Ion microprobe U-Pb dating of monazite with about five micrometer spatial resolution

Transcription

1 Geochemical Journal, Vol. 40, pp. 597 to 608, 2006 Ion microprobe U-Pb dating of monazite with about five micrometer spatial resolution YUJI SANO, 1 * NAOTO TAKAHATA, 1 YUKIYASU TSUTSUMI 2 and TOMOHARU MIYAMOTO 3 1 Center for Advanced Marine Research, Ocean Research Institute, The University of Tokyo, Nakano-ku, Tokyo , Japan 2 Department of Geology, The National Science Museum, Shinjuku-ku, Tokyo , Japan 3 Department of Earth and Planetary Sciences, Kyushu University, Higashi-ku, Fukuoka , Japan (Received February 7, 2006; Accepted July 31, 2006) We have developed 238 U- 206 Pb and 207 Pb- 206 Pb dating method of monazite by using a Cameca NanoSIMS NS50 ion microprobe. A ~4 na O primary beam was used to sputter a 5~7-µm-diameter crater and secondary positive ions were extracted for mass analysis using a Mattauch-Herzog geometry. The multi-collector system was modified to simultaneously detect 140 Ce, 204 Pb, 206 Pb, 238 U 16 O, and 238 U 16 O 2 ions. A mass resolution of 4100 at 1% peak height was attained with a flat peak top, while the sensitivity of Pb was about 4 cps/na/ppm. A monazite from North-Central Madagascar with a U-Pb age of ± 3.1 Ma (2σ) obtained by thermal ionization mass spectrometry was used as a reference for Pb /UO - UO 2 /UO calibration. Based on the positive correlation, we have determined the 206 Pb/ 238 U ratios of samples. 207 Pb/ 206 Pb ratios were measured by a magnet scanning with a single collector mode. Then 44 monazite grains extracted from a sedimentary rock in Taiwan were analyzed. Observed ages were compared with the U-Th-Pb chemical ages by electron microprobe. 238 U- 206 Pb ages agree well with those of the chemical ages except for some samples. The discrepancy may be due to an over-estimation of radiogenic Pb by the chemical method. 207 Pb- 206 Pb ages also agree with the chemical ages while there are a few discordant samples. Taking into account the concordant samples, there are three main age groups, 230 Ma, 440 Ma and 1850 Ma of monazites. The age distribution suggests that the provenance of detrital monazites is possibly the North China Craton or the Qinling-Dabie-Sulu zone between the North China and South China blocks. Keywords: SIMS, monazite, U-Pb age, Pb-Pb age, U-Th-Pb chemical age INTRODUCTION In situ U-Th-Pb dating of accessory minerals such as zircon, monazite and apatite has provided significant information on Earth Sciences, since the linkage between the age, textures and petrology may quantify the dynamics of several geologic processes (Muller, 2003). There are a few methods of in situ U-Th-Pb dating with ~30 µm scale such as laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS: Hirata and Nesbitt, 1995; Horn et al., 2000; Ballard et al., 2001), secondary ion mass spectrometry (SIMS: Compston et al., 1984; Whitehouse et al., 1997; Williams, 1998), and electron microprobe (EMP: Suzuki and Adachi, 1991; Montel et al., 1996). Among them, LA-ICPMS method has several advantages such as easy to operate, less expensive and smaller matrix effect than SIMS method. However it consumes more sample amount (worse depth resolution) and *Corresponding author ( ysano@ori.u-tokyo.ac.jp) Copyright 2006 by The Geochemical Society of Japan. suffers from elevated Hg backgrounds interfering at 204 Pb (Comspton, 1999). On the other hand, the advantages of U-Th-Pb chemical dating using EMP are rapid analysis, the lowest cost and excellent spatial resolution (spot size 3~5 µm) compared with SIMS method, while the major drawback is a relatively low sensitivity of Pb and in case of discordant sample, the chemical ages are only apparent ages. The most powerful tool for precise in situ U-Th-Pb dating is the SIMS method, although laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) may provide in the future an alternative technique. A pionier work of the SIMS U-Pb dating was carried out by using an ARL machine (Hinthorne et al., 1979). The commonest application of the SIMS U-Pb dating has been on zircons with 20~30 µm scale using the Sensitive High Resolution Ion MicroProbe (SHRIMP) instrument built at the Australian National University (Compston et al., 1984; Williams, 1998). The SIMS method has been also extended to several minerals such as monazite (DeWolf et al., 1993), perovskite (Ireland et al., 1990), titanite (Kinny et al., 1994) and apatite (Sano 597

