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Research paper

40Ar/39Ar dating of the SP and Bar Ten lava flows AZ, USA: Laying thefoundation for the SPICE cosmogenic nuclide production-ratecalibration project

Cassandra R. Fenton a,*, Darren F. Mark b, Dan N. Barfod b, Samuel Niedermann a,Mirjam M. Goethals a, Finlay M. Stuart b

aHelmholtz-Zentrum Potsdam e Deutsches GeoForschungsZentrum, Telegrafenberg, D-14473 Potsdam, Germanyb Scottish Universities Environmental Research Centre, Rankine Avenue, East Kilbride G75 0QF, UK

a r t i c l e i n f o

Article history:Received 5 June 2012Received in revised form18 January 2013Accepted 22 January 2013Available online 5 February 2013

Keywords:CosmogenicProduction ratesNoble gasesHeliumNeonOlivinePyroxeneQuartzQuaternaryBasaltKeAr40Ar/39ArOSLSingle-grain datingErosion rateUinkaretSan Francisco volcanic field

a b s t r a c t

Precise 40Ar/39Ar age determinations made on basalt groundmass collected from the SP and upper andlower Bar Ten lava flows in the San Francisco and Uinkaret volcanic fields of Arizona, USA, yield ages of72 � 4, 97 � 10, and 123 � 12 ka (2s; relative to Renne et al., 2010, 2011, full external precision),respectively. Previous ages of the SP lava flow include a KeAr age of 70 � 8 ka and OSL ages of 5.5e6 ka.40Ar/39Ar age constraints, relative to the optimization model of Renne et al. (2010, 2011), of 81 � 50 and118 � 64 ka (2s; full external precision) were previously reported for the upper and lower Bar Ten lavaflows, respectively. The new 40Ar/39Ar ages are within uncertainty of previous age constraints, and aremore robust, accurate, and precise. Preliminary cosmogenic 3He and 21Ne production rates from the BarTen flows reported by Fenton et al. (2009) are updated here, to account for the improved quality of the40Ar/39Ar data. The new 40Ar/39Ar age for the SP flow yields cosmogenic 3He and 21Ne production ratesfor pyroxene (119 � 8 and 26.8 � 1.9 at/g/yr; error-weighted mean, 2s uncertainty; Dunai (2000) scalingmethod) that are consistent with production rate values reported throughout the literature. The40Ar/39Ar and cosmogenic 3He and 21Ne data support field observations indicating the SP flow has un-dergone negligible erosion. The SP flow contains co-existing phenocrysts of olivine and pyroxene, as wellas xenocrysts of quartz in a fine-grained groundmass facilitating cross-calibration of cosmogenic pro-duction rates and production-rate (3He, 10Be, 14C, 21Ne, 26Al, and 36Cl). Thus, we propose the SP flow is anexcellent location for a cosmogenic nuclide production-rate calibration site (SPICE: the SP Flow Pro-duction-Rate Inter-Calibration Site for Cosmogenic-Nuclide Evaluations).

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

The EU-funded Marie Curie Research and Training Network“CRONUS-EU”, as well as the American partner network “CRONUS-Earth”, have spent the past 7 years locating sites all over the globewhere production rates of terrestrial cosmogenic nuclides (mainly3He, 10Be, 14C, 21Ne, 26Al, and 36Cl) can be determined withimproved accuracy. It is more important than ever that the pro-duction rates be well-established and accurate (to �5%), not in theleast because exposure-age data have contributed significantly to

the climate-change debate (e.g., Easterbrook et al., 2011), but alsobecause exposure ages are increasingly being compared to otherQuaternary age-dating methods in global-climate studies. In fact,Balco (2011) states, “it is rare to find a study of glacial geology orglacier chronology, or any paleoclimate synthesis that makes use ofsuch studies, that does not involve exposure dating”.

The Quaternary Earth Science community is now soundlydependent upon cosmogenic nuclides, whether they are used astracers or as dating tools. Though the cosmogenic nuclide com-munity has made impressive progress over the past 20 years inevaluating and better understanding cosmogenic nuclide produc-tion rates and systematics, efforts are still required to minimizesystematic uncertainties (to <5%) that stem from cosmogenic-

* Corresponding author.E-mail address: cassiefenton@gmail.com (C.R. Fenton).

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nuclide production-rate estimates and scaling schemes (e.g., Balco,2011). Continued refinement aimed at providing accurate andprecise primary calibrations of production rates is required. TheSPICE project (the SP Flow Production-Rate Inter-Calibration Sitefor Cosmogenic-Nuclide Evaluations), focused at the formallynamed SP lava flow, provides just such an opportunity.

The San Francisco and Uinkaret volcanic fields are located innorthern Arizona along the southern boundary of the ColoradoPlateau, in the southwestern USA (Fig. 1). Both fields containnumerous well-preserved basaltic lava flows. Volcanism in the SanFrancisco volcanic field began 5600 ka, but the majority of volcanicactivity is younger than ca. 780 ka and is basaltic to andesitic incomposition; there has been roughly one eruption every 15 ka since780 ka (Conway et al., 1998). In the Uinkaret volcanic field, flowsrange in age from approximately 3,700 ka to 1 ka (Fenton et al.,2001; Crow et al., 2008). Both the San Francisco and Uinkaret andvolcanic fields are considered still active, with the most recenteruptions occurring w1 ka at the Bonito lava flow (Sunset Crater)and Little Springs flow, respectively (Elson et al., 2002, 2009;Fenton et al., 2001; Fenton and Niedermann, in press).

The SP and Bar Ten lava flows e located in the Uinkaret and SanFrancisco volcanic fields, respectively (Figs. 1e3) e were initiallyselected as potential calibration sites to be studied within theCRONUS-EU research framework. The SP lava flow has a reportedKeAr age of 70 � 8 ka (2s; Baksi, 1974), but recent OSL ages

determined from quartz xenocrysts in the lava suggested the SPflow could be as young as 5.5e6 ka (Rittenour et al., 2012). Volca-nologists and geoscientists who have studied Arizona volcanicshave judged the 70 � 8 ka (KeAr) age of the SP flow to be incon-sistent with the unweathered appearance of the cone and flow(Rittenour et al., 2012). The basaltic cinder cone and flow both havean unweathered geomorphic appearance (Fig. 2), retaining a well-defined rim and crater, and lava-flow levee, blocky aa, and agglu-tinate features. No soils have formed on the flow, and its surface hasawell-developed desert varnish. Concentrations of cosmogenic 3Heand 21Ne (3Hec and 21Nec) in pyroxene from the SP flow support theKeAr age (Fenton and Niedermann, in press).

