This award will support the development of techniques for making in situ analyses of oxygen and sulfur isotopes in oxide, silicate and sulfide minerals using fluorine gas and laser heating for the liberation of oxygen and sulfur. The technique development work will be carried out in the Geophysical Laboratory of the Carnegie Institution of Washington which will provide necessary equipment as well as technical and scientific personnel for the project. It is anticipated that the improved method of stable isotope analysis will have application in a wide variety of investigations where high-resolution, in situ, isotopic analyses are required including, for example, siliceous cements binding detrital grains in sandstones, phenocrysts in plutonic and volcanic rocks, zoned porphyroblasts in metamorphic rocks, and crustiform deposits of sulfide, silicate, and oxide minerals in ore deposits.