O¹⁸/0¹⁶ Ratios in Coexisting Minerals of Igneous and Metamorphic Rocks

The flourine extraction technique for determination of oxygen isotopic abundances has been investigated, and it has been used to obtain O18/O16 analyses of the common rock-forming minerals. A direct comparison with an independent method of analysis, that of reduction with carbon at high temperatures, has shown that both these methods are reliable for quartz samples. Most minerals can be analyzed by the flourine method to a precision of +/- 0.2 per mil. Only olivine and magnetite, of the common igneous and metamorphic minerals, are difficult to react. Oxygen isotopic analyses of all the major minerals of the four principal rock types of the Southern California batholith have been obtained. For comparison, the same technique has been used upon similar rock types from widely scattered localities, to test the general applicability of the results. All the main rock types of the Skaergaard intrusion have been analyzed, as well as many of the minerals. Various samples of ultramafic rocks, anorthosites, and metamorphic amphibolites have also been run. It has been found that the crystallization history and mineral paragenesis of igneous rocks are reflected by the oxygen isotopic data. In every instance where conclusive information exists, the isotopic results are in excellent agreement with the sequence and history determined by conventional geological criteria. Most rocks analyzed fell into a consistent pattern with respect to petrologic type. The Skaergaard rocks deviated markedly from the normal isotopic trends, as a direct result of the unusual course of fractional crystallization which they have undergone