Field and geochemical data from study of the Bar Ten flow(Fig. 3) showed it was produced by least two eruptive events(Fenton et al., 2009). Previous 40Ar/39Ar age determinations failedto temporally resolve the unit ages, yielding 81 � 50 ka (2s) and118 � 64 ka (2s) for the upper and lower lava flows, respectively.Cosmogenic 3He and 21Ne concentrations in pyroxene extractedfrom the upper flow, in combination with the imprecise chrono-logical constraints, lead to the estimation of a 21Nec production ratein pyroxene (25 � 8 at/g/yr; Fenton et al., 2009).

Here, we present new 40Ar/39Ar age data for the SP lava flow andthe upper and lower Bar Ten flows. Combined with 3Hec and 21Necconcentrations (Fenton et al. (2009); Fenton and Niedermann (inpress) they place constraints on the timing of volcanism and

Fig. 1. (a) Map showing the geographic locations of the Uinkaret and San Francisco volcanic fields in northern Arizona, USA, in which the Bar Ten and SP lava flows are located,respectively. (b) Shaded relief image showing the geographic locations of the South Sheba and SP lava flows in the San Francisco volcanic field (modified from Fenton andNiedermann, in press).

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cosmogenic noble gas production rates in the San Francisco andUinkaret volcanic fields.

2. Methods

Large hand-sized samples were collected from the SP lava flow(Fig. 2) and both the upper and lower Bar Ten flows (Fig. 3) fol-lowing shallow excavation below the flow surface to avoid alter-ation. Samples CFSP01 and CFSP04 were collected from the interiorof the SP flow levee, approximately 40 cm (horizontal) from itsexposed edge and approximately 3e5 m below the top of the levee.CFSP02 was collected from 30 to 40 cm depth from the top ofa pressure ridge.

The samples contained abundant phenocrysts and xenocrysts(e.g., olivine, pyroxene, quartz, feldspar). These can often containsignificant amounts of atmospheric or excess argon. The presenceof excess argon has been previously reported during KeAr dating ofthe young basalts in both the Uinkaret and the San Francisco vol-canic fields of Arizona (Dalrymple and Hamblin, 1998; Fenton et al.,2001; Duffield et al., 2006). Visibly altered areas of the whole-rockbasalt samples were removed using a slow rotating saw. Thesamples were crushed and sieved, and the 125e250 mm fractionwas washed in de-ionized water, and then passed through a mag-netic separator to remove olivine and pyroxene phenocrysts. Thephenocryst-free fraction was leached in 10% HNO3 and washed

(sonicated) in de-ionized water. This process was repeated sixtimes until the water remained clear. Small aggregates of ground-mass, free of phenocrysts (e.g. olivine, pyroxene) and xenocrysts(e.g. quartz, feldspar) were hand-picked under a binocular micro-scope (further procedural details in Mark et al. (2010), Hicks et al.(2012)).

Between 300 and 500 mg of groundmass for each sample wasparceled into small Cu packets, placed within Al discs, and stackedin a flame sealed glass vial for irradiation. International standardAlder Creek sanidine (ACs) with an age of 1.193 � 0.001 Ma(Nomade et al., 2005) was placed within wells adjacent to thesamples and throughout the entire stack to permit detailed char-acterization of the irradiation flux to the samples. The SP Flowsamples were irradiated in the Cd-lined (CLICIT) facility of theOregon State University (USA) TRIGA reactor for 180 s. The Bar TenFlow samples were irradiated in the Cd-lined (RODEO) facility ofthe McMaster reactor for 360 s. The duration between irradiationand analyses was 16 weeks.

ACs grains were analyzed by single crystal total fusion witha focused CO2 laser. Forty crystals from the five different irradiationdiscs (1 sample per disc) were analyzed (n ¼ 200) to facilitate a J-parameter precision of c. 0.1% for each sample. Data were collectedusing a MAP 215-50 noble gas mass spectrometer. Smith et al.(2011) provides a full description of the MAP 215-50 mass spec-trometer system.

Fig. 2. An ASTER image of the SP lava flow and its crater, with sample sites where 40Ar/39Ar samples (CFSP-01, -02, and -04) cosmogenic 3He and 21Ne samples (Fenton andNiedermann, in press) were collected (modified from NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team. http://photojournal.jpl.nasa.gov/catalog/PIA13700).Sample 290406-06 was collected on the lava flow levee near the slope of SP Crater. It cannot be ruled out that the surface of the levee was at one point covered with ash or cindersfrom the adjacent slope/crater.

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The SP and Bar Ten flow samples were step-heated using a resis-tively heated double-vacuum furnace over a temperature range from500 to 1750 �C. Isotope datawere collected using aGVI ARGUSmulti-collector mass spectrometer which has a measured sensitivity of7�10�14mol/volt (Mark et al., 2009). Sampleswere heated for 5minprior to 10 min cleanup. Extracted gases were cleaned using 3 GP50SAES getters (two operated at 450 �C and one at room temperature)and a cold finger maintained at �95 �C using an acetone-solid CO2slush trap. The extraction, clean up and data collection processes areentirely automated. Experiments were conducted over 7 h periodswith hot furnace blanks (500e1750 �C) collected prior to everysample run. Average backgrounds � standard deviations from allfive blank runs (n ¼ 30) were used to correct isotope abundances.Complete raw data reported according to Renne et al. (2009b) aregiven in Supplementary Table 1. Air calibrations were collectedin batches (n ¼ 10) immediately before and after individualexperiments to monitor mass discrimination. Average 40Ar/36Arvalues � standard deviation (300.08 � 0.19, n ¼ 100) was used tocalculate discrimination factor using the power law (Renne et al.,2009a). The atmospheric argon ratios of Lee et al. (2006) that haverecently been independently verified by Mark et al. (2011a) wereused for discrimination factor determinations. The decay constantsof Steiger and Jäger (1977) were utilized.

