Mantle heterogeneities beneath the eastern SWIR

The eastern termination of the South West Indian Ridge (SWIR) is characterized by very slow spreading rates, reduced magma production and a peculiar accretionary tectonics[1]. Oblique spreading sectors separate orthogonal segments characterized by an anomalous abundance of variably extended core complexes that accommodate spreading through low angle, long-lived, detachment faulting. High ridge depths, along with abundant mantle exposures at the seafloor and reduced crustal thickness suggest mantle temperatures to be the lowest of the entire South West Indian Ridge [2]. This region is bounded by the Melville fracture zone to the west. Based on basalt isotopic fingerprint it appears that this fracture zone represents a major thermal and compositional boundary [3]. We have carried out a detailed major and trace element study of mantle residua from the two domains east and west of the Melville fracture zone recognizing different compositional domains in the mantle underlying this region accompanied by different melting styles. The western domain reflects the high degree of depletion due to the higher mantle potential temperature. By contrast the domain East of the Melville fracture zone is characterized by mantle that undergone low degrees of partial melting mirrored by the overall lower crustal thickness associated to large obliquity of the spreading axis. These mantle batches show a large interaction with variably aggregated melts. A marked garnet signature and incompatible element enrichment is locally present interpreted as source refertilization by garnet-derived melts. Basalts with compositions close to these melts have been collected in the region but they are not spatially associated to the enriched mantle. These basalts also show the enriched-most isotopic signature suggesting that melting of enriched heterogeneities and significant garnet field refertilization are two aspects of the same process. However these basalts are chemically depleted nearly similar to isotopically depleted basalts from other sectors of the Indian Ridge. This observation requires enriched melts to be generated by large extent of melting of the generating lithology without an important contribution from the surrounding mantle. The prevailing contribution of the enriched lithologies can be explained by the lower solidus but also by a probable more depleted character of the surrounding mantle resulting in a selective extraction of the first. The scattered presence of mantle rocks too depleted for resulting from the melting process beneath the region east of Melville confirms the presence of relict domains of ancient depletions. The entire range of basalt compositions from this stretch of SWIR overlaps the calculated range of the melts in equilibrium with the peridotites strongly suggesting that the variability of the mantle composition is likely similar at local (dredge) and regional scales. References. [1] Cannat, M., Sauter D., Mendel V., Ruellan E., Okino K., Escartin J., Combier V., Baala M., (2006). Geology 34(7), 605-608. [2] Cannat, M., Rommevaux-Jestin C., Sauter D., Deplus C., Mendel V., (1999). Journal of Geophysical Research 104(B10), 22825-22843. [3] Meyzen C.M., Toplis M.J., Humler E., Ludden J.N., Catherine M., (2003) Nature 731-733., 100-110