Petrology and Geochemistry of the Galápagos Islands : portrait of a pathological mantle plume

We report new major element, trace element, isotope ratio, and geochronological data on the Galapagos Archipelago. Magmas erupted from the large western volcanos are generally moderately fractionated tholeiites of uniform composition; those erupted on other islands are compositionally diverse, ranging from tholeiites to picritic basanitoids. While these volcanoes do not form a strictly linear age progressive chain, the ages of the oldest dated flows on any given volcano do form a reasonable progression from youngest in the west to oldest in the east, consistent with motion of the Nazca plate with respect to the fixed hotspot reference frame. Isotope ratios in the Galapagos display a considerable range, from values typical of mid-ocean ridge basalt on Genovesa (⁸⁷Sr/⁸⁶Sr: 0.70259, εNd: +9.4, ²⁰⁶Pb/²⁰⁴Pb: 18.44), to typical oceanic island values on Floreana (⁸⁷Sr/⁸⁶Sr: 0.70366, εNd: +5.2, ²⁰⁶Pb/²⁰⁴Pb: 20.0). La/SmN ranges from 0.45 to 6.7; other incompatible element abundances and ratios show comparable ranges. Isotope and incompatible element ratios define a horseshoe pattern with the most depleted signatures in the center of the Galapagos Archipelago and the more enriched signatures on the eastern, northern, and southern periphery. These isotope and incompatible element patterns appear to reflect thermal entrainment of asthenosphere by the Galapagos plume as it experiences velocity shear in the uppermost asthenosphere. Both north-south heterogeneity within the plume itself and regional variations in degree and depth of melting also affect magma compositions. Rare earth systematics indicate that melting beneath the Galapagos begins in the garnet peridotite stability field, except beneath the southern islands, where melting may occur entirely in the spinel peridotite stability field. The greatest degree of melting occurs beneath the central western volcanos and decreases both to the east and to the north and south. Si₈.₀, Fe₈.₀, and Na₈.₀, values are generally consistent with these inferences. This suggests that interaction between the plume and surrounding asthenosphere results in significant cooling of the plume. Superimposed on this thermal pattern produced by plume-asthenosphere interaction is a tendency for melting to be less extensive and to occur at shallower depths to the south, presumably reflecting a decrease in ambient asthenospheric temperatures away from the Galapagos Spreading Center.Copyrighted by American Geophysical Union

Geochemistry of lavas from the 2005–2006 eruption at the East Pacific Rise, 9°46′N–9°56′N : implications for ridge crest plumbing and decadal changes in magma chamber compositions

Author Posting. © American Geophysical Union, 2010. 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 11 (2010): Q05T09, doi:10.1029/2009GC002977.Detailed mapping, sampling, and geochemical analyses of lava flows erupted from an ∼18 km long section of the northern East Pacific Rise (EPR) from 9°46′N to 9°56′N during 2005–2006 provide unique data pertaining to the short-term thermochemical changes in a mid-ocean ridge magmatic system. The 2005–2006 lavas are typical normal mid-oceanic ridge basalt with strongly depleted incompatible trace element patterns with marked negative Sr and Eu/Eu* anomalies and are slightly more evolved than lavas erupted in 1991–1992 at the same location on the EPR. Spatial geochemical differences show that lavas from the northern and southern limits of the 2005–2006 eruption are more evolved than those erupted in the central portion of the fissure system. Similar spatial patterns observed in 1991–1992 lavas suggest geochemical gradients are preserved over decadal time scales. Products of northern axial and off-axis fissure eruptions are consistent with the eruption of cooler, more fractionated lavas that also record a parental melt component not observed in the main suite of 2005–2006 lavas. Radiogenic isotopic ratios for 2005–2006 lavas fall within larger isotopic fields defined for young axial lavas from 9°N to 10°N EPR, including those from the 1991–1992 eruption. Geochemical data from the 2005–2006 eruption are consistent with an invariable mantle source over the spatial extent of the eruption and petrogenetic processes (e.g., fractional crystallization and magma mixing) operating within the crystal mush zone and axial magma chamber (AMC) before and during the 13 year repose period. Geochemical modeling suggests that the 2005–2006 lavas represent differentiated residual liquids from the 1991–1992 eruption that were modified by melts added from deeper within the crust and that the eruption was not initiated by the injection of hotter, more primitive basalt directly into the AMC. Rather, the eruption was driven by AMC pressurization from persistent or episodic addition of more evolved magma from the crystal mush zone into the overlying subridge AMC during the period between the two eruptions. Heat balance calculations of a hydrothermally cooled AMC support this model and show that continual addition of melt from the mush zone was required to maintain a sizable AMC over this time interval.This work has been supported by NSF grants OCE‐0525863 and OCE‐0732366 (D. J. Fornari and S. A. Soule), OCE‐0636469 (K. H. Rubin), and OCE‐ 0138088 (M. R. Perfit), as well as postdoctoral fellowship funds from the University of Florida

Igneous petrology, 2nd. ed/ McBirney

x, 503 hal.: ill.; 24 cm

What Is the Probability of Explosive Eruption at a Long-Dormant Volcano?

One of the most difficult problems we face in assessing volcanic hazards is that of evaluating the potential activity of volcanoes with little or no record of Holocene eruptions. Is there some minimum period of inactivity after which we can safely rule out a future eruption of large magnitude? Or, failing that, can we say how likely it is that such a volcano will return to activity within a particular span of time? Violent explosive eruptions are uncommon during the youthful stage of active growth. They are confined almost entirely to large mature volcanoes. Here, we use the global record of volcanic activity (Simkin & Siebert 1994) to evaluate the duration of repose intervals preceding such explosive volcanic eruptions. This analysis indicates that the hazard rate for explosive eruptions is not constant with time, but depends on the time since last eruption and that explosive eruptions may occur at volcanoes that have been quiescent for 10 ka or more. The techniques we employ are common to a class of problems in survival analysis (Cox & Oakes 1984; Woo 1999), and can be applied to a variety of hazard problems on volcanoes (e.g. Hill et al. 1998; Connor et al. 2003; Calder et al. 2005). One of the major lessons of this type of analysis is that applied statistical methods can teach us much about the time scales of volcanic activity, and about the underlying physical mechanisms governing these

Volcanic and Seismic Hazards at a Proposed Nuclear Power Site in Central Java

A nuclear power plant site has been proposed near the base of Mount Muria, a long-dormant volcano in Indonesia. Over a period of eight years the volcanic and seismic hazards were investigated, first by the contractor and later by a joint team of Indonesian geologists and consultants to the International Atomic Energy Agency. In order to assess the risk posed by a large volcano for which there is no record of historical eruptions, it was necessary to determine the age of the last activity by geological and geochronological means and to deduce from this whether the volcano posed a credible risk. Similarly, because there was no adequate record of seismic activity, the seismic hazards were investigated mainly by geological, geomorphological, and geophysical methods that identified and characterized potential seismogenic sources related to the volcano or tectonic movements (i.e. active/capable faults). Muria Volcano has not erupted since about two thousand years ago, but the last activity was sufficiently recent to rule out any assumption that the volcano is extinct. Detailed studies indicated that the proposed site may be vulnerable to the effects of air-borne tephra, pyroclastic flows and surges, debris flows, lahars, and opening of new vents. A more serious factor, however, was the poor geotechnical properties of the foundation material that required a careful analysis of the seismic hazards. Although the project was suspended, the study proved useful, because it provided an opportunity to develop procedures and techniques that could be applied in similar studies elsewhere