Berkeley Geochronology Centre software ‘mass spec’ was usedto regress and reduce age data; isotope data are corrected for blank,radioactive decay, mass discrimination and interfering reactions.Data are reported according to Renne et al. (2009a). Plateau cal-culations are based on the acceptance criteria outlined in Market al. (2011b): n ¼ 3 for minimum number of contiguous stepswith no resolvable slope, F ¼ 0.6 (that is>60% of 39Ar released) andP ¼ 0.05 for the probability of fit.

3. 40Ar/39Ar results

40Ar/39Ar data are presented in Figs. 4 and 5. SupplementaryTable 1 provides a summary of all raw 40Ar/39Ar age data. Allsamples yielded statistically robust plateau ages with 100% 39Ar;integrated ages are therefore indistinguishable from plateau ages

(Figs. 4 and 5). Data were cast on isotope correlation plots withinverse isochron ages overlapping with plateau ages and initialtrapped components overlapping with accepted atmospheric argonvalues (Lee et al., 2006).

3.1. SP lava flow

The 3 samples yielded 40Ar/39Ar ages that are indistinguishableat the 1s confidence level (Fig. 4) supporting field observations thatwe dated a single lava flow. The 40Ar/39Ar sample sites are close inspace, but most importantly, they are near three exposure-datingsample sites and the OSL samples sites of Rittenour et al. (2012)(Fig. 2).

Samples CFSP-01 and CFSP-04 were each collected near samplesOSL samples USU-194 and -196 of Rittenour et al. (2012), respec-tively, and near exposure-age samples 290406-06 and AZ01-SP-106of Fenton and Niedermann (in press), respectively. These lattersamples yielded exposure ages (w55 and 40 ka) that were youngerthan the cluster of ages (61e82 ka) yielded by samples 290406-09to -14 (Fig. 2), where 40Ar/39Ar sample CFSP-02 was also collected.The OSL sample USU-195 (Rittenour et al., 2012) was also collectedin this vicinity.

Because the age of the SP flow has been debated (Fenton andNiedermann, in press; Rittenour et al., 2012), our 40Ar/39Ar sam-ple selection was designed to quantify whether the youngerexposure ages of samples 290406-06 and AZ01-SP-106 were rela-ted to (a) a lava flow of a younger age, or (b) erosional/burial pro-cesses. The agreement in all three 40Ar/39Ar ages shows that allthree sample sites are part of the same w70 ka lava flow, and thatthe younger exposure ages are likely due to surface processes, suchas erosion or ash-cover.

CFSP04 is the most potassic of the 3 samples and yielded higherradiogenic 40Ar age steps than CFSP02 and CFSP01, hence the rel-ative improved age precision. The 27 plateau steps were cast ona relative probability diagram and an isotope correlation plot(Fig. 6). A weighted average (�standard error of the mean givenMSWD < 1) for plateau steps defines an age of 71 � 4 ka (2 sigma,MSWD¼ 0.18, n¼ 27, analytical precision only). The ‘global’ inverse

Fig. 3. A digital orthophotograph of the Bar Ten lava flow complex, located in the Uinkaret volcanic field. The lava complex is made up of at least two lava flows (40Ar/39Ar agesshown): the upper and lower Bar Ten flows (Fenton et al., 2009). Points A and B indicate where 40Ar/39Ar samples (051101-02 and 051201-05, respectively) were collected. Twostraight dashed white lines point out a major strand of the normal Hurricane Fault that crosscuts the lower Bar Ten flow, and has up to 10 � 3 m of offset (Fenton et al., 2001).

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isochron that contains all data points defines an age of 71 � 6 ka(analytical precision) and an initial trapped component of298.6 � 0.5. Owing to the limited spread of the low-radiogenic 40Arage steps that require significant extrapolation of the isochron tothe 40Ar/39Ar axis (Fig. 6), but the high-precision trapped compo-nent measurement (36Ar/40Ar axis) that overlaps with accepted40Ar/36Ar values (298.6� 0.3; Lee et al., 2006), we use the weightedaverage of all plateau steps to define the age of the SP flow as71 � 4 ka.

3.2. Bar Ten lava flows

The lower Bar Ten flow (stratigraphically oldest) yielded an40Ar/39Ar age of 122 � 12 ka (analytical precision). The upper Bar

Ten flow (stratigraphically youngest) yielded an 40Ar/39Ar age of96 � 10 ka (analytical precision; Fig. 5). These ages overlap withthe previously reported 40Ar/39Ar ages for the lower and upperflows (samples 051110-02 and 051210-05 from Fenton et al.(2009)), but are significantly more precise (Fig. 7). Furthermore,the two new 40Ar/39Ar ages are mutually exclusive at the 2s-con-fidence level and support petrologic and geochemical evidence(Fenton et al., 2009) showing the Bar Ten flow is composed of (atleast) two different eruptive units. There is a minimum 4 kaduration between eruption of the lower (123e12 ka, 2s confidencelevel) and upper flow (97 þ 10 ka, 2s confidence level). Thecompositional data (39ArK/37ArCa) show the upper Bar Ten flow isslightly more potassic than the lower flow (SupplementaryTable 1; Fig. 5).

Fig. 4. 40Ar/39Ar step-heating spectra and isotope correlation plots for the SP lava flow samples. Data reported against an FCs age of 28.02 Ma and the decay constants of Steiger andJager (1977).

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3.3. 40Ar/39Ar ages relative to optimization model approach

The 40Ar/39Ar method is a relative dating techniquewith all agesreferenced back to a standard of known age. Recently Renne et al.(2010, 2011) published an optimization model that used con-straints from 40K activity, KeAr isotopic data, and pairs of 238Ue206Pb and 40Ar/39Ar data as inputs for estimating the partial decayconstants of 40K and 40Ar*/40K ratio of FCs. This calibration hasreduced systematic uncertainties (i.e., accuracy) in the 40Ar/39Arsystem and has yielded an age for FCs that is indistinguishable at

the 2s confidence level from the astronomically tuned FCs age ofKuiper et al. (2008). To present the most accurate 40Ar/39Ar ages forthe SP and Bar-Ten flows we have recalculated our ages (originallyagainst Steiger and Jäger, 1977; Renne et al., 1998) relative to theoptimization model of Renne et al. (2010, 2011) using the approachof Ellis et al. (2012). Data define an age for the SP flowof 72� 4 ka (2sigma, full external precision) and for the Upper and Lower Bar-Tenflows, 97 � 10 ka and 123 � 12 ka (2 sigma, full external precision),respectively. All 40Ar/39Ar data are presented at the 95% confidenceinterval and include full external uncertainty unless otherwise

Fig. 5. 40Ar/39Ar step-heating spectra and isotope correlation plots for the upper and lower Bar Ten lava flows. Data (analytical precision only) reported against an FCs age of28.02 Ma and the decay constants of Steiger and Jager (1977).

