Ar-Ar age of mylonites along the Merens Fault, central Pyrenees

Seven mylonitic samples and two coarse muscovites from the central Pyrenees have been dated by the ⁴⁰Ar-³⁹Ar method. Whole rock specimens of mylonite were cut out of thin-section chips allowing complete characterisation of mineralogy and texture. Several specimens showed rising staircase patterns in the range 50–90 Ma, with much higher ages in the highest temperature steps. This is believed to reflect mixing of argon released from micas with excess argon contained in plagioclase and released mainly at high temperatures. One biotite-quartz mylonite gave a plateau age of 93 ± 2 Ma. Other inferred mica ages are about 60–73 Ma for biotite and 50–60 Ma for muscovite; it is probable that biotite contains excess argon and that 50 Ma approximates to the cooling age in the mylonites. One coarse muscovite collected immediately below the major Mérens shear zone gave a Hercynian plateau age, while another collected within the Mérens zone gave a partially reset Hercynian age.Taken together, the data indicate that the shear zones were active in Alpine times < 100 Ma and probably about 50 Ma ago. They are believed to have formed during the early stages of Eocene compression in the Pyrenees. Deformation and resultant uplift probably terminated an important thermal event in this part of the Pyrenean basement, which may have begun at the time of the mid-Cretaceous North Pyrenean metamorphism (90–100 Ma).     

Metamorphic evolution of aluminous granulites from Labwor Hills,Uganda

Sapphirine-cordierite-quartz and spinel-cordierite-quartz form relic assemblages of probable Archaean age in Fe-rich aluminous metapelites from Labwor Hills, Uganda, and reflect an unusually high temperature metamorphism (1,000° C) at pressures in the vicinity of 7–9 kbars and a(O₂) near the magnetite-hematite buffer. Subsequent reaction textures include the replacement of spinel and cordierite by sillimanite and hypersthene and formation of sapphirine-hypersthene-K-feldspar-quartz symplectites which are interpreted as pseudomorphs after osumilite. A petrogenetic grid appropriate to these assemblages suggests these reaction textures may be due to cooling at constant or increasing pressure and constant a(O₂), or decreasing a(O₂) at constant temperature and pressure. The former interpretation is supported by the coexistence of ilmenohematite and magnetite during the development of the reaction textures, and by the comparatively low Al₂O₃-contents of secondary hypersthene. This pressure-temperature path implies that: (1) metamorphism occurred at deep levels within normal thickness crust, probably less than 40–45 km thick, due to an extreme thermal perturbation induced either by emplacement of mantle-derived magmas or by thinning of the subcontinental lithosphere in an extensional tectonic regime, (2) the excavation and surface exposure of the granulites is due to a subsequent, postgranulite facies metamorphism, crustal thickening most probably involving their incorporation into an allochthonous upper crustal thrust sheet during the formation of the Mozambique foldbelt.     

Paleohydrological implications of Holocene peatland development in northern Michigan

Sediment, pollen, and plant macrofossil stratigraphies from two small oligotrophic Chamaedaphne-Sphagnum peatlands provide data about local hydrologic changes in northern Michigan during the Holocene. Gleason Bog started about 8000 yr B.P. as a shallow pond that supported rich fen vegetation. After it was partly filled with peat and sand (about 4000 yr B.P.), the vegetation changed to oligotrophic bog. At Gates Bog paludification starting about 3800 yr B.P. caused peat accumulation over sand without an initial pond phase. The onset of peat accumulation at both sites is attributed to a rise in the water table resulting from the onset of cool and moist late Holocene climates. The water table of Gleason Bog is linked to the water level of adjacent Douglas Lake, which may have undergone a simultaneous rise. The results emphasize the individuality of hydrological conditions and hydroseral development in northern Michigan peatlands.     

The natural occurrence of hydroxide in olivine

Polarized infrared (IR) spectra of olivine single crystals from 17 different localities show a tremendous variability in both mode and abundance of hydroxide (OH) incorporation. Kimberlitic olivines contain the most total OH at an estimated concentration level of 976 H/10⁶Si, whereas olivines from basalts contain the least at 3 H/10⁶Si. Olivines of metamorphic and hydrothermal origin have widely varying concentration levels intermediate between those of basalts and kimberlites. Over 30 distinct OH absorption bands have been identified. Most of these bands are not unique to individual localities but may be found in samples from several different localities. Pleochroism is consistent among localities, but relative band intensities vary. No evidence is found for molecular H₂ in olivine. Hydrous minerals have been identified in olivine by their characteristic OH absorption bands. Serpentine is commonly found and is clearly distinguishable from intrinsic OH. Talc is present in one sample. Prominent OH bands at 3572 and 3525 cm^?1 are attributed to humite group minerals. San Carlos, Arizona, olivines annealed in the presence of H₂O develop absorption bands which are found in natural samples, however the OH absorption spectra of these annealed olivines are not identical to those of any single natural crystal. Sharp-band OH abundances in annealed samples are an order of magnitude lower than the maximum measured in natural specimens. The mechanical properties determined from these annealed olivines may not be directly applicable to mantle olivine because both the OH sites and concentrations are different.     

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.     

Controls on englacial sediment deposition during the November 1996 jökulhlaup,Skeiđarárjökull,Iceland

This paper presents sedimentary evidence for rapid englacial debris entrainment during jökulhlaups. Previous studies of jökulhlaup sedimentology have focused predominantly on proglacial impact, rather than depositional processes within glaciers. However, observations of supraglacial floodwater outbursts suggest that englacial sediment emplacement is possible during jökulhlaups. The November 1996 jökulhlaup from Skei?arárjökull, Iceland presented one of the first opportunities to examine englacial flood deposits in relation to former supraglacial outlets. Using observations from Skei?arárjökull, this paper identifies and explains controls on the deposition of englacial flood sediments and presents a qualitative model for englacial jökulhlaup deposition. Englacial jökulhlaup deposits were contained within complex networks of upglacier‐dipping fractures. Simultaneous englacial deposition of fines and boulder‐sized sediment demonstrates that englacial fracture discharge had a high transport capacity. Fracture geometry was an important control on the architecture of englacial jökulhlaup deposits. The occurrence of pervasively frozen flood deposits within Skei?arárjökull is attributed to freeze‐on by glaciohydraulic supercooling. Floodwater, flowing subglacially or through upglacier‐dipping fractures, would have supercooled as it was raised to the surface faster than its pressure‐melting point could increase as glaciostatic pressure decreased. Evidence for floodwater contact with the glacier bed is supported by the ubiquitous occurrence of sheared diamict rip‐ups and intra‐clasts of basal ice within jökulhlaup fractures, deposited englacially some 200–350 m above the bed of Skei?arárjökull. Evidence for fluidal supercooled sediment accretion is apparent within stratified sands, deposited englacially at exceptionally high angles of rest in the absence of post‐depositional disturbance. Such primary sediment structures cannot be explained unless sediment is progressively accreted to opposing fracture walls. Ice retreat from areas of former supraglacial outbursts revealed distinct ridges characterized by localized upwellings of sediment‐rich floodwater. These deposits are an important addition to current models of englacial sedimentation and demonstrate the potential for post‐jökulhlaup landform development. Copyright © 2001 John Wiley & Sons, Ltd.