Local equilibrium in polymetamorphic gneiss and the titanium substitution in biotite

Abstract

Biotite is a mineral of widespread occurrence and considerable petrological importance in metamorphic rocks. Numerous substitutions occur in biotite, its composition reflecting both rock bulk composition and metamorphic grade. Equilibria involving these substitutions can be used to monitor metamorphic conditions, provided that reliable thermodynamic data exist for biotite end-members. However, successful thermodynamic modeling of biotite solid solutions must be based on a sound knowledge of the cation and anion substitutions in order to define an appropriate set of end-members.The nature of the titanium substitution in biotite, however, is still an open question despite a long history of analytical and experimental investigation (e.g., Forbes and Flower 1974;Dymek 1983;Patino Douce 1993). Ti 4+ substitutes for cations of lower charge in the octahedral layer of the mica structure, but the manner in which this charge is compensated is not clear (Dymek 1983;Guidotti 1984). Suggested coupled substitutions are of various types: (1) within octahedral sites, by creation of vacancies, e.g., Ti 4+ + ■ ■ AE 2Mg 2+ ; (2) between octahedral and tetrahedral sites, as in variants of the Tschermak substitution such as Ti 4+ + 2 IV Al 3+ AE Mg 2+ + 2Si 4+ ; and (3) by loss of hydrogen from the hydroxyl site: Ti 4+ + 2O 2-+ H 2 AE M 2+ + 2OH -. This third type is sometimes called an "oxy-substitution" because it can also be written Ti 4+ + 2O 2-+ H 2 O AE M 2+ + 2OH -+ 1/2O 2 . However, according to Dyar et al. (1993), it is more appropriately termed a "deprotonation", a description made more obvious by writing Ti 4+ + H 2 AE M 2+ + 2H + .Similar considerations apply to other high-charge cations such as Al 3+ and Fe 3+ in the octahedral sites of dioctahedral mica. The nature of the substitutions cannot easily be deduced from routine analytical studies by electron microprobe, because the oxidation state of Fe is unknown, and hydrogen cannot be determined directly. The normal practice of assuming full occupancy of the hydroxyl site by monovalent anions (OH, F, Cl) means that substitutions of type 3 cannot be assessed, and the importance of octahedral vacancies may have been overstated. Recent studies in which both Fe 3+ and H + content have been independently determined (e.g., Dyar et al. 1993;Virgo and Popp 2000) suggest the probable importance of deprotonation substitutions.This study describes a natural occurrence where biotite grew and equilibrated in local domains of greatly varying Ti content as the result of the polymetamorphic transformation of granulite-facies garnet-cordierite gneiss into amphibolite-facies biotite-garnet-sillimanite rock. First, we describe the petrography and petrology of the occurrence, to establish the conditions * ABSTRACT Secondary metamorphic biotite grew with sillimanite and garnet at the expense of cordierite during a Pan-African metamorphic event overprinting Grenville age granulite-facies metapelites at the western margin of the Namaqualand Metamorphic Complex, South Africa. Fe-Mg exchange ther...

Surprisingly young Rb/Sr ages from the Simav extensional detachment fault zone, northern Menderes Massif, Turkey

“…Brown biotites form relatively larger crystals than green biotites while green biotite form later in aggregates of small flakes. Similar observations have been made by others where the coexistence of brown and green biotites is classically attributed to retrogression of brown to green biotites (e.g., Verschure et al, 1980;Satır and Friedrichsen, 1986;Edwards and Essene, 1988;Ikeda, 1998;Thomson, 2001;Brown, 2002;Waters and Charnley, 2002;Henry et al, 2005). The dated green biotites (G4-01, 02 and 03) contain titanite and rutile in a rather sagenitic pattern whereas chloritization, particularly around the crystal margins, is a common feature associated with brown biotites (Fig.…”

“…The authors also documented coexisting red--brown matrix biotite and green biotite next to cordierite and concluded that the Ti-in-biotite geothermometer for red-brown matrix biotites gives temperatures of 758-771 • C while temperatures for green biotite is well below the 450 • C isotherm. Similar compositional relations between red-brown and green biotite were attributed to development of local equilibrium during cooling in the presence of a fluid (Waters and Charnley, 2002) where green biotite recrystallizes from brown biotite with incorporation of Ti into biotite by means of different substitution mechanisms (Satır and Friedrichsen, 1986;Henry et al, 2005). Fig.…”

“…The presence of both brown and green biotites in the same thin section indicates the role of retrogressive processes during which the brown biotites have been transformed into green biotites (cf. Verschure et al, 1980;Veblen and Ferry, 1983;Edwards and Essene, 1988;Shau et al, 1991;Pitra and Guiraud, 1996;Ikeda, 1998;Thomson, 2001;Brown, 2002;Waters and Charnley, 2002;Henry et al, 2005). The transformation of brown biotite to green biotite is evidenced by Ti-exsolution (±chloritization) in the form of titanite and rutile where sagenitic texture is very characterisitic (Fig.…”

Titanium in phengite: a geobarometer for high temperature eclogites

“…2O 2-? H 2 ) is a major substitution in Fe-rich biotite at high temperature (Dyar et al 1993;Waters and Charnley 2002;Cesare et al 2003;Henry et al 2005). Our analysies does not entirely rule out this possibility.…”

Mineralogy and crystal chemistry of micas from the A-type El Portezuelo Granite and related pegmatites, Catamarca (NW Argentina)

“…% (as TiO 2 ); however, a full evaluation of substitutions involving Ti requires complete chemical analyses, as some of the possible exchanges involve deprotonation and octahedral vacancies (as reviewed in Waters and Charnley 2002).…”