Extreme geochemical heterogeneity in Afro-Arabian Oligocene tephras: Preserving fractional crystallization and mafic recharge processes in silicic magma chambers

Four Indian Ocean deep-sea tephras can be correlated to individual Oligocene on-land silicic pyroclastic units found in the Afro-Arabian flood volcanic province (Yemen and Ethiopia), providing valuable stratigraphic marker horizons. They also preserve among the largest geochemical heterogeneities observed in individual eruption events (SiO2: 43 to 75 wt.% and 56 to 79 wt.%). The major, trace element and isotopic variations preserved in individual shards in these ash layers provide a unique series of snapshots of magma chamber processes that are used to elucidate the formation and petrogenetic evolution of large volume, chemically zoned silicic magmatic systems. Banded shards within individual ash layers represent up to 85% of the SiO2 variation observed in the entire tephra on the scale of < 1 mm3, clearly demonstrating that the observed compositional variations found within individual layers represent individual eruptive events. The silicic magmas are compositionally related to the underlying Main Flood Basalt phase of volcanism in Yemen, whereas the basaltic end-member found in the tephras is compositionally distinct and related to the Upper Series mafic lavas, found intercalated with the Main Silicic Series on-land ignimbrites and tuffs. The intermediate to silicic component of the tephras was generated by extreme fractional crystallization (F = ca. 60%) of plagioclase, anothoclase, augite, magnetite and ilmenite, as observed in on-land phenocryst assemblages.