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NanoSIMS U-Pb dating of fossil-associated apatite crystals from Ediacaran (~570 Ma) Doushantuo Formation
Journal Pre-proofs NanoSIMS U-Pb dating of fossil-associated apatite crystals from Ediacaran (∼570 Ma) Doushantuo Formation Chuan-Hsing Chung, Chen-Feng You, James William Schopf, Naoto Takahata, Yuji Sano PII: DOI: Reference:
S0301-9268(18)30673-9 https://doi.org/10.1016/j.precamres.2019.105564 PRECAM 105564
To appear in:
Precambrian Research
Received Date: Revised Date: Accepted Date:
31 December 2018 25 September 2019 4 December 2019
Please cite this article as: C-H. Chung, C-F. You, J. William Schopf, N. Takahata, Y. Sano, NanoSIMS U-Pb dating of fossil-associated apatite crystals from Ediacaran (∼570 Ma) Doushantuo Formation, Precambrian Research (2019), doi: https://doi.org/10.1016/j.precamres.2019.105564
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NanoSIMS U-Pb dating of fossil-associated apatite crystals from Ediacaran (~570 Ma) Doushantuo Formation Chuan-Hsing Chung1,*, Chen-Feng You1, James William Schopf2, Naoto Takahata3, Yuji Sano3
1 Department
of Earth Sciences, National Cheng Kung University, Tainan, Taiwan
2 Department
of Earth and Space Sciences, University of California, Los Angeles, Los Angeles,
USA 3 Atmosphere
and Ocean Research Institute, University of Tokyo, Chiba, Japan
1
Abstract Sedimentary rocks record a wealth of information on the evolution of lithosphere, hydrosphere, atmosphere and biosphere. However, uncertainty about the age of many Precambrian sequences makes it very difficult to establish the accurate temporal framework which can dramatically improve our understanding of the scale and duration of geological and biological events. The Neoproterozoic Doushantuo Formation, South China, preserve unique assemblages of early multicellular fossils and records valuable paleoenvironmental information. The age of these formations is thus critical for understanding the important biological and climatic events that occurred towards the end of the Proterozoic Eon. In this study, we used NanoSIMS to date samples from the Doushantuo Formations by apatite U-Pb geochronology. Fossil-associated precipitated apatite grains were identified using optical microscope and Raman spectrometer. The resultant age is 305 ± 26 Ma. High common Pb and low U concentration in those apatite specimens limit the precision of age determination, while the yielded age suggests that the Doushantuo Formation may have experience a post-depositional hydrothermal event.
Keywords: U-Pb geochronology, apatite, NanoSIMS, Doushantuo Formation
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1. Introduction Sedimentary rocks record wealth of information on the evolution of lithosphere, hydrosphere, atmosphere and biosphere. The ages of sedimentary formations especially in the Phanerozoic period are conventionally estimated from chemo- or biostratigraphy correlation. In older rocks, the age determination using this method become inapplicable due to the absence of fossil records. Some indirect methods, such as dating of contemporaneous volcanic rocks, bracketing relationships of igneous and metamorphic rocks, and the dating of detrital and diagenetic minerals (Rasmussen, 2005), can be used to constrain the age of sedimentary rocks. But many basins lack suitable volcanic rocks and sometimes the range of constrained ages can be up to hundreds of millions of years. Dating detrital minerals shows very limited information about the depositional age since it can only yield maximum ages. The feasibility of the radiometric dating methods for diagenesis were established with ArAr age of glauconies (Smith et al., 1993), Rb-Sr age of illite (Morton, 1985), K-Ar age of diagenetic K-feldspar (Girard et al., 1988), Pb-Pb age of monazite nodules (Evans et al., 2002) and U-Pb age of apatite (Chandler and Parrish, 1989). Most diagenetic minerals form in low temperature environment and have low closure temperature, making them prone to later thermal resetting, therefore are not suitable for geochronology. A recent development in sedimentary geochronology has been the identification of the U–Pb chronometer, xenotime, as a diagenetic phase in sedimentary rocks (Fletcher et al., 2000; Rasmussen et al., 2004; Rasmussen, 2005). Being a common U-bearing accessory mineral in sedimentary rocks, U–Pb dating of apatite has potential application in sedimentary provenance studies. Apatite also has been widely used in thermochronology studies (Chew and Spikings, 2015). The relatively high closure temperature of ca. 325-550 °C ( McDougall and Harrison, 1999; Chamberlain and Bowring, 2001; Schoene and 3
Bowring, 2007) makes U-Pb apatite system a useful tool to study the deposition ages of sedimentary or low-grade meta-sedimentary rocks. A variety of methods are currently available for U-Pb geochronology of diagenetic minerals. Three main techniques used are Thermal Ionization Mass Spectrometry (TIMS), Secondary Ion Mass Spectrometry (SIMS) and Multicollector Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS). Traditionally the uranium and lead isotope of apatite has been measured by isotope dilution mass spectrometry ( Oosthuyzen and Burger, 1973; Romer, 1996; Corfu and Stone, 1998). Using an ion microprobe (Li et al., 2012; McNaughton et al., 1999; Sano et al., 1999a; Vallini et al., 2002) or LA-ICPMS (Willigers et al. 2002; Chew et al. 2011; Thomson et al. 2012), it is possible to date xenotime and apatite with a spatial resolution of 530μm. Isotope dilution TIMS involves the chemical digestion of entire crystals and is more precise than in situ methods. However, heterogeneity and small sizes which are typical characteristics of diagenetic minerals make the microprobe techniques more appropriate. Under ideal conditions, the LA-ICPMS can yield U–Pb ages that approach the accuracy and precision delivered by SIMS in a fraction of the time with the cost of much larger sample consumption. Analysis by ion microprobe is the method used in this study because of the fine spatial resolution, shallow penetration depth and high precision. The primary limitation of apatite U-Pb geochronology is poor dating precision cause by low U concentration and high common lead content. The Ediacaran marks the final geological period of the Proterozoic eon between the termination of the global Marinoan glaciation at ∼635 Ma and the beginning of the Cambrian at 542 Ma. The Ediacaran Earth experienced major biological changes, including radiation of complex multicellular life such as globally distributed Ediacaran-type soft-bodied animals, and 4
the environmental oxygen level was also suggested to be increasing base on the physiological requirement of these complex animals and other geochemical evidence. However, the history of this oxygenation event is not adequately constrained due to insufficient information in terms of radiometric age and oxygen concentration. The Neoproterozoic Doushantuo Formation, South China, preserve unique assemblages of early multicellular fossils and overlies rocks, which are thought to have formed during an ice age of global extent. It is the main phosphorite-hosting strata in South China and preserves the earliest diverse eukaryote assemblage (Zhang, 1989; Li, 1998; Xiao et al., 1998; Chen et al., 2000). This allow a more complete understanding of the late Proterozoic biosphere. Recently, a novel apatite-based oxygen paleobarometer which use the fluorescence signatures of fossilassociated apatite specimens to quantify the ambient O2 concentrations, has been established. Fluorescence signatures associated with an oxygen-dependent mechanism for Sm3+-substitution in apatite are used to devise a semi-quantitative apatite oxygen paleobarometer indicative of ambient O2 concentrations during Sm3+ emplacement (Garcia, 2018). The age of these formations is thus critical for understanding the important biological and climatic events that occurred towards the end of the Proterozoic Eon. However, uncertainty about the age of many Precambrian sequences makes it very difficult to establish the accurate temporal framework which can dramatically improve our understanding of the scale and duration of geological and biological events. The purpose of this study is to determine the formation age of fossil associated apatite found in Neoproterozoic Doushantuo Formations in Yangtze Block using U-Pb dating method. Here, we report U-Pb analysis of thirteen apatite specimens extracted from phosphorite of Doushantuo
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Formation using NanoSIMS. These results will help us to reconstruction the oxygenation events of Ediacaran Ocean.
