MAGMA MIXING AND THE CONTRASTING CHEMISTRIES OF BASALT AT MOST SPREADING RIDGES AND THE NORTH ATLANTIC IGNEOUS PROVINCE

Abstract:

Shallow differentiation of depleted MORB and later assimilation or mixing beneath spreading ridges does not produce enriched (E-) MORB. Strongly differentiated plutonic rock and lavas do not have the geochemical attributes of E-MORB, which are produced in the mantle. Enriched melt strains at depth (where high-pressure minerals including garnet influence chemistry) migrate upward as small partial melts and collect at the relatively impermeable bases of plates, mainly old and thick. The migrating melts come from something old, which influences isotopes. The result is an enriched layer that thickens with age at the bases of plates. Continental portions of plates are not immune (e.g., East African Rift). Break open and tap the layer, and what comes out is sometimes voluminous olivine nephelinite, basanite and alkalic olivine basalt, commonly lumped with "ocean-island basalts".

Mantle upwelling at Hawaii and other “hot spots” punches up the layer, which can cause geochemical “bulls-eye” patterns over the space of several volcanoes and the Hawaiian stages of eruption.

Spreading ridges migrate, causing mantle material both above and below this "layer" to melt, scattering it as blobs, schlieren and streaks. Ridges have mainly depleted lavas, some of which never mix with melt strains from the blobs. Truly depleted lava usually erupts on near-axis seamounts and in transform faults, but mainly away from spreading centers. There, because of restricted axial geometries, some mixing with enriched melt almost invariably occurs, so that most “depleted” MORB are ever so slightly enriched. Enrichment may be tied to minor deep minerals such as phlogopite and kaersutitic (titanian) amphibole. These are not important factors in the chemistry of rift basalts at continental margins. Instead, mixing occurs between depleted basalt and rhyolitic partial melts of ancient crust. Such crust, stretched out from continental margins, has to be present to some degree beneath Iceland. This is mainly a low-pressure phenomenon, in the crust. It shows up in diagrams of La/Ta versus Th/Ta, but also affects isotopes, [K/Ti]_N, ΔNb and [La/Sm]_N. Mixing between parental magma stems at almost all other spreading ridges, between depleted and enriched, although widespread, is different.