新元古代(5.45-10億年前)蓋帽白雲岩廣泛分布在全球的地層中，其直接覆蓋在雪球地球事件的冰磧物上，顯示於雪球地球事件結束後沉積。由於冰雪地球結束後的環境變化非常劇烈，此時沉積的蓋帽白雲岩其特殊的地球化學特徵可以反映冰雪地球後沉積環境變化(如:海水面變化、陸源風化強度變化)。本研究樣本為中國南部藍田層石盂段之蓋帽白雲岩(約6.35億年前)，藉分析樣品中化學元素濃度、鍶同位素(87Sr/86Sr)、鎂同位素(δ26Mg)資料，探討冰雪消融後，古海洋大陸邊緣的地球化學特徵變化。首先，進行拉曼光譜分析(RAMAN)、X光粉末繞射儀(XRD)分析及掃描式電子顯微鏡-能量散佈光譜儀(SEM/EDS)，了解樣品間礦物相組成差異。為避免非碳酸岩礦物相的影響並得到白雲石的原始化學訊號，我們利用低濃度醋酸進行3步驟序列萃取，並量測鎂鈣比(Mg/Ca)、錳鍶比(Mn/Sr)……等地球化學指標，確保各萃取相之成分。使用AG50W-X8陽離子交換樹脂及Srspec樹脂，從各步驟萃取中分別純化鎂及鍶元素，並量測純化後溶液的元素以評估元素回收率(recovery)、基質效應(matrix effect)的影響。再由多接收器感應耦合電漿質譜分析儀(MC-ICP-MS)分析樣品中的鎂、鍶同位素組成。利用白雲石與方解石相對含量不同的樣本(SY-8、14、16及22)在各步驟萃取之間的地球化學指標及同位素值，評估序列萃取之各步驟成效以及非白雲石相的干擾。由於，成岩作用相關指標與鍶、鎂同位素間無明顯相關性，樣品中較高的錳鍶比可能由於高濃度錳海水及白雲石較高的錳鈣比分配係數，故推測樣本無受到後期成岩作用影響。樣品中鍶和鎂同位素間的正相關可以連結至冰期結束後的陸源風化。利用鍶、鎂同位素在地層中的變化，可以探究在冰雪消融時，大陸邊緣環境的陸源風化強度變化。

Cap-dolostones precipitated from the Neoproterozoic ocean is a good archive for recording the environmental conditions after the deglaciation of the 635 Ma Marinoan Snowball Earth event. The distinctive geochemical features could reconstruct the extensive sedimentary environmental change during and after the retreating of glaciation and potential enhancements in continental weathering. Here we presented δ26Mg and 87Sr/86Sr data of the cap-dolostone samples from the Lantian Formation at Shiyu, southern Anhui, to examine the geochemical features and further confirm the hypotheses of paleo-oceanography in the continental margin area. Raman spectrometer, X-ray diffractometer (XRD) and scanning electron microscopy with x-ray microanalysis (SEM/EDS) were applied to identify the mineral composition of sample specimens.
Our results showed that most of these samples were composed of dolomite with minor fractions of calcite, silicates. The sample powders were drilled by a hand-hold microdrill to avoid vein and spar. The powder extraction procedure was processed by a multiple-step sequential extraction using diluted acetic acid to obtain pristine signal of the sedimentary conditions. To evaluate the diagenetic effect on the samples, Sr/Ca, Mn/Sr, 87Sr/86Sr and δ26Mg were performed to ensure the isolation of the pristine signals from the cap-dolostone specimens. The positive correlation between Sr and Mg isotopes could help us focus on the intensity of continental weathering. With the variation of Mg and Sr isotopes, we could constrain the intensity of weathering event in the profile.

Alene, M., Jenkin, G. R. T., Leng, M. J., and Darbyshire, D. P. F. (2006). The Tambien Group, Ethiopia: An early Cryogenian (ca. 800–735Ma) Neoproterozoic sequence in the Arabian–Nubian Shield. Precambrian Research, 147(1-2), 79–99.
Bailey, T. R., Mcarthur, J. M., Prince, H., and Thirlwall, M. F. (2000). Dissolution methods for strontium isotope stratigraphy : whole rock analysis, 313–319.
