Relict magnesian calcite oolite and subsidence of the Amazon shelf

Oolitic sediments occur over large portions of the outer continental shelf off northernmost Brazil and southern French Guiana. Carbon-14 dates, stable isotope values and analogies with similar deposits from other areas in the world indicate that the ooids were formed in shallow water (possibly protected hypersaline environments) during low stands of sea level. In contrast to both modern ooids and other relict ooids, however, the Amazon shelf ooids are composed entirely of magnesian calcite (12 mol % MgCO₃) and possess radial crystal microstructures. A comparison of water depths and ages of the Amazon ooids with the generally accepted eustatic sea level curve indicates that the Amazon shelf subsided more than 100 m 16-21 thousand years ago. Prior to and subsequent to the subsidence, however, the outer shelf remained relatively stable.     

Pressure effect on magnesian calcite coating calcite and synthetic magnesian calcite seeds added to seawater in a closed system

When pure crystalline calcite seeds are added to supersaturated seawater, precipitation results in a coating which with time equilibrates at atmospheric pressure with seawater and corresponds to a calcite containing probably only 2 or 3% of MgCO₃ (mole fraction).If synthetic crystalline magnesian calcite is added, the surface layer equilibrates not only with respect to seawater but also in relation with the crystalline sites initiating precipitation. Adding Mg_0.03Ca_0.97CO₃ results in a coating with a solubility close to that of calcite. This confirms that the surface coating on pure calcite seeds contains about 2 or 3% MgCO₃ (K'_sp = 10?6.30).The surface layer precipitated on a synthetic Mg_0.08Ca_0.92CO₃ equilibrates finally with a carbonate more soluble than calcite (K'_sp = 10?5.94) corresponding to the seeds composition.Experiments at 1000 kg cm t-2 imply that when magnesian calcites are precipitated at the surface of calcite or magnesian calcite seeds, the precipitate must be hydrated, otherwise pressure accelerated recrystallization or rearrangement with loss of Mg would thermodynamically be impossible.By changing the pressure of a seawater sample originally saturated with a solid carbonate phase, changes in pH result from the effect of pressure on the dissociation constants of carbonic acid and boric acid causing either undersaturation or supersaturation with respect to the solid. By changing pressure we can show whether precipitation, dissolution and recrystallization are reversible processes if pH is taken as criteria of reversibility.     

Precipitation and lithification of magnesian calcite in the deep-sea sediments of the eastern Mediterranean Sea*

Magnesian calcite is an important sedimentary component in the deep-sea sediments of the eastern Mediterranean Sea, comprising an average of 20–50% of the carbonate fraction in most areas. The lack of any obvious biogenic source, plus similarities with magnesian-rich lutites from the Red Sea and deep-sea cements from other areas suggest that this magnesian calcite was precipitated inorganically. Although the exact mode of precipitation is not understood at present, it probably occurred at the water-sediment interface under elevated salinity and temperature conditions, such as those present in the modern eastern Mediterranean. Precipitation did not occur during periods of lower temperatures and/or salinities such as during the stagnant conditions caused by the influx of fresh waters from melting Pleistocene glaciers. The eastern Mediterranean magnesium-rich sediments appear to represent an intermediate stage between normal deep-sea sediments and those from the warm hpyersaline Red Sea. Normal deep-sea carbonates are composed almost entirely of biogenic calcite, whereas the Red Sea magnesian calcite alternates with layers of aragonite that were precipitated under elevated salinity and temperature conditions brought about by lowered stands of sea level.     

Biological uptake and accumulation of silica on the Amazon continental shelf

Water column and seabed samples were obtained from 92 stations on the Amazon continental shelf during October of 1979. Uptake of silica near and southeast of the river mouth began at a salinity of 8%. and accounted for 17% of the riverine silica flux to this region. Uptake northwest of the river mouth began at a salinity of 20%. and resulted in 33% removal of the riverine silica flux. Examination of filtered suspended solids revealed abundant diatoms in the surface waters, including Coscinodiscus. Skeletonema, Synedra. and Thalassiosira. The biological uptake of silica appears to be dependent on three factors: turbidity, turbulence, and nutrient availability. There was no evidence of abiological removal of silica in the Amazon estuary. 75 to 88% of the silica removed from surface waters by diatoms dissolves prior to accumulation in the seabed. Based on the mean biogenic silica content of shelf sediment (0.25%) and estimates of rates of sediment accumulation, the biogenic silica accumulation rate on the shelf is 2 × 10¹² g/yr, which represents only 4% of the dissolved silica supplied by the Amazon River. Biological uptake of silica in estuarine surface waters may not accurately reflect permanent removal of biogenic silica to the seabed because of dissolution which occurs in bottom waters and near the sediment-water interface.     