2 et al., 1999). Cameca ims-3f and ims-1270 instruments were also applied to the U-Pb dating (Wiedenbeck and Goswami, 1994; Schuhmacher et al., 1994; Tsunogae and Yurimoto, 1995; Whitehouse et al., 1997). Recently, NanoSIMS NS50 ion microprobe, with supreme lateral resolution up to 0.05 µm has been develped by Cameca (Hillion et al., 1993) and mostly applied to the field of cosmochemistry (Nguyen and Zinner, 2004; Stadermann et al., 2005), even though geochemical applications are significantly less (Meibom et al., 2004; Stern et al., 2005; Sano et al., 2005). It is desirable that the instrument would apply to in situ U-Pb dating with its high lateral resolution. We report here U-Pb dating of monazite samples with various formation ages using a NanoSIMS NS50 instrument installed at Ocean Research Institute, The University of Tokyo. Experimental details of the ion microprobe analysis at spot size of 5~7 µm are given. We assess the accuracy and precision of the 238 U- 206 Pb and 207 Pb- 206 Pb ages obtained this work as compared with those determined by the U-Th-Pb chemical dating using EMP at The National Science Museum, Tokyo. Then we discuss the provenance of monazites based on the concordant ages. 22 N 100km SAMPLES Standard sample was derived from a large crystal of monazite kept as one of Sakurai Mineral Collection at The National Science Museum, Tokyo. It was collected in North-Central Madagascar and said to be derived from the Andriamena unit that experienced a thermal event at Ma (Paquette et al., 2004). A sedimentary rock sample (sandstone of Middle Miocene) was derived from Chunhuangkeng Formation, Juifang Group in Western foothills of Taiwan. Figure 1 shows the sampling site and geologic province of Taiwan. ANALYTICAL METHOD Monazite grains were separated from their matrix of sandstone using standard crushing and heavy-liquid techniques. The separated monazites were hand-picked and then cast into epoxy resin discs together with standard monazite grains. The monazites were polished until the mid-sections of the grains were exposed. Final polishing was with 0.25 µm diamond paste. After carbon coating, major element compositions, U, Th, and Pb abundances were analyzed by EMP (JOEL JCMA8800 at The National Science Museum). Measurements were performed under conditions of 15 kv accelerating voltage, 200 na specimen current and about five micrometer spatial resolution (see Fig. 2). Peak intensities of U, Th and Pb were derived from Mα-line peaks using wavelength-dispersive spectrometers with 100~300 second counting time. E Fig. 1. Sampling point of Middle Miocene sedimentary rock in the Western foothills of Taiwan together with major geological provinces. Hand-picked grains of the standard monazite were dissolved by acid using standard chemical techniques and separated by anion exchange resin after addition of selected isotope spikes. U and Pb abundances, and Pb isotopic compositions were measured by a thermal ionization mass spectrometer, TIMS (JEOL JMS05RB Mass spectrometer at Kyushu University). Experimental details were given elsewhere (Miyamoto and Yanagi, 1996). Observed 238 U- 206 Pb* and 235 U- 207 Pb* ages are 531 ± 5 Ma (2σ) and 521 ± 4 Ma (2σ), respectively. Taking into account of error weighted average, the TIMS U-Pb age of the standard monazite should be ± 3.1 Ma (2σ). After EMP measurements, the monazite mounts were polished slightly to remove carbon coat and damaged layer, and cleaned with petroleum spirit, detergent and pure water to reduce surface contaminants and then gold coated to dissipate charge during SIMS analysis. The samples were evacuated in the air-rock system of NanoSIMS to reduce water absorbed onto the surface of the mount. Using a critical illumination mode, a ~4 na mass filtered O primary beam was used to sputter a 5~7 µm-diameter crater (see Fig. 2b) and secondary positive ions were extracted for mass analysis using a Mattauch-Herzog ge- 598 Y. Sano et al.

