Petrochemistry of Jurassic and Cretaceous tholeiites from Kong Karls Land, Svalbard, and their relation to Mesozoic magmatism in the Arctic

A study of the whole-rock geochemistry and mineral chemistry of high-TiO₂ Upper Jurassic and medium-TiO₂ Lower Cretaceous basalts from Kong Karls Land, Svalbard, is presented. Geochemical criteria indicate that the basalts are initial rifting tholeiites with weak signs of crustal contamination. The Upper Jurassic basalts appear to be associated with the Olga Rift, part of a trans-Barents rift system which failed to link the proto-Atlantic and proto-Arctic basins. The Lower Cretaceous basalts may be more closely related to initial rifting tholeiites on Franz Josef Land and Spitsbergen generated during the rifting stage of opening of the Canada Basin. During break-up of the Barents Shelf, the sequence of magma types corresponds to the pre-, syn- and post-rifting stages established in other areas of continental break-up. Evidence for a possible hot-spot or plume trail, extending from Siberia to the Yermak Plateau over 250 Ma, is assembled.     

Monitoring of rapid salt weathering in the central Namib Desert using limestone blocks

Pre-weighed blocks of a Jurassic Limestone were exposed on the ground surface in the coastal Namib Desert for a period of 2 years. The environment is both salty and foggy. Some of the blocks suffered extensive disintegration, and laboratory analyses (including geochemistry, XRD and SEM) indicate that the weathered samples have a high halite (sodium chloride) content. Cycles of wetting and drying associated with the frequent fog events of the area cause cycles of halite crystallization. Rocks exposed at the surface absorb salts from the surrounding desert surface and then disintegrate, contributing to planation of the landscape.     

Trends In U.S. Hazardous Materials Transportation Spills

This paper examines historical and spatial trends in hazardous materials transportation spills from 1971 to 1991. While the number of spills Inc.reased steadily during the 1970s, peaking in 1978–1979, there has been a decline in frequency sInc.e then largely due to modifications in reporting. Monetary damages have the opposite temporal pattern, with major Inc.reases recorded from 1982 onward. Death and injury statistics are more variable. Spatially, accidents are more prevalent in the Rust Belt extending from the Northeast corridor westward to the Great Lakes states, as well as in the Southeast. The greatest potential risk to the public is found in smaller, more densely populated industrial states such as New Jersey, Delaware, and Maryland. The spatial distribution, however, has not changed. Potential risk sources (e.g., chemical industry, number of hazardous waste facilities, number of railroad miles) are the best predictors of hazmat Inc.ident frequency. Mitigation efforts (statewide regulatory and/or management policies) also help explain the variability in hazmat Inc.idents.     

Palaeosecular variation over the past 5 Myr based on a new generalized database

A generalized database of global palaeomagnetic data from 3719 lava flows and thin dykes of age 0–5 Ma has been constructed for use with a relational database management system. The database includes all data whose virtual geomagnetic poles (VGP) lie within 45 of the spin axis and can be used for studies of palaeosecular variation and for geomagnetic field modelling. Because many of these data were collected and processed more than 15–20 years ago, each result has been characterized according to the demagnetization procedures carried out. Analysis of these data in terms of the latitude variation of the angular dispersion of VGPs (palaeosecular variation from lavas) strongly suggests that careful data selection is required and that many of the older studies may need to be redone using more modern methods. Differences between the angular dispersions for separate normal- and reverse-polarity data sets confirm that many older studies have not been adequately cleaned magnetically. Therefore, the use of the database for geomagnetic field modelling should be carried out with some caution. Using a VGP cut-off angle that varies with latitude, the best data set consists of 2636 results that show a smooth increase of VGP angular dispersion with latitude. Model G for palaeosecular variation, which is based on modelling of the antisymmetric (dipole) and symmetric (quadrupole) dynamo families, provides a good fit to these results.     

Multifractal analysis of the spatial distribution of earthquakes in southern Italy

We evaluate the complete spectrum of the generalized fractal dimension of the spatial pattern of microearthquakes in Southern Italy, revealing a multifractal distribution structure. Our analysis is focused on the dependence of the multifractal distribution on the size of the selected area and the kind of seismicity in the area. As the size of the window varies, we observe that the capacity, information and correlation dimensions vary significantly, while both d _∞ and d _−infin; remain unchanged within their errors limits. We interpret this result in terms of the observation that our data are mainly clustered around a linear fault (the Sisifo fault). When we restrict the selected windows around the fault, clustering around a line (the fault) is highlighted. The capacity dimension changes from about 1.8 to about 1.4 and the correlation dimension decreases because we observe in detail the clustering of the seismicity along the fault, which approximates the maximum intense clustering of the whole data set. Although our results are strongly influenced by the fact that the data are dominated by the epicentres located on the fault, we can conclude that multifractal analysis can be a very useful tool to discriminate the seismicity linked to a particular fault in a given area.     

