Henry, 2005 Ti en Biotite

Henry, 2005 Ti en Biotite - American Mineralogist, Volume...

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0003-004X/05/0203&316$05.00/DOI: 10.2138/am.2005.1498 316 INTRODUCTION The concentration of Ti in biotite has long been considered to be a function primarily of changing temperature conditions in metamorphic rocks and has been suggested as a potential geo- thermometer (Engel and Engel 1960; Kwak 1968; Robert 1976; Dymek 1983; PatiAo Douce 1993). However, the factors that in uence the incorporation of Ti in biotite are more multifaceted than simple temperature change, and involve a relatively complex interplay among temperature, pressure, biotite crystal chemistry, and coexisting mineral assemblages (Guidotti et al. 1977, 1988; Dymek 1983; Labotka 1983; Guidotti 1984; Tracy and Robinson 1988; Guidotti and Sassi 2002; Henry and Guidotti 2002). Experimental investigators have examined how temperature, pressure, and composition control the amount of Ti in biotite. The temperature effect appears to be the most in uential. In phlogopite, the incorporation of Ti is relatively dramatic at high temperatures, particularly higher than 800 ±C. For in- stance, Robert (1976) found that Ti solubility is relatively low, 0.07 Ti atoms per formula unit (apfu), at 600 ±C and 1 kbar, but increases signi ± cantly to 0.2 Ti apfu at 800 ±C and 1 kbar, and to 0.7 Ti apfu at 1000 ±C and 1 kbar. Increasing pressure has the opposite effect, with Ti concentrations decreasing substantially with increasing pressure (Forbes and Flower 1974; Robert 1976; * E-mail: dhenry@geol.lsu.edu The Ti-saturation surface for low-to-medium pressure metapelitic biotites: Implications for geothermometry and Ti-substitution mechanisms DARRELL J. HENRY, 1, * CHARLES V. GUIDOTTI, 2 AND JENNIFER A. THOMSON 3 1 Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Lousianna 70803, U.S.². 2 Department of Geological Sciences, University Of Maine, Orono, Maine 04469, U.S.². 3 Department of Geology, Sci 130, Eastern Washington University, Cheney, Washington 99004, U.S.². ABSTRACT The Ti content of biotite can serve as a geothermometer for graphitic, peraluminous metapelites that contain ilmenite or rutile and have equilibrated at roughly 4&6 kbar. The relationship between Ti- content, temperature, and Mg/(Mg + Fe) value was calibrated empirically using an extensive natural biotite data set (529 samples) from western Maine and south-central Massachusetts in combination with the petrogenetic grid of Spear et al. (1999). The calculated Ti-saturation surface is curved such that for a given Mg/(Mg + Fe) value, Ti concentration increases as a function of temperature in a nonlinear fashion, and for a given temperature Ti concentrations decrease with an increase in Mg/(Mg + Fe). The ± t to the Ti-saturation surface can be reformulated as the geothermometric expression: T = {[ln(Ti) & a & c ( X Mg ) 3 ]/ b } 0.333 , in which T is temperature in degrees Celsius, Ti is the number of atoms per formula unit (apfu) normalized on the basis of 22 O atoms,
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This note was uploaded on 03/19/2012 for the course GEOLOGY 6 taught by Professor Rm during the Spring '12 term at UNAM MX.

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Henry, 2005 Ti en Biotite - American Mineralogist, Volume...

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