Construction of the Galapagos platform by large submarine volcanic terraces

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 9 (2008): Q03015, doi:10.1029/2007GC001795.New multibeam bathymetric and side-scan sonar data from the southwestern edge of the Galápagos platform reveal the presence of ∼60 large, stepped submarine terraces between depths of 800 m and 3500 m. These terraces are unique features, as none are known from any other archipelago that share this geomorphic form or size. The terraces slope seaward at 3000 m) lava flow fields west of Fernandina and Isabela Islands. The terraces are formed of thick sequences of lava flows that coalesce to form the foundation of the Galápagos platform, on which the subaerial central volcanoes are built. The compositions of basalts dredged from the submarine terraces indicate that most lavas are chemically similar to subaerial lavas erupted from Sierra Negra volcano on southern Isabela Island. There are no regular major element, trace element, or isotopic variations in the submarine lavas as a function of depth, relative stratigraphic position, or geographic location along the southwest margin of the platform. We hypothesize that magma supply at the western edge of the Galápagos hot spot, which is influenced by both plume and mid-ocean ridge magmatic processes, leads to episodic eruption of large lava flows. These large lava flows coalesce to form the archipelagic apron upon which the island volcanoes are built.This work was supported by the National Science Foundation grants OCE0002818 and EAR0207605 (D.G.), OCE0002461 (D.J.F. and M.K.), OCE05-25864 (M.K.), and EAR0207425 (K.H.)

Submarine Fernandina : magmatism at the leading edge of the Galapagos hot spot

Author Posting. © American Geophysical Union, 2006. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 7 (2006): Q12007, doi:10.1029/2006GC001290.New multibeam and side-scan sonar surveys of Fernandina volcano and the geochemistry of lavas provide clues to the structural and magmatic development of Galápagos volcanoes. Submarine Fernandina has three well-developed rift zones, whereas the subaerial edifice has circumferential fissures associated with a large summit caldera and diffuse radial fissures on the lower slopes. Rift zone development is controlled by changes in deviatoric stresses with increasing distance from the caldera. Large lava flows are present on the gently sloping and deep seafloor west of Fernandina. Fernandina's submarine lavas are petrographically more diverse than the subaerial suite and include picrites. Most submarine glasses are similar in composition to aphyric subaerially erupted lavas, however. These rocks are termed the “normal” series and are believed to result from cooling and crystallization in the subcaldera magma system, which buffers the magmas both thermally and chemically. These normal-series magmas are extruded laterally through the flanks of the volcano, where they scavenge and disaggregate olivine-gabbro mush to produce picritic lavas. A suite of lavas recovered from the terminus of the SW submarine rift and terraces to the south comprises evolved basalts and icelandites with MgO = 3.1 to 5.0 wt.%. This “evolved series” is believed to form by fractional crystallization at 3 to 5 kb, involving extensive crystallization of clinopyroxene and titanomagnetite in addition to plagioclase. “High-K” lavas were recovered from the southwest rift and are attributed to hybridization between normal-series basalt and evolved-series magma. The geochemical and structural findings are used to develop an evolutionary model for the construction of the Galápagos Platform and better understand the petrogenesis of the erupted lavas. The earliest stage is represented by the deep-water lava flows, which over time construct a broad submarine platform. The deep-water lavas originate from the subcaldera plumbing system of the adjacent volcano. After construction of the platform, eruptions focus to a point source, building an island with rift zones extending away from the adjacent, buttressing volcanoes. Most rift zone magmas intrude laterally from the subcaldera magma chamber, although a few evolve by crystallization in the upper mantle and deep crust.This work was supported by the National Science Foundation grants OCE0002818 and EAR0207605 (D.G.), OCE0002461 (D.J.F. and M.K.), OCE9811504 (D.J.F. and M.R.P.), and EAR0207425 (K.H.) and WHOI postdoctoral support for Soule

A Preliminary Survey of the Northeast Seamounts, Galápagos Platform

A simple plume model cannot fully explain the origin of the Galápagos Islands because magmatism continues east of the current position of the hotspot and young volcanoes occur north of the main platform between the hotspot and the Galápagos Spreading Center (GSC). The evolution of this pattern over time can be elucidated by a set of 3-6Ma seamounts that lie off the northeast margin of the Galápagos platform. The three largest seamounts appear to be drowned islands that host twelve different lava types. Three of these have slightly enriched trace element compositions, while the rest are primitive basalts with highly depleted incompatible element compositions-more depleted than western GSC MORB and different from any on the islands. These ultra-depleted lavas could have been produced by partial melting of a previously depleted component of the Galápagos mantle plume as it interacted with the thinner lithosphere at the northeastern boundary of the platform. Regional paleoreconstructions over the past 6 My show that the smallest seamount formed over the present position of the hotspot, consistent with its enriched lava compositions. The three larger seamounts formed east of the hotspot-evidence that significant magmatism east of the hotspot has been continuous-but a major difference from the present islands is that these volcanoes did not form on the shallow platform. Magmatism north of the hotspot did not begin until the encroachment of the 90°50\u27W Galápagos transform fault and an apparent 60-70km shift northward of the hotspot

