Analysis of volcanic activity patterns using MODIS thermal alerts

We investigate eruptive activity by analysis of thermal-alert data from the MODIS (moderate resolution imaging spectrometer) thermal infrared satellite instrument, detected by the MODVOLC (MODIS Volcano alert) algorithm. These data are openly available on the Internet, and easy to use. We show how such data can plug major gaps in the conventional monitoring record of volcanoes in an otherwise generally poorly documented region (Melanesia), including: characterising the mechanism of lava effusion at Pago; demonstrating an earlier-than-realised onset of lava effusion at Lopevi; extending the known period of lava lake activity at Ambrym; and confirming ongoing activity at Bagana, Langila and Tinakula. We also add to the record of activity even at some generally better-monitored volcanoes in Indonesia, but point out that care must be taken to recognise and exclude fires.Editorial responsibility: J Stix     

Pro-poor sanitation technologies

It is estimated that at least two billion people have inadequate sanitation. The current situation in water and sanitation services for millions of peri-urban residents is starkly anti-poor and represents a major challenge for the 21st century. By virtue of its cost and water requirements, we would argue that conventional sewerage is an implicitly anti-poor technology. This paper summarises low-cost sanitation technologies that have been developed by engineers from around the world, and seeks to provide evidence that there is such a thing as a pro-poor technology. We argue that simplified sewerage is often the only sanitation technology that is technically feasible and economically appropriate for low income, high-density urban areas. Simplified sewerage will only truly be a pro-poor technology if issues such as lack of investment in sanitation, insufficient cost recovery for sanitation services, conservative technical standards favoured over innovation, low-cost technologies perceived as second class provision, the nature of peri-urban settlements, and lack of engagement with users, are addressed. So often, peri-urban sanitation schemes fail to exist, fail to be sustainable, or fail to be pro-poor. The challenge is for engineers, social scientists and other professionals to work together to make pro-poor sanitation a reality and interdisciplinarity the norm.     

Terminal Pleistocene/Early Holocene Environmental Change at the Sunshine Locality, North-Central Nevada, U.S.A.

Sedimentological, faunal, and archaeological investigations at the Sunshine Locality, Long Valley, Nevada reveal a history of human adaptation and environmental change at the last glacial–interglacial transition in North America's north-central Great Basin. The locality contains a suite of lacustrine, alluvial, and eolian deposits associated with fluvially reworked faunal remains and Paleoindian artifacts. Radiocarbon-dated stratigraphy indicates a history of receding pluvial lake levels followed by alluvial downcutting and subsequent valley filling with marsh-like conditions at the end of the Pleistocene. A period of alluvial deposition and shallow water tables (9,800 to 11,000 ¹⁴C yr B.P.) correlates to the Younger Dryas. Subsequent drier conditions and reduced surface runoff mark the early Holocene; sand dunes replace wetlands by 8,000 ¹⁴C yr B.P. The stratigraphy at Sunshine is similar to sites located 400 km south and supports regional climatic synchroneity in the central and southern Great Basin during the terminal Pleistocene/early Holocene. Given regional climate change and recurrent geomorphic settings comparable to Sunshine, we believe that there is a high potential for buried Paleoindian features in primary association with extinct fauna elsewhere in the region yet to be discovered due to limited stratigraphic exposure and consequent low visibility.     

Swedish Ordovician limestones: Problems in clarifying their directions of magnetizations

The easterly region of the Swedish Ordovician limestones (the Orthoceras Limestone) shows a fairly stable bulk magnetization of high positive (downward) inclination. A closer scrutiny reveals, however, that this magnetization is of polyphase structure, involving at least two axes of magnetization. The components have similar stability spectra, and alternating field demagnetization cannot separate them. Thermal demagnetization is more effective but no stable end-point has been reached.     

