Relationship Between O¹⁸/O¹⁶ Ratios in Coexisting Minerals of Igneous and Metamorphic Rocks: Part 1: Principles and Experimental Results

A fluorine-extraction technique for determination of oxygen-isotopic abundances has been used to obtain O¹⁸/O¹⁶ analyses of the common minerals of igneous and metamorphic rocks. 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 fluorine method with a precision of 0.2 per mil. Only olivine and magnetite, of the common rock-forming minerals, are difficult to react with fluorine.

Consistent oxygen-isotopic relationships exist among the various igneous rock types and their constituent minerals. Mafic rocks have lower O¹⁸/O¹⁶ ratios than do the more salic rocks. Quartz is invariably the mineral richest in O¹⁸, and magnetite is the lowest. Other coexisting minerals have intermediate O¹⁸/O¹⁶ ratios; the usual sequence from highest ratio to lowest is K feldspar, plagioclase, pyroxene, hornblende, and biotite. Commonly, a spread of 4-5 per mil is found between coexisting biotite and quartz. These consistencies are observed in all the rocks studied, regardless of age or geographic location.

Analyses of the minerals in the four principal rock types of the Southern California batholith (gabbro, tonalite, granodiorite, and quartz monzonite) have been obtained. For comparison, the same technique has been used on similar rock types from widely scattered localities, to test the general applicability of the results. The main rock types of the Skaergaard intrusion have been analyzed, as well as various mineral samples from ultramafic rocks, anorthosites, pegmatites, metamorphic amphibolites, and metamorphic arkosic quartzites.