IIB TRIOCTAHEDRAL CHLORITE FROM THE BARBERTON GREENSTONE-BELT - CRYSTAL-STRUCTURE AND ROCK COMPOSITION CONSTRAINTS WITH IMPLICATIONS TO GEOTHERMOMETRY

IIb trioctahedral chlorite in the Barberton greenstone belt (BGB) meta volcanic rocks was formed during pervasive greenschist metamorphism. T he chemical composition of the chlorite is highly variable, with the F e/(Fe+Mg) ratio ranging from 0.12 to 0.8 among 53 samples. The chemica l 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 struct ure of the chlorite. All major cation abundances in the chlorite are s trongly correlated with each other. Silicon increases with Mg and decr eases with Fe, while Al-IV and Al-IV decrease with Mg and increase wit h Fe2+ A complex exchange vector explains over 90% of the chlorite com positional variation: ME(4)SiFe(-3)(2+)Al(=1)(VI) Al--1(IV), which has 3 parts Fe-ME substitution coupled with one part tschermakite substit ution. 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 B ased on 47 analysed chlorite-bearing rock samples, chlorite (Chi) comp osition is strongly correlated with the MgO content of the host rock. Calculated correlation coefficients are +0.91 for SiO2Chl-MgORock, -0. 87 for Al2O3Chl-MgORock, +0.89 for MgOChl-MgORock, and -0.85 for FeOCh l-MgORock. Only weak correlations have been found between chlorite oxi des and other oxides of rock (between same oxides in chlorite and rock : SiO2 -0.67, Al2O3 + 0.59, FeO -0.41). However, MgOChl is saturated a t about 36 wt% in rocks that have MgO above 22 wt%. The MgOChl is abou t 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 chlori tes formed under metamorphic conditions because it can predict the che mical composition of the chlorite from basaltic and dacitic samples in this study. An estimated temperature of about 320 degrees C for the g reenschist metamorphism of the greenstone belt through this geothermom eter is consistent with that obtained by other geothermometers.