Zircon U–Pb dating using LA-ICP-MS: Quaternary tephras in Boso Peninsula, Japan

Quaternary Geochronology xxx (2016) 1e11

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Zircon UePb dating using LA-ICP-MS: Quaternary tephras in Boso Peninsula, Japan Hisatoshi Ito a, *, Futoshi Nanayama b, Hiroomi Nakazato c a

Nuclear Risk Research Center, Central Research Institute of Electric Power Industry, Chiba 270-1194, Japan Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8567, Japan c Laboratory of Disaster Prevention, National Institute for Rural Engineering, Tsukuba 305-8609, Japan b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 February 2016 Received in revised form 31 May 2016 Accepted 27 July 2016 Available online xxx

Zircon UePb dating using LA-ICP-MS was applied to six Quaternary tephras in Boso Peninsula, central Japan: J1, Ks4, Ks5, Ks10, Ks11, and Ch2 in descending order. Accurate age determination of these tephras is of critical importance because they are widespread tephras in Japan and also relevant to a candidate site for the global boundary stratotype section and point of the earlyemiddle Pleistocene boundary. Twenty grains were dated for each tephra and the following results were obtained. The J1 tephra had only 5 grains that yielded <2 Ma. The obtained age was ~0.2 m.y. older than the stratigraphic age. No Quaternary ages were obtained from the Ks4 tephra. The Ks5 and Ks10 tephras had 10e12 grains that were ~0.1e0.3 m.y. older than the stratigraphic age. The Ks11 tephra had 14 grains that yielded a weighted mean age of 0.52 ± 0.04 Ma (error reported as 95% confidence level), which was in agreement with the stratigraphic age. The Ch2 tephra had 16 grains that yielded a weighted mean age of 0.61 ± 0.02 Ma, which was also in agreement with the stratigraphic age. The good agreement between zircon UePb ages and the stratigraphy for Ks11 and Ch2 tephras validates the reliability of the established stratigraphy and our dating approach. The other tephras that yielded ~0.1e0.3 m.y. older ages than the stratigraphy may indicate that the analyzed zircons were antecrysts that crystallized before eruption or they were detrital zircons incorporated during deposition. © 2016 Elsevier B.V. All rights reserved.

Keywords: Zircon UePb dating LA-ICP-MS Tephrochronology Boso Peninsula

1. Introduction In-situ zircon UePb dating using secondary ion mass spectrometry (SIMS) and laser ablation-inductively coupled plasmamass spectrometry (LA-ICP-MS) is now widely used for Quaternary tephrochronology (e.g., Lanphere et al., 2004; Bachmann et al., 2007; Cocherie et al., 2009; Guillong et al., 2014; Ito, 2014; Suganuma et al., 2015). Among them, Ito (2014) demonstrated that UePb dating using LA-ICP-MS can yield reliable ages as young as ~0.1 Ma using Toya tephra, widely distributed in northern Japan. Here, we follow Ito (2014)’s UePb dating approach for six Quaternary tephras distributed in Boso Peninsula, central Japan (Fig. 1). In Boso Peninsula, thick Pleistocene marine sediments, the Kazusa Group and the overlying Shimosa Group, were deposited (Figs. 1B and 2). The Kazusa Group is well exposed and contains a remarkably continuous stratigraphic succession with well-

* Corresponding author. E-mail address: [email protected] (H. Ito).

preserved marine microfossils, pollen, paleomagnetic reversal events, and a large number of tephra layers, allowing a robust chronological and stratigraphic framework. It contains the MatuyamaeBrunhes (MB) boundary and is considered as an excellent candidate for the global boundary stratotype section and point of the earlyemiddle Pleistocene boundary (Kazaoka et al., 2015; Hyodo et al., 2016). Suganuma et al. (2015) reported a SIMS UePb zircon age of 0.773 ± 0.007 Ma from a tephra (Byakubi-E or Byk-E) just below the MB boundary in Boso Peninsula. This study reports new LA-ICP-MS UePb zircon ages from some of important tephras above the Byakubi-E tephra in Boso Peninsula, which will contribute to further establish Japanese and worldwide Pleistocene chronostratigraphy. 2. Geology and samples Thick marine sediments were deposited in response to the westnorth-westward subduction of the Pacific plate beneath the Philippine Sea and North American Sea plates during the Pleistocene (Fig. 1A). In Boso Peninsula, the Kazusa Group of ~3000 thick deep-

http://dx.doi.org/10.1016/j.quageo.2016.07.002 1871-1014/© 2016 Elsevier B.V. All rights reserved.

Please cite this article in press as: Ito, H., et al., Zircon UePb dating using LA-ICP-MS: Quaternary tephras in Boso Peninsula, Japan, Quaternary Geochronology (2016), http://dx.doi.org/10.1016/j.quageo.2016.07.002

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Fig. 1. Location of study site, Japan. A: Tectonic setting, B: Distribution of the Shimosa and Kazusa Groups (Boso Peninsula) and sampling points. a: J1 tephra, b: Ks4 tephra, c: Ks5 tephra, d: Ks10 and Ks11 tephras, e: Ch2 tephra.

and shallow-water marine succession was deposited in the early and middle Pleistocene time. Evidence based on calcareous nannofossils (Sato et al., 1988), planktonic foraminifera (Oda, 1977), diatoms (Cherepanova et al., 2002), magnetostratigraphy (Niitsuma, 1976), and oxygen isotope stratigraphy (Okada and Niitsuma, 1989; Pickering et al., 1999; Tsuji et al., 2005) has provided estimated depositional ages of ca. 2.4 to 0.5 Ma for the Kazusa Group. Furthermore numerous tephra beds facilitate detailed stratigraphic correlation and the compilation of different types of age data (Machida et al., 1980; Satoguchi and Nagahashi, 2012). So far many zircon fission-track (ZFT) ages were reported on tephras in the Kazusa Group (e.g., Tokuhashi et al., 1983; Kasuya, 1990; Watanabe and Danhara, 1996; Suzuki et al., 1998) but these have >10% uncertainties (2 s) (Fig. 2). The stratigraphy of the Kazusa Group is well summarized in Kazaoka et al. (2015). It is subdivided into 14 formations: in stratigraphically ascending order, these are the Kurotaki, Katsuura, Namihana, Ohara, Kiwada, Otadai, Umegase, Kokumoto, Kakinokidai, Ichijiku, Chonan, Mandano, Kasamori, and Kongochi Formations. The MB boundary is observed in the Kokumoto Formation. In this study, five tephras were sampled in the Chonan and Kasamori Formations from the upper part of the Kazusa Group in and around the Mobara district, Chiba Prefecture, Japan (Figs. 1B and 2). These tephras were selected because they are widespread tephras or candidates of marker tephras and it was known that they contain abundant zircons. The Chonan Formation is 65e110 m thick and composed of sand-dominated alternating thin (up to a few cm) sands and siltstones. Several sandstone beds up to 4 m thick are intercalated in the upper part of the formation. Three tephra beds (Ch1e3) are recognized. The Ch2, a 10e13 cm thick white fine-sand to silt tephra in the middle of the Chonan Formation, was sampled (Figs. 1B and 2). A detailed sampling locality and an outcrop photograph are shown in Supplementary Figs. S1 and S2. This tephra is correlated to the Seiganji-Toga (Se-Tg) tephra of Kyushu Island (Fig. 1A) origin. Based on a comparison of oxygen isotope

