Compositional and Dynamic Controls on Mafic—Silicic Magma Interactions at Continental Arc Volcanoes: Evidence from Cordón El Guadal, Tatara-San Pedro Complex, Chile

Heterogeneous andesitic and dacitic lavas on Cordón El Guadal bear on the general problem of how magmas of differing compositions and physical properties interact in shallow reservoirs beneath continental arc volcanoes. Some of the lavas contain an exceptionally large proportion (<40%) of undercooled basaltic andesitic magma in various states of disaggregation. Under-cooled mafic magma occurs in the silicic lavas as large (<40 cm) basaltic andesitic magmatic inclusions, as millimeter-sized crystal-clots of Mg-rich olivine phenocrysts plus adhering Carich plagioclase microphenocrysts (An50-70), and as uniformly distributed, isolated phenocrysts and microphenocrysts. Compositions and textures of plagioclase phenocrysts indicate that inclusion-forming magmas are hybrids formed by mixing basaltic and dacitic melts, whereas textural features and compositions of groundmass phases indicate that the andesitic and dacitic lavas are largely mechanical mixtures of dacitic magma and crystallized basaltic andesitic magma. This latter observation is significant because it indicates that mechanical blending of undercooled mafic magma and partially crystallized silicic magma is a possible mechanism for producing the common porphyritic texture of many calc-alkaline volcanic rocks. The style of mafic-silicic magma interaction at Cordon El Guadal was strongly dependent upon the relative proportions of the endmembers. Equally important in the Guadal system, however, was the manner in which the contrasting magmas were juxtaposed. Textural evidence preserved in the plagioclase phenocrysts indicates that the transition from liquid-liquid to solid-liquid mixing was not continuous, but was partitioned into periods of magma chamber recharge and eruption, respectively. Evidently, during periods of recharge, basaltic magmas rapidly entrained small amounts of dacitic magma along the margins of a turbulent injection fountain. Conversely, during periods of eruption, dacitic magma gradually incorporated small parcels of basaltic andesitic magma. Thus, the coupled physical-chemical transition from mixed inclusions to commingled lavas is presumably not coincidental. More likely, it probably provides a partial record of the dynamic processes occurring in shallow magma chambers beneath continental are volcanoe

PARAS program: Phased array radio astronomy from space

An orbiting radio telescope is proposed which, when operated in a Very Long Baseline Interferometry (VLBLI) scheme, would allow higher (than currently available) angular resolution and dynamic range in the maps, and the ability of observing rapidly changing astronomical sources. Using a passive phases array technology, the proposed design consists of 656 hexagonal modules forming a 150 meter diameter dish. Each observatory module is largely autonomous, having its own photovoltaic power supply and low-noise receiver and processor for phase shifting. The signals received by the modules are channeled via fiber optics to the central control computer in the central bus module. After processing and multiplexing, the data is transmitted to telemetry stations on the ground. The truss frame supporting each observatory pane is a hybrid structure consisting of a bottom graphite/epoxy tubular triangle and rigidized inflatable Kevlar tubes connecting the top observatory panel and bottom triangle. Attitude control and stationkeeping functions are performed by a system of momentum wheels in the bus and four propulsion modules located at the compass points on the periphery of the observatory dish. Each propulsion module has four monopropellant thrusters and six hydrazine arcjets, the latter supported by a nuclear reactor. The total mass of the spacecraft is 22,060 kg

Contemporaneous Trachyandesitic and Calc-alkaline Volcanism of the Huerto Andesite, San Juan Volcanic Field, Colorado, USA

