Mineralogy and Petrology

, Volume 98, Issue 1–4, pp 91–110 | Cite as

Iron isotope compositions of carbonatites record melt generation, crystallization, and late-stage volatile-transport processes

  • Clark M. Johnson
  • Keith Bell
  • Brian L. Beard
  • Aaron I. Shultis
Original Paper

Abstract

Carbonatites define the largest range in Fe isotope compositions yet measured for igneous rocks, recording significant isotopic fractionations between carbonate, oxide, and silicate minerals during generation in the mantle and subsequent differentiation. In contrast to the relatively restricted range in δ56Fe values for mantle-derived basaltic magmas (δ56Fe = 0.0 ± 0.1‰), calcite from carbonatites have δ56Fe values between −1.0 and +0.8‰, similar to the range defined by whole-rock samples of carbonatites. Based on expected carbonate-silicate fractionation factors at igneous or mantle temperatures, carbonatite magmas that have modestly negative δ56Fe values of ~ −0.3‰ or lower can be explained by equilibrium with a silicate mantle. More negative δ56Fe values were probably produced by differentiation processes, including crystal fractionation and liquid immiscibility. Positive δ56Fe values for carbonatites are, however, unexpected, and such values seem to likely reflect interaction between low-Fe carbonates and Fe3+-rich fluids at igneous or near-igneous temperatures; the expected δ56Fe values for Fe2+-bearing fluids are too low to produced the observed positive δ56Fe values of some carbonatites, indicating that Fe isotopes may be a valuable tracer of redox conditions in carbonatite complexes. Further evidence for fluid-rock or fluid-magma interactions comes from the common occurrence of Fe isotope disequilibrium among carbonate, oxide, silicate, and sulfide minerals in the majority of the carbonatites studied. The common occurrence of Fe isotope disequilibrium among minerals in carbonatites may also indicate mixing of phenocyrsts from distinct magmas. Expulsion of Fe3+-rich brines into metasomatic aureols that surround carbonatite complexes are expected to produce high-δ56Fe fenites, but this has yet to be tested.

Notes

Acknowledgments

C.M.J., B.L.B., and A.I.S. thank the organizers of this special volume in honor of our co-author Keith Bell, including guest editor Antonio Simonetti. This work was supported by the Department of Geology and Geophysics (U.W. Madison), the Geological Society of America, and the National Science Foundation (grant EAR-0525417). In addition to samples in the collection of K.B., samples were provided by J.B. Dawson, D. Moecher, E. Haynes, and M. Spicuzza. Journal reviews were provided by R. Schoenberg and an anonymous reviewer, whose comments helped to improve the manuscript. We thank A. Simonetti for editorial handling of the paper.

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Copyright information

© Springer-Verlag 2009

Authors and Affiliations

  • Clark M. Johnson
    • 1
  • Keith Bell
    • 2
  • Brian L. Beard
    • 1
  • Aaron I. Shultis
    • 1
  1. 1.Department of Geology and GeophysicsLewis G. Weeks Hall for Geological SciencesMadisonUSA
  2. 2.Isotope Geochemistry and Geochronology Research Centre2117 Herzberg Laboratories, Carleton UniversityOttawaCanada

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