Palaeomagnetism of some Miocene rocks, Qattara depression, Western Desert, Egypt

Eleven sites comprising 76 core samples of Lower (sandstone) and Middle (limestone) Miocene age were collected from the northeastern tip of the Qattara depression in the north of the Western Desert of Egypt. The majority of samples showed weak to very weak remanent magnetization with geothite, haematite and titanomagnetite as the main magnetic carriers. However, with a careful detailed thermal demagnetization, they yielded stable, probably primary, magnetization. The resultant overall mean direction of 2.9°/31.2° with α₉₅=2.3° corresponds to a palaeomagnetic pole position of 76.5°N and 198.0°E with A₉₅=2.0°, which seems to agree with other known African Miocene poles. This result implies that the Qattara area has been stable at least since Early Miocene. The presence of goethite as the main magnetic carrier supports the assumption that weathering has been playing an important role in the development and shaping of the depression.     

The contributions of H.N. Fisk to engineering geology in the Lower Mississippi Valley

Fisk's work provided a three-dimensional pattern of occurrence and composition of sediments in the alluvial valley and deltaic plain of the lower Mississippi River. He also interpreted the processes by which the sediments were deposited and the relevance of tectonism in their history. These data provide site characterizations that are basic for all major categories of engineering. His work has found applications in better methods for control of the river, stabilizing its banks, locating sources of aggregate for concrete, management of groundwater, maintenance of wetlands, and generally for more reliable, timely and economical evaluations in selecting optimum site locations and determining foundation conditions. Since then, work in the Lower Mississippi Valley (LMV) has expanded enormously and continues to be greatly indebted to formative interpretations by Fisk.     

Effects of heat/water flow interaction on compacted bentonite: Preliminary results

Compacted bentonite blocks have been heated and hydrated in a stainless steel cell in order to simulate, in the laboratory, the conditions of the clay barrier in a high-level radioactive waste repository. Temperature distributions at different times, rate of hydration, final water content and dry density have been measured. Some chemical parameters, as electrical conductivity in an aqueous extractable amorphous silica, have also been obtained. For the periods of time considered (up to 2500 h), the hydration process is not affected by the thermal gradient, the high suction of the bentonite being the critical factor in the initial water uptake of the clay barrier. A remarkable saline environment has been detected near the heater, due to salt migration towards dried areas. This phenomenon should be taken into account in further investigations of the mechanical and geochemical behaviour of the clay barrier.     

Trace element and isotope geochemistry of depleted peridotites from an N-MORB type ophiolite (Internal Liguride,N. Italy)

 Mantle peridotites of the Internal Liguride (IL) units (Northern Apennines) constitute a rare example of the depleted lithosphere of the Jurassic Ligurian Tethys. Detailed chemical (ICP-MS and SIMS techniques) and isotopic investigations on very fresh samples have been performed with the major aim to constrain the timing and mechanism of their evolution and to furnish new data for the geodynamic interpretation. The data are also useful to discuss some general geochemical aspects of oceanic-type mantle. The studied samples consist of clinopyroxene-poor spinel lherzolites, showing incipient re-equilibration in the plagioclase-facies stability field. The spinel-facies assemblage records high (asthenospheric) equilibration temperatures (1150–1250° C). Whole rocks, and constituent clinopyroxenes, show a decoupling between severe depletion in highly incompatible elements [light rare earth elements (LREE), Sr, Zr, Na, Ti] and less pronounced depletion in moderate incompatible elements (Ca, Al, Sc, V). Bulk rocks also display a relatively strong M(middle)REE/H(heavy)REE fractionation. These compositional features indicate low-degree (<10%) fractional melting, which presumably started in the garnet stability field, as the most suitable depletion mechanism. In this respect, the IL ultramafics show strong similarity to abyssal peridotites. The Sr and Nd isotopic compositions, determined on carefully handpicked clinopyroxene separates, indicate an extremely depleted signature (⁸⁷Sr/⁸⁶Sr=0.702203–0.702285; ¹⁴³Nd/¹⁴⁴Nd=0.513619–0.513775). The Sm/Nd model ages suggest that the IL peridotites melted most likely during Permian times. They could record, therefore, the early upwelling and melting of mid ocean ridge basalt (MORB) type asthenosphere, in response to the onset of extensional mechanisms which led to the opening of the Western Tethys. They subsequently cooled and experienced a composite subsolidus evolution testified by multiple episodes of gabbroic intrusions and HT-LP retrograde metamorphic re-equilibration, prior to their emplacement on the sea floor. The trace element chemistry of IL peridotites also provides useful information about the composition of oceanic-type mantle. The most important feature concerns the occurrence of Sr and Zr negative anomalies (relative to “adjacent” REE) in both clinopyroxenes and bulk rocks. We suggest that such anomalies reflect changes in the relative magnitude of Sr, Zr and REE partition coefficients, depending on the specific melting conditions. Received: 15 February 1995/Accepted: 4 August 1995     

