Dissolution methods for strontium isotope stratigraphy: Guidelines for the use of bulk carbonate and phosphorite rocks

Chemical Geology 290 (2011) 133–144

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Research paper

Dissolution methods for strontium isotope stratigraphy: Guidelines for the use of bulk carbonate and phosphorite rocks Da Li a, b,⁎, Graham A. Shields-Zhou b, Hong-Fei Ling a, Matthew Thirlwall c a b c

School of Earth Sciences and Engineering, Nanjing University, 210093, Nanjing, China Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT, UK Department of Earth Sciences, Royal Holloway, University of London, Egham, Surrey, TW20 0EX, UK

a r t i c l e

i n f o

Article history: Received 25 January 2011 Received in revised form 8 September 2011 Accepted 12 September 2011 Available online 17 September 2011 Editor: U. Brand Keywords: Strontium isotope stratigraphy Limestone Dolostone Phosphorite Dissolution methods

a b s t r a c t Diverse carbonate dissolution methods have been applied to bulk carbonate rocks in order to target least altered components for strontium isotope stratigraphy (SIS). This is especially important for Precambrian and Cambrian studies for which no suitable skeletal material is available. One proven method for bulk limestones involves the removal of up to a third of the powdered sample using an acid pre-leach before partial dissolution of the rest of the sample using a weak acid or acid buffer solution. We applied a similar technique to dissolve a range of lithologies (limestone, dolostone and phosphorite) and compared the strontium isotopic composition of various leaches to contemporaneous seawater. Our results vindicate this approach and allow us to conclude that some dolomitic and phosphatic rock components may retain a near-primary 87 86 seawater Sr/ Sr composition once contaminant strontium from secondary calcite and other phases has been removed. Commonly applied trace element (e.g. Mn/Sr, Mg/Ca) and isotopic (C, O) screening cut-offs were also examined in the light of these results, and proved unreliable particularly in the case of dolostones and partially dolomitized limestones. We recommend that rigid sample selection and sequential leaching procedures be applied to all SIS studies of bulk materials and propose a general protocol for strontium isotope studies on marine authigenic rocks which may also be applicable to rare earth element and other geochemical studies of marine authigenic minerals in the future. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Strontium isotope stratigraphy (SIS) is now an established chemostratigraphic tool (Elderfield, 1986; McArthur, 1994; Veizer et al., 87 86 1999) that serves two basic purposes. First, the Sr isotope ( Sr/ Sr) ratio of a sample can constrain its age by comparison with the global 87 86 seawater Sr/ Sr curve (McArthur et al., 2001). Second, if we know 87 86 the approximate age of a sample, we can use its Sr/ Sr ratio to distinguish marine from terrestrial depositional environments (e.g. Zhao et al., 2009). As an additional bonus, changes in the seawater 87 86 Sr/ Sr curve reflect global tectonic and environmental events (Shields, 2007) and so can be used to force or test geochemical and conceptual models of Earth system dynamics (Peucker-Ehrenbrink and Miller, 2006; Halverson et al., 2007). 87 86 SIS is based on the assumption that seawater Sr/ Sr has always been homogeneous on a global scale. This assumption is plausible because the residence time of Sr in the ocean today (~4 Ma) is more than 1000 times longer than the ocean circulation time (~ 1500 a) (Elderfield, 1986). SIS is also based on the premise that at least

⁎ Corresponding author. Tel.: + 86 25 83592660; fax: + 86 25 83592393. E-mail address: [email protected] (D. Li). 0009-2541/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2011.09.004

some sedimentary components in a given rock suite will have 87 86 retained the Sr/ Sr composition of seawater at the time of formation. However, Sr isotopic exchange after deposition is commonplace and so diagenetic alteration needs to be constrained in every SIS study (Brand, 2004). The problem of sample fidelity is reduced in Phanerozoic studies which benefit from the relative abundance of suitable materials such as microscopically well preserved low Mg calcite tests, e.g. foraminifera, brachiopods and belemnites, and apatitic fossils such as conodonts (Veizer et al., 1999; McArthur and Howarth, 2004). Cambrian trilobites, although largely untested, may also pre87 86 serve more faithfully contemporaneous seawater Sr/ Sr than their enclosing whole rock (Brand, 2004). However, for Precambrian and even early Cambrian studies, it is necessary to analyze bulk rock or selected rock components (Derry et al., 1989; Kaufman et al., 1993). SIS can be a crucial correlation tool in the absence of adequate biostratigraphic control and especially in combination with carbon iso13 tope (δ C) stratigraphy (Brasier et al., 1996; Melezhik et al., 2009; Sawaki et al., 2010). As a consequence, SIS is being applied increasingly to late Precambrian and early Cambrian age rocks, during which 87 86 time seawater Sr/ Sr rose sharply from ~ 0.705 to ~0.709 (Shields, 1999; Halverson et al., 2007). Despite such obvious potential, many SIS studies on older successions result in a jumble of meaningless, 87 86 radiogenic Sr/ Sr data which are clearly altered as they deviate

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significantly from the global seawater Sr/ Sr curve (e.g. Wotte et al., 2007). Because of this, it is important not only to identify the least altered samples and sample components but also to limit contamination during sample preparation from non-target phases (Bailey et al., 2000). The most appropriate sample preparation techniques for each type of marine authigenic rock (limestones, dolostones, phosphorites) are still under debate with significant variations in the published literature. Our intention in this article is to provide new data from sequential leaching experiments in order to establish guidelines for future SIS studies on Precambrian and Cambrian successions. SIS follows a six step procedure (Banner, 2004): 1) sample selection using petrographic and geochemical criteria, 2) physical extraction from the host sample, 3) chemical pre-treatment to remove ions from exchangeable or leachable sites within or on mineral surfaces, 4) dissolution of targeted components in the sample, 5) chemical separation of the element Sr, and 6) mass spectrometric analysis. In this study, we deal with the first four steps because the last two steps generally follow standard procedures and so affect measured values to a lesser extent. We widen the scope of previous leaching experiments on limestones (Bailey et al., 2000) to cover dolostone and phosphorite as well as limestone samples from the same section. Our results and published data allow us to recommend a new analytical procedure for future SIS studies, while holding commonly applied trace element and isotopic cut-offs for diagenetic screening up to scrutiny.

