SO208 PLUMEFLUX: Ausbreitung von Galápagosplumematerial im äquatorialem Ostpazifik

The main objective of this Dissertation is the investigation of the distribution of Galápagos plume material in the equatorial East Pacific. It provides a more detailed view on the Galápagos plume and the adjacent Galápagos Spreading Center (GSC; also known as Cocos-Nazca-Spreading Center (CNS)) and contributes to further our understanding of plume-ridge interaction and mantle material transport within the earth upper mantle. Of special interest in this context is the far-field lateral transport of plume material (subproject 1) and the temporal evolution of plume-ridge interaction at the Galápagos plume-Galápagos Spreading Center system (sub-project 2). The necessary sample material in order to address these questions was recovered during the cruise SO208 with the German research vessel R/V Sonne in July/August 2010.Gegenstand der vorliegenden Dissertation ist die Untersuchung der Verbreitung von Galápagosplumematerial im äquatorialen Ostpazifik. Am Beispiel des Galápagosplume in Verbindung mit dem benachbarten Galápagosspreizungzentrum (GSC; auch als Cocos-Nazca-Spreizungszentrum (CNS) bezeichnet) soll diese Arbeit zu einem besseren Verständnis von Plume-Rücken-Wechselwirkungen und Materialtransport im oberen Mantel beitragen. Der Schwerpunkt wurde dabei auf die räumliche Ausdehnung von Galápagosplumematerial im äquatorialen Ostpazifik (Teilprojekt 1) und die zeitliche Entwicklung des Galápagosplume-Galápagosspreizungszentrum-Systems (Teilprojekt 2) gelegt. Die dafür analysierten Proben wurden während der Forschungsfahrt SO208 mit dem deutschen Forschungsschiff Sonne im Juli/August 2010 gewonnen

SO208 PLUMEFLUX - Distribution of Galápagos plume material in the equatorial East Pacific Ocean

The main objective of this Dissertation is the investigation of the distribution of Galápagos plume material in the equatorial East Pacific. It provides a more detailed view on the Galápagos plume and the adjacent Galápagos Spreading Center (GSC; also known as Cocos-Nazca-Spreading Center (CNS)) and contributes to further our understanding of plume-ridge interaction and mantle material transport within the earth upper mantle. Of special interest in this context is the far-field lateral transport of plume material (subproject 1) and the temporal evolution of plume-ridge interaction at the Galápagos plume-Galápagos Spreading Center system (sub-project 2). The necessary sample material in order to address these questions was recovered during the cruise SO208 with the German research vessel R/V Sonne in July/August 2010

Temporal variations in Galápagos plume-ridge interaction at the Cocos-Nazca spreading center

The major goals of cruise SO208 with the German research vessel Sonne were to investigate 1) plume-ridge interaction through time at the Cocos-Nazca spreading center (CNS) north of the Galápagos Islands by sampling across axis profiles of the seafloor and 2) off axis volcanism at the East Pacific Rise (EPR) versus far field effects of the Galápagos hotspot documented in seamounts off the coast of N Costa Rica and Nicaragua. Overall the nature of material transfer from the plume to the ridge and its large scale distribution throughout the Eastern Pacific is being investigated by means of major and trace element and Sr-Nd-Pb (double spike) isotope data. The seamounts on the EPR generated part of the Cocos plate appear to originate on one hand from a depleted MORBlike source consistent with their formation near the EPR axis, while other seamounts formed through lower degrees melting of an enriched OIB source either more distant from the EPR or by intraplate volcanism. Geochemical profiles across the Western and Eastern CNS indicate the participation of two different Galápagos plume components with a change in the amount this material entering the CNS with time. While at the western profile element ratios of more to less incompatible elements show an overall decrease of a plume component, Wolf-Darwin or Northern domain [1], with increasing age, the opposite is observed at the eastern profile. The Central domain component [1] increases with increasing age of the crust in this area. These observations indicate variable flux of specific Galápagos plume components to the CNS over the past 800 000 years. Sr-Nd-Pb isotope data to verify these observations are currently being generated and will be presented at the conference. [1] Hoernle et al. (2000) Geology 28, 435–43

