Preservation of Fe isotope heterogeneities during diagenesis and metamorphism of banded iron formation

We present the iron isotope composition of primary, diagenetic and metamorphic minerals in five samples from the contact metamorphosed Biwabik Iron Formation. These samples attained peak metamorphic temperatures of <200, <340, ∼500, <550, and <740°C respectively. δ⁵⁶Fe of bulk layers ranges from −0.8 to +0.8‰; in some samples the layers may differ by >1‰ on the millimeter scale. Minerals in the lowest grade samples consistently show a sequence in which δ⁵⁶Fe of magnetite > silicate ≥ carbonate. The inter-mineral Fe isotope differences vary in a fashion that cannot be reconciled with theoretical temperature-dependent fractionation factors. Textural evidence reveals that most, if not all, magnetite in the Biwabik Formation is diagenetic, not primary, and that there was tremendous element mobility during diagenesis. The short duration of contact metamorphism allowed diagenetic magnetite compositions to be preserved throughout prograde metamorphism until at least the appearance of olivine. Magnetite compositions therefore act as an isotope record of the environment in which these sediments formed. Larger-scale fluid flow and longer timescales may allow equilibration of Fe isotopes in regionally metamorphosed rocks to lower temperatures than in contact metamorphic environments, but weakly regionally metamorphosed rocks may preserve small-scale Fe isotopic heterogeneities like those observed in the Biwabik Iron Formation. Importantly, Fe isotope compositions that are characteristic of chemical sedimentation or hydrothermal processes are preserved at low grade in the form of large inter-mineral variations, and at high grade in the form of unique bulk rock compositions. This observation confirms earlier work that has suggested that Fe isotopes can be used to identify sedimentary processes in the Precambrian rock record. An erratum to this article can be found at     

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Iron,calcium, and potassium atom densities in the trails of Leonids and other meteors: Strong evidence for differential ablation

Abstract— We report on studies of the Fe, Ca, and K atom densities in the trails of meteors. The measurements of the densities were taken simultaneously and in a common volume by three ground-based lidars. We report and analyze the data obtained during two nights of Leonid showers (1996 and 1998 November 16/17) and of one night five days after the 1998 Leonids. The lidar-observed trails of Leonids differ from those of other meteor showers in both their mean altitude and in mean metal composition. The Leonid trails show a highly depressed Ca/Fe abundance ratio in comparison to CI meteoritic composition. Our observations are interpreted with the help of a numerical model that describes the ablation processes occurring during the high-speed entry of meteoroids into the Earth's atmosphere. We conclude that for the lidar-observed meteoroids, the ablation process occurs differentially for the three elements. This leads to a mixture of metals in the meteor trails, the composition of which is strongly altitude dependent and at any one altitude deviates significantly from a CI meteoritic composition. The model predicts differing altitudes and durations of trail observations for different showers, allowing us to tentatively assign the origin to the observed trails.     

Further investigation into a recent model for toroidal rings of Saturn

A previous paper described a toroidal ringlet consisting of a large number of charged particles distributed uniformly along slightly inclined, slightly eccentric, orbits around Saturn. The orbits have identical semimajor-axis, identical eccentricity and identical inclination, but their ascending nodes are distributed uniformly around Saturn. Assuming an inclination (in radians) much smaller than the eccentricity, it was shown that the toroidal cross-section could rock slowly back and forth as the inclination fluctuates. In this paper a similar result is obtained for the more general case when inclination is comparable with eccentricity and both fluctuate.     

Amazonian climate: results and future research

Summary Some of the results from the climate component of the Large Scale Biosphere-Atmosphere Experiment in Amazonia (LBA), which are presented in this Special Issue are summarised. Recent advances in Amazonian climate modelling are also discussed. There is a range of papers which fall into three groups: surface fluxes and boundary layer growth; convection, clouds and rainfall; and climate modelling. The new insight given by this work is discussed and an argument is made for future research to employ a wider approach to Amazonian climate modelling.     

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