2. Geological setting and stratigraphy The samples were collected from Xiaotan section (103°28′16″E, 28°07′17″N), located in the northeastern corner of Yunnan Province, SW China (Fig. 1a). Strata of the Xiaotan section are exposed along the southern bank of the Jingsha River. Palaeogeographic investigations show that the sediments of this section were generally deposited in relatively shallow seawater with low energy, representing continental shelf facies on the western part of the Yangtze Platform (Zhu et al., 2003).

The Xiaotan section consists of – in upward stratigraphic order – the Donglongtan Member, Jiucheng Member and Baiyanshao Member of the Dengying Formation, Daibu Member, Zhongyicun Member and Dahai Member of the Zhujiaqing Formation, which underlies the Shiyantou Formation as well as other Cambrian formations that do not form the subject of this article (Fig. 1b). The Dahai Member comprises gray thickly bedded to massive limestones. The underlying Zhongyicun Member is a phosphorite unit with dolostone interlayers. The Daibu Member contains interbedded siliceous and dolomitic layers. The Baiyanshao and Donglongtan Members both comprise thickly bedded dolostones and are separated by the Jiucheng Member which has been eroded away by the river in this section. The Ediacaran– Cambrian boundary in South China is generally placed at the first appearance of small shelly fossils at the base of the Zhongyicun Member, below which occurs a prominent negative carbon isotope excursion that correlates with many sections globally (Li et al., 2009). Based on carbon isotope stratigraphic correlation between the Xiaotan section (Zhou et al., 1997; Li et al., in revision) and the global compilation of Maloof et al. (2010), the strata containing upper Dengying Formation and Zhujiaqing Formation cover an interval from the late Ediacaran to the early Meishucunian (Fortunian). Geochronological calibrations (Maloof et al., 2010) indicate that the deposition of our samples spanned an interval from approximately 545 Ma to 525 Ma. Samples from the Xiaotan section contain a wide range of lithologies, including limestone, dolostone, phosphorite, siliceous and siliciclastic rock types. The thin phosphatic layers within the Zhongyicun Member mostly exhibit the same texture as fine grained limestones and so may represent early diagenetic replacement of carbonate minerals by phosphate. 3. Methods 3.1. Sample selection and extraction More than two hundred bulk rock samples from one stratigraphic section (Xiaotan) were cut in order to obtain fresh slabs. Mineral compositions and textures of samples (including limestones, dolostones and phosphorites) were studied in thin-section, permitting

Fig. 1. a) Section location on the Yangtze Platform during the late-Neoproterozoic to early-Cambrian interval. b) Stratigraphic column with sample stratigraphic levels marked.

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the recognition of primary and secondary components. Targeted primary rock components were then located on the counterpart slabs and subsequently micro-drilled or micro-milled to extract powder, with careful attention to avoid visibly altered portions of the rock such as calcitic micro-veins and recrystallized calcite or interstitial cements. We used either a static micro-drill assembly or a computer controlled micro-mill which could follow automatically a given pathway and fixed depth on the slab surface. We avoided deep drilling so that the powder we obtained was as similar as possible to petrographic observations. After drilling, we ground the sample further because coarse flakes are not suitable for sequential leaching studies. The powder extractions were carried out at the Bloomsbury Environmental Isotope Facility (BEIF) at UCL. Powders of each sample were dissolved in an excess of 10% hydrochloric acid. The concentrations of some diagnostic major and trace elements (Ca, Mg, Fe, Mn and Sr) were determined using ICP-AES at the Wolfson Laboratory of University College London. The analyses were accurate within an error of b5% for the analyzed elements based on long-term reproducibility of the lab measurement. Based on the principle that samples with high Sr concentrations and low Mn/Sr ratios tend to preserve more primary seawater information, and are thus best to reconstruct SIS (Kaufman and Knoll, 1995), we then selected the 35 samples listed in Table 1, while attempting also to include a variety of lithologies, such as limestones, dolostones and phosphorites. The nineteen limestones are mainly from the Dahai Member except for two from interbedded calcareous layers of the underlying Zhongyicun Member. Five dark brown to black phosphorite samples are all from the Zhongyicun Member. Eleven dolostone samples derive from various stratigraphic levels from the Zhongyicun, Daibu, Baiyanshao and Donglongtan members (Fig. 1b).

3.2. Dissolution of samples and mass spectrometric analysis The chemical experiments were conducted at two different laboratories: at Royal Holloway, University of London (RHUL) and at the State Key Laboratory for Mineral Deposits Research, Nanjing University (NJU) (Fig. 2). These two slightly different studies at separate laboratories permit us to develop a generally applicable protocol and showed no inter-laboratory bias after comparing elemental and isotopic data. Limestone, dolostone and phosphorite samples are given the abbreviations L, D and P, respectively, in the following text and Table 1. We used a simple acetic acid pre-leach instead of the commonly used cation exchange pre-treatment step using ammonium acetate. At RHUL, about 20–60 mg of 18 sample powders of 3 different rock types was used for analysis. As illustrated in Fig. 2, each weighed limestone sample was leached using 0.3% acetic acid of specific volumes on the order of milliliters calculated to sequentially dissolve 30% and then 40% of CaCO3 (assuming total Ca exists as calcite). The cut-offs of around one-third were estimated based on the cumulative (sequential) step-leaching study by Bailey et al. in 2000. Phosphorite samples were leached in the same way to remove all carbonate phases, and then treated with 0.05 M nitric acid in order to dissolve the phosphate phase only. Dolostone samples were leached using 0.2% acetic acid of volumes calculated to sequentially dissolve 30% and then 40% of the CaCO3 + MgCO3 (assuming all measured Ca and Mg is in the carbonate fraction) in each sample. In order to test what minerals and how much of them had been leached during these leaching steps, the leached solutions were collected after passing through cation exchange columns prior to Sr collection, and then analyzed for major and trace elements, such as Mg, Ca and Sr (for Mg/ Ca ratios) by ICP-MS (Fig. 2). The Sr released during each leaching step was purified through resin columns and converted to nitrate using nitric acid, before being loaded on tungsten filaments for isotopic analysis. Sr isotope ratios were determined using a VG354 ® Thermal Ionization Mass Spectrometer (TIMS). Repeated measurement of

