Processes controlling precipitation of carbonate cement and dissolution of silica in reef and near-reef settings

Carbonate cementation in the surface layer of reefs and beachrock eliminates porosity and partially replaces detrital quartz grains. The uptake and release of CO₂ by photosynthesis and respiration in reef communities cause a shift in the carbonate buffer system of seawater. Field studies and experimentation simulating the natural settings show minimum values of CO₂ (1.9 mmoles 1^?1) and HCO₃^? (2.4 meq 1^?1) in association with maximum values of pH (9.8) and O₂ (> 100% oxygen saturation) in waters covering corals and algae prior to sunset. The converse is true for these variables prior to sunrise, when minimum values of pH (7.6) and O₂ (<66% oxygen saturation) occur with maximum values of CO₂ (2.7 mmoles 1^?1) and HCO₃^? (2.7 meq 1^?1). Experimental tanks containing plain seawater showed almost no diurnal variability in pH (a constant 7.5–7.6) or O₂ (80–90% oxygen saturation) measurements. Seawater adjacent to reef biomass, with elevated pH and supersaturated with calcium carbonate, is periodically pumped into the underlying reef and beach sediments due to pressure-buildups between the reef framework or algal zones and open waters. Carbonate precipitation and quartz dissolution follow. As the system equilibrates, pH values progressively decrease.     

Proterozoic aeolian quartz arenites from the Hornby Bay Group, Northwest Territories, Canada: Implications for Precambrian aeolian processes

The Hornby Bay Group is a Middle Proterozoic 2.5 km-thick succession of terrestrial siliciclastics overlain by marine siliciclastics and carbonates. A sequence of conglomeratic and arenaceous rocks at the base of the group contains more than 500 m of mature hematitic quartz arenite interpreted to have been deposited by migrating aeolian bedforms. Bedforms and facies patterns of modern aeolian deposits provided a basis for recognizing two sequences of aeolian arenite. Both sequences interfinger with alluvial—wadi fan conglomerates and arenites deposited by braided streams. Depositional processes, facies patterns and paleotopographic position of the arenites are consistent with modern sand sea dynamics.Distal aeolian facies in both sequences are composed of trough crossbed megasets deposited by climbing, sinuous-crested, transverse dunes. Megasets comprise a gradational assemblage of tabular to wedge-planar cosets formed by deflation/reactivation of dune lee slopes and migration of smaller superposed aeolian bedforms (small dunes and wind ripples). Megasets in the proximal facies are thinner, display composite internal stratification and have a tabular-planar geometry which suggests that they were formed by smaller, straight-crested transverse dunes. Most stratification within the crossbeds is inferred to have formed by the downwind climbing of aeolian ripples across the lee slopes of dunes.Remarkably few Precambrian aeolian deposits have been reported previously. This seems anomalous, because most Precambrian fluvial sediments appear to have been deposited by low sinuosity (braided) streams, the emergent parts of which are prime areas for aeolian deflation. Frequent floods and rapid lateral migration of Precambrian humid climate fluvial systems probably restricted aeolianite deposition to arid paleoclimates. Thus the apparent anomaly may reflect non-recognition and/or non-preservation of aeolianites and/or variations in some aspect of sand sea formation and migration unique to the Precambrian. Reconstruction of the Hornby Bay Group aeolianites using recently developed criteria for their recognition suggests that the latter reason did not exert a strong influence.     

Electronic absorption spectra of Fe2+ ions in oxygen-based rock-forming minerals at temperatures between 297 and 600 K

 Polarized electronic single crystal spectra of natural Fe²⁺ ion-bearing oxygen-based minerals, in which ferrous ions enter octahedral sites of different symmetry and distortion (olivine, cordierite, ortho- and clinopyroxene, amphibole), eightfold sites in garnet (almandine) and clinopyroxene (M2), and tetrahedral sites in spinel, were studied at temperatures from 300 to ca. 600 K. In the minerals studied, the spin-allowed bands of Fe²⁺ display rather variable temperature behaviour. In most cases, due to the thermal expansion of the Fe²⁺-bearing polyhedra, bands shift to lower energies upon increasing temperature, though there are some exceptions to this rule: in cases of other than sixfold octahedral or close to octahedral coordination, in almandine and spinel the bands shift to higher energies, which can be explained by an increase in distortions of the Fe²⁺-bearing polyhedra. Splitting of the excited ⁵ E _g-level of Fe²⁺ ions usually, but not always, increases with temperature, reflecting thermally induced increase in distortion of the Fe²⁺-bearing sites in the minerals studied. Integral intensities of the bands in question do not always obey the general rule, according to which intensity should increase with temperature, when the 3d ^N-centred site is centrosymmetric, or should remain unchanged when the 3d ^N site lacks an inversion centre. The experimental results show that the response of the characteristics of absorption bands such as width, intensity and energy caused by dd transitions of Fe²⁺ in oxygen-based minerals to increasing temperature is not always uniform and is at variance with expectation. This temperature dependence cannot be used directly to solve band assignment problems, as earlier proposed in the literature. Received: 22 December 1999 / Accepted: 30 October 2000     

