TEM-AEM OBSERVATIONS OF CL-RICH AMPHIBOLE AND BIOTITE AND POSSIBLE PETROLOGIC IMPLICATIONS

Cl-rich amphibole and biotite (2.9 and 2.8 wt% Cl, respectively) were analyzed with high-resolution transmission electron microscopy (HRTEM) , selected-area electron diffraction (SAED), and X-ray emission analyt ical electron microscopy (AEM). The samples are from Archean granulite -facies metamorphosed iron formations located in the Bear-tooth Mounta ins, Montana, U.S.A. Two types of amphibole and biotite were found: (1 ) high-Cl prograde hastingsite and biotite and (2) retrograde amphibol e of grunerite composition containing no detectable amount of Cl but i ntergrown with submicrometer-sized slabs of Cl-rich annite (2.8 wt%), Fe-rich talc, Fe-rich clinojimthompsonite, and very wide chain silicat e. TEM-AEM study of prograde hastingsite and biotite suggests that Cl is dissolved in the crystal structures and that Cl distribution is hom ogeneous. AEM analyses show that Cl occupies up to about 30 and 15% of the OH positions in hastingsite and biotite, respectively. Calculatio ns of the Cl-OH exchange reaction for apatite indicate a high log(a(HC I0)/a(H2O) ratio of about -1.8 approximately -2.2, suggesting equilibr ium with a Cl-rich fluid. Speciation and solubility modeling of the eq uilibrium between an aqueous fluid and the Cl-rich apatite, biotite, a nd hastingsite suggest that reactions with a saline brine of about 25 wt% NaCl could explain the Cl enrichment in these hydrous phases, wher eas the brine may coexist with an immiscible CO2-dominated vapor durin g the granulite-facies metamorphism, in light of low calculated H-2O a ctivities and CO2-rich fluid inclusions. This explanation is consisten t with the results from studies of natural and synthetic fluid inclusi ons and phase equilibrium experiments reported in the literature. The widespread occurrence of Cl-rich amphibole and biotite in granulite-fa cies rocks thus has significant implications for the nature of high-gr ade metamorphism. TEM-AEM observations of submicrometer-sized inclusio ns of Cl-rich phases within retrograde amphibole may help to explain t he commonly observed heterogeneities of electron microprobe Cl analyse s, found in samples from a variety of geologic settings. Microprobe an alyses using typical beam diameters may sample areas with different pr oportions of submicroscopic intergrowths. This will give apparent Cl h eterogeneities and fortuitous correlations between Cl and concentratio ns of K, Al, and Si. Although compositional correlations sometimes ind icate reciprocal effects that are likely important for amphibole, we e mphasize the potential danger of artifacts resulting from submicroscop ic solid inclusions. Additional TEM-AEM studies on complex samples are needed to explain the compositional heterogeneities with confidence.