curves between central and northernmost Boso Peninsula (Kameo et al., 2006), the Ch2 tephra was deposited during the marine isotope stage (MIS) 16.2 which is at ~0.63 Ma (Bassinot et al., 1994) (Fig. 2). The Kasamori Formation is approximately 300 m thick and is mainly composed of silty sandstone interbedded with sandstones (5e30 m). The depositional environments of the Kasamori Formation have been interpreted as representing a storm-influenced shelf (Ito, 1998). Numerous tephra beds are recognized. Ks11, 10, 5, and 4 tephras in ascending order were sampled in this study (Fig. 2). The Ks11 is a ~10 cm thick pale pink silt tephra of Kyushu Island origin and also referred to as Kobayashi-Kasamori (Kb-Ks) tephra or as Kamiogi I (Kmg 1) tephra, which corresponds to MIS 14.2 or 0.53e0.54 Ma (Kameo et al., 2006; Nakazawa et al., 2009). Machida and Arai (2011) estimated its age as 0.52e0.53 Ma based on compilation of ZFT age data. The Ks10 is a ~10 cm thick pale gray silt tephra of Kyushu Island origin and also referred to as Kamiogi II (Kmg 2) tephra. It also corresponds to MIS 14.2. The Ks5 is a ~20 cm white silt tephra of Kyushu Island origin and also referred to as Ikadachi II tephra (Satoguchi and Hattori, 2008). It corresponds to MIS 13 or ~0.50 Ma (Nanayama et al., 2016). On the contrary, Machida (2002) estimated a younger age of 0.43e0.45 Ma based on ZFT age data and stratigraphy. The Ks4 is a ~30 cm thick white fine-sand to silt tephra which lies at the boundary between Kasamori and Kongochi Formations at MIS 13 (Nakazato and Sato, 2010). The Kazusa Group is overlain by the middle and late Pleistocene Shimosa Group which comprises shallow marine to coastal sediments that represent the infilling of the paleo-Tokyo Bay (Ito, 1995; Ito et al., 1999). The Shimosa Group is subdivided into 7 formations and a ~10 cm white silt tephra (J1) was sampled from the lowest Jizodo Formation (Fig. 2). The Jizodo Formation is approximately 50 m thick and unconformably overlies the Kongochi Formation of the Kazusa Group. It is mainly composed of mudstone in the lower part and sandstone in the upper part, deposited in the shallow marine environment mainly during the MIS 11.3 transgression

Please cite this article in press as: Ito, H., et al., Zircon UePb dating using LA-ICP-MS: Quaternary tephras in Boso Peninsula, Japan, Quaternary Geochronology (2016), http://dx.doi.org/10.1016/j.quageo.2016.07.002

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Fig. 2. Chronostratigraphic correlations for the upper part of the Kazusa Group and the lowermost part of the Shimosa Group (Jizodo Formation) in and around the Mobara district, Chiba Prefecture, Japan. Marine isotope stage (MIS) 12.2 in the oxygen isotope curve (Bassinot et al., 1994) was fixed to the J1 tephra bed, and MatuyamaeBrunhes (MB) boundary was fixed to 0.77 Ma. Zircon fission-track (ZFT) age uncertainties are shown as 2s. FAD and LAD denote first and last appearance datum, respectively. Tephras shown in red are sampled in this study. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Please cite this article in press as: Ito, H., et al., Zircon UePb dating using LA-ICP-MS: Quaternary tephras in Boso Peninsula, Japan, Quaternary Geochronology (2016), http://dx.doi.org/10.1016/j.quageo.2016.07.002

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stage (Tokuhashi and Endo, 1984; Nakazato and Sato, 2001). The J1 tephra lies in the lower part of the Jizodo Formation and is assumed to have been deposited during MIS 12.2 or at ~0.43 Ma (Nakazato and Sato, 2001; Kameo et al., 2006). 3. Methodology 3.1. Sample preparation Zircons were separated from each tephra sample of ~1e1.5 kg using standard heavy liquid and magnetic techniques. After HF treatment, several hundreds of zircons were obtained from each tephra. Zircons were mostly small (~100 mm in length and ~40 mm in width), euhedral and contains small amounts of inclusions (Fig. 3), whereas some large zircons (200e300 mm in length) were found in J1 tephra (Fig. 4).

Zircons of ~100e200 grains were handpicked and embedded in a PFA Teflon sheet. Because sufficient zircon crystals had clear and flat crystal surfaces, they were analyzed on unpolished surfaces except for Ks11.Two embedded zircon sheets were prepared for Ks11 (unpolished Ks11-1 and polished Ks11-2). Glass shards from the 6 tephras were handpicked, mounted in epoxy, polished by diamond paste, and then analyzed to determine Th/U ratios. 3.2. UePb dating LA-ICP-MS UePb dating was performed at the Central Research Institute of Electric Power Industry (CRIEPI), using a Thermo Fisher Scientific ELEMENT XR magnetic sector-field ICP-MS coupled to a New Wave Research UP-213 Nd-YAG laser (Ito, 2014). Data were acquired using instrumental parameters as shown in Table 1. All

Fig. 3. Representative raw UePb data and corresponding transmitted light images for Ks5 (A), Ks10 (B), Ks11 (C), and Ch2 (D) zircons. Grain ages with 2s error are shown. Ablation pits in the images are ~40 mm in diameter. C, D: Images before laser ablation are also shown. Uncertainties of UePb age were calculated from fluctuation of 206Pb/238U ratio for 40e50 s. Therefore small uncertainties were obtained for Ks5-12 (A), Ks11-1-3 (C), and Ch2-3 (D) zircons and a large uncertainty was obtained for Ks10-17 (B) zircon.

Please cite this article in press as: Ito, H., et al., Zircon UePb dating using LA-ICP-MS: Quaternary tephras in Boso Peninsula, Japan, Quaternary Geochronology (2016), http://dx.doi.org/10.1016/j.quageo.2016.07.002

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Fig. 4. Representative transmitted light (left panel) and corresponding cathodoluminescence (CL; right panel) images for J1 zircons (left panel: unpolished; right panel: polished). Sample name and its age (2s error) are shown. Ablation pits in the left panel are ~40 mm in diameter. A, B: autocrystic or antecrystic zircons. C, D: xenocrystic zircons.