Fig. 6. Relative probability plot (left) showing all age steps from the three 40Ar/39Ar step-heating experiments for the SP flow. The isotope correlation plot (right) also shows all agesteps from the three step-heating experiments for the SP flow. Data (analytical precision only) reported against an FCs age of 28.02 Ma and the decay constants of Steiger andJager(1977).

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stated. These ages are discussed throughout the remainder of themanuscript and are used for calculation of cosmogenic nuclideproduction rates.

4. 3He and 21Ne production rates

We use the 40Ar/39Ar age-determinations, relative to the opti-mization model of Renne et al. (2010, 2011), for the SP (72 � 4 ka)and upper (97 � 10 ka) and lower Bar Ten (123 � 12 ka) flows torecalculate 3Hec and 21Nec production rates that were originallyreported in Fenton et al. (2009) and Fenton and Niedermann (inpress) using the scaling scheme of Dunai (2000). Scaling factorsin this study (Table 1) are calculated in CosmoCalc (http://cosmocalc.googlepages.com; Vermeesch, 2007) using sample-specific geographic data from Table 2.

3Hec and 21Nec are both stable nuclides, and therefore accu-mulate linearly over time, in the absence of burial and/or erosion.The following equation is used to determine production rates at theSP and Bar Ten lava flow sites:

J0 ¼ C0ðtÞt

(1)

where C0(t) is the concentration of either 3Hec or 21Nec at the sur-face of the lava flow (depth¼ 0 cm), t is the 40Ar/39Ar age of the lavaflow of interest, and J0 is the surface production rate. Table 1 liststhe 3Hec or 21Nec production rates, scaled to sea level and highlatitude (SLHL), for individual samples from the SP and upper andlower Bar Ten flows. All production rates are given with 2s unlessstated otherwise.

4.1. SP lava flow (pyroxene and olivine)

SP flow pyroxene yields error-weighted mean SLHL 3Hec and21Nec production rates of 119 � 8 at/g/yr and 26.8 � 1.9 (Table 1;Figs. 8 and 9). The average Mg content (wt. %) for SP flow pyroxene(En47-48) is 9.91% (Fenton and Niedermann, in press). The co-existing olivine in samples 290406-06 and 290406-09 yield 3Heproduction rates of 116 � 10 and 109 � 14 at/g/yr, respectively.Sample 06 yielded a 21Ne production rate of 38.6 � 4.6 at/g/yr(including uncertainty of the 40Ar/39Ar age). Erosion could not beruled out at these two sample sites (Fenton and Niedermann, inpress), however, so these are considered minimum productionrates, even though they agree within error with the error weighted

mean rates and with other rates in the literature. Technical diffi-culties were experienced during the neon analysis of sample -09,thus no 21Ne determination was made.

4.2. Lower and upper Bar Ten flows (olivine and pyroxene)

The new error-weighted mean 3Hec production rates calculatedfor the upper and lower Bar Ten flows are 118 � 13 at/g/yr (in py-roxene) and 107 � 11 at/g/yr (in olivine), respectively (Table 1;Figs. 8 and 9), when corrected for nucleogenic 3He and 4He con-tributions over the past 97 and 123 ka, respectively (SupplementaryTable 2). The corresponding error-weighted mean 21Nec productionrates (1s) are 22.8 � 2.5 (in pyroxene) and 40.9 � 4.1 (in olivine).One olivine sample from the upper Bar Ten lava flow yielded 3Hecand 21Nec production rates of 114�14 at/g/yr and 47.2� 6.1 at/g/yr.These 2s uncertainties include the uncertainty associated with thenew 40Ar/39Ar ages (Table 1). Pyroxene in the upper Bar Ten flowhas an average Mg content of 9.15 wt. % yielding pyroxene with anenstatite composition of En43-44 (Fenton et al., 2009). Forsteritecontents in upper and lower Bar Ten flow olivine are Fo82 and Fo80-84, with corresponding Mg contents of 25.96 and 25.61 wt.%,respectively.

Fenton et al. (2001) report an 40Ar/39Ar age of 190 � 390 ka fora whole-rock sample (97-AZ-329-BT) that was collected from thelower Bar Ten flow of Fenton et al. (2009). The very large uncer-tainty associated with the age was likely a result of poor sampleselection, as well as from alteration of glass in the groundmass.Samples 051110-02 and 051210-05 collected from the lower andupper Bar Ten flows, respectively, were used in both the study ofFenton et al. (2009) and this present one. Improved sample prep-aration (i.e., complete removal of phenocrysts) as well as the use ofstate-of-the art technology at SUERC are the likeliest reasons forimproved uncertainties.

5. Discussion

5.1. Age of the SP flow

The age of the SP lava flow has been under debate since Baksi(1974) dated two 100-mg whole rock, which yielded a KeAr iso-chron of 70 � 8 ka. Because the KeAr technique does not allow forthe correction of excess Ar, the 70 ka agemay be older than the trueage. Recently, Rittenour et al. (2012) reported single-grain SAR OSLages of 5.5e6 ka for quartz xenocrysts present in the SP flow, whilea study by Fenton and Niedermann (in press) showed that the SPflow has an average exposure age of 70� 7 ka based on cosmogenic3He and 21Ne measurements in pyroxene. The present study pro-vides 40Ar/39Ar ages that are in excellent agreement with the KeArand exposure-age data for the SP flow, thus adding to the number ofindependent dating techniques (i.e., KeAr, 40Ar/39Ar, 3Hec, and21Nec) that indicate the SP flow is w70 ka.