3 Fig. 2. (a) Back-scattered electron image of sample No. 1 obtained by EMP where analyzed spot (~5 µm diameter) is shown. (b) Optical photomicrograph of standard monazite where analyzed spot (5~7 µm diameter) is shown. Because of primary beam tunning to be the brightest in this study, the pit is triangular. Note that the spot sizes are comparable. ometry. Before the actual analysis, the sample surface was rastered by 10 µm square for 3 min in order to reduce the contribution of surface contaminant Pb to the analysis. There are two advantages of NanoSIMS over any conventional SIMS. First, co-axial signature of primary and secondary beams makes short working distance of the probe forming lens/extraction system giving smaller spot size for a provided beam current with higher collection efficiency. For example SHRIMP can produce ~1 na of mass filtered O primary beam at 5 µm spot at maximum (Terada, personal communication), while the primary beam intensity of NanoSIMS is 4 times larger than that of SHRIMP. Second, multi-collector system with a Mattauch-Herzog geometry should cover wide range of mass number. In case of calcite analysis, we can detect 26 Mg (a detector called EM#1 whose position is movable), 43 Ca (EM#2 movable), 88 Sr (EM#3 movable), 138 Ba (EM#4 movable), 238U 16O (EM#5 fixed position) and U O2 (LD fixed position) at the same time under static magnetic field (Sano et al., 2005). In case of monazite analysis, it is possible to detect Pb isotopes, UO and UO2 together with a matrix peak of Ce at the same time. Then it is not necessary to scan the magnetic field from Ce to UO2, which can reduce measurement time. There is a drawback that the distance of ion beams at high mass number after the magnet should become significantly short by using NanoSIMS. For example, the distance between 204Pb and 206Pb at the focal plane is only 2 mm, which makes impossible to detect them at the same time by an original configuration of NanoSIMS. So we have developed a dual collector system (EM#4 and 4b) to detect 204Pb and 206Pb at the same time by two ion counting system located very closely. Difference in the sensitivity of EM#4 and EM#4b is calibrated by using 208Pb beam with a magnet scanning, which was ± at the time of experiment. In addition the position of EM#5 (fixed) was adjusted to collect 238U beam when 238U16O beam could enter LD detector at the outermost position. Then modified multi-collector system can detect 140Ce (Faraday cup), 204Pb (EM#4) 206 Pb (EM#4b), 238U (EM#5) and 238U16O (LD) ions of monazite at the same time. It is also possible to detect 146Nd, 204Pb, 206Pb, U O, and 238U 16O2 ions simultaneously. The entrance and exit slits (for Faraday cup, EM#1~EM#5, and LD) were set to about 40 µm and 50 µm, respectively. A mass resolution of 4100 at 1% peak height was attained to separate 206Pb from 143Nd31P 16O2 with adequate flat topped peaks (see Fig. 3). 204Pb beams are also discriminated from 172Yb16O2 beams. The Pb sensitivity =4 cps/1 na/ppm Pb was obtained by an intensitiy of 208Pb ion beam and abundance of Pb in the standard monazite (Table 1), which is about 1/5 of Pb sensitivity in zircon using SHRIMP instrument. Difference of sensitivity may be due to the fact that we use a O primary while the SHRIMP value is derived from O2. In addition the transmission of Pb ions at high mass resolution mode may be higher in SHRIMP than NanoSIMS, even though we cannot calculate the difference precisely. Taking into account of the 4 times bright primary beam at 5~7 µm spot (see Fig. 2b), however, total performance of NanoSIMS is equivalent with SHRIMP. It takes 6 min including pre-analysis raster to complete single analysis of 140Ce, 204Pb, 206Pb, 238 U16O, and 238U 16O2 ions with statistically enough counts (e.g., 206 Pb: counts) by Nano-SIMS. Then the magnet was cyclically peak-stepped through a series of mass numbers 204, 206, Ion microprobe U-Pb dating of monazite 599

