Solid Solution and Cation Ordering Limits in High-Temperature Sodic Pyroxenes from the Nybö Eclogite Pod, Norway

M. A. Carpenter and D. C. Smith*
Dept. of Earth Sciences, University of Cambridge, Downing St., Cambridge CB2 3EQ
Grant Institute of Geology, Edinburgh University, West Mains Road, Edinburgh EH9 3JW, Scotland
*New address: Laboratoire de Minéralogie, Muséum National d'Histoire Naturelle, 61 Rue de Buffon, 75005 Paris, France.

Abstract: A range of sodic pyroxenes (Jd78Ac2Aug20Jd24Ac9Aug67) from the Nybö eclogite pod, Norway, have been examined by electron-microprobe analysis and by transmission electron microscopy.. Five different microprobes give generally compatible results and new analyses completely fill the natural composition gap in jadeite-rich omphacites with ∼ 3–12% acmite, confirming complete miscibility at high temperatures (⩾ 700 °C). Crystals with omphacite compositions around jadeite: augite = 1:1 contain antiphase domains resulting from the C2/c → P2/n cation ordering transformation. Exsolution microstructures were not observed, from which it is concluded that there are no two-phase regions separating the P2/n and C2/c stability fields at high temperatures (∼ 620–750 °C) and that cooling was too rapid for exsolution to occur in the jadeite/omphacite and omphacite/augite low-temperature solvi. Crystals whose compositions depart significantly from Jd:Aug= 1:1 have weak, diffuse h + k = odd reflections in selected area electron diffraction patterns which are interpreted as being due to short-range ordering outside the true P2/n stability field. The short-range ordering and lack of exsolution are consistent with a previous suggestion that the order/disorder transformation in omphacite is second (or higher) order in character. The average antiphase domain sizes (up to ∼ 3500 Å) are larger than any previously recorded in omphacites and are consistent with available petrological evidence for a long period of annealing at high temperatures before tectonic uplift and cooling.

Mineralogical Magazine; March 1981 v. 44; no. 333; p. 37-44; DOI: 10.1180/minmag.1981.44.333.04
© 1981, The Mineralogical Society
Mineralogical Society (