Table 1 LA-ICP-MS operating conditions at CRIEPI. Laboratory & Sample Laboratory name Sample type/mineral Sample preparation Laser ablation system Make, model & type Ablation cell & volume Laser wavelength Pulse width Energy density/fluence Repetition rate Ablation duration Ablation pit depth Spot size Sampling mode Carrier gas and flow ICP-MS Instrument Make, model & type Sample introduction RF power Make-up gas flow Detection system Masses measured Integration time per peak Total integration time per reading Detector deadtime Data Processing Gas blank Calibration strategy Reference material information Data processing package used Mass discrimination Common-Pb correction Uncertainty level & propagation Quality control/validation Other information

Central Research Institute of Electric Power Industry, Japan Zircons Conventional separation, 1 cm PFA Teflon mount, unpolished or 1 mm polish to finish New Wave Research UP-213 NWR standard 1-volume cell, volume ~3 cm3 213 nm 4 ns 5e6 J/cm2 10 Hz 30 s; 10e20 s from the start of ablation were used for signal data ~24 mm pit depth, measured using confocal laser microscopy 40 mm Single hole drilling, laser beam focused at the surface 100% He, 0.5 l/min Thermo Fisher Scientific ELEMENT XR, SF-ICP-MS Ablation aerosol only 1200 W 0.8 l/min Ar Single detector triple mode 202, 204, 206, 207, 208, 230, 232, 238 10 ms ~1 s 11 ns 30 s prior to each ablation spot Fish Canyon Tuff (FCT) used as primary reference material, Plesovice & 91500 used as secondaries for quality control FCT 206Pb/238U 0.004421 & 207Pb/235U 0.02867 (Schmitz and Bowring, 2001) In-house spreadsheet data processing Mass bias correction normalized to the primary reference material 207 Pb method (Williams, 1998) Ages are quoted at 95% conf., propagation is by quadratic addition. Plesovice: Wtd ave. 206Pb/238U age ¼ 329.5 ± 6.4 Ma (95% conf.) Drift of Pb/U ratio corrected by NIST SRM 610 glass standard

Please cite this article in press as: Ito, H., et al., Zircon UePb dating using LA-ICP-MS: Quaternary tephras in Boso Peninsula, Japan, Quaternary Geochronology (2016), http://dx.doi.org/10.1016/j.quageo.2016.07.002

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Table 2 Zircon LA-ICP-MS U-Pb analytical results. Data in italics (>75% common Pb contamination) were excluded for further U-Pb analyses. Data in bold face (<2 Ma) are used in Fig. 5. Sample name

Th (ppm) U (ppm)

232

Th/238U fa

f206%b Total 207

J1 measured on 10/02/2015 J1-1-1 290 J1-1-2 362 J1-1-3 24 J1-1-4 69 J1-1-5 80 J1-1-6 323 J1-1-7 178 J1-1-8 509 J1-1-9 328 J1-1-10 396 J1-1-11 49 J1-1-12 401 J1-1-13 115 J1-1-14 226 J1-1-15 167 J1-1-16 198 J1-1-17 298 J1-1-18 99 J1-1-19 485 J1-1-20 839 Number of grains ¼ 5 Ks4 measured on 13/02/2015 Ks4-1 168 Ks4-2 673 Ks4-3 231 Ks4-4 387 Ks4-5 493 Ks4-6 108 Ks4-7 194 Ks4-8 334 Ks4-9 325 Ks4-10 171 Ks4-11 2081 Ks4-12 3195 Ks4-13 122 Ks4-14 245 Ks4-16 211 Ks4-17 29526 Ks4-18 311 Ks4-19 608 Ks4-20 1004 Ks5 measured on 25/05/2015 Ks5-1 331 Ks5-2 95 Ks5-3 245 Ks5-4 254 Ks5-5 131 Ks5-6 176 Ks5-7 174 Ks5-8 349 Ks5-9 118 Ks5-10 186 Ks5-11 254 Ks5-12 2924 Ks5-14 508 Ks5-16 134 Ks5-17 243 Ks5-18 33 Ks5-19 220 Ks5-20 119 Number of grains ¼ 12 Ks10 measured on 28/08/2015 Ks10-1 114 Ks10-2 130 Ks10-3 86 Ks10-4 192 Ks10-5 183 Ks10-6 196 Ks10-7 93 Ks10-8 369 Ks10-9 96

Pb/206Pb

Age (Ma)c

Radiogenic 207

Pb/235U 2s

206

Pb/238U 2s

206

Pb/238U 2s

206

MSWDd

Pb/238U 2s

700 997 48 92 818 417 509 391 383 887 60 629 109 309 157 301 810 111 586 1531

0.41 0.36 0.50 0.75 0.10 0.78 0.35 1.30 0.86 0.45 0.81 0.64 1.05 0.73 1.06 0.66 0.37 0.89 0.83 0.55

0.08 0.07 0.10 0.15 0.02 0.16 0.07 0.27 0.17 0.09 0.16 0.13 0.21 0.15 0.22 0.13 0.08 0.18 0.17 0.11

7.7 1.2 41.6 1.1 4.7 1.3 1.4 1.1 0.4 1.3 36.2 0.4 14.9 0.1 36.9 0.5 1.4 25.3 1.3 1.2

0.10698 0.03630 0.37298 0.03739 0.08323 0.03604 0.03510 0.03713 0.04891 0.03616 0.33067 0.04323 0.16288 0.04690 0.33604 0.04181 0.03535 0.24517 0.03582 0.03704

0.00166 0.07972 0.00999 0.05215 2.39596 0.08271 0.07739 0.09364 0.01224 0.08038 0.00408 0.05990 0.00244 0.10701 0.00519 0.01185 0.06777 0.03216 0.04894 0.00792

0.00068 0.00260 0.00370 0.00464 0.51613 0.00435 0.00686 0.00370 0.00293 0.00352 0.00249 0.00198 0.00138 0.02668 0.00398 0.00138 0.00105 0.00822 0.00322 0.00072

0.00011 0.01594 0.00019 0.01012 0.20887 0.01665 0.01600 0.01830 0.00182 0.01613 0.00009 0.01005 0.00011 0.01655 0.00011 0.00206 0.01391 0.00095 0.00991 0.00155

0.00001 0.00010 0.00001 0.00036 0.01594 0.00036 0.00005 0.00011 0.00005 0.00032 0.01012 0.00032 0.04332 0.19901 0.04332 0.00057 0.01665 0.00057 0.00066 0.01600 0.00066 0.00071 0.01830 0.00071 0.00011 0.00181 0.00011 0.00062 0.01613 0.00062 0.00003 0.00006 0.00003 0.00022 0.01005 0.00022 0.00005 0.00009 0.00005 0.00075 0.01654 0.00075 0.00004 0.00007 0.00004 0.00007 0.00206 0.00007 0.00024 0.01391 0.00024 0.00006 0.00071 0.00006 0.00048 0.00991 0.00048 0.00005 0.00155 0.00005 Weighted mean for 'bold' data

