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Isotopic Variations Along the Southeast Indian Ridge

Isotopic Figure 2

Click image to enlarge. Locations of samples recovered during the GEISEIR I and WW10 expeditions. The new GEISEIR sampling campaign has increased the earlier sampling density by nearly an order of magnitude; along segments C14-C16 the average sample spacing is 10 km, and along segment C13 it is 5 km. Inset shows the study area in the context of the Indian Ocean seafloor.

This is a newly funded study, in collaboration with Dr. Barry Hanan (San Diego State University), focusing on the He, Pb, Hf, Nd and Sr isotopic compositions of basalts erupted along the Southeast Indian Ridge (SEIR) between 89°E and 99°E. A new sample suite, recovered in collaboration with French colleagues during 2009 aboard the R/V Marion Dufresne, provides us with an unprecedented opportunity to characterize upper mantle heterogeneity beneath the Indian Ocean. Fresh basalt glass was recovered from ~130 localities along ~1200 km of the SEIR axis. Previous work at lower sampling density revealed significant physical and geochemical variations along this section of the SEIR; our investigation of the new sample suite will be used to constrain mantle melting conditions and source composition/lithology at high resolution.

The main questions to be addressed in this project are:

  1. What are the characteristic length scales of isotopic variability in the Indian Ocean upper mantle?
  2. Does significant (>10 km) redistribution of melt occur along axis, and how does it obscure the scales of mantle isotopic variability?

Our rationale for this study is that the characteristic length scales of upper mantle heterogeneity beneath the SEIR, revealed by analysis of He, Pb, Hf, Nd and Sr isotopes in SEIR basalts, may be significantly different from those beneath the Atlantic and Pacific Oceans. If He, Pb, Hf, Nd and Sr isotopes reveal that portions of the upper mantle beneath different sections of the ocean ridge system have different characteristic length scales, or "textures", then it suggests that we can begin to characterize differences in long-term evolutionary history of those regions and the impact of lithospheric recycling vs. deep mantle input via upwelling at hotspots. One simple expectation might be that differences in upper mantle heterogeneity may be related to tectonic history of continents that have bordered the respective ocean basins, and/or to the speed of mantle convection.

References

Graham, D. W., J. E. Lupton, F. J. Spera and D. M. Christie (2001) Upper mantle dynamics revealed by helium isotope variations along the Southeast Indian Ridge. Nature 409, 701-703. PDF

Mahoney, J. J., D. W. Graham, D. M. Christie, K. T. M. Johnson, L. S. Hall and D. L. VonderHaar (2002) Between a hot spot and cold spot: isotopic variation in the Southeast Indian Ridge asthenosphere, 86°-118°E. J. Petrol. 43, 1155-1176. PDF

Burnard, P. G., D. W. Graham and K. A. Farley (2004) Fractionation of noble gases (He, Ar) during MORB mantle melting? A case study on the Southeast Indian Ridge. Earth Planet. Sci. Lett. 227, 457-472. PDF

Graham, D. W., J. Blichert-Toft, C. J. Russo, K. A. Rubin and F. Albarède (2006) Cryptic striations in the upper mantle revealed by hafnium isotopes in Southeast Indian Ridge basalts. Nature 440, 199-202. PDF

Russo, C. J., K. H. Rubin and D. W. Graham (2009) Mantle melting and magma supply to the Southeast Indian Ridge: the roles of lithology and melting conditions from U-series disequilibria. Earth Planet. Sci. Lett. 278, 55-66. PDF

 

Projects

Mid-Ocean Ridges and Upper Mantle Processes

Mantle Plumes and Geodynamics of Earth's Interior