Showing papers by "Eizo Nakamura published in 2014"

Supervolcano eruptions driven by melt buoyancy in large silicic magma chambers

Abstract: Supervolcano eruptions dwarf all historical eruptions, but their trigger mechanisms are unclear. Experimental measurements of magma density at high pressures and temperatures show that the buoyancy of magma alone can impose sufficient pressure at the roof of a supervolcano magma chamber to induce an eruption. Super-eruptions that dwarf all historical volcanic episodes in erupted volume1 and environmental impact2 are abundant in the geological record. Such eruptions of silica-rich magmas form large calderas. The mechanisms that trigger these super-eruptions are elusive because the processes occurring in conventional volcanic systems cannot simply be scaled up to the much larger magma chambers beneath supervolcanoes. Over-pressurization of the magma reservoir, caused by magma recharge, is a common trigger for smaller eruptions3, but is insufficient to generate eruptions from large supervolcano magma chambers4. Magma buoyancy can potentially create sufficient overpressure4, but the efficiency of this trigger mechanism has not been tested. Here we use synchrotron measurements of X-ray absorption5 to determine the density of silica-rich magmas at pressures and temperatures of up to 3.6 GPa and 1,950 K, respectively. We combine our results with existing measurements of silica-rich magma density at ambient pressures6,7 to show that magma buoyancy can generate an overpressure on the roof of a large supervolcano magma chamber that exceeds the critical overpressure of 10–40 MPa required to induce dyke propagation4, even when the magma is undersaturated in volatiles. We conclude that magma buoyancy alone is a viable mechanism to trigger a super-eruption, although magma recharge and mush rejuvenation8, volatile saturation9 or tectonic stress10 may have been important during specific eruptions.

Origin of the ‘Ghost Plagioclase’ Signature in Galapagos Melt Inclusions: New Evidence from Pb Isotopes

Abstract: Olivine-hosted melt inclusions from both Fernandina and Santiago islands in the Galapagos Archipelago have compositions indicating that plagioclase played an important role in the magmatic evolution of these volcanic islands.The major and trace element chemistry of the Santiago melt inclusions indicates simple plagioclase assimilation. In contrast, Fernandina inclusions have compositions for which the plagioclase appears to be present only as a ‘ghost’ trace element signature (i.e. ‘ghost plagioclase’signature).Two competing hypotheses have been proposed to explain this unique signature: (1) incorporation of an ancient recycled plagioclase-rich cumulate into the mantle; (2) shallow-level interaction between melts and plagioclase-rich cumulates in the present-day lower oceanic crust. Here we present new Pb isotope measurements for olivine-hosted melt inclusions from Fernandina and Santiago islands to distinguish between the two models.The new Pb isotope data are within the range previously reported for whole-rock basalts from those islands. Melting and mixing models involving ancient ( 0·5^1 Ga) recycled plagioclase-rich cumulates cannot reproduce the observed trace element and Pb isotopic characteristics of the Fernandina melt inclusions with a ghost plagioclase signature. Shallow-level diffusive interactions between basalt and present-day plagioclase-rich cumulates provide the simplest explanation for the observed trace element compositions and Pb isotope ratios of melt inclusions from Fernandina and Santiago islands.

Software Dedicated for the Curation of Geochemical Data Sets in Analytical Laboratories

Abstract: Software designed for analytical laboratories to guarantee traceability and accessibility of rocks with their geochemical properties has been developed. The software documents the sample origin, current sample location and the location of any sample subsets (e.g., thin sections, solutions, etc.), and archives all associated geochemical data sets. The software can be installed on a personal computer so is available for use in any laboratory and allows curation before and after publication. The software will be of use in integrating and sharing geological reference materials within and among institutes. In this article, the system design and implementation are detailed. All source codes for the software are available at http://dream.misasa.okayama-u.ac.jp/. Un logiciel concu pour les laboratoires d'analyse afin de garantir la tracabilite et l'accessibilite des echantillons de roches et de leurs caracteristiques geochimiques a ete developpe. Le logiciel documente l'origine de l'echantillon, son emplacement actuel et l'emplacement de tous ses derives (par exemple, des lames minces, des solutions, etc.), et toutes les bases de donnees, geochimiques associees. Le logiciel qui peut etre installe sur un ordinateur personnel est donc disponible pour une utilisation dans n'importe quel laboratoire et permet la conservation avant et apres publication. Le logiciel sera utile dans l'integration et le partage de materiaux de reference geologiques a l'interieur et entre des instituts. Dans cet article, la conception et la mise en œuvre du systeme sont detaillees. Tous les codes sources du logiciel sont disponibles a l'adresse: http://dream.misasa.okayama-u.ac.jp/.