Why Melts Differ Across the Azores Triple Junction

The Azores archipelago exposes a central ambiguity in the interpretation of ocean-island basalt: is inter-island geochemical diversity a record of mantle source heterogeneity, or a signature of lithospheric architecture filtering where melting stops and magmas stall? In the Azores, the two are geometrically inseparable: the isotopic enrichment gradient runs parallel to the lithospheric thickness gradient. Here we test which control is first-order using a screened nine-island whole-rock compilation (278 samples), together with new XRF and ICP-MS analyses and olivine EPMA, anchored to published receiver-function lithosphere-asthenosphere boundary depths. Garnet retains Yb far more than Gd, making Gd/Yb a garnet-sensitive index less inflated than La/Yb by light-REE source enrichment; primitive-mantle-normalized values climb from 2.37 at Faial, above a ~40 km lid, to 3.15 at São Miguel, above ~80 km, while adjacent plume-influenced Mid-Atlantic Ridge basalts anchor the ridge endpoint near 1.5 and extend the trend (r ≈ 0.79 among the islands, ≈ 0.88 with the ridge anchor).  The eruptive and crystal records are consistent with contrasting delivery histories.  Faial whole rocks reach 12–14 wt% MgO and at the crystal scale one Faial sample carries disequilibrium, reverse-zoned olivine cargo while two São Miguel samples contain host-equilibrated olivine recording ~1180–1290 °C.  The sharpest test lies where the gradients diverge: Terceira and São Miguel share an ~80 km lid, yet Terceira erupts lower-Gd/Yb lavas. Recent seismic tomography images segmented crustal magma bodies beneath Terceira’s Santa Bárbara and Guilherme Moniz–Pico Alto systems, while crystal-zoning evidence documents multilevel storage, recharge, and mixing along its active rift; São Miguel preserves higher garnet-field ratios consistent with a different plumbing history. These results identify lithospheric architecture modulated by island-scale structure as a first-order control on melt chemistry, with source heterogeneity acting within that framework rather than instead of it. Sr-Nd data across all nine islands (259 paired analyses) document real source heterogeneity, but the island-scale Gd/Yb pattern remains more closely associated with the lithospheric framework than with the isotopic-enrichment gradient it parallels.  Clinopyroxene barometry, in progress, will test whether the studied lavas record the storage levels already imaged beneath Terceira and how those levels compare with São Miguel.