Exsolution structures in calcic pyroxenes from the Bjerkreim-Sokndal lopolith, SW Norway

Abstract

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 '00l ' 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 ~ and 900 ~ 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 ~ for stage 1, and 600 400 ~ for stage 3. We believe this geothermometer to be unreliable for the low temperatures involved in stage 4. THE mechanism of precipitation (exsolution) and the nature of the subsolidus phases in lunar and terrestrial pyroxenes have been the subject of many studies in the past two decades. Extensive petrographic, single-crystal X-ray diffraction and, more recently, transmission electron microscopic (TEM) studies have shown that subsolidus phase relations in pyroxenes from slowly cooled igneous and metamorphic rocks may be complex (e.g. Ross et al., 1972;Jaffe et al., 1975;Robinson et al., 1977; Rietmeijer and Champness, 1980a;and Nakajima and Hafner, 1980). Copyright the Mineralogical SocietyRobinson et al. (1971, 1977) have shown that the habit planes of exsolved Ca-rich and Ca-poor clinopyroxenes are not always parallel to (001) and (100) as had previously been assumed, but are (h01) planes that may deviate several degrees from the exact (001) and (100) orientations. The relative equilibrium cell dimensions of the phases are such that, for coherent exsolution of one phase from another, one principal strain is of opposite sign to the other two (the strain quadric is a hyperboloid). There are thus two directions perpendicular to the intermediate principal axis of strain (the y-axis) for which the strain is zero. Because the strain parallel to y, the elastic anisotropy and the chemical-energy component of the interracial energy are small, the orientations of the exsolution lamellae are essentially those of the 'planes of best fit' at nucleation. Robinson et al. (1971Robinson et al. ( , 1977 showed that slowly cooled...

Chemical distinction between igneous and metamorphic orthopyroxenes especially those coexisting with Ca-rich clinopyroxenes: a re-evaluation

“…5) support the lower temperature estimate of 600 ~ Also compositions of the coexisting (Rietmeijer, 1979). SOLID L~NES.…”

“…1 (marked ER) are orthopyroxene which forms a partial rim on Carich clinopyroxenes from the upper part of the Layered Intrusion of Bjerkreim-Sokndal. Both pyroxenes are of igneous origin (Rietmeijer and Champness, 1982). In fig.…”

“…The Layered Intrusion of Bjerkreim-Sokndal intruded during and after the peak of metamorphism (Rietmeijer, 1979;Kars et al, 1980). Throughout the intrusion, small sub-to anhedral orthopyroxene crystals may occur next to large crystals of Ca-poor pyroxene.…”

“…Fe-rich igneous rocks are commonly associated with anorthosite masses in Precambrian terranes (cf. Smith, 1974;Maim and Ormaasen, 1978;Rietmeijer, 1979). Subsolidus re-equilibration and recrystallization are commonly extensive in these rocks.…”

Progressive or continual exsolution in pyroxenes: an indicator of polybaric igneous crystallization for the Perinthatta anorthositic gabbro, northern Kerala, southwestern India

“…Pyroxenes constitute one of the best known multicomponent systems in mineralogy (Lindsley, 1983) and the result of unmixing in Ca-Mg-Fe pyroxenes is commonly visible as sets of thin exsolution lamellae (Robinson et al, 1977;Rietmeijer and Champness, 1982). The study of microstructures, for example the coarsening of exsolution lamellae showing an irregular width or bulging, provides evidence not only of changing cooling rates, but also for interactions between rocks and fluid phases (e.g.…”