There are several possible reasons why the OSL ages are youngerthan the 40Ar/39Ar ages presented here. Rittenour et al. (2012)acknowledge that volcanic quartz is often unsuitable for OSL dat-ing (e.g., Fattahi and Stokes, 2003; Tsukamoto et al., 2003), and thatquartz extracted directly from plutonic and sedimentary rocks isweakly luminescent (Sawakuchi et al., 2011). While the SAR pro-tocol has been shown to be robust for OSL samples dominated bythe fast decaying component, it is not suited for samples withsignificant medium- and slow-decaying components (Murray andWintle, 2003; Jain et al., 2003). Initial analysis of SP Crater quartzxenocrysts generally produced falling De(t) plots and significantrecuperation, signaling a potential for age underestimation(Rittenour et al., 2012), though Rittenour et al. (2012) also state thatanalyses where the cut heat was increased to 220 �C and a 280NC

Fig. 7. Summary of previous and current 40Ar/39Ar ages for the upper and lower BarTen flows. SUERC data show the upper and lower flows to be temporally distinct. Data(analytical precision only) reported against an FCs age of 28.02 Ma and the decayconstants of Steiger and Jager (1977).

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Table 1Cosmogenic 3He and 21Ne concentrations measured in olivine and pyroxene separates obtained from the SP lava flow in the San Francisco volcanic field and the upper and lower Bar Ten flows in the Uinkaret volcanic field innorthern Arizona, USA. Cosmogenic 3He and 21Ne concentrations are scaled to sea-level and high-latitude using the scaling method of Dunai (2000) as calculated in CosmoCalc (Vermeesch, 2007). Concentrations of cosmogenic3He and 21Ne were originally reported in Fenton et al. (2009) and Fenton and Niedermann (in press), and are here corrected for total shielding and nucleogenic/radiogenic helium contributions according to new 39Ar/40Ar ages(Supplementary Table 2).

Location/Sample Phasea CosmoCalcDunai (2000)scaling factor

40Ar/39Arages of lavaflows (ka)

Uncertainty(2s, ka)

3Hecb

(106

atoms/g)

Uncertainty(2s, 106

atoms/g)c

21Necb

(106

atoms/g)

Uncertainty(2s, 106

atoms/g) c

3Hec productionrate (at/g/yr)

Uncertainty(2s, at/g/yr)

21Nec productionrate (at/g/yr)

Uncertainty(2s, at/g/yr)

SP lava flow290406-06d ol 3.62 72 4 8.4 0.7 2.78 0.33 116d 10 38.6d 4.6

px 3.62 72 4 7.3 0.8 1.57 0.21 101d 11 21.9d 3.0290406-09 ol 3.52 72 4 7.8 1.0 e e 109 14 e e

px 3.52 72 4 8.1 0.6 2.04 0.17 112 8 28.3 2.4290406-11d px 3.52 72 4 7.0 0.9 1.78 0.34 98d 13 24.7d 4.7290406-13 px 3.52 72 4 8.7 0.7 1.87 0.19 121 10 26.0 2.7290406-14 px 3.52 72 4 8.8 0.8 1.60 0.18 122 11 22.2 2.5290406-16 px 3.34 72 4 8.9 0.7 2.13 0.15 124 10 29.5 2.1290406-18 px 3.36 72 4 9.1 0.8 1.89 0.20 127 11 26.0 2.7

Error-weighted mean 119 8e 26.8 1.9e

Upper Bar Ten flow250406-17 px 3.17 97 10 11.9 0.5 2.38 0.1 123 6 24.5 1.4250406-15 px 3.43 97 10 10.4 0.8 2.29 0.1 107 9 23.6 2.6250406-16 ol 3.43 97 10 11.1 0.7 4.58 0.1 114 14e 47.2 6.1e

px 3.43 97 10 11.5 0.8 2.23 0.1 119 8 23.0 1.6250406-13 px 3.52 97 10 10.8 0.5 2.07 0.1 112 5 21.3 1.4

Error-weighted mean (for pyroxene) 118 13e 22.8 2.5e

Lower Bar Ten flow240406-10 ol 2.10 123 12 e e 4.80 0.31 e e 39.0 2.5051110-01 ol 2.16 123 12 13.6 0.6 4.95 0.38 111 5 40.2 3.1051110-03 ol 2.16 123 12 e e 5.13 0.30 e e 41.7 2.4240406-06 ol 2.17 123 12 12.0 0.6 5.14 0.35 98 5 41.8 2.8240406-08 ol 2.17 123 12 13.8 0.6 4.77 0.35 112 5 38.5 2.8240406-09 ol 2.17 123 12 e e 5.12 0.33 e e 41.6 2.7240406-11 ol 2.24 123 12 14.3 0.7 5.22 0.28 116 6 42.4 2.3260406-20 ol 2.44 123 12 12.2 0.6 5.05 0.31 100 5 41.1 2.5

Error-weighted mean 107 11e 40.9 4.1e

Note: ‘e’ indicates no available data. All samples were analyzed at the GFZ noble gas laboratory. Uncertainties (2s) associated with individual production rates do not include the uncertainty of the 40Ar/39Ar age. See footnote e.a ol ¼ olivine, px ¼ pyroxene.b 3Hec and 21Nec atom concentrations are corrected to the surface of each sample (selfeshielding) and for topographic shielding. Values are scaled to sea-level, high latitude (SLHL) using the time-independent scaling method

of Dunai (2000) with scaling factors calculated by CosmoCalc (Vermeesch, 2007).c Analytical uncertainty (2s) includes precision of mass spectrometer sensitivity and corrections for blanks, mass discrimination and isobaric interferences.d Sample was not included in the error-weighted mean production rates, because erosion or ash cover could not be ruled out (Fenton and Niedermann, in press).e Error-weighted means (in bold) have uncertainties (2s) that include the analytical errors reported for 3Hec and 21Nec atom concentrations as well as the uncertainties associated with the 40Ar/39Ar age of the lava flow of

interest.

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bleach was inserted at the end of each SAR cycle produced stable toincreasing De(t) plots with no increase in recuperation. From thesedata, Rittenour et al. (2012) conclude that unstable medium to slowcomponents are largely removed by pre-treatment steps prior tonumerically isolating the fast OSL signal component. Despite thecautious pre-treatment steps, it appears that the SAR OSL ages forSP flow quartz underestimate the eruption age by an order ofmagnitude.