2. Geological setting and previous dating works The Doushantuo Formation is a recently discovered lagerstätte in Guizhou Province, China that is most notable for its scientific contributions in the hunt for Precambrian life. Doushantuo is of particular interest because its fossils, dating from about 565 to 590 million years ago, predate the Cambrian Explosion by at least 20 million years. The most fossiliferous zones are estimated to be 570 million years old. The Yangtze Platform evolved from a rift basin to a passive continental margin during the Ediacaran-Cambrian transition. The Doushantuo Formation is represented by a phosphate-dolostone sequence at Weng'an, where it is 33 to 55 m thick and consists mainly of dark phosphate, cherty phosphate, chert, and gray dolomite. The overlying Dengying Formation, of Ediacaran (~565 Ma to 544 Ma) age and containing rare Ediacaran body fossils in the lower part and basal Cambrian shelly fossils near the top, is a 180 m thick dolomite sequence. The Doushantuo Formation is further divided into 4 Members in ascending order: Member 1 consists of a ~5m thick cap carbonate which overlies the tillites deposited during the Nantuo (Marinoan) glaciation; Member 2 is composed of an alternating shale-mudstone-dolostone sequence; Member 3 consists mainly of dolostone and limestone, Member 4 (also called the Miaohe Member) consists of ~13m thick black shales (Sawaki et al., 2010; Wang et al., 1998; Zhu et al., 2003).
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Precise zircon U–Pb dating of volcanic tuff beds from the lower and uppermost Doushantuo Formation has constrained the age of the Doushantuo Formation to between 635.2±0.6 and 551.1±0.7 Ma (Condon et al., 2005). SIMS zircon U-Pb analysis for a tuffaceous bed immediately above the upper phosphorite unit in the Doushantuo Formation yield an age of 609±5 Ma (Zhou et al., 2017). The Re–Os age for the base of Doushantuo Member 4 has been determined to be 591.1±5.3 Ma (Zhu et al., 2013). Dating of Doushantuo phosphorites by a LuHf dating method and Pb-Pb geochronometry independently yielded ages of 584±26 Ma and 599.3±4.2 Ma, respectively (Barfod et al., 2002). Pb isotopic composition analysis of Doushantuo black shales yielded a Pb isotope isochron ages of 572±36Ma (Chen et al., 2009) and the Pb-Pb age of upper part of Doushantuo phosphorites was determined to be 576±14Ma (Chen et al., 2004). The apatite samples used in this study were collected from Phosphorite Unit B of Doushantuo Formation at Chuanyandong, Weng’an area, Guizhou Province, China. The stratigraphic level is 4.5m above base of Phosphorite Unit B (Fig. 1). Each apatite specimen is several hundred micrometers in size (Fig. 2).
3. Analytical methods
Thin sections are prepared from collected rock samples and fossil-associated precipitated apatite grains were identified using optical microscope and Raman spectrometer. Selected apatite specimens are polished until the mid-sections of the grains were exposed. The thin sections were cleaned by deionized water in an ultrasonic bath and gold-coated. Then the sample was heated
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by a lamp in a vacuum chamber for ~12 hours before introducing it into NanoSIMS 50 ion microprobe installed at the Atmosphere and Ocean Research Institute, The University of Tokyo. The NanoSIMS analytical method used in this study is well-established and has been applied to date biogenic apatites and other phosphate minerals (Koike et al., 2016, 2014; Sano et al., 2014; Terada et al., 2018). Prior to ion microprobe analysis, sample was kept evacuated in the air-rock system of NanoSIMS overnight in order to reduce possible hydride interference cause by water absorbed onto the surface of the sample mount. During preliminary NanoSIMS test, we found that the U concentrations of our apatites are roughly one tenth of a typical sedimentary apatite. The low U contents which limited production of radiogenic Pb may hinder the attempts to apply in situ SIMS U-Pb technique. To acquire statistically sufficient counts, a higher primary O- beam current of about 10nA was focused onto the sample with a diameter of about 15µm. Before the actual analysis, the sample surface was rastered by 15 µ m square for 5 min in order to reduce the contribution of surface contaminant Pb to the analysis. The sputtered secondary positive ions were extracted using an acceleration voltage of 8kV. The generated secondary ions are focused and transferred for mass analysis and detection. The mass resolution of mass spectrometer was set to 4100 at 1% peak height to separate 206Pb from 143Nd31P16O2 with adequate flat topped peaks. 204Pb beams are also discriminated from 172Yb16O2 beams. The Mattauch-Herzog geometry mass analyzer used in NanoSIMS makes the simultaneous collection of 31P+, 43Ca+, 204Pb+, 206Pb+, 238U16O+, 238U16O2+
under a static magnetic field possible which reduced cycle
time dramatically. All these ions were detected by low noise ion counter. In NanoSIMS, the ion dispersion at high mass number after the magnet should become significantly short which makes it impossible to detect 204Pb and 206Pb at the same time by an original configuration of NanoSIMS, since the distance between them at the focal plane is only 2 mm. To improve
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analytical efficiency and precision, a customer ordered dual collector system to detect 204Pb and 206Pb
at the same time was installed in NanoSIMS at The University of Tokyo. The integration
time for each analysis was set to be 10mins to acquire statistically sufficient counts. At this setting, the total time needed for each analysis was 15min including rastering. We did not apply the 206Pb-207Pb measurement because peak jump mode which is necessary to measure the extra 207Pb
signal would take a much longer time of about 60mins. Furthermore, previous study
suggested that the 206Pb–207Pb isochron of biogenic apatite did not provide a precise formation age (Sano and Terada, 1999; Sano and Terada, 2001). We decided to process as many spots as possible using the 238U-206Pb systematics under the allocated machine time. Precision and accuracy in SIMS U-Pb geochronology strongly depend on the method of determination of the inter-element ion ratios from the measured secondary ion ratios (Jeon and Whitehouse, 2015). Therefore, a well-characterized matrix-matched standard, “PRAP”, derived from an alkaline rock of the Prairie Lake circular complex in the Canadian Shield dated at 1155±20 Ma by SHRIMP II (Sano et al., 2006b; Sano and Terada, 1999) was mount along with samples in each analytical session and used to correct for elemental fractionation. The background of ion counters and intensity of 204Pb were also been accurately determined to ensure robust isotope ratio correction
4. Results and discussion
4.1 UO2/UO-Pb/UO calibration
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Previous studies show that the secondary Pb+/U+ ratio produced by a primary 16O- beam can differ by as much as a factor of two for a target of constant Pb/U ratio (Williams, 1998). However, it was possible to calculate the Pb/U ratio using a correction factor determined by the relationship between UO+/U+ and Pb+/U+ in SHRIMP analysis (Sano et al., 1999a). For NanoSIMS, Pb is emitted almost entirely as Pb+ while UO2+ : UO+:U+ ions are produced by oxygen bombardments are about 10:10:1 (Sano et al., 2014). It was difficult to accumulate statistically sufficient U+ counts within 10 min due to the low concentration in the specimens. We decided to apply an UO2/UO–Pb/UO calibration instead of the UO/U–Pb/U system used for SHRIMP. The new calibration showed good performance for monazite (Sano et al., 2006a) and zircon (Takahata et al., 2008) using NanoSIMS. Correlation was found between the 206Pb+/238U16O+
and 238U16O2+/238U16O+ ratios of PRAP standard apatite. The variations are
expressed using the empirical quadratic relation as follows. ( 206Pb+ / 238U16O+ )obs = a × ( 238U16O2+ / 238U16O+
+b
Therein, obs is the observed ratio, and a and b are constants. The 206Pb/238U ratio of an unknown sample is calculated using the following equation. 206Pb / 238U
= c× ( 238U16O2+ / 238U16O+)obs / [a × ( 238U16O2+ / 238U16O+
+ b]
In that equation, the constants a, b and c are determined by repeated measurements of the PRAP standard and using the 238U–206Pb age of 1155 ± 20 Ma (Sano et al., 2006b). Table 1 lists 206Pb+/238U16O+
and 238U16O2+/238U16O+ ratios of PRAP apatite sample. The relation between the
206Pb+/238U16O+
and 238U16O2+/238U16O+ ratios of the PRAP standard is shown as Fig. 3. The
reproducibility of standard was very well (less than 5%). There are apparently two curves which represent two different ionization relationships. This was because that the primary ion source 10
(Duoplasmatron source) of the instrument need to be repaired during the analytical session therefore changed the ionization behaviors. The best fit results of these two curves were used to calculate 238U/206Pb ratios accordingly. The uncertainty in the 206Pb/238U ratio was derived from the external reproducibility of the standard and internal error of sample analysis. We also estimate the U concentration of sample based on the observed 238U16O+/43Ca+ ratio calibrated against that of the PRAP standard.