Mineral inclusions in diamonds from the Sputnik kimberlite pipe, Yakutia

The Sputnik kimberlite pipe is a small “satellite” of the larger Mir pipe in central Yakutia (Sakha), Russia. Study of 38 large diamonds (0.7-4.9 carats) showed that nine contain inclusions of the eclogitic paragenesis, while the remainder contain inclusions of the peridotitic paragenesis, or of uncertain paragenesis. The peridotitic inclusion suite comprises olivine, enstatite, Cr-diopside, chromite, Cr-pyrope garnet (both lherzolitic and harzburgitic), ilmenite, Ni-rich sulfide and a Ti-Cr-Fe-Mg-Sr-K phase of the lindsleyite-mathiasite (LIMA) series. The eclogitic inclusion suite comprises omphacite, garnet, Ni-poor sulfide, phlogopite and rutile. Peridotitic ilmenite inclusions have high Mg, Cr and Ni contents and high Nb/Zr ratios; they may be related to metasomatic ilmenites known from peridotite xenoliths in kimberlite. Eclogitic phlogopite is intergrown with omphacite, coexists with garnet, and has an unusually high TiO₂ content. Comparison with inclusions in diamonds from Mir shows general similarities, but differences in details of trace-element patterns. Large compositional variations among inclusions of one phase (olivine, garnet, chromite) within single diamonds indicate that the chemical environment of diamond crystallisation changed rapidly relative to diamond growth rates in many cases. P-T conditions of formation were calculated from multiphase inclusions and from trace element geothermobarometry of single inclusions. The geotherm at the time of diamond formation was near a 35 mW/m² conductive model; that is indistinguishable from the Paleozoic geotherm derived by studies of xenoliths and concentrate minerals from Mir. A range of Ni temperatures between garnet inclusions in single diamonds from both Mir and Sputnik suggests that many of the diamonds grew during thermal events affecting a relatively narrow depth range of the lithosphere, within the diamond stability field. The minor differences between inclusions in Mir and Sputnik may reflect lateral heterogeneity in the upper mantle.     

Late Cretaceous rift-related upwelling and melting of the Trindade starting mantle plume head beneath western Brazil

High mantle potential temperatures and local extension, associated with the Late-Cretaceous impact of the Trindade mantle plume, produced substantial widespread and voluminous magmatism around the northern half of the Paraná sedimentary basin. Our previous studies have shown that, above the central and eastern portions of the postulated impact zone where lithosphere extension is minimal, heat conducted by the plume caused large-scale melting of the more fusible parts of the subcontinental lithospheric mantle beneath the margin of the São Francisco craton and the surrounding Brasilía mobile belt. Here we combine geochemical data and field evidence from the Poxoreu Igneous Province, western Brazil to show how more intense lithospheric extension above the western margin of the postulated impact zone permitted greater upwelling and melting of the Trindade plume than further east. Laser ⁴⁰Ar/³⁹Ar age determinations indicate that rift-related basaltic magmas of the Poxoreu Igneous Province were emplaced at ? 84 Ma. Our detailed geochemical study of the mafic magmas shows that the parental melts underwent polybaric crystal fractionation within the crust prior to final emplacement. Furthermore, some magmas (quartz-normative) appear to have assimilated upper crust whereas others (nepheline- and hypersthene-normative) appear to have been unaffected by open-system crustal magma chamber processes. Incompatible trace element ratios (e.g. chondrite-normalised La/Nb?=?1) and isotopic ratios (⁸⁷Sr/⁸⁶Sr?=?0.704 and ¹⁴³Nd/¹⁴⁴Nd?=?0.51274) of the Hy-normative basalts resemble those of oceanic islands (OIB). We therefore propose that these “OIB-like” magmas were predominantly derived from convecting-mantle-source melts (i.e. Trindade mantle plume). Inverse modelling of rare-earth element (REE) abundances suggests that the initial melts were predominantly generated within the depth range of ?80–100 km, in mantle with a potential temperature of ?1500 °C.