Granulite and pyroxenite xenoliths from the Deccan Trap: insight into the nature and composition of the lower lithosphere beneath cratonic India

Granulite and pyroxenite xenoliths in lamprophyre dykes intruded during the waning stage of Deccan Trap volcanism are derived from the lower crust beneath the Dharwar craton of Western India. The xenolith suite consists of plagioclase-poor mafic granulites (55% of the total volume of xenoliths), plagioclase-rich felsic granulites (25%), and ultramafic pyroxenites and websterites (20%) with subordinate wehrlites. Rare spinel peridotite xenoliths are also present, representing mantle lithosphere. The high Mg #, low SiO₂/Al₂O₃ and low Nb/La (<1) ratios suggest that the protoliths of the mafic granulites broadly represent cumulates of sub-alkaline magmas. All of the granulites are peraluminous and light rare-earth element-enriched. The felsic granulites may have resulted from anatexis of the mafic lower crustal rocks; thus, the mafic granulites are enriched in Sr whereas the felsic ones are depleted. Composite xenoliths consisting of mafic granulites traversed by veins of pyroxenite indicate intrusion of the granulitic lower crust by younger pyroxenites. Petrography and geochemistry of the latter (e.g. presence of phlogopite) indicate the metasomatised nature of the deep crust in this region.Thermobarometric estimates from phase equilibria indicate equilibration conditions between 650 and 1200 °C, 0.7-1.2 GPa suggestive of lower crustal environments. These estimates provide a spatial context for the sampled lithologies thereby placing constraints on the interpretation of geophysical data. Integration of xenolith-derived P-T results with Deep Seismic Soundings (DSS) data suggests that the pyroxenites and websterites are transitional between the lower crust and the upper mantle. A three-layer model for the crust in western India, derived from the xenoliths, is consistent with DSS data. The mafic nature of this hybrid lower crust contrasts with the felsic lower crustal composition of the south Indian granulite terrain.     

Development of a Continental Volcanic Field: Petrogenesis of Pre-caldera Intermediate and Silicic Rocks and Origin of the Bandelier Magmas, Jemez Mountains (New Mexico, USA)

The Miocene–Quaternary Jemez Mountains volcanic field(JMVF) is the site of the Valles caldera and associated BandelierTuff. Caldera formation was preceded by > 10 Myr of volcanismdominated by intermediate composition rocks (57–70% SiO₂)that contain components derived from the lithospheric mantleand Precambrian crust. Simple mixing between crust-dominatedsilicic melts and mantle-dominated mafic magmas, fractionalcrystallization, and assimilation accompanied by fractionalcrystallization are the principal mechanisms involved in theproduction of these intermediate lavas. A variety of isotopicallydistinct crustal sources were involved in magmatism between13 and 6 Ma, but only one type (or two very similar types) ofcrust between 6 and 2 Ma. This long history constitutes a recordof accommodation of mantle-derived magma in the crust by meltingof country rock. The post-2 Ma Bandelier Tuff and associatedrhyolites were, in contrast, generated by melting of hybridizedcrust in the form of buried, warm intrusive rocks associatedwith pre-6 Ma activity. Major shifts in the location, styleand geochemical character of magmatism in the JMVF occur withina few million years after volcanic maxima and may correspondto pooling of magma at a new location in the crust followingsolidification of earlier magma chambers that acted as trapsfor basaltic replenishment. KEY WORDS: crustal anatexis; fractional crystallization; Jemez Mountain Volcanic Field; Valles Caldera; radiogenic isotopes; trace elements     

Arsenic contamination in groundwater and its proposed remedial measures

Arsenic contamination occurs in groundwater of Bangladesh mainly from the alluvial and deltaic sediments. Arsenic contamination of groundwater in Bangladesh was first detected more than a decade ago and the ’shallow tubewells’ were reported as the main source of arsenic contaminated water. From the nutritional and metabolic points of view, arsenic is likely to adversely affect human health and nutrition. Up to now, several studies have been carried out on this context; however, inadequate knowledge on arsenic sources, mobilization and transport still remains as a constraint due to lack of data, information and technological advances. Thus, a review study has been undertaken on the sources of arsenic, its causes, mobilization, transport, effects on human health, arsenic test procedures and removal methods, in the context of groundwater contamination in Bangladesh, and finally sustainable remedial measures of arsenic have been proposed. This study suggests that laboratory facilities for testing of arsenic and effects of enhanced groundwater pumping, phosphate fertilizer etc., need to be updated, expanded and studied. This review work is significant to further knowledge improvement, as the topic is general and worldwide. It can be concluded that the integration of the proposed remedial measures with the national geographic information system interface database relating to arsenic for analysis, production of hazard maps, and dissemination on television show for the planners, engineers, managers, field supervisors and affected people, can reach at the sustainable solution for mitigating arsenic and associated problems successfully in Bangladesh.     