4 [c/s] 141Pr 31 P 16 O 2 [c/s] 143 Nd 31 P 16 O Pb 172 Yb 16 O Pb 174 Yb 16 O 2 mass mass Fig. 3. Mass spectrum in the vicinity of m/e = 206 and 204 in a monazite standard. Mass resolution for 206 Pb = 4100 at 1% peak height. 206 Pb is well separated from 143 Nd 31 P 16 O 2 with adequate flat topped peak. Tail of 172 Yb 16 O 2 is significantly small at 204 Pb. Table 1. Chemical compositions and U-Th-Pb chemical ages of standard monazite derived from Madagascar P 2 O 5 La 2 O 3 Ce 2 O 3 Pr 2 O 3 Nd 2 O 3 Sm 2 O 3 Gd 2 O 3 UO 2 ThO 2 PbO Chemical age (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (Ma) std ± 18.4 std ± 19.4 std ± 18.8 std ± 19.0 std ± 16.4 av ± 8.2 Errors are 2σ. and 207 to measure Pb isotopes by using EM#5 as a single collector mode. More precisely we detect 204 Pb for 10 sec, 206 Pb for 2 sec and 207 Pb for 10 sec with 4 sec, 2 sec, and 2 sec of waiting time before measurements, respectively, in a single cycle. It takes about 14 min to carry out the Pb isotope analysis by 28 cycles. Total time required for U-Pb dating procedure is about 20 min. RESULTS AND DISCUSSION Table 1 lists chemical compositions and U-Th-Pb chemical ages of standard monazite grains from North- Central Madagascar. Individual EMP chemical ages are calculated from the U, Th and Pb concentrations assuming that unradiogenic Pb in the monazite is negligible and that no partial Pb loss occurred since its initial crystallisation or last complete resetting (closed system evolution). Then the chemical ages should be considered as apparent ages. The weighted mean of five measurements is ± 8.2 Ma (2σ), which agrees well with the TIMS U-Pb age of ± 3.1 Ma, and is consistent with a thermal event that affected the sample at Ma (Paquette et al., 2004). Table 2 shows chemical compositions and U-Th-Pb chemical ages of 44 monazite grains extracted from a sedimentary rock in Taiwan. The chemical ages are refered 600 Y. Sano et al.

5 Table 2. Chemical compositions and U-Th-Pb chemical ages of monazite samples extracted from a sedimentary rock in Taiwan No. P 2 O 5 La 2 O 3 Ce 2 O 3 Pr 2 O 3 Nd 2 O 3 Sm 2 O 3 Gd 2 O 3 # UO 2 # ThO 2 PbO # Chemical age # (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (Ma) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 23 # Tsutsumi et al. (2004). Errors are 2σ. from the other work (Tsutsumi et al., 2004). Generally sample monazites have higher La, U and P 2 O 5 abundances than those of standard, while Nd concentrations are contrary. Observed chemical ages vary significantly from 124 Ma to 2022 Ma, even though they are apparent ages. Table 3 lists 238 U 16 O / 140 Ce, 238 U 16 O 2 / 238 U 16 O, and 206 Pb / 238 U 16 O ratios of the standard monazite obtained by NanoSIMS within successive three days. In this work Pb is emitted almost entirely as Pb while formed UO 2 :UO :U are 10:10:1. Generally in SIMS U-Pb dating, Pb /UO could differ as much as a factor two for a target of constant Pb/U ratio (Willaims, 1998; Sano et al., 1999). However it is possible to determine a correction factor as a function of UO 2 /UO for calculating Pb/ U ratio from the Pb /UO, which was well documented in titanite and perovskite using SHRIMP (Ireland et al., Ion microprobe U-Pb dating of monazite 601

6 Table U 16 O/ 140 Ce, 238 U 16 O 2 / 238 U 16 O and 206 Pb/ 238 U 16 O ratios of the standard monazite U 16 O/ 140 Ce ( 10 3 ) 238 U 16 O 2 / 238 U 16 O 206 Pb/ 238 U 16 O ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Errors are 2σ. 206 Pb/ 238 U 16 O U 16 O 2 / 238 U 16 O Fig. 4. A correlation diagram between Pb /UO and UO 2 / UO ratios of standard monazite. Errors are portrayed at two sigma level. A dotted line shows best fit by a simple linear regression (y = ax b), where a = and b = Pb/ 204 Pb Pb/ 204 Pb ; Kinny et al., 1994). Williams et al. (1996) reported that the Pb/U calibration curve for monazite is a Pb /U - UO /U power law with exponent 2. On the other hand we decide to use Pb /UO - UO 2 /UO calibration since the U ion beam is significantly weak and Pb /U ratio is errorneous. Figure 4 shows the correlation between observed secondary Pb /UO and UO 2 /UO ratios. It is noted that the variation of Pb/UO ratios is up to a factor three, which is larger than those of previsou works (Willaims, 1998; Sano et al., 1999). This may be due to the wide variation of U concentrations (see Table 1). However the mechanism is not well understood. In order Fig. 5. Repeated measurements of 206 Pb/ 204 Pb and 207 Pb/ 204 Pb ratios in NIST SRM610 glass. Mass discriminations are significantly small, 0.1 ± 2.6 for 206 Pb/ 204 Pb and 1.5 ± 2.6 for 207 Pb/ 204 Pb, within experimental error margins. 602 Y. Sano et al.