Current controlled sediment deposition from the shelf to the deep ocean: the cenozoic evolution of circulation through the SW pacific gateway

 The circulation of cold, deep water is one of the controlling factors of the Earth's climate. Forty percent of this water enters the world ocean through the Southwest Pacific as a deep western boundary current (DWBC) flowing northwards at bathyal to abyssal depths, east of the New Zealand microcontinent. South of latitude 50°S, the DWBC is intimately linked with the Antarctic circumpolar current (ACC), which is the prominent force for the shallow-water circulation. The Pacific DWBC is presently the largest single contributor of deep ocean water, and deciphering its evolution is of fundamental importance to understanding ocean and climate history, and global ocean hydrography. The evolution of the DWBC system, and of related circum-Antarctic currents, has taken place since 30–25 Ma when plate movements created the first oceanic gaps south of Australia and South America. The stratigraphic record preserved in sediment drifts of the Southwest Pacific, in eastern New Zealand, is the best available for deciphering the Neogene history of Southern Ocean water masses, and of the circulation of the ACC, DWBC and their precursor systems. Major current activity commenced on the New Zealand margin in the late Eocene or early Oligocene (Hoiho Drift; early ACC) and was widespread by the mid-late Oligocene (Marshall Paraconformity and Weka Pass Limestone drift; ACC). During the Neogene the eastern South Island continental shelf built seawards by accretion at its outer edge of large Miocene current drifts up to tens of kilometres long and hundreds of metres thick (Canterbury drifts). Also commencing in the mid-Cenozoic, but in depths >2000 m, the DWBC emplaced large deep-water sediment drifts. Rates of drift deposition accelerated considerably in the late Neogene, when climatic change (and particularly glacial sea-level falls) caused the delivery of large volumes of turbiditic sediment into the path of the DWBC via the Bounty and Hikurangi channels. Received: 9 August 1995 / Accepted: 15 January 1996     

Thermodynamic Mixing Models for Muscovite--Paragonite Solutions Based on Solution Calorimetric and Phase Equilibrium Data

Enthalpies of solution have been measured on a series of muscovite—paragonitemicas in 20.1% HF at 50C under isoperibolic conditions. Themolar enthalpy of formation of paragonite at 298 K, for whichno calorimetrically measured value is currently available, hasbeen determined to be –5937.5 (3) kJ. An inversion ofall calorimetric, volumetric and phase equilibrium data hasbeen performed, giving a range of mixing models compatible withmost experimental data. The following expressions of the mixingproperties of 2M₁ micas for enthalpy (H_ex) and volume (V_ex)at pressures up to 10 kbar, forcing excess entropy (S_ex) tobe zero and using a subregular mixing model are favoured: H_ex(kJ) = [10.6+4.45(1–2X_ms)]X_ms(1–X_ms) V_ex(J/bar) = 0.452X_ms(1–X_ms). However, mixing models of higher order with asymmetric negativeS_ex are also possible. KEY WORDS: muscovite; paragonite; solvus; calorimetry; solid solution ^*Corresponding author.     