Transects of the ancient and modern continental margins of eastern Canada

Rocks of the west flank of the northern Appalachian Orogen (miogeocline) record the history of the late Precambrian-early Paleozoic passive continental margin of Eastern North America. The ancient margin was destroyed by ophiolite obduction and arc collision during the Ordovician Taconic Orogeny. The present sinuous form of the miogeocline is interpreted to reflect ancient promontories and re-entrants of a previous orthogonal margin bounded by rifts and transforms.Four major terranes are recognized east of the miogeocline in Newfoundland and Nova Scotia. From west to east, these are the Dunnage, Gander, Avalon and Meguma. The Dunnage and Gander terranes were linked to the miogeocline during the Middle Ordovician Taconian Orogeny. The Avalon terrane arrived later, possibly during the mid-Paleozoic Acadian Orogeny. The Meguma terrane of southern Nova Scotia had docked with the Avalon terrane by Carboniferous time. The Dunnage terrane contains arc volcanics which lie above an ophiolitic substrate. The Gander terrane comprises a thick sequence of clastic sedimentary rocks, underlain by basement rocks with continental affinities. It has been interpreted as a continental margin, perhaps once on the eastern side of the Paleozoic Iapetus ocean. The Avalon terrane consists of belts of sedimentary and volcanic rocks which are probably underlain by Grenvillian basement. Its tectonic affinities are unclear. The Meguma terrane comprises a thick sequence of sediments, derived from the south-east. It is found only in southeastern Atlantic Canada. The boundaries between terranes are compressional in the west and steep, transcurrent faults in the east.The surface extent of the geological terranes is grossly correlative with deep structural zones, although no direct evidence exists for linking the two because most surface structures can be traced geophysically to only a few kilometres depth. A striking feature of the deep crustal structure is a lower, high velocity crustal layer beneath the Dunnage and Gander terranes.The modern margin of Atlantic Canada developed by rifting and by transform motion between adjacent continents. Stretching and thinning of the lithosphere, and the consequent production of basaltic magma that in places intrudes or underplates the thinned continental crust, are the most likely processes responsible for the evolution of the modern margin. These processes predict the observed deep sedimentary basins along the margin, the thinning of continental crust, and the high seismic velocities found within the ocean-continent transition zones.Rifting adjacent to Nova Scotia began in Late Triassic-Early Jurassic time between the present African and North American plates. These plate motions are also responsible for the major transform margin south of the Grand Banks. Separation between Iberia and the eastern Grand Banks occurred in mid-Cretaceous time, before the Late Cretaceous opening of the Labrador Sea. While the rifted segments of the margin exhibit deep sedimentary basins and thinned continental crust, the Grand Banks transform segment is characterized by a sharp transition zone and a relatively thin sediment cover. Numerous volcanic seamounts are built on the ocean crust adjacent to this transform segment.Mimicry of Paleozoic promontories and re-entrants by modern rift and transform margin segments, the location of Mesozoic sedimentary basins on ancestral Appalachian structures, and the reactivation and propagation of major Precambrian and Paleozoic structural boundaries during the latest phase of ocean opening attest to ancestral controls of the modern margins.The rift phase of both the ancient and modern passive margins is characterized by volcanism, mafic dike intrusion and by the development of basins filled with clastic sediments. The drift phase of both the ancient margin and the present Nova Scotia margin is marked by a change in sedimentary environment, such that carbonates replaced the rift phase clastic sediments. Two of the markers used to delineate the ancient ocean-continent transition zone; carbonate banks and steep gravity anomaly gradients, should be used with caution as the modern analogs of these markers may lie 100 km or more of this transition zone. Furthermore, it is naive to view the ancient transition as simple and narrow, for the modern margins exhibits complex transition zones between 30 and 300 km wide.In general, the evolution of the ancient and modern passive margins appear to be remarkably similar. Predictably, closing the present Atlantic will mimic the evolution of the Appalachian Orogen.     

Formation of sapphirine during retrogression of a basic high-pressure granulite,Roan, Western Gneiss Region,Norway

Sapphirine-bearing rocks occur in the northern part of the Western Gneiss Region, Vestranden, central Norway. The sapphirine-bearing rocks are characterized by a high MgO/(MgO + FeO) ratio, high Al₂O₃, MgO and CaO, and low SiO₂ contents. These rocks form layers within larger complexes which originated as layered magmatic rocks. High PT-metamorphism produced a cpx+ky+gt assemblage. The P and T estimates are P = 14.5±2 kbar and T= 870±50° C. During retrogression, the high-P granulite assemblage broke down to form an intermediate-P granulite mineralogy comprising orthopyroxene, spinel, anorthite, andesine, sapphirine and corundum. Textural relationships suggest that sapphirine formed by the reaction: spinel+kyanite sapphirine+corundum, and probably also by a reaction between corundum, spinel and orthopyroxene. All reactions took place within the stability field of kyanite. Textural and micro-chemical relationships indicate equilibrium, conditions during the peak metamorphism, whereas pronounced disequilibrium characterizes the mineral associations formed during the early retrogression at low P _H2O. The investigation shows that parts of the northern segment of the Western Gneiss Region underwent a metamorphic evolution similar to the Caledonian one recorded from eclogite/granulite terrains further south.     

New Insights into Crustal Contributions to Large-volume Rhyolite Generation in the Mid-Tertiary Sierra Madre Occidental Province, Mexico, Revealed by U Pb Geochronology

