IRON SITE OCCUPANCY AND VALENCE STATE IN METAPELITIC CHLORITE FROM WESTERN MAINE

The chemical composition of chlorite is believed to be a function of bulk chemistry and the environmental conditions of crystallization of the host. This study presents results on carefully-handpicked chlorite separates from metapelites from western Maine, with a focus on the changes in Fe valence and site occupancy over a range of temperatures (lower garnet zone to staurolite zone) and oxygen fugacities. Samples come from the well-studied Rangeley (Ra), Oquossoc (Oq), Perry Mountain (PM), and Smalls Falls (SmFs) localities. All are saturated with respect to Al and most are also Ti-saturated. The opaque mineral assemblages vary: ilmenite or rutile are present, graphite is sometimes absent, and magnetite and/or hematite are sometimes present as primary or secondary phases. Results of this work in progress include the following: (1) As noted in previous work by Guidotti, the main thing that varies (as a function of grade or sulfidation reactions) is the Mg/Mg+Fe ratio. (2) The ^[6]Al/^[6]Al+Fe+Mg ratio in these chlorites is independent of the Mg/Mg+Fe ratio--just as is the case for biotite. (3) There is little or no compelling indication that the total Al, ^[4]Al, or Si are co-varying with Mg/Mg+Fe ratio. (4) In graphite/ilmenite-bearing samples from garnet and staurolite grade rocks in the Ra and Oq, Fe³⁺ contents of chlorite remain constant at roughly 10+/-4% Fe³⁺ and are independent of metamorphic grade. (5) All PM chlorites contain elevated Fe³⁺, ranging from 18-28% of the total Fe. Major element compositions of these samples are otherwise analogous to those of the Ra and Oq suites, so the difference must be in the oxide assemblage, which is now being characterized. (6) In the Mg-rich samples from the SmFs rocks, the Mg/Mg+Fe ratio goes as high as 0.94 -- an aluminous phlogopite. Si is also systematically higher, and total Al is lower. Samples in this data set contain hematite and/or magnetite, and their Fe³⁺ contents range from 13-30% of the total iron.