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Modeling incompatible trace-element abundances in plagioclase in the Skaergaard intrusion using the trapped liquid shift effect


Incompatible trace-element abundances in minerals and whole rocks from layered intrusions have been used to model the fractionation processes and evolving liquid compositions. Many such models assume that the analyzed concentration in a mineral represents that of the mineral when it first crystallized. However, overgrowth from residual liquid and subsequent diffusive equilibration can result in significant changes to the bulk mineral compositions (the more incompatible the element the more dramatic the subsequent changes). The proportion of that residual liquid relative to the cumulus minerals is the most important parameter in determining the magnitude of this effect (trapped liquid shift effect). Calculations involving Ba and La contents in plagioclase quantitatively demonstrate this effect. For Ba and La (partition coefficients of 0.4 and 0.04), 50% trapped liquid in a sample can result in two and sevenfold increases, respectively, in concentration between original and final bulk mineral compositions. Different cumulus assemblages also have a major effect on final compositions. We use examples of the concentrations of Ba and La in plagioclase from the Skaergaard intrusion from previous publications to illustrate the importance of this effect. Specifically, the La content of bulk plagioclase steadily decreases upward from the Lower Zone to Upper Zone c, and Ba in plagioclase shows no increase from the Lower Zone to the top of the Middle Zone. Such results are not explicable by fractionation processes, but can be modeled by the trapped liquid shift effect, assuming the well-established evidence for upward decrease in trapped liquid proportion through these zones.

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RGC acknowledges financial support from AngloPlatinum, Implats and Lonplats companies and NRF (South Africa). CT acknowledges funding from the Danish Council for Independent Research (Natural Sciences), the Carlsberg Foundation, and the Danish National Research Foundation. We thank Richard Naslund, Steven Barnes and Jonas Møller Pedersen for their comments and suggestions, and Christian Ballhaus for his editorial management.

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Correspondence to R. Grant Cawthorn.

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Communicated by Chris Ballhaus.

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Cawthorn, R.G., Tegner, C. Modeling incompatible trace-element abundances in plagioclase in the Skaergaard intrusion using the trapped liquid shift effect. Contrib Mineral Petrol 172, 93 (2017).

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  • Skaergaard intrusion
  • Plagioclase
  • Trace elements
  • Trapped liquid shift effect