Stolper, Daniel A. and Eiler, John M. (2016) Constraints on the formation and diagenesis of phosphorites using carbonate clumped isotopes. Geochimica et Cosmochimica Acta, 181 . pp. 238-259. ISSN 0016-7037. http://resolver.caltech.edu/CaltechAUTHORS:20160315-083934513
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The isotopic composition of apatites from sedimentary phosphorite deposits has been used previously to reconstruct ancient conditions on the surface of the earth. However, questions remain as to whether these minerals retain their original isotopic composition or are modified during burial and lithification. To better understand how apatites in phosphorites form and are diagenetically modified, we present new isotopic measurements of δ^(18)O values and clumped-isotope-based (Δ_(47)) temperatures of carbonate groups in apatites from phosphorites from the past 265 million years. We compare these measurements to previously measured δ^(18)O values of phosphate groups from the same apatites. These results indicate that the isotopic composition of many of the apatites do not record environmental conditions during formation but instead diagenetic conditions. To understand these results, we construct a model that describes the consequences of diagenetic modification of phosphorites as functions of the environmental conditions (i.e., temperature and δ^(18)O values of the fluids) during initial precipitation and subsequent diagenesis. This model captures the basic features of the dataset and indicates that clumped-isotope-based temperatures provide additional quantitative constraints on both the formational environment of the apatites and subsequent diagenetic modification. Importantly, the combination of the model with the data indicates that the δ^(18)O values and clumped-isotope temperatures recorded by phosphorites do not record either formation or diagenetic temperatures, but rather represent an integrated history that includes both the formation and diagenetic modification of the apatites.
|Additional Information:||© 2016 Elsevier Ltd. Available online 2 March 2016. We wish to thank Yehoshua Kolodny and Boaz Luz of the Hebrew University in Jerusalem for providing the samples and helpful discussions. We additionally wish to thank Aldo Shemesh of the Weizmann Institute for helpful discussions. We thank two anonymous reviewers, Cedric John, and our associate editor, Hagit Affek, for helpful comments. DAS would like to thank his PhD thesis committee of Jess Adkins, Woodward Fischer, John Grotzinger, and Alex Sessions (along with John Eiler) for sage advice and guidance. DAS acknowledges the NSF GRFP for support. JME acknowledges the support of the NSF-EAR program for instrumentation.|
|Official Citation:||Daniel A. Stolper, John M. Eiler, Constraints on the formation and diagenesis of phosphorites using carbonate clumped isotopes, Geochimica et Cosmochimica Acta, Volume 181, 15 May 2016, Pages 238-259, ISSN 0016-7037, http://dx.doi.org/10.1016/j.gca.2016.02.030. (http://www.sciencedirect.com/science/article/pii/S001670371630076X)|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Tony Diaz|
|Deposited On:||15 Mar 2016 16:45|
|Last Modified:||19 Nov 2016 00:48|
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