The provenance and crustal residence ages of British sediments in relation to palaeogeographic reconstructions

24 SmNd isotope analyses of fine-grained Phanerozoic and modern clastic sediments from Britain and Quebec are presented. In combination with published data, they have been used to calculate “crustal residence ages” (t_CR) and to assess the provenance of the British sedimentary mass. Sediments now preserved on either side of the suture formed by the closure of the Iapetus Ocean ca. 400 Ma ago were derived from isotopically distinct source regions. Sediments north of the suture are characterised by1.7 < t_CR < 2.8 Ga, whereas those to the south exhibit a smaller range and an average value of 1.6 Ga. The northern and southern source regions were most probably Laurentia and Gondwana respectively. It seems likely that a third source, perhaps Baltica, provided the lower Palaeozoic Southern Uplands succession. Sediments deposited in southern Britain after the closure of Iapetus were derived mainly from the recycling of older sediments. The tectonic rearrangements which occurred during the Phanerozoic are not reflected in the SmNd isotopic structure of the southern British sedimentary mass, suggesting that the Caledonian and Hercynian orogenies, and even the Grenville orogeny, involved minimal accretion of new mantle-derived material into the British and adjacent continental crust. SmNd analyses of fine-grained clastic sediments provide a powerful sedimentological tool for elucidating palaeogeography, clastic source areas, sediment recycling and maturity, and some aspects of sediment transport.     

Extreme fractionation in felsic magma chambers: a product of liquid-state diffusion or fractional crystallization?

Extreme fractionation of minor and trace elements commonly accompanies very modest changes in major element concentrations in highly felsic igneous sequences. In such sequences, Si increases by only a few percent while, for example, Sr, Ba, Mg, and light rare earth elements decrease drastically, commonly by a factor of 10 or more. It has been argued, most notably by Hildreth (e.g. [1]), that such trends observed in tuffs were not induced by fractional crystallization (FC), but rather are a manifestation of compositional gradients in parental magma chambers which form via liquid-state thermogravitational diffusion (LSTD). The strongest arguments against FC are that (1) crystal settling is not a viable mechanism for crystal-liquid separation, and (2) extensive recrystallization is required to produce the observed trends, yet the tuffs are relatively crystal-poor. Many workers have noted trends in plutonic as well as volcanic rocks which are strikingly similar to those for which LSTD has been proposed, and some have concluded that LSTD was the fractionating mechanism.Several lines of evidence lead us to the conclusion that FC is the dominant differentiating process in high-silica magmas: (1) elemental trends are strikingly consistent with those predicted for FC; it would be a remarkable coincidence if diffusion-induced trends mimicked FC so closely; (2) large phenocryst assemblages in high-silica tuffs indicate low-variance liquid compositions that would be improbable if crystal-liquid equilibria were not controlling differentiation; (3) highly evolved plutonic rocks in many cases do not form the caps expected for LSTD, but rather occur in dikes and pods where they apparently segregated as late liquids; (4) recent experimental studies suggest that trends induced by diffusion differ drastically from observed felsic igneous trends.We do not believe that the principal arguments against FC in high-silica systems (unlikelihood of crystal settling; crystal-poor nature of tuffs) refute the reality of the chemical process, but rather emphasize the need for a better understanding of the physical mechanisms of crystal-liquid fractionation and eruption.     

Concentration of Utah tar sands by an ambient temperature flotation process

Physical separation of bitumen from low-grade Utah tar sand deposits containing a relatively high viscosity bitumen phase (Sunnyside and Tar Sand Triangle deposits) has been accomplished by traditional size reduction and froth flotation techniques. At appropriate experimental conditions more than 90% of the bitumen can be recovered in a concentrate, containing more than 20 wt. % bitumen, which should be a suitable feed material for subsequent hot water or thermal processing. The efficiency of bitumen recovery depends on the extent of size reduction, as well as promoter and dispersant addition. Rejection of greater than 60% of the sand at ambient temperature and ease of water removal from the concentrate make such a process strategy both energy and cost effective. The energy required to achieve effective separation by the ambient temperature process is significantly less than the energy required for the recently developed hot water process which is being evaluated in a 100-tpd pilot plant this year.The flotation behaviour of the tar sand in this ambient temperature process has been correlated with contact angle measurements and the apparent point-of-zero-charge of the bitumen. The best flotation response at pH 7.8 to 9.0 occurs when the contact angle between the air bubble and bitumen surface is a maximum, corresponding to the apparent point-of-zero-charge of the bitumen as determined by titration.     

Gravity in one dimension: Stability of a three particle system

The autonomous dynamical system consisting of parallel planes of constant mass density moving under their mutual gravitational attraction is of interest for testing diverse astrophysical models of gravitational relaxation. The simplest non-integrable system consists of three sheets. In this paper the dynamics and stability of this one-dimensional solar system is systematically investigated. A linear transformation of the coordinates reduces the problem to that of a falling body constrained by oblique boundaries. By constructing a Poincare surface of section, it is found that regions of stability and chaos coexist. This behavior is predicted by the KAM theorem for systems having smoother trajectories in phase space, but does not apply here because of the discontinuity in acceleration experienced by each sheet during an encounter. The results of numerical experiments indicate that chaotic regions may be associated with trajectories which contain nearly triple collisions of the three particles.     

Little Ice Age recorded in summer temperature reconstruction from vared sediments of Donard Lake, Baffin Island, Canada

Clastic varved sediments from Donard Lake, in the Cape Dyer region of Baffin Island, provide a 1250 yr record of decadal-to-centennial scale climate variability. Donard Lake experiences strong seasonal fluctuations in runoff and sediment fluxes due to the summer melting of the Caribou Glacier, which presently dominates its catchment. The seasonal variation in sediment supply results in the annual deposition of laminae couplets. A radiocarbon date measured on moss fragments, with a calibrated age of 860 ± 80 yrs before present (BP), is in close agreement with the age based on paired-layer counts. Together with the fabric of the laminae determined from microscope analysis, the age agreement demonstrates that the laminae couplets are annually deposited varves. Comparisons of varve thickness and average summer temperature from nearby Cape Dyer show a significant positive correlation (r= 0.57 for annual records, r = 0.82 for 3-yr averages), indicating that varve thickness reflects changes in average summer temperature. Varve thickness was used to reconstruct average summer temperatures for the past 1250 yrs, and shows abrupt shifts and large amplitude decadal-to-centennial scale variability throughout the record. The most prominent feature of the record is a period of elevated summer temperatures from 1200-1375 AD, followed by cooler conditions from 1375-1820 AD, coincident with the Little Ice Age.     

Ion and relativistic electron acceleration by Alfven and whistler turbulence in solar flares

We show that protons can be accelerated to several GeV in 10 s by Alfven turbulence whose energy density is greater than a few erg/cm³. We also show that electrons can be accelerated to tens of MeV on similar time scales by whistler and Alfven turbulence.