Basaltic Rocks from Different Tectonic Settings

5 Basaltic Rocks from Different Tectonic Settings 

Harmon and Hoefs (1995) have assembled a database consisting of 2,855 O-isotope analyses of Neogene volcanic rocks worldwide. They observed a 5%c variation in the 5 0-values of fresh basalts and glasses, which they have taken as evidence of significant oxygen isotope heterogeneities in the mantle sources of the basalts. This is documented in Fig. 3.8, which plots 8 0-values vs. Mg-numbers (Harmon and Hoefs 1995). 

The usage of whole rock data has, however, its ambiguities. Estimates of original magmatic 8 0-values is best achieved through analysis of unaltered phenocrysts within rocks. Laser-based extraction methods on small amounts of separated mineral phases have documented subtle, but resolvable differences among different types of basaltic lavas (EUer et al. 1996, 2000 Dorendorf et al. 2000 Cooper et al. 2004 Bindeman et al. 2004, 2005, and others). 

Continental basalts tend to be enriched in relative to oceanic basalts and exhibit considerably more variability in O-isotope composition, a feature attributed to interaction with 0-enriched continental crust during magma ascent (Harmon and Hoefs 1995 Baker et al. 2000). 

separate levels of the oceanic crust are simultaneously enriched and depleted in relative to normal mantle values because of reaction with sea water at different temperatures. Muehlenbachs and Clayton (1976) and Gregory and Taylor (1981) concluded that the 0 enrichments are balanced by the depletions which acts like a buffer for the oxygen isotope composition of ocean water. 

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