Chemical disequilibrium in the mineral assemblage olivine + orthopyroxene + augite + quartz from the contact metamorphosed Biwabik Iron-Formation, northeastern Minnesota

The mineral assemblage olivine + orthopyroxene + augite + quartz occurs locally in the Biwabik Iron-Formation within the aureole of the mafic intrusive Duluth Complex. The minerals are described adequately by chemical components SiO2, MgO, FeO, and CaO, and, since metamorphism was essentially isobaric, the assemblage at equilibrium is effectively univariant. Six such samples investigated in the study variably show evidence of textural equilibrium or disequilibrium. Application of the olivine-orthopyroxene-quartz barometer via the QUILF database yields spurious or non-convergent results for all six samples, suggesting that the compositions of the minerals are not representative of a state of chemical equilibrium. The low variance of the system allows for the possibility that the composition of a single mineral may serve as a monitor of equilibrium. Neither olivine nor augite is suitable for this purpose. Compositions of olivine included in quartz are consistently more magnesian than those of matrix grains, indicating that all matrix olivine has been altered by means of post-growth exchange of cations either with adjacent minerals or with an intergranular fluid phase. Augite is altered pervasively by exsolution of orthopyroxene and presents evidence that its temperature of closure to exchange of cations with olivine and of closure to Fe2+-Mg2+ interdiffusion is lower than that of orthopyroxene. If compositions of the chemically zoned orthopyroxene grains from the various samples are representative of equilibrium with olivine, augite, and quartz, then they must plot within the pyroxene quadrilateral along a curvilinear feature – representing an isobar – over a range of temperatures of equilibration. However, most of the analyzed orthopyroxene compositions do not plot along such a feature, but rather show a great degree of scatter. Despite this fact, the scatter of analyses possesses a relatively sharp boundary at high [special characters omitted] and high [special characters omitted]. Through analysis of phase relations within the quadrilateral and analysis of compositional profiles of individual grains, several processes that could account for deviation of orthopyroxene compositions from an isobar may be identified. These include (1) growth while equilibrium among all four minerals was not maintained, (2) post-growth exchange of cations with adjacent minerals, particularly olivine, (3) intragranular interdiffusion of Fe 2+ and Mg2+ during and after growth, and (4) exsolution of augite. Several conclusions are obtained in the study. (1) A given analysis from an orthopyroxene grain interior cannot necessarily be assumed representative of equilibrium in the system containing olivine, augite, and quartz. (2) The sharp boundary exhibited by orthopyroxene compositions within the quadrilateral roughly represents an isobar; however its position is obscured by the effects of intragranular diffusion of Fe2+ and Mg2+, which in some cases has forced orthopyroxene composition to higher [special characters omitted] for given [special characters omitted]. (3) Orthopyroxene currently present in the rocks is uniformly of retrograde origin. Most grains show decreasing [special characters omitted] concomitant with increasing [special characters omitted] progressing from interior to rim, which can only be associated with growth with falling T regardless of whether that growth occurred while equilibrium was maintained with olivine, augite, and quartz. (4) Maximum [special characters omitted] in a given sample decreases with distance from the contact with the Duluth Complex, indicating that a near-peak T profile may be preserved within the orthopyroxene zone of the aureole. (5) The correlation of textural and chemical equilibrium is ambiguous in many cases. The proximity of the four minerals does not ensure that chemical equilibrium was maintained among them during growth of orthopyroxene. (6) Isobars predicted by QUILF lie roughly parallel to the apparent isobar identified from the orthopyroxene compositions and indicate that, where applicable, and assuming accuracy of the QUILF model and the chemical analyses, orthopyroxene equilibrated with olivine, augite, and quartz at P = 1300 ± 200 bars over T ranging between ~780 and