The b0 lattice parameter and chemistry of phengites from HP/LT metapelites

Abstract: The statistical b0 method is based on the fact that the cumulative b0 values of white K-micas (WKM) from metapelites of a specific bulk composition can be used, in the lowermost T range of metamorphism, as a semiquantitative P indicator: mean b0 values (and related celadonite content) inWKMincrease with increasing P, thus characterizing the baric type of metamorphism. The solid petrological framework of this method includes specific constraints on rock bulk composition and need of other conditions. In this way, a reference b0 scale for LT metapelites has become available in the literature, covering the whole range of metamorphic P. However, aftermany publications based on it, whichmade a large amount of data available, revision of this b0 scale is now opportune, and the primary need is a better definition of the reference b0 values andWKMchemistry for HP/LTmetapelites. Such a better definition is the aim of this paper. The considered HP/LT metapelites come from various terrains: Schistes Lustr´es ofWAlps, Diablo Range (Franciscan Complex of California), Phyllite-Quartzite Unit of Crete and Peloponnese, UHP gneiss from Dora Maira, Attic-Cycladic Complex of Sifnos, Tavsanli Zone and Nilüfer area of NW Turkey. In addition, WKM chemical and b0 data were used fromvarious sources. The new chemical and b0 data (EMPAand XRPD) allowed us: (i) to establish the reference b0 and chemistry ofWKMfromHP/LT metapelites: b0 turns out to be around 9.040 Å; (ii) to confirm and extend to HP/LTmetapelites the petrological basis of the b0 method, particularly the control ofmineral assemblages on b0 values; (iii) to glimpse more complex relationships between b0 and Si than those currently known, and particularly the importance of the Fe2+/(Mg + Fe2+) ratio; (iv) to ascertain the validity of the b0 method for detecting generic HP/LT conditions; (v) to ascertain the inability of the b0 method to discriminate, within the above HP field, differences between various P/T regimes, mainly due to still poorly known crystal-chemical problems, and the fact that comparative b0 interpretations of various HP/LT sample populations from different belts are impeded by poor understanding of the extent of reequilibration of WKM at higher T and lower P during exhumation. Key-words: phengite, XRPD, b0 parameter, EMPA chemistry, HP/LT metapelites, semiquantitative geobarometry.