Trace element constraints on pegmatite genesis: Tin Mountain pegmatite,Black Hills,South Dakota

The Tin Mountain pegmatite is a small, zoned granitic body that is extremely enriched in Rb and Li, but has moderate concentrations of Sr and Ba. These trace elements are modelled using granitic distribution coefficients in order to test the potentials of partial melting of metasedimentary rocks and fractionation of a less-evolved granitic melt to have produced the parental liquid to the Tin Mountain pegmatite. Batch melting of any reasonable metasedimentary source rock would likely have yielded melts that were either insufficiently enriched in Rb and Li to be the parental liquid, or that had Sr and Ba concentrations that were much higher than those estimated for the parental liquid. The modelling of simple fractional crystallization and equilibrium crystallization of a granitic melt within the compositional range of the spatially associated Harney Peak Granite gives calculated melt compositions with either lower Sr and Ba concentrations or inadequate Rb and Li enrichments, to be the parent liquid of the pegmatite. At least two variants from simple crystal-liquid fractionation models can, however, successfully account for the derivation of the parent liquid: 1) generation of a Rb-, Li-, Ba- and Sr-rich granitic melt (outside of the compositional range of the sampled portions of the Harney Peak Granite complex) by low degrees of partial melting of metasedimentary rocks found in the Black Hills, followed by moderate extents of fractional or equilibrium crystallization, 2) derivation from Harney Peak granite via a complex, multi-stage crystal-liquid fractionation process, such as progressive equilibrium crystallization.     

Electric potential gradient changes during explosive activity at Sakurajima volcano,Japan

We report electric potential gradient measurements carried out at Sakurajima volcano in Japan during: (1) explosions which generated ash plumes, (2) steam explosions which produced plumes of condensing gases, and (3) periods of ashfall and plume-induced acid rainfall. Sequential positive and negative deviations occurred during explosions which generated ash plumes. However, no deflections from background were found during steam explosions. During periods of ashfall negative electric potential gradients were observed, while positive potential gradients occurred during fallout of plume-induced acid rain from the same eruption. These results suggest that a dipole arrangement of charge develops within plumes such that positive charges dominate in the volcanic gas-rich top and negative charges in the following ash-rich part of the plume. The charge polarity may be reversed for other volcanoes (Hatakeyama and Uchikawa 1952). We suggest that charge is generated by fracto-emission (Donaldson et al. 1988) processes probably during magma fragmentation within the vent, rather than by frictional effects within the plume.     

Late Quaternary barrier and fringing reef development of Bora Bora (Society Islands,south Pacific): First subsurface data from the Darwin‐type barrier‐reef system

The universally known subsidence theory of Darwin, based on Bora Bora as a model, was developed without information from the subsurface. To evaluate the influence of environmental factors on reef development, two traverses with three cores, each on the barrier and the fringing reefs of Bora Bora, were drilled and 34 uranium‐series dates obtained and subsequently analysed. Sea‐level rise and, to a lesser degree, subsidence were crucial for Holocene reef development in that they have created accommodation space and controlled reef architecture. Antecedent topography played a role as well, because the Holocene barrier reef is located on a Pleistocene barrier reef forming a topographic high. The pedestal of the fringing reef was Pleistocene soil and basalt. Barrier and fringing reefs developed contemporaneously during the Holocene. The occurrence of five coralgal assemblages indicates an upcore increase in wave energy. Age–depth plots suggest that barrier and fringing reefs have prograded during the Holocene. The Holocene fringing reef is up to 20 m thick and comprises coralgal and microbial reef sections and abundant unconsolidated sediment. Fringing reef growth started 8780 ± 50 yr bp ; accretion rates average 5·65 m kyr^?1. The barrier reef consists of >30 m thick Holocene coralgal and microbial successions. Holocene barrier‐reef growth began 10 030 ± 50 yr bp and accretion rates average 6·15 m kyr^?1. The underlying Pleistocene reef formed 116 900 ± 1100 yr bp , i.e. during marine isotope stage 5e. Based on Pleistocene age, depth and coralgal palaeobathymetry, the subsidence rate of Bora Bora was estimated to be 0·05 to 0·14 m kyr^?1. In addition to subsidence, reef development on shorter timescales like in the late Pleistocene and Holocene has been driven by glacioeustatic sea‐level changes causing alternations of periods of flooding and subaerial exposure. Comparisons with other oceanic barrier‐reef systems in Tahiti and Mayotte exhibit more differences than similarities.     

Datation Ar/Ar de la province magmatique de l’Atlantique central dans le Sud-Ouest algérien

⁴⁰Ar/³⁹Ar step-heating analyses performed on plagioclase separates from seven doleritic basalts (four sills, one dyke and two lava flows) from southwestern Algeria display disturbed age spectra, reflecting various contributions of alteration by sericite and/or excess argon. Weighted mean ages corresponding to the less altered plagioclase fractions yielded minimum ages ranging from 192.7 ± 3.0 to 197.9 ± 2.0 Ma and a minimum date of 198.9 ± 1.8 Ma was obtained on a saddle-shaped age spectrum (excess argon). These ages are in accordance with those previously obtained on the CAMP province and partly in agreement with the peak activity of the CAMP at 198 Ma, highlighted in the neighbouring Taoudenni basin (Mali). In Algeria, the eastern boundary of the CAMP seems to coincide with the Pan-African suture zone.     

