Investigations concerning the Breakdown of Albite at Depth in the Earth

Fine-grained jadeite alterations of detrital sodic plagioclase in several specimens of metasediments from the Franciscan formation have been analyzed with the electron microprobe. Iron is the major "impurity," ranging from 1 to 8 wt. per cent. An "average" jadeite has the composition Jd₈₂Ac₁₄Di₄. Surviving albite contains only 0.0-0.1 per cent Fe; higher Fe analyses were obtained for albite containing disseminated amphibole, but the analyses probably suffer from overlapping. An improved equilibrium curve for the reaction albite → jadeite + quartz, which includes new experimental data in the range 500°-600°C. and which takes account of the entropy change of disordering of albite, falls about 2 kb. lower in the 200°C. range than the result of previous investigators. An equilibrium diagram of the join NaAlSi₃O₈-NaFeSi₃O₈ at 600∘C., giving the stabilities of jadeite-acmite solid solutions with quartz, shows that these pyroxenes are nearly ideal solid solutions and that the presence of 10 mol. per cent acmite has at most a few hundred bars effect on the stability of jadeite with quartz. From these analytical and experimental data it is concluded that pressures of 6-8.5 kb. are reached in the genesis of some glaucophane schists. Estimates of the extreme thicknesses of geosynclinal sedimentary and volcanic accumulations indicate that an appeal to tectonic overpressure to account for pressures of this magnitude is not necessary.