Do clay mineral dissolution rates reach steady state?

The temporal evolution of natural illite du Puy dissolution rates was measured from Si release rates in single-pass flow-through experiments lasting at least 100 days at 25°C and pH ranging from 2 to 12. Si release rates decreased by a factor of five and three at pH 12 and 2, respectively, during the experiments. These observations are interpreted to stem from changes in illite du Puy reactive surface area during these experiments. As the edges of clay minerals dissolve faster than the basal planes, dissolution tends to change clay mineral morphology decreasing the percentage of reactive edge sites. This continuously changing morphology prevents illite dissolution rates from attaining steady state during laboratory experiments lasting 100 to 200 days. A similar temporal decrease in dissolution rates is evident for many different sets of clay mineral dissolution rate data available in the literature. It seems reasonable, therefore, to expect that clay mineral dissolution does not attain steady state in nature, but rather their dissolution rates decrease continuously during their dissolution.     

The oxygen isotope fractionation behaviour of kyanite in experiment and nature

The oxygen isotope fractionation between kyanite and calcium carbonate has been investigated experimentally at four temperatures in the range between 625 and 775 °C at 13 kbar. Because of low exchange rates, the isotopic reaction was enhanced by polymorphic transformation of andalusite to kyanite. With this experimental modification a close approach to equilibrium was reached in all runs. The temperature dependence of the equilibrium fractionation is described by the equation 1000 ln _ky-cc=−2.62×10⁶/T ². Application of the experimental results to natural quartz-kyanite-garnet assemblages indicates the preservation of the oxygen isotope composition of kyanite acquired during its formation, reflecting its extremely low oxygen diffusivity. This refractory behaviour restricts the use of kyanite for thermometry but opens the possibility to use its O-isotope composition as an indicator for recognition of polymetamorphic rock histories and reconstruction of the prograde evolution of a metamorphic sequence. Received: 8 June 1998 / Accepted: 24 August 1998     

On the last explosions of carbonatite pipe G3b, Gross Brukkaros, Namibia

 Pipe G3b is part of the Upper Cretaceous carbonatitic Gross Brukkaros Volcanic Field in southern Namibia. The pipe represents the root zone of a diatreme and is located 2800 m west of the rim of Gross Brukkaros, a downsag caldera. The pipe is exposed approximately 550 m below the original Upper Cretaceous land surface. It cuts down into its own feeder dyke, 0.3 m thick. The pipe coalesced from two small pipes and in plan view is 19 m long and 12 m wide. It consists of fragmented Cambrian Nama quartzites and shales of the Fish River subgroup. Despite intensive brecciation, the stratigraphic sequence of the country rocks is almost preserved in the pipe. In addition, the feeder dyke became fragmented too and can be traced in a 2- to 3-m-wide zone full of carbonatite blocks along the southern margin of the pipe. The void space of the breccia is 30–50% in volume. Finally, after the disruption of country rocks and feeder dyke, a little carbonatite magma intruded some of the void space. The breccia of pipe G3b is considered to represent a root zone at the transition from the feeder dyke into a diatreme above. Formation of the breccia required a shock wave thought to have been associated with a last explosion of the diatreme immediately above the present level of exposure. The explosion can be shown to have been phreatomagmatic in origin. Received: 11 October 1996 / Accepted: 6 March 1997     

Volcanological features of a low-viscosity melt: the carbonatitic Gross Brukkaros Volcanic Field, Namibia

 In the Upper Cretaceous Gross Brukkaros Volcanic Field, southern Namibia, a radial dyke system surrounds a dome structure and its 74 closely related carbonatite diatremes. This paper focuses on volcanological features which seem to be typical for a low-viscosity melt in various settings such as dykes, sills and diatremes. The total or near absence of vesicles in carbonatite ash grains and lapilli inside the diatremes is evidence against explosive exsolution of volatile phases and in favour of a phreatomagmatic fragmentation mechanism and thus for a phreatomagmatic eruption mechanism of the carbonatite diatremes. Received: 15 August 1996 / Accepted: 13 January 1997     

Erratum: the 2.27 day period of WR-134 (HD 191765)

The original temporal analysis of a 12 night spectral timeseries of WR-134 has been found to be flawed and a re-analysis shows that the line profile variations are indeed periodic. When combined with a 4 night timeseries taken 45 days earlier, a period near 2.27 d is found in periodograms of the Heii 5412 line centroid,rms line width, and line skew variations. When the emission line residuals are ordered as a function of phase, a sinuous feature appears to snake about the line center with an amplitude of ± 500 km s^–1. This is 20 larger than the line centroid amplitude; the calculation of which is heavily weighted by static portions of the line profile. In addition to the snake, emission residuals appear that move away from line center on unbound trajectories and are thought to result from the interaction of a periodic driver with the unstable flow of the radiation driven wind. The nature of the periodic driver is a topic for discussion.     

In situ Sr/Sr investigation of igneous apatites and carbonates using laser-ablation MC-ICP-MS

In situ Sr isotopic compositions of coexisting apatite and carbonate for carbonatites from the Sarfartoq alkaline complex, Greenland, have been determined by laser-ablation multicollector inductively coupled plasma mass spectrometry. This study is the first to examine the extent of Sr isotopic homogeneity among coexisting igneous minerals containing high Sr (>3000 ppm) and low Rb (?1 ppm) contents within a single ∼50-μm-thick thin-section mount. This technique is capable of producing measured ⁸⁷Sr/⁸⁶Sr values with analytical precision (∼0.005%, 2σ) approaching those obtained by thermal ionization mass spectrometry but in a much shorter interval of time (100 s vs. >1 h, respectively). The combined total analyses (n = 107) of apatite and carbonate yield ⁸⁷Sr/⁸⁶Sr compositions ranging from ∼0.7025 to ∼0.7031. This relatively large variation in Sr isotopic compositions (∼0.0006) is ∼1 order of magnitude larger than the estimated external reproducibility (∼0.00005, 2σ) of the method. The large range in ⁸⁷Sr/⁸⁶Sr values suggests that apatite and carbonate precipitated predominantly under nonequilibrium conditions. The isotopic variations observed within individual hand specimens may therefore reflect larger (regional) scale open-system processes, possibly involving mixing of carbonatitic melts derived from distinct mantle sources or from a common isotopically heterogeneous mantle.