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A major issue facing our community is that there are biases between chronometers (and laboratories) that have become significant as we interrogate the rock record with ever increasing levels of precision.  Despite much progress there are still major issues with building a timescale with multiple chronometers.  A central issue in the quest for a highly resolved and accurate time scale of Earth history is how directly comparable are ages from different chronometers and different laboratories.  The most widely applied radio-isotopic chronometers for the Geologic Time Scale are U-Pb zircon and 40Ar/39Ar sanidine.  Until relatively recently, there was little overlap in the timescales at which these chronometers were used (Gradstein et al., 2004).  While 40Ar/39Ar was the chronometer of preference for the Cenozoic and into the Mesozoic, U-Pb was most commonly used in ash layers from older strata.  Improvements in methods for determining U-Pb (zircon) dates has led to their application at precisions of 0.2% or better in rocks even younger than a million years (Crowley et al., 2007), and significantly better than 0.1% in some cases (e.g., Bowring et al., 2005), resulting in increased overlap between the U-Pb and 40Ar/39Ar chronometers. These advances have greatly extended the need for cross calibrations of the two chronometers and ultimately seamless integration into the Geologic Time Scale.  Therefore we propose a new and comprehensive initiative to compare high-precision 40Ar/39Ar (sanidine) and U-Pb (zircon) dates on between 10 and 20 volcanic rocks to assess and quantify systematic (and non-systematic) differences in order to facilitate increased inter-comparability.

At the heart of 40Ar/39Ar geochronology is the assumption of a known absolute age of a standard, and all unknowns are referenced to it.  The Fish Canyon sanidine has served as the most common standard in this regard for at least the last decade. Based on astronomical tuning of cyclic Miocene deposits from Morocco, dated by 40Ar/39Ar sanidine, Kuiper et al. (2008) proposed a revised age of Fish Canyon sanidine of 28.201±0.046 Ma.  This refinement shifts the nominal age previously used, (28.02±0.28 Ma; Renne et al., 1998), but not outside the stated uncertainty of the previous estimate.  Importantly, the new estimate has a much-reduced uncertainty, facilitating comparison of U-Pb and 40Ar/39Ar at the 0.2% level. Renne et al. (2010, in press) have proposed a further revision of the Fish Canyon sanidine to 28.305±0.031 Ma, based on a combination of 40Ar/39Ar - U-Pb (from a single laboratory) comparisons, counting based estimates of uncertainty, and a mathematical approach that combines all these constraints.  While the Renne et al. approach is cogent, the implied age of 28.305 Ma for Fish Canyon sanidine presents some clear problems.  It pushes the 40Ar/39Ar age estimates for several important events to values that are significantly older than either U-Pb or astronomical estimates.  For example, the implied 40Ar/39Ar age of the Bishop Tuff is already “too old” compared to astronomical calibrations of the Matuyama-Brunhes geomagnetic reversal (791±4 ka based on 28.201, Kuiper et al., 2008 supplement; compared to 773.2±3 ka, Channell et al., in review; this jumps to 797±4 with 28.305 Ma) and based on U-Pb dating (767.1 ± 0.9 ka, Crowley et al., 2007).  With 28.305, the implied 40Ar/39Ar ages of the Green River ashes (e.g., Smith et al., 2010), Cretaceous-Paleogene boundary (Bowring et al., unpublished), and Carboniferous Fire Clay tonstein (Table 1) would be significantly older than those based on U-Pb.  A central conclusion of the Renne et al. work is that, while this is a promising approach that may lead to improved accuracy of the 40Ar/39Ar chronometer, it is now essential to deal with the question of inter-laboratory biases and systematic biases between the two methods. 

