127-10 Whole-Rock and Xenocryst Geochemistry Across the Terceira Rift, Azores Triple Junction

Session: Mineralogy, Geochemistry, Petrology, and Volcanology Student Session

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The Terceira Rift, a segment of the Azores rift–oblique rift–transform triple junction, is an ultraslow oblique spreading axis where heterogeneous mantle upwelling and depleted mantle components interact beneath variably thick oceanic lithosphere. In this setting, erupted basalts reflect both primary mantle signatures and secondary crustal overprinting during magma storage and ascent. Although Terceira Rift lavas have been extensively characterized geochemically, direct comparisons between them remain limited. This study characterizes and compares lavas from multiple islands using a combined whole-rock and crystal-scale approach to evaluate mantle signatures versus crustal modification. 

Whole-rock ICP–MS analyses from São Miguel, Terceira, and Faial basalts reveal broadly coherent trace element systematics consistent with derivation from a heterogeneous mantle source modified during crustal differentiation. São Miguel basalts are the least evolved, with high Mg and trace element compositions resembling those of St. Helena OIBs. Faial lavas display similar bulk compositions but show a distinct negative Eu anomaly, and Dy/Dy* values suggestive of MORB with LREE-enrichment. Terceira samples are the most evolved, exhibiting pronounced negative Eu anomaly, the lowest La/Yb ratios, and similar Dy/Dy* values to Faial. Terceira basalts also have similar trace element compositions to the trachytic composition rocks.  

Millimeter-scale clinopyroxene xenocrysts extracted from xenoliths from São Miguel basalts diverge from their host whole-rock chemistry. Individual crystals display incompatible element enrichments and fractionation trends inconsistent with the enclosing melt, indicating crystallization in earlier, more evolved magma batches. Their incorporation into primitive basalts demonstrates open-system recycling of antecrysts during ascent. This juxtaposition highlights the dual nature of TR magmatism: primitive whole-rocks represent rapid mantle-to-surface transport, while xenocrysts record extended crustal residence, mixing, and differentiation. These results indicate that geochemical diversity across the Terceira Rift can be explained through open-system crustal processing superimposed on mantle heterogeneity. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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