4.2 U-Pb age of fossil-associated apatite
Sixty-two NanoSIMS analyses were conducted on 13 apatite specimens. Table 2 presents the U contents, 204Pb/206Pb and 238U/206Pb ratios of the sample spot, where the error assigned to the ratio is reported to two sigma. The U contents range from 5.5 ppm to 15 ppm with the average of 8.3 ppm, which is on the lower end of typical uranium concentration of apatite used in geochronology studies (Li et al., 2012; Sano et al., 2014, 2006b, 1999a; Sano and Terada, 1999). The 204Pb/206Pb ratios vary from 0.0149 to 0.0432. Our apatite grains contain high proportion of common lead, ranging from 25% to 72%. The 238U/206Pb ratios are highly variable, range from 4.7 to 14.6 due to different amount of common Pb. Fig. 4 shows the relation between the 204Pb/206Pb
and 238U/206Pb ratios. A least-squares fit of these five data using the York method
gives the 238U–206Pb isochron age of 305 ± 26 Ma (2σ; MSWD = 1.6). The initial 206Pb/204Pb ratio was estimated to be 17.5 ± 3.0 (2σ), which is consistent with the average common Pb of 18.315 at 250 Ma (Stacey and Kramers, 1975) within experimental error. The age is significantly younger than previously estimated deposition ages of Doushantuo Formation (550~635Ma). The
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dating results indicating closure for U-Pb system in an event as late as 300Ma. This suggests either that the apatite in Doushantuo Formation crystallized about 300Ma ago, or that they grew earlier than that but some subsequent thermal process reset the U-Pb system. Some submarine hydrothermal activity has produced a depositional sequence dominated by carbonates in the first stage (calcite and dolomite) with subsequent apatite and late barite veins (Núñez-Cornú et al., 2000). But the 200Ma time span of various mineral deposition stages is highly implausible in submarine hydrothermal system. Also, the selected apatite grains in this study are all infill microscopic fossil, showing their very early emplacement, prior to decay and disintegration of their fossil host. Therefore, the hypothesis that apatite grew at 250Ma later than the deposition of Doushantuo can be rule out. The other possibility is the post-depositional alteration event in Carboniferous which might be related to the overgrowth or the reset of radiogenic age of the phosphate that is originally precipitated in Neoproterozoic-Cambrian transition. However, the extraordinary fossil preservation (Xiao and Knoll, 2000, 1999) and REE patterns in Doushantuo phosphorites (Chen et al., 2003) have also previously been interpreted as being free from postdepositional alteration. Sano et al. (1999c) reported the 1500Ma U-Pb age obtained on apatites from sediment sequences of Akilia island, southwest Greenland, that are originated more than 3850Ma old. They conclude that, about 1500Myr ago, these apatites experienced a metamorphic event of about 600°C. The apatite specimens used in this study may have also experienced a similar thermal event about 300Myr ago. The closure temperature for apatite was found to between 550°C and 325°C (McDougall and Harrison, 1999). Given the small size of the apatite samples used in this study, the closure temperature could be even lower. Clumped isotope study on Member 2 of Doushantuo Formation indicate that the carbonate grew from hydrothermal fluid with ambient temperature of 86-476°C (Bristow et al., 2011). Clay mineral, organic matter and
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K-Ar dating studies show that the depositional geochemical records preserved in Doushantuo Formation have been overprinted by hydrothermal event at ~325Ma (Derkowski et al., 2013), which is within the error of U-Pb isochron age of Doushantuo apatites obtained in this study. Therefore, the U-Pb system of Doushantuo apatite may have been reset by thermal induced activities.
5. Conclusion
In this paper we present NanoSIMS U-Pb dating results on Doushantuo fossil-associated apatite. Sixty-two spots on thirteen apatite grains yield a 238U/206Pb isochron age of 305 ± 26 Ma. The age is significantly younger than previously estimated deposition ages of Doushantuo Formation (550~635Ma). It appears that there was a post-depositional hydrothermal event at about 300Ma which reset the U-Pb system.
Acknowledgments
We thank Der-Chuen Lee, Zan Peeters and Sung-Yun Hsiao at IESAS for the help on preliminary NanoSIMS test on apatite samples. The authors express thanks to Takanori Kagoshima for
assistance in the NanoSIMS analysis at University of Tokyo. Supported by MOST 105-2116-M006-011 to Chen-Feng You.
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Lagerstatte, South China. Lethaia 32, 219–240. https://doi.org/10.1111/j.15023931.1999.tb00541.x Xiao, S., Zhang, Y., Knoll, A.H., 1998. Three-dimensional preservation of algae and animal embryos in a neoproterozoic phosphorite. Nature 391, 553-558. https://doi.org/10.1038/35318 Yun, Z., 1989. Multicellular thallophytes with differentiated tissues from Late Proterozoic phosphate rocks of South China. Lethaia 22, 113-132. https://doi.org/10.1111/j.15023931.1989.tb01674.x Zhang, Y., Yuanl, X., 1992. New data on multicellular thallophytes and fragments of cellular tissues from Late Proterozoic phosphate rocks, South China. Lethaia 25, 1–18. https://doi.org/10.1111/j.1502-3931.1992.tb01788.x Zhou, C., Li, X.H., Xiao, S., Lan, Z., Ouyang, Q., Guan, C., Chen, Z., 2017. A new SIMS zircon U–Pb date from the Ediacaran Doushantuo Formation: age constraint on the Weng’an biota. Geol. Mag. 154, 1193-1201. https://doi.org/10.1017/S0016756816001175 Zhu, B., Becker, H., Jiang, S.Y., Pi, D.H., Fischer-Gödde, M., Yang, J.H., 2013. Re-Os geochronology of black shales from the Neoproterozoic Doushantuo Formation, Yangtze platform, South China. Precambrian Res. 225, 67–76. https://doi.org/10.1016/j.precamres.2012.02.002 Zhu, M., Zhang, J., Steiner, M., Yang, A., Li, G., Erdtmann, B.-D., 2003. Sinian-Cambrian stratigraphic framework for shallow- to deep-water environments of the Yangtze Platform : an integrated approach. Prog. Nat. Sci. 13, 75–84.