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the Sr standard SRM 987 yielded Sr/ Sr = 0.710250 (±13) (2σ external standard deviation, n = 8) during the period of sample analysis. At NJU, sample powders of another 12 limestones and 5 dolostones were leached in a similar way (Fig. 2) with 0.2 M acetic acid. The 0.2 M (~ 1%) and 0.3% acetic acid are both very dilute (weak) acidic solutions, and so are comparable. One other difference was that the amount of dissolved carbonate in the first and second steps was 40% and 30%, respectively, which is reversed compared to the amounts at RHUL, but isotopic results showed no significant differences. This design of cut-offs is instructive for future practical use when it is difficult to get precisely one-third targeted components dissolved. Sr isotope ratios were determined on a Triton TIMS (Finnigan Thermo, Germany). The recent half-year long-run 2 S.D. of the standard SRM 987 was 0.710252 (± 16) (n = 40). We also determined carbon and oxygen isotopes for all these 35 samples in parallel experiments. Sample powders were reacted with orthophosphoric acid at 70 °C for 1 hour to extract CO2 following the principles first determined by McCrea (1950) and Craig (1953). C- and O-isotope compositions of the liberated CO2 were measured using an online analysis system, Finnigan Gasbench II + Delta Plus 13 18 XP at NJU, and reported as δ CV-PDB and δ OV-PDB values, respectively. The deviations of C- and O-isotope results of repeated analyses of standards are both within 0.1‰.

4. Results 4.1. Elemental results The Ca, Mg, Mn and Sr concentrations, acquired in advance for each sample by HCl dissolution (see Table 1), together with scrutinization of thin-sections, provide a better understanding of the rock components and microfabrics. All limestone samples have CaO N30% (except one b18.6%) and MgO b1.1%; Mg/Ca ratios are all b0.05, indicating relatively pure low-Mg calcite in limestones of variable purity. Thin-sections of these samples show three types of limestone: partially recrystallized pelsparite, coarse sparite and siliciclastic sparite (Table 1). All eleven dolostones have CaO + MgO N30%, MgO contents around 10%, and Mg/Ca N0.21 and b0.41, indicating varying degrees of calcite or phosphate admixture. Thin-sections of these dolostones show dolosparite, dolomicrite, or dolomicrosparite. The five phosphorite samples contain dolomitic spar and/or a significant siliciclastic component together with largely peloidal phosphorite (or collophanite). An aliquot of the step-leached solutions from some of the limestone, dolostone and phosphorite samples was analyzed using ICP-MS for Mg/ Ca ratios, and the results are shown in Table 1. For samples L6 and L7, the two step leachates yielded low Mg/Ca ratios, but slightly higher than those of the original limestone samples, suggesting that the weak acid tended to dissolve calcite first, before dissolving trace dolomite or secondary sparry calcite and or dedolomite (dolomite-shaped calcite pseudomorphs). The 1st and 2nd weak acid leachates of samples D1–D6 have Mg/Ca ratios approaching 0.6, which is much higher than for the bulk carbonates, suggesting that all the trace calcite had been dissolved in the 1st leach leaving only dolomite remaining. The Mg/Ca ratios of the nitric acid leachates of samples P1–P5 are near zero, which suggests that there was no dolomite left after preleaching. In this case, there cannot have been any calcite left either since calcite tends to react with acetic acid more readily than dolomite. Thus, the third (nitric acid) leachate for these samples should have contained the phosphate component only. We were also able to estimate the amount of CaMg(CO3)2 that was dissolved in the first and second leachates for samples D1–D6 at RHUL, and so can estimate the percentage of dolomite which was dissolved during each step (Table 1). These rough estimates confirm our objective which was to dissolve 30% and 40% dolomite after the first and second leaches, respectively.

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Table 1 Lithology, chemical concentrations and ratios, C-, O-isotopic ratios, and various leach Sr isotopic compositions of samples from one section (Xiaotan). 13

18

Lithology

a CaO (%)

a MgO (%)

a Mg/ Ca

a Mn/ Sr

b [Sr] (ppm)

δ Ccarb (VPDB ‰)

δ Ocarb (VPDB ‰)

L1 L2 L3 L4 L5 L6 L7 L8 L9 L10 L11 L12 L13 L14 L15 L16 L17 L18 L19 P1 P2 P3 P4 P5 D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11

Sparite Siliciclastic sparite Partially recrystallized pelsparite Partially recrystallized pelsparite Partially recrystallized pelsparite Siliciclastic sparite Pelmicrosparite Siliciclastic microsparite Siliciclastic microsparite Partially recrystallized pelsparite Partially recrystallized pelsparite Microsparite Partially recrystallized pelsparite Partially recrystallized pelsparite Partially recrystallized pelsparite Partially recrystallized pelsparite Pelsparite Phosphatic pelsparite Phosphatic pelsparite Peloidal phosphorite Peloidal phosphorite Peloidal phosphorite Sparry phosphorite Siliciclastic phosphorite Dolomicrosparite Dolosparite Dolomicrosparite Peloidal dolosparite Dolosparite Dolosparite Dolomicrosparite Dolosparite Dolosparite Dolosparite Partially recrystallized dolomicrospar

45.7 45.7 51.0 49.5 50.2 18.6 32.8 38.7 43.0 49.9 47.5 51.8 50.3 49.2 48.7 48.4 47.9 50.5 46.5 39.9 40.6 50.2 30.3 22.2 37.5 27.8 26.3 28.3 23.9 29.8 17.1 25.7 21.7 25.9 29.0

0.7 0.7 0.4 0.3 0.3 1.0 0.2 1.1 1.0 0.3 0.3 0.6 0.6 0.3 0.4 0.4 0.8 0.3 0.3 0.2 0.3 0.4 0.2 0.6 11.1 6.9 10.5 8.6 7.6 13.8 8.4 10.5 9.3 8.0 13.2