Experimental data have shown that the luminescence prop-erties of quartz are highly variable with geological source and varyeven at a grain-to-grain level within a sediment (Preusser et al.,2009); a variety of defects in quartz that are either intrinsic (e.g.,Si and O vacancies) or related to impurity atoms (e.g., Al or Ti)causes complex luminescence signals. Problematic luminescenceproperties in quartz can arise from low and changing luminescencesensitivity, thermal transfer of trapped electrons, thermal insta-bility of the trapped electrons, and low saturation dose, thusmaking quartz problematic, or even unsuitable, for certain datingapplications (Preusser et al., 2009). Steffen et al. (2009) demon-strated that OSL from quartz extracts yielded ages younger thanthose yielded by IRSL analysis of feldspar from the same sediment,despite samplesmeeting standard dose recovery (DRT) and preheatplateau (PPT) tests.

Though the OSL ages of the SP flow do not appear to provide anaccurate determination of the SP flow eruption age, they mayprovide information about fading rates of quartz in volcanic rockswith further study.

5.2. 3Hec production rates

Goehring et al. (2010) report 3He production rates that com-bine all published 3He production-rate determinations in botholivine and pyroxene (Kurz et al., 1990; Poreda and Cerling, 1992;Cerling and Craig, 1994; Licciardi et al., 1999; Dunai and Wijbrans,2000; Ackert et al., 2003; Blard et al., 2006; Licciardi et al., 2006).Of the 113 samples in this dataset (Fig. 10), 78% of the productionrates are calibrated at locales with independent ages that are lessthan 25 ka (Fig. 10b). The remaining samples (n ¼ 25; from Dunai

Table 2Information on sampling locations and sample types collected from the SP, upper Bar Ten, and lower Bar Ten lava flows in the San Francisco and Uinkaret volcanic fields innorthern Arizona, USA. Original data were reported in Fenton et al. (2009) and Fenton and Niedermann (in press).

Location/Sample Sample type Latitude (�N) Longitude (�W) Elevation (km) Sample thickness (cm) Total shielding factora

SP lava flow290406-06 Scoriaceous pahoehoe (flow levee) 35.5877 111.6342 1.884 4 0.955290406-09 Pressure ridge 35.5903 111.6354 1.845 6 0.944290406-11 Pressure ridge 35.5904 111.6353 1.845 6 0.944290406-13 Pressure ridge 35.5905 111.6353 1.845 4 0.944290406-14 Pressure ridge 35.5904 111.6352 1.845 6 0.944290406-16 Pressure ridge 35.6292 111.6036 1.773 4 0.944290406-18 Pressure ridge 35.6304 111.6046 1.782 5 0.963AZ01-SP-106 Flow levee 35.5905 111.6361 1.848 4 0.955Upper Bar Ten flow250406-17 Pahoehoe/tumulus 36.2382 113.1938 1.680 6 0.947250406-15 Pahoehoe/tumulus 36.2408 113.1904 1.782 4 0.964250406-16 Pahoehoe/tumulus 36.2408 113.1904 1.782 6 0.947250406-16 Pahoehoe/tumulus 36.2408 113.1904 1.782 6 0.947250406-13 Pahoehoe/tumulus 36.2417 113.1889 1.818 6 0.948Lower Bar Ten flow240406-10 Pahoehoe/tumulus 36.2167 113.2384 1.148 4 0.963051110-01 Pressure ridge 36.2279 113.2391 1.183 4 0.963051110-03 Pressure ridge 36.2279 113.2391 1.183 4 0.963240406-06 Pressure ridge 36.2244 113.2357 1.189 4 0.964240406-08 Pressure ridge 36.2244 113.2357 1.189 4 0.964240406-09 Pressure ridge 36.2244 113.2357 1.189 4 0.964240406-11 Pressure ridge 36.2314 113.2268 1.232 4 0.964260406-20 Pressure ridge 36.2307 113.2145 1.338 4 0.964

a The total shielding factor includes corrections for sample depth (self-shielding) and topographic shielding (total shielding factor ¼ 1.0 equates to no shielding correction).

Fig. 8. Plot showing cosmogenic noble gas production rates as derived in this studyfrom individual SP, upper and lower Bar Ten samples; production rates for samplescollected from the South Sheba flow, also located in the San Francisco volcanic field(Fenton and Niedermann, in press) are included for comparison. It is likely that thelower Bar Ten lava flow yields a minimum cosmogenic 3He production rate in olivine,due to the effects of faulting.

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and Wijbrans, 2000; Ackert et al., 2003; Blard et al., 2006) haveindependent ages ranging from 23 to 281 ka. The SP and upperand lower Bar Ten flows erupted during this latter time period;however, samples in this subset were collected from locations inthe Canary Islands, southern South America, and Europe(Fig. 10a), whereas our study site is located in the southwesternUSA. Production rates of cosmogenic nuclides are dependentboth on geographic location and time (Gosse and Phillips, 2001).When production rates from this subset (with independent agesof 23e281 ka) are scaled to SLHL with the scaling method Dunai(2000) in CosmoCalc (Vermeesch, 2007), the 3He productionrate for olivine þ pyroxene is 127 � 26 at/g/yr (arithmeticmean � standard deviation). If samples that were collected onlyin western North America (n ¼ 41; Poreda and Cerling, 1992;Cerling and Craig, 1994; Licciardi et al., 1999) are selected fromthe whole dataset of Goehring et al. (2010), the arithmeticalmean 3He production rate with standard deviation is 109 � 15 at/g/yr, however all samples in this subset have independent ages<20 ka (Fig. 10). In either case, the new 3Hec production rates inpyroxene from the SP flow (119 � 8 at/g/yr; � 6.7% (2s)) and inolivine and pyroxene from the upper and lower Bar Ten flows(118 � 13 and 107 � 11 at/g/yr, respectively) are in good agree-ment (overlapping within 2s with each other), with productionrates from each subset, and with the combined global productionrates in olivine and pyroxene (Goehring et al., 2010) (Figs. 9 and10; Table 1).

Though it has been suggested that pyroxene should have lowerproduction rates than those in olivine, due to the lower Mg contentof pyroxene (Masarik and Reedy, 1996; Masarik, 2002), Goehringet al. (2010) report no statistically significant differences between

production rates in 3He in pyroxene and those in olivine, even if theminerals are co-existing. Furthermore, neither the findings ofFenton et al. (2009) nor of Fenton and Niedermann (in press) showany correlation between the Mg content of individual olivine andpyroxene samples and their corresponding 3He production rates(Figs. 9 and 11). There exists only a difference in 21Nec productionrates between olivine and pyroxene (Fig. 11c and d); olivine con-tains moreMg and has higher production rates than does pyroxene.Foeken et al. (in press) report a small discrepancy between 3Heproduction rates in olivine and pyroxene, where the productionrate in pyroxene (98.8 � 3.5 at/g/yr; 7.8 wt.% Mg; Dunai (2000)scaling method) is 7% lower than that of co-existing olivine(108.9 � 8.6 at/g/yr; 27.0 wt. % Mg); however, Foeken et al. (inpress) do not correct for nucleogenic 3He and 4He contributionsin their samples.