Locally, voluminous andesitic volcanism both preceded and followed large eruptions of silicic ash-flow tuff from many calderas in the San Juan volcanic field. The most voluminous post-collapse lava suite of the central San Juan caldera cluster is the 28 Ma Huerto Andesite, a diverse assemblage erupted from at least 5-6 volcanic centres that were active around the southern margins of the La Garita caldera shortly after eruption of the Fish Canyon Tuff. These andesitic centres are inferred, in part, to represent eruptions of magma that ponded and differentiated within the crust below the La Garita caldera, thereby providing the thermal energy necessary for rejuvenation and remobilization of the Fish Canyon magma body. The multiple Huerto eruptive centres produced two magmatic series that differ in phenocryst mineralogy (hydrous vs anhydrous assemblages), whole-rock major and trace element chemistry and isotopic compositions. Hornblende-bearing lavas from three volcanic centres located close to the southeastern margin of the La Garita caldera (Eagle Mountain-Fourmile Creek, West Fork of the San Juan River, Table Mountain) define a high-K calc-alkaline series (57-65 wt % SiO2) that is oxidized, hydrous and sulphur rich. Trachyandesitic lavas from widely separated centres at Baldy Mountain-Red Lake (western margin), Sugarloaf Mountain (southern margin) and Ribbon Mesa (20 km east of the La Garita caldera) are mutually indistinguishable (55-61 wt % SiO2); they are characterized by higher and more variable concentrations of alkalis and many incompatible trace elements (e.g. Zr, Nb, heavy rare earth elements), and they contain anhydrous phenocryst assemblages (including olivine). These mildly alkaline magmas were less water rich and oxidized than the hornblende-bearing calc-alkaline suite. The same distinctions characterize the voluminous precaldera andesitic lavas of the Conejos Formation, indicating that these contrasting suites are long-term manifestations of San Juan volcanism. The favoured model for their origin involves contrasting ascent paths and differentiation histories through crustal columns with different thermal and density gradients. Magmas ascending into the main focus of the La Garita caldera were impeded, and they evolved at greater depths, retaining more of their primary volatile load. This model is supported by systematic differences in isotopic compositions suggestive of crust-magma interactions with contrasting lithologie

Approaches to Improving School Attendance: Insights From Australian Principals

School absenteeism has been concerning educators in the Global North (including Australia) as research suggests a relationship between school attendance, academic achievement and subsequent life chances. This paper focuses on the perspectives of 50 school leaders in Queensland, Australia about approaches to improving attendance. Strategies reflected the cultural, economic and social diversity of their school communities. In general, quality curricula and pedagogies were considered important, but were not explicitly linked to attendance. This suggests the need for schools to develop strategies to enhance student engagement in meaningful learning through quality curricula and pedagogies within a positive school environment

Petrological Insights into Shifts in Eruptive Styles at Volcán Llaima (Chile)

Tephra and lava pairs from two summit eruptions (ad 2008 and 1957) and a flank fissure eruption (∼ ad 1850) are compared in terms of textures, phenocryst contents, and mineral zoning patterns to shed light on processes responsible for the shifts in eruption style during typical eruptive episodes at Volcán Llaima (Andean Southern Volcanic Zone, Chile). The mineralogy and whole-rock compositions of tephra and lavas are similar within eruptive episodes, suggesting a common magma reservoir for Strombolian paroxysms and lava effusion. The zoning profiles and textures of plagioclase record successive and discrete intrusions of volatile-rich mafic magma accompanied by mixing of these recharge magmas with the resident basaltic-andesitic crystal mushes that are commonly present at shallow levels in the Llaima system. Each recharge event destabilizes the plagioclase in equilibrium with the resident crystal mush melt and stabilizes relatively An-rich plagioclase, as is recorded by the numerous resorption zones. Lavas typically have ∼15-20 vol. % more phenocrysts than the tephra. Differences in plagioclase and olivine textures and zoning, combined with different phenocryst contents, indicate that a greater volume fraction of recharge magma is present in the explosively erupted magma than in subsequent effusively erupted magma. We propose that Strombolian paroxysms at Volcán Llaima are triggered by interactions with large volume fractions of recharge magma, which decrease the bulk viscosity and increase the volatile contents of the erupted magmas, leading to the conditions required for the fragmentation of basaltic-andesite. Lava effusion ensues from reduced interactions with the recharge magma, after it has partially degassed and crystallized, thereby impeding rapid ascent. This process could be operating at other steady-state basaltic volcanoes, wherein shallow reservoirs are periodically refilled by fresh, volatile-rich magma

Spectral Network (SpecNet)—What is it and why do we need it?

