QUARTZ-BEARING BASALTS - OXYGEN ISOTOPIC EVIDENCE FOR CRUSTAL CONTAMINATION OF CONTINENTAL MAFIC ROCKS

Continental mafic volcanic rocks commonly contain partially resorbed g rains of quartz surrounded by reaction rims of granular pyroxene. delt a(18)O values for single grains and for grain-composites of quartz fro m a lherzolite-bearing flow from Mount Taylor, NM, are 8.18-10.54 part s per thousand and 9.77-10.68 parts per thousand, respectively, indica ting that the quartz is crustally derived. In contrast, the delta(18)O values for xenocrystic olivine and pyroxene are 4.95-5.39 parts per t housand and 4.99-5.22 parts per thousand, respectively. Because only a small amount of contamination is required (<3-5%), major elements and strontium and neodymium isotopes do not yield a clear crustal signatu re. Yet contamination is sufficient to shift some trace element patter ns and may affect lead isotopes and K/Ar dates. Our work suggests that crustal contamination of continental basalts may be cryptic and more widespread than previously recognized.