Brasier, M., McCarron, G., Tucker, R., Leather, J., Allen, P., and Shields, G. (2000). New U-Pb zircon dates for the Neoproterozoic Ghubrah glaciation and for the top of the Huqf Supergroup, Oman. Geology, 28(2), 175–178.
Bristow, T. F., Bonifacie, M., Derkowski, A., Eiler, J. M., and Grotzinger, J. P. (2011). A hydrothermal origin for isotopically anomalous cap dolostone cements from south China. Nature, 474(7349), 68–71.
Bristow, T. F., Kennedy, M. J., Derkowski, A., Droser, M. L., Jiang, G., and Creaser, R. a. (2009). Mineralogical constraints on the paleoenvironments of the Ediacaran Doushantuo Formation. Proceedings of the National Academy of Sciences of the United States of America, 106(32), 13190–13195.
Condon, D., Zhu, M., Bowring, S., Wang, W., Yang, A., and Jin, Y. (2005). U-Pb ages from the neoproterozoic Doushantuo Formation, China. Science (New York, N.Y.), 308(5718), 95–98.
Drits, V. I. A., Carty, D. O. K. M. C., and Milliken, K. I. L. (2005). NEW INSIGHT INTO STRUCTURAL AND COMPOSITIONAL VARIABILITY IN SOME ANCIENT EXCESS-Ca DOLOMITE, 43, 1255–1290.
Elderfield, H. (1986). Strontium isotope stratigraphy. Palaeogeography, Palaeoclimatology, Palaeoecology, 57(1), 71–90.
Faure, G. (1986). Principles of isotope geology. United States: John Wiley and Sons Inc.,New York, NY.
Frimmel, H. E. (2009). Trace element distribution in Neoproterozoic carbonates as palaeoenvironmental indicator. Chemical Geology, 258(3-4), 338–353.
Galy, A., Bar-Matthews, M., Halicz, L., and Oand apos;Nions, R. K. (2002). Mg isotopic composition of carbonate: Insight from speleothem formation. Earth and Planetary Science Letters, 201(1), 105–115.
Gao, T., Ke, S., Teng, F. Z., Chen, S., He, Y., and Li, S. G. (2016). Magnesium isotope fractionation during dolostone weathering. Chemical Geology, 445, 14–23.
Geske, A., Zorlu, J., Richter, D. K., Buhl, D., Niedermayr, A., and Immenhauser, A. (2012). Impact of diagenesis and low grade metamorphosis on isotope (δ26Mg, δ13C, δ18O and 87Sr/ 86Sr) and elemental (Ca, Mg, Mn, Fe and Sr) signatures of Triassic sabkha dolomites. Chemical Geology, 332-333, 45–64.
Gray, C. M., and Compston, W. (1974). Excess 26Mg in the Allende Meteorite. Nature, 251(5475), 495–497.
Halverson, G. P., Dudás, F. Ö., Maloof, A. C., and Bowring, S. A. (2007). Evolution of the 87Sr/86Sr composition of Neoproterozoic seawater. Palaeogeography, Palaeoclimatology, Palaeoecology, 256(3-4), 103–129.
Halverson, G. P., Hoffman, P. F., Schrag, D. P., Maloof, A. C., and Rice, A. H. N. (2005). Toward a Neoproterozoic composite carbon-isotope record. Bulletin of the Geological Society of America, 117(9-10), 1181–1207.
Hayes, J. M., Strauss, H., and Kaufman, A. J. (1999). The abundance of in marine organic matter and isotopic fractionation in the global biogeochemical cycle of carbon during the past 800 Ma. Chemical Geology, 161(1-3), 103–125.
Hess, J., Bender, M. L., and Schilling, J.-G. (1986). Evolution of the Ratio of Strontium-87 to Strontium-86 in Seawater from Cretaceous to Present Author ( s ): Jennifer Hess , Michael L . Bender and Jean-Guy Schilling Reviewed work ( s ): Source : Science , New Series , Vol . 231 , No . 4741 ( Feb . 28 , 1, 231(4741), 979–984.
Hoefs, J. (2004). Stable Isotope Geochemistry.
