Evidence from rare gases for magma-chamber degassing of highly evolved mid-ocean-ridge basalt

Abstract

HIGHLY evolved lavas have been recovered from moderate- to fast-spreading centres along the mid-ocean ridge system, most commonly near propagating rifts and overlapping spreading centres^1–7. Their origin and occurrence are generally explained by extensive modification of a primary basalt magma by fractional crystallization at shallow levels in the oceanic crust (model I)^1–4,6–9. Other crustal processes have also been invoked, including periodic magma mixing of a highly fractionated liquid with multiple primary liquids (model 2)^10,11, and seawater interaction with the mantle-derived magma (model 3)^12–14. Here we report the first ⁴He and argon isotope data for several highly evolved mid-ocean-ridge basalts (MORBs) and andesites. The samples were recovered with the submersible Alvin from a very limited section (<8 km²) of the axial valley of the eastern Galapagos Rift, between 85° 52′ and 85° 50′ W. Although they range in composition from moderately evolved normal (N) MORB to ferrobasalts, Fe–Ti basalts and high-silica andesites, geochemical studies indicate that the suite is cogenetic, facilitating interpretation of the data¹⁵. We find distinct differences in the rare-gas data between the evolved and N-MORBs, indicating that the evolved magmas have undergone pre-eruption degassing in a process not common to the source magmas for normal MORBs. Our data fit well with model 1, but provide no evidence for models 2 and 3.