Dating young eruptions by (U-Th)/He on xenolithic zircons

Madalyn S. Blondes* Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, Peter W. Reiners† New Haven, Connecticut 06511, USA Benjamin R. Edwards* Department of Geology, Dickinson College, James Center, Carlisle, Pennsylvania 17013, USA Adrian Biscontini*

ABSTRACT young and/or low-K samples often have large Accurate ages for young (e.g., Pleistocene) volcanic eruptions are important for geomorphic, errors due to the small proportion of radiogenic tectonic, climatic, and hazard studies. Existing techniques can be time-consuming and expen- Ar to total Ar (McDougall and Harrison, 1999). sive when many ages are needed, and in the case of K/Ar and 40Ar/ 39Ar dating, extraneous Ar In contrast, ZHe can be performed relatively often can limit precision, especially for continental erupted through old lithosphere. rapidly on replicate single grains, requires no We present a new technique for dating young basaltic eruptions by (U-Th)/He dating of zir- irradiation, and the greater diffusivity and lower cons (ZHe) from crustal xenoliths. Single-crystal ZHe dates generally have lower precision atmospheric abundance of He may lead to a than typical 40Ar/ 39Ar dates, but can be determined relatively easily on multiple replicate grain lower inherited daughter abundance. Because aliquots. We dated zircons from xenoliths from four volcanic centers in western North Amer- inherited He is likely to be less of a problem ica: , British Columbia (157 ± 3.5 [2.2%] ka weighted 95% confi dence than inherited Ar, ZHe ages should be younger interval [CI], mean square of weighted deviates [MSWD] = 1.7) and , Alaska than Ar ages in these samples. (176 ± 16 [8.9%] ka, MSWD = 13), in the northern Cordilleran volcanic province, and Fish ZHe dating is typically applied to thermo- Springs (273 ± 23 [8.6%], MSWD = 43) and Oak Creek (179 ± 8.1 [4.5%] ka, MSWD = 12), in chronologic studies of tectonic and erosional the Big Pine , California. All ZHe ages are either equivalent to or younger than exhumation, where it has a closure temperature previously determined K/Ar or 40Ar/ 39Ar ages, indicating the possibility of inherited 40Ar in (for typical crystal sizes and assuming a cooling some of the previous measurements. Zircons from upper crustal xenoliths in the Oak Creek rate of ~10 °C/m.y.) of ~180 °C (Reiners et al., and Fish Springs vents show poorer reproducibility and multiple apparent age distribution 2004; Reiners, 2005; Stockli, 2005). It has also peaks, consistent with either intracrystalline U-Th zonation or <99.99% He degassing (assum- been used for geochronology, to date volcanic ing ca. 100 Ma pre-entrainment ZHe ages) of some zircons during magmatic entrainment. rocks and resetting from intrusions and sub surface Removal of clear outliers in the older age-distribution peaks of the upper crustal xenoliths, coal fi res (Farley et al., 2002; Tagami et al., 2003; most of which have extremely high U compared to other zircons of the same xenolith, improve Heffern et al., 2007). In rare cases, reproducibil- the reproducibilities of Fish Springs to 4.7% (95% CI, MSWD = 4.8) and Oak Creek to 3.4% ity of ZHe ages on replicate single-grain analyses (95% CI, MSWD = 6.2). Coupled thermal and He diffusion modeling using appropriate xeno- approaches that of analytical precision (<4%, 2σ) lith sizes and magma temperatures and assuming published diffusion kinetics for zircon indi- (e.g., Nasdala et al., 2004; one example in this cate that incomplete He degassing would require entrainment times <1 h. However, the obser- study), but in most cases, reproducibility of typi- vation of extremely high U in most zircons with older ages raises the possibility that zircons cal zircons requiring alpha-ejection corrections with high radiation dosages may have more retentive He diffusion characteristics. such as those of the Fish Canyon Tuff (FCT) is ~8%–9% (2σ) (Reiners, 2005). In the case of Keywords: (U-Th)/He, geochronology, thermochronology, Big Pine Volcanic Field, northern Cor- zircons reset by short-duration reheating, such dilleran volcanic province, xenoliths. as in this study, incomplete resetting is another potential source of scatter. Zircons retaining some INTRODUCTION and Cornette, 1986; Duncan and Hogan, 1994; fraction of their pre-xenolithic He after magmatic Dating young volcanic rocks is important for Guillou et al., 1997; Heizler et al., 1999; Singer entrainment and eruption may yield signifi cantly a variety of