0.67 101.92 0.73 64.91 1169.98 106.46 102.32 116.88 11.66 103.15 0.37 64.49 0.60 105.73 0.46 13.24 89.04 4.58 63.59 9.99 0.65

0.04 2.31 0.35 2.03 273.36 3.68 4.27 4.55 0.71 4.01 0.17 1.39 0.30 4.82 0.25 0.43 1.54 0.41 3.07 0.34 0.10 3.9

176 2383 292 286 470 137 292 263 161 213 2157 966 140 1515 191 36233 379 918 1129

0.96 0.28 0.79 1.35 1.05 0.79 0.66 1.27 2.02 0.81 0.96 3.31 0.87 0.16 1.11 0.81 0.82 0.66 0.89

0.20 0.06 0.16 0.28 0.21 0.16 0.14 0.26 0.41 0.16 0.20 0.68 0.18 0.03 0.23 0.17 0.17 0.14 0.18

6.1 6.1 3.3 1.9 0.1 24.3 6.4 0.8 43.6 32.4 0.8 14.5 43.9 0.1 64.3 1.7 0.2 1.6 0.0

0.09434 0.09403 0.07193 0.06120 0.04693 0.23719 0.09652 0.03970 0.38894 0.30083 0.05222 0.16043 0.39148 0.04499 0.55149 0.05908 0.04799 0.05858 0.04627

0.24660 0.01357 0.18223 0.14506 0.22859 0.11379 0.24259 0.06143 0.10291 0.06205 0.07428 0.28217 0.06512 0.09775 0.28649 0.03158 0.11347 0.13460 0.06392

0.03054 0.00460 0.04709 0.00433 0.01396 0.01927 0.04206 0.00352 0.01953 0.01594 0.00807 0.18872 0.02632 0.00267 0.05580 0.00458 0.00612 0.02104 0.00890

0.01897 0.00105 0.01838 0.01720 0.03534 0.00348 0.01824 0.01123 0.00192 0.00150 0.01032 0.01276 0.00121 0.01577 0.00377 0.00388 0.01716 0.01667 0.01002

0.00053 0.00005 0.00078 0.00037 0.00120 0.00045 0.00081 0.00017 0.00098 0.00032 0.00019 0.00149 0.00025 0.00037 0.00065 0.00087 0.00037 0.00035 0.00023

0.00053 0.00005 0.00078 0.00037 0.00120 0.00045 0.00081 0.00017 0.00098 0.00032 0.00019 0.00149 0.00025 0.00037 0.00065 0.00087 0.00037 0.00035 0.00023

113.75 6.34 113.60 107.84 223.65 16.96 109.09 71.98 6.97 6.52 65.67 69.92 4.36 100.84 8.67 24.54 109.41 104.90 64.27

3.41 0.32 5.03 2.41 7.75 2.93 5.21 1.07 6.29 2.03 1.25 9.59 1.61 2.40 4.18 5.58 2.40 2.26 1.47

234 93 326 212 130 199 194 294 121 123 204 1540 391 124 233 70 128 100

1.41 1.02 0.75 1.20 1.00 0.88 0.89 1.19 0.98 1.50 1.25 1.90 1.30 1.08 1.04 0.47 1.72 1.19

0.29 0.21 0.15 0.24 0.20 0.18 0.18 0.24 0.20 0.31 0.25 0.39 0.27 0.22 0.21 0.10 0.35 0.24

69.3 64.2 1.2 39.3 52.3 23.9 25.0 47.9 68.5 55.9 18.5 1.0 52.2 72.6 50.4 33.9 25.3 71.0

0.59066 0.55029 0.03667 0.35516 0.45702 0.23357 0.24244 0.42262 0.58441 0.48561 0.19110 0.05430 0.45673 0.61689 0.44211 0.31268 0.24484 0.60379

0.01134 0.02859 0.14100 0.00783 0.01502 0.00483 0.00376 0.01398 0.14942 0.01235 0.00380 0.00067 0.02000 0.51108 0.00816 0.00633 0.00381 0.37219

0.00417 0.00809 0.00990 0.00280 0.00351 0.00149 0.00080 0.00462 0.07342 0.00300 0.00190 0.00020 0.01009 0.31290 0.00266 0.00286 0.00192 0.17329

0.00014 0.00003 0.00004 0.00003 0.00038 0.00013 0.00014 0.00013 0.02790 0.00208 0.02790 0.00208 0.00016 0.00004 0.00010 0.00004 0.00024 0.00003 0.00011 0.00003 0.00015 0.00002 0.00011 0.00002 0.00011 0.00001 0.00008 0.00001 0.00024 0.00007 0.00013 0.00007 0.00186 0.00086 0.00058 0.00086 0.00018 0.00006 0.00008 0.00006 0.00014 0.00005 0.00012 0.00005 0.00009 0.00001 0.00009 0.00001 0.00032 0.00013 0.00015 0.00013 0.00601 0.00363 0.00165 0.00363 0.00013 0.00002 0.00007 0.00002 0.00015 0.00004 0.00010 0.00004 0.00011 0.00003 0.00008 0.00003 0.00447 0.00206 0.00130 0.00206 Weighted mean for 'bold' data without no.1, 6

0.28 0.87 177.40 0.63 0.73 0.74 0.54 0.81 3.77 0.52 0.76 0.57 0.98 10.60 0.43 0.63 0.54 8.37 0.57

0.17 0.82 13.39 0.28 0.18 0.12 0.07 0.43 5.54 0.40 0.29 0.04 0.83 23.35 0.15 0.24 0.17 13.30 0.03

0.01639 0.00347 0.00132 0.00166 0.00054 0.00079 0.00072 0.00031 0.00046

102.30 19.82 3.46 3.12 1.51 1.49 1.68 0.90 1.27

78.69 4.60 2.94 1.91 0.78 0.93 0.92 0.55 1.00

109 132 77 212 143 124 83 217 92

1.04 0.99 1.12 0.91 1.28 1.58 1.12 1.70 1.05

0.21 0.20 0.23 0.18 0.26 0.32 0.23 0.35 0.21

2.4 11.3 59.4 70.8 57.1 70.8 63.8 55.5 56.9

0.06509 0.13484 0.51317 0.60261 0.49472 0.60231 0.54763 0.48232 0.49327

0.14704 0.06450 0.09356 0.13755 0.03710 0.06560 0.05437 0.02087 0.03115

0.07681 0.01168 0.02774 0.02491 0.00819 0.02078 0.02154 0.00779 0.01224

0.01228 0.00071 0.00046 0.00030 0.00012 0.00014 0.00014 0.00009 0.00015

0.01780 0.00098 0.01778 0.01687 0.03530 0.00263 0.01707 0.01123 0.00108 0.00101 0.01024 0.01091 0.00068 0.01577 0.00135 0.00381 0.01712 0.01641 0.01002

