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Journal of Petrology Volume 41 Number 12 Pages 1805-1820 2000
© Oxford University Press 2000
Resolving Crustal and Mantle Contributions to Continental Flood Volcanism, Yemen; Constraints from Mineral Oxygen Isotope Data
1DEPARTMENT OF GEOLOGY, ROYAL HOLLOWAY UNIVERSITY OF LONDON, EGHAM HILL, EGHAM TW20 0EX, UK
2DEPARTMENT OF GEOLOGY, UNIVERSITY OF SANA’A, SANA’A, YEMEN
Oxygen isotope ratios determined by laser fluorination analysis on olivine, clinopyroxene and plagioclase separated from 31 Oligocene flood basalts and rhyolites from Yemen display small but significant variations (5·1–6·2
for olivine; 5·5–6·9 for clinopyroxene; 5·9–6·9 for plagioclase). The range in 
18O values exceeds: (1) the analytical reproducibility of the technique (±0·15; 2 SD); (2) the range expected for minerals that would have crystallized from uncontaminated oceanic basalts or primary magmas in equilibrium with mantle peridotite; (3) the range in melt values and equilibrium phenocryst compositions that could be produced by fractional crystallization of these magmas. Samples with the highest 18O values exhibit increases in 87Sr/86Sr ratio, decreases in 143Nd/144Nd ratio, and increasing Pb isotopic heterogeneity. Samples with the lowest 18O values have radiogenic isotope ratios that approach those inferred for the Afar plume. The oxygen isotope data provide unequivocal evidence that assimilation of heterogeneous lower and upper Pan-African crust was the primary control on isotopic variation in this continental flood basalt province. Moreover, new radiogenic and oxygen isotope data for Pan-African crustal samples from Yemen have appropriate crustal isotopic compositions to generate the observed isotopic variations in the volcanic rocks. A near-primary high-MgO basalt with low 
Nd and extreme Pb isotope ratios contains strongly zoned clinopyroxene crystals that range from green cores through to greenish brown, brownish green and dark brown or black rims. Handpicked crystals of each colour type display the following correlated range in isotope ratios: 87Sr/86Sr = 0·7036–0·7049; 143Nd/144Nd = 0·5129–0·5127; 206Pb/204Pb = 18·6–17·9; 18O = 5·67–6·86. The Sr–Nd–Pb–O isotope variations are attributed to rapid assimilation of 
25% Pan-African continental crust by hot mafic magma during clinopyroxene crystallization. Contamination in this flood basalt province varied from combined assimilation and fractional crystallization to rapid assimilation of crust by hot mafic magmas with little fractionation. Laser fluorination oxygen isotope analysis of mineral separates allows small differences in 18O to be correlated with radiogenic isotope data and is a powerful tool for evaluating the relative roles of enriched lithospheric mantle and continental crust in suites of continental flood basalts.
KEY WORDS: oxygen isotopes; phenocrysts; flood volcanism; crustal contamination; Yemen
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