Graduate Thesis Or Dissertation
 

Geochemical expression of volcanism in an on-axis and intraplate hotspot : Cobb and Marquesas

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  • The Pacific Ocean basin is home to a set of hotspots diverse in their eruption rate, duration of volcanism, and basalt chemistry. Pacific hotspots are found in a spectrum of distinct plate tectonic settings, from near a spreading ridge to intraplate. Cobb hotspot, which resulted in formation of the Cobb-Eickelberg seamount (CES) chain, is currently located beneath Axial seamount, on the Juan de Fuca ridge. The Marquesas hotspot, which formed the Marquesas archipelago and, perhaps, older seamounts to the west, is a member of the cluster of hotspots found within French Polynesia, located well away from the nearest spreading ridge. Geochemical and geochronological studies of volcanism at these two hotspots contribute to an understanding of the effect of plate tectonic environment on hotspot volcanism. Cobb hotspot has the temporal but not the isotopic characteristics usually attributed to a mantle plume. The earlier volcanic products of the hotspot show a westward age progression away from the hotspot and a westward increase in the age difference between the seamounts and the crust on which they formed. These results are consistent with movement of the Pacific plate over a fixed Cobb hotspot and encroachment by the westwardly migrating Juan de Fuca ridge. CES lavas are slightly enriched in alkali and incompatible elements relative to those of the Juan de Fuca ridge but they have Sr, Nd, and Pb isotopic compositions virtually identical to those found along the ridge. Cobb hotspot is a stationary upper mantle melting anomaly whose volcanic products have strong mid-ocean ridge basalt affinity. Basaltic lavas from dredge hauls in the Marquesas archipelago decrease in age from northwest to southeast, with the youngest volcanism at a group of seamounts southeast of the island of Fatu Hiva. Tholeiitic and transitional basalts compose the shield of a typical Marquesan volcano; alkalic lavas form a later phase. Tholeiitic and transitional basalts result from variable degrees of melting of a relatively depleted mantle source with pods of incorporated radiogenic-Pb enriched material. Alkalic lavas represent smaller degrees of melting of a radiogenic Sr and Pb enriched mantle source. Volcanism at both hotspots can be explained by variable amounts of entrainment of a heterogeneous upper mantle into a mantle plume of isotopically depleted (Cobb) or enriched (Marquesas) composition.
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