0.01600 0.00308 0.00054 0.00048 0.00023 0.00023 0.00026 0.00014 0.00020

0.01228 0.00071 0.00046 0.00030 0.00012 0.00014 0.00014 0.00009 0.00015

1.1

Please cite this article in press as: Ito, H., et al., Zircon UePb dating using LA-ICP-MS: Quaternary tephras in Boso Peninsula, Japan, Quaternary Geochronology (2016), http://dx.doi.org/10.1016/j.quageo.2016.07.002

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Table 2 (continued ) Sample name

Th (ppm) U (ppm)

232

Th/238U fa

f206%b Total 207

Ks10-10 147 Ks10-11 78 Ks10-12 106 Ks10-13 110 Ks10-14 99 Ks10-15 208 Ks10-16 162 Ks10-17 91 Ks10-18 536 Ks10-19 245 Ks10-20 80 Number of grains ¼ 10 Ks11 measured on 13/10/2015 Ks11-1-1 175 Ks11-1-2 222 Ks11-1-3 783 Ks11-1-4 565 Ks11-1-5 350 Ks11-1-6 270 Ks11-1-7 442 Ks11-1-8 485 Ks11-1-9 644 Ks11-1-10 343 Ks11-2-1 494 Ks11-2-2 117 Ks11-2-3 230 Ks11-2-4 1726 Ks11-2-5 189 Ks11-2-6 354 Ks11-2-7 308 Ks11-2-8 2097 Ks11-2-9 514 Ks11-2-10 467 Number of grains ¼ 14

124 88 102 109 101 162 134 83 380 208 74

136 199 449 380 343 178 271 245 373 245 250 140 246 908 189 261 267 961 311 272

Ch2 measured on 30/10/2015 Ch2-1 423 191 Ch2-2 351 133 Ch2-3 1185 412 Ch2-4 1154 614 Ch2-5 320 164 Ch2-6 269 162 Ch2-7 510 229 Ch2-8 121 83 Ch2-9 330 147 Ch2-10 921 466 Ch2-11 1648 576 Ch2-12 220 146 Ch2-13 177 120 Ch2-14 427 275 Ch2-15 103 71 Ch2-16 236 138 Ch2-17 340 185 Ch2-18 937 389 Ch2-19 232 131 Ch2-20 424 239 Number of grains ¼ 16 Plesovice (reference age: 337.13 ± 0.37 Ma; P3-1-47 113 1131 P3-1-48 73 719 P3-1-49 68 692 P3-1-50 138 1101 P3-1-51 115 1100 P3-1-52 183 1418 P3-1-53 184 1398 P3-1-54 132 1163 P3-1-70 128 1211 P3-1-71 96 879 P3-1-72 81 755 P3-1-73 99 918 P3-1-92 123 1084 P3-1-93 110 970 P3-1-94 102 933

1.18 0.89 1.04 1.01 0.98 1.29 1.20 1.10 1.41 1.18 1.08

1.29 1.12 1.74 1.49 1.02 1.51 1.63 1.98 1.73 1.40 1.97 0.83 0.93 1.90 1.00 1.36 1.15 2.18 1.66 1.72

2.22 2.64 2.88 1.88 1.95 1.66 2.22 1.46 2.25 1.98 2.86 1.51 1.48 1.55 1.46 1.71 1.84 2.41 1.78 1.78

0.24 0.18 0.21 0.21 0.20 0.26 0.25 0.22 0.29 0.24 0.22

0.26 0.23 0.36 0.30 0.21 0.31 0.33 0.40 0.35 0.29 0.40 0.17 0.19 0.39 0.20 0.28 0.24 0.45 0.34 0.35

0.45 0.54 0.59 0.38 0.40 0.34 0.45 0.30 0.46 0.40 0.58 0.31 0.30 0.32 0.30 0.35 0.38 0.49 0.36 0.36

55.4 3.5 59.8 66.6 70.6 59.4 59.4 48.7 46.8 34.9 55.5

53.7 62.9 6.7 33.7 52.0 71.4 73.1 68.2 57.5 30.2 14.3 8.5 18.4 22.8 52.3 39.3 55.3 10.1 40.4 31.9

68.1 45.6 7.5 34.6 13.0 4.1 15.1 75.4 60.7 7.2 20.6 10.9 24.4 12.9 73.1 13.0 33.4 28.2 22.4 5.7

Pb/

Age (Ma)c

Radiogenic 206

0.48125 0.07322 0.51641 0.56924 0.60057 0.51255 0.51290 0.42917 0.41391 0.32028 0.48224

0.46780 0.54008 0.09888 0.31100 0.45438 0.60726 0.62044 0.58220 0.49838 0.28365 0.15826 0.11292 0.19080 0.22564 0.45726 0.35522 0.48090 0.12557 0.36363 0.29654

0.58144 0.40421 0.10520 0.31818 0.14824 0.07846 0.16506 0.63904 0.52332 0.10237 0.20771 0.13167 0.23747 0.14774 0.62072 0.14856 0.30841 0.26741 0.22195 0.09105

Pb

207

Pb/

235

0.02506 0.06798 0.04168 0.12235 0.22750 0.02272 0.02540 0.00963 0.01806 0.00919 0.04722

0.11298 0.02197 0.00118 0.00627 0.01511 0.45847 0.34258 0.09770 0.02583 0.00746 0.00250 0.00139 0.00599 0.00326 0.01628 0.00948 0.03674 0.00146 0.01915 0.00466

0.13241 0.01484 0.00140 0.00763 0.00220 0.00119 0.00262 0.10698 0.03223 0.00152 0.00367 0.00195 0.00369 0.00208 1.21561 0.00211 0.00680 0.00564 0.00350 0.00122

U 2s 0.00615 0.01676 0.00831 0.04434 0.03408 0.00470 0.01886 0.00423 0.01133 0.00598 0.01658

0.07369 0.01186 0.00047 0.00239 0.00479 0.22239 0.07816 0.02165 0.01793 0.00451 0.00111 0.00126 0.00125 0.00134 0.00618 0.00394 0.00772 0.00031 0.01306 0.00322

0.02249 0.00622 0.00028 0.00301 0.00097 0.00059 0.00077 0.03015 0.01836 0.00029 0.00054 0.00049 0.00070 0.00058 0.26880 0.00094 0.00300 0.00137 0.00085 0.00023

206

Pb/

238

0.00038 0.00674 0.00059 0.00156 0.00275 0.00032 0.00036 0.00016 0.00032 0.00021 0.00071

