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

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) in WKM increase 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 IT metapelites has become available in the literature, covering the whole range of metamorphic P. However, after many publications based on it, which made 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 and WKM chemistry for HP/LT metapelites. Such a better definition is the aim of this paper. The considered HP/LT metapelites come from various terrains: Schistes Lustrés of W Alps, 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 from various sources. The new chemical and b₀ data (EMPA and XRPD) allowed us: (i) to establish the reference b₀ and chemistry of WKM from HP/LT metapelites: b₀ turns out to be around 9.040 Å; (ii) to confirm and extend to HP/LT metapelites the petrological basis of the b₀ method, particularly the control of mineral 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 re-equilibration of WKM at higher T and lower P during exhumation.