Mantle compositional control on the extent of mantle melting, crust production, gravity anomaly, ridge morphology, and ridge segmentation: a case study at the Mid-Atlantic Ridge 33 - 35°N

Abstract

Mantle temperature variation and plate spreading rate variation have been considered to be the two fundamental variables that determine the extent of mantle melting and ocean crust production. Along the length of a 200 km portion of the Mid-Atlantic Ridge (MAR) between the Oceanographer (35°N) and Hayes (33°N) transforms, the mantle potential temperature is the same, the plate spreading rate is the same, but the extent of mantle melting and crustal production vary drastically. In addition to the typical crustal thickness variation on ridge segment scales at the MAR, i.e. thicker at segment centers and thinner at segment ends, there exist between-segment differences. For example, the 90 km long segment OH-1 is magmatically robust with a central topographic high, thick crust, and a large negative gravity anomaly whereas the 45 km long segment OH-3 is magmatically starved with a deep rift valley, thin crust and a weak negative gravity anomaly. We demonstrate that the observed differences in the extent of mantle melting, melt production and crustal mass between segments OH-1 and OH-3 are ultimately controlled by their fertile mantle source compositional difference as reflected by the lava compositional differences between the two segments: >70% of OH-1 samples studied (N=57) are enriched MORB with [La/Sm]N>1, but >85% of OH-3 samples studied (N=42) are depleted MORB with [La/Sm]N<1. Calculations show that the mean OH-1 source is more enriched in incompatible elements, total alkalis (0.36 wt% Na2O and 0.09% K2O) and H2O content (280 ppm) than the mean OH-3 source, which is depleted of incompatible elements, total alkalis (<0.17% Na2O and <0.01% K2O) and H2O content (70 ppm). These fertile compositional differences result in significantly reduced solidus temperature of OH-1 source over that of OH-3 source, and allows melting to begin at a significantly greater depth beneath OH-1 (90 km) than beneath OH-3 (<60 km), leading to a taller melting column, higher degrees of decompression melting, greater melt production, thus thicker crust and more negative gravity anomaly at OH-1 than at OH-3. We emphasize that fertile mantle source compositional variation is as important as mantle temperature variation and plate spreading rate variation in governing the extent of mantle melting, crustal production, and MORB chemistry. The buoyancy-driven focused mantle upwelling model better explains the observations than the subcrustal melt migration model. Future mantle flow models that consider the effect of fertile mantle compositional variation are expected to succeed in producing along-axis wavelengths of buoyant flow comparable to the observed size of ridge segments at the MAR. We propose that the size and fertility of the enriched mantle heterogeneities may actually control the initiation and evolution of ridge segments bounded by non-rigid discontinuities at slow-spreading ridges.