purposes, including understanding et al., 2000, 2004). In a different approach, Gil- older ages (though this would still provide a geomorphic, tectonic, and climatic chronolo- lespie et al. (1982, 1983, 1984) circumvented maximum eruption age estimate). U and Th gies as well as hazard assessments. Pleistocene the lack of U-, Th-, or K-bearing phenocrysts zonation may also affect ZHe ages by as much as eruptions have typically been dated using K/Ar, in Pleistocene basalts in the Big Pine Volcanic 35% in extreme cases, though age scatter arising 40Ar/ 39Ar, U-series disequilibria, 14C, fi ssion Field by using the 40Ar/ 39Ar step-heating method from zonation is typically smaller than this (e.g., track of volcanic glasses (Renne, 2000, and ref- of K-feldspars extracted from granitic xenoliths. ~± 5% for FCT; Hourigan et al., 2005). erences therein), and (U-Th)/He (Farley et al., Early-released gas yielded precise apparent 2002). Young basaltic eruptions can be particu- 40Ar/ 39Ar ages interpreted as eruption age, and SAMPLE LOCATIONS larly problematic due to the rarity of U-, Th-, or later steps revealed inherited Ar in more reten- AND PETROLOGY K-bearing phenocrysts such as zircon, apatite, tive domains that was not completely outgassed We measured ZHe ages from felsic xenoliths or K-feldspar within them, though precise tech- during heating. This approach was an important from four basaltic centers: two in the Big Pine niques have been developed for whole-rock K/Ar step in the development of multidomain dif- Volcanic Field (BPVF) of eastern California, and 40Ar/ 39Ar analyses of young basalts (Gillot fusion theory in 40Ar/ 39Ar thermochronology and two from the northern Cordilleran volcanic (McDougall and Harrison, 1999). province (NCVP) in British Columbia, Canada, Although in certain circumstances modern and eastern Alaska. Units from each locality *E-mails: [email protected]; edwardsb@ 40Ar/ 39Ar approaches can yield precise ages have been dated previously, though in some dickinson.edu; [email protected]. †Current address: Department of Geosciences, of young mafi c eruptions, in many cases zir- cases data are only reported as average ages in University of Arizona, Tucson, Arizona 85721, USA; con (U-Th)/He (ZHe) may be more expedient. abstracts, and in at least one case multiple erup- [email protected]. The 40Ar/ 39Ar method requires irradiation, and tions of different age may have come from a

©2007 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, January January 2007; 2007 v. 35; no. 1; p. 17–20; doi: 10.1130/G22956A.1; 2 fi gures; 1 table; Data Repository item 2007013. 17 single vent complex (Table 1). (See Data Repos- but are signifi cantly younger than the Gillespie 16 itory Table DR11 for detailed sample locations et al. (1983) 40Ar/ 39Ar ages (Table 1). Because Oak Creek 14 12 and descriptions.) Oak Creek consists of many separate outcrops 176 ± 6.1 [3.4%] ka of basalt fl ows that are not clearly related, this MSWD = 6.2 10 METHODS suggests that fl ows in the Oak Creek area repre- 8 Single zircon crystals with c-axis-perpendicu- sent several distinct eruptions with widely vary- 6 μ lar dimensions ranging from 61 to 210 m were ing ages. The fl ow remnants dated in this study 4 179 ± 8.1 [4.5%] ka photographed, measured, and wrapped in Nb and those dated by K/Ar all may be the same 2 foil using methods outlined in Reiners (2005). fl ow, but the 40Ar/ 39Ar ages from the fl ow farther MSWD = 12 0 Alpha-ejection corrections were made using the west that was studied by Gillespie et al. (1983) 14 approach of Farley (2002) and Hourigan et al. indicate at least one additional earlier eruption. Fish Springs (2005). Analytical uncertainties (1σ) propagated Recent whole-rock 40Ar/ 39Ar ages of the same 12 243 ± 11 [4.7%] ka 4 from uncertainties on U, Th, and He measure- three Gillespie et al. (1983) Oak Creek outcrops MSWD = 4.8 10 ments (including original manometric calibra- confi rm the presence of two distinct fl ows with 8 tions) are typically <2.5%, but a few crystals, ages of ca. 1.2 and 0.20 Ma (Blondes et al., 6 4 σ 273 ± 23 [8.6%] ka most with He contents <3 fmol, have 1 uncer- unpublished data). MSWD = 43 4 tainties as high as 4.5%. We analyzed 15 zircons from 1 xenolith from 2 Using the statistics defi ned in Isoplot (Lud- the Fish Springs cone. The ZHe ages range wig, 2003), we calculated weighted mean ages, from 233 to 381 ka and the entire population 0 95% confi dence intervals, and mean square of has a weighted mean age of 273 ± 23 ka (8.6%, 8 weighted deviates (MSWD) values using 1σ weighted 95% CI, MSWD = 43) (Fig. 1, Table 1; Little Bear 7 157 ± 3.5 [2.2%] ka 6 analytical error as weights. These weights rep- Table DR1 [see footnote 1]). The weighted mean probability Relative MSWD = 1.7 resent only formal analytical uncertainties and age with the eight oldest measurements of the 5 do not take into account uncertainties on alpha- two distinct older age peaks removed is 243 ± 4 ejection corrections and the effects of U-Th 11 ka (4.7%, weighted 95% CI, MSWD = 4.8). 