Hoffman, P. F., Kaufman, A. J., Halverson, G. P., and Schrag, D. P. (1998). A Neoproterozoic Snowball Earth. Source: Science, New Series, 281(5381), 1342–1346.
Hoffman, P. F., and Li, Z. X. (2009). A palaeogeographic context for Neoproterozoic glaciation. Palaeogeography, Palaeoclimatology, Palaeoecology, 277(3-4), 158–172.
Hoffman, P. F., and Schrag, D. P. (2002). The snowball Earth hypothesis : testing the limits of global change. Terra, 14(3), 129–155.
Hu, Z., Hu, W., Wang, X., Lu, Y., Wang, L., Liao, Z., and Li, W. (2017). Resetting of Mg isotopes between calcite and dolomite during burial metamorphism: Outlook of Mg isotopes as geothermometer and seawater proxy. Geochimica et Cosmochimica Acta, 208, 24–40.
Huang, F., Chakraborty, P., Lundstrom, C. C., Holmden, C., Glessner, J. J. G., Kieffer, S. W., and Lesher, C. E. (2010). Isotope fractionation in silicate melts by thermal diffusion. Nature, 464(7287), 396–400.
Huang, F., Lundstrom, C. C., Glessner, J., Ianno, A., Boudreau, A., Li, J., … DeFrates, J. (2009). Chemical and isotopic fractionation of wet andesite in a temperature gradient: Experiments and models suggesting a new mechanism of magma differentiation. Geochimica et Cosmochimica Acta, 73(3), 729–749.
Huang, K.-J., Teng, F.-Z., Shen, B., Xiao, S., Lang, X., Ma, H.-R., … Peng, Y. (2016). Episode of intense chemical weathering during the termination of the 635 Ma Marinoan glaciation. Proceedings of the National Academy of Sciences, 113(December), 201607712.
Jacobsen, S. B. (2001). Gas hydrates and deglatiations. Nature, 412(September 2001), 691–693.
Jacobsen, S. B., and Kaufman, A. J. (1999). The Sr, C and O isotopic evolution of neoproterozoic seawater-comment. Chemical Geology, 181(1-4), 193–195.
Jiang, G., Kennedy, M. J., and Christie-Blick, N. (2003). Stable isotopic evidence for methane seeps in Neoproterozoic postglacial cap carbonates. Nature, 426(6968), 822–826.
Jiang, G., Shi, X., and Zhang, S. (2006). Methane seeps, methane hydrate destabilization, and the late Neoproterozoic postglacial cap carbonates. Chinese Science Bulletin, 51(10), 1152–1173.
Kaufman, A. J., Jacobsen, S. B., and Knoll, A. H. (1993). The Vendian record of Sr and C isotopic variations in seawater: Implications for tectonics and paleoclimate. Earth and Planetary Science Letters, 120(3-4), 409–430.
Kennedy, M. J., Christie-Blick, N., and Prave, A. R. (2001). Carbon isotopic composition of Neoproterozoic glacial carbonates as a test of paleoceonographic models for snowball Earth phenomena. Geology, 29(12), 1135–1138.
Kennedy, M., Mrofka, D., and von der Borch, C. (2008). Snowball Earth termination by destabilization of equatorial permafrost methane clathrate. Nature, 453(7195), 642–5.
Kirschvink, J. L. (1992). Late Proterozoic low-latitude global glaciation: the snowball Earth. The Proterozoic Biosphere, 52, 51–52.
Li, D., Shields-Zhou, G. A., Ling, H. F., and Thirlwall, M. (2011). Dissolution methods for strontium isotope stratigraphy: Guidelines for the use of bulk carbonate and phosphorite rocks. Chemical Geology, 290(3-4), 133–144.
Li, W. Y., Teng, F. Z., Ke, S., Rudnick, R. L., Gao, S., Wu, F. Y., and Chappell, B. W. (2010). Heterogeneous magnesium isotopic composition of the upper continental crust. Geochimica et Cosmochimica Acta, 74(23), 6867–6884.
Liu, C., Wang, Z., and Raub, T. D. (2013). Geochemical constraints on the origin of Marinoan cap dolostones from Nuccaleena Formation, South Australia. Chemical Geology, 351, 95–104.
