-fidelity datasets that allow for resolution of time distributed within LIPs. Within LIPs no one chronometer can date all geologic processes of interest, but we demonstrate through strategic deployment of a dual-chronometer approach and an interpretive framework built on geologic observations that events can be resolved at the 10 kyr level.

5. Conclusion

Our new high-precision U-Pb and 40Ar/39Ar age model for the CRBG offers a uniquely detailed view of the dynamics of LIP emplacement. Our concordant geochronological data for the CRBG from both techniques demonstrates that 83% of the total volume of the CRBG (bracketed by our Upper Steens 40Ar/39Ar sample, and our U-Pb sample overlying the Roza Member) was emplaced in ~720 ka. The GRB, representing 72% of total CRBG volume, was emplaced over 458+38/-42 kyr. The longest hiatus that occurred during our modeled interval was most likely 60-120 kyr in duration, in the early stages of Wanapum Basalt emplacement. New trace element and hafnium isotope geochemistry from CRBG interbed zircons suggest a magmatic, continental arc provenance for these grains, indicating that interbeds contain volcaniclastic material that record primary eruptive ages. The volumetric emplacement rates of 0.2-0.9 km3/yr calculated through the main phase of CRBG volcanism likely contributed to its differing environmental effects from other LIPs; rather than association with a cataclysmic mass extinction, the CRBG instead was likely partly responsible for establishing the conditions of the MCO. Our work highlights the importance of integrating multiple high-precision chronometers to present the most highly resolved age models possible for LIP emplacement.


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