5.3. 21Ne production rates

Aside from a 21Ne production rate reported in Fenton et al.(2009), there is only one reported production rate of 21Ne in oli-vine (45 � 4; Poreda and Cerling, 1992), and only three in pyroxene(ranging from 23 � 3 to 34 � 3 at/g/yr; Poreda and Cerling, 1992;Amidon et al., 2009; Fenton et al., 2009). The new 21Nec productionrate in SP flow pyroxene is 26.8 � 1.9 at/g/yr (�7.1%; 2s), whichagrees within uncertainty with these values, as do 21Nec productionrates determined for olivine (47.2 � 6.1 at/g/yr) and pyroxene(22.8 � 2.5 at/g/yr) in the upper Bar Ten flow. The proposed 21Neproduction-rate estimate initially reported for pyroxene from theupper Bar Ten flow is 25 � 8 at/g/yr (Fenton et al., 2009). This es-timate was based on the initial 40Ar/39Ar age of 051210-05

Fig. 9. Error-weighted mean production rates of 3Hec and 21Nec in olivine and pyroxene at the SP and upper and lower Bar Ten lava flows. Uncertainties include uncertainties of40Ar/39Ar measurements. Olivine data for the SP flow are based on two samples (290406-06 and -09) represented by “Ol (-06)” and “Ol (-09), respectively; sample -09 has no 21Necdetermination. Though sample 290406-06 has likely been affected by erosion and/or burial, the data are included in this diagram for comparison. Olivine data for the upper Bar Tenflow is based on only one sample. Lower Bar Ten flow production rates may be affected by erosion due to faulting. The gray rectangle with the large black arrow illustrates theaverage 3Hec production rate and its standard deviation determined from all calibration sites in Goehring et al. (2010) with independent ages are older than 25 ka. The smallerrectangular (with dashed lines) shows the average 3Hec production rate and its standard deviation for calibration sites (from Goehring et al., 2010) in western North America.Average 21Nec production rates in olivine and pyroxene are shown by the light gray bars in the lower part of the graph. See text for details.

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(80 � 50 ka). The 21Nec production rate for the upper Bar Ten flow(22.8� 2.5 at/g/yr) is recalculated here using the new, more precise40Ar/39Ar data (97 � 10 ka). Though both rates agree, the originalproduction rate estimate (25 � 8 at/g/yr) is far less precise andshould not be included in the growing database of world-wideterrestrial cosmogenic nuclide production rates.

Olivine from the lower Bar Ten lava flow yielded a nominallylower 21Nec production rate 40.9 � 4.1 at/g/yr, and it agrees withvalues in the literature within the 95% confidence interval (Figs. 8and 9; Table 1). The range of production rates reported in ourstudy, in Poreda and Cerling (1992), and in Amidon et al. (2009),like that of 3He production rates, may be a result of surface pro-cesses (e.g., erosion, burial, etc.), fluctuations in the earth’s mag-netic field, and/or interlaboratory differences.

5.4. Geologic factors: erosion, burial, and tectonic activity

Though the upper and lower Bar Ten flows have 3Hec or 21Necproduction rates that overlap within uncertainty with the pro-duction rate for the SP flow, the lower Bar Ten flow may have beenaffected by erosion or block-removal from tectonic activity. Erosionrates of 0.07e0.14 cm/ka are calculated here using the new 3Hecproduction rate of 119 � 8 at/g/yr determined for the SP flow, an

attenuation coefficient 160 g/cm3, and a rock density of 3.0 g/cm3.We use the SP flow production rate because the SP flow is close inboth proximity and eruption age to the Bar Ten lava flows. Erosionrates at the Bar Ten flows indicate that 17 and 7 cm of surfaceerosion is required to produce the average measured cosmogenicnuclide concentrations in the lower and upper Bar Ten lava flows,respectively. Such rates of erosion are slightly lower than thosereported for a 60e70 ka basalt flow in New Mexico (0.17e0.5 cm/ka; Sims et al., 2007).

The Bar Ten flow complex is west of a long chain of cindercones, responsible for the vast volumes of lava erupted in theUinkaret volcanic field. The complex has also been ruptured by thestill-active Hurricane Fault (Fig. 2; Fenton et al., 2001, 2009). Ashcover from eruptive centers and/or bits of rock being shaken freeduring ground-rupturing earthquakes could result in temporaryburial or enhanced erosion that may explain the slightly lowercosmogenic nuclide concentrations in the Bar Ten lava flows. It ispossible that decimeter-sized blocks of basalt could have beenshaken free from the sample sites on the lower Bar Ten flow(Fig. 12a). Erosion likely has some effect on the concentrations ofcosmogenic nuclides in upper Bar Ten lava flow sites (Fig. 12b andc), however, it seems implausible, given the field evidence (e.g.,intact aa and pahoehoe structures) that 7 cm of the lava flow

Fig. 10. 3Hec production-rate data versus independent age control at production-rate calibration sites, as summarized in Goehring et al. (2010). The dark grey bar illustrates theaverage production rate and its standard deviation for all samples in this dataset that have independent ages older than 25 ka. The light grey, transparent bar shows the averageproduction rate and its standard deviation for all samples collected in western North America.

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surface has been removed at sample sites. Interestingly, field siteson both the lower Bar Ten and SP lava flows have very similargeomorphic appearances, however, cosmogenic concentrations atthe SP flow indicate negligible erosion, whereas 3Hec concentra-tions recorded in lower Bar Ten flow samples indicate the possi-bility of substantial erosion. The SP flow is not ruptured by anactive fault.

