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Plagioclase in the Skaergaard intrusion. Part 1: Core and rim compositions in the layered series

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Abstract

The anorthite content of plagioclase grains (XAn) in 12 rocks from the layered series of the Skaergaard intrusion has been studied by electron microprobe (typically ∼30 core and ∼70 rim analyses per thin section). Mean core compositions vary continuously from An66 at the base of the layered series (LZa) to An32–30 at the top. On the other hand, crystal rims are of approximately constant composition (An50 ± 1) from the LZa to the lower Middle Zone (MZ). Above the MZ, core and rim compositions generally overlap. Profiles across individual plagioclase grains from the lower zone show that most crystals have an external zone buffered at XAn ∼50 ± 1. The simplest explanation for these features is that during postcumulus crystallization in the lower zone, interstitial liquids passed through a density maximum. This interpretation is consistent with proposed liquid lines of descent that predict silica enrichment of the liquid associated with the appearance of cumulus magnetite.

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Acknowledgments

We are particularly indebted to our friends and colleagues who generously donated rock samples, before our memorable visit to the intrusion in 2001. Those people include Graham Chinner (University of Cambridge), Alex McBirney (University of Oregon), Ian Parsons, Brian Upton and Peder Aspen (University of Edinburgh) and Kevin Walsh (University of Oxford Museum). Thanks also to S. Barda, F. Diot and A. Kohler of the Service Commun d’analyse of the Université Henri Poincaré in Nancy for help with the electron microprobe and SEM. S.A. Morse and an anonymous reviewer are thanked for their comments which were greatly appreciated. Financial support from the CNRS-INSU (“Intérieur de la Terre” and “Dyeti” programmes) is also acknowledged.

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Authors and Affiliations

  1. UMR 5562 Dynamique Terrestre et Planétaire, Observatoire Midi-Pyrénées, 14 avenue Edouard Belin, 31400, Toulouse, France

    Michael J. Toplis

  2. UPR 2300 CRPG-CNRS, BP 20, 54501, Vandoeuvre-Lès-Nancy, France

    William L. Brown & Elsa Pupier

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  1. Michael J. Toplis
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  2. William L. Brown
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  3. Elsa Pupier
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Correspondence to Michael J. Toplis.

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Communicated by T.L. Grove.

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Toplis, M.J., Brown, W.L. & Pupier, E. Plagioclase in the Skaergaard intrusion. Part 1: Core and rim compositions in the layered series. Contrib Mineral Petrol 155, 329–340 (2008). https://doi.org/10.1007/s00410-007-0245-1

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