STABILITY OF YODERITE IN THE ABSENCE AND IN THE PRESENCE OF QUARTZ - AN EXPERIMENTAL-STUDY IN THE SYSTEM MGO-AL2O3-FE2O(3)-SIO2-H2O

The water-pressure temperature stability field of yoderite, ideally Mg (2)Al5.6Fe(0.4)(3+)Si(4)O(18)(OH)(2), was determined at high oxygen fu gacities by high-pressure bracketing runs on eight possible breakdown reactions involving the phases chlorite, kyanite, talc, staurolite, py rope, enstatite, boron-free kornerupine, cordierite, quartz, and invar iably an excess of hematite. Yoderite was found to be stable over the surprisingly large PT range from similar to 6 to 25 kbar water pressur e and 590 to 795 degrees C. It is thus a high-pressure mineral coverin g the upper amphibolite and portions of the eclogite facies. In the pr esence of quartz its upper pressure stability is reduced to some 15 kb ar, and its upper temperature stability to 715 degrees C. Two of the y oderite-producing reactions are anomalous as they show dehydration in the direction towards lower temperatures. Importantly, this is also tr ue for the reaction kyanite+talc+hematite+H2O=yoderite+quartz which is responsible for the only yoderite occurrence in nature at Mautia Hill , Tanzania. Preliminary thermodynamic calculations indicate that-owing to this unusual dehydration behavior-the stability field for the asse mblage yoderite+quartz disappears for water activities lower than simi lar to 0.5. The rarity of yoderite in natural rocks, which is in contr ast to its large PT stability field, must be explained on chemical rat her than on physical grounds. Yoderite can only occur in whiteschist-t ype bulk compositions rich in MgO, Al2O3, SiO2, and containing some ir on, but poor in alkalis and CaO. Oxygen fugacities must be unusually h igh to keep Fe trivalent, and-at least for rocks with excess quartz-th e water activity must be high as well. In an environment of this kind, yoderite formation in the Mautia Hill whiteschist may have occurred e ven at constant total pressure and temperature simply by an influx of hydrous fluid during the late stages of metamorphism under amphibolite facies conditions.