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Table 1 238U16O2/238U16O and 206Pb*/238U16O ratios of PRAP standard apatite sample 238U16O /238U16O 2
PRAP_1 PRAP_2 PRAP_3 PRAP_4 PRAP_5 PRAP_6 PRAP_7 PRAP_8 PRAP_9 PRAP_10
0.767 0.629 0.527 0.435 0.771 0.636 0.605 0.736 0.707 0.645
± ± ± ± ± ± ± ± ± ±
0.005 0.004 0.003 0.003 0.006 0.007 0.005 0.004 0.004 0.004
206Pb*/238U16O
0.191 0.159 0.139 0.113 0.209 0.169 0.147 0.187 0.184 0.168
± ± ± ± ± ± ± ± ± ±
0.002 0.001 0.001 0.001 0.002 0.002 0.002 0.001 0.001 0.001
Primary ion source maintenance PRAP_11 0.639 ± 0.003 0.172 ± 0.001 PRAP_12 0.761 ± 0.004 0.216 ± 0.002 PRAP_13 0.851 ± 0.005 0.244 ± 0.002 PRAP_14 0.487 ± 0.003 0.142 ± 0.001 PRAP_15 0.713 ± 0.004 0.190 ± 0.001 PRAP_16 0.712 ± 0.004 0.202 ± 0.001 Error assigned to the ratio is one sigma estimated by counting statistics.
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Table 2. Observed U contents, common lead percentages, 204Pb/206Pb and 238U/206Pb ratios. Spot
204Pb/206Pb
238U/206Pb
U (ppm)
fcom Pb
Gray Rim_1 Gray Rim_2 Gray Rim_3 Gray Rim_4 Gray Rim_5
9.1 8.7 8.3 8.3 8.5
49% 65% 50% 52% 56%
0.0289 0.0385 0.0299 0.0307 0.0333
± ± ± ± ±
0.0028 0.0030 0.0025 0.0026 0.0029
10.16 7.95 8.51 9.16 8.72
± ± ± ± ±
0.30 0.24 0.27 0.29 0.27
Vertical Crack_1 Vertical Crack_2 Vertical Crack_3 Vertical Crack_4 Vertical Crack_5
9.6 10 9.8 8.9 9.5
44% 49% 35% 52% 46%
0.0259 0.0292 0.0211 0.0310 0.0275
± ± ± ± ±
0.0027 0.0025 0.0023 0.0028 0.0029
12.17 13.08 12.91 11.00 12.14
± ± ± ± ±
0.37 0.39 0.39 0.34 0.35
Rim Cap_1 Rim Cap_2 Rim Cap_3 Rim Cap_4 Rim Cap_5
7.0 6.8 6.3 5.9 6.6
55% 50% 53% 55% 53%
0.0326 0.0297 0.0313 0.0327 0.0317
± ± ± ± ±
0.0029 0.0038 0.0027 0.0033 0.0034
6.14 7.46 7.80 9.48 9.99
± ± ± ± ±
0.20 0.42 0.61 0.92 0.80
Circle Crack_1 Circle Crack_2 Circle Crack_3 Circle Crack_4 Circle Crack_5
9.2 10 8.2 8.6 9.2
48% 49% 45% 41% 49%
0.0287 0.0292 0.0269 0.0243 0.0290
± ± ± ± ±
0.0030 0.0028 0.0026 0.0025 0.0028
10.78 12.01 11.40 11.67 11.49
± ± ± ± ±
0.41 0.47 0.57 0.49 0.41
Egg_1 Egg_2 Egg_3
9.8 15 13
53% 57% 69%
0.0318 ± 0.0030 0.0338 ± 0.0031 0.0412 ± 0.0028
Cracked_1 Cracked_2 Cracked_3 Cracked_4 Cracked_5 Cracked_6
11 12 11 11 12 12
61% 63% 62% 72% 59% 60%
0.0361 0.0372 0.0367 0.0432 0.0353 0.0360
Oblate_1 Oblate_2 Oblate_3
11 9.9 9.6
65% 52% 51%
0.0385 ± 0.0038 0.0311 ± 0.0033 0.0307 ± 0.0028
7.98 ± 0.64 8.28 ± 0.57 8.24 ± 0.40
Fuzzy Rim_1
8.8
36%
0.0217 ± 0.0037
13.97 ± 0.55
24
± ± ± ± ± ±
0.0030 0.0029 0.0028 0.0037 0.0026 0.0029
9.82 ± 0.40 5.89 ± 0.89 4.71 ± 0.46 9.22 7.32 7.47 6.53 6.65 8.32
± ± ± ± ± ±
0.38 0.48 0.32 0.24 0.33 0.41
Fuzzy Rim_2 Fuzzy Rim_3 Fuzzy Rim_4 Fuzzy Rim_5
9.3 10 7 7.6
45% 39% 49% 26%
0.0269 0.0230 0.0293 0.0155
± ± ± ±
0.0036 0.0032 0.0033 0.0032
13.93 14.04 13.80 14.62
± ± ± ±
0.75 0.70 0.78 0.94
Sphere Crack_1 Sphere Crack_2 Sphere Crack_3 Sphere Crack_4 Sphere Crack_5 Sphere Crack_6
7.3 6.8 7.3 6.8 7.2 5.8
40% 33% 42% 46% 42% 50%
0.0237 0.0198 0.0247 0.0275 0.0250 0.0300
± ± ± ± ± ±
0.0028 0.0026 0.0027 0.0022 0.0030 0.0029
10.99 9.57 11.41 10.22 10.80 7.97
± ± ± ± ± ±
0.82 0.75 0.61 0.53 0.81 0.42
Central Band_1 Central Band_2 Central Band_3 Central Band_4 Central Band_5 Central Band_6
6.6 6.2 6.3 6.0 6.2 6.2
32% 57% 35% 45% 51% 35%
0.0191 0.0342 0.0211 0.0269 0.0303 0.0208
± ± ± ± ± ±
0.0032 0.0029 0.0024 0.0029 0.0030 0.0020
12.12 11.75 11.03 11.07 11.51 11.55
± ± ± ± ± ±
0.71 0.66 0.66 0.90 0.82 0.70
Sphere Vein_1 Sphere Vein_2
7.1 7.5
32% 41%
0.0188 ± 0.0020 0.0244 ± 0.0026
Bean_1 Bean_2 Bean_3 Bean_4 Bean_5 Bean_6
7.2 6.0 6.0 5.5 6.4 6.5
54% 47% 46% 48% 50% 36%
0.0320 0.0277 0.0276 0.0286 0.0298 0.0215
± ± ± ± ± ±
0.0035 0.0029 0.0028 0.0028 0.0031 0.0022
10.34 11.69 11.61 10.05 11.79 11.92
± ± ± ± ± ±
0.73 0.60 0.60 0.56 0.55 0.54
Extra_1 Extra_2 Extra_3 Extra_4 Extra_5
6.4 7.3 6.8 6.6 6.4
31% 25% 41% 29% 36%
0.0187 0.0149 0.0243 0.0175 0.0216
± ± ± ± ±
0.0028 0.0024 0.0024 0.0027 0.0023
12.98 13.04 11.88 13.26 10.73
± ± ± ± ±
0.78 0.70 0.69 0.72 0.63
25
12.33 ± 0.96 12.85 ± 0.92
Figure captions Fig. 1 Geological map (modified from Zhang and Yuanl, 1992) and schematic section of Doushantuo phosphorites at Weng’an. Fig. 2a Optical images of fossil-associated apatite of Doushantuo Formation. Fig. 2b Representative image showing the sputtering spot
Fig. 3 Relation between the 238U16O2/238U16O and 206Pb/238U16O ratios for the PRAP standard apatite. Two curves, before and after Duo maintance, show the best fit with the parameter “a=0.289, b=-0.006” and “a=0.264, b=-0.004” respectively, where the equation is as follows: (206Pb+/238U16O+)obs = a x (238U16O2+/238U16O+)2obs + b.