Liu, C., Wang, Z., Raub, T. D., Macdonald, F. A., and Evans, D. A. D. (2014). Neoproterozoic cap-dolostone deposition in stratified glacial meltwater plume. Earth and Planetary Science Letters, 404, 22–32.
Liu, S. A., Teng, F. Z., He, Y., Ke, S., and Li, S. (2010). Investigation of magnesium isotope fractionation during granite differentiation: Implication for Mg isotopic composition of the continental crust. Earth and Planetary Science Letters, 297(3-4), 646–654.
Lumsden, D. N., and Lloyd, R. V. (1984). Mn(II) partitioning between calcium and magnesium sites in studies of dolomite origin. Geochimica et Cosmochimica Acta, 48(9), 1861–1865.
Melezhik, V. A., Fallick, A. E., Rychanchik, D. V., and Kuznetsov, A. B. (2005). Palaeoproterozoic evaporites in Fennoscandia: Implications for seawater sulphate, the rise of atmospheric oxygen and local amplification of the δ13C excursion. Terra Nova, 17(2), 141–148.
Ornes, S., Perdew, J. P., Yang, W., Burke, K., Yang, Z., Gross, E. K. U., … Lu, C. E. (2017). From the Cover, 114(11).
Palmer, M. R., and Edmond, J. M. (1989). The strontium isotope budget of the modern ocean. Earth and Planetary Science Letters, 92(1), 11–26.
Palmer, M. R., and Elderfield, H. (1985). Sr isotope composition of sea water over the past 75 Myr. Nature, 314(6011), 526–528.
Richter, F. M., Dauphas, N., and Teng, F. Z. (2009). Non-traditional fractionation of non-traditional isotopes: Evaporation, chemical diffusion and Soret diffusion. Chemical Geology, 258(1-2), 92–103.
Richter, F. M., Watson, E. B., Mendybaev, R. A., Teng, F.-Z., and Janney, P. E. (2008). Magnesium isotope fractionation in silicate melts by chemical and thermal diffusion. Geochimica et Cosmochimica Acta, 72(1), 206–220.
Rosman, K. J. R., and Taylor, P. D. P. (1998). Isotopic compositions of the elements 1997 (Technical Report). Pure and Applied Chemistry, 70(1).
Sawaki, Y., Ohno, T., Tahata, M., Komiya, T., Hirata, T., Maruyama, S., … Li, Y. (2010). The Ediacaran radiogenic Sr isotope excursion in the Doushantuo Formation in the Three Gorges area, South China. Precambrian Research, 176(1-4), 46–64.
Scholle, P. A., and Ulmer-Scholle, D. S. (2003). Color Guide to the Petrography of Carbonate Rocks: Grains, Textures, Porosity, Diagenesis. AAPG, 2003.
Shen, B. (2009). The Mg isotopic systematics of granitoids incontinental arcs and implications for the roleof chemical weathering in crust formation, 1–6.
Shields, G. A. (2005). Neoproterozoic cap carbonates: A critical appraisal of existing models and the plumeworld hypothesis. Terra Nova, 17(4), 299–310.
Svensen, H., Sverre Planke, A. M.-S., Jamtveit, B., Myklebust, R., Eidem, T. R., and Rey, and S. S. (2004). Release of methane from a volcanic basin as a mechanism for initial Eocene global warming. Nature, 429(June), 3–6.
Teng, F. Z., Li, W. Y., Ke, S., Marty, B., Dauphas, N., Huang, S., … Pourmand, A. (2010). Magnesium isotopic composition of the Earth and chondrites. Geochimica et Cosmochimica Acta, 74(14), 4150–4166.
Teng, F. Z., Li, W. Y., Rudnick, R. L., and Gardner, L. R. (2010). Contrasting lithium and magnesium isotope fractionation during continental weathering. Earth and Planetary Science Letters, 300(1-2), 63–71.
Teng, F. Z., Wadhwa, M., and Helz, R. T. (2007). Investigation of magnesium isotope fractionation during basalt differentiation: Implications for a chondritic composition of the terrestrial mantle. Earth and Planetary Science Letters, 261(1-2), 84–92.
Tipper, E. T., Galy, A., and Bickle, M. J. (2006). Riverine evidence for a fractionated reservoir of Ca and Mg on the continents: Implications for the oceanic Ca cycle. Earth and Planetary Science Letters, 247(3-4), 267–279.