7 Table Pb/ 206 Pb and 207 Pb/ 206 Pb ratios of the standard monazite Errors are 2σ. 204 Pb/ 206 Pb 207 Pb*/ 206 Pb f Pb*/ 206 Pb* 207 Pb*/ 206 Pb* age ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 41 av ± 13 to describe the positive correlation between the Pb/UO and UO 2 /UO ratios, a simple linear regression with R = is more appropriate than the quadratic relation with R = Thus we take the simple linear regression as follow: (Pb /UO ) obs = a(uo 2 /UO ) obs b where obs is the observed ratio, and a and b are constant. Best fit shows a = and b = We can determine the 206 Pb/ 238 U ratio of unknown sample based on the following equation: 206 Pb/ 238 U = A ( 206 Pb / 238 UO ) sample /{a(uo 2 /UO ) sample b} where constant A is determined by repeated measurements of the standard. Figure 5 shows repeated measurements of Pb isotopes in NIST SRM 610 glass. Based on the data, isotopic mass discriminations of 206 Pb/ 204 Pb and 207 Pb/ 204 Pb ratios are 0.1 ± 2.6 and 1.5 ± 2.6, respectively. Stern et al. (2005) have measured Pb isotopes of SRM Pb metals and zirconolite and suggested that the mass discrimination is undetectable. Thus we did not make any corrections for the instrumental isotopic mass discrimination of Pb isotopes by using NanoSIMS. Table 4 lists observed 204 Pb/ 206 Pb and 207 Pb/ 206 Pb ratios together with common 206 Pb fraction (f 206 ), 207 Pb*/ 206 Pb* ratio (where * denotes radiogenic) and 207 Pb*- 206 Pb* age of standard monazite from North-Central Madagascar. Generally subtraction of common Pb from measured Pb is required to estimate the accurate age (Williams, 1998). In this study a measured 204 Pb/ 206 Pb ratio, ( 204 Pb/ 206 Pb) obs was used for the correction of common Pb, ( 204 Pb/ 206 Pb) com whose isotopic composition were assumed by a two-stage evolution model (Stacey and Kramers, 1975). Then f 206 deonotes ( 204 Pb/ 206 Pb) obs /( 204 Pb/ 206 Pb) com. As is shown in Table 4, the weighted mean of 14 measurements is 528 ± 13 Ma (2σ), which is consistent with the TIMS U-Pb age of ± 3.1 Ma (2σ). Thus the standard sample shows its concordant signature. Table 5 lists observed 204 Pb/ 206 Pb, 207 Pb/ 206 Pb, 206 Pb/ 238 U ratios, and 238 U- 206 Pb* and 207 Pb*- 206 Pb* ages of 44 monazite grains extracted from a sedimentary rock in Taiwan. Error of the 206 Pb/ 238 U ratio is estimated by the counting statistics of 206 Pb and 238 U and the external reproducibility of the relation between Pb /UO and UO 2 / UO ratios in measuring standard monazite, which was propagated to the individual sample measurements based on the equation above. Data reduction are following as SHRIMP is doing in Hiroshima (Sano et al., 2000). Common Pb correction was made as following the way of standard monazite. The 238 U- 206 Pb* and 207 Pb*- 206 Pb* ages vary significantly from 128 Ma to 1986 Ma and 61 Ma to 1910 Ma, respectively. There is a general agreement between the 238 U- 206 Pb* and 207 Pb*- 206 Pb* ages except for No. 1 and 5. Figure 6 shows the comparison between the U-Th-Pb chemical ages measured by EMP (Tsutsumi et al., 2004) and 238 U- 206 Pb* ages obtained from pits in close proximity on the same monazite grain from a sedimentary rock in Taiwan by using Nano-SIMS. Except for some samples such as No. 4, 19, 38 and 44, excellent agreement between the EMP chemical ages and NanoSIMS 238 U- 206 Pb* ages is evident within experimental error. This means that the both U-Th-Pb chemical and NanoSIMS 238 U- 206 Pb* ages are reliable. Outliers such as No. 4, 19, Ion microprobe U-Pb dating of monazite 603

