Trace element variations along the Central Indian Ridge: deciphering Indian mantle heterogeneities
Abstract
We report more than 50 ICP-MS and 70 LA-ICP-MS new analyses of basalt glasses along the Central Indian (CIR) and Carlsberg Ridges. The new high precision data further document the chemical characteristics, geographical distribution and origin of mantle heterogeneity beneath the Indian Ocean. Comparison with other data shows the importance of interlab differences in interpreting ICP-MS and particularly LA-ICP-MS data. Samples from the Carlsberg Ridge and north of 10°S along the CIR display both incompatible element enriched and depleted compositions. Between 10°S and 20°S, all compositions are enriched. South of 20°S, bimodal compositions abruptly reappear. The most-enriched trace element compositions are found within and south of the Marie Celeste FZ, confirming the effect of the fracture zone on the mantle flow originating from the neighboring Reunion plume. The progressive diminishment of the incompatible enrichment towards the north of the fracture zone suggests a plume influence over more than 1000km of ridge length. Samples from the Carlsberg Ridge are virtually indistinguishable from the large set of EPR samples analyzed in our laboratory, and reflect two component mixing of an enriched end member generated by low degree melt metasomatism with a depleted end member. CIR basalts also demonstrate linear mixing arrays, but both enriched and depleted end members are distinct from the EPR. The enriched component is marked by high Ba, Rb and Th (but not U) relative to the REE. The depleted component is less depleted in light REE and relatively more depleted in U and high field strength elements, including Nb, Ta, Zr. The combination of these two effects creates data arrays distinct from the EPR for trace element ratios such as Ba/Nb, Th/U, Th/Ta and Th/Nb. Th/Nb varies substantially in this region, in contrast to the almost constant ratio on other ocean ridges. The enriched member is not accounted for by recycled sediment or ocean crust, and does not have significant Nb depletion. Therefore recycled sediment or other contamination by continental materials does not account for the distinct flavor of Indian heterogeneity. The linear data arrays and strong fractionation of highly incompatible elements also rule out recycled ocean crust. While there are clearly multiple enriched components along the CIR, the fact that the data have a similar topology to the other ocean ridges suggests the addition and substraction of low degree melts to the mantle sources. The details must differ, however, perhaps owing to differing in residual phases and oxidation state over Earth history. The generally high Th/U ratio and relative Rb enrichment and U depletion in this region appear to reflect the overall isotopic characteristics of the Indian Ocean. The abrupt changes in signature associated with fracture zones reaffirm the significance of these surface features to mantle composition and flow. It is not entirely clear how the trace element depletion is related to the event responsible for the enriched isotopic signature observed in the Indian Oceans. Higher density regional coverage of other Indian ridges should lead to much higher definition of mantle domain boundaries and place further constraints on the cause of their formation.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2005
- Bibcode:
- 2005AGUFM.V33D..05E
- Keywords:
-
- 1025 Composition of the mantle;
- 3610 Geochemical modeling (1009;
- 8410);
- 3614 Mid-oceanic ridge processes (1032;
- 8416);
- 3619 Magma genesis and partial melting (1037);
- 3621 Mantle processes (1038)