Exsolution Structures in Calcic Pyroxenes from the Bjerkreim-Sokndal Lopolith, SW Norway

F. J. M. Rietmeijer and P. E. Champness
Department of Petrology, Instituut voor Aardwetenschappen, Budapestlaan 4, 3508 TA Utrecht, The Netherlands
Department of Geology, University of Manchester, Manchester M13 9PL

Abstract: Iron-rich (Fs:En ∼0.8) calcic pyroxenes that have been subjected to granulite-facies metamorphism contain up to seven generations of exsolution lamellae. They can be grouped into four stages. In stage 1 pigeonite exsolved parallel to ‘001’ and ‘100’ (where ‘hkl’ signifies ∼ (hkl)) and mostly inverted later to orthopyroxene. During stage 2 orthopyroxene exsolved parallel to (100), while during stage 3 orthopyroxene was quickly followed by metastable ‘001’ pigeonite. The stage 3 precipitates clearly grew and thickened together for some time. During stage 4 a ‘100’ pigeonite was exsolved. The stage 3 and 4 precipitates show evidence of reheating, dissolution and later, renewed growth. Sometimes orthopyroxenes of stage 3 have crossed a ‘001’ pigeonite lamella and caused it to invert by a shear mechanism.

Chemical analysis shows no rotation of the tie lines between Ca-rich and Ca-poor phases, in contrast to previous studies of Skaergaard and Bushveld pyroxenes. The geothermometers of Wood and Banno (1973) and Wells (1977) indicate solidus temperatures of about 850°C and 900°C respectively, but the geothermometers were found to be unsuitable for subsolidus conditions. We estimate the pressure to have been about 9 kbar during solidification. Estimates of nucleation temperatures obtained from the orientations of the exsolved lamellae (Robinson et al., 1977) were 850–700°C for stage 1, and 600–400°C for stage 3. We believe this geothermometer to be unreliable for the low temperatures involved in stage 4.

Mineralogical Magazine; 1982 v. 45; no. 337; p. 11-24; DOI: 10.1180/minmag.1982.045.337.02
© 1982, The Mineralogical Society
Mineralogical Society (www.minersoc.org)