Chemical and isotopic studies of Deccan Traps dikes (India) and Louisville Seamounts (South Pacific): Aspects of the youth and maturity of hotspots.

摘要

The source of intraplate volcanism is commonly postulated to be melting anomalies, referred to as hotspots, relatively fixed in the Earth's mantle and fed by deep-seated thermal plumes. In plume models, flood basalts are thought to form when a rising mantle plume-head breaches the lithosphere, and age-progressive volcanic chains represent the trace of these mantle plume "tails" as lithospheric plates move over the melting anomaly. In this dissertation, I studied both aspects: the Deccan Traps (India), representing volcanism related to a plume-head, and the Louisville Seamount Chain (Southwest Pacific) tracking plume-tail activity, to gain insights into aspects of hotspot initiation and maturity.;The Deccan Traps of India is one of the Earth's largest flood basalt province. Using elemental and Pb-Nd-Sr isotope characteristics, feeder dikes for Deccan flow formations were identified and dike strikes were measured to inferred tectonic conditions at the time of their emplacement. Lower and middle lava formations were fed by E-W or N-S striking dikes. Feeder dikes for upper lava formations displayed no clear preferred strike. The absence of a preferred dike orientation for the voluminous upper formation lavas favors a plume-head origin for the Deccan Traps.;The Louisville Seamount Chain is the second longest hotspot track in the Pacific. This geochemical study was undertaken to characterize the mantle source evolution of a hotspot that has been active for ∼80 Myr and compare those findings with generic models of hotspot volcanism. I focused on lavas dredged from scarps and steep slopes of seamounts along the chain between 72 Ma and 24 Ma. I found little evidence of chemical variations that would be caused by mantle source heterogeneity. In fact, age-corrected Sr, Nd and Pb isotope ratios are extraordinarily homogeneous. The small variations that do exist, do not correlate with the age of the seamounts, degree of partial melting or age of the lithosphere at the time of seamount emplacement.