Capital and Empire: Geographies of Imperial London

The notion of empire has often been regarded in Europe as a matter of diffusion and expansion; something which happened over there rather than close to home. Yet the form, use and representation of modern European cities have been shaped by the global history of imperialism in ways that continue to matter even in an apparently post-imperial age. The signs of empire were prominently displayed within the built environments of all the major cities of late-nineteenth century Europe, as they came (in different ways) to play the role of regional, national and imperial capitals. In what was evidently a pan-European discourse on the imperial city between the mid-nineteenth century and the mid-twentieth, national models were defined in relation to other national models, in a spirit of competition as much as emulation. This paper examines the case of London. British architects and planners frequently complained that London lagged behind its rivals in the struggle for imperial primacy, given the absence of state-sponsored projects to parallel Haussmann's rebuilding of Paris or Leopold's grand plans for Brussels. At the intra-urban scale, the imperial city had a geography which mattered: in the case of London, different parts of the city were associated with different aspects of empire. More generally, it is clear that national debates over imperial urbanism were conditioned not simply by understandings of the global reach of European empires, but also by attitudes towards social, cultural and political change within Europe itself.     

Fehler der Eisen-Konstantan-Thermoelemente bei Feuchtemessungen mit dem thermoelektrischen Psychrometer

Zusammenfassung Drei verschiedene Eisen-Konstantan Thermoelemente wurden in Petroleum und in Wasser geeicht, wobei die passive Lötstelle einmal auf 0° und einmal auf 50° C lag. Die beiden Eichkurven in Wasser zeigen bei dem extremsten Thermoelement eine maximale Temperaturdifferenz von 1,1°C. Die Eichung in Petroleum ergibt gegenüber Wasser eine etwas geringere Temperaturdifferenz, die aber mit ausreichender Genauigkeit vernachlässigt werden kann. Auffallend ist das Auseinanderlaufen der Eichkurven der einzelnen Thermoelemente. Der Unterschied hierbei ist direkt proportional zur Temperaturdifferenz (t _akt.–t _pass.) und ist wohl auf die Vorgeschichte der Materialien zurückzuführen.

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Summary Three different iron-constantan thermocouples were gauged in both petroleum and water, the passive soldered joint lying at 0°C and subsequently at 50°C. The two gauging curves in water show, for the extremest thermocouple, a highest temperature difference of 1,1°C. Gauging in petroleum yields, with regard to water, a somewhat smaller temperature difference which can be neglected with sufficient accuracy. There is an obvious divergence of the gauging curves of the three thermocouples. The difference is directly proportional to the temperature difference (t _active–t _passive) and may be accounted for by the previous state and treatment of the material in question.

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Résumé On a étalonné trois thermoéléments différents de ferro-constantan dans du pétrole et dans de l'eau, les soudures passives étant une fois à 0° et une autre fois à 50° C. Les deux courbes d'étalonnage dans l'eau accusent pour le thermoélément le plus extrême une différence de température n'excédant pas 1,1°, tandis que dans le pétrole cette différence est un peu plus faible, mais peut être pratiquement négligée. La divergence des courbes d'étalonnage de chaque thermoélément est remarquable; l'écart est proportionnel à la différence de température des soudures et s'explique probablement par l'état antérieur des matériaux.

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Mit 5 Textabbildungen.     

A Synthetic Haematite Reference Material for LA‐ICP‐MS U‐Pb Geochronology and Application to Iron Oxide‐Cu‐Au Systems

In both nature and synthetic experiments, the common iron oxide haematite (α‐Fe₂O₃) can incorporate significant amounts of U into its crystal structure and retain radiogenic Pb over geological time. Haematite is a ubiquitous component of many ore deposit types and, therefore, represents a valuable hydrothermal mineral geochronometer, allowing direct constraints to be placed on the timing of ore formation and upgrading. However, to date, no suitable natural haematite reference material has been identified. Here, a synthetic haematite U‐Pb reference material (MR‐HFO) is characterised using LA‐ICP‐MS and ID‐TIMS. Centimetre‐scale ‘chips’ of synthesised α‐Fe₂O₃ were randomly microsampled via laser ablation‐extraction and analysed using ID‐TIMS. Reproducible U/Pb and Pb/Pb measurements were obtained across four separate chips (n = 13). Subsequently, an evaluation of the suitability MR‐HFO in constraining U‐Pb data via LA‐ICP‐MS is presented using a selection of natural samples ranging from Cenozoic to Proterozoic in age. The MR‐HFO normalised U‐Pb ratios are more concordant and ages more accurate versus the same LA‐ICP‐MS spot analyses normalised to zircon reference material, when compared with independently acquired ID‐TIMS data from the same natural haematite grains. Results establish MR‐HFO as a suitable reference material for LA‐ICP‐MS haematite U‐Pb geochronology.     

Projections of Future Trends in Biogeochemical Conditions in the Northwest Atlantic Using CMIP5 Earth System Models

We use the results from eight of the Earth System Models (ESMs) made available for the Fifth Assessment Report of the Intergovernmental Panel on Climate Change to analyze the projected changes in biogeochemical conditions over the next 50 years in the northwest Atlantic. We looked at the projected changes using the Representative Concentration Pathway 8.5 scenario in the 100–400?m depth range over a large region and at more specific locations to assess the relevance of using these outputs to force a regional climate downscaling model of the Gulf of St. Lawrence. The projected trends for dissolved oxygen (decrease), pH (decrease), and nitrate (variable although negative in general) represent a continuation of the recently observed trends in the area. For primary production, no firm conclusions can be drawn because of large differences in the trends from one model to another. The consistency of the trends near the regional model lateral boundaries leads us to conclude that the ESM trends can be used to set up future boundary conditions to evaluate regional impacts of climate change although the uncertainty of the results for the Scotian Shelf will be greater than for the Gulf of St. Lawrence.