Die Paläogeographische Auswertung der Altvulkanischen Bildungen zur Rekonstruktion der Korrelaten Sedimentationsräume im Bereich Späterer Abtragung

Ohne ZusammenfassungVortrag, gehalten am 14. März 1955 auf der Hauptversammlung der Geologischen Vereinigung in Mainz.     

Calcite solubility and speciation in supercritical NaCl-HCl aqueous fluids

The solubility of calcite in NaCl-H₂O and in HCl-H₂O fluids was measured using an extraction-quench hydrothermal apparatus. Experiments were conducted at 2 kbar, between 400° C and 600° C. Measurements in NaCl-H₂O were conducted in two ways: 1) at constant pressure and NaCl concentration, as a function of temperature; and 2) at constant pressure and temperature, as a function of NaCl concentration. In both the NaCl-H₂O and the HCl-H₂O systems, the solubility of calcite increases with increasing chlorine concentrations. For example, the log calcium molality in equilibrium with calcite increases from –3.75 at 2 kbar and 500° C, in pure H₂O to –3.10 at 2 kbar and 500° C at log NaCl molality=–1.67. At fixed pressure and NaCl molality, the solubility of calcite is almost constant from 400° C to 550° C, but increases somewhat at higher temperatures. The results can be used to determine the dominant calcium species in the experimental solutions as a function of NaCl concentration and to obtain values for the second dissociation constant of CaCl_2(aq). At 2 kbar, 400° C, 500° C, and 600° C, we calculate values for the log of the dissociation constant of CaCl⁺ of –2.1, –3.2, and –4.3, respectively. The 400° C and 500° C values are consistent with those obtained by Frantz and Marshall (1982) using electrical conductance techniques. However, our 600° C value is 0.8 log units higher than that reported by Frantz and Marshall. The calcite solubilities in the NaCl-H₂O and HCl-H₂O systems are inconsistent with the solubilities of calcite in pure H₂O reported by Walther and Long (1986). They are, however, consistent with the measurements of calcite solubilities in pure H₂O presented in this study. These results allow for the calculation of the solubilities of calcium silicates and carbonates in fluids that contain CO₂ and NaCl.     

Rate and mechanism in prograde metamorphism

For a given rate of heat input into a prograde metamorphic sequence the extent of overstep of reaction temperature (disequilibrium) depends on the slowest of three sequential steps: (a) surface detachment of reactant minerals, (b) transport of material to the site of mineral growth, and (c) nucleation and growth of the product mineral. We have developed analytical expressions which enable determination of the rates of mineral dissolution and growth and of advective and diffusive mass transport during metamorphism. The dissolution and growth steps are linear functions of the driving force (– G) of the overall reaction while diffusion may take place either through a grain boundary fluid film or through the disorganized grain boundary itself.While little is known about heterogeneous nucleation, we argue from field observations that the rate of nucleation is not in general rate limiting. Additionally, if a fluid phase is present true grain boundary diffusion cannot be the mechanism which transports material over the mm to cm distances observed between reactant and product minerals.Simple models of contact (200° C temperature rise in 10,000 years) and regional (10° C per million years) metamorphic events lead to several conclusions concerning the rate determining step. Firstly, growth and dissolution are extremely rapid, dehydration reactions at 500° C going to completion in 2×10² years (contact) and 1×10⁴ years (regional), if all solutes are readily transported. Secondly, the effect of substantial fracture flow of fluid is to divert the transporting medium away from the grain boundary region and hence to retard the transport step. Under most such circumstances it appears that diffusive transport of aqueous SiO₂ or Mg species will be rate controlling. Despite this retardation of reaction rates, the extent of disequilibrium is rarely more than a few degrees C. Extensive disequilibrium (40° C) can only occur for reactions such as the andalusite sillimanite transformation which have very small entropy changes and which occur in rapid metamorphic events.     

Mineral magnetic record of paleoclimate variation in loess and paleosol from the Buenos Aires formation (Buenos Aires, Argentina)

Environmental magnetic techniques were applied to a loess–paleosol sequence of the Chacopampean plain (Buenos Aires, Argentina). Mineral magnetic carriers and their grain size were identified in order to detect magnetic mineral fluctuations associated with climatic changes. Multidomain magnetite of detrital origin dominates the record. In paleosols, a high coercivity fraction was identified. Horizons with no visual evidence of pedogenesis, but showing magnetic behavior analogous to that of paleosols were observed and are thought to represent environmental conditions similar to those prevailing during paleosol formation.The results suggest that the magnetic signal yielded by paleosols in these South American loess deposits is different from that in the Chinese loess. This may be due to differences in parent materials, diagenetic processes and/or differences in paleoclimatic conditions in both regions.