O ' NEIL AND TAYLOR Oxygen Isotope Fractionetlon

Oxygen isotopes have been equilibrated between muscovite and aqueous alkali chloride solution and between paragonite and alkali chloride solution in the temperature range of 400ø-650øC at I and 1.5 kb fluid pressure. Isotopic equilibrium was inferred from the fact that compatible fractionation factors were obtained using 3 different chemical reactions to produce the mica: (1) muscovite or paragonite was prepared by reacting natural kaolinire with 2-3 molal KC1 or NaC1 solutions; (2) muscovite was crystallized in pure water from a gel; and (3) synthetic paragonite was reacted with 2-3 molal KC1 solution, producing muscovite by an alkali ion exchange reaction. The I M modification of the mica was made in every experiment. In several cases the extent of oxygen exchange was traced by running companion equilibrations in solutions of unusually low O's/o x6 ratio. No isotopic fractionation was discernible between muscovite and paragonite in the temperature range studied. Per mille fracrionations between muscovite and water are given by the expression 10 s In a • 2.38(106T -2) -3.89. These data can be combined with the results of other laboratory equilibration studies to establish a set of calibrated oxygen isotope geothermometers. Analogous to the alkali feldspar systems previously reported, the direct relationship between cation and oxygen isotope exchange suggests that some type of solution-redeposition mechanism operated during muscovite-paragonite transformations in aqueous solutions. Also, the extensive oxygen isotope exchange (with the solution) that accompanies the formation of muscovite from kaolinite implies a breakdown of the kaolinitc structure. This notion does not concur with hypotheses based on rate studies and X-ray measurements that the unaltered kaolinitc structure is partially inherited by the mica.