Tipper, E. T., Galy, A., and Bickle, M. J. (2008). Calcium and magnesium isotope systematics in rivers draining the Himalaya-Tibetan-Plateau region: Lithological or fractionation control? Geochimica et Cosmochimica Acta, 72(4), 1057–1075.
Tipper, E. T., Galy, A., Gaillardet, J., Bickle, M. J., Elderfield, H., and Carder, E. A. (2006). The magnesium isotope budget of the modern ocean: Constraints from riverine magnesium isotope ratios. Earth and Planetary Science Letters, 250(1-2), 241–253.
Walter, M. R., Veevers, J. J., Calver, C. R., Gorjan, P., and Hill, A. C. (2000). Dating the 840-544 Ma Neoproterozoic interval by isotopes of strontium, carbon, and sulfur in seawater, and some interpretative models. Precambrian Research (Vol. 100).
Wang, W., Zhou, C., Guan, C., Yuan, X., Chen, Z., and Wan, B. (2014). An integrated carbon, oxygen, and strontium isotopic studies of the Lantian Formation in South China with implications for the Shuram anomaly. Chemical Geology, 373, 10–26.
Wen, B., Evans, D. A. D., Li, Y., Wang, Z., and Liu, C. (2015). Newly discovered Neoproterozoic diamictite and cap carbonate ( DCC ) couplet in Tarim Craton , NW China : Stratigraphy , geochemistry , and paleoenvironment. Precambrian Research, 271, 278–294.
Xiao, S., Bao, H., Wang, H., Kaufman, A. J., Zhou, C., Li, G., … Ling, H. (2004). The Neoproterozoic Quruqtagh Group in eastern Chinese Tianshan: Evidence for a post-Marinoan glaciation. Precambrian Research, 130(1-4), 1–26.
Xu, B., Xiao, S., Zou, H., Chen, Y., Li, Z. X., Song, B., … Yuan, X. (2009). SHRIMP zircon U-Pb age constraints on Neoproterozoic Quruqtagh diamictites in NW China. Precambrian Research, 168(3-4), 247–258.
Yang, W., Teng, F. Z., and Zhang, H. F. (2009). Chondritic magnesium isotopic composition of the terrestrial mantle: A case study of peridotite xenoliths from the North China craton. Earth and Planetary Science Letters, 288(3-4), 475–482.
Yuan, X., Chen, Z., Xiao, S., Zhou, C., and Hua, H. (2011). An early Ediacaran assemblage of macroscopic and morphologically differentiated eukaryotes. Nature, 470(7334), 390–3.
Zhang, S., Jiang, G., Zhang, J., Song, B., Kennedy, M. J., and Christie-Blick, N. (2005). U-Pb sensitive high-resolution ion microprobe ages from the Doushantuo Formation in south China: Constraints on late Neoproterozoic glaciations. Geology, 33(6), 473–476.
Zhao, Y. Y., and Zheng, Y. F. (2010). Stable isotope evidence for involvement of deglacial meltwater in Ediacaran carbonates in South China. Chemical Geology, 271(1-2), 86–100.
Zhao, Y. Y., Zheng, Y. F., and Chen, F. (2009). Trace element and strontium isotope constraints on sedimentary environment of Ediacaran carbonates in southern Anhui, South China. Chemical Geology, 265(3-4), 345–362.
Zhao, Y., and Zheng, Y. (2015). Geochemistry of vein and wallrock carbonates from the Ediacaran system in South China : Insights into the origins of depositional and post-depositional fl uids. Chemical Geology, 404, 71–87.
Zheng, Y.-F., Wu, R.-X., Wu, Y.-B., Zhang, S.-B., Yuan, H., and Wu, F.-Y. (2008). Rift melting of juvenile arc-derived crust: Geochemical evidence from Neoproterozoic volcanic and granitic rocks in the Jiangnan Orogen, South China. Precambrian Research, 163(3), 351–383.
Zhou, C., Tucker, R., Xiao, S., Peng, Z., Yuan, X., and Chen, Z. (2004). New constraints on the ages of Neoproterozoic glaciations in south China. Geology, 32(5), 437–440.