Fig. 4 Correlation diagram between the 204Pb/206Pb and 238U/206Pb ratios of the apatites. The error assigned to the symbol is 2 sigma. A least-squares fitting of the line gives the Y-Intercept of 17.5 ±3.0, calculated by York method (IsoPlot/Ex).
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Fig. 1
27
Fig. 2a
28
Fig. 2b
29
Fig.3
30
Fig.4
31
32
33
34
35
Highlights
Fossil-associated precipitated apatite of Doushantuo Formation, China, were in situ analyzed using NanoSIMS.
Sixty-two spots on thirteen apatite grains yield a 238U/206Pb isochron age of 305 ± 26 Ma.
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High common Pb and low U concentration in those apatite specimens limit the precision of age determination.
The resultant age suggests that the Doushantuo Formation may have experience a postdepositional hydrothermal event.
To The Editor Precambrian Research
Sub: Revised submission of PRECAM_2018_550
Dear Editor, Greetings!
The authors declare no competing interests.
Thanking you Sincerely yours
Chuan-Hsiung Chung Postdoctoral researcher Department of Earth Sciences, National Cheng Kung University No. 1, University Rd., 701 Tainan, Taiwan
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S0301-9268(18)30673-9 https://doi.org/10.1016/j.precamres.2019.105564 PRECAM 105564
To appear in:
Precambrian Research
Received Date: Revised Date: Accepted Date:
31 December 2018 25 September 2019 4 December 2019
Please cite this article as: C-H. Chung, C-F. You, J. William Schopf, N. Takahata, Y. Sano, NanoSIMS U-Pb dating of fossil-associated apatite crystals from Ediacaran (∼570 Ma) Doushantuo Formation, Precambrian Research (2019), doi: https://doi.org/10.1016/j.precamres.2019.105564
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NanoSIMS U-Pb dating of fossil-associated apatite crystals from Ediacaran (~570 Ma) Doushantuo Formation Chuan-Hsing Chung1,*, Chen-Feng You1, James William Schopf2, Naoto Takahata3, Yuji Sano3
1 Department
of Earth Sciences, National Cheng Kung University, Tainan, Taiwan
2 Department
of Earth and Space Sciences, University of California, Los Angeles, Los Angeles,
USA 3 Atmosphere
and Ocean Research Institute, University of Tokyo, Chiba, Japan
1
Abstract Sedimentary rocks record a wealth of information on the evolution of lithosphere, hydrosphere, atmosphere and biosphere. However, uncertainty about the age of many Precambrian sequences makes it very difficult to establish the accurate temporal framework which can dramatically improve our understanding of the scale and duration of geological and biological events. The Neoproterozoic Doushantuo Formation, South China, preserve unique assemblages of early multicellular fossils and records valuable paleoenvironmental information. The age of these formations is thus critical for understanding the important biological and climatic events that occurred towards the end of the Proterozoic Eon. In this study, we used NanoSIMS to date samples from the Doushantuo Formations by apatite U-Pb geochronology. Fossil-associated precipitated apatite grains were identified using optical microscope and Raman spectrometer. The resultant age is 305 ± 26 Ma. High common Pb and low U concentration in those apatite specimens limit the precision of age determination, while the yielded age suggests that the Doushantuo Formation may have experience a post-depositional hydrothermal event.
Keywords: U-Pb geochronology, apatite, NanoSIMS, Doushantuo Formation
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1. Introduction Sedimentary rocks record wealth of information on the evolution of lithosphere, hydrosphere, atmosphere and biosphere. The ages of sedimentary formations especially in the Phanerozoic period are conventionally estimated from chemo- or biostratigraphy correlation. In older rocks, the age determination using this method become inapplicable due to the absence of fossil records. Some indirect methods, such as dating of contemporaneous volcanic rocks, bracketing relationships of igneous and metamorphic rocks, and the dating of detrital and diagenetic minerals (Rasmussen, 2005), can be used to constrain the age of sedimentary rocks. But many basins lack suitable volcanic rocks and sometimes the range of constrained ages can be up to hundreds of millions of years. Dating detrital minerals shows very limited information about the depositional age since it can only yield maximum ages. The feasibility of the radiometric dating methods for diagenesis were established with ArAr age of glauconies (Smith et al., 1993), Rb-Sr age of illite (Morton, 1985), K-Ar age of diagenetic K-feldspar (Girard et al., 1988), Pb-Pb age of monazite nodules (Evans et al., 2002) and U-Pb age of apatite (Chandler and Parrish, 1989). Most diagenetic minerals form in low temperature environment and have low closure temperature, making them prone to later thermal resetting, therefore are not suitable for geochronology. A recent development in sedimentary geochronology has been the identification of the U–Pb chronometer, xenotime, as a diagenetic phase in sedimentary rocks (Fletcher et al., 2000; Rasmussen et al., 2004; Rasmussen, 2005). Being a common U-bearing accessory mineral in sedimentary rocks, U–Pb dating of apatite has potential application in sedimentary provenance studies. Apatite also has been widely used in thermochronology studies (Chew and Spikings, 2015). The relatively high closure temperature of ca. 325-550 °C ( McDougall and Harrison, 1999; Chamberlain and Bowring, 2001; Schoene and 3
Bowring, 2007) makes U-Pb apatite system a useful tool to study the deposition ages of sedimentary or low-grade meta-sedimentary rocks. A variety of methods are currently available for U-Pb geochronology of diagenetic minerals. Three main techniques used are Thermal Ionization Mass Spectrometry (TIMS), Secondary Ion Mass Spectrometry (SIMS) and Multicollector Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS). Traditionally the uranium and lead isotope of apatite has been measured by isotope dilution mass spectrometry ( Oosthuyzen and Burger, 1973; Romer, 1996; Corfu and Stone, 1998). Using an ion microprobe (Li et al., 2012; McNaughton et al., 1999; Sano et al., 1999a; Vallini et al., 2002) or LA-ICPMS (Willigers et al. 2002; Chew et al. 2011; Thomson et al. 2012), it is possible to date xenotime and apatite with a spatial resolution of 530μm. Isotope dilution TIMS involves the chemical digestion of entire crystals and is more precise than in situ methods. However, heterogeneity and small sizes which are typical characteristics of diagenetic minerals make the microprobe techniques more appropriate. Under ideal conditions, the LA-ICPMS can yield U–Pb ages that approach the accuracy and precision delivered by SIMS in a fraction of the time with the cost of much larger sample consumption. Analysis by ion microprobe is the method used in this study because of the fine spatial resolution, shallow penetration depth and high precision. The primary limitation of apatite U-Pb geochronology is poor dating precision cause by low U concentration and high common lead content. The Ediacaran marks the final geological period of the Proterozoic eon between the termination of the global Marinoan glaciation at ∼635 Ma and the beginning of the Cambrian at 542 Ma. The Ediacaran Earth experienced major biological changes, including radiation of complex multicellular life such as globally distributed Ediacaran-type soft-bodied animals, and 4
the environmental oxygen level was also suggested to be increasing base on the physiological requirement of these complex animals and other geochemical evidence. However, the history of this oxygenation event is not adequately constrained due to insufficient information in terms of radiometric age and oxygen concentration. The Neoproterozoic Doushantuo Formation, South China, preserve unique assemblages of early multicellular fossils and overlies rocks, which are thought to have formed during an ice age of global extent. It is the main phosphorite-hosting strata in South China and preserves the earliest diverse eukaryote assemblage (Zhang, 1989; Li, 1998; Xiao et al., 1998; Chen et al., 2000). This allow a more complete understanding of the late Proterozoic biosphere. Recently, a novel apatite-based oxygen paleobarometer which use the fluorescence signatures of fossilassociated apatite specimens to quantify the ambient O2 concentrations, has been established. Fluorescence signatures associated with an oxygen-dependent mechanism for Sm3+-substitution in apatite are used to devise a semi-quantitative apatite oxygen paleobarometer indicative of ambient O2 concentrations during Sm3+ emplacement (Garcia, 2018). The age of these formations is thus critical for understanding the important biological and climatic events that occurred towards the end of the Proterozoic Eon. However, uncertainty about the age of many Precambrian sequences makes it very difficult to establish the accurate temporal framework which can dramatically improve our understanding of the scale and duration of geological and biological events. The purpose of this study is to determine the formation age of fossil associated apatite found in Neoproterozoic Doushantuo Formations in Yangtze Block using U-Pb dating method. Here, we report U-Pb analysis of thirteen apatite specimens extracted from phosphorite of Doushantuo
5
Formation using NanoSIMS. These results will help us to reconstruction the oxygenation events of Ediacaran Ocean.
2. Geological setting and previous dating works The Doushantuo Formation is a recently discovered lagerstätte in Guizhou Province, China that is most notable for its scientific contributions in the hunt for Precambrian life. Doushantuo is of particular interest because its fossils, dating from about 565 to 590 million years ago, predate the Cambrian Explosion by at least 20 million years. The most fossiliferous zones are estimated to be 570 million years old. The Yangtze Platform evolved from a rift basin to a passive continental margin during the Ediacaran-Cambrian transition. The Doushantuo Formation is represented by a phosphate-dolostone sequence at Weng'an, where it is 33 to 55 m thick and consists mainly of dark phosphate, cherty phosphate, chert, and gray dolomite. The overlying Dengying Formation, of Ediacaran (~565 Ma to 544 Ma) age and containing rare Ediacaran body fossils in the lower part and basal Cambrian shelly fossils near the top, is a 180 m thick dolomite sequence. The Doushantuo Formation is further divided into 4 Members in ascending order: Member 1 consists of a ~5m thick cap carbonate which overlies the tillites deposited during the Nantuo (Marinoan) glaciation; Member 2 is composed of an alternating shale-mudstone-dolostone sequence; Member 3 consists mainly of dolostone and limestone, Member 4 (also called the Miaohe Member) consists of ~13m thick black shales (Sawaki et al., 2010; Wang et al., 1998; Zhu et al., 2003).
6
Precise zircon U–Pb dating of volcanic tuff beds from the lower and uppermost Doushantuo Formation has constrained the age of the Doushantuo Formation to between 635.2±0.6 and 551.1±0.7 Ma (Condon et al., 2005). SIMS zircon U-Pb analysis for a tuffaceous bed immediately above the upper phosphorite unit in the Doushantuo Formation yield an age of 609±5 Ma (Zhou et al., 2017). The Re–Os age for the base of Doushantuo Member 4 has been determined to be 591.1±5.3 Ma (Zhu et al., 2013). Dating of Doushantuo phosphorites by a LuHf dating method and Pb-Pb geochronometry independently yielded ages of 584±26 Ma and 599.3±4.2 Ma, respectively (Barfod et al., 2002). Pb isotopic composition analysis of Doushantuo black shales yielded a Pb isotope isochron ages of 572±36Ma (Chen et al., 2009) and the Pb-Pb age of upper part of Doushantuo phosphorites was determined to be 576±14Ma (Chen et al., 2004). The apatite samples used in this study were collected from Phosphorite Unit B of Doushantuo Formation at Chuanyandong, Weng’an area, Guizhou Province, China. The stratigraphic level is 4.5m above base of Phosphorite Unit B (Fig. 1). Each apatite specimen is several hundred micrometers in size (Fig. 2).
3. Analytical methods
Thin sections are prepared from collected rock samples and fossil-associated precipitated apatite grains were identified using optical microscope and Raman spectrometer. Selected apatite specimens are polished until the mid-sections of the grains were exposed. The thin sections were cleaned by deionized water in an ultrasonic bath and gold-coated. Then the sample was heated
7
by a lamp in a vacuum chamber for ~12 hours before introducing it into NanoSIMS 50 ion microprobe installed at the Atmosphere and Ocean Research Institute, The University of Tokyo. The NanoSIMS analytical method used in this study is well-established and has been applied to date biogenic apatites and other phosphate minerals (Koike et al., 2016, 2014; Sano et al., 2014; Terada et al., 2018). Prior to ion microprobe analysis, sample was kept evacuated in the air-rock system of NanoSIMS overnight in order to reduce possible hydride interference cause by water absorbed onto the surface of the sample mount. During preliminary NanoSIMS test, we found that the U concentrations of our apatites are roughly one tenth of a typical sedimentary apatite. The low U contents which limited production of radiogenic Pb may hinder the attempts to apply in situ SIMS U-Pb technique. To acquire statistically sufficient counts, a higher primary O- beam current of about 10nA was focused onto the sample with a diameter of about 15µm. Before the actual analysis, the sample surface was rastered by 15 µ m square for 5 min in order to reduce the contribution of surface contaminant Pb to the analysis. The sputtered secondary positive ions were extracted using an acceleration voltage of 8kV. The generated secondary ions are focused and transferred for mass analysis and detection. The mass resolution of mass spectrometer was set to 4100 at 1% peak height to separate 206Pb from 143Nd31P16O2 with adequate flat topped peaks. 204Pb beams are also discriminated from 172Yb16O2 beams. The Mattauch-Herzog geometry mass analyzer used in NanoSIMS makes the simultaneous collection of 31P+, 43Ca+, 204Pb+, 206Pb+, 238U16O+, 238U16O2+
under a static magnetic field possible which reduced cycle
time dramatically. All these ions were detected by low noise ion counter. In NanoSIMS, the ion dispersion at high mass number after the magnet should become significantly short which makes it impossible to detect 204Pb and 206Pb at the same time by an original configuration of NanoSIMS, since the distance between them at the focal plane is only 2 mm. To improve
8
analytical efficiency and precision, a customer ordered dual collector system to detect 204Pb and 206Pb
at the same time was installed in NanoSIMS at The University of Tokyo. The integration
time for each analysis was set to be 10mins to acquire statistically sufficient counts. At this setting, the total time needed for each analysis was 15min including rastering. We did not apply the 206Pb-207Pb measurement because peak jump mode which is necessary to measure the extra 207Pb
signal would take a much longer time of about 60mins. Furthermore, previous study
suggested that the 206Pb–207Pb isochron of biogenic apatite did not provide a precise formation age (Sano and Terada, 1999; Sano and Terada, 2001). We decided to process as many spots as possible using the 238U-206Pb systematics under the allocated machine time. Precision and accuracy in SIMS U-Pb geochronology strongly depend on the method of determination of the inter-element ion ratios from the measured secondary ion ratios (Jeon and Whitehouse, 2015). Therefore, a well-characterized matrix-matched standard, “PRAP”, derived from an alkaline rock of the Prairie Lake circular complex in the Canadian Shield dated at 1155±20 Ma by SHRIMP II (Sano et al., 2006b; Sano and Terada, 1999) was mount along with samples in each analytical session and used to correct for elemental fractionation. The background of ion counters and intensity of 204Pb were also been accurately determined to ensure robust isotope ratio correction
4. Results and discussion
4.1 UO2/UO-Pb/UO calibration
9
Previous studies show that the secondary Pb+/U+ ratio produced by a primary 16O- beam can differ by as much as a factor of two for a target of constant Pb/U ratio (Williams, 1998). However, it was possible to calculate the Pb/U ratio using a correction factor determined by the relationship between UO+/U+ and Pb+/U+ in SHRIMP analysis (Sano et al., 1999a). For NanoSIMS, Pb is emitted almost entirely as Pb+ while UO2+ : UO+:U+ ions are produced by oxygen bombardments are about 10:10:1 (Sano et al., 2014). It was difficult to accumulate statistically sufficient U+ counts within 10 min due to the low concentration in the specimens. We decided to apply an UO2/UO–Pb/UO calibration instead of the UO/U–Pb/U system used for SHRIMP. The new calibration showed good performance for monazite (Sano et al., 2006a) and zircon (Takahata et al., 2008) using NanoSIMS. Correlation was found between the 206Pb+/238U16O+
and 238U16O2+/238U16O+ ratios of PRAP standard apatite. The variations are
expressed using the empirical quadratic relation as follows. ( 206Pb+ / 238U16O+ )obs = a × ( 238U16O2+ / 238U16O+
+b
Therein, obs is the observed ratio, and a and b are constants. The 206Pb/238U ratio of an unknown sample is calculated using the following equation. 206Pb / 238U
= c× ( 238U16O2+ / 238U16O+)obs / [a × ( 238U16O2+ / 238U16O+
+ b]
In that equation, the constants a, b and c are determined by repeated measurements of the PRAP standard and using the 238U–206Pb age of 1155 ± 20 Ma (Sano et al., 2006b). Table 1 lists 206Pb+/238U16O+
and 238U16O2+/238U16O+ ratios of PRAP apatite sample. The relation between the
206Pb+/238U16O+
and 238U16O2+/238U16O+ ratios of the PRAP standard is shown as Fig. 3. The
reproducibility of standard was very well (less than 5%). There are apparently two curves which represent two different ionization relationships. This was because that the primary ion source 10
(Duoplasmatron source) of the instrument need to be repaired during the analytical session therefore changed the ionization behaviors. The best fit results of these two curves were used to calculate 238U/206Pb ratios accordingly. The uncertainty in the 206Pb/238U ratio was derived from the external reproducibility of the standard and internal error of sample analysis. We also estimate the U concentration of sample based on the observed 238U16O+/43Ca+ ratio calibrated against that of the PRAP standard.
