Petrological evidence for focussed mid-crustal magma intrusion in the Main Ethiopian Rift

Rifting in Ethiopia is predominantly driven by magmatic intrusion into the rifting crust. Unravelling the dynamics of lithospheric melt migration and storage is paramount to understanding the late-stage development of continental rifts. In particular, extensive geophysical observations of the structure and composition of rifting crust must be supported by petrology to provide a complete picture of rift-related magmatism. We present major element, trace element, and volatile element compositional data for olivine-hosted melt inclusions from the Boku Volcanic Complex (BVC), a monogenetic cone field in the north Main Ethiopian Rift. Through combined CO₂-density-calibrated Raman spectroscopy and secondary ion mass spectrometry we assess the total CO₂ concentrations within the melt inclusions allowing us to estimate pressures of entrapment via CO₂-H₂O solubility models. Our results show that primitive BVC melts carry up to 0.58 wt% CO₂ (mean ~0.2 wt%), with as much as half of the CO₂ in the melt inclusion present within shrinkage bubbles. Volatile solubility models suggest that these melts are stored over a narrow range of depths (10-15 km), consistent with geophysical data and implying the existence of focussed zone of magma intrusion at mid-crustal depths. The expansive range of trace element concentrations in the inclusions illustrate that, at the time of entrapment, compositional heterogeneity remains extant, and melts must therefore be stored in discrete magmatic bodies with limited mixing. Our results have implications for understanding the interplay between magma intrusion and extensional tectonics during continental break-up, such as magmatic compensation of crustal thinning and the thermo-mechanical effects of melt emplacement into the rifting crust.