Thermal modification of hematite-ilmenite intergrowths in the Ecstall pluton, British Columbia, Canada

TitleThermal modification of hematite-ilmenite intergrowths in the Ecstall pluton, British Columbia, Canada
Publication TypeJournal Article
Year of Publication2010
AuthorsBrownlee SJ, Feinberg JM, Harrison RJ, Kasama T, Scott GR, Renne PR
JournalAmerican Mineralogist
Volume95
Pagination153-160
Abstract

In this study, we examine the effects of reheating on finely exsolved hematite-ilmenite intergrowths from the  91 Ma Ecstall pluton using reflected light microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). As a result of the emplacement of the younger adjacent  52 Ma Quottoon pluton, samples closer to a thermal boundary have experienced greater degrees of thermal alteration. Five main microstructural features characterize hematite-ilmenite intergrowths from the Ecstall: (I) exsolution lamellae of hematite and ilmenite; (II) oxidation of ilmenite to form hematite, rutile, and other Ti-rich phases; (III) 20–50 nm magnetite precipitates in hematite; (IV) rutile blitz texture; and (V) exsolution of hematite in rutile. Based on spatial relationships, textures II through V appear to be related to reheating of the Ecstall by the Quottoon, and samples up to  14 km from the thermal boundary intrusive contact have been affected. We propose a mechanism, similar to that of Kontny and Dietl (2001), in which reheating has driven T-fO2 conditions across the hematite-magnetite buffer to lower fO2 resulting in the reduction of hematite. Higher temperatures also enhanced oxidation in ilmenite. The formation of magnetite altered the bulk magnetic properties of these samples, increasing NRM intensity. This study underscores the need to consider a pluton's post-emplacement thermal history before making tectonic interpretations based on paleomagnetic data.

URLhttp://ammin.geoscienceworld.org/content/95/1/153.abstract
DOI10.2138/am.2010.3191