4.2 U-Pb age of fossil-associated apatite
Sixty-two NanoSIMS analyses were conducted on 13 apatite specimens. Table 2 presents the U contents, 204Pb/206Pb and 238U/206Pb ratios of the sample spot, where the error assigned to the ratio is reported to two sigma. The U contents range from 5.5 ppm to 15 ppm with the average of 8.3 ppm, which is on the lower end of typical uranium concentration of apatite used in geochronology studies (Li et al., 2012; Sano et al., 2014, 2006b, 1999a; Sano and Terada, 1999). The 204Pb/206Pb ratios vary from 0.0149 to 0.0432. Our apatite grains contain high proportion of common lead, ranging from 25% to 72%. The 238U/206Pb ratios are highly variable, range from 4.7 to 14.6 due to different amount of common Pb. Fig. 4 shows the relation between the 204Pb/206Pb
and 238U/206Pb ratios. A least-squares fit of these five data using the York method
gives the 238U–206Pb isochron age of 305 ± 26 Ma (2σ; MSWD = 1.6). The initial 206Pb/204Pb ratio was estimated to be 17.5 ± 3.0 (2σ), which is consistent with the average common Pb of 18.315 at 250 Ma (Stacey and Kramers, 1975) within experimental error. The age is significantly younger than previously estimated deposition ages of Doushantuo Formation (550~635Ma). The
11
dating results indicating closure for U-Pb system in an event as late as 300Ma. This suggests either that the apatite in Doushantuo Formation crystallized about 300Ma ago, or that they grew earlier than that but some subsequent thermal process reset the U-Pb system. Some submarine hydrothermal activity has produced a depositional sequence dominated by carbonates in the first stage (calcite and dolomite) with subsequent apatite and late barite veins (Núñez-Cornú et al., 2000). But the 200Ma time span of various mineral deposition stages is highly implausible in submarine hydrothermal system. Also, the selected apatite grains in this study are all infill microscopic fossil, showing their very early emplacement, prior to decay and disintegration of their fossil host. Therefore, the hypothesis that apatite grew at 250Ma later than the deposition of Doushantuo can be rule out. The other possibility is the post-depositional alteration event in Carboniferous which might be related to the overgrowth or the reset of radiogenic age of the phosphate that is originally precipitated in Neoproterozoic-Cambrian transition. However, the extraordinary fossil preservation (Xiao and Knoll, 2000, 1999) and REE patterns in Doushantuo phosphorites (Chen et al., 2003) have also previously been interpreted as being free from postdepositional alteration. Sano et al. (1999c) reported the 1500Ma U-Pb age obtained on apatites from sediment sequences of Akilia island, southwest Greenland, that are originated more than 3850Ma old. They conclude that, about 1500Myr ago, these apatites experienced a metamorphic event of about 600°C. The apatite specimens used in this study may have also experienced a similar thermal event about 300Myr ago. The closure temperature for apatite was found to between 550°C and 325°C (McDougall and Harrison, 1999). Given the small size of the apatite samples used in this study, the closure temperature could be even lower. Clumped isotope study on Member 2 of Doushantuo Formation indicate that the carbonate grew from hydrothermal fluid with ambient temperature of 86-476°C (Bristow et al., 2011). Clay mineral, organic matter and
12
K-Ar dating studies show that the depositional geochemical records preserved in Doushantuo Formation have been overprinted by hydrothermal event at ~325Ma (Derkowski et al., 2013), which is within the error of U-Pb isochron age of Doushantuo apatites obtained in this study. Therefore, the U-Pb system of Doushantuo apatite may have been reset by thermal induced activities.
5. Conclusion
In this paper we present NanoSIMS U-Pb dating results on Doushantuo fossil-associated apatite. Sixty-two spots on thirteen apatite grains yield a 238U/206Pb isochron age of 305 ± 26 Ma. The age is significantly younger than previously estimated deposition ages of Doushantuo Formation (550~635Ma). It appears that there was a post-depositional hydrothermal event at about 300Ma which reset the U-Pb system.
Acknowledgments
We thank Der-Chuen Lee, Zan Peeters and Sung-Yun Hsiao at IESAS for the help on preliminary NanoSIMS test on apatite samples. The authors express thanks to Takanori Kagoshima for
assistance in the NanoSIMS analysis at University of Tokyo. Supported by MOST 105-2116-M006-011 to Chen-Feng You.
13
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https://doi.org/10.1080/10020070312331344710
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Table 1 238U16O2/238U16O and 206Pb*/238U16O ratios of PRAP standard apatite sample 238U16O /238U16O 2
PRAP_1 PRAP_2 PRAP_3 PRAP_4 PRAP_5 PRAP_6 PRAP_7 PRAP_8 PRAP_9 PRAP_10
0.767 0.629 0.527 0.435 0.771 0.636 0.605 0.736 0.707 0.645
± ± ± ± ± ± ± ± ± ±
0.005 0.004 0.003 0.003 0.006 0.007 0.005 0.004 0.004 0.004
206Pb*/238U16O
0.191 0.159 0.139 0.113 0.209 0.169 0.147 0.187 0.184 0.168
± ± ± ± ± ± ± ± ± ±
0.002 0.001 0.001 0.001 0.002 0.002 0.002 0.001 0.001 0.001
Primary ion source maintenance PRAP_11 0.639 ± 0.003 0.172 ± 0.001 PRAP_12 0.761 ± 0.004 0.216 ± 0.002 PRAP_13 0.851 ± 0.005 0.244 ± 0.002 PRAP_14 0.487 ± 0.003 0.142 ± 0.001 PRAP_15 0.713 ± 0.004 0.190 ± 0.001 PRAP_16 0.712 ± 0.004 0.202 ± 0.001 Error assigned to the ratio is one sigma estimated by counting statistics.
