Stratigraphic modelling of platform architecture and carbonate production: a Messinian case study (Sorbas Basin,SE Spain)

The late Messinian mixed carbonate‐siliciclastic platforms of the Sorbas Basin, known as the Terminal Carbonate Complex, record significant changes in carbonate production and geometry. Their facies and stratigraphic architecture result from complex interactions between base‐level fluctuations, evaporite deformation/dissolution and detrital inputs. A 3D quantitative approach (with DIONISOS software) is used to explore the basin‐scale platform architecture and to quantify the carbonate production of the Terminal Carbonate Complex. The modelling strategy consists in integrating detailed 2D field‐based transects and modern carbonate system parameters (e.g. carbonate production rates, bathymetric and hydrodynamic ranges of production). This approach limits user impact and so provides more objective output results. Tests are carried out on carbonate production rates, subsidence and evaporite deformation/dissolution. Numerical modelling provides accurate predictions of geometries, facies distributions and depositional sequence thicknesses, validated by field data. Comparative statistical testing of the field transects and of the various model outputs are used to discern the relative contribution of the parameters tested to the evolution of basin filling. The 3D visualization and quantification of the main carbonate producers (ooids and microbialites) are discussed in terms of changes in base‐level and detrital supply. This study demonstrates that base‐level fluctuations have the greatest impact on the carbonate budget. Evaporite deformation/dissolution affects the type and amount of carbonate production, inducing a transition from an ooid‐ to microbialite‐dominated system and also has a major effect on stratigraphic architecture by inducing the migration of depocentres. The numerical modelling results obtained using modern carbonate system parameters could also be applied to subsurface ooid‐microbialite reservoirs, and the Terminal Carbonate Complex is a good analogue for such systems.     

External controls on the distribution,fabrics and mineralization of modern microbial mats in a coastal hypersaline lagoon,Cayo Coco (Cuba)

Active, carbonate‐mineralizing microbial mats flourish in a tropical, highly evaporative, marine‐fed lagoonal network to the south of Cayo Coco Island (Cuba). Hypersaline conditions support the development of a complex sedimentary microbial ecosystem with diverse morphologies, a variable intensity of mineralization and a potential for preservation. In this study, the role of intrinsic (i.e. microbial) and extrinsic (i.e. physicochemical) controls on microbial mat development, mineralization and preservation was investigated. The network consists of lagoons, forming in the interdune depressions of a Pleistocene aeolian substratum; they developed due to a progressive increase in sea‐level since the Holocene. The hydrological budget in the Cayo Coco lagoonal network changes from west to east, increasing the salinity. This change progressively excludes grazers and increases the saturation index of carbonate minerals, favouring the development and mineralization of microbial mats in the easternmost lagoons. Detailed mapping of the easternmost lagoon shows four zones with different flooding regimes. The microbial activity in the mats was recorded using light–dark shifts in conjunction with microelectrode O₂ and HS^? profiles. High rates of O₂ production and consumption, in addition to substantial amounts of exopolymeric substances, are indicative of a potentially strong intrinsic control on mineralization. Seasonal, climate‐driven water fluctuations are key for mat development, mineralization, morphology and distribution. Microbial mats show no mineralization in the permanently submersed zone, and moderate mineralization in zones with alternating immersion and exposure. It is suggested that mineralization is also driven by water‐level fluctuations and evaporation. Mineralized mats are laminated and consist of alternating trapping and binding of grains and microbially induced magnesium calcite and dolomite precipitation. The macrofabrics of the mats evolve from early colonizing Flat mats to complex Cerebroid or Terrace structures. The macrofabrics are influenced by the hydrodynamic regime: wind‐driven waves inducing relief terraces in windward areas and flat morphologies on the leeward side of the lagoon. Other external drivers include: (i) storm events that either promote (for example, by bioclasts covering) or prevent (for example, by causing erosion) microbial mat preservation; and (ii) subsurface degassing, through mangrove roots and desiccation cracks covered by Flat mats (i.e. forming Hemispheroids and Cerebroidal structures). These findings provide in‐depth insights into understanding fossil microbialite morphologies that formed in lagoonal settings.     

