Skip to main content

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

Abstract

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.

This is a preview of subscription content, access via your institution.

Fig. 1
Fig. 2
Fig. 3

References

  • Barnes SJ (1986) The effect of trapped liquid crystallization on cumulus mineral compositions in layered intrusions. Contrib Miner Pet 93:524–531

    Article  Google Scholar 

  • Bédard JH (1994) A procedure for calculating the equilibrium distribution of trace elements among the minerals in cumulate rocks, and the concentration of trace elements in the coexisting liquids. Chem Geol 118:143–153

    Article  Google Scholar 

  • Bédard JH (2006) Trace element partitioning in plagioclase feldspar. Geochim Cosmochim Acta 70:3717–3742

    Article  Google Scholar 

  • Blundy JD, Wood B (2003) Partitioning of trace elements between crystals and melts. Earth Planet Sci Lett 210:383–397

    Article  Google Scholar 

  • Cawthorn RG (1996a) Models for incompatible trace-element abundances in cumulus minerals and their application to plagioclase and pyroxene compositions in the Bushveld Complex. Contrib Miner Pet 123:109–115

    Google Scholar 

  • Cawthorn RG (1996b) Re-evaluation of magma compositions and processes in the uppermost Critical Zone of the Bushveld Complex. Mineral Mag 60:131–148

    Article  Google Scholar 

  • Cawthorn RG (2013) REE abundances in apatite in the Bushveld Complex—a consequence of the trapped liquid shift effect. Geology 41:603–606

    Article  Google Scholar 

  • Godel B, Barnes S-J, Maier WD (2011) Parental magma composition inferred from in situ traceelements in cumulus and intercumulus silicate minerals; an example from the Lower and Lower Critical Zones of the Bushveld Complex, South Africa. Lithos 125:537–552

    Article  Google Scholar 

  • Hill E, Blundy JD, Wood BJ (2011) Clinopyroxene-melt trace-element partitioning and the development of a predictive model for HFSE and Sc. Contrib Miner Pet 161:423–438

    Article  Google Scholar 

  • Holness MB, Tegner C, Namur O, Pilbeam L (2015) The earliest history of the Skaergaard magma chamber: a textural and geochemical study of the Cambridge drill core. J Pet 56:1199–1227

    Article  Google Scholar 

  • Hunter RH (1996) Texture development in cumulate rocks. In: Cawthorn RG (ed) Layered intrusions. Elsevier, Amsterdam, pp 77–102

    Chapter  Google Scholar 

  • Jang YD, Naslund HR (2001) Major and trace element composition of Skaergaard plagioclase: geochemical evidence for changes in magma dynamics during the final stages of crystallization of the Skaergaard intrusion. Contrib Miner Pet 140:441–457

    Article  Google Scholar 

  • Lundgaard KL, Tegner C, Cawthorn RG, Kruger FJ, Wilson JR (2006) Trapped intercumulus liquid in the Main Zone of the eastern Bushveld Complex, South Africa. Contrib Miner Pet 151:352–369

    Article  Google Scholar 

  • McBirney AR (1996) The Skaergaard intrusion. In: Cawthorn RG (ed) Layered intrusions. Elsevier, Amsterdam, pp 147–180

    Chapter  Google Scholar 

  • McBirney AR (2002) The Skaergaard layered series. Part VI. Excluded trace elements. J Pet 43:535–556

    Article  Google Scholar 

  • Morse SA (1984) Cation diffusion in plagioclase feldspar. Science 27:1183–1214

    Google Scholar 

  • Nielsen TFD (2004) The shape and volume of the Skaergaard intrusion, Greenland: implications for the mass balance and bulk composition. J Pet 45:507–530

    Article  Google Scholar 

  • Prowatke S, Klemme S (2006) Trace element partitioning between apatite and silicate melts. Geochim Cosmochim Acta 70:4513–4527

    Article  Google Scholar 

  • Rudge JF, Holness MB, Smith GC (2008) Quantitative textural analysis of packing of elongate crystals. Contrib Miner Pet 156:413–429

    Article  Google Scholar 

  • Tegner C, Thy P, Holness MG, Jakobsen JK, Lesher CE (2009) Differentiation and compaction in the Skaergaard intrusion. J Pet 50:813–841

    Article  Google Scholar 

  • Wager LR, Brown GM, Wadsworth WJ (1960) Types of igneous cumulate. J Pet 1:73–85

    Article  Google Scholar 

Download references

Acknowledgements

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.

Author information

Authors and Affiliations

Authors

Corresponding author

Correspondence to R. Grant Cawthorn.

Additional information

Communicated by Chris Ballhaus.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

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). https://doi.org/10.1007/s00410-017-1411-8

Download citation

  • Received:

  • Accepted:

  • Published:

  • DOI: https://doi.org/10.1007/s00410-017-1411-8

Keywords

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