Although the sanidine from the Fish Canyon tuff is the most extensively applied 40Ar/39Ar monitor standard, the reported age for this sanidine standard ranges from 27.51 to 28.305 Ma (Cebula et al., 1986; Renne et al., 1994, 1998, 2010; Villenueve et al., 2000; Lanphere and Baadsgard, 2001; Spell and McDougall, 2003; Kuiper et al., 2004, 2005, 2008; Nomade et al., 2005; Jourdan and Renne, 2007; Lipman and McIntosh 2008; Smith et al., 2010), and there is evidence for complexity in the eruptive system for the Fish Canyon tuff (e.g., Lipman et al., 1997; Schmitz and Bowring, 2001; Bachmann et al., 2007; Charlier et al., 2007; DeVincenzo et al., 2009).  The accepted age and associated uncertainty for Fish Canyon was first suggested to be 27.9±1.2 Ma (referenced to Mmhb age of 519.5 Ma, Cebula et al, 1986; Renne et al. (1994) comment that the Berkeley Geochronology Center estimate, updated to reflect the Samson and Alexander (1987) estimate of 520.4±3.4 Ma for Mmhb is 27.84 Ma).  This estimate was revised to 27.95±0.18 Ma by using the ages of magnetic chron boundaries calibrated to an astronomical time scale (Renne et al., 1994), and was further revised to 28.02±0.28 Ma (Renne et al., 1998, who suggested that a re-evaluation of the decay constants would be in order). The 28.02 Ma value was used as a reference value for approximately the last decade, by agreement within the Ar geochronology community.  However, it was recognized that this estimate led to an apparent bias of close to 1% with U-Pb zircon estimates.  Based on a survey of nuclear physics literature on 40K decay constant uncertainties (Min et al., 2000), comparisons with U-Pb ages (Min et al., 2000; Schmitz and Bowring 2001; Schoene et al., 2006) and astronomical calibration (Kuiper et al., 2008), it has been suggested that the accepted age of the Fish Canyon sanidine monitor standard be revised from the previous estimate of 28.02±0.28 Ma (Renne et al., 1998) to 28.201±0.046 Ma (Kuiper et al., 2008).  This 0.68% revision of the nominal Fish Canyon age largely eliminates much of the apparent discrepancy between 40Ar/39Ar and U-Pb chronometers, in many cases, providing empirical support for the revision (e.g., Smith et al., 2010; Machlus et al., in prep. a, b). 

The true age of this important monitor standard is still in flux.  The newest estimate proposed by Renne et al. (2010, in press) of 28.305±0.031 Ma is significantly older than that proposed by Kuiper et al. (2008), but is approaching the youngest zircon dates reported by Schmitz and Bowring (2001) who acknowledged complex zircons and pre-eruptive residence made their dates too old.   Unpublished work done at MIT indicates the youngest zircon crystals from the Fish Canyon are slightly younger than 28.30 Ma.   The new Renne et al. (2010, in press) date for the Fish Canyon Tuff yields a recalibrated Cretaceous-Paleogene boundary age of 66.236 Ma, older than any previous estimate.  The best U-Pb zircon age estimate for this boundary is 66.08 (Bowring et al., unpublished).  Furthermore, with 28.201, the 40Ar/39Ar sanidine age estimate of the Bishop Tuff is 791 ± 4 or 790 ± 4 ka (Kuiper et al., 2008, supplement), which is significantly older than the weighted mean 206Pb/238U zircon age of 767.1 ± 0.9 ka (Crowley et al., 2007).  Additionally, the Bishop Tuff is several meters above the Matuyama-Brunhes reversal boundary in the western US, and has a normal polarity of magnetization- too old based on the 773.2±3 ka estimate from Pleistocene astronomical calibrations (Channell et al., in review).  Thus, we are left in the unsatisfying position that simply adjusting the age of the sanidine from the Fish Canyon Tuff will not resolve this issue, and a new approach is needed.  We fully understand that there may never be a magic bullet or correction factor but that understanding and quantifying differences between chronometers is of the utmost importance for earth history.
 