The geochemistry of gem opals as evidence of their origin

Seventy-seven gem opals from ten countries were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) through a dilution process, in order to establish the nature of the impurities. The results are correlated to the mode of formation and physical properties and are instrumental in establishing the geographical origin of a gem opal. The geochemistry of an opal is shown to be dependant mostly on the host rock, at least for examples from Mexico and Brazil, even if modified by weathering processes. In order of decreasing concentration, the main impurities present are Al, Ca, Fe, K, Na, and Mg (more than 500 ppm). Other noticeable elements in lesser amounts are Ba, followed by Zr, Sr, Rb, U, and Pb. For the first time, geochemistry helps to discriminate some varieties of opals. The Ba content, as well as the chondrite-normalized REE pattern, are the keys to separating sedimentary opals (Ba > 110 ppm, Eu and Ce anomalies) from volcanic opals (Ba < 110 ppm, no Eu or Ce anomaly). The Ca content, and to a lesser extent that of Mg, Al, K and Nb, helps to distinguish gem opals from different volcanic environments. The limited range of concentrations for all elements in precious (play-of-color) compared to common opals, indicates that this variety must have very specific, or more restricted, conditions of formation. We tentatively interpreted the presence of impurities in terms of crystallochemistry, even if opal is a poorly crystallized or amorphous material. The main replacement is the substitution of Si⁴⁺ by Al³⁺ and Fe³⁺. The induced charge imbalance is compensated chiefly by Ca²⁺, Mg²⁺, Mn²⁺, Ba²⁺, K⁺, and Na⁺. In terms of origin of color, greater concentrations of iron induce darker colors (from yellow to “chocolate brown”). This element inhibits luminescence for concentrations above 1000 ppm, whereas already a low content in U (≤ 1 ppm) induces a green luminescence.     

Oxygen isotopes composition of sapphires from the French Massif Central: implications for the origin of gem corundum in basaltic fields

Alluvial and colluvial gem sapphires are common in the basaltic fields of the French Massif Central (FMC) but sapphire-bearing xenoliths are very rare, found only in the Menet trachytic cone in Cantal. The O-isotope composition of the sapphires ranges between 4.4 and 13.9‰. Two distinct groups have been defined: the first with a restricted isotopic range between 4.4 and 6.8‰ (n = 22; mean δ¹⁸O = 5.6 ± 0.7‰), falls within the worldwide range defined for blue-green-yellow sapphires related to basaltic gem fields (3.0 < δ¹⁸O < 8.2‰, n = 150), and overlaps the ranges defined for magmatic sapphires in syenite (4.4 < δ¹⁸O < 8.3‰, n = 29). A second group, with an isotopic range between 7.6 and 13.9‰ (n = 9), suggests a metamorphic sapphire source such as biotite schist in gneisses or skarns. The δ¹⁸O values of 4.4–4.5‰ for the blue sapphire-bearing anorthoclasite xenolith from Menet is lower than the δ¹⁸O values obtained for anorthoclase (7.7–7.9‰), but suggest that these sapphires were derived from an igneous reservoir in the subcontinental spinel lherzolitic mantle of the FMC. The presence of inclusions of columbite-group minerals, pyrochlore, Nb-bearing rutile, and thorite in these sapphires provides an additional argument for a magmatic origin. In the FMC lithospheric mantle, felsic melts crystallized to form anorthoclasites, the most evolved peraluminous variant of the alkaline basaltic melt. The O-isotopic compositions of the first group suggests that these sapphires crystallized from felsic magmas under upper mantle conditions. The second group of isotopic values, typified for example by the Le Bras sapphire with a δ¹⁸O of 13.9‰, indicates that metamorphic sapphires from granulites were transported to the surface by basaltic magma.     

The UBV orbital phase curves of Rhea,Dione, and Tethys

Orbital phase curves are reported for Rhea (SV), Dione (SIV), and Tethys (SIII). Variations of 0.2 mag. and 0.8 mag. were found for Rhea and Dione respectively, while no variation of Tethys was detected. Some color variations are also reported.     

Effects of scattered solar radiation and absorption by SO2 on the photodissociation processes in the stratosphere of Venus

In the stratosphere of Venus, the available luminous flux which locally produces the photodissociation processes at a given altitude may be divided into three parts: direct incoming downward flux, flux resulting from the reflection on the surface of the clouds, and flux due to molecular scattering. A relatively simple computation method has been used to evaluate the relative importance of these three parts at altitudes between 65 and 100 km. It is shown that the extra contribution of the reflected and scattered fluxes to photodissociation processes cannot be neglected in the uv and visible regions. In the case of SO₂, for instance, which presents an absorption band in the uv, the photodissociation coefficient is increased 30% due to these effects. Calculations of the photodissociation coefficients of CO₂, O₃, H₂S, and SO₂ are presented. As a result of the increase by 60% in the ozone photolysis rate, the calculated O₂ infrared band at 1.27 μm is larger by a factor of nearly 2 than is expected from a calculation without taking albedo or scattering into account.