8 Table Pb/ 206 Pb, 207 Pb/ 206 Pb, 206 Pb/ 238 U ratios, and 238 U- 206 Pb* and 207 Pb*- 206 Pb* ages of monazite samples No. 204 Pb/ 206 Pb 207 Pb/ 206 Pb 206 Pb/ 238 U f U- 206 Pb* age (Ma) 207 Pb*- 206 Pb* age (Ma) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 9 61 ± ± ± ± ± ± ± ± ± ± ± 14 Errors are 2σ. 38 and 44 are all located below the dotted line (Fig. 6), suggesting that the discrepancy may be due to an overestimation of radiogenic Pb by the chemical method, since it is assumed that common Pb in the monazite is negligibly small. If the common Pb was incorporated at the time of initial crystallization, there would be a negative correlation between the extent of discrepancy and the observed age of grain. However this is not the case. Common Pb may also be trapped in the grain after crystallization during geological time. The mechanism of incorporation is not easy to assess since we have selected the analyzed spot as non-metamicted state by using EMP. Figure 7 shows the comparison between the U-Th-Pb chemical ages (Tsutsumi et al., 2004) and 207 Pb*- 206 Pb* ages obtained by Nano-SIMS. Generally the chemical ages are consistent with 207 Pb*- 206 Pb* ages, even though 604 Y. Sano et al.

9 Fig. 6. The comparison between U-Th-Pb chemical ages and NanoSIMS 238 U- 206 Pb* ages obtained from pits in close proximity on the same monazite grain from a sedimentary rock in Taiwan. Errors are portrayed at two sigma level. NANO-SIMS Pb-Pb (Ma) #5 # # #12 # Chemical age (Ma) 2500 Fig. 7. The comparison between U-Th-Pb chemical ages and NanoSIMS 207 Pb*- 206 Pb* ages obtained from pits in close proximity on the same monazite grain from a sedimentary rock in Taiwan. Errors are portrayed at two sigma level. there is larger discrepancy than the comparison with 238 U- 206 Pb* ages. For example, No. 1 and 5 indicate significantly older 207 Pb*- 206 Pb* ages than the chemical ages, suggesting discordant signature (probably partial Pb loss due to recent thermal events). Outliers such as No. 4, 12, and 13 may again be attributable to common Pb incorporation. However the 238 U- 206 Pb* ages of No. 12 and 13 are comparable to those of chemical ages. Common Pb incorporation as well as discordant signature may occur in these samples, which makes its explanation difficult. Figure 8 shows a Tera-Wasserburg U-Pb monazite concordia diagram for the sedimentary rock in Taiwan. Most grains indicate a concordant signature except for two samples No. 1 and 5 which may have a recent Pb loss. It is noted that there are three main age groups, 232 ± 12 Ma, 436 ± 11 Ma and 1848 ± 57 Ma of concordant monazite grains, even though the oldest group (1848 ± 57 Ma) may be composed of few sub-groups. Geological province of Taiwan is relatively simple with five major blocks (Western foothills, Hsuehshan Range, Backbone Range, Eastern Central Range and Coastal Range) located from west to east (Fig. 1). Western foothills where the sample rock was collected is mainly composed of clastic rocks of neritic signature between Miocene and Quaternary. A cumulative probability of the monazite ages is similar to those of monazite ages in sandstone from the mouth of the Chang Jiang (Yangtze river), which flows on the Yangtze craton (Tsutsumi et al., 2004). Based on the bulk chemical compositions, however, most sandstones are considered to be derived from the North China Platform (Tsutsumi et al., 2004). Fig. 8. Terra-Wasserburg U-Pb monazite concordia diagrams for the sedimentary rock in Taiwan. Errors are portrayed at two sigma level. Dotted curve indicates concordant ages. Taking into account observed age groups (232 ± 12 Ma, 436 ± 11 Ma and 1848 ± 57 Ma), one may assess the provenance of detrital monazites. There are two main blocks characterized by distinctive geological epoch in China; the North China and the South China blocks. The South China block is subdivided into two units: Yangtze carton to the northwest and Cathaysia block to the south- Ion microprobe U-Pb dating of monazite 605

