Gabbroic Xenoliths and Host Ferrobasalt from the Southern Juan de Fuca Ridge

Rare isotropic gabbroic xenoliths occur in sheet and lobate flow fragments of nearly aphyric ferrobasalt collected along a 12‐km section of the southern Juan de Fuca Ridge. Xenoliths comprise ≪ 1% of the dredge contents and range in size from 1 cm3 (glomerocryst) to 240 cm3. The xenoliths have ophitic to intersertal texture with 5–50% interstitial glass of ferrobasaltic composition more evolved than the host lava. On the basis of texture and mineralogy, the xenoliths have been subdivided into three types: type I, plagioclase + olivine + glass; type II, plagioclase + augite + glass ± olivine; and type III, plagioclase + augite + olivine + glass ± pigeonite (partially inverted) + Fe‐Ti oxides. Mineral and glass inclusion compositions suggest a sequence of evolution for the three xenolith types in which type I is the least evolved and type III is the most evolved. Application of a graphical pyroxene geothermometer to augite in xenolith types II and III yields crystallization temperatures of 1100°–1200°C and to host‐lamellae pairs in inverted pigeonite yields subsolidus equilibrium temperatures of 1100°–1150°C. Coexisting titanomagnetite‐ilmenite pairs in type III xenoliths yield temperature estimates of 1000°–1070°C and log f02 = −9.7 to −10.8. We infer that the xenoliths represent the partially crystalline “mush” boundary zone of a magma chamber based on the abundance of interstitial glass, zonation of mineral grains in the most crystalline samples, and coherence of chemical trends between interstitial glass, glass inclusions, and mineral phases. The evolved composition of the xenoliths provides evidence for the presence of melts more fractionated than the host ferrobasalt in the magma chamber. The erupted ferrobasalt is a hybrid lava formed by mixing these highly evolved melts with more primitive melts