Our project aimed at characterizing the geochemical variability, at moderately high resolution, along a ~1800 km section of the Southeast Indian Ridge. This area was known to contain significant physical and chemical variability along the ridge axis that can be used to understand mantle melting processes that produce seafloor basalt, and variations in their mantle source composition. We previously participated in sampling expeditions in 2009 and 2010 aboard the French research vessel R/V Marion Dufresne, providing us with a new suite of closely spaced basalt glasses along the SEIR.  With colleagues in France we produced a high-precision, multi-isotopic data set (for helium, lead, hafnium, strontium and neodymium) of the recovered basalts. The new results provide a first look at variability over ~1800 km of the spreading mid-ocean ridge system at unprecedented resolution, with a continuous sample spacing of 5-10 km throughout the study region.

The isotopic variations in this sample suite confirmed our working hypothesis for the presence of heterogeneous streaks of mantle beneath this region of the global mid-ocean ridge system.  The results reveal streaks having a characteristic thickness of ~20-30 km. The Indian Ocean upper mantle was found to contain two resolvable components arising from plate tectonic recycling; these originate as 1) continental lithosphere or lower crust that was remobilized into the upper mantle during the breakup of the supercontinent Gondwana at ~180 million years ago, and 2) subducted ocean lithosphere (crust+sediment) that has been recycled continually into the upper mantle through deep mantle plume upwellings since the ocean basin formed. Remarkably, mantle convection has folded together these distinct composite reservoirs of heterogeneous mantle, and stretched them into streaks that still remain coherent and discernible units.

The helium isotope variations reveal that there are often very abrupt transitions over short distances (10-20 km) along the ridge (Fig. 1). The abrupt variations are distinguished chemically by the most depleted vs. enriched basalts in the sample, and are evidence for an additional linkage between isotope composition and melting processes beneath the SEIR.  We obtained detailed, quantitative information about the characteristic length scales using different spectral analysis techniques. Long-wavelength variations at ~1000 km and 500 km are well-resolved in the data set, and resemble those previously observed along the Mid-Atlantic Ridge. We also found that statistically significant variation (>95% confidence level) is present at 25-30 km, similar to the length scales inferred from the independent hafnium and lead isotope analyses. Collectively, our results show that the scales of isotope variability along mid-ocean ridges reflect both the heterogeneity associated with folding and stretching of material during large scale mantle flow, and the response of melting processes to smaller-scale, solid state mantle convection.

The isotope data obtained in this study are also fundamental to understanding crustal accretion processes along mid-ocean ridges. These processes include melt retention in the upper mantle and lower crust, high-pressure crystallization of magma, magma mixing, and magma-wallrock interactions. Observed variations in isotope composition along mid-ocean ridges are central to evaluating the accuracy of mantle convection models. The project supported the development of fully utilized state-of-the-art instrumentation in laboratories at Oregon State University and San Diego State University, ensuring that they remain valuable sites for mentoring students and post-docs, and for fostering both national and international collaboration.

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