0.01 0.01 0.01 0.00 0.00 0.05 0.00 0.02 0.02 0.00 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.01 0.00 0.01 0.01 0.01 0.02 0.25 0.21 0.34 0.26 0.27 0.39 0.41 0.34 0.36 0.26 0.38

0.48 0.13 0.01 0.03 0.04 0.15 0.01 0.27 0.15 0.31 0.16 0.02 0.03 0.03 0.03 0.02 0.12 0.03 0.54 0.01 0.41 0.22 0.02 0.06 0.27 1.20 1.57 2.85 8.99 3.42 2.29 13.47 8.94 6.18 3.03

757.0 1806.6 4998.8 3191.6 2517.5 4117.5 3743.8 1497.6 1848.8 950.7 2185.3 4632.4 4782.6 4457.3 4033.7 3954.8 1772.8 3661.8 2383.2 6568.9 4858.5 2056.1 3567.7 4451.3 2570.9 1586.9 1222.9 136.8 117.9 82.3 1199.1 83.7 109.7 85.6 76.3

− 4.1 − 0.2 6.4 6.7 5.8 − 4.2 − 10.4 − 0.9 − 0.1 4.2 5.7 7.2 7.1 6.7 6.7 7.2 6.0 6.5 1.4 − 6.0 − 2.9 − 3.0 − 7.9 − 5.3 − 5.9 − 5.2 − 5.3 − 12.2 1.2 3.0 − 4.2 0.2 1.4 1.4 1.3

− 15.0 − 15.1 − 12.2 − 12.3 − 11.8 − 9.6 − 12.7 − 14.5 − 15.1 − 11.9 − 12.5 − 11.5 − 11.0 − 12.1 − 11.3 − 12.2 − 12.1 − 11.4 − 11.5 − 13.5 − 11.1 − 10.0 − 11.9 − 9.0 − 6.5 − 8.4 − 8.2 − 7.1 − 6.6 − 3.5 − 9.0 − 7.8 − 6.5 − 6.3 − 4.4

Mg/Ca

1st leach

a b c d e f g

d87

c

0.06 0.02

2nd leach

3rd leach

0.19 0.00 e e e e e e e e e e e e

0.00 0.01 0.01 0.00 0.03 0.51 0.49 0.63 0.56 0.55 0.57

86

f Deviation from seawater

Sr/ Sr

0.45 0.53 0.59 0.56 0.56 0.58 e e e e e

1st leach

2nd leach

0.711371 0.711967 0.708283 0.708323 0.708313 0.708823 0.708882 0.712170 0.712029 0.708303 0.708394 0.708311 0.708351 0.708354 0.708284 0.708326 0.708634 0.708269 0.708544 0.708698 0.708432 0.708608 0.708983 0.708854 0.708659 0.712401 0.708762 0.708550 0.710219 0.710659 0.708871 0.712998 0.713931 0.710973 0.710954

0.710796 0.711320 0.708270 0.708314 0.708284 0.708832 0.708768 0.711959 0.711718 0.708305 0.708382 0.708366 0.708261 0.708344 0.708282 0.708288 0.708577 0.708279 0.708486 0.708674 0.708412 0.708588 0.708878 0.708814 0.708607 0.708654 0.708531 0.709669 0.711379 0.710222 0.708718 0.712326 0.711988 0.710007 0.710339

3rd leach

0.708507 0.708470 0.708664 0.708749 0.708726

0.002538 0.003062 0.000003 0.000046 0.000000 0.000370 0.000281 0.003701 0.003460 0.000047 0.000123 0.000106 0.000000 0.000082 0.000018 0.000022 0.000310 0.000010 0.000151 0.000298 0.000000 0.000162 0.000438 0.000364 0.000131 0.000128 0.000000 0.000000 0.001669 0.001672 0.000191 0.003776 0.003438 0.001457 0.001789

Proxies of the bulk samples determined by ICP-AES. Sr concentrations in the HCL-soluble part of the samples. Mg/Ca ratios of the step-wise leaching solutions. Sr isotope values of each step-leached solution determined by TIMS. Two italicized data mean no precise analysis due to the small amount of loaded Sr. The Sr isotope chemistry and determination of these samples were carried out at Nanjing University (NJU), while the others were carried out at Royal Holloway, University of London (RHUL). 87 86 The deviation of the measured Sr/ Sr on target component from the estimated seawater baseline. Estimated percentage of dolomite having been dissolved in sequential leaching steps based on the column method and the elemental concentrations of the leaching solution.

g % of dissolved dolomite in

1st leach

2nd leach

31.7 40.5 29.6 33.9 33.4 23.1

31.0 47.1 32.4 40.9 39.0 23.3

0.000131 0.000058 0.000238 0.000309 0.000276

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Samples

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Fig. 2. Flow chart depicting sequential leaching methods for different sedimentary rock types. Lab RHUL: Dept. Earth Sciences, Royal Holloway, University of London, Lab NJU: State Key Lab for Mineral Deposits Research, Nanjing University.

4.2. Sr isotope results The Sr isotope results of carbonate samples (L1–L19, D1–D11) show 87 86 that the second leachates have significantly lower Sr/ Sr values than the first leachates in 15 out of 19 limestone, and 9 out of 11 dolostone samples (Table 1, Fig. 3a). This relative decrease was up to 647×10−6 for limestone (except samples L6, L10, L12 and L18) and up to 3747×10−6 for dolostone (except D4 and D5) samples (Fig. 3a). Note that the measured increase between the 1st and 2nd leachates for samples L6, L10, L18 was only 9×10−6, 2×10−6, 10× 10−6, respectively (Fig. 3a), which are all lower than the 2σ analytical errors using TIMS (13× 10−6 at RHUL and 16× 10−6 at NJU) (within 2 S.D. line in Fig. 3b). The only significant increase was for sample L12 at 55×10−6. Only two dolostone samples (D4 and D5) have second leachates with significantly higher values by up to 0.0011 (Fig. 3a), but this increase was not precisely constrained because of the small amount of loaded Sr, i.e. no precise analysis of the 2nd leachates of samples D4 and D5 proved possible using TIMS. For the phosphorite samples (P1–P5), the last leach (phosphate only) was the least radiogenic in three samples, but not for samples P2 and P3 where it was significantly more radiogenic (58×10−6 and 76 ×10−6, respectively) than the 2nd leach (values on calcite) (Fig. 3a). 5. Discussion It can be discerned from our results that the second, partial leach using weak acetic acid after first pre-leaching corresponds in most 87 86 cases to the lowermost Sr/ Sr ratios for the analyzed carbonate rocks. All samples are from different stratigraphic levels, so these lowermost values do not reflect alteration from one fixed seawater 87 86 Sr/ Sr. Instead, these lowermost values trace a decreasing trend, in a stratigraphic sense, which is perfectly consistent with the global seawater curve that falls almost linearly from ~ 0.7085 to ~ 0.7081 between about 545 Ma and 525 Ma (Shields, 1999; Halverson et al.,