The Evolution of Galápagos Volcanoes: An Alternative Perspective

The older eastern Galápagos are different in almost every way from the historically active western Galápagos volcanoes. Geochemical, geologic, and geophysical data support the hypothesis that the differences are not evolutionary, but rather the eastern volcanoes grew in a different tectonic environment than the younger volcanoes. The western Galápagos volcanoes have steep upper slopes and are topped by large calderas, whereas none of the older islands has a caldera, an observation that is supported by recent gravity measurements. Most of the western volcanoes erupt evolved basalts with an exceedingly small range of Mg#, Lan/Smn, and Smn/Ybn. This is attributed to homogenization in a crustal-scale magmatic mush column, which is maintained in a thermochemical steady state, owing to high magma supply directly over the Galápagos mantle plume. In contrast, the eastern volcanoes erupt relatively primitive magmas, with a large range in Mg#, Lan/Smn, and Smn/Ybn. These differences are attributed to isolated, ephemeral magmatic plumbing systems supplied by smaller magmatic fluxes throughout their histories. Consequently, each batch of magma follows an independent course of evolution, owing to the low volume of supersolidus material beneath these volcanoes. The magmatic flux to Galápagos volcanoes negatively correlates to the distance to the Galápagos Spreading Center (GSC). When the ridge was close to the plume, most of the plume-derived magma was directed to the ridge. Currently, the active volcanoes are much farther from the GSC, thus most of the plume-derived magma erupts on the Nazca Plate and can be focused beneath the large young shields. We define an intermediate sub-province comprising Rabida, Santiago, and Pinzon volcanoes, which were most active about 1 Ma. They have all erupted dacites, rhyolites, and trachytes, similar to the dying stage of the western volcanoes, indicating that there was a relatively large volume of mush beneath them. The paradigm established by the evolution of Hawaiian volcanoes as they are carried away from the hotspot does not apply to most archipelagos

Earth’s mantle composition revealed by mantle plumes

International audienceEarth's mantle is integral to many processes that shape our planet, including convection, crustal formation, crustal recycling, and global heat and volatile budgets. Still, many questions remain unresolved about mantle composition and its influence on geodynamics today and throughout geologic time. Because they originate at depths that can extend to the core-mantle boundary, mantle plumes provide invaluable information about the composition of the deep mantle. In this review, we discuss the effectiveness and challenges of using isotopic analyses of plume-derived rocks to document the origin and composition of mantle heterogeneities, earlyformed mantle reservoirs, crustal recycling processes, core-mantle interactions, and mantle evolution. We discuss isotopic methods and improvements to existing isotopic systems to characterize plume-derived ocean island basalts. Nevertheless, because mantle plumes vary in many properties, including magmatic flux, temperature, tectonic environment, and compositions, geochemical observations on plume-generated systems must be considered carefully before making interpretations about Earth's interior. Consequently, plumes and their melts should be evaluated along a spectrum that acknowledges and contextualizes their differences, particularly mantle flux. Ultimately, the most important advancements in mantle geochemistry, architecture, and dynamics will emerge from cross-disciplinary studies in diverse fields such as experimental petrology, mineral physics, numerical geodynamics, seismology, and geochemistry

Persistently well-ventilated intermediate-depth ocean through the last deglaciation

This study was funded by the European Research Council, the Natural Environment Research Council (NE/S001743/1; NE/N011716/1), the Philip Leverhulme Trust, the Strategic Priority Research Program of Chinese Academy of Sciences (XDB40010200), the National Natural Science Foundation of China (41822603), the US National Science Foundation (OCE-0926637, OCE-10309040 and OCE-0926491), a Marie Curie Reintegration Grant and the NOAA (National Oceanic and Atmospheric Administration) Ocean Exploration Trust.During the last deglaciation (~18–11 thousand years ago), existing radiocarbon (14C) reconstructions of intermediate waters in the mid- to low-latitude oceans show widely diverging trends, with some broadly tracking the atmosphere and others suggesting extreme depletions. These discrepancies cloud our understanding of the deglacial carbon cycle because of the diversity of hypotheses needed to explain these diverging records, for example, injections of 14C-dead geological carbon, mixing of extremely isolated waters from the abyssal ocean or changes in sites of deep-water ventilation. Here we present absolutely dated deglacial deep-sea coral 14C records of intermediate waters from the Galápagos Platform—close to the largest reported deglacial 14C depletions—together with data from the low-latitude Atlantic. Our records indicate coherent, well-equilibrated intermediate-water 14C ventilation in both oceans relative to the atmosphere throughout the deglaciation. The observed overall trend towards 14C-enriched signatures in our records is largely due to enhanced air–sea carbon isotope exchange efficiency under increasing atmospheric pCO2. These results suggest that the 14C-depleted signatures from foraminifera are likely sedimentary rather than water mass features, and provide tight 14C constraints for modelling changes in circulation and carbon cycle during the last deglaciation.PostprintPeer reviewe