Microtexture of dolomite within exsolved magnesian calcite — examples from the Damara orogen (Namibia)

Exsolved magnesian calcite grains containing rods of dolomite were etched to remove the calcite matrix and investigated with a scanning electron microscope. The rods were found to be composed of small rhombohedral dolomite crystals. The single rhombohedrons are stacked with the threefold axis subparallel in all rods of the exsolved host crystal. The curvature of exsolution rods in the host is due to a step by step displacement of the single rhombohedrons.     

Uranium geochemistry on the Amazon shelf: Evidence for uranium release from bottom sediments

In Amazon-shelf waters, as salinity increases to 36.5 × 10⁻³, dissolved uranium activities increase to a maximum of 4.60 dpm 1⁻¹. This value is much higher than the open-ocean value (2.50 dpm 1⁻¹), indicating a source of dissolved uranium to shelf waters in addition to that supplied from open-ocean and riverine waters. Uranium activities are much lower for surface sediments in the Amazon-shelf seabed (mean: 0.69 ±.09 dpm g⁻¹) than for suspended sediments in the Amazon River (1.82 dpm g⁻¹). Data suggest that the loss of particulate uranium from riverine sediments (and the consequent input of dissolved uranium to shelf waters) is probably the result of uranium desorption from the ferric-oxyhydroxide coatings on sediment particles, and/or uranium release by mobilization of the ferric oxyhydroxides. The total flux (i.e., riverine flux plus desorbed-remobilized particulate flux) of dissolved ²³⁸U from the Amazon shelf (about 1.2 × 10¹⁵ dpm yr⁻¹) constitutes about 15% of uranium input to the world ocean, commensurate to the Amazon River's contribution to world river-water discharge (approximately 18%). Measurement of only the riverine flux of dissolved ²³⁸U underestimates, by a factor of about 5, the flux of dissolved²³⁸U from the Amazon shelf to the open ocean.     

白云岩的残余结构及由此引发的孔隙分类问题

对碳酸盐岩的结构分类方法进行了评述,但目前的分类并未对白云岩的残余结构问题进行充分论述.为了能更准确地恢复岩相古地理和进行精细的成岩研究,有必要对残余结构进行细分.白云岩的残余结构是由于原始粒屑结构的灰岩,经白云岩化、重结晶、溶蚀、胶结等成岩作用之后,岩石原始结构遭到不同程度的破坏,但仍残留有部分原始结构信息而形成的....     

Stratigraphic simulations of the shelf of the Gulf of Lions: testing subsidence rates and sea‐level curves during the Pliocene and Quaternary

Determining the relative importance of factors interacting to control stratigraphic organization is a key issue in sedimentology. The Pliocene‐Quaternary chronostratigraphy on the Gulf of Lions platform is still poorly constrained, giving rise to different interpretations of the evolution of its subsidence through time. This paper examines the Pliocene‐Quaternary sedimentary filling of the Gulf of Lion's shelf with Dionisos, a numerical stratigraphic model. Our results show that a constant subsidence rate accurately reproduces the observed geometries, whereas a varying subsidence rate reproduces them only if the acceleration of subsidence is limited. At this time‐scale, a third‐order eustatic curve is also reappraised: a higher resolution curve (built using δ¹⁸O measurements) gives a more realistic restitution of our stratigraphic markers. Finally, the constant subsidence rate and sediment fluxes implied in these modellings are discussed relative to climate and local factors of sedimentation.     