2007; Maloof et al., 2010). Because of this consistency, we can estimate a trend of least alteration, approximating seawater, through 87 86 the lowest points of our Sr/ Sr data, which is compatible with the compilation trend given in Maloof et al. (2010) through the same Ediacaran–Cambrian boundary interval. This estimated 13 trend of least alteration is made more plausible by the δ C pro13 file through Xiaotan section, which can be correlated with δ C profiles across the Ediacaran–Cambrian boundary globally (Maloof et al., 87 86 2010; Li et al., in revision). We calculated each sample's Sr/ Sr devia87 86 tion from seawater (Table 1) as the difference between the Sr/ Sr values of the final leaches (the 2nd leach for L and D samples, but the 2nd and the 3rd leach for P samples) and the estimated seawater trend. With the exception of 4 limestone and 6 dolostone samples, all the other 25 samples, including also dolomitic and phosphatic 87 86 rocks, exhibit Sr/ Sr deviations from the seawater curve of less than 0.0004; only two dolomites and one phosphate sample show deviations of less than 0.0001 from this idealized trend 87 86 (Table 1, Fig. 4a). The Sr/ Sr deviations of 0.0004 or 0.0001 seem to be unacceptably large, especially when we take into account the relatively narrow range of ~0.00005 shown by modern brachiopod specimens which cover a broad spectrum of latitudes, salinity, temperature regimes, and depths (Brand et al., 2003). However, it 87 86 is unrealistic to expect that the Sr/ Sr ratios of near-primary rock phases will normally vary by as little as 0.00005 and so we consider deviations lower than 0.0004, and especially lower than 0.0001, as acceptable, at least for the purpose of evaluating petrologic and geochemical parameters in order to establish a general protocol for future study. According to the results of Table 1, and based on the assumption 87 86 above, only 9 of 19 limestone samples carry a Sr/ Sr signature that is within modern natural variations (~ 0.00005) and thus deemed near-primary, for a success rate of 47%. The equivalent success rate for dolomites is only 9% and for the phosphorites it is 20%. If we

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Fig. 3. a) Sr/ Sr values plotted in stepwise leaching sequence. Note that the chemical experiments and Sr-isotope determinations for samples L1–L7, P1–P5, D1–D6 were carried out at the RHUL lab at Royal Holloway; the others were carried out at the NJU lab at Nanjing University. b) Diagram showing measured Sr isotope values and those of each sample pre-leach. The green line represents the line of best fit (2 S.D.).

assume acceptable deviations up to 0.00025, then the success rate for limestones, dolomites, and phosphorites would be 63%, 36% and 60%, respectively. It clearly underscores the difficulty in obtaining nearprimary values (even with intensive leaching), and a single analysis (or just a few) will be inadequate in identifying suitable material and results.

Our results illustrate the potential of the sequential leaching method for SIS using bulk carbonate and phosphorite samples, while highlighting the need for demonstrably well preserved authigenic components. In the following section, we discuss in detail the four initial steps necessary for SIS using bulk marine carbonates and phosphorites in order to establish a generally applicable protocol for bulk rock SIS.

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Fig. 4. Sr/ Sr deviation is here defined as the difference in Sr isotope compositions between measured values on target components and the seawater baseline estimated from the 87 86 lowermost, and presumably “least-altered,” Sr/ Sr values. Diagrams a to f show the correlation between the deviation and other geochemical proxies. The filled symbols refer to 87 8686 87 86 87 86 samples having Sr/ Sr values near seawater baseline ( Sr/ Sr deviation b 0.0004), while the open symbols refer to samples having Sr/ Sr values significant above seawater base87 86 line ( Sr/ Sr deviation N 0.0004).

5.1. Selection criteria 5.1.1. Petrographic criteria One important key message from the published literature is the importance of selecting “least altered” authigenic marine components for SIS analysis. For example, published differences between the 87 86 Sr/ Sr values of pure calcitic, relatively fine-grained components and bulk rock dissolution (e.g. Wotte et al., 2007) underscore the need for careful sample component selection in SIS studies. Our

results show that carbonate (including calcite and dolomite) and phosphatic parts of bulk sedimentary rocks can be useful in SIS studies, but only if early authigenic phases can be identified petrographically and isolated chemically. In our study, partially recrystallized fine sparite limestone samples yielded consistently better Sr isotope 87 86 results (with Sr/ Sr deviations b0.0004), while siliciclastic sparite (with a significant mud component) and dolosparite samples yielded higher, obviously altered values far above the estimated seawater curve (between 0.0004 and 0.004). For our dolostone samples,

