A TEM and microprobe study of a two-perthite alkali gabbro: Implications for the ternary feldspar system |
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| Authors: | I Parsons W L Brown |
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| Institution: | 1. Department of Geology and Mineralogy, Marischal College, AB9 1AS, Aberdeen, Scotland
2. Laboratoire de Pétrologie - Géochimie, Université de Nancy 1, BP 239, 54506, Vandoeuvre-les-Nancy, Cédex, France
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| Abstract: | An unusual magmatic three-feldspar syenogabbro occurs 3 m inside the contact of the Klokken gabbro-syenite stock. It contains plagioclase, mesoperthite and cryptoperthite, together with a low temperature symplectite intergrowth. Textural relationships have been investigated by cathodoluminescence, bulk chemistry by microprobe, and exsolution microtextures and intracrystalline boundaries by TEM. The mesoperthite has a bulk composition around Or26Ab52An22, well outside accepted limits of ternary feldspar solid solution. It is a mainly coherent, lamellar intergrowth of sodic andesine and orthoclase with incipient Mtwinning. The cryptoperthite, bulk composition around Or61Ab33An6, is a coherent lamellar intergrowth of orthoclase and sodic low oligoclase. The compositions of the exsolved phases have been estimated. The meso- and cryptoperthite crystals have sharp boundaries with each other. Plagioclase (zoned An 55-35, with 2–3% Or) defines a solidus fractionation path. It behaved as an inert phase during crystallization of the perthites, which grew as homogeneous monoclinic phases in equilibrium on the strain-free ternary solvus, defining a solvus isotherm at ～ 950° C. Both of the monoclinic phases exsolved on reaching the coherent ternary spinodal at temperatures estimated to be close to 800–850° C and 700–830° C respectively. Lamellar periodicities (529±149 nm and 148±18 nm respectively) are considerably finer scale than predicted by coarsening experiments, suggesting that An and/or Al-Si order greatly inhibit coarsening. The symplectite is a coarse, incoherent intergrowth of sodic andesine and nearly pure K-feldspar, probably produced by simultaneous crystallization at <400° C. The new data and literature analyses are used to construct the geometry of the ternary feldspar system. Solvus isotherms implied by existing experimental data approach the Ab apex too closely at high temperature. The critical solution curve becomes slightly more albitic after leaving the binary Ab-Or join, and then turns sharply towards the An-Or join. It intersects the proposed new limit of feldspar solid solution near Or36Ab44An20 at 1,060° C. This probably approximates to the highest temperature on the ternary critical curve at 1 bar. |
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