U 2s

206

Pb/

238

U 2s

206

Pb/

238

MSWDd

U 2s

0.00006 0.00017 0.00006 0.00276 0.00650 0.00276 0.00009 0.00024 0.00009 0.00053 0.00052 0.00053 0.00041 0.00081 0.00041 0.00005 0.00013 0.00005 0.00021 0.00015 0.00021 0.00004 0.00008 0.00004 0.00014 0.00017 0.00014 0.00005 0.00014 0.00005 0.00022 0.00032 0.00022 Weighted mean for 'bold' data

1.09 41.79 1.52 3.36 5.22 0.84 0.94 0.54 1.09 0.87 2.04 0.87

0.39 17.78 0.59 3.40 2.63 0.31 1.36 0.24 0.91 0.33 1.45 0.24

2.4

0.00175 0.00125 0.00081 0.00125 0.00030 0.00010 0.00011 0.00010 0.00009 0.00001 0.00008 0.00001 0.00015 0.00003 0.00010 0.00003 0.00024 0.00004 0.00012 0.00004 0.00548 0.00255 0.00157 0.00255 0.00401 0.00080 0.00108 0.00080 0.00122 0.00027 0.00039 0.00027 0.00038 0.00011 0.00016 0.00011 0.00019 0.00006 0.00013 0.00006 0.00011 0.00002 0.00010 0.00002 0.00009 0.00002 0.00008 0.00002 0.00023 0.00003 0.00019 0.00003 0.00010 0.00001 0.00008 0.00001 0.00026 0.00006 0.00012 0.00006 0.00019 0.00005 0.00012 0.00005 0.00055 0.00004 0.00025 0.00004 0.00008 0.00001 0.00008 0.00001 0.00038 0.00015 0.00023 0.00015 0.00011 0.00001 0.00008 0.00001 Weighted mean for 'bold' data without no.2e3, 2e7

5.23 0.71 0.52 0.62 0.75 10.09 6.95 2.49 1.03 0.86 0.63 0.53 1.20 0.52 0.79 0.76 1.60 0.49 1.47 0.50 0.52

8.03 0.64 0.05 0.16 0.27 16.42 5.13 1.74 0.72 0.36 0.15 0.12 0.20 0.08 0.38 0.33 0.23 0.05 0.97 0.08 0.04

1.7

0.00165 0.00027 0.00010 0.00017 0.00011 0.00011 0.00012 0.00121 0.00045 0.00011 0.00013 0.00011 0.00011 0.00010 0.01421 0.00010 0.00016 0.00015 0.00011 0.00010

0.00027 0.00053 0.00027 0.00009 0.00015 0.00009 0.00001 0.00009 0.00001 0.00004 0.00011 0.00004 0.00001 0.00009 0.00001 0.00003 0.00011 0.00003 0.00001 0.00010 0.00001 0.00034 0.00030 0.00034 0.00022 0.00018 0.00022 0.00001 0.00010 0.00001 0.00001 0.00010 0.00001 0.00001 0.00010 0.00001 0.00002 0.00009 0.00002 0.00001 0.00009 0.00001 0.00307 0.00382 0.00307 0.00001 0.00009 0.00001 0.00004 0.00011 0.00004 0.00002 0.00011 0.00002 0.00001 0.00009 0.00001 0.00001 0.00009 0.00001 Weighted mean for 'bold' data

3.40 0.94 0.58 0.73 0.60 0.68 0.63 1.92 1.13 0.65 0.66 0.62 0.55 0.57 24.58 0.58 0.69 0.71 0.57 0.59 0.61

1.77 0.56 0.04 0.26 0.07 0.17 0.06 2.22 1.42 0.05 0.06 0.06 0.10 0.06 19.77 0.06 0.23 0.10 0.07 0.05 0.02

1.3

0.00300 0.00113 0.00179 0.00119 0.00104 0.00071 0.00104 0.00171 0.00217 0.00131 0.00141 0.00210 0.00094 0.00269 0.00164

323.8 328.1 334.4 331.1 336.3 326.4 324.7 315.0 315.2 332.0 331.7 333.7 347.5 341.6 326.3

19.3 7.2 11.5 7.7 6.7 4.6 6.7 11.0 14.0 8.5 9.1 13.5 6.0 17.3 10.5

Sl
ama et al., 2008) measured from 10/02/2015 to 30/10/2015 0.10 0.9 0.03925 0.27867 0.01809 0.05151 0.10 0.8 0.03959 0.28488 0.00858 0.05221 0.10 0.9 0.03929 0.28832 0.01026 0.05324 0.13 0.9 0.03923 0.28493 0.00672 0.05270 0.10 0.9 0.03936 0.29054 0.00554 0.05356 0.13 0.3 0.04405 0.31530 0.01070 0.05194 0.13 0.8 0.03982 0.28352 0.00572 0.05167 0.11 0.8 0.04020 0.27745 0.01180 0.05008 0.11 0.9 0.03928 0.27124 0.01154 0.05011 0.11 0.9 0.03930 0.28631 0.00830 0.05286 0.11 0.9 0.03899 0.28379 0.00607 0.05281 0.11 0.9 0.03907 0.28606 0.00862 0.05313 0.11 1.1 0.05501 0.42481 0.01620 0.05603 0.11 0.8 0.05241 0.39622 0.02417 0.05486 0.11 0.9 0.03918 0.28030 0.01024 0.05192

0.05151 0.05221 0.05324 0.05270 0.05356 0.05194 0.05167 0.05008 0.05011 0.05286 0.05281 0.05313 0.05539 0.05442 0.05192

0.00300 0.00113 0.00179 0.00119 0.00104 0.00071 0.00104 0.00171 0.00217 0.00131 0.00141 0.00210 0.00094 0.00269 0.00164

(continued on next page)

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8

H. Ito et al. / Quaternary Geochronology xxx (2016) 1e11

Table 2 (continued ) Sample name

Th (ppm) U (ppm)

232

Th/238U fa

f206%b Total 207

P3-1-98 P3-1-99 P3-1-100 P3-1-119 P3-1-120 P3-1-121 Number of grains ¼ 20

133 127 130 123 132 145 121

1082 1055 1089 882 912 967 1022

0.12 0.12 0.12 0.14 0.15 0.15 0.12

0.6 0.1 0.6 0.8 0.8 0.8

Pb/

Age (Ma)c

Radiogenic 206

0.04110 0.04557 0.05073 0.04020 0.03964 0.03982

Pb

207

Pb/

235

0.30722 0.32941 0.35879 0.29045 0.28271 0.29260

U 2s 0.00488 0.00884 0.01556 0.00670 0.00349 0.00736

206

Pb/

238

U 2s

0.05424 0.05245 0.05132 0.05242 0.05174 0.05332

206

238

Pb/

U 2s

0.00070 0.05424 0.00070 0.00108 0.05245 0.00108 0.00085 0.05102 0.00085 0.00108 0.05242 0.00108 0.00090 0.05174 0.00090 0.00133 0.05332 0.00133 Weighted mean without no.92