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dolomicrosparite preserved primary seawater composition more reliably than dolosparite. When combined with the above observations for limestones, this appears to suggest a role for late-stage recrystallization in strontium isotopic exchange. Thin-section analysis should therefore be an essential prerequisite for any bulk SIS study as many previous studies have documented. Some carbonate values overshoot the entire range exhibited by the 87 86 Phanerozoic seawater Sr/ Sr evolution curve (Shields, 2007), which must indicate the presence of secondary components or alteration in those samples. For example, Ehrenberg et al. (2006) show that the earliest dissolved parts of sparry dolostones are the crystal rims which may have formed several million years later than the crystal core (as well as possibly later calcite in the form of vein calcite or dedolomite). As dolo87 86 mitization has also been known to decrease the Sr/ Sr of Cambrian marine carbonates (Gao and Land, 1991; Nicholas, 1996), petrographic and geochemical criteria need to be applied to distinguish primary from secondary rock components for SIS studies. 5.1.2. Geochemical selection As described above, rigid geochemical criteria were used to choose our samples. Only samples containing high Sr concentrations and low Mn/Sr ratios, high Ca and/or Mg contents were used for Sr isotopic analysis. It is important to note, however, that only some of our 87 86 Sr/ Sr results are close to the seawater curve, while 10 samples have 87 86 Sr/ Sr values which are obviously higher than the seawater curve. Of these 10 carbonate samples, the 4 limestones have CaO b45.7%, Sr concentrations b1848.8 ppm and Mn/Sr ratios ranging between 0.13 and 0.48, while the 6 dolostones have Sr concentrations b117.9 ppm and Mn/Sr ratios ranging between 3.03 and 13.47 (Fig. 4b, c). On the basis of commonly applied trace element criteria for screening carbonate samples for SIS, our clearly altered limestone samples would be considered acceptable, having Mn/Sr ratios b2–3 (e.g. Kaufman et al., 1993), while only the dolostones would have been rejected. By contrast, the nine best-preserved limestone samples, showing 87 86 Sr/ Sr deviations b0.0001, have CaO N48.4%, Sr concentrations N2517.5 ppm (except sample L10 = 950 ppm), Mn/Sr b0.04 (except sample L10 = 0.31) (Fig. 4b, c), while the five best-preserved 87 86 dolostone samples, showing Sr/ Sr deviations b0.0002, have Sr concentrations N1199.1 ppm (except sample D4= 136.8 ppm), and Mn/Sr b2.85 (Fig. 4b, c). This allows us, at least in our case, to perceive a lower cut-off for CaO of around 47%, for Sr concentration of around 2500 ppm, and an upper cut-off for Mn/Sr of around 0.1 for limestone. For dolostones, a lower cut-off for Sr concentration of around 1000 ppm would be necessary, and an upper cut-off for Mn/Sr ratio of around 3. Although this confirms several previous studies, which have indicated how Sr concentration, Mn/Sr ratio and sample purity can be key indicators of isotopic fidelity in carbonates, limestone samples clearly need to be treated differently from dolostones during screening with much stricter cut-offs applied to limestones. Such constraints may seem useful, but are also likely to vary between sample sets as initial Sr concentrations are unknown, and likely to vary considerably due to differences in the initial carbonate mineralogy (aragonite vs. calcite) and environmental conditions. Three phosphorite samples with deviations b0.00025 have CaO N39.9%, and the other two phosphorite samples with deviations N0.00025 have CaO b30.3%. This suggests that sample purity also has a role to play in SIS studies using phosphorite. No trace element cutoffs pertaining to phosphorite samples are apparent from our study. Late diagenesis is believed to alter rock chemistry by fluid interaction, which can change Sr isotope ratios to either higher (more radiogenic) or lower values (Brand et al., 2010). Diagenetic water– rock interactions can also leave a potentially useful stable isotopic footprint. Our limestone, dolostone and phosphate samples exhibit a wide range of C-isotope values between − 12.2‰ and + 7.2‰, and between −4.1‰ and +3.0‰, respectively, no matter whether their 87 86 Sr/ Sr deviations were b0.0004 or N0.0004 (Fig. 4e). To a great

extent, this variability reflects the oscillatory nature of early Cambrian carbon isotope cycling, and no useful C-isotope constraints on diagenetic alteration emerge from our study. In general, the oxygen isotope system in marine authigenic minerals is more susceptible to diagenetic alteration than the carbon isotope system (Brand, 2004). In our study, limestone, dolostone and 87 86 phosphate samples with Sr/ Sr deviations b0.0004 have O-isotope values ranging between −13.5‰ and − 6.5‰ (Fig. 4f). However, 4 87 86 limestone samples with Sr/ Sr deviations N0.0004 have significantly more negative O-isotope values ranging from −14.5‰ to − 15.1‰ (Fig. 4f), which suggests that fluid alteration has lowered the oxygen isotopic composition of these rocks during late diagenesis. Although 87 86 this observation supports the common assumption that Sr/ Sr ratios 18 and δ O values should be correlated in diagenetically altered samples, our data indicate that the commonly applied cut-off at − 10‰ (Kaufman et al., 1993) cannot be used for diagenetic screening in SIS studies as the best preserved samples from our study show consistent18 ly low δ O values of around − 12‰. Furthermore, the general 18 assumption that lower δ O values reflect a greater degree of alteration becomes less useful in the case of dolomitization. Dolomites 18 in our study are all O-enriched compared with limestones, ranging 3–8‰ higher than the limestone average (Fig. 4f). As dolomite is 18 commonly O-enriched with respect to coeval calcite by up to 2– 5‰ or more (Vasconcelos et al., 2005), it is difficult to apply firm 18 δ O cut-offs to dolostone samples, considering also potential variations in salinity, temperature and conditions of dolomitization. 18 Partially dolomitized limestones will also be affected by O-exchange, and this can be seen in our study whereby the most altered limestone sample has the highest Mg/Ca ratio among the limestones 18 (Fig. 4d). As it also has the highest δ O value (Fig. 4f), it would erroneously be expected to be the most suitable sample for SIS. 87 86 In this study, we have compared deviations from seawater Sr/ Sr 18 to other potentially useful parameters, such as δ O, Mn/Sr, [Sr], etc. in order to evaluate the reliability of alteration criteria. However, in Fig. 5, we have also provided traditional cross-plots showing the mea87 86 sured Sr/ Sr ratios versus the various other geochemical parameters. Open symbols in Fig. 5 relate to Sr isotope values which deviate further from contemporaneous seawater (with deviations above 0.0004). Note that the diagenetic (dolomitic and otherwise) origin of O-isotopic variation, as well as the correlation between Srand O-isotopic alteration in our samples can only be fully appreciated after studying the seawater-corrected deviation plot (Fig. 4f) rather than a conventional plot showing measured ratios only (Fig. 5b). 5.2. Physical extraction Micro-drilling or micro-milling is essential to ensure that only the required component has been powdered. In this study, we obtained the powders of targeted components from shallow surfaces of fresh rock thin-section counterparts, using the micro-drill or computercontrolled micro-mill assembly under a microscope at a resolution of submillimeter scale. After drilling, it is important to ensure that the powder is as fine-grained as possible as drilling can chip off coarser flakes. In such cases, pre-leaching techniques may dissolve the fine powder first leaving the un-pre-leached coarse grains to dissolve in the second dissolution. Although we did not test methods of physical extraction specifically, published studies and theoretical considerations can be brought to bear on this topic. High spatial resolution sampling is needed to extract powders from the targeted components seen in the counterparts of thin-sections. For example, Carpenter et al. (1991) used a microscopemounted drill assembly to obtain microsamples of powdered calcite. In their extraction, a 500 μm dental burr and, when necessary, a faceted 20 μm drill bit were used for sampling. Dettman and Lohmann (1995) discussed the early use of physical extraction of carbonates at the spatial resolution of 1 μm using computer-controlled micropositioning