A 1.5 Ma record of plume-ridge interaction at the Western Galápagos Spreading Center (91°40’-92°00’W)

Shallow (elevated) portions of mid-ocean ridges with enriched geochemical compositions near hotspots document the interaction of hot geochemically enriched plume mantle with shallow depleted upper mantle. Whereas the spatial variations in geochemical composition of ocean crust along the ridge axis in areas where plume-ridge interaction is taking place have been studied globally, only restricted information exists concerning temporal variations in geochemistry of ocean crust formed through plume-ridge interaction. Here we present a detailed geochemical study of 0-1.5 Ma ocean crust sampled from the Western Galápagos Spreading Center (WGSC) axis to 50 km north of the axis, an area that is presently experiencing a high influx of mantle material from the Galápagos Hotspot. The tholeiitic to basaltic andesitic fresh glass and few bulk rock samples have incompatible element abundances and Sr-Nd-Pb isotopic compositions intermediate between depleted normal mid-ocean-ridge basalt (N-MORB) from >95.5°W along the WGSC and enriched lavas from the Galápagos Archipelago, displaying enriched (E-)MORB type compositions. Only limited and no systematic geochemical variations are observed with distance from the ridge axis for <1.0 Ma old WGSC crust, whereas 1.0-1.5 Ma old crust trends to more enriched isotopic compositions in 87Sr/86Sr, 143Nd/144Nd, 207Pb/204Pb and 208Pb/204Pb isotope ratios. On isotope correlation diagrams, the data set displays correlations between depleted MORB and two enriched components. Neither the geographically referenced geochemical domains of the Galápagos Archipelago nor the end members used for principle component analysis can successfully describe the observed mixing relations. Notably an off-axis volcanic cone at site DR63 has the appropriate composition to serve as the enriched component for the younger WGSC and could represent a portion of the northern part of the Galápagos plume not sampled south of the WGSC. Similar compositions to samples from volcanic cone DR63 have been found in the northern part of the 11-14 Ma Galápagos hotspot track offshore Costa Rica, indicating that this composition is derived from the northern portion of the Galápagos plume. The older WGSC requires involvement of an enriched mantle two (EMII) type source, not recognized thus far in the Galápagos system, and is interpreted to reflect entrained material either from small-scale heterogeneities within the upper mantle or from the mantle transition zone. Overall the source material for the 0-1.5 Ma WGSC ocean crust appears to represent mixing of depleted upper mantle with Northern Galápagos Plume material of relatively uniform composition in relatively constant proportions

Cocos Plate Seamounts offshore NW Costa Rica and SW Nicaragua: Implications for large-scale distribution of Galápagos plume material in the upper mantle

The origin of intraplate volcanism not directly part of a hotspot track, such as diffuse seamount provinces, and the extent of mantle plume influence on the upper mantle remain enigmatic. Here we present new 40Ar/39Ar age data and geochemical (major and trace-element and Sr-Nd-Pb isotopic) data from seamounts on the Cocos Plate presently located offshore of NW Costa Rica and SW Nicaragua. The seamounts (~ 7-24 Ma) require mixing of an enriched ocean island basalt composition, similar to that of the Northern Galápagos Domain, with two depleted components. One of the depleted components is similar to East Pacific Rise normal mid-ocean ridge basalt and the other has more depleted incompatible elements, either reflecting secondary melting of MORB or a depleted Galápagos plume component. Seamounts with ages significantly younger than the ocean crust formed in an intraplate setting and can be explained by northward transport of Galápagos plume material along the base of the Cocos Plate up to 900 km away from the hotspot and 250-500 km north of the Galápagos hotspot track. We propose that melting occurs due to decompression as the mantle upwells to shallower depth as it flows northwards, either due to changes in lithospheric thickness or as a result of upwelling at the edge of a viscous plug of accumulated plume material at the base of the lithosphere. The tholeiitic to alkaline basalt compositions of the Cocos Plate Seamounts compared to the more silica under-saturated compositions of Hawaiian rejuvenated and arch (alkali basalts to nephelinites) lavas is likely to reflect the significant difference in age (< 25 vs ~ 90 Ma) and thus thickness of the lithosphere on which the lavas were erupted