206

Pb/

238

MSWDd

U 2s

340.5 329.5 320.8 329.4 325.2 334.9 329.5

4.5 7.0 5.5 7.0 5.8 8.6 6.4

3.2

a

f ¼ (Th/U)zircon/(Th/U)magma. (Th/U)magma ¼ 4.9 was used. f206% denotes the percentage of 206Pb that is common Pb. Correction for common Pb was made using measured 206Pb/238U and 207Pb/206Pb ratios. Data calculated less than 0% were treated as 0%. c Error of weighted mean is shown as 95% confidence level. d MSWD: mean square weighted deviation. b

samples were ablated in helium gas by pulses at a 10 Hz repetition rate with 40 mm laser spot and ~5e6 J/cm2 energy density. The focus of the laser beam was fixed at the sample surface throughout the data acquisition. Data were acquired in electrostatic scanning (E-scan) mode over 1080 mass scans during a 30 s background measurement, followed by 30 s sample ablation and then a 45 s background measurement. 235U was calculated from 238U assuming 238 235 U/ U ¼ 137.818 (Hiess et al., 2012). Data for the first 10 s of ablation were omitted to avoid surface Pb contamination and signal instability, and the following 10 s (40e50 s from the start of the analysis) of data were used for age calculation. Drilling depths of some ablated pits measured by confocal laser microscopy (Ito et al., 2013a) were ~24 mm, therefore depths of ~8e16 mm were analyzed. Raw data were processed offline using an Excel spreadsheet program created by the first author. After gas blank correction, laser-induced elemental fractionation and instrumental mass discrimination for 207Pb/235U and 206Pb/238U ratios were corrected by normalization to the Fish Canyon Tuff (FCT) zircon (Schmitz and Bowring, 2001). The drift of the Pb/U ratio during the analytical session was monitored and corrected by the NIST SRM 610 glass standard, which was analyzed every 12 (in-between 10 unknowns,
ma et al., 2008) and 91500 two standard zircons of Plesovice (Sla (Wiedenbeck et al., 2004)) ablations. No down-hole isotope ratios (Pb/U, Th/U) fractionation correction was performed because data from the same depth range was used for standards and unknowns in each analysis. U and Th concentrations were quantified by comparing counts of 238U and 232Th for the sample relative to the standard 91500, which is assumed to have homogeneous U and Th concentrations of 80 and 30 ppm respectively (Wiedenbeck et al., 2004). FCT zircon was used as a UePb reference material because it has been precisely dated by the ID-TIMS UePb method (Schmitz and Bowring, 2001) and our repeated analyses of FCT confirmed its suitability as a UePb reference zircon (Ito et al., 2013a, b; Ito, 2014). FCT zircons show a wide range of U and Th concentration and Th/U ratio (Schmitz and Bowring, 2001), whereas the standard 91500 zircons show a very narrow range. Therefore, for these values 91500 data were used instead of FCT data. Data processing and age calculations were completed using Isoplot v. 3.75 (Ludwig, 2012). Individual 206Pb/238U grain ages were determined using the 206Pb/238U ratio, and the uncertainty was calculated based on the time-series fluctuation (standard error) of 206Pb/238U for the time range adopted (i.e., 40e50 s from the start of the analysis). Calculated uncertainties are given at the 2s level (95% confidence limit) and the mean square weighted deviation (MSWD) is used as a statistical test of validity (Wendt and Carl, 1991).

Common-lead correction was done using the 207Pb method (Williams, 1998; Cocherie et al., 2009). The percentage of common 206 Pb is shown as f206% in Table 2. Data with <75% for f206% were adopted (Ito, 2014). Young (e.g., <2 Ma) UePb zircon ages are strongly affected by disequilibrium of 230 Th at the time of zircon crystallization from €rer, 1984). Therefore, individual zircon UePb ages the magma (Scha were corrected using a factor f, where f ¼ (Th/U)zircon/(Th/U)magma €rer, 1984). The (Th/U)magma was assumed to be 4.9 ± 0.4 (error (Scha shown as 2 standard errors; Table 3), which was determined by analyzing glass shards of each tephra using the same analytical conditions for zircon dating. The (Th/U)zircon was substituted by the measured 232Th/238U of individual zircons. Twenty grains were analyzed for each tephra, which would be a minimum number for samples that may contain xenocrystic zircons but would be enough to test our dating approach and yield meaningful age data.

Table 3 LA-ICP-MS analytical results for volcanic glass shards. All data were obtained with a 40 mm laser beam and ~5e6 J/cm2 energy density. Sample name J1 Tephra J1-1 J1-2 J1-3 Ks4 Tephra Ks4-1 Ks4-2 Ks4-3 Ks5 Tephra Ks5-3 Ks10 Tephra Ks10-1 Ks10-2 Ks11 Tephra Ks11-1 Ks11-2 Ks11-3 Ch2 Tephra Ch2-1 Ch2-2 Ch2-3

Th (ppm)

U (ppm)

232

Th/238U

232

Th (cps)

238

U (cps)

1.3 0.3 14.5 2.9 1.2 0.3 Arithmetic mean

3.9 5.0 4.4 4.4

15,328 165,199 13,053

6,137 50,975 4,617

2.2 0.4 11.1 1.7 6.6 1.3 Arithmetic mean

5.2 6.5 5.1 5.6

24,492 126,747 73,625

7,273 29,957 22,268

9.3

1.9

4.9

102,730

32,600

6.7 1.5 10.6 2.6 Arithmetic mean

4.6 4.1 4.3

72,992 117,973

24,711 44,249

3.1 0.6 4.7 0.9 7.1 1.3 Arithmetic mean

5.6 5.3 5.5 5.5

35,631 52,961 81,499

9,855 15,548 22,871

10.9 2.7 11.2 2.4 14.6 3.5 Arithmetic mean Overall mean (±2 S.E.)

4.1 4.6 4.2 4.3 4.9 ± 0.4

121,129 129,516 164,300

45,869 43,084 60,473

Please cite this article in press as: Ito, H., et al., Zircon UePb dating using LA-ICP-MS: Quaternary tephras in Boso Peninsula, Japan, Quaternary Geochronology (2016), http://dx.doi.org/10.1016/j.quageo.2016.07.002

H. Ito et al. / Quaternary Geochronology xxx (2016) 1e11

9

4. Results

4.2. UePb ages of J1 and Ks4

4.1. UePb age of Plesovice

J1 and Ks4 tephras contained many xenocrystic zircons much older than 2 Ma (Table 2). Fig. 5 shows ages that are <2 Ma with analytical uncertainty (2s) of <2 Ma. The J1 tephra yielded only 5 grains that are <2 Ma. The weighted mean age of 0.65 ± 0.10 Ma (MSWD ¼ 3.9) is ~0.2 m.y. older than the stratigraphically extrapolated age of ~0.43 Ma. Although the analytical uncertainties are large, the youngest ages of J1 are ~0.4 Ma. Therefore analyzing more zircon crystals would be required to obtain a reliable representation of the population. Cathodoluminescence (CL) images were taken after polishing to mid-section for the zircons used for dating the J1 tephra (Fig. 4).