Sediment flux distribution in the Southern Brazil Basin during the late Quaternary: the role of deep-sea currents

Detailed sedimentological and stratigraphic analyses were carried out on seven Kullenberg cores collected across the Brazilian continental margin during the French cruises Byblos and Apsara III, in order to highlight the factors controlling the sediment flux distribution in the Southern Brazil Basin during the late Quaternary. On the continental slope and upper continental rise above 3000 m depth, sediment fluxes are important and highly variable (4·2–14·2 g cm^?2 10^?3 yr). The sediments show a pelagic or turbiditic character, depending on the width of the shelf and proximity of canyons. The material is characterized by high kaolinite contents, and originates from the coastal rivers draining the South American continent north of Rio de Janeiro. On the middle continental rise between 3000 and 4000 m depth, sediment fluxes are the lowest observed in the area (0·9 g cm^?2 10^?3 yr), because terrigenous input is trapped at shallower depths on the São Paulo Plateau. Pelagic settling is the dominant process. In the deep domains, below 4000 m depth, contouritic accumulations are developed on the path of the northwards moving Antarctic bottom water (AABW) currents. The deposits consist of fine-grained silty-clayey muds with very low carbonate contents. The sediment fluxes (1·45 g cm^?2 10^?3 yr) are higher than on the middle continental rise, as a consequence of fine-grained terrigenous supply derived from higher latitudes (Argentine Basin and Southern Ocean), and transported in the basin through the Vema Channel by the AABW currents. This material is characterized by high smectite and chlorite contents. These data reveal large sediment flux variations which are linked to distinct depth-related domains. Such a distribution is the consequence of the presence of two available sources of terrigenous sediments: (1) the Brazilian continental areas with a downslope material transport and a sediment distribution controlled by the morphology of the margin, and (2) the Argentine Basin with an alongslope material transport by deep-sea currents which dominate the sedimentation in the abyssal domains.     

Mineral inclusions in diamonds from the Sputnik kimberlite pipe, Yakutia

The Sputnik kimberlite pipe is a small “satellite” of the larger Mir pipe in central Yakutia (Sakha), Russia. Study of 38 large diamonds (0.7-4.9 carats) showed that nine contain inclusions of the eclogitic paragenesis, while the remainder contain inclusions of the peridotitic paragenesis, or of uncertain paragenesis. The peridotitic inclusion suite comprises olivine, enstatite, Cr-diopside, chromite, Cr-pyrope garnet (both lherzolitic and harzburgitic), ilmenite, Ni-rich sulfide and a Ti-Cr-Fe-Mg-Sr-K phase of the lindsleyite-mathiasite (LIMA) series. The eclogitic inclusion suite comprises omphacite, garnet, Ni-poor sulfide, phlogopite and rutile. Peridotitic ilmenite inclusions have high Mg, Cr and Ni contents and high Nb/Zr ratios; they may be related to metasomatic ilmenites known from peridotite xenoliths in kimberlite. Eclogitic phlogopite is intergrown with omphacite, coexists with garnet, and has an unusually high TiO₂ content. Comparison with inclusions in diamonds from Mir shows general similarities, but differences in details of trace-element patterns. Large compositional variations among inclusions of one phase (olivine, garnet, chromite) within single diamonds indicate that the chemical environment of diamond crystallisation changed rapidly relative to diamond growth rates in many cases. P-T conditions of formation were calculated from multiphase inclusions and from trace element geothermobarometry of single inclusions. The geotherm at the time of diamond formation was near a 35 mW/m² conductive model; that is indistinguishable from the Paleozoic geotherm derived by studies of xenoliths and concentrate minerals from Mir. A range of Ni temperatures between garnet inclusions in single diamonds from both Mir and Sputnik suggests that many of the diamonds grew during thermal events affecting a relatively narrow depth range of the lithosphere, within the diamond stability field. The minor differences between inclusions in Mir and Sputnik may reflect lateral heterogeneity in the upper mantle.     