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Table 2. Observed U contents, common lead percentages, 204Pb/206Pb and 238U/206Pb ratios. Spot
204Pb/206Pb
238U/206Pb
U (ppm)
fcom Pb
Gray Rim_1 Gray Rim_2 Gray Rim_3 Gray Rim_4 Gray Rim_5
9.1 8.7 8.3 8.3 8.5
49% 65% 50% 52% 56%
0.0289 0.0385 0.0299 0.0307 0.0333
± ± ± ± ±
0.0028 0.0030 0.0025 0.0026 0.0029
10.16 7.95 8.51 9.16 8.72
± ± ± ± ±
0.30 0.24 0.27 0.29 0.27
Vertical Crack_1 Vertical Crack_2 Vertical Crack_3 Vertical Crack_4 Vertical Crack_5
9.6 10 9.8 8.9 9.5
44% 49% 35% 52% 46%
0.0259 0.0292 0.0211 0.0310 0.0275
± ± ± ± ±
0.0027 0.0025 0.0023 0.0028 0.0029
12.17 13.08 12.91 11.00 12.14
± ± ± ± ±
0.37 0.39 0.39 0.34 0.35
Rim Cap_1 Rim Cap_2 Rim Cap_3 Rim Cap_4 Rim Cap_5
7.0 6.8 6.3 5.9 6.6
55% 50% 53% 55% 53%
0.0326 0.0297 0.0313 0.0327 0.0317
± ± ± ± ±
0.0029 0.0038 0.0027 0.0033 0.0034
6.14 7.46 7.80 9.48 9.99
± ± ± ± ±
0.20 0.42 0.61 0.92 0.80
Circle Crack_1 Circle Crack_2 Circle Crack_3 Circle Crack_4 Circle Crack_5
9.2 10 8.2 8.6 9.2
48% 49% 45% 41% 49%
0.0287 0.0292 0.0269 0.0243 0.0290
± ± ± ± ±
0.0030 0.0028 0.0026 0.0025 0.0028
10.78 12.01 11.40 11.67 11.49
± ± ± ± ±
0.41 0.47 0.57 0.49 0.41
Egg_1 Egg_2 Egg_3
9.8 15 13
53% 57% 69%
0.0318 ± 0.0030 0.0338 ± 0.0031 0.0412 ± 0.0028
Cracked_1 Cracked_2 Cracked_3 Cracked_4 Cracked_5 Cracked_6
11 12 11 11 12 12
61% 63% 62% 72% 59% 60%
0.0361 0.0372 0.0367 0.0432 0.0353 0.0360
Oblate_1 Oblate_2 Oblate_3
11 9.9 9.6
65% 52% 51%
0.0385 ± 0.0038 0.0311 ± 0.0033 0.0307 ± 0.0028
7.98 ± 0.64 8.28 ± 0.57 8.24 ± 0.40
Fuzzy Rim_1
8.8
36%
0.0217 ± 0.0037
13.97 ± 0.55
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± ± ± ± ± ±
0.0030 0.0029 0.0028 0.0037 0.0026 0.0029
9.82 ± 0.40 5.89 ± 0.89 4.71 ± 0.46 9.22 7.32 7.47 6.53 6.65 8.32
± ± ± ± ± ±
0.38 0.48 0.32 0.24 0.33 0.41
Fuzzy Rim_2 Fuzzy Rim_3 Fuzzy Rim_4 Fuzzy Rim_5
9.3 10 7 7.6
45% 39% 49% 26%
0.0269 0.0230 0.0293 0.0155
± ± ± ±
0.0036 0.0032 0.0033 0.0032
13.93 14.04 13.80 14.62
± ± ± ±
0.75 0.70 0.78 0.94
Sphere Crack_1 Sphere Crack_2 Sphere Crack_3 Sphere Crack_4 Sphere Crack_5 Sphere Crack_6
7.3 6.8 7.3 6.8 7.2 5.8
40% 33% 42% 46% 42% 50%
0.0237 0.0198 0.0247 0.0275 0.0250 0.0300
± ± ± ± ± ±
0.0028 0.0026 0.0027 0.0022 0.0030 0.0029
10.99 9.57 11.41 10.22 10.80 7.97
± ± ± ± ± ±
0.82 0.75 0.61 0.53 0.81 0.42
Central Band_1 Central Band_2 Central Band_3 Central Band_4 Central Band_5 Central Band_6
6.6 6.2 6.3 6.0 6.2 6.2
32% 57% 35% 45% 51% 35%
0.0191 0.0342 0.0211 0.0269 0.0303 0.0208
± ± ± ± ± ±
0.0032 0.0029 0.0024 0.0029 0.0030 0.0020
12.12 11.75 11.03 11.07 11.51 11.55
± ± ± ± ± ±
0.71 0.66 0.66 0.90 0.82 0.70
Sphere Vein_1 Sphere Vein_2
7.1 7.5
32% 41%
0.0188 ± 0.0020 0.0244 ± 0.0026
Bean_1 Bean_2 Bean_3 Bean_4 Bean_5 Bean_6
7.2 6.0 6.0 5.5 6.4 6.5
54% 47% 46% 48% 50% 36%
0.0320 0.0277 0.0276 0.0286 0.0298 0.0215
± ± ± ± ± ±
0.0035 0.0029 0.0028 0.0028 0.0031 0.0022
10.34 11.69 11.61 10.05 11.79 11.92
± ± ± ± ± ±
0.73 0.60 0.60 0.56 0.55 0.54
Extra_1 Extra_2 Extra_3 Extra_4 Extra_5
6.4 7.3 6.8 6.6 6.4
31% 25% 41% 29% 36%
0.0187 0.0149 0.0243 0.0175 0.0216
± ± ± ± ±
0.0028 0.0024 0.0024 0.0027 0.0023
12.98 13.04 11.88 13.26 10.73
± ± ± ± ±
0.78 0.70 0.69 0.72 0.63
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12.33 ± 0.96 12.85 ± 0.92
Figure captions Fig. 1 Geological map (modified from Zhang and Yuanl, 1992) and schematic section of Doushantuo phosphorites at Weng’an. Fig. 2a Optical images of fossil-associated apatite of Doushantuo Formation. Fig. 2b Representative image showing the sputtering spot
Fig. 3 Relation between the 238U16O2/238U16O and 206Pb/238U16O ratios for the PRAP standard apatite. Two curves, before and after Duo maintance, show the best fit with the parameter “a=0.289, b=-0.006” and “a=0.264, b=-0.004” respectively, where the equation is as follows: (206Pb+/238U16O+)obs = a x (238U16O2+/238U16O+)2obs + b.
Fig. 4 Correlation diagram between the 204Pb/206Pb and 238U/206Pb ratios of the apatites. The error assigned to the symbol is 2 sigma. A least-squares fitting of the line gives the Y-Intercept of 17.5 ±3.0, calculated by York method (IsoPlot/Ex).
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Fig. 1
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Fig. 2a
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Fig. 2b
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Fig.3
30
Fig.4
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32
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Highlights
Fossil-associated precipitated apatite of Doushantuo Formation, China, were in situ analyzed using NanoSIMS.
Sixty-two spots on thirteen apatite grains yield a 238U/206Pb isochron age of 305 ± 26 Ma.
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High common Pb and low U concentration in those apatite specimens limit the precision of age determination.
The resultant age suggests that the Doushantuo Formation may have experience a postdepositional hydrothermal event.
To The Editor Precambrian Research
Sub: Revised submission of PRECAM_2018_550
Dear Editor, Greetings!
The authors declare no competing interests.
Thanking you Sincerely yours
Chuan-Hsiung Chung Postdoctoral researcher Department of Earth Sciences, National Cheng Kung University No. 1, University Rd., 701 Tainan, Taiwan
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