Mantle melting and heterogeneity along mid-ocean ridges: Insights from basalt geochemistry along axial depth and morphologic gradients for intermediate spreading rate systems.

摘要

This investigation focuses on gaining a better understanding of the complex relationship between melt generation, source variability and mid-ocean ridge morphology. The approach adopted here uses a variety of geochemical techniques to evaluate the ability of 'global' models to predict regional scale geochemical variability associated with axial depth and axial morphologic changes. Three separate regional scale studies were conducted along ridges characterized by intermediate spreading rates (where the system is very sensitive to variations in magma production).; The first study focuses on the development of the Australian-Antarctic Discordance (AAD), an anomalously deep portion of the modern global ridge system located in the eastern Indian Ocean, over the period from 28 to 14 Ma during which the eastern Indian Ocean basin was in a relatively young stage of formation. Major and trace element results from this study suggest a more magmatically robust ridge was present during this period.; The second study investigates the link between U-series disequilibria and axial ridge depth. In this study U and Th isotopic compositions and elemental concentrations were analyzed along the Southeast Indian Ridge. The results of this study suggest that a simple relationship, such as that predicted from global MORB variations, does not exist on a regional scale in this part of the Indian Ocean. Plausible explanations of this data set require the consideration of other intrinsic variables such as residual porosity and mantle melting rates.; The third investigation focuses on the western Galapagos Spreading Center (GSC), an intermediate spreading ridge whose axial morphology is affected by the addition of heat and material from the nearby Galapagos hotspot. This study investigates the origin and nature of the transfer of this material through analysis of rare earth element concentrations in melt inclusions. The results from this study support a deep (≥ 60 km), strong lateral flow of hotspot-derived mantle toward the GSC.; Collectively these studies support a strong link between axial morphology and melt generation and verify that geochemical investigations along regional morphologic gradients provide a meaningful 'window' into the underlying mantle, while demonstrating that although 'global' models succeed in providing a valid platform from which to evaluate regional-scale observations, they do not accurately describe the complex process of melt generation on a regional scale.