5.5. Geomagnetic field strength

Fluctuations in the earth’s magnetic field and the effects oncosmogenic production rates are another factor requiring consid-eration. Between 100 and 150 ka, the earth’s magnetic field wasweaker than it was during the period between 50 and 100 ka(Guyodo and Valet, 1999; Gosse and Phillips, 2001; Carcaillet et al.,2004). Because the strength of the magnetic field partially controlscosmogenic production rates, this would mean that cosmogenicproduction rates should be higher at 100e150 ka than 50e100 ka.Production rates in this study are integrated over the past 72 ka,97, and 123 ka. In theory, if a weakened magnetic field had anyeffect on the production rates determined for the Bar Ten lava flows(integrated over the past 97 and 123 ka), they would be relativelyhigher than the production rate determinations at the SP flow site,integrated over the past 72 ka. Yet, this is not the case (Figs. 8 and

10). Furthermore, Fenton and Niedermann (in press) report 3Hecand 21Nec production rates in the South Sheba flow, located in theSan Francisco volcanic field, of 133 � 27 and 51 � 11 at/g/yr (inolivine) and 131 � 41 and 28 � 7 at/g/yr (in pyroxene). Theseproduction rates (arithmetical means and standard deviations) arebased on an 40Ar/39Ar age of 175� 36 ka (Fig. 8) and include its 22%uncertainty. The South Sheba flow has undergone minimal erosion,where these production-rate samples were collected, based on fieldevidence and cosmogenic nuclide data (Fenton and Niedermann, inpress). These rates agree well with production-rate determinationsat the SP flow and with rates published in Goehring et al. (2010), inPoreda and Cerling (1992), and in Amidon et al. (2009). It appearsthat production rates in Arizona integrated over the past 175 ka and120 ka are similar to those values integrated over the past 70 ka(Figs. 8 and 9).

The arithmetical mean and standard deviation for the entiredataset of Goehring et al. (2010), regardless of independent age orgeographic location, is 127 � 37 at/g/yr (scaled with Dunai, 2000).Though it is possible that variations in production rates may resultfrom differences in surface processes, such as erosion or burial, orfrom temporal fluctuations in the earth’s magnetic field, it is pos-sible that variations could be explained by inter-laboratory differ-ences in noble-gas measurements. For example, Vermeesch et al.(submitted for publication) report a 7.1% over-dispersion of

Fig. 11. Plots showing SLHL production rates versus Mg content (wt. %) in olivine and pyroxene extracted from the SP, Sheba, and upper Bar Ten lava flows. All cosmogenic noble gasmeasurements were made at the GeoForschungsZentrum-Potsdam.

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cosmogenic 21Ne measurements made on the CREU-1 naturalquartz standard when comparing external reproducibility betweenparticipating laboratories. Furthermore, in a recent Ïround-robinÓexercise conducted by Merchel et al. (2012) indicates that thecosmogenic 10Be value of any given sample can vary from 6 to 25%depending on which AMS laboratory makes the measurement;similar discrepancies occur in cosmogenic 36Cl determinations(Merchel et al., 2011). Such an interlaboratory comparison is stilllacking for cosmogenic 3He.

6. Conclusions

In summary, the upper and lower lava flows within the Bar Tenlava complex have 40Ar/39Ar ages of 97 � 10 ka and 123 � 12 ka,respectively. These ages do not overlap at the 2s-confidence leveland confirm that the flows are distinct, as originally proposed byFenton et al. (2009). These new and more precise 40Ar/39Ar agesindicate that both the upper and the lower Bar Ten flows haveundergone some erosion, and that erosion is likely a factor more

Fig. 12. Photographs of cosmogenic nuclide samples sites at the (a and b) upper Bar Ten, (c) lower Bar Ten, and (d and e) SP lava flows [modified from Fenton et al. (2009) andFenton and Niedermann (in press)].

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greatly affecting the production rate determinations for olivine inthe lower flow. Production rates determined for both the upper andlower Bar Ten flows agree within uncertainty with production ratesdetermined for the SP and South Sheba lava flows, also located inArizona. The rates also agree well with those in the literature(Fig. 10). The original 21Nec production rate estimate (25 � 8 at/g/yr) in upper Bar Ten flow pyroxene is inaccurate and should not beincluded in the list of world-wide terrestrial cosmogenic nuclideproduction rates.

Based on three new 40Ar/39Ar measurements, the age of the SPflow is 72 � 4 ka. 3Hec and 21Nec concentrations in pyroxeneextracted from the SP flow yield error-weighted mean productionrates (119 � 8 and 26.8 � 1.9 at/g/yr), based on the new 40Ar/39Arage. These rates are consistent with production rates in the liter-ature, suggesting the flow has likely undergone negligible erosionand/or burial at most sample sites. Production rates integrated overthe past 72 ka and 175 ka at the SP and South Sheba flows areessentially the same, and they agree well with average productionrates yielded by the global dataset of Goehring et al. (2010 andauthors therein). This indicates that there is little effect on cos-mogenic nuclide production rates in Arizona due to fluctuations inthe Earth’s magnetic field between 0 and 175 ka. Alternatively, thegeomagnetic correction over the past 72 ka is the same as thecorrection over the past 175 ka for this region of North America.

Because of the robust independent age control provided bythese new precise 40Ar/39Ar ages, the SP flow can and will be usedas the primary calibration site for the SPICE project. In theory, thereshould be essentially no “lag time” between the 40Ar/39Ar eruptionage of the SP flow and the point in time when cosmogenic nuclideproduction in SP flow minerals began. This is in stark contrast to“lag times” that may occur at calibration sites reliant on radio-carbon dating, for example. Production rates and production-rateratios of multiple cosmogenic nuclides (3He, 10Be, 14C, 21Ne, 26Al,and 36Cl) will be cross-calibrated in co-existing olivine, pyroxene,quartz, and whole-rock basalt sampled from the SP flow. The SPICEproject will provide the first study where production rates of themost commonly used terrestrial cosmogenic nuclides are cross-calibrated at one calibration site, and whose rates are based onprecise 40Ar/39Ar age control. We aim to substantially improve theaccuracy of cosmogenic nuclide production rates.

Acknowledgments

We gratefully acknowledge field, technical, and/or laboratorysupport from Luigia Di Nicola, Jim Imlach, Wendell Duffield, SimonMerrall, Michael Ort. We wish to thank two anonymous reviewersfor their comments, which greatly improved the paper. This studywas partly funded by the EU Marie Curie Research and TrainingNetwork “CRONUS-EU” (RTN Project Reference 511927).

Editorial handling by: D. Bourlès

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.quageo.2013.01.007.

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