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87

86

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18

Fig. 5. Actual measured Sr/ Sr values (last leach) versus various parameters: a) Sr concentrations, b) δ O, c) Mn/Sr, d) Mg/Ca. The filled and open symbols have the same meaning as in Fig. 4.

technology. Other kinds of pure calcite samples, such as brachiopod shells can be sampled for isotope studies using a stainless steel dental pick under a binocular microscope (Banner and Kaufman, 1994; Mii and Grossman, 1994). Potentially, laser ablation MC-ICP-MS could be applied to SIS studies, which provides rapid, texturally sensitive, high precision Sr analysis without the need for chemical preparation of samples. However, initial attempts suggest that this approach is compromised by the inadvertent ablation of non-target contaminant phases such as clay minerals. The poor reproducibility and isotopic interference by organics in the sample are potential problems (Powell and Kyser, 1991; Smalley et al., 1992). The crater diameter from the laser ablation of the carbonates is approximately 150 μm, while the precision of the ICP-MS is only at the 10 −5 level (Christensen et al., 1995). The spatial resolution of micro-drilling is equivalent to, or can even surpass that of laser ablation, while organic contamination poses fewer problems. TIMS analysis yields better reproducibility and precision. Secondary ionization mass spectrometry (SIMS) have also been used to measure Sr isotopic compositions in situ but the precision (0.2%) is of limited application and does not compare well with thermal ionization mass spectrometry (TIMS) (Exley and Jones, 1983). 5.3. Chemical pre-treatment Several research groups have hit upon the idea of an initial preleach to remove exchangeable or easily soluble Sr from diagenetic or detrital rock components. The most commonly used pre-leach involves ammonium acetate (Gorokhov et al., 1995; Montanez et al.,

1996; Kuznetsov et al., 1997; Gorokhov et al., 1998; Semikhatov et al., 1998; Kuznetsov et al., 2010). Kuznetsov et al. (1997) report data for the ammonium acetate pre-leaches as well as subsequent acetic acid leaches, which allows us to assess the efficacy of their pre-leach. For 22 bulk carbonates in their study, the pre-leach 87 86 resulted in a decrease in Sr/ Sr of between 1 × 10 −6 and 62 × 10 −6, with a mean decrease of 25 × 10 −6, which is narrowly, but significantly above the analytical reproducibility between laboratories (McArthur, 1994). In a study by Gorokhov et al. (1995) where more of the sample was dissolved by the pre-leach, a greater effect could be observed. The Sr isotope compositions from 54 samples ranged between 3 × 10 −6 and 546 × 10 −6 lower, averaging 126 × 10 −6, as a result of the ammonium acetate pre-leach. In the study of Kuznetsov 87 86 et al. (2010), the difference between measured Sr/ Sr ratios of the ammonium acetate pre-leached fraction and the subsequent acetic acid leached fraction of 4 dolostones were even higher, varying between 840 × 10 −6 and 1640 × 10 −6. Some recent work has shown that the ammonium acetate leaching technique may not remove all contaminant Sr (Bailey et al., 2000). These authors suggest that the initial phases of weak acid leaching after ammonium acetate pre-leach may release additional radiogenic Sr, and that less soluble Sr associated with silicate minerals compared with carbonate minerals may also be leached if carbonate dissolution reaches totality before pH can be neutralized. On the basis of their leaching experiments, these authors propose a preleach of acetic acid, which ought to dissolve up to 40% of the sample. The sample for analysis would be taken from the next 30% that dissolves using the same acid, but avoiding complete dissolution of the

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sample. Note, this sequential leaching method does not remove the unknown diagenetic factor, apart from perhaps some secondary overgrowths that might be present. For this method to work efficiently, the sample must be a fine powder. Otherwise, contaminants would continually reappear during the course of leaching due to the appearance of fresh surfaces. Our test on pre-leaches of 19 limestones, 5 phosphorites and 11 dolostones by 0.2–1% acetic acid corroborates previous conclusions (Bailey et al., 2000), but using a greater variety of lithologies. The initial release by acetic acid should have contained not only contaminant water-soluble Sr on ion-exchange sites (ammonium acetate soluble) but also Sr from secondary calcite (in the case of dolostones). As bulk carbonate samples are seldom pure carbonates, it is important to know the carbonate content first and calculate the amount of weak acid (acetic acid) needed to pre-leach 30–40% of the carbonates before incomplete (about another 30–40%) dissolution. The pre-leach was more radiogenic than the second step dissolution by up to 647× 10 −6 for limestones and 3747× 10 −6 for dolostones, with a mean value of 145 × 10−6 for limestones and 980 × 10−6 for dolostones. Sequential leaching on ~40 mg sample powders using dilute (0.2– 1%) acetic acid with volumes on the orders of milliliters in this study is more appropriate in SIS studies, than the 20% acetic acid on the order of tens of microliters for dissolving ~100 mg sample powder used in the study of Bailey et al. (2000). Note that the difference between the pre-leach and second leach is higher for dolostones than for limestones, 87 86 which could result from secondary calcite with high Sr/ Sr values in dolostone samples, but may also be exacerbated by the generally much lower Sr concentrations of dolomite. These effects can be seen in the extraordinarily large difference between leaches in sample D2 (Fig. 3a), which also exhibited the lowest initial Mg/Ca ratio (0.21) of all the dolomites and so contained significant amounts of calcite. Ruppel et al. (1996) presented the results of step-wise leaching of 87 86 bioapatites (conodonts in this case) on measured Sr/ Sr ratios. When working with bioapatites, it is suggested to automatically leach out the carbonate component first and then develop a strategy for dealing with the potential diagenetic alteration of the phosphatic component. With regard to our phosphorite samples, complete removal of carbonate contaminants improved the result in three cases significantly, suggesting that the phosphate component was isotopically better preserved than carbonate components. In two cases though, including the best preserved example, it was the second

