Nature Geosci. 2, 785–789 (2009); published online: 25 October 2009; corrected online: 30 October 2009. In the version of this Letter originally published, in Fig. 2 the elongated closed contour line should have been labelled +4, as shown here. This error has been corrected in the HTML and PDF versions.

The cause of high electrical conductivity in the middle crust beneath the Pacific Northwest region of the US is not clear. New electrical-resistivity data reveal a connection between this regional conductor and a localized conductor beneath a prominent volcano in the region, suggesting that the anomalous conductivity is due to the presence of partial melts.

Integrating isotopic microanalysis with other analytical techniques creates powerful new methodologies for understanding the evolution of rock samples at the sub-grain scale. Here we present Crystal Size Distribution (CSD) data for a 26,000 year old sample from Stromboli Volcano and accompanying isotopic microanalysis of the phenocrysts. A technique, called the ICSD plot, is introduced which given stated assumptions allows the integration of both sets of data to generate timelines of isotopic evolution through the volcanic system. The combined approach is powerful, allowing investigation of the magma supply, mixing, crystallisation and contamination processes prior to eruption of a volcanic sample. For Stromboli Volcano, the combined analysis suggests that the change in magma type following a cone collapse took roughly five years to complete, similar to the timescale of changes seen in recent decades. (C) 2007 Elsevier B.V. All rights reserved.

The use of crystal isotope ‘microstratigraphy’, through microanalysis for Sr isotopes, shows that interand intracrystalline isotopic and compositional heterogeneities exist within many volcanic rocks. Here we report Sr isotope data for sanidine, plagioclase, sphene, apatite, glass, biotite and hornblende crystals separated from representative samples of the 5000km, 28 Ma Fish Canyon Tuff and the pre-caldera Pagosa Peak Dacite, from the La Garita Caldera, San Juan Volcanic Field, U.S.A. Age-corrected bulk-rock Sr/Sr values define a small range (0.70632 to 0.70652), whereas interand intra-grain variations among plagioclase, sanidine and apatite crystals exhibit a much larger range (0.706070 to 70672). These ranges in Sr/Sr cannot be solely attributed to radiogenic ingrowth during residence in the Fish Canyon magma reservoir, since measured Rb/Sr are too low to significantly affect Sr/Sr over ~30Myrs. In contrast, biotite, glass and hornblende exhibit much higher agecorrected Sr isotope ratios (0.70692 to 0.70738), a wider range of Rb/Sr ratios and can be used to construct a Rb-Sr isochron which yields an eruption age of 28.18±0.86 Ma and an Sr/Sri of 0.707255. The Sr/Sri of the isochron is much higher than any of the Sr/Sri ratios recorded by the plagioclase, sanidine, apatite and sphene crystals, and is taken as evidence consistent with textural observations, that these crystals were remobilised from an older igneous protolith shortly prior to eruption. The biotite and hornblende crystals which appear to be in isotopic equilibrium with the glass (by virtue of defining the isochron) appear to have grown from a liquid generated at ~28Ma from partial melting of this protolith. Thus, we interpret the isotopic variations to represent open system processes in the generation of the Fish Canyon magma by the direct incorporation of xenocrystic phases from earlier periods of magmatism to produce an isotopically heterogeneous magma (and rock) at the mineral scale. 5.4.56

Volcán Ceboruco, Mexico, erupted ~1,000 years ago, producing the Jala pumice and forming a ~4-km-wide caldera. During that eruption, 2.8 to 3.5 km3 of rhyodacite (~70 wt% SiO2) magma and 0.2 to 0.5 km3 of mixed dacite (~67 wt% SiO2) magma were tapped and deposited as the Jala pumice. Subsequently, the caldera was partially filled by extrusion of the Dos Equis dome, a low-silica (~64 wt% SiO2) dacite dome with a volume of ~1.3 km3. Petrographic evidence indicates that the Jala dacite and Dos Equis dacite originated largely through the mixing of three end-member magmas: (1) rhyodacite magma, (2) dacite magma, and (3) mafic magma. Linear least-squares modeling and detailed modal analysis indicate that the Jala dacite is predominantly a bimodal mixture of rhyodacite and dacite with a small additional mafic component, whereas the Dos Equis dacite is composed of mostly dacite mixed with subordinate amounts of rhyodacite and mafic magma. According to Fe–Ti oxide geothermometry, before the caldera-forming eruption the rhyodacite last equilibrated at ~865 °C, whereas the dacite was originally at ~890 °C but was heated to ~960 °C by intrusion of mafic magma as hot as ~1,030 °C. Zoning profiles in plagioclase and/or magnetite phenocrysts indicate that mixing between mafic and dacite magma occurred ~34–47 days prior to eruption, whereas subsequent mixing between rhyodacite and dacite magmas occurred only 1–4 days prior to eruption. Following the caldera-forming eruption, continued inputs of mafic magma led to effusion of the Dos Equis dome dacite. In this case, timing between mixing and eruption is estimated at ~93–185 days based on the thickness of plagioclase overgrowth rims.