IIb trioctahedral chlorite from the Barberton greenstone belt: crystal structure and rock composition constraints with implications to geothermometry

Abstract

IIb trioctahedral chlorite in the Barberton greenstone belt (BGB) metavolcanic rocks was formed during pervasive greenschist metamorphism. The chem‐ical composition of the chlorite is highly variable, with the Fe/(Fe+Mg) ratio ranging from 0.12 to 0.8 among 53 samples. The chemical variation of the chlorite results from the chemical diversity of the host rock, especially the MgO content of the rock, but major details of the variation pattern of the chlorite are due to the crystal structure of the chlorite. All major cation abundances in the chlorite are strongly correlated with each other. Sil‐icon increases with Mg and decreases with Fe, while Al^IV and Al^VI decrease with Mg and increase with Fe²⁺. A complex exchange vector explains over 90% of the chlorite compositional variation: Mg₄SiFe²⁺ ₋₃Al^VI ₋₁ Al^IV ₋₁, which has 3 parts Fe-Mg substitution coupled with one part tschermakite substitution. This ratio is required to maintain the charge and site balances and the dimensional fit between the tetrahedral and octahedral sheets. The subtle change in Al substitution in chlorite implies that Al^VI is preferentially ordered in the M(4) site, and about 84% of the Al^VI present is in the M(4) sites when they are nearly filled with Al^VI. Based on 47 analyzed chlorite-bearing rock samples, chlorite (Chl) composition is strongly correlated with the MgO content of the host rock. Calculated correlation coefficients are +0.91 for SiO_2Chl-MgO_Rock, −0.87 for Al₂O_3Chl-MgO_Rock, +0.89 for MgO_Chl-MgO_Rock, and −0.85 for FeO_Chl-MgO_Rock. Only weak correlations have been found between chlorite oxides and other oxides of rock (between same oxides in chlorite and rock: SiO₂−0.67, Al₂O₃ + 0.59, FeO −0.41). However, MgO_Chl is saturated at about 36 wt% in rocks that have MgO above 22 wt%.The MgO_Chl is about 5 wt% when the host rock approaches 0 wt% of MgO. This implies that Mg substituting into the chlorite is approximately limited to 1.5–9.2 Mg atoms per formula unit and 1.0–3.2 Al^IV. Chlorite geothermometers can not be applied to all BGB samples. However, the empirical chlorite geothermometer based on Al^IV of chlorite may be applicable to chlorites formed under metamorphic conditions because it can predict the chemical composition of the chlorite from basaltic and dacitic samples in this study. An estimated temperature of about 320°C for the greenschist metamorphism of the greenstone belt through this geothermometer is consistent with that obtained by other geothermometers.