It remains important to apply U-Pb, 40Ar/39Ar, and astronomical approaches where possible in order to fully test the comparability.  As suggested by Schoene et al. (2006), tests between the U-Pb and 40Ar/39Ar systems in older rocks is highly desirable because uncertainties due to processes in the magma chamber become a vanishingly small fraction of the total age.  On the other hand, much is to be gained by using the youngest possible rocks as we overlap astronomical models that do not depend on radio-isotopic geochronology.  We also argue that tests between these systems where both are uncomplicated, that is that both systems yield simple age populations, are essential to eliminate interpretation of geological factors as much as possible.  To our knowledge all currently published comparisons of the two methods have to deal with some sort of geological complexity in one or both of the systems (Min et al., 2000; Schmitz and Bowring, 2001; Schoene et al., 2006; Renne et al., 2010).

Initial results on inter-laboratory and inter-method comparisons

Recent inter-calibration experiments of 40Ar/39Ar dating of Alder Creek and Taylor Creek sanidines, using Fish Canyon sanidine as a monitor standard, have revealed inter-laboratory variations; reported values are 1.1734±0.0027 to 1.2032±0.0015 Ma (2.5% range) and 28.199±0.029 to 28.508±0.057 Ma (1.1% range) for Alder Creek and Taylor Creek, respectively, but most labs reported errors of better than 0.5% for Alder Creek and all reported errors better than 0.3% for Taylor Creek (Matt Heizler, personal communication).  It is clear that there is a need for careful tests of what causes the inter-laboratory biases, and that there is a need to find quality minerals that represent the age and compositional range of the materials that are being used for 40Ar/39Ar dating.  It would be particularly desirable to have an older standard that would allow bracketing the age range of the main applications of 40Ar/39Ar ash dating for geologic time scale work.  We suggest that for calibrating the Phanerozoic time scale, it is important to use at least two monitor standards of different age in order to allow internal assessment of the reliability of the age of the unknown.  In our joint LDEO/MIT efforts evaluation of the ca. 315 Ma Fire Clay Tonstein as a potential inter-laboratory standard for 40Ar/39Ar sanidine and U-Pb zircon ages, we co-irradiated with Fish Canyon and Taylor Creek sanidines and GA1550 biotite (Table 1).  While our estimate of Taylor Creek is consistent with that of Renne et al. (1998), our estimate of GA1550 is approximately 0.6% older.  Such an offset is similar to that between the LDEO and Wisconsin estimate of the Fire Clay sanidine, and may partly explain the apparently older 40Ar/39Ar age of the Fire Clay relative to the U-Pb zircon age, and points to the need for further scrutiny of factors that may lead to biases.

Laboratory inter-calibration experiments are valuable exercises, but it is very difficult to achieve a perfect experiment.  Circumstances inevitably get in the way of a timely submission of data, from lab downtime to urgent priorities in the lab to other factors that are out of the control of the people participating in the experiment.  Two inter-laboratory, inter-monitor calibration experiments were conducted between 2004 and 2008.  A wide international cross section of argon labs participated with the logistics of the experiments handled at NM Tech under the direction of Matt Heizler. These EARTHTIME inter-calibration experiments have been discussed at length at EARTHTIME working group meetings and presented at international meetings (Heizler et al., 2005, 2008, 2010; Bowring et al., 2008). The experiments were successful in obtaining good participation and in generating on-going synergies. However, the data revealed a sobering ca. 2% spread among laboratories that all participants agree needs to be better understood in order to achieve the community goal of highly reliable ages from 40Ar/39Ar laboratories at the 0.1% level.  For the first inter-calibration experiment each lab was provided un-irradiated samples of several monitor standards that represent a wide spread of ages and a mix of sanidine, biotite and hornblende.  In this experiment, choice of irradiation map pattern, reactor, reactor position and duration of irradiation were all free parameters left up to the individual laboratories.  When evaluating the results at an EARTHTIME meeting, it was considered that one possibility for the ~2% range of reported values was that there were too many uncontrolled variables in the experiment.  Subsequently a second experiment was conducted on Fish Canyon, Taylor Creek and Alder Creek sanidine monitors.  Matt Heizler packaged a large number of aliquots for a single irradiation and sent irradiated samples to the participating labs. Despite significant reduction for free parameters the range of results remained at the 1-2% level. The NM Tech lab has conducted numerous internal examinations of methodologies, adopted other laboratory procedures, collected data with other laboratory equipment and has yet to identify a simple explanation for their results relative to other facilities. Both Lamont and Rutgers have also conducted additional experiments that may be suggestive of detector nonlinear behavior as a potential cause of dispersion (e.g., Turrin et al., 2010).  The Ar geochronology community is taking these biases seriously, and there are several parallel efforts underway that will lead to further tests and improvement of our results.  One is a roving pipette experiment, led by Brent Turrin, that will allow us all to “smoke from the same pipe” thus eliminating biases associated with extraction line cleanup, irradiation, and gas extraction.

