Geochemistry of basalt from the North Gorda segment of the Gorda Ridge: Evolution toward ultraslow spreading ridge lavas due to decreasing magma supply

Abstract

High‐density, precisely located, dive and rock‐corer basalt samples from the 65‐km‐long North Gorda ridge segment reveal compositional diversity as great as for the entire Gorda Ridge. Lava compositions along the ridge axis show considerable major and minor element diversity (MgO 9.2–4.4%, K2O 0.04–0.36%) for lavas erupted in close proximity. Although they form a near‐continuum in the higher MgO range, the samples can be separated into two groups; one is typical N‐type mid‐ocean ridge basalt (MORB) (K2O/TiO2 < 0.09), and the other is a more enriched T‐MORB (K2O/TiO2 > 0.09). Incompatible elements also reflect this grouping with (Ce/Yb)N < 1 and Zr/Nb > 20 for N‐MORB and (Ce/Yb)N > 1 and Zr/Nb < 20 for T‐MORB. Samples collected from off‐axis, over a distance of 4 km up the eastern rift valley wall, are all light rare earth element (LREE)‐depleted N‐MORB with a narrower compositional range (MgO of glasses 7.7 ± 0.3%, Zr/Nb = 38–50; (Ce/Yb)N < 1), although isotopic ratios are comparable to those on‐axis. Lavas erupted in the past, before the present‐day deep axial valley formed on this part of the ridge, were more uniform N‐MORB, generated by larger degrees of melting when magma supply was greater. Basalts from the adjoining southern Juan de Fuca Ridge segment, with comparable spreading rate but distinctly different ridge morphology, are also all LREE‐depleted N‐MORB, but the narrow range of evolved compositions of the sheet flows covering the broad, U‐shaped valley suggests shallower, more steady state magma reservoirs underlying this ridge segment. Basalts from Escanaba, the slowest spreading segment of Gorda Ridge, include N‐, T‐, and E‐MORB that were erupted from isolated volcanic centers. The pattern of incompatible element enrichment, especially in LREE, K, Ba, and 87Sr/86Sr, with decreasing spreading rate and magma supply is even more pronounced at the ultraslow spreading Arctic ridges where most lavas are E‐MORB (Zr/Nb < 10, (Ce/Yb)N >1.0–3.0). Arctic E‐MORB compositions lie along a common mixing trend with those from North Gorda. As the magma budget and/or partial melting decreases, a similar enriched component, especially in K, Ba, and LREE, widely present in the oceanic mantle is apparently incorporated to a greater degree. At North Gorda, morphology and chemical characteristics appear to evolve with time toward that of ultraslow spreading ridges.