Hydrothermal alteration of biotite and plagioclase as inferred from intragranular oxygen isotope- and cation-distribution patterns

Abstract

The alteration processes of biotite to chlorite and plagioclase to albite that have occurred during hydrothermal alteration of granites were investigated on a scale of less than or equal to250 mum using a combination of in situ oxygen isotope- and electron microprobe analyses. On an intragranular scale, oxygen isotope compositions parallel variations in the cationic composition. Steep chemical and isotopic gradients are obtained between secondary and primary phases, whereas both secondary and primary phases themselves appear chemically and isotopically homogeneous. Oxygen isotope- and cation-exchange between fluid and biotite or plagioclase is, therefore, best explained by dissolution-reprecipitation mechanisms, taking place along microfractures, micropores and grain boundaries. The breakdown of biotite to chlorite occurs once K+ is removed from biotite interlayers. This process is most likely accompanied by a redistribution of elements including oxygen isotopes in both tetrahedral and octahedral sheets of the biotite. Distribution patterns of oxygen isotopes within altered plagioclase indicate that the albitization of plagioclase does not simply reflect addition of SiO2 and Na+ to plagioclase. Instead, albite is precipitated from dissolved plagioclase accompanying a restructuring of aluminosilicate tetrahedra within the plagioclase.