Oxygen-isotope ratios in metamorphic eclogites

Rocks having O¹⁸-values less than those of normal basalts (5 to 7 permil) are very rare. However, anomalously low -values are common in European B-type eclogites, including those from N. W. Spain (1.5 to 5.6 permil), W. France, S. Germany, and W. Norway. Since isotopic exchange with sediments during metamorphism would have increased their -values, we tentatively conclude that the isotopically light eclogites descended from basaltic rocks which interacted with light meteoric waters at high temperatures, as did the Tertiary igneous rocks of W. Scotland. Isotopically heavy eclogites from S. Germany (8.6 to 10.5 permil) are isotopically similar to calc-silicate rocks from N. W. Spain and are possibly derived from dolomitic pelites.Coexisting quartz and rutile effectively fractionate the isotopes of oxygen and thereby constitute a potentially useful geothermometer. The average quartz-rutile fractionation in B-type eclogites is 6.5 permil.Lamont-Doherty Geological Observatory of Columbia University Contrib. No. 1559.