3 zonation that arise through these corrections; The 314 ± 36 ka K/Ar age of the Fish Springs 2 these are effects that are extremely diffi cult or basalt is older than the mean ages determined in 1 impossible to estimate a priori. Thus the formal this study (Martel et al., 1987) (Table 1). 0

MSWDs are expected to be signifi cantly greater Five zircons from Prindle Volcano gave ages 5 than unity in most cases. Where the age proba- of 166–190 ka, with a weighted mean age of Prindle bility plots show multiple apparent age distribu- 176 ± 16 ka (8.9%, 95% weighted CI, MSWD 176 ± 16 [8.9%] ka 4 MSWD = 13 tion peaks, we assume that the youngest, largest = 6.4) (Table 1; Table DR1 [see footnote 1]). 3 peaks represent the actual eruption age and the These ages are several million years younger older peaks are due to incomplete degassing or than those of previous studies (6.26 ± 0.15 Ma 2 σ] σ]; zonation with cores enriched in U-Th. We then [2 and 3.57 ± 0.14 Ma [2 Hunt and Rod- 1 removed outlying old ages in the secondary dick, 1992), but are similar to those estimated peaks and recalculated, presenting both sets of from fi eld relations (Foster et al., 1966). 0 100 200 300 400 500 data for discussion. Eight zircons from Little Bear Mountain show Age (ka) a tightly clustered age range from 151 to 163 ka, RESULTS AND DISCUSSION and a weighted mean age of 157 ± 3.5 ka (2.2%, Figure 1. Age probability plots for all four For all four samples, ZHe ages are younger weighted 95% CI, MSWD = 1.7) (Table 1; Table sample locations. Where two weighted mean 40 39 ages are presented, age without surrounding than or the same as the previously determined DR1 [see footnote 1]). Previous Ar/ Ar ages box represents all samples (black and gray Ar/Ar or K/Ar ages (Table 1). We analyzed 16 are as much as a factor of 40 older than this, points); age in surrounding box was recalcu- zircons from 1 xenolith in an Oak Creek fl ow. but both the ZHe age and the occurrence of pil- lated after discarding older age peaks (black The ZHe ages range from 163 to 224 ka and the low lava and hyaloclastite are consistent with an points only). Error bars are 1σ formal analyti- entire population has a weighted mean age of eruption during the Pleistocene marine isotopic cal precision and do not include propagated uncertainties arising from alpha-ejection 179 ± 8.1 ka (4.5%, weighted 95% confi dence stage 6 glaciation (Edwards, 1997). corrections (and associated U-Th zonation). interval [CI], MSWD = 12) (Fig. 1, Table 1; The source of the scattered apparent age dis- Vertical line is corrected weighted mean with Table DR1 [see footnote 1]). The weighted mean tributions for Oak Creek and Fish Springs and 95% confi dence interval for analytical error age with the two outliers of the secondary peak implications for entrainment times and He dif- shown as gray fi eld. MSWD—mean square of weighted deviates. removed (Fig. 1) is 176 ± 6.1 ka (3.4%, weighted fusion kinetics are discussed in the following. 95% CI, MSWD = 6.2). These ages are within Where the ZHe ages are similar to or younger the range of whole-rock K/Ar ages (Turrin and than the K/Ar or 40Ar/ 39Ar ages, it is possible that Gillespie, 1986; Connor and Conway, 2000), the youngest ZHe age represents the best esti- this is diffi cult to prove without more detailed mate of maximum eruption age, and older ZHe data on some of the previous ages. In general, 1GSA Data Repository item 2007013, the thermal ages represent either intracrystalline zonation excess Ar is a greater problem for samples with and ZHe-diffusion model discussed in the paper, with cores enriched in U-Th (e.g., Reiners et al., younger ages, simply because of the lower ratio sample locations and petrology, and Table DR1 2004; Hourigan et al., 2005) or He retained from of (posteruption) radiogenic to extraneous Ar. (individual zircon age determinations), is available premagmatic entrainment. At least in the case of Oak Creek, however, it is online at www.geosociety.org/pubs/ft2007.htm, or on request from [email protected] or Docu- The ZHe ages systematically younger than probable that the age discrepancy between the 40 39 40 39 ments Secretary, GSA, P.O. Box 9140, Boulder, CO both the K/Ar and Ar/ Ar dates are consistent ZHe ages and the Ar/ Ar is not due to inher- 80301, USA. with inherited Ar in these samples, although ited Ar, but rather to the presence of fl ows from