Participation in the EARTHTIME initiative has inspired us to seek the highest possible quality of measurements, and we have made steps to contribute to the community goal of a highly refined time scale for Earth history.  Initial data from two projects are included for example here:

1) Fire Clay tonstein: We have put extensive effort into evaluating the Fire Clay tonstein as a possible sanidine monitor standard as well as a target for inter-calibration of the 40Ar/39Ar and U-Pb systems (Table 1). Tonsteins are kaolinized volcanic ash deposits that occur mainly associated with coal (Spears and Kanaris-Sotiriou, 1979; Bohor and Triplehorn, 1993; Lyons et al., 1994).  They are common in Carboniferous rocks worldwide and are important for regional correlations and geochronology (Hess and Lippolt, 1986; Lyons et al., 2006; Davydov et al., 2010).

The Fire Clay is a Middle Pennsylvanian coal bed of the Hyden Formation in the Breathitt Group (Greb et al., 1999).  The Fire Clay tonstein was described extensively by Rice et al. (1994).  It contains approximately 95% kaolinite and 5% quartz.  Trace minerals include gemmy sanidine, zircon and apatite.  Webster et al. (1995) reported on the elemental composition of melt inclusions, including Th and U concentrations, within the volcanic quartz in the Fire Clay as well as other tonsteins in the Appalachian Basin.  The Fire Clay tonstein was previously dated by 40Ar/39Ar in the Heidelberg lab in four separate step heating experiments as 312.1±2.0 (Hess et al., 1988; Keiser, 1989; Lyons et al., 1992) and by the USGS Reston lab from 7 separate step heated samples as 310.9±0.5 Ma (Kunk and Rice, 1994).  In particular, the Kunk and Rice (1994) data are impressive in that the range of reported plateaus is 310.3±0.5 to 311.4±0.5 for samples taken over a 300 km distance (the ages have not been recalculated because they were not reported relative to Fish Canyon).

Table 1.  Results of LDEO monitor standard/Fire Clay inter-calibration.
figure
(n = number of individual crystals, nFC = number of individual Fish Canyon crystals for J-value. Dates are referenced to Fish Canyon sanidine age of 28.201 (Kuiper et al., 2008).  Published dates from Renne et al. (1998 are corrected using the recalculator from McLean http://www.earth-time.org/phpBB3/viewforum.php?f=4.)

Rice et al. (1994) reported U-Pb dates from large (ca. 1 mg) zircon samples that showed clear signs of inheritance of approximately 1 Ga, and Lyons et al. (2006) reported data for four single crystals.  One of the four crystals gave concordant 206Pb/238U-207Pb/235U dates with a reported 206Pb/238U date of 314.6±0.9 Ma.  The other three crystals showed substantial discordance, with an upper intercept age of approximately 1 Ga.  Although this might seem bleak for using the Fire Clay zircons as an inter-laboratory and inter-method standard, we had already started on the U-Pb zircon analyses on individual crystals before we knew about the Lyons et al. (2006) efforts.  Our initial results were presented at the Melbourne Goldschmidt meeting (Machlus et al., 2006, Goldschmidt) and the GSA meeting (Machlus et al., 2006, GSA), and although we did find some inherited grains, 7 of 11 individual crystals analyzed at that time were concordant and reproducible yielding a weighted mean 206Pb/238U age of 314.77±0.11 Ma, entirely consistent with the single crystal estimate from Lyons et al. (2006).  It appears that selection of acicular crystals with elongate melt inclusions minimizes the inheritance problem.