Voluminous (3·9 x 10⁵ km³), prolonged (18 Myr) explosivesilicic volcanism makes the mid-Tertiary Sierra Madre Occidentalprovince of Mexico one of the largest intact silicic volcanicprovinces known. Previous models have proposed an assimilation–fractionalcrystallization origin for the rhyolites involving closed-systemfractional crystallization from crustally contaminated andesiticparental magmas, with <20% crustal contributions. The lackof isotopic variation among the lower crustal xenoliths inferredto represent the crustal contaminants and coeval Sierra MadreOccidental rhyolite and basaltic andesite to andesite volcanicrocks has constrained interpretations for larger crustal contributions.Here, we use zircon age populations as probes to assess crustalinvolvement in Sierra Madre Occidental silicic magmatism. Laserablation-inductively coupled plasma-mass spectrometry analysesof zircons from rhyolitic ignimbrites from the northeasternand southwestern sectors of the province yield U–Pb agesthat show significant age discrepancies of 1–4 Myr comparedwith previously determined K/Ar and ⁴⁰Ar/³⁹Ar ages from thesame ignimbrites; the age differences are greater than the errorsattributable to analytical uncertainty. Zircon xenocrysts withnew overgrowths in the Late Eocene to earliest Oligocene rhyoliteignimbrites from the northeastern sector provide direct evidencefor some involvement of Proterozoic crustal materials, and,potentially more importantly, the derivation of zircon fromMesozoic and Eocene age, isotopically primitive, subduction-relatedigneous basement. The youngest rhyolitic ignimbrites from thesouthwestern sector show even stronger evidence for inheritancein the age spectra, but lack old inherited zircon (i.e. Eoceneor older). Instead, these Early Miocene ignimbrites are dominatedby antecrystic zircons, representing >33 to 100% of the datedpopulation; most antecrysts range in age between 20 and 32 Ma.A sub-population of the antecrystic zircons is chemically distinctin terms of their high U (>1000 ppm to 1·3 wt %) andheavy REE contents; these are not present in the Oligocene ignimbritesin the northeastern sector of the Sierra Madre Occidental. Thecombination of antecryst zircon U–Pb ages and chemistrysuggests that much of the zircon in the youngest rhyolites wasderived by remelting of partially molten to solidified igneousrocks formed during preceding phases of Sierra Madre Occidentalvolcanism. Strong Zr undersaturation, and estimations for veryrapid dissolution rates of entrained zircons, preclude coevalmafic magmas being parental to the rhyolite magmas by a processof lower crustal assimilation followed by closed-system crystalfractionation as interpreted in previous studies of the SierraMadre Occidental rhyolites. Mafic magmas were more probablyimportant in providing a long-lived heat and material flux intothe crust, resulting in the remelting and recycling of oldercrust and newly formed igneous materials related to Sierra MadreOccidental magmatism. KEY WORDS: ignimbrite; rhyolite; Sierra Madre Occidental; Tertiary; U–Pb geochronology; zircon; antecryst; crustal melting     

Multimillion year thermal history of a porphyry copper deposit: application of U–Pb, 40Ar/39Ar and (U–Th)/He chronometers, Bajo de la Alumbrera copper–gold deposit, Argentina

Application of multiple chronometers (including U–Pb and ⁴⁰Ar/³⁹Ar geochronology and zircon and apatite (U–Th)/He thermochronology) to porphyry intrusions at the Bajo de la Alumbrera porphyry copper–gold deposit, Argentina, reveals a complex history of reheating that spans millions of years. Previous U–Pb geochronology, combined with our new ⁴⁰Ar/³⁹Ar data, shows that the multiple porphyritic intrusions at Bajo de la Alumbrera were emplaced during two episodes, the first at about 8.0 Ma (P2 and associated porphyries) and the second about a million years later (Early and Late P3 porphyries). Complex overprinting alteration events have obscured the earliest hydrothermal history of the deposit. By contrast, ⁴⁰Ar/³⁹Ar data reveal the close temporal relationship of ore-bearing potassic alteration assemblages (7.12 ± 0.13 Ma; biotite) to the emplacement of the P3 intrusions. Consistent with low closure temperatures, younger ages have been determined for associated hydrothermal alkali feldspar (6.82 ± 0.05 Ma and 6.64 ± 0.09 Ma). The temperature-sensitive Ar data also record an unexpected prolonged cooling history (to below 200°C) extending to 5.9 Ma. Our data suggest that the Bajo de la Alumbrera system underwent protracted cooling, after the collapse of the main hydrothermal system, or that one or more low-temperature (~100–200°C) reheating events occurred after emplacement of the porphyritic intrusions at Bajo de la Alumbrera. These have been constrained in part by our new ⁴⁰Ar/³⁹Ar data (including multidomain diffusion modeling) and (U–Th)/He ages. Single-grain (U–Th)/He ages (n = 5) for phenocrystic zircon from P2 and P3 intrusive phases bracket these thermal events to between 6.9 (youngest crystallization of intrusion) and 5.1 Ma. Multidomain modeling of alkali feldspar data (from both igneous and hydrothermal crystals) is consistent with the deposit cooling rapidly from magmatic temperatures to below about 300°C, with a more protracted history down to 150°C. We conclude that the late-stage low-temperature (150 to 200°C) thermal anomaly localized at Bajo de la Alumbrera resulted from radiation of heat and/or fluids sourced from deeper-seated magma bodies, emplaced beneath the deposit. To produce the observed thermal longevity of the porphyry system, magma bodies underlying the Bajo de la Alumbrera deposit must have been repeatedly replenished by new magma batches. Without replenishment, crystallization of the source magma will occur, and heat release will stop, leading to rapid cooling (in less than ten thousand years). The influx of deep-seated magma may have caused the development of late low-temperature hydrothermal alteration assemblages at Bajo de la Alumbrera, at the same time that mineralization formed at Agua Rica, some 25 km away. All available chronologic data for the Bajo de la Alumbrera deposit suggest that the hydrothermal system was active episodically over at least a three-million and possibly up to a four-million-year period. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.