Recrystallization Of Aragonite Research Articles

Uranium loss and aragonite–calcite age discordance in a calcitized aragonite stalagmite

We analyzed uranium-series concentrations and isotopic ratios in a mixed aragonite and calcite stalagmite from Juxtlahuaca Cave, from the Sierra Madre del Sur of Mexico. The U-series data for the aragonite layers return highly precise and stratigraphically correct ages over the past ca. 4300 years. In contrast, age determinations from calcite layers are too old by several hundred years relative to the precise aragonite ages, have analytical uncertainties an order of magnitude larger than aragonite ages, and yield ages that do not overlap the aragonite ages within analytical uncertainties. Based on geochemical and petrographic observations, we interpret the calcite layers to have formed from recrystallization of aragonite soon after primary aragonite deposition. Calcite occurs as discontinuous lenses on and off the growth axis, and laminae can be traced between aragonite and calcite layers, demonstrating that visible growth banding is not effaced in the recrystallization process. Paired aragonite-calcite U-series data from coeval stratigraphic layers demonstrate that uranium concentrations decrease by two orders of magnitude during calcitization, and result in decreased (234U/238U). Uranium loss during diagenesis mimics a need for an age correction using an initial 230Th/232Th ratio one to two orders of magnitude larger than the bulk Earth ratio of 4.4 ± 2.2 × 10−6. A need for apparent high initial 230Th/232Th ratios results from ingrowth of 230Th during 234U decay.

Interactions between dissolved silica and carbonate minerals: An experimental study at 25–50°C

Replacement of carbonate minerals by silica polymorphs in marine and meteoric environments is a common early diagenetic feature of sedimentary rocks. Although relatively few experimental investigations have been made of interactions between carbonate minerals and dissolved SiO 2 at moderate temperatures, available data suggest that silica uptake onto carbonate surfaces can be significant. We present results of experiments conducted between 25° and 50°C of the uptake of aqueous SiO 2 from artificial seawater (MgNaClCaHCO 3, I=0.7) and meteoric-type (CaCl 2Na 2CO 3, I<0.1) solu onto well-characterized CaMg-carbonates. Silica concentrations ranged from 1.5 to 3.0 m M, a reasonable upper limit for natural diagenetic waters. Dissolved silica-carbonate mineral interactions were investigated under three experimental conditions: (1) in solutions near equilibrium with respect to calcite; (2) in calcite-seeded precipitation experiments at fixed degrees of calcite supersaturation; and (3) during closed-system, dissolution-precipitation of metastable carbonate minerals (aragonite and 18 mole% Mg-calcite) to low-Mg calcite (LMC). In artificial seawater (ASW) maintained near equilibrium with respect to calcite at various P CO2-values, silica uptake from solution onto calcite surfaces (∼1–15 μmol Si/gram seed) was irreversible and strongly time-and pH-dependent. The rate of uptake was greatest in the first 5 hr and in solutions of lower pH (higher P CO2). Silica uptake was only nominally dependent on solution [Mg] and temperature over the range studied. Silica uptake was greater in ASW than in otherwise comparable experiments conducted in low-ionic-strength solutions. Controlled calcite precipitation rates were unaffected by the adsorption of silica. During recrystallization of aragonite and Mg-calcite in low-ionic-strength solutions, silica uptake (1–6 μmol g −) was similar to values frorn near-equilibrium experiments for synthetic calcite in ASW. Experimental Si adsorption values were comparable to Si adsorption values determined for modem metastable sediments (3–4μmol g −1). Despite considerable variation in the experimental carbonate equilibrium-disequilibrium, mineralogy and solution chemistry, Si uptake at near-surface and moderate burial conditions (25–50°C) was limited to ∼15 μmol g −1 Si even in solutions maintained near opal-CT saturation over a period of days to weeks.

Geochemistry of Formation Waters from the Lower Silurian Clinton Formation (Albion Sandstone), Eastern Ohio (1)

Waters of the petroleum-bearing Clinton formation (Albion Sandstone) of eastern Ohio are highly concentrated brines with average total dissolved solids (TDS) of 250,000 ppm. Sodium, calcium, and chloride account for 97% of the TDS. Distribution of divalent metal chlorides (MCl[2]) allows inference of an up-dip paleoflow direction, although present-day flow is probably down-dip. Solute distribution may have been emplaced during early basin development; it thus provides a snapshot of paleoflow. Formation structure alone can not explain MCl[2] trends; they probably also are controlled by regional variation in salt thickness. Major constituent data do not indicate that membrane filtration affected the waters. High bromide content (mean = 1860 ppm) of the waters indicates that they originated from evaporating seawater. They probably are related genetically to the Salina evaporite group. Calculations show that several subsequent diagenetic reactions can account for the observed major ion composition. Recrystallization of aragonite and dolomitization of calcite probably occurred as the waters moved through the Big Lime and/or the Packer Shell carbonates. Cation exchange and chlorite formation probably altered water composition during interaction with shales of the Cabot Head formation and within the Clinton. Minor constituents of the waters appear to be controlled by reactions with clays.

Electron Microprobe Studies of Magnesium Distribution in Carbonate Cements and Recrystallized Skeletal Grainstones from the Pleistocene of Barbados, West Indies

ABSTRACT The electron microprobe has been used successfully to investigate the MgCO3 distribution within cements of Pleistocene carbonate skeletal grainstones from West Indies. The resultant MgCO3 distributions are accounted for by a model involving precipitation of carbonate cement within one or more chemically discrete diagenetic environments (phreatic, early vadose, late vadose). The MgCO3 content of carbonate cements precipitated within a particular diagnetic environment has also been estimated from the Mg2+/Ca2+ ratio of the water in the environment and the distribution coefficient of Mg2+. These estimates are in good agreement with the microprobe data. In the rock samples under consideration, almost all cementation occurred during recrystallization of aragonite and high-Mg calcite to low-Mg calcite rock. MgCO3 distribution patterns acquired during the recrystallization of samples now 240,000 to 700,000 years in age remain unaltered. Further application and refinement of the model may allow quantitative evaluation of the relative importance of various diagenetic environments in the cements of ancient rocks.