The application of cathodoluminescence to interpreting the diagenesis of an ancient calcrete profile

A laterally extensive calcrete profile has been identified in the Late Asbian (Lower Carboniferous) shallow marine shelf limestones of the Llangollen area, North Wales. The upper surface of the profile is defined by a laterally discontinuous palaeokarstic surface and by laminated calcareous crusts which developed within the underlying limestone. The profile contains a unique series of early pore-filling vadose cements which only occur down to 1 m below the palaeokarstic surface. Cathodoluminescence reveals that these cements pre-date the late pore-filling meteoric phreatic cements which occur throughout local Asbian lithologies. A spar cement stratigraphy has been established for the calcrete profile. Subaerial vadose cements comprise two generations of non-luminescent cement, followed by a brightly luminescent generation which occasionally shows an acicular habit. This needle-fibre calcite represents the final stage of vadose cementation. Precipitation of vadose cements was contemporary with subaerial alteration and micritization of the limestone. Textures, visible only with cathodoluminescence, provide evidence of recurrent periods of fabric dissolution. The most extensive phase of dissolution occurred immediately after the precipitation of the non-luminescent subaerial vadose cements. Several different textures have been recorded, each reflecting the morphology of a partially dissolved substrate. Dissolution textures are generally confined to the walls of the larger pores and to early brecciation fractures. These probably acted as fluid pathways in the calcrete during early subaerial diagenesis. Much of the non-marine micrite in the calcrete profile appears as needle-fibre calcite under cathodoluminescence. This acicular calcite was probably formed in response to localized supersaturation of meteoric pore fluids caused by periods of near-surface evaporation. Since needle-fibre luminescence is strongly variable, these ambient conditions are not believed to have directly controlled the activator ion concentrations of cementing pore waters. Needle-fibre calcite is considered to be a cement precipitate which has almost completely recrystallized to micrite, probably during the late stages of subaerial diagenesis. Two generations of subaerial micrite which define a ‘micrite stratigraphy’, have been distinguished under cathodoluminescence. Reconstructing the diagenetic history of this ancient calcrete profile has revealed that subaerial alteration was multistaged, with many diagenetic processes acting simultaneously during a single phase of emergence.     

Counting and Sampling Errors in Modal Analysis by Point Counter

There are three principal sources of errors involved in modalanalyses by point counting: (a) operator variation, (b) thedetermination of area by grid counting, (c) the determinationof volumes from areal analyses. Operator variation is likely to be negligible but (b) and (c)may combine to give a variance equal to, or less than where A=measurement area, a=grid spacing, R=grain radius, andp=the fraction of the particular mineral in the rock (Hasofer,1963).     

OBLIQUE EMISSION OF WHISTLER-MODE WAVES AROUND INTERPLANETARY SHOCKS

We present a study of whistler-mode wave generation and wave particle interaction in the vicinity of interplanetary shocks as observed by the Ulysses spacecraft. Generally the whistler-mode waves (measured in the frequency range 0.22–448 Hz) are observed downstream of the shocks where they persist for some hours. From the electron distribution functions (EDF) in the energy range 1.6 to 862 eV measured by the spacecraft, we compute the wave growth rate of the electromagnetic electron cyclotron and Landau instabilities for the case of oblique propagation of the waves with respect to the interplanetary magnetic field (IMF) B. In general, in agreement with the wave measurements, the instability grows mostly downstream of the shock fronts. Following the wave activity, velocity space diffusion of the electrons results in a marginally stable state with some sporadic fluctuations.     

A procedure for determining the nature of Mercury's core

Abstract— We review the assertion that the precise measurement of the second degree gravitational harmonic coefficients, the obliquity, and the amplitude of the physical libration in longitude, C₂₀, C₂₂, θ, and φ₀, for Mercury are sufficient to determine whether or not Mercury has a molten core (Peale, 1976). The conditions for detecting the signature of the molten core are that such a core not follow the 88‐day physical libration of the mantle induced by periodic solar torques, but that it does follow the 250 000‐year precession of the spin axis that tracks the orbit precession within a Cassini spin state. These conditions are easily satisfied if the coupling between the liquid core and solid mantle is viscous in nature. The alternative coupling mechanisms of pressure forces on irregularities in the core‐mantle boundary (CMB), gravitational torques between an axially asymmetric mantle and an assumed axially asymmetric solid inner core, and magnetic coupling between the conducting molten core and a conducting layer in the mantle at the CMB are shown for a reasonable range of assumptions not to frustrate the first condition while making the second condition more secure. Simulations have shown that the combination of spacecraft tracking and laser altimetry during the planned MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, Ranging) orbiter mission to Mercury will determine C₂₀, C₂₂, and θ to better than 1% and φ₀ to better than 8%—sufficient precision to distinguish a molten core and constrain its size. The possible determination of the latter two parameters to 1% or less with Earth‐based radar experiments and MESSENGER determination of C₂₀ and C₂₂ to 0.1% would lead to a maximum uncertainty in the ratio of the moment of inertia of the mantle to that of the whole planet, C_m/C, of ?2% with comparable precision in characterizing the extent of the molten core.     

The early diagenetic origin of Lower Carboniferous mottled limestones (pseudobreccias)

Dark mottles are a prominent and widespread feature of the regressive, cyclic, shallow marine limestones which form the late Asbian succession in many parts of Britain. The colour difference which defines mottles in outcrop is caused by distinct but often subtle petrographic differences in the limestone fabric. Specifically, mottles contain a light brown coloured opaque inclusion-rich calcite spar with characteristic dull brown luminescence; ‘mottle spar’. Outside mottle margins this calcite spar is absent, with clear inequant blocky cements forming the pore filling phase. ‘Mottle spar’ comprises a fabric of irregular crystals predominantly 5–40 μm in diameter, with more regular crystals up to 100 μm diameter often occurring in intraparticle and large interparticle pores. Under cathodoluminescence, ‘mottle spar’ displays crystal morphologies and growth patterns which indicate that both localized neomorphism and patchy cementation contributed to mottle formation. Cathodoluminescence cement stratigraphy shows that ‘mottle spar’ pre-dates all other major pore filling cements in the local Asbian succession, but post-dates marine micritization. ‘Mottle spar’ sharply defines the moulds of former aragonitic allochems which are now filled by the later clear, inequant spar cements. This shows that aragonite dissolution occurred after the formation of ‘mottle spar’. Mottles in calcretes contain unaltered allochems which have been protected from the effects of subaerial micritization by ‘mottle spar’, although mottles are often affected by subaerial brecciation. This evidence shows that mottles formed during early diagenesis; after marine micritization, but before dissolution of aragonite, subaerial exposure and meteoritic phreatic cementation. Mottles represented lithified patches of very low porosity which are interpreted to have formed in the marine/freshwater mixing zone, during the repeated phases of regression and emergence in the late Asbian.