H2O activity in concentrated KCl and KCl-NaCl solutions at high temperatures and pressures measured by the brucite-periclase equilibrium

H₂O activities in supercritical fluids in the system KCl-H₂O-(MgO) were measured at pressures of 1, 2, 4, 7, 10 and 15  kbar by numerous reversals of vapor compositions in equilibrium with brucite and periclase. Measurements spanned the range 550–900 °C. A change of state of solute KCl occurs as pressures increase above 2 kbar, by which H₂O activity becomes very low and, at pressures of 4 kbar and above, nearly coincident with the square of the mole fraction (x _H2O). The effect undoubtedly results primarily from ionic dissociation as H₂O density (ρ_H2O) approaches 1 gm/cm³, and is more pronounced than in the NaCl-H₂O system at the same P-T-X conditions. Six values of solute KCl activity were yielded by terminal points of the isobaric brucite-periclase T-x _H2O curves where sylvite saturation occurs. The H₂O mole fraction of the isobaric invariant assemblage brucite-periclase-sylvite-fluid is near 0.52 at all pressures, and the corresponding temperatures span only 100 °C between 1 and 15 kbar. This remarkable convergence of the isobaric equilibrium curves reflects the great influence of pressure on lowering of both KCl and H₂O activities. The H₂O and KCl activities can be expressed by the formulas: a _H2O = γ_H2O[x _H2O+(1 + (1 + α)x _KCl)], and a _KCL = γ_KCl[(1 + α)x _KCl/(x _H2O +(1 + α)x _KCl)]^(1 + α), where α is a degree of dissociation parameter which increases from zero at the lowest pressures to near one at high pressures and the γ's are activity coefficients based on an empirical regular solution parameter W: ln γ_i = (1 − x_i)²W. Least squares fitting of our H₂O and KCl activity data evaluates the parameters: α = exp(4.166 −2.709/ρ_H2O) − 212.1P/T, and W = (−589.6 − 23.10P) /T, with ρ_H2O in gm/cm³, P in kbar and T in K. The standard deviation from the measured activities is only ± 0.014. The equations define isobaric liquidus curves, which are in perfect agreement with previous DTA liquidus measurements at 0.5–2 kbar, but which depart progressively from their extrapolation to higher pressures because of the pressure-induced dissociation effect. The great similarity of the NaCl-H₂O and KCl-H₂O systems suggests that H₂O activities in the ternary NaCl-KCl-H₂O system can be described with reasonable accuracy by assuming proportionality between the binary systems. This assumption was verified by a few reconnaissance measurements at 10 kbar of the brucite-periclase equilibrium with a Na/(Na + K) ratio of 0.5 and of the saturation temperature for Na/(Na + K) of 0.35 and 0.50. At that pressure the brucite-periclase curves reach a lowest x _H2O of 0.45 and a temperature of 587 °C before salt saturation occurs, values considerably lower than in either binary. This double-salt eutectic effect may have a significant application to natural polyionic hypersaline solutions in the deep crust and upper mantle in that higher solute concentrations and very low H₂O activities may be realized in complex solutions before salt saturation occurs. Concentrated salt solutions seem, from this standpoint, and also because of high mechanical mobility and alkali-exchanging potential, feasible as metasomatic fluids for a variety of deep-crust and upper mantle processes. Received: 9 August 1996 / Accepted: 15 November 1996     

Effects of geology and human activity on the dynamics of salt-water intrusion in three coastal aquifers in southern Spain

 Mathematical modelling of salt-water intrusion processes in three aquifers on the southern coast of Spain (Río Verde, Río Vélez and Castell de Ferro) reveals that, although all three systems are subject to the same climate and seasonal over-exploitation, geological and human factors have very different effects on the dynamics of contamination. In the Río Verde aquifer, the most important influence is the high volume of extractions occurring during the dry season; in Río Vélez, the intrusion is strongly controlled by infiltration of water from the river to the aquifer, and, in the Castell de Ferro system, an intensely karstified carbonate massif lying in contact with both the sea water and the detrital aquifer represents the main entrance point for influx of sea water and subsequent washing of the aquifer. We have undertaken a mathematical simulation of various possible measures to counteract intrusion, according to the specific characteristics of the process in each aquifer. These measures include artificial recharge, use of natural recharge from the river as a hydraulic barrier, and the construction of a low-permeability barrier. Received: 5 December 1995 · Accepted: 12 April 1996