Scientific applications of downhole measurements in the ocean basins

Abstract The well logging activity of the Ocean Drilling Programme, which deploys the most technically advanced suite of downhole measurements available for routine use, is providing new opportunities for advancement in marine geoscience. Particular examples are cited of the application of wireline data to problems associated with global environmental changes, crust/ mantle interactions, crustal fluid circulation in the context of a global geochemical budget, lithospheric stress and deformation, and evolutionary processes in oceanic communities. Further technological developments will expand the scientific role of downhole measurements still further, especially in terms of the integration of geophysical data at different scales of measurement, and the interpretation of these data in accordance with; global scientific themes.     

Side-scan targets in Lake Superior—evidence for bedforms and sediment transport

A high-resolution side-scan sonar survey of the lake bed off the Keweenaw Peninsula, Lake Superior, demonstrates that bottom currents are affecting lake bed morphology at depths up to 240 m. Numerous lineations which run parallel to the shore appear to be sand ribbons. A field of sedimentary furrows which occurs in one area demonstrates the long-term directional stability of the near-bottom flow. Large (100–300 m in diameter, 2–5 m deep), unusual ring-like or arcuate depressions are common throughout the western half of Lake Superior. These rings themselves do not appear to have been formed by bottom currents, but may have developed as water was released by the rapid compaction of glacial sediments which underlie the lake bed. Off the Keweenaw Peninsula the forms of the rings have been modified by bottom currents. The bottom currents which have modified the lake bed are probably generated when storms cross the lake at times when it is poorly stratified.     

A garnet–hornblende geothermometer: calibration, testing, and application to the Pelona Schist, Southern California

Abstract A garnet–hornblende Fe–Mg exchange geothermometer has been calibrated against the garnet–clinopyroxene geothermometer of Ellis & Green (1979) using data on coexisting garnet + hornblende + clinopyroxene in amphibolite and granulite facies metamorphic assemblages. Data for the Fe–Mg exchange reaction between garnet and hornblende have been fitted to the equation. In K_D=Δ (X_Ca,g) where K_D is the Fe–Mg distribution coefficient, using a robust regression approach, giving a thermometer of the form: with very satisfactory agreement between garnet–hornblende and garnet–clinopyroxene temperatures. The thermometer is applicable below about 850°C to rocks with Mn-poor garnet and common hornblende of widely varying chemistry metamorphosed at low a_O2. Application of the garnet–hornblende geothermometer to Dalradian garnet amphibolites gives temperatures in good agreement with those predicted by pelite petrogenetic grids, ranging from 520°C for the lower garnet zone to 565–610°C for the staurolite to kyanite zones. These results suggest that systematic errors introduced by closure temperature problems in the application of the garnet–clinopyroxene geothermometer to the ‘calibration’data set are not serious. Application to ‘eclogitic’garnet amphibolites suggests that garnet and hornblende seldom attain Fe–Mg exchange equilibrium in these rocks. Quartzo-feldspathic and mafic schists of the Pelona Schist on Sierra Pelona, Southern California, were metamorphosed under high pressure greenschist, epidote–amphibolite and (oligoclase) amphibolite facies beneath the Vincent Thrust at pressures deduced to be 10±1 kbar using the phengite geobarometer, and 8–9kbar using the jadeite content of clinopyroxene in equilibrium with oligoclase and quartz. Application of the garnet–hornblende thermometer gives temperatures ranging from about 480°C at the garnet isograd through 570°C at the oligoclase isograd to a maximum of 620–650°C near the thrust. Inverted thermal gradients beneath the Vincent Thrust were in the range 170 to 250°C per km close to the thrust.     

A model to estimate the depositional brine depths of ancient halite rocks: Implications for ancient subaqueous evaporite depositional environments

A model is derived which allows a more general approach to the interpretation of bromine profiles in halite rocks than is possible with the models of Kuhn or Holser. Allowing for an open system, with influx and reflux, reduces the depth of brine necessary to generate a given salt deposit. Even very small and regular bromine gradients, which historically have been interpreted as the result of deposition from deep brine bodies, can be generated in an open system from brine no more than a few tens of metres deep. The bromine gradient produced from a given depth of brine is strongly dependent on the composition and amount of influx, but less so on the composition of brine already in the basin. A Zechstein profile is analysed which in a closed basin would require brine 2665 m deep. It can equally be modelled by influx of normal seawater which fully replenishes a basin no more than 140 m deep. A somewhat irregular profile from the Paradox Basin, instead of requiring a basin 100–400 m deep, can be modelled as being produced by deposition from brines only 10 m deep. Reflux in both cases can only be a few per cent of influx at a maximum.     

DISTINCTIVE TYPES OF RIPPLE-DRIFT CROSS-LAMINATION

From work on two Upper Carboniferous formations in southwest England, three morphologically distinct types of ripple-drift cross-lamination have been recognised. Type 1 is characterised by strong erosion of laminae on the stoss (or up-current) side of the ripples, and absence of grading. Type 3 is characterised by an absence of erosion on the stoss sides, concentration of mud in the ripple troughs and an upward gradual decrease in grain size and amplitude of rippling. Type 2 is an intermediate form with some characteristics in common with types 1 and 3. An examination of the illustrated literature has shown that types 1 and 3 occur frequently, and that type 2 has not pre- viously been recorded.
It is suggested that type 1 is formed in fluvial and shallow water environments at times of net deposition of sediment, and that type 3 is formed by deposition from a tur- bidity current. Type 2 suggests hydrodynamic conditions intermediate between fluvial or shallow water traction currents, and turbidity currents.
Since all current ripples move forwards, or "drift", it is suggested that the term "ripple-drift" should be used to describe ripple cross-lamination where the ripples can be seen to climb onto the stoss slope of the ripple immediately downstream, there having been a net deposition, and not merely a forward drift of sediment.     

