HR: 0800h
AN: GP21B-0158 INVITED     [Abstracts]
TI: Significance of Elevated Contents of $\Box$Si$_{4}$O$_{8}$ and AlAl$_{3}$SiO$_{8}$ End Members in Black Clouded Plagioclase From Anorthosite: Evidence for Oxidation-Induced Postmagmatic Exsolution of Fe-Ti Oxide Inclusions From Plagioclase
AU: Murakami, H
EM: hidekim@rimr.akita-u.ac.jp
AF: Research Institute of Materials and Resources, Faculty of Engineering and Resource Science, Akita University, 1-1 Tegata Gakuen-cho, Akita, 010-8502 Japan
AU: * Lalonde, A E
EM: andre.e.lalonde@uottawa.ca
AF: Department of Earth Sciences, Ottawa-Carleton Geoscience Centre, University of Ottawa, 140, rue Louis-Pasteur, Ottawa, ON K1N 6N5 Canada
AB: Plagioclase phenocrysts from large Proterozoic massif anorthosites that have escaped metamorphic recrystallization commonly have a distinctive black color that is attributed to myriads of minute Fe-Ti oxide inclusions. These inclusions are responsible for the black color of the feldspar. The orientation of these inclusions along specific crystallographic directions in the plagioclase is strong evidence that these formed by exsolution from the plagioclase. Plagioclase from the Proterozoic Lac Saint-Jean anorthosite in Qu$\'{e}$bec is a good example of such clouded feldspar. In this plagioclase we recognize four types of Fe-Ti oxide inclusions: 1) Myriads of fine, submicroscopic and unidentifiable dust-like inclusions, presumably Fe-Ti oxides, and responsible in great part for the black color. 2) Small $\sim$1 $\mu$m sized crystals of hemo-ilmenite that occur in regions of the plagioclase that are free of type 1 dust, suggesting that these originated by Ostwald ripening from the dusty material. 3) Larger aligned and acicular crystals of hematite, $\sim$10 $\mu$m in diameter and with good rhombohedral sections. These occur, like type 2 inclusions, in zones free of type 1 dust suggesting again that they result from Ostwald ripening. 4) Millimetric inclusions of ilmenite or magnetite of magmatic origin, often rimmed by amphibole or biotite, signs of late hydration. As part of our study, EPMA analyses of plagioclase from Lac Saint-Jean were done. In addition, a large single crystal, $\sim$30 cm in diameter was analyzed by bulk XRF. Our results show that plagioclase contains significant amounts of the $\Box$Si$_{4}$O$_{8}$ (up to 6.6 mole $%$) and AlAl$_{3}$SiO$_{8}$ (up to 2.2 mole $%$) endmembers. A positive relationship is observed between the content of these two endmembers. Furthermore, the ratio of $\Box$Si$_{4}$O$_{8}$ to AlAl$_{3}$SiO$_{8}$ is approximately 3:4. We believe that the high content of $\Box$Si$_{4}$O$_{8}$ and AlAl$_{3}$SiO$_{8}$ in plagioclase, and the presence of minute Fe-Ti oxide inclusions are both products of a late postmagmatic oxidation via the following chemical reactions: 1) 4Fe$^{2+}$Al$_{2}$Si$_{2}$O$_{8}$ + 4CaFe$^{2+}$Si$_{3}$O$_{8}$ + 2O$_{2}$ $\rightarrow$ 4Fe$_{2}$O$_{3}$ + 4CaAl$_{2}$Si$_{2}$O$_{8}$ + 3$\Box$Si$_{4}$O$_{8}$ 2) 8Fe$^{2+}$Al$_{2}$Si$_{2}$O$_{8}$ + 2O$_{2}$ $\rightarrow$ 4Fe$_{2}$O$_{3}$ + 3$\Box$Si$_{4}$O$_{8}$ + 4 AlAl$_{3}$SiO$_{8}$ 3) 8CaAl$_{2}$SiTiO$_{8}$ + 8Fe$^{2+}$Al$_{2}$Si$_{2}$O$_{8}$ $\rightarrow$ 8CaAl$_{2}$Si$_{2}$O$_{8}$ + 8FeTiO$_{3}$ + $\Box$Si$_{4}$O$_{8}$ + 4 AlAl$_{3}$SiO$_{8}$ These reactions indicate that significant amounts of Fe$^{2+}$ were originally present in the tetrahedral and cavity sites of the plagioclase and that Ti$^{4+}$ was also present in the tetrahedral sites. We conclude that plagioclase in the Lac Saint-Jean anorthosite crystallized under fairly reducing conditions and later underwent postmagmatic oxidation when the plutonic mass was uplifted and came into contact with crustally-derived oxygenated fluids. These findings have important implications for studies of magnetic remanence and anisotropy fabrics in anorthositic rocks.
DE: 3620 Crystal chemistry
DE: 3640 Igneous petrology
DE: 1518 Magnetic fabrics and anisotropy
DE: 1519 Magnetic mineralogy and petrology
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting