20-1 Basalt Melt Evolution and Magma Storage Across the Terceira Rift, Azores Triple Junction

Session: Undergraduate Research

Presenting Author:

Authors:

Abstract:

The Terceira Rift is a key segment of the Azores Triple Junction. This ultraslow, oblique spreading system juxtaposes plume-influenced mantle upwelling with depleted mantle domains and oceanic lithosphere of variable thickness. Basaltic lavas from the rift record mantle-source heterogeneity and crustal modification during ascent and storage. Whole-rock and xenolith samples from São Miguel, Terceira, and Faial constrain melt evolution along the rift and assess magma transport and storage beneath this plate boundary.

Major and trace element data resolve three melt families. Silica-saturated trachybasalts reflect shallow-level differentiation and mixing. Strongly undersaturated basanites contain high normative olivine and commonly occur with Fe–Ti–P–rich cumulate xenoliths. Olivine-normative tholeiite basalts exhibit N-MORB-like affinities. These groups indicate contributions from both enriched OIB-like mantle and N-MORB-like mantle contaminated by crustal material during magma storage in shallow reservoirs.

Crystal-scale observations of xenolith and xenocryst populations indicate open-system behavior during ascent. Compositional divergence between whole-rock basalts and their enclosed crystals shows that primitive magmas entrained material from evolved cumulates or mush-zone assemblages. This pattern supports rapid ascent and limited crustal residence for basanites, whereas trachybasalts record longer storage and differentiation within shallow crustal reservoirs. These relationships define a distributed magma plumbing architecture beneath the Terceira Rift, with variable ascent rates, storage durations, and degrees of melt mixing.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1, 2026

doi: 10.1130/abs/2026SE-12818

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.