Glauconitization of detrital silica substrates in the Barton Formation (upper Eocene) of the Hampshire Basin, southern England

Upper Eocene detrital silica grains (chert and quartz) of the Hampshire Basin display alteration and replacement fabrics by glauconite. Silica grains have etched surfaces due to glauconitization which appear green in reflected light and thin section. Quartz grains were glauconitized by surface nucleation and replacement, which spreads from the margin with progressive glauconitization, replacing the quartz grain interior. Chert grains were glauconitized by surface replacement and nucleation internally along cracks and in pores. Different forms of glauconite are associated with the two minerals; glauconite associated with quartz is generally highly-evolved whereas glauconite associated with chert is of the evolved variety. This is interpreted as being due to different surface-reaction control mechanisms associated with the two forms of silica. There is no evidence to suggest that glauconite evolved in stages from a nascent form. Two crystalline morphological forms of glauconite are found associated with both quartz and chert. Glauconite growing within a confined space has a laminated morphology whilst glauconite occurring on the surface has a rosette morphology.     

Peridotite Melting at 1 GPa: Reversal Experiments on Partial Melt Compositions Produced by Peridotite-Basalt Sandwich Experiments

One of the goals of igneous petrology is to use the subtle andmore obvious differences in the geochemistry of primitive basaltsto place constraints on mantle composition, melting conditionsand dynamics of mantle upwelling and melt extraction. For thisgoal to be achieved, our first-order understanding of mantlemelting must be refined by high-quality, systematic data oncorrelated melt and residual phase compositions under knownpressures and temperatures. Discrepancies in earlier data onmelt compositions from a fertile mantle composition [MORB (mid-oceanridge basalt) Pyrolite mg-number 87] and refractory lherzolite(Tinaquillo Lherzolite mg-number 90) are resolved here. Errorsin earlier data resulted from drift of W/Re thermocouples at1 GPa and access of water, lowering liquidus temperatures by30–80°C. We demonstrate the suitability of the ‘sandwich’technique for determining the compositions of multiphase-saturatedliquids in lherzolite, provided fine-grained sintered oxidemixes are used as the peridotite starting materials, and thechanges in bulk composition are considered. Compositions ofliquids in equilibrium with lherzolitic to harzburgitic residueat 1 GPa, 1300–1450°C in the two lherzolite compositionsare reported. Melt compositions are olivine + hypersthene-normative(olivine tholeiites) with the more refractory composition producinga lower melt fraction (7–8% at 1300°C) compared withthe model MORB source (18–20% at 1300°C). KEY WORDS: mantle melting; sandwich experiments; reversal experiments; anhydrous peridotite melting; thermocouple oxidation; olivine geothermometry     

Volatiles in Basaltic Glasses from Loihi Seamount, Hawaii: Evidence for a Relatively Dry Plume Component

New H₂O, CO₂ and S concentration data for basaltic glasses fromLoihi seamount, Hawaii, allow us to model degassing, assimilation,and the distribution of major volatiles within and around theHawaiian plume. Degassing and assimilation have affected CO₂and Cl but not H₂O concentrations in most Loihi glasses. Waterconcentrations relative to similarly incompatible elements inHawaiian submarine magmas are depleted (Loihi), equivalent (Kilauea,North Arch, Kauai–Oahu), or enriched (South Arch). H₂O/Ceratios are uncorrelated with major element composition or extentor depth of melting, but are related to position relative tothe Hawaiian plume and mantle source region composition, consistentwith a zoned plume model. In front of the plume core, overlyingmantle is metasomatized by hydrous partial melts derived fromthe Hawaiian plume. Downstream from the plume core, lavas tapa depleted source region with H₂O/Ce similar to enriched Pacificmid-ocean ridge basalt. Within the plume core, mantle components,thought to represent subducted oceanic lithosphere, have waterenrichments equivalent to (KEA) or less than (KOO) that of Ce.Lower H₂O/Ce in the KOO component may reflect efficient dehydrationof the subducting oceanic crust and sediments during recyclinginto the deep mantle. KEY WORDS: basalt; Hawaii; mantle; plumes; volatiles     