18 GEOLOGY, January 2007 TABLE 1. WEIGHTED MEAN ZHe AGES FROM FOUR VOLCANIC CENTERS COMPARED TO PREVIOUSLY DETERMINED K/Ar OR 40Ar/39Ar AGES Sample Location Xenolith Previous ages (ka) ± 2σ Previous dating n ZHe age (ka) weighted mean ± MSWD rock type method 95% conf. analytical error Oak Creek BPVF, CA, U.S. Granodiorite 1180 ± 70; 270 ± 140 Ar/Ar; K/Ar 16 → 14 197 ± 8.1 → 176 ± 6.1 12 → 6.2 Fish Springs BPVF, CA, U.S. Granodiorite 314 ± 36 K/Ar 15 → 7 273 ± 23 → 243 ± 11 43 → 4.8 Prindle NCVP, U.S. Fel. granulite 3570 ± 140; 6260 ± 150 K/Ar; K/Ar 5 176 ± 16 13 Little Bear NCVP, B.C. Pyx. syenite 253 ± 47 Ar/Ar 8 157 ± 3.5 1.7 Note: All ZHe ages fall within the range of, or are younger than, previous cited ages (Connor and Conway, 2000; Edwards et al., 1999b; Edwards et al., 2002; Gillespie et al., 1983; Hunt and Roddick, 1992; Martel et al., 1987; Turrin and Gillespie, 1986; Villeneuve et al., 1998). If there is a number to the right of an arrow, it represents a recalculation of ages and errors with older peaks removed. Errors are 95% confi dence interval analytical error. Bold type signifi es the fi nal accepted age determination. See the Data Repository (text footnote 1) for all individual crystal ages. vents with multiple eruption ages. The ZHe ages history for each position in the xenolith was samples, only 0.01% retention of pre-eruptive determined here show good agreement with the then used to calculate the Dt/a2 of an equivalent He would be required for ZHe ages more than K/Ar ages in nearby fl ows (Turrin and Gillespie, square thermal pulse, and fractional He loss for ~10% older than the true eruption age. Even 1986; Connor and Conway, 2000), but are much each zircon. very small intracrystalline domains, such as younger than the 40Ar/ 39Ar dated outcrops up the For typical basaltic magma temperatures of defects with increased He retentivity in some canyon to the west (Gillespie et al., 1983; Con- 1150–1250 °C, magmatic entrainment for <50 zircons, could lead to incomplete degassing. In nor and Conway, 2000). min is necessary to prevent complete degassing this light it may be important that in both of the The scattered age distributions and high of zircons in the interior of xenoliths as large as BPVF xenoliths, zircons with the oldest ages MSWDs for zircons from the Oak Creek and 10 cm in radius (Fig. 2). Subsequent slow cool- have some of the highest U concentrations. In Fish Springs volcanoes (Fig. 1) have several ing at the surface after eruption has a negligible the Fish Springs xenolith, the two zircons with possible origins. One is that some of the zircons contribution to He degassing (Table DR1; see the oldest ages have U concentrations 3.2 times are strongly zoned in U and/or Th. Conventional footnote 1). For xenoliths entrained at depths higher than the average concentration of other alpha-ejection corrections on crystals with strong between ~3 and 9 km (potentially within the zircons. If this is due to increased He retentivity (factor of ~3–5) core-to-rim zonation could lead zircon He partial retention zone), incomplete with radiation dosage (as suggested by Farley, to age inaccuracies (Reiners et al., 2004; Houri gan degassing requires ascent velocities of 1–3 m/s, 2000; Crowley et al., 2002), as confi rmed for et al., 2005). However, the fact that only xenoliths within the bounds of estimated ascent rates apatite by Shuster et al. (2006), dating zircons with upper crustal affi nities (Data Repository; (Rutherford and Gardner, 2000; Demouchy with different U-Th concentrations from the see footnote 1) show large age scatter raises the et al., 2006). Magma ascent rates could further same partially reset xenolith could be an effec- alternative possibility that some zircons from be constrained using fractional resetting varia- tive way to elucidate such effects. shallowly derived xenoliths may not have been tions in zircons from different core-to-rim dis- completely reset during magmatic en trainment. tances in xenoliths of different sizes CONCLUSIONS AND FURTHER WORK In both the Prindle and