The weighted mean age of the standard Plesovice obtained during the entire analytical periods was 329.5 ± 6.4 Ma (n ¼ 20, MSWD ¼ 3.2) (Table 2), which is slightly beyond the analytical uncertainty and is 2% younger than its reference age of
ma et al., 2008). Based on this comparison 337.13 ± 0.37 Ma (Sla between the ages for the secondary reference and its recommended age, the zircon ages reported here may be biased to younger ages by ~2%.

Fig. 5. Zircon UePb ages from five tephras in Boso Peninsula. Ages <2 Ma with analytical uncertainty (2s) of <2 Ma are shown. An arrow to the right of each diagram denotes the stratigraphic age.

Please cite this article in press as: Ito, H., et al., Zircon UePb dating using LA-ICP-MS: Quaternary tephras in Boso Peninsula, Japan, Quaternary Geochronology (2016), http://dx.doi.org/10.1016/j.quageo.2016.07.002

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H. Ito et al. / Quaternary Geochronology xxx (2016) 1e11

Autocrystic or antecrystic (Miller et al., 2007) J1 zircons show sector zoning (Fig. 4A, B), while xenocrystic zircons show concentric zoning (Fig. 4C, D). All Ks4 zircons were >2 Ma and therefore they are interpreted as xenocrystic zircons. Fig. 6 shows age distributions for J1 and Ks4. Both tephras contain zircons of 60e75 Ma and 90e120 Ma. Suganuma et al. (2015) showed that ~0.77 Ma Byakubi-E tephra from Boso peninsula contains xenocrystic zircons of 60e75 Ma. Our data indicates that there are many 90e120 Ma zircons besides 60e75 Ma zircons in Quaternary tephras in Boso peninsula. 4.3. UePb age of Ks5 A weighted mean age of 0.57 ± 0.03 Ma (MSWD ¼ 1.1) was obtained for 12 zircons (Fig. 5B). This is ~0.1 m.y. older than the stratigraphically extrapolated age of 0.50 Ma. A zircon named Ks512 had the smallest uncertainty. Both 238U and 206Pb signals for this zircon decreased rapidly after 50 s (Fig. 3A), indicating that the zircon was drilled through at ~50 s by laser ablation. The youngest age of 0.28 ± 0.17 Ma from grain Ks5-1 was omitted as an outlier. Although this age may represent the depositional age of Ks5, it is treated as insignificant because of its large uncertainty and a high common Pb contamination of 69% (Table 2). 4.4. UePb age of Ks10 Large uncertainties of U-Pb zircon ages were found in Ks10 tephra (Fig. 5C). The weighted mean age of this sample is 0.87 ± 0.24 Ma (n ¼ 10, MSWD ¼ 2.4). This is ~0.3 m.y. older than the stratigraphically extrapolated age of ~0.52 Ma. Grain Ks10-17 yielded the youngest age of 0.54 ± 0.24 Ma, which may best represent the Ks10 eruption age. A large fluctuation of 206Pb/238U ratios during 40e50 s for this grain yielded a large uncertainty of ±0.24 Ma (Fig. 3B) and a high common Pb contamination of 49% (Table 2). More grain analyses would be required to obtain a better age constraint. 4.5. UePb age of Ks11 A weighted mean age of 0.52 ± 0.04 Ma (MSWD ¼ 1.7) was obtained for 14 grains (Fig. 5D). This agrees with the stratigraphically constrained age of 0.52e0.54 Ma. Grains Ks11-1-3 (unpolished; Fig. 3C) and Ks11-2-8 (polished) had the smallest

uncertainty of ±0.05 Ma and their ages agree with the stratigraphic age. This indicates that whether grains are polished or not does not affect the UePb age. 4.6. UePb age of Ch2 A weighted mean age of 0.61 ± 0.02 Ma (MSWD ¼ 1.3) was obtained for 16 grains (Fig. 5E). This is consistent with the stratigraphically constrained age of ~0.63 Ma. Many U-Pb zircon ages for this sample yielded ages with small uncertainties, and all zircon ages agree with the stratigraphic age. An example of grain age with a small uncertainty (Ch2-3) is shown in Fig. 3D. 5. Discussion and conclusions Zircons extracted from six tephras in Boso Peninsula were dated using the LA-ICP-MS UePb method. Among them, UePb ages for the Ks11 and Ch2 tephras are in good agreement with the established chronostratigraphy. Although a ~2% younger age is possible based on the offset between the U-Pb age of Plesovice standard obtained here and the age recommended in the literature, this difference is negligible compared to other sources of uncertainty. The UePb ages were calculated using a fixed value of 4.9 for (Th/ U)magma although it was determined to be 4.9 ± 0.4 (Table 3). Substituting the (Th/U)magma of 4.9 with 4.5 or 5.3 shifts the UePb age within 0.5% and hence the effect on UePb age by change of (Th/ U)magma is negligible. All other tephras yielded UePb ages older than the chronostratigraphy. Since zircon UePb age gives the time of zircon crystallization in the magma and tephra eruption postdates or is coeval with zircon crystallization, it can be reasonably assumed that zircon UePb age for a tephra is equal or older than its eruption date. From these considerations, the obtained UePb ages for the Quaternary tephras in this study seem accurate. The LA-ICP-MS zircon UePb ages of 0.52 ± 0.04 Ma for the Ks11 tephra and of 0.61 ± 0.02 Ma for the Ch2 tephra have uncertainties of <8% and <4%, respectively, which are the most tightly constrained radiometric ages for Quaternary tephras in Boso Peninsula except for the SIMS zircon UePb age of 0.773 ± 0.007 Ma (uncertainty of <1%) for the Byakubi-E tephra (Suganuma et al., 2015). These UePb ages should help to further establish Japanese and worldwide Pleistocene chronostratigraphy.

Fig. 6. Age distributions (cumulative probability distributions) for zircons younger than 150 Ma. A: J1 tephra, B: Ks4 tephra.

Please cite this article in press as: Ito, H., et al., Zircon UePb dating using LA-ICP-MS: Quaternary tephras in Boso Peninsula, Japan, Quaternary Geochronology (2016), http://dx.doi.org/10.1016/j.quageo.2016.07.002

H. Ito et al. / Quaternary Geochronology xxx (2016) 1e11

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Please cite this article in press as: Ito, H., et al., Zircon UePb dating using LA-ICP-MS: Quaternary tephras in Boso Peninsula, Japan, Quaternary Geochronology (2016), http://dx.doi.org/10.1016/j.quageo.2016.07.002