Brad Singer’s lab has recently measured a sample of the Fire Clay sanidine and obtained a precise age of 313.53 +/- 0.35 Ma (2 sigma, n=24/25) relative to Fish Canyon = 28.201 Ma.  This is 0.5% younger than the LDEO estimate, but the LDEO estimate is very close to the U-Pb zircon estimate.  The LDEO estimate for GA1550 is 0.6% older than the Renne et al. (1998) estimate (recalculated) although the Taylor Creek estimate is consistent.  The proposed work plan will yield extensive cross calibration of the Fire Clay, Taylor Creek and Fish Canyon sanidines, and should thoroughly address this issue.

Inter-laboratory comparison of 40Ar/39Ar dates from Mesozoic ashes from China: The Cretaceous terrestrial Jehol Biota (Chen, 1988; Zhou et al. 2003) and the Jurassic Haifanggou-Lanqi fossils (HLF) (Chang et al., 2009) in East Asia include abundant and varied vertebrates, invertebrates and plants with a simply spectacular degree of preservation. As a Ph.D. student at the Berkeley Geochronology Center, co-PI Chang analyzed eight tuff samples and two basalt samples collected from the Jehol and the HLF related formations near the classic outcrops in western Liaoning, NE China and obtained high-precision 40Ar/39Ar ages (Chang, 2008). She continues to have a strong interest in constraining the chronology of Mesozoic biological evolution and correlations among disparate sections in Asia.  As participants of EARTHTIME, we seek to test the inter-laboratory comparisons between the Ar lab at LDEO and other Ar geochronology laboratories. Thus, we have reanalyzed one Cretaceous ash sample (YX07-4) and one Jurassic ash sample (LQ07-1) at LDEO, that had been previously analyzed by Chang at the Berkeley Geochronology Center.   The results show excellent agreement between these two labs (Figure 1).  When Chang presented the results at the GSA meeting in Portland, Brad Singer offered to measure LQ07-1 at University of Wisconsin, and he has recently produced an age of 161.10±0.27 Ma (2 sigma, n=19/20).

Figure 1: Tests of inter-laboratory consistency for samples YX07-1 and LQ07-1. The LDEO irradiation was 8 hours at the Denver TRIGA reactor and the BGC irradiation was 10 hours at the Oregon State TRIGA reactor.

figure

Collaborators:

LDEO: Sidney Hemming, Su-chin Chang, Kaori Tsukui

Elsewhere: Malka Machlus (Schlumberger), James Crowley (MIT), Samuel Bowring (MIT),Troy Rasbury (Stony Brook), Carl Swisher (Rutgers), Brent Turrin (Rutgers), Klaus Mezger (University of Bern)

 

References:

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Schmitz, M. D., and Bowring, S. A., 2001. U-Pb zircon and titanite systematics of the Fish Canyon Tuff: an assessment of high-precision U-Pb geochronology and its application to young volcanic rocks.  Geochimica et Cosmochimica Acta, 65, 2571-2587.

Schoene, B., Crowley, J. L., Condon, D. J., Schmitz, M. D., and Bowring, S. A., 2006. Reassessing the uranium decay constants for geochronology using ID-TIMS U-Pb data. Gechimica et Cosmochimica Acta 70, 426-445.

Smith, M. E., Carroll, A. R., and Singer, B. S., 2008. Synoptic reconstruction of a major ancient lake system: Eocene Green River Formation, western United States. GSA Bulletin 120, 54-84.

Smith, M. E., Chamberlain, K. R., and Singer, B. S., 2010. Eocene clocks agree: Coeval 40Ar/39Ar, U/Pb and astronomical ages from the Green River Formation. Geology 38, 527-530.

 

 

 

 

 


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