ALBERT

Description: Rocks from the 23 Ma Lake City caldera show diverse chemical affinities attesting to a complex magmatic system beneath the caldera. Field and geochemical data from ignimbrites and intrusions constrain magma storage and magma interactions during the formation of the caldera. Two geochemically distinct magma batches erupted during caldera formation: batch A, consisting of rhyolites and trachytes, and batch B, consisting of dacites and trachyandesites. The ignimbrites of the Lower, Middle, and Upper Sunshine Peak Tuff represent the bulk of erupted batch A magma, with an increasing proportion of trachyte to rhyolite as the eruption progressed. Overall, the observed trends of major and trace elements are consistent with the sequential eruption of a magmatic system with a rhyolitic upper portion and trachytic lower portion. The Middle Sunshine Peak Tuff contains two distinct types of pumice clast, while the Upper Sunshine Peak Tuff contains four distinct pumice clast types, with one type chemically related to batch B magma. The link between the rhyolite and trachyte of batch A is supported by major- and trace-element geochemical modeling of an initially trachytic magma that fractionated and was subjected to crystal/melt segregation following 50%–60% crystallization. Compositional gaps and chemical heterogeneity in the bulk ignimbrite composition show that the proportions of these different magma types varied significantly during eruption. We propose that the fractionating batch A and B magmas formed distinct magma pods, some containing residual magma mush, that were tapped during different phases of caldera formation. After collapse, dacite lavas of batch B were erupted concurrent with resurgent uplift from shallow intrusion of both residual mingled batch A and batch B magma. In summary, our observations suggest (1) a complex magma chamber geometry from two fractionating magma batches, and (2) magma replenishment and accelerated periods of magma reorganization in the shallow magma plumbing system during a single caldera cycle at Lake City.

Description: A series of Cu-substituted goethites, single and co-substituted with Cr, Zn, Cd and/or Pb was prepared, having molar ratios equal to 2.00, 3.33 and 5.00 mol%. All the samples contained only goethite, except Cu-, (Cu,Zn)- and (Cu,Pb)-samples synthesized at 5.00 mol% where hematite was also formed. The presence of Cr/Cd suppressed the hematite-forming effects of Cu. The general sequence of metal entry into the single-metal-substituted goethites was Zn = Cr 〉 Cd 〉 Cu 〉 Pb and in di- (5.00 mol%) and tri- (3.33 mol%) metal-substituted goethites was Cu 〉 Zn 〉 Cd 〉 Cr 〉〉 Pb. Cu incorporation increased all the unit-cell parameters in single-metal-substituted goethite, and these parameters increased in combination with other metals as follows: Cd 〉 Zn 〉 Cr 〉 Pb in the multimetal-substituted goethites. The Cu-substituted goethite dissolved faster than pure goethite. Co substitutions of Cr/Pb reduced the dissolution rate (kFe), while substitutions of Cd/Zn increased kFe.

Description: The unexpected intersection of rhyolitic magma and retrieval of quenched glass particles at the Iceland Deep Drilling Project-1 geothermal well in 2009 at Krafla, Iceland, provide unprecedented opportunities to characterize the genesis, storage, and behavior of subsurface silicic magma. In this study, we analyzed the complete time series of glass particles retrieved after magma was intersected, in terms of distribution, chemistry, and vesicle textures. Detailed analysis of the particles revealed them to represent bimodal rhyolitic magma compositions and textures. Early-retrieved clear vesicular glass has higher SiO2, crystal, and vesicle contents than later-retrieved dense brown glass. The vesicle size and distribution of the brown glass also reveal several vesicle populations. The glass particles vary in δD from −120‰ to −80‰ and have dissolved water contents spanning 1.3−2 wt%, although the majority of glass particles exhibit a narrower range. Vesicular textures indicate that volatile overpressure release predominantly occurred prior to late-stage magma ascent, and we infer that vesiculation occurred in response to drilling-induced decompression. The textures and chemistry of the rhyolitic glasses are consistent with variable partial melting of host felsite. The drilling recovery sequence indicates that the clear magma (lower degree partial melt) overlays the brown magma (higher degree partial melt). The isotopes and water species support high temperature hydration of these partial melts by a mixed meteoric and magmatic composition fluid. The textural evidence for partial melting and lack of crystallization imply that magma production is ongoing, and the growing magma body thus has a high potential for geothermal energy extraction. In summary, transfer of heat and fluids into felsite triggered variable degrees of felsite partial melting and produced a hydrated rhyolite magma with chemical and textural heterogeneities that were then enhanced by drilling perturbations. Such partial melting could occur extensively in the crust above magma chambers, where complex intrusive systems can form and supply the heat and fluids required to re-melt the host rock. Our findings emphasize the need for higher resolution geophysical monitoring of restless calderas both for hazard assessment and geothermal prospecting. We also provide insight into how shallow silicic magma reacts to drilling, which could be key to future exploration of the use of magma bodies in geothermal energy.