The Two-Phase Region and Excess Mixing Properties of Paragonite-Muscovite Crystalline Solutions

The chemical compositions of synthetic paragonite-muscovitepairs were obtained by electron probe microanalysis of run productsprepared hydrothermally at 300, 400, 500, and 600 ?C and 2.07kbar. The microcrystalline run products were dispersed on polishedberyllium rods, and Na, K, and Si were determined simultaneously.Compositions were determined from K/Si and Na/Si ratios referredto standards. Tentative excess thermodynamic mixing properties of paragonite-muscovitecrystalline solutions, based on the two-phase composition dataand on X-ray diffraction data, are represented by the followingthird-order Margules formulation: which leads to a critical temperature of 833 ?C and a criticalcomposition of 39.0 mole per cent Mu at 2.07 kbar (the criticalphase is probably metastable with respect to alkali feldsparand corundum). These results are quantitatively in agreementwith Iiyama's (1964) ion-exchange data. From relationships amongthe quantities (arctanh s)/s, In r², and 1/T we obtain criticalconditions which are in good agreement with the above (T_c =829 ?C, N_2c = 0.396). The critical curve obtained from the aboveMargules parameters is given by: T_c(?C) = 768.8 + 31.00P(kbar). The above results are complicated by polymorphism and by possiblelack of complete equilibrium between the two-mica synthesisproducts and possible substitution of hydronium for alkalies.We emphasize, therefore, that the phase diagrams and derivedmixing properties should be applied with caution to naturalmuscovite- and paragonite-bearing assemblages.     

Holocene pisoliths and encrustations associated with spring-fed surface pools, Pastos Grandes, Bolivia

Calcite pisoliths, with diameters ranging from 1 to 200 mm, are forming now on the surface of a playa (salar) in the Andean Altiplano (4500 m above MSL) of Bolivia. They are associated with active or recently active hot springs (20-75°C) which flow onto the playa surface. Encrustations of pieces of an older caliche-type crust, of pisoliths, of indurated mud and of older concretions are also found as well as series of small (1-3 cm high) sinter terraces (rimstone dams). Arborescent concretions and overgrowths are common and they are reminiscent of drip-stone textures. Water analyses demonstrate that calcite supersaturation (about twenty times) occurs mainly through CO₂ loss, with photosynthesis by algae and degassing the main removal mechanisms. The two available analyses indicate slight evaporation and a calcium loss between spring and pool of 2.3 mmol per litre of water. It is thought that the hot springs pick up much of their solute load from the playa sediments. The closest analogues to these deposits have been reported from caves (cave pearls and concretions). Although the depositional processes may be similar, the environment on an evaporitic playa surface is quite different. The geological implications for this newly observed pisolith environment may be considerable.     

Mineral Equilibria in the Searles Lake Evaporites, California

The Searles Lake evaporites (late Quaternary) consist of salineand mud layers permeated by brines. Two mineral pairs (gaylussite-pirssonite,mirabilite-thenardite) are used as indicators of relative aH₂othroughout the stratigraphic column. Variations are attributedmostly to changes in brine salinity and partly to temperature. These aH₂o-sensitive minerals locally coexist with trona, nahcolite,burkeite, northupite, tychite, hanksite, and aphthitalite, whichare sensitive to both aH₂o and a_co. Such assemblages permitconstruction of schematic isothermal aH₂o-a_co, diagrams. Fieldboundary sequences are derived from both theoretical considerationsand observed assemblages; their slopes are determined by thestoichiometry of possible reactions. Predicted assemblages invariablyagree with observed assemblages. By means of these diagrams, present-day lateral and stratigraphicvariations in relative aH₂o and aco₂ in the deposit are reconstructed.They show that aH₂o and a_co2 vary independently. Many of thepresent activities reflect depositional conditions; some indicatepost-depositional events.     

Isotopic composition and origin of polar precipitation in present and glacial climate simulations

The Hamburg atmospheric general circulation model (AGCM) ECHAM‐4 is used to identify the main source regions of precipitation falling on Greenland and Antarctica. Both water isotopes H₂¹⁸O and HDO are explicitly built into the water cycle of the AGCM, and in addition the capability to trace water from different source regions was added to the model. Present and LGM climate simulations show that water from the most important source regions has an isotopic signature similar to the mean isotope values of the total precipitation amount. But water from other source regions (with very different isotopic signatures) contributes an additional, non‐negligible part of the total precipitation amount on both Greenland and Antarctica. Analyses of the temperature‐isotope‐relations for both polar regions reveal a solely bias of the glacial isotope signal on Greenland, which is caused by a strong change in the seasonal deposition of precipitation originating from nearby polar seas and the northern Atlantic. Although the performed simulations under LGM boundary conditions show a decrease of the δ ¹⁸O values in precipitation in agreement with ice core measurements, the AGCM fails to reproduce the observed simultaneous decrease of the deuterium excess signal.     

Evidence for common breakup events of the acapulcoites‐lodranites and chondrites

Abstract— Acapulcoites and lodranites are believed to originate on a common parent body and to represent some of the earliest events in the differentiation of the chondritic asteroids. We have conducted isotopic studies of the noble gases He, Ne, Ar, Kr, and Xe, and determinations of the concentrations of the major elements and of the radionuclides ¹⁰Be, ²⁶Al, and ³⁶Cl in an attempt to constrain the cosmic‐ray exposure history of two members of the acapulcoite‐lodranite clan recovered in Antarctica: Frontier Mountain (FRO) 95029 and Graves Nunataks (GRA) 95209. From cosmic‐ray‐produced ³He, ²¹Ne, and ³⁸Ar and appropriate production rates, we derive parent‐body breakup times of 4.59 ± 0.60 and 6.82 ± 0.60 Ma for FOR 95029 and GRA 95209, respectively. These times are consistent with those obtained from the pairs ¹⁰Be‐²¹Ne and ²⁶Al‐²¹Ne; whereas the times inferred from the pair ³⁶Cl‐³⁶Ar are slightly longer, perhaps because the ³⁶Cl activities decreased as a result of decay on Earth. Terrestrial ages up to ～50 ka for the two meteorites are consistent with the measured ³⁶Cl activities of the metal phases. All acapulcoites and lodranites dated until now show cosmic‐ray exposure ages in the range of 4–10 Ma. This is the same range as that found for the major exposure age cluster of the H chondrites. As a common parent body is improbable on the basis of the O‐isotopic systematics, a common set of impactors might have affected the asteroid belt 4–10 Ma ago.     

Theoretical post-glacial sedimentation rates for a semi-enclosed sea; example of the Kieler Bucht, Western Baltic

The theoretical rates of deposition for fine sediments over the last 10 ka have been deduced and plotted for the Kieler Bucht. Assumptions are that the bay has remained as a closed sedimentary system, and that the fine sediments deposit in water deeper than 10 m. A sharp peak of sedimentation activity is indicated between 7.5 and 8.5 ka B.P. with low rates prior to 9.5 ka and since 6 ka. Comparison of rates obtained from dated cores extracted from different parts of the Kieler Bucht with the theoretical curve shows general conformity, and confirms that peak sedimentation rates exceeding 3 mm/a, as averaged over 100–200 years, occurred between 8 and 9 ka, with the suggestion of a minor activity peak between 3 and 4 ka. The overall consistency supports the view that the bay has acted essentially as a closed sedimentary basin during the Holocene marine transgression and subsequently.