Resolving Crustal and Mantle Contributions to Continental Flood Volcanism, Yemen; Constraints from Mineral Oxygen Isotope Data

Abstract

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 δ¹⁸O 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 δ¹⁸O values exhibit increases in ⁸⁷Sr/⁸⁶Sr ratio, decreases in ¹⁴³Nd/¹⁴⁴Nd ratio, and increasing Pb isotopic heterogeneity. Samples with the lowest δ¹⁸O 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 εNd 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; δ¹⁸O = 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 δ¹⁸O 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.