Variations in diagenetic environments of the coral rag and underlying oolite of the osmington oolite formation (upper oxfordian) of the Oxford—Berkshire area,U.K., and their implications

Petrographic and geochemical evidence shows convincingly that the diagenetic environment changes from marine phreatic through freshwater phreatic to vadose during lithification of the oolites and the overlying Coral Rag of the Osmington Oolite Formation in the Oxford—Berkshire area. This change is thought to be a consequence of tectonic uplift following deposition of the Coral Rag. Extensive neomorphic replacement of the corals took place in a freshwater phreatic environment created by tectonic uplift. Retention of Sr in the Coral Rag is related to differing diagenesis: pore-fluids remained for protracted periods within the Coral Rag as the downward flow was greatly retarded because of obliteration of porosity in the oolites by marine phreatic cementation before deposition of the Coral Rag.     

Genesis and composition of lazurite in magnesian skarns

The genesis and composition of lazurite was considered in the metasomatic rocks and deposits of the Baikal area, Pamirs, Hindu Kush, and other regions. It was shown that formation of lazurite is related to the prograde bimetasomatic or contact-metasomatic magnesian skarn processes developing in dolomites. The skarnized sedimentary or metamorphic rocks should be salt-bearing, contain sulfur-bearing minerals, and be affected by Cl-bearing hydrothermal solutions. Both abyssal and hypabyssal skarn bodies show distinct zoning. Depending on the PT parameters, their metasomatic columns can vary, with the preservation of the Mg/Ca ratio from the carbonate precursor. The formation of contact-metasomatic lazurite mineralization is promoted by high-alkali magmatic solutions, which cause the replacement of dolomites by skarn. Lazurite can also be formed during the postmagmatic stage, owing to a change in the mineral composition of the adjacent aluminosilicate and magnesian-skarn rocks at abyssal and hypabyssal deposits around the world. The occurrence of authigenic lazurite emphasizes its polygenetic formation. Original Russian Text ? S.M. Aleksandrov, V.G. Senin, 2006, published in Geokhimiya, 2006, No. 10, pp. 1053–1067.     

The origin of ferroan and magnesian anorthosites in the lunar crust

<正>1 Introduction The widely accepted standard model for the lunar feldspathic crust is:the early Moon was wholly or mostly molten,forming Lunar Magma Ocean(LMO).Olivine and pyroxene crystallized first from that magma ocean and sank     

Determination of presence of pyrope and magnesian olivine in diamond prospecting

The paper discusses use of a glass tube (2-4 mm diameter and 3-5 mm deep cup at end) and heavy liquids for the identification of pyrope and magnesian olivine in prospecting for diamonds deposits.—V. E. Alexandrov.     

Geodynamic significance of spasmodic, cretaceous, rapid subsidence rates, continental shelf U.S.A.

Subsurface Cretaceous shallow-water sedimentary rocks of the exposed continental shelf between New York and southern Florida provide evidence of two periods of relatively high sedimentation and subsidence rates. North and south of the Cape Fear and Peninsula Arches, Late Aptian to Early Albian rates range from 50 to over 150 meters/ million years; in Coniacian/Santonian time, rates exceeding 30 meters/million years occur sporadically, even across the area of the Arches. A common, global phenomenon that affected both cratons and continental shelves is believed to have caused these periods of relatively rapid basin subsidence and consequent rapid sediment accumulation.     

Relict sulfides in terrigenous rocks of the Upper Kolyma Region

In terrigenous flysch strata of the Upper Kolyma Region in northeastern Russia, microaggregates of sulfides with spherical form are widespread. During dynamic metamorphism, spherical sulfide units were segregated into lenticular aggregates along the cleavage planes, were as seed, or were recrystallized into faceted metacrystals. Further stress on the rock sometimes led to a morphologically expressed particular dissolution of pyrite crystals along the cleavage directions, and to their depletion with admixture elements. A more substantial effect led to transformation of pyrite into pyrrotine with inclusions of chalcopyrite and Fe-Ni-Co-sulfoarsenides. Relict sulfides reflect, to a certain degree, the primary geochemical condition of the sedimentation period and its further evolution. When studying the numerous sulfidization zones, the composition of relict sulfides allows us to predict the geochemical specialization and the degree of inheritance of post-sedimentation mineralization.     

Deep subsidence of the early catagenesis zones of organic matter in the sedimentary rocks of Northeastern Sakhalin shelf at high thermal regime of the basin

Doklady Earth Sciences -