leachate which proved to be less radiogenic. It is possible that some phosphate was leached by the second acetic acid leachate, although this seems unlikely. It may be more likely in these two cases that the final strong acid leach also sampled contaminant strontium from silicate minerals. More research into the strontium systematics of phosphorites is required before we can address this question adequately. Pre-leaching is also important as it generally reduces, or completely avoids the need for a correction due to Rb decay. In the study by Gorokhov et al. (1995), Rb-correction of the more radiogenic pre87 86 leaches occasionally overcorrected the measured Sr/ Sr ratio, possibly due to radiogenic Sr loss from the structure of clays, or the presence of a Rb phase that formed after deposition (Kuznetsov et al., 2010). The commonly observed removal of Rb with the pre-leach tends to indicate that clay-associated ions are removed by this leach rather than diagenetic carbonate (which is commonly more coarsely crystalline and thus harder to dissolve) as proposed by Gorokhov et 87 86 al. (1995). The Rb/ Sr ratio in carefully cleaned carbonates is generally of the order of 0.001 (as this is in our case) and no correction is necessary (Elderfield, 1986). 5.4. Sample dissolution The subsequent partial dissolution of targeted carbonate or phosphate was carried out using a weak acid attack after first pre-leaching with weak acetic acid. The partially dissolution of carbonates prevents the leaching of strontium from non-carbonate phases, such as aluminosilicates (Bailey et al., 2000). Several previous studies using strong acids have shown that these are more aggressive than weak 87 86 acids and generally lead to more radiogenic Sr/ Sr ratios in the leachate. However, this does not mean that strong acids should not be used for the pre-leach, and this has been attempted by the authors with comparable success (e.g. data in Kouchinsky et al., 2008). 6. Conclusions Based on our results and on published studies, we propose a general protocol for the analysis of bulk carbonate and phosphorite rocks for SIS (Fig. 6). This involves the removal of up to a third of the carbonate component of the sample, followed by partial dissolution of the target component using a weak acid or acid buffer for analysis.

Fig. 6. Experimental flow chart showing the protocol for SIS studies.

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Common secondary minerals such as dolomite or francolite may be successfully analyzed for SIS. However, rigid sample selection using stratigraphic, petrographic and geochemical criteria and careful physical extraction are important prerequisites. Commonly applied diage18 netic controls (e.g. Mn/Sr and δ O) provide ambiguous information in the case of dolostones, or partially dolomitized limestones, while cutoffs are likely to vary significantly from case to case. The importance of analyzing rock components rather than bulk rock powders is emphasized. Near-primary components, such as early marine cements or unrecrystallized micrite seem to be the most suitable for SIS; however, in some cases, dolomitic and phosphatic components could be used. Sample selection (1) can be managed with the help of petrographic analysis, focusing on pure, fine-grained components, while geochemical screening can be used to identify those samples 87 86 most likely to provide seawater Sr/ Sr values: high Sr content, high total Ca and/or Mg content in the bulk sample (i.e. high carbonate or 18 phosphate purity), low Mn/Sr ratios, and plausible δ O. Caution is advised when applying elemental and isotopic screening to sample selection and/or identification of altered Sr isotopic compositions as simplistic cut-offs can be misleading, especially in the case of dolostones and in the absence of obvious diagenetic trends. After physically extracting components (2) from the unweathered counterparts of thin-sections, fine-grained carbonate powders may be pre-leached (3) with a dilute, weak acid (e.g. diluted acetic acid) or a dilute, strong acid (e.g. HCl) calculated to remove about one third of the total carbonate component, estimated from total Ca + Mg content or some other method. This is especially important in the case of dolostones, whereby the pre-leach will remove all secondary calcite phases, provided that the sample has been ground finely enough. Subsequently, a weak acid, such as dilute, weak HAc, can then be used as the main leach (4) calculated to dissolve only partially the carbonate fraction in the sample. For other rock components (such as phosphorite), preleaching of all non-target carbonate components and other easily soluble Sr-bearing phases using an excess of acetic acid will clean the target 87 86 phases (such as phosphate) for Sr/ Sr determination, but here again incomplete dissolution is recommended. It is possible that the approach used in this study could be applied to other geochemical studies of carbonate and phosphate rocks, such as rare earth element studies, including neodymium isotope studies. As SIS becomes more widely used, especially for Proterozoic studies, 87 86 it is important that the methods used to constrain seawater Sr/ Sr continue to be refined as our success rate in this study was still far from satisfactory. Acknowledgments This study was supported by the NSFC grant 40872025 and the DFG Forschergruppe 736 “The Precambrian–Cambrian Ecosphere Revolution.” We thank Uwe Brand and two anonymous reviewers for their constructive comments. Maoyan Zhu is thanked for his help in the fieldwork. D.L. is grateful to Christina Manning for her assistance with Sr isotope analysis at Royal Holloway, Tim Atkinson for the technical support with the sample preparations at BEIF of UCL, Tony Osborn for his assistance with the ICP-AES determinations at UCL and Wei Pu for her help with the TIMS work at Nanjing University. D.L. also acknowledges the scholarship from China Scholarship Council for overseas study. G.S.Z. acknowledges the support of the Chinese Academy of Sciences visiting professorship scheme during 2011. References Bailey, T.R., McArthur, J.M., Prince, H., Thirlwall, M.F., 2000. Dissolution methods for strontium isotope stratigraphy: whole rock analysis. Chemical Geology 167 (3–4), 313–319. Banner, J.L., 2004. Radiogenic isotopes: systematics and applications to earth surface processes and chemical stratigraphy. Earth-Science Reviews 65 (3–4), 141–194.

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