Small scale mantle heterogeneities sampled through oceanic basalts from ridge-transform intersections

Studies of MORB have shown that the oceanic upper mantle is heterogeneuos. The nature, distribution and scale of these heterogeneities are still a matter of debate. Short-wavelength chemical variability has been observed along mid-ocean ridges. Melt inclusions show isotopic and chemical variabilty within a single basalt sample and even within a single olivine grain. The distance along axis affected by the ridge-transform intersection (RTI) cold edge effect is proportional to slip rate and offset length. Numerical calculations show a strong decrease of crustal production as a ridge approaches a transform. When the amount of melt produced is small, the capability of melt to aggregate and to mix in magma chambers on the way to the surface is low; therefore, the chemical and isotopic signatures of preaggregated melts can be detected. These predictions make long-offset RTI's potential areas to detect chemical variability in the mantle through the study of MORB. Basalts sampled in the equatorial Atlantic along a ridge segment south of the Romanche RTI (ERRS), show an increase in trace element variability within a single dredge haul, with wavelengths comparable to that observed in suites of melt inclusions. This variability implies that melt transport beneath the ERRS is inefficient at mixing melts and suggests melt transport in channelized systems that limit interaction between melt and solid. The correlation between trace element and isotopic enrichement suggests that most of the chemical variability is probably due to source heterogeneity. Melt transport preserves correlations present in the source, suggesting channels nucleation in the deepest part of the molten region just within melting heterogeneities. Correlations will be probably destroyed if channel networks cross source heterogeneities. Volatile-rich low degree partial melts, generated at depth from enriched peridotite or from pyroxenite lumps, react with and metasomatiz