Some effects of compaction and geological time on the pore parameters of argillaceous rocks

Graphical statistics have been applied to the pore-size distribution curves of argillaceous rocks to characterize the changes in pore parameters that result from compaction and geological time. The most striking characteristic of recently deposited sediment is the high variability in mean pore size and in the sorting and skewness of the pore system. The mean pore size ranges from 15 to 980 nm, sorting ranges from very well sorted to poorly sorted, and skewness varies from systems in which small pores predominate over large ones to systems in which large pores predominate. This high variability in pore structure represents the many environmental and mineral-related variables that affect the pore system of newly deposited sediment. The mean pore size of shales decreases with increasing compaction and approaches a limiting value of about 3·5 nm at depth. Within a geological time span of 50 m.y. and/or depth of burial of about 1200 m, most sediments have reached an irreversible, well sorted pore-size distribution. Early diagenetic processes apparently affect the skewness of pore systems more than compaction, such that within about 50 m.y. the pore system is negatively skewed, with small pores predominating over large. Sediments buried to a depth of 500 m or less exhibit a porosity range of 40--85%; below 500 m, porosity decreases linearly with burial depth. No correlation exists between the surface area of shale pore systems and depth of burial, geological age, and the pore parameters mean pore size, sorting, and skewness.     

JAPANESE PRECIOUS WOOD AND THE PARADOXES OF ADDED VALUE*

ABSTRACT. Japanese precious wood — meiboku — is cherished for its grain, color, and other characteristics. The value of meiboku is augmented in finding and selecting the wood; in specialized silviculture; in the aging, treatment, and fashioning of the wood; in distribution, display, and consultation; and, especially, in its integration into the design of Japanese-style rooms. Historically, forest communities embraced a labor-intensive tradition of meiboku silviculture and processing, but modern technology has reduced the labor required, the cost of meiboku, and, potentially, the rarity and value of the wood. In explaining paradoxes of value creation, Thorstein Veblen's institutionalism illuminates an iterative interplay between a changing culture of demand and divisions of labor, alternately traditional and rationalized, in meiboku.     

Occurrence, thermal evolution and primary migration processes derived from studies of organic matter in the Lucaogou source rock at the southern margin of the Junggar Basin, NW China

The Lucaogou Formation carbonate-rich oil shale source rock is exposed at the southern margin of the Junggar Basin, Xinjiang, NW China. We have sampled it in detail and conducted microstructural, mineralogical and geochemical studies, including thin section petrography, UV fluorescence petrography, X-ray diffraction, vitrinite reflectance, bitumen reflectance, fluid inclusion analysis and Raman spectroscopy. Organic matter is disseminated through the carbonate-bearing siltstone source rocks and concentrated in numerous bedding parallel stylolites and in two sets of carbonate veins, one along bedding parallel fractures and the other cross-cutting stylolites and bedding. The research about maturity of organic matter finds vitrinite reflectance values increase from the dispersed kerogen (0.64%) to the stylolites (the one of oriented vitrinite is 0.72% and the one of migrated bitumen is 2.38%); Homogenization temperatures of fluid inclusions in veins containing hydrocarbon fluid inclusions show an increase from 178.5℃ in the bedding parallel veins to 222℃ in the cross-cutting veins, confirmed by Raman spectroscopy. These results support a model of progressive heating accompanied by fluid loss during later stages of thermal maturation of source rock and the onset of primary migration. Obviously, the occurrence of organic matter is the trace of hydrocarbon primary migration, and the bedding lamination surfaces and cross-cutting fissures are the principal pathways of hydrocarbon-bearing fluids migration. Bedding lamination surfaces evolved into stylolites along the earliest primary migration pathways, followed by bedding parallel and cross-cutting fissures.     

Calculated Phase Relations in High-Pressure Metapelites in the System NKFMASH (Na2O-K2O-FeO-MgO-Al2O3-SiO2-H2O)

Using an internally consistent thermodynamic dataset and updatedmodels of activity–composition relation for solid solutions,petrogenetic grids in the system NKFMASH (Na₂O–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O)and the subsystems NKMASH and NKFASH have been calculated withthe software THERMOCALC 3.1 in the P–T range 5–36kbar and 400–810°C, involving garnet, chloritoid,biotite, carpholite, talc, chlorite, kyanite/sillimanite, staurolite,phengite, paragonite, albite, glaucophane, jadeite, with quartz/coesiteand H₂O in excess. These grids, together with calculated AFMcompatibility diagrams and P–T pseudosections, are shownto be powerful tools for delineating the phase equilibria andP–T conditions of Na-bearing pelitic assemblages for avariety of bulk compositions from high-P terranes around theworld. These calculated equilibria are in good agreement withpetrological studies. Moreover, contours of the calculated phengiteSi isopleths in P–T pseudosections for different bulkcompositions confirm that phengite barometry is highly dependenton mineral assemblage. KEY WORDS: phase relations; HP metapelite; NKFMASH; THERMOCALC; phengite geobarometry     

Chondrule precursor minerals as anhydrous phases

Abstract— Chondrules might be generated from hydrous or anhydrous precursor minerals, but vesicles due to dehydration during melting are absent in natural chondrules. The occurrence of vesicles in experimental charges made from serpentine-rich starting material that was (a) flash heated and remelted several times, (b) melted for long durations and (c) melted in a vacuum has been studied in order to test whether vesicles could be eliminated in chondrules made from hydrous precursors. High percentages of vesicles in all experiments indicate that chondrules, whether melted once, multiple times, for a brief or extended period, at solar nebula or higher pressure, were made from anhydrous phases.