A topical evaluation and discussion of data movement technologies for data-intensive scientific applications

Transferring large volumes of information from one location to potentially many others that are geographically distributed and across varying networks is still prevalent in modern scientific data systems. This is despite the movement to push computation to the data and to reduce data movement needed to compute answers to challenging scientific problems, to disseminate information to the scientific community, and to acquire data for curation and enrichment. Because of this, it is imperative that decisions made regarding data movement systems and architectures be backed by both analytical rigor, and also by empirical evidence and measurement. The purpose of this study is to expand on the work performed by our research team over the last decade and to take a fresh look at the evaluation of multiple topical data transfer technologies in use cases derived from data-intensive scientific systems and applications in the areas of Earth science. We report on the evaluation of a set of data movement technologies against a set of empirically derived comparison dimensions. Based on this evaluation, we make recommendations towards the selection of appropriate data movement technologies in scientific applications and scenarios.     

Shale basins,sulfur-deficient ore brines and the formation of exhalative base metal deposits

The massive sulfide deposits of the Iberian Pyrite Belt are interbedded with felsic volcanic rocks and shale, and underlain by several thousand meters of siliciclastic sedimentary rocks known as the PQ Group. Isotope geochemistry and regional geology are both consistent with equilibration of the ore-forming fluids with the PQ Group, prior to ore deposition near the former seafloor. The average Cu:Zn:Pb ratio of the PQ Group rocks (ca. 26:55:19) is similar to the weighted average of all the massive sulfide orebodies combined (ca. 25:52:23).The genetic relationship between massive sulfide deposits and a siliciclastic sedimentary metal source is explained here by a thermodynamic model, proposing that mildly reducing redox conditions imposed by equilibration with the sedimentary rocks are most critical for the formation of an effective ore-forming fluid. Relatively metal-rich but organic-poor pyrite-bearing shale undergoing dewatering of saline pore fluids is an effective source for the generation of sulfur-deficient but relatively iron and base metal-rich brines. Thus, we propose that the giant deposits of the Iberian Pyrite Belt owe their existence not to exceptionally metal-enriched (e.g., magmatic) fluids, but to the existence of a fairly ordinary but large metal source in reactive siliciclastic sediments, combined with an underlying igneous heat source and a particularly efficient mechanism of sulfide precipitation by mixing with H₂S-rich fluids at or near the seafloor.Essentially similar mineral equilibria are imposed when saline fluids are buffered by typical continental basement rocks. Leaching of retrograde minerals and possibly residual salts from their magmatic or metamorphic prehistory is expected to generate similar, variably metal-rich but relatively sulfide-deficient fluids. Thus, the existence of mildly reducing rocks can be the dominant chemical control in the source of fluids generating many volcanogenic, Irish-type or sedex deposits, many of which are known to precipitate their metal load in response to biogenic sulfide addition at the ore deposition site.     

Chronology and organic chemistry of the Black Creek Swamp Megafauna site (Late Pleistocene), Kangaroo Island, Australia

The timing of the extinction of the Australian Megafauna and whether it was simultaneous and widespread has been a much researched topic in Quaternary geoscience. The Black Creek Swamp Megafauna site on Kangaroo Island was thought to be a refugium for Megafauna; however, recent and multidiscipline age determinations have established that the fossils are considerably older than the well-quoted extinction age of 45 kyr. Further radiocarbon age determinations, δ¹³C isotope analysis and ¹³C-NMR spectroscopy of the fossil containing organic matter demonstrates that it is highly soluble and accumulated as recently as 31–18 cal. kyr BP. These radiocarbon ages are much younger than the 100–50 kyr age bracket ascertained for the fossil material itself, implying separate episodes of death, deposition and burial. The soluble nature of the organic matter and increasing radiocarbon ages with depth suggests lateral accumulation, probably transported by subsurface waters from elevated areas proximal to the low-lying swamp. Such depositional conditions and ¹⁴C age range implies that the site may have experienced an unusually wet Last Glacial Maximum, due maybe to its proximity to the continental shelf and thus to maritime conditions. C3 vegetation dominates the Black Creek Swamp and its organic matter (δ¹³C; −30‰ and −23‰); however, variations in δ¹³C may indicate climatic shifts. ¹³C-enrichment and an abundance of salt-tolerant gastropods within the site's final phase of sediment accumulation (<6 cal. kyr BP) suggest that conditions during this most recent period were not as wet as those of the Last Glacial Maximum.     

TRACKING THE BABY BOOM,THE BABY BUST,AND THE ECHO GENERATIONS: HOW AGE COMPOSITION REGULATES US MIGRATION*

US regional and state migration data from the 1940s–80s, when members of the baby boom generation aged into their years of peak labor force mobility, suggest ways in which changing age composition regulates geographical mobility and interregional migration. Labor supply pressure plays a key role in the dynamics of the national migration system. A “delayed mobility” effect in the 1980s similar to the delayed fertility of the baby boom cohorts appears to be a result of the depressed rates of mobility experienced by members of this generation when they flooded regional labor markets with record numbers of entrants in the 1970s. Recent temporal shifts in age-specific volumes of interregional migration help predict the future pace of migration based upon the projected age distribution of the nation.     

Tectonic and Geodynamic Settings of Formation of the Mesozoic Cumulative Dunite–Wehrlite–Olivine Clinopyroxenite—Gabbro Massif in Eastern Chukotka

Geotectonics - The dunite–wehrlite–clinopyroxenite–gabbro massif in Eastern Chukotka, a key object for geodynamic reconstructions of the Vel’may terrane, which represents...