Gonadal and nongonadal zinc concentrations in mussels

Seasonal measurement of zinc levels in mussels (Mytilus edulis) from the Tyne estuary showed a small spring peak and an annual low in September. Zinc concentration was found to be significantly lower in gonadal tissue than in nongonadal tissue. Although gonadal dry weight was strongly negatively correlated with gonadal zinc concentration there was no significant relationship between gonadal dry weight and whole body zinc levels, indicating that gonadal state has little effect on zinc variability in these animals. However, a strong negative correlation was found between log gonadal zinc concentration and log nongonadal dry weight. Construction of a gonadal index (gonadal/nongonadal dry weight) shows that the seasonal peak zinc concentration in early May occurs before major spawning has taken place. It is concluded that the seasonal dynamics of zinc in this species can be explained in terms of growth of somatic rather than germinal tissue.     

Remanent magnetism of poorly sorted deposits from the Minoan eruption of Santorini

Thick, poorly sorted ash deposits produced during the Minoan cruption have differing palacomagnetic histories. Included clasts from deposits produced in the second phase of the eruption have random NRM and cleaned directions and were emplaced as cold mudflows. Clasts from ignimbrite flow units erupted in the third phase have significant directions and their palacomagnetic pole is consistent with today’s geomagnetic pole. These deposits acquired a TRM on deposition and were emplaced as hot pumice flows.     

Presentation and interpretation of chemical data for igneous rocks

Arguments are made in favor of using variation diagrams to plot analyses of igneous rocks and their derivatives and modeling differentiation processes by least-squares mixing procedures. These methods permit study of magmatic differentiation and related processes in terms ofall of the chemical data available. Data are presented as they are reported by the chemist and specific processes may be modeled and either quantitatively described or rejected as inappropriate or too simple.Examples are given of the differing interpretations that can arise when data are plotted on an AEM ternary vs. the same data on a full set of MgO variation diagrams. Mixing procedures are illustrated with reference to basaltic lavas from the Columbia Plateau.     

A computational study of oxygen diffusion in olivine

Atomistic modelling techniques are used to study the rate-determining steps that limit diffusion of oxygen in forsterite. The activation energies for diffusion parallel to all three crystallographic axes by the vacancy and interstitial mechanisms are calculated. The activation energy for extrinsic vacancy diffusion is predicted to be isotropic with a barrier height of 119 kJ mol^–1. Conversely, in the interstitial case it is found to be anisotropic, with extrinsic activation energies that range between 94 and 178 kJ mol^–1. The effect of intrinsic defects and two typical impurities, iron and hydrogen, upon diffusion is also considered. We find that the migration energy is slightly higher in iron-rich fayalite compared with forsterite and that the presence of hydrogen defects will not affect the diffusion mechanism. These observations lead us to reinterpret existing experimental results on oxygen diffusion in natural olivine. We suggest that at low oxygen partial pressure the mechanism observed is a vacancy mechanism, while at high oxygen partial pressure the mechanism is interstitial. We believe that this change in mechanism is mediated by iron redox reactions. Taking this process into account, we derive activation energies in excellent agreement with those found experimentally in natural samples of olivine. The anisotropy of activation barriers and hence the change in diffusion rates with temperature could be used to distinguish between the two mechanisms in future experimental work.     

A computer simulation study of OH defects in Mg2SiO4 and Mg2GeO4 spinels

Classical atomistic simulation techniques have been used to investigate the energies of hydrogen defects in Mg₂SiO₄ and Mg₂GeO₄ spinels. Ringwoodite (γ-Mg₂SiO₄) is considered to be the most abundant mineral in the lower part of the transition zone and can incorporate large amounts of water in the form of hydroxyls, whereas the germanate spinel (γ-Mg₂GeO₄) corresponds to a low-pressure structural analogue for ringwoodite. The calculated defect energies indicate that the most favourable mechanisms for hydrogen incorporation are coupled either with the reduction of ferric iron or with the creation of tetrahedral vacancies. Hydrogen will go preferentially into tetrahedral vacancies, eventually leading to the formation of the hydrogarnet defect, before associating with other negatively charged point defects. The presence of isolated hydroxyls is not expected. The same trend is observed for germanate, and thus γ-Mg₂GeO₄ could be used as a low-pressure analogue for ringwoodite in studies of water-related defects and their effect on physical properties.