10 east. Geochronology of the Fuping Complex revealed that there is Archean to Paleoproterozoic magmatic arc system that has been subsequently tectonically disrupted and juxtaposed during the collision of the eastern and western North China blocks at similar to 1850 Ma, which resulted in the final assembly of the North China Craton (Zhao et al., 2002; Guan et al., 2002). Recently Liu et al. (2004) reported that zircon U-Pb ages of Ma of olivine pyroxenite from Hannuoba could be records of the subduction of Mongolia oceanic crust under the North China Craton. On the other hand, Li (1997) suggested that the Paleoproterozoic basement rocks in Cathaysia block were most likely formed at 1.77 Ga through cratonization by granite formation. And then, the basement rocks were covered by anorogenic magmatic rocks dated at ca Ma (e.g., Li et al., 2005). The Yangtze craton has a Late Archean to Paleoproterozoic core surrounded mostly by younger orogenic belts which are characterized by igneous activities about 800 Ma U-Pb zircon (Li, X.-H. et al., 2003; Li, Z. X. et al., 2003). The Ma U-Pb zircon ages also observed in the ultrahigh-pressure metamorphic unit of the Dabie Shan, eastern China (Schmid et al., 2003; Bryant et al., 2004). From the Qinling-Dabie-Sulu zone which formed between North China and South China blocks, detrital zircons with about 400 Ma U-Pb ages are derived in Carboniferous sedimentary rocks (Li et al., 2004), and Triassic zircon with Ma U-Pb ages are also found in Ultra High Pressure metamorphism (Hacker et al., 1998). Li et al. (2004) suggested that the detrital zircon were supplied from the southern margin of North China block. Observed age groups (436 ± 11 Ma and 1848 ± 57 Ma) agree well with those of the North China Craton, suggesting that these detrital monazites were derived from the craton. The youngest age group (232 ± 12 Ma) may be related to Triassic syn-collisional monzogranites, possibly representing the collision of the Central Asian Orogenic Belt with the North China Craton and final closure of the Paleo-Asian Ocean (Zhang et al., 2004). However, zircons with U-Pb ages of Ma and Ma were also found in South China block. The Chang Jiang flows on the Yangtze craton along the Qinling- Dabie-Sulu zone, which is another source of zircons with 400 Ma and Ma U-Pb ages. This area is an alternative provenance for western foothills sediments in Taiwan. CONCLUSIONS A pilot study of NanoSIMS 238 U*- 206 Pb* and 207 Pb*- 206 Pb* dating of monazite samples with various formation ages has been reported here. After taking the Pb / UO and UO 2 /UO calibration, we obtain consistent 238 U- 206 Pb* ages at 5~7 µm pits, which agree with the U- Th-Pb chemical ages from spots in close proximity on the same monazite grains by using EMP. The 207 Pb*- 206 Pb* ages are also comparable with the chemical ages while there are a few discordant samples. U-Pb dating is routinely undertaken by the IDTIMS method as well as SHRIMP instrument. However, in cases where the monazite grains are smaller than 10 µm or the textural context of the grains are significantly complex, then it is possible to use NanoSIMS to obtain accurate and reasonably precise U-Pb ages. For instance based on the U-Pb age distribution of 44 monazite grains extracted from a sedimentary rock in Taiwan, the provenance of detrital monazites is attributed to either North China Craton or Qinling-Dabie-Sulu zone between North China and South China blocks. Acknowledgments We appreciate K. Yokoyama for providing a standard monazite; K. Terada and R. A. Stern for helpful comments and discussion. Earlier version of the paper was reviewed by an anonymous referee. Constructive comments by H. Yurimoto and P. W. O. Hoskin were useful to revise the manuscript. 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