Genetic relations among basic lavas and ultramafic nodules: Evidence from oxygen isotope compositions

¹⁸O values of unaltered basic lavas range from 4.9 to 8.3 but different types of basalts are usually restricted to narrow and distinct ranges of isotopic composition. The average ¹⁸O values for Hawaiian tholeiites, mid-ocean ridge tholeiites, and alkali basalts are 5.4, 5.7, and 6.2 permil, respectively. Potassic lavas and andesites tend to be more ¹⁸O rich with ¹⁸O values between 6.0 and 8.0 permil. The differences among the oxygen isotopic compositions of most of these lavas can be attributed to partial melting of isotopically distinct sources. The oxygen isotope compositions of the sources may be a function of prior melting events which produce ¹⁸O-depleted partial melts and ¹⁸O-enriched residues as a consequence of relatively large isotopic fractionations that exist at high temperatures. It is proposed that lavas with relatively low ¹⁸O values are derived from primitive, ¹⁸O-depleted sources whereas ¹⁸O-rich basalts are produced from refractory sources that have already produced partial melts. High temperature fractionations among silicate liquids and coexisting minerals can be used in conjunction with the oxygen isotope compositions of ultramafic nodules to place constraints on the genetic relations between some nodules and different types of basic lavas.