Effective integration of optical remote sensing with flux measurements across multiple scales is essential for understanding global patterns of surface–atmosphere fluxes of carbon and water vapor. SpecNet (Spectral Network) is an international network of cooperating investigators and sites linking optical measurements with flux sampling for the purpose of improving our understanding of the controls on these fluxes. An additional goal is to characterize disturbance impacts on surface–atmosphere fluxes. To reach these goals, key SpecNet objectives include the exploration of scaling issues, development of novel sampling tools, standardization and intercomparison of sampling methods, development of models and statistical methods that relate optical sampling to fluxes, exploration of component fluxes, validation of satellite products, and development of an informatics approach that integrates disparate data sources across scales. Examples of these themes are summarized in this review

The Upper Crustal Evolution of a Large Silicic Magma Body: Evidence from Crystal-scale Rb-Sr Isotopic Heterogeneities in the Fish Canyon Magmatic System, Colorado

Batholith-sized bodies of crystal-rich magmatic ‘mush' are widely inferred to represent the hidden sources of many large-volume high-silica rhyolite eruptive units. Occasionally these mush bodies are ejected along with their trapped interstitial liquid, forming the distinctive crystal-rich ignimbrites known as ‘monotonous intermediates'. These ignimbrites are notable for their combination of high crystal contents (35-55%), dacitic bulk compositions with interstitial high-silica rhyolitic glass, and general lack of compositional zonation. The 5000 km3 Fish Canyon Tuff is an archetypal eruption deposit of this type, and is the largest known silicic eruption on Earth. Ejecta from the Fish Canyon magmatic system are notable for the limited compositional variation that they define on the basis of whole-rock chemistry, whereas ∼ 45 vol. % crystals in a matrix of high-silica rhyolite glass together span a large range of mineral-scale isotopic variability (microns to millimetres). Rb/Sr isotopic analyses of single crystals (sanidine, plagioclase, biotite, hornblende, apatite, titanite) and sampling by micromilling of selected zones within glass plus sanidine and plagioclase crystals document widespread isotopic disequilibrium at many scales. High and variable 87Sr/86Sri values for euhedral biotite grains cannot be explained by any model involving closed-system radiogenic ingrowth, and they are difficult to rationalize unless much of this radiogenic Sr has been introduced at a late stage via assimilation of local Proterozoic crust. Hornblende is the only phase that approaches isotopic equilibrium with the surrounding melt, but the melt (glass) was isotopically heterogeneous at the millimetre scale, and was therefore apparently contaminated with radiogenic Sr shortly prior to eruption. The other mineral phases (plagioclase, sanidine, titanite, and apatite) have significantly lower 87Sr/86Sri values than whole-rock values (as much as −0·0005). Such isotopic disequilibrium implies that feldspars, titanite and apatite are antecrysts that crystallized from less radiogenic melt compositions at earlier stages of magma evolution, whereas highly radiogenic biotite xenocrysts and the development of isotopic heterogeneity in matrix melt glass appear to coincide with the final stage of the evolution of the Fish Canyon magma body in the upper crust. Integrated petrographic and geochemical evidence is consistent with pre-eruptive thermal rejuvenation of a near-solidus mineral assemblage from ∼720 to 760°C (i.e. partial dissolution of feldspars + quartz while hornblende + titanite + biotite were crystallizing). Assimilation and blending of phenocrysts, antecrysts and xenocrysts reflects chamber-wide, low Reynolds number convection that occurred within the last ∼10 000 years before eruptio

Spectral Network (SpecNet)—What is it and why do we need it?

Effective integration of optical remote sensing with flux measurements across multiple scales is essential for understanding global patterns of surface–atmosphere fluxes of carbon and water vapor. SpecNet (Spectral Network) is an international network of cooperating investigators and sites linking optical measurements with flux sampling for the purpose of improving our understanding of the controls on these fluxes. An additional goal is to characterize disturbance impacts on surface–atmosphere fluxes. To reach these goals, key SpecNet objectives include the exploration of scaling issues, development of novel sampling tools, standardization and intercomparison of sampling methods, development of models and statistical methods that relate optical sampling to fluxes, exploration of component fluxes, validation of satellite products, and development of an informatics approach that integrates disparate data sources across scales. Examples of these themes are summarized in this review