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 degrees N

Mantle temperature variation and plate spreading rate variation have been c onsidered to be the two fundamental variables that determine the extent of mantle melting and ocean crust production. Along the length of a similar to 200 km portion of the Mid-Atlantic Ridge (MAR) between the Oceanographer ( 35 degreesN) and Haves (33 degreesN) transforms, the mantle potential tempe rature 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 betw een-segment differences. For example, the similar to 90 km long segment OH- 1 is magnatically robust with a central topographic high, thick crust, and a large negative gravity anomaly whereas the similar to 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 segm ents OH-1 and OH-3 are ultimately controlled by their fertile mantle source compositional difference as reflected by the lava compositional difference s between the two segments: > 70% of OH-1 samples studied (N = 57) are enri ched MORE with [La/Sm](N) > I, but > 85% of OH-3 samples studied (N = 42) a re depleted MORB with [La/Sm](N) < I. Calculations show that the mean OH-I source is more enriched in incompatible elements, total alkalis (<similar t o> 0.36 wt% Na2O and similar to 0.09% K2O) and H2O content (similar to 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 (similar to 7 0 ppm). These fertile compositional differences result in significantly red uced solidus temperature of OH-1 source over that of OH-3 source, and allow s melting to begin at a significantly greater depth beneath OH-1 (similar t o 90 km) than beneath OH-3 (< 60 km), leading to a taller melting column, h igher degrees of decompression melting, greater melt production, thus thick er crust and more negative gravity anomaly at OH-1 than at OH-3. We emphasi ze that fertile mantle source compositional variation is as important as ma ntle temperature variation and prate spreading rate variation in governing the extent of mantle melting, crustal production, and MORB chemistry. The b uoyancy-driven focused mantle upwelling model better explains the observati ons than the subcrystal melt migration model. Future mantle flow models tha t consider the effect of fertile mantle compositional variation are expecte d 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 siz e and fertility of the enriched mantle heterogeneities may actually control the initiation and evolution of ridge segments bounded by non-rigid discon tinuities at slow-spreading ridges. (C) 2001 Elsevier Science B.V. All righ ts reserved.