Abyssal peridotites, very slow spreading ridges and ocean ridge magmatism

Abstract

Summary: The SW Indian and American-Antarctic Ridges are two of the world’s slowest spreading ocean ridges (less than 1 cm a −1 ), making them the low end-members for rate of ocean ridge magma supply. Two-thirds of the rocks dredged at the numerous large offset transforms along the ridges are residual mantle peridotites. Gabbroic rocks, however, representing layer 3 and possible palaeo-magma chambers are rare. This suggests a highly segmented crustal structure, with anomalously thin crust near fracture zones that may consist of only a thin veneer of pillow basalt erupted over mantle peridotite. The dredged peridotites underwent high degrees of melting, spanning the range believed to produce abyssal basalt. Their depleted compositions show that the melt was almost entirely removed. At the same time, the spatially associated basalts have a large range of compositions, similar to those from the rift valleys, requiring extensive shallow-level fractional crystallization. Since there is little evidence for magma chambers at these fracture zones, it is concluded that melts formed in the underlying mantle flowed laterally through the mantle beneath the crust towards a magmatic centre at the mid-point of an adjacent ridge segment. Magma was then subsequently intruded down the rift valley fissure system from the magmatic centre to erupt onto the fracture zone floor. Alternatively, the melt was drained from a mantle diapir beneath the midpoint of a ridge segment, prior to lateral flow of the residual peridotite beneath the ridge axis to the fracture zone. These processes suggest behaviour of the partially molten layer beneath ocean ridges analogous to Rayleigh-Taylor fluid instability, where a light less viscous fluid layer floating upwards in a denser medium goes unstable and drains at regularly spaced points into protrusions which rise rapidly to the surface. Evidence for such dynamically driven non-uniform melt flow in the mantle is seen in locally-abundant plagioclase peridotites, where the plagioclase crystallized from impregnated trapped melt. These rocks can contain up to 30% trapped melt, contrasting sharply with the typical abyssal peridotite which contains virtually none. Basalts erupted along these ridges provide a classic case of trace- and major-element decoupling during magma genesis. Despite trace-element and isotopic diversity, basalts from individual ridge segments were derived from primary magmas with similar major-element compositions. These observations can be explained if melt flows locally through the depleted mantle at the end of melting towards the midpoint of a ridge segment. This would cause melts originating at different points in an initially heterogeneous mantle to migrate through and equilibrate with the same section of mantle immediately prior to segregation—which, for the most part, would homogenize the melt’s major-element compositions. However, by virtue of the lever rule, this would have little effect on critical incompatible-trace-element or isotopic ratios of the migrating melts because of the very low incompatible-trace-element content of residual peridotite. Ocean ridges, then, appear to be marked by strings of regularly spaced volcanic centres overlying instability points in the partially molten upwelling asthenosphere much as has been postulated for arc volcanism and early continental rifting. Unlike arcs, the asthenosphere upwells to the base of the crust and the magmatic centres undergo continuous extension. Thus, large volcanoes are not constructed, and instead, ribbons of basaltic crust form parallel to the spreading direction. This is most evident at the SW Indian and American-Antarctic Ridges because of their highly attenuated magma supply. Where the magma supply is more robust and the magma chambers are correspondingly larger, the chambers may merge and eliminate the surficial morphological and chemical expression of punctuated magmatism at ocean ridges.