Effect of Fe3+-bearing point defects on the UV-spectra of two natural olivines

Polarized near-UV spectra have been recorded on 20 μm diameter spots on oriented crystals of microprobe-analyzed olivines from Baikhal Rift, Fa_8.8 (I) and Seberget, Fa_9.3 (II), which have formed under different fO₂ and, therefore, are expected to contain Fe³⁺-bearing point defects in different concentrations. These should be reflected in the UV-spectra of such minerals (Cemic et al. 1986). The spectra obtained confirm these predictions: The difference in α _{Y, 26500} in both samples indicates a difference in Fe³⁺-site fractions of ΔX_Fe. = 1.78-10⁻⁴, which may be related to an fO₂ about ten times higher for the formation of olivine II compared to I.     

Aluminum-26 in meteorites—VII. Ureilites,their unique radiation history

Cosmogenic ²⁶A1 activities have been measured by γ-γ coincidence counting in the three ureilites which had not previously been studied. The values in dpm/kg are: Dingo Pup Donga, 38.4 ± 2.4; North Haig, 39.3 ± 4.8; Dyalpur, 55.8 ± 4.8. Five of the six known ureilites thus have lower ²⁶A1 contents, 63 per cent to 77 per cent, than the calculated saturation values, in marked contrast to most other stony meteorites. This cannot be attributed to short cosmic ray exposure ages. Nor do size and depth effects account for the narrow range of ²⁶A1 activities, because a nuclear particle track study indicates that preatmospheric radii were highly variable, from ≥ 40 cm for Goalpara to only a few cm for Dingo Pup Donga. By default, the most likely explanation is that the ureilites had much smaller or much larger orbits than all other stony meteorites.     

Magnetism and thermal evolution of the terrestrial planets

Of the terrestrial planets, Earth and probably Mercury possess substantial intrinsic magnetic fields generated by core dynamos, while Venus and Mars apparently lack such fields. Thermal histories are calculated for these planets and are found to admit several possible present states, including those which suggest simple explanations for the observations; whule the cores of Earth and Mercury are continuing to freeze, the cores of Venus and Mars may still be completely liquid. The models assume whole mantle convection, which is parameterized by a simple Nusselt-Rayleigh number relation and dictates the rate at which heat escapes from the core. It is found that completely fluid cores, devoid of intrinsic heat sources, are not likely to sustain thermal convection for the age of the solar system but cool to a subadiabatic, conductive state that can not maintain a dynamo. Planets which nucleate an inner core continue to sustain a dynamo because of the gravitational energy release and chemically driven convection that accompany inner core growth. The absence of a significant inner core can arise in Venus because of its slightly higher temperature and lower central pressure relative to Earth, while a Martian core avoids the onset of freezing if the abundance of sulfur in the core is ?15% by mass. All of the models presented assume that (I) core dynamos are driven by thermal and/or chemical convection; (ii) radiogenic heat production is confined to the mantle; (iii) mantle and core cool from initially hot states which are at the solidus and superliquidus, respectively; and (iv) any inner core excludes the light alloying material (sulfur or oxygen) which then mixes uniformly upward through the outer core. The models include realistic pressure and composition-dependent freezing curves for the core, and material parameters are chosen so that the correct present-day values of heat outflow, upper mantle temperature and viscosity, and inner core radius are obtained for the earth. It is found that Venus and Mars may have once had dynamos maintained by thermal convection alone. Earth may have had a completely fluid core and a dynamo maintained by thermal convection for the first 2 to 3 by, but an inner core nucleates and the dynamo energetics are subsequently dominated by gravitational energy release. Complete freezing of the Mercurian core is prohibited if it contains even a small amount of sulfur, and a dynamo can be maintained by chemical convection in a thin, fluid shell.