U-series Isotope Data on Lau Basin Glasses: the Role of Subduction-related Fluids during Melt Generation in Back-arc Basins

New thermal ionization mass spectrometry U-series disequilibriumdata are presented for 24 basaltic to dacitic glasses from activespreading centres in the back-arc Lau Basin (SW Pacific), togetherwith additional inductively coupled plasma mass spectrometrytrace element analyses and Sr–Nd–Pb isotope data.Valu Fa Ridge samples, adjacent to the arc front, have highU/Th and (²³⁰Th/²³⁸U) <1, implying a recent (<<350ka) addition of a U-rich slab-derived fluid. The Valu Fa datacan be combined with existing ²³⁰Th–²³⁸U data for theCentral Tonga arc to infer a fluid addition event at     

NUMERICAL SIMULATION OF TURBIDITY CURRENT IN RESERVOIR

I. INTRODUCTIONWhen a sediment--laden flow reaches the backwater zone of a reservoir, the suddenreduction of flow velocity causes sediment particles to settle toward the river bed. Undercertain circumstsnces, it will plunge and form a layer of sediment--water mixture flowingbeneath the water surface. This flowing layer is called the turbidity current. A turbiditycurrent is relatively stable and has important impacts on reservoir sedimentation.In the case of deep reservoirs, due to temper…     

Coherent slumping of early postglacial lake sediments at Hall Lake, Ontario, Canada

Cores of organic postglacial sediment from Hall Lake, southern Ontario, Canada, are described. The laminated, calcareous sediments deposited between 9,000 and 8,000 B.P. have coherently slumped into the deep part of the lake, causing overturning and replication of parts of the sequence. This process may be considered as a mechanism for sediment focusing, since it involves reworking of marginal sediments into deeper areas. Unlike previously described mechanisms, it may be a source of error in the interpretation of diagrams of pollen percentages as well as accumulation rates. The slumping has only been recognized through the distortion of the laminae. It might not be possible to recognize slumping in unstratified sequences.     

Anomalous luminescence of subglacial sediment at Haut Glacier d'Arolla,Switzerland – a consequence of resetting at the glacier bed?

Swift, D. A., Sanderson, D. C. W., Nienow, P. W., Bingham, R. G. & Cochrane, I. C. 2010: Anomalous luminescence of subglacial sediment at Haut Glacier d'Arolla, Switzerland – a consequence of resetting at the glacier bed? Boreas, Vol. 40, pp. 446–458. 10.1111/j.1502‐3885.2010.00196.x. ISSN 0300‐9483. Luminescence has the potential to elucidate glacial geomorphic processes because primary glacial sediment sources and transport pathways are associated with contrasting degrees of exposure to light. Most notably, sediment entrained from extraglacial sources should be at least partially reset, whereas sediment produced by glacial erosion of subglacial bedrock should retain substantial luminescence commensurate with a geological irradiation history. We set out to test the validity of this assumption at Haut Glacier d'Arolla, Switzerland using sediment sampled extraglacially and from the glacier bed. Contrary to our expectations, the subglacial samples exhibited natural signals that were substantially lower than those of other sample groups, and further (albeit limited) analyses have indicated no obvious differences in sample‐group luminescence characteristics or behaviour that could account for this observation. For glaciological reasons, we can eliminate the possibilities that the subglacial sediment has been extraglacially reset or exposed in situ to heat or light. We therefore advocate investigation of possible resetting processes related to subglacial crushing and grinding, and speculate that such processes, if more generally present, may enable the dating of subglacially deposited tills using luminescence‐based techniques.     

Stegosaur tracks and the persistence of facies — the Lower Cretaceous of Western Australia

Sometimes between January and June 1996, the only known stegosaur tracks were stolen from Broome in north-western Australia (Fig. 1). Vandalism or loss of any type material is always to be lamented, yet there is good cause to suspect that the trackmaker at Broome was not a stegosaur. The reasons why this should be so relate to some of the factors involved in the formation of dinosaur trace fossils and the difficulties encountered in their interpretation. The stratigraphy at Broome is notable for preserving evidence of potential Wealden-type deposition in Australia.