Lithospheric Structure Controls Geochemical Diversity Across the Terceira Rift, Azores Triple Junction

The Azores archipelago sits at the intersection of three tectonic plates, where the Terceira Rift connects the Mid-Atlantic Ridge to the East Azores Fracture Zone. Volcanic compositions across the rift vary systematically, but the controls on this diversity remain debated. This study evaluates whether lithospheric thickness, the depth to the lithosphere-asthenosphere boundary (LAB), is a primary control on melt composition across the rift. Whole-rock geochemistry, phenocryst electron probe microanalysis, xenolith geochemistry, and pMELTS thermodynamic modeling were applied to samples from Faial (LAB ~40 km), Terceira, and São Miguel (LAB ~80 km). Results show a systematic west-to-east gradient in garnet-sensitive trace element ratios consistent with deeper melt initiation under thicker lithosphere. Thermodynamic modeling independently reproduces this gradient, yielding liquidus temperatures at Faial approximately 75–100°C higher than São Miguel. Terceira’s anomalously low liquidus temperatures are consistent with extended crustal storage at the Pico Alto central edifice, suggesting LAB depth and crustal architecture operate as independent controls. Results support lithospheric thickness variation as a primary control on rift geochemical diversity. Radiogenic isotope analysis is identified as the required next step to evaluate source heterogeneity.