Little Bear cases, min- It is possible that incomplete He loss during Xenolithic zircon (U-Th)/He ages from four eral assemblages, thermobarometry, and regional magmatic entrainment may be due to greater different volcanic eruptions are equivalent to geologic considerations strongly suggest deriva- He retentivity in some zircons than suggested or younger than previously determined K/Ar or tion from the middle or lower crust, where tem- by laboratory diffusion experiments. For these 40Ar/ 39Ar ages, consistent with inherited Ar in the peratures are high enough to induce complete He previous measurements. In addition to providing loss prior to magmatic entrainment (Data Reposi- a relatively fast and straightforward method of Time (min) 0 1020 30 40 50 tory). The BPVF xenoliths, however, are similar 1 dating eruptions, ZHe dating of xenoliths could to surface exposures of Sierran granitoids. If they Xenolith Radius provide an independent test for inherited Ar in 0.9 2.5 cm came from shallow depths, they would have con- 5.0 cm these other dating methods. 0.8 7.5 cm tained signifi cant amounts of pre-entrainment 10 cm Two eruptions with upper crustal xenoliths 0.7

He, and had shorter magmatic entrainment times. Temperature yield relatively imprecise xenolith ZHe ages, as 0.6 For typical Sierran zircons with pre- eruption 1150 ˚C well as multiple age distribution peaks, whereas 0.5 1200 ˚C ZHe ages of ca. 100 Ma (e.g., Maheo et al., 1250 ˚C zircons from middle to lower crustal xenoliths 2004; Cecil et al., 2006), more than 99.99% He 0.4 yield more reproducible ages. This is consistent loss would be required during magmatic entrain- 0.3 with retention of small amounts of pre- eruption ment to reduce pre-eruptive He contents to <10% 0.2 He in some upper crustal zircons, though it of posteruption ingrowth, assuming an eruption 0.1 could also be due to intracrystalline U-Th zona- Fraction of total zircon with retained (f < 0.9999) helium with retained zircon of total Fraction 0 age of ca. 100 ka. 0 500 1000 1500 2000 2500 3000 tion. Using laboratory derived He-in-zircon dif- Although partial resetting of some xenolithic Time (s) fusion kinetics, incomplete He loss in xenolithic zircons can complicate eruption age measure- Figure 2. Fraction of zircons in xenolith with zircons would require magmatic entrainment ments, it has the potential, when combined with He loss <99.99%, as function of entrainment times of <1 h. Identifi cation of ZHe age gradi- He diffusion kinetics in zircon, to constrain mag- time of xenolith in basaltic host magma. ents in xenoliths of varying size could poten- matic entrainment times and ascent rates. Using Central curves of different thickness rep- tially constrain magmatic entrainment times resent xenolith radii. Gradient shaded fi eld estimates of magma and initial xenolith tem- shows range of temperature for each xeno- or ascent rates in greater detail, for a range of peratures, thermal diffusivity, and He diffusion lith size. For xenoliths with 2.5, 5.0, 7.5, and xenolith- bearing magma types. However, the parameters for zircon, we modeled maximum 10 cm radii and basaltic host temperatures observation of extremely high U in most zircons magmatic entrainment times required to prevent of 1150, 1200, and 1250 °C, entrainment with older ages raises the possibility that zircons times <50 min are required for any of zircons 99.99% resetting of zircons in 2.5-, 5.0-, 7.5-, to retain signifi cant fraction (0.01%) of their with high radiation dosages and signifi cant pre- and 10-cm-radius spherical xenoliths (Table pre- eruptive He, even for largest xenolith in eruptive He may have more retentive He diffu- DR1; see footnote 1). The time- temperature coolest melt modeled. sion characteristics.

GEOLOGY, January 2007 19 ACKNOWLEDGMENTS Foster, H.L., Forbes, R.B., and Ragan, D.L., 1966, Nasdala, L., Reiners, P.W., Garver, J.I., Kennedy, This work was supported by National Science Granulite and peridotite inclusions from Prindle A.K., Stern, R.A., Balan, E., and Wirth, R., Foundation grant EAR-0236965 to Reiners, a Geo- Volcano, Yukon-Tanana Upland, Alaska: U.S: 2004, Incomplete retention of radiation dam- logical Society of America Lipman Research award U.S. Geological Survey Professional Paper, age in zircon from Sri Lanka: American Miner- and a Sigma XI GIAR to Blondes, and a Dana Sum- v. 550B, p. B115–B119. alogist, v. 89, p. 219–231. mer Internship to Biscontini and Edwards. 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