Geological and palaeomagnetic studies indicate that ice sheets may have reached the Equator at the end of the Proterozoic eon, 800 to 550 million years ago, leading to the suggestion of a fully ice-covered 'snowball Earth'. Climate model simulations indicate that such a snowball state for the Earth depends on anomalously low atmospheric carbon dioxide concentrations, in addition to the Sun being 6 per cent fainter than it is today. However, the mechanisms producing such low carbon dioxide concentrations remain controversial. Here we assess the effect of the palaeogeographic changes preceding the Sturtian glacial period, 750 million years ago, on the long-term evolution of atmospheric carbon dioxide levels using the coupled climate-geochemical model GEOCLIM. In our simulation, the continental break-up of Rodinia leads to an increase in runoff and hence consumption of carbon dioxide through continental weathering that decreases atmospheric carbon dioxide concentrations by 1,320 p.p.m. This indicates that tectonic changes could have triggered a progressive transition from a 'greenhouse' to an 'icehouse' climate during the Neoproterozoic era. When we combine these results with the concomitant weathering effect of the voluminous basaltic traps erupted throughout the break-up of Rodinia, our simulation results in a snowball glaciation.
We have observed remarkably consistent patterns of concentric zonation in the values of low-field magnetic susceptibility measurements over the Variscan age Mont-Louis Andorra granite pluton of the eastern Pyrenees of Andorra, Spain and France. This zonation is a function of the rock petrology since a close correlation is shown between the petrographic nature (as defined by chemical analysis) and the magnetic susceptibility magnitude of the rocks. It reflects the dominantly paramagnetic nature of the granite, carried by Fe-bearing silicates, and this is demonstrated by the fact that low-field susceptibilities of representative specimens are almost equal to (1) their high-field susceptibilities and (2) their calculated susceptibilifies using Fe contents, assuming a null ferromagnetic contribution. We conclude that this technique accurately reflects the modal abundances of ferromagnesian phases in rocks of the ilmenite series and that it represents a powerful and efficient tool for the reconnaissance surveying of petrological variations in granitoid plutons.
]NTRODU•ONModeling the origin and emplacement of granite plutons is usually based on a structured study, beginning with the field characterization of the principal rock types and their spatial distribution and followed by detailed structural and/or petrological investigations. The pertinence of any petrogenetic or emplacement model is indeed dependent on the accuracy of the original field observations and laboratory characterizations of the rocks. Important field constraints include knowledge of whether the pluton is compositionally zoned and whether any such zoning is composite or gradational. Acquisition of such information is highly dependent on the quality of outcrop and the collection of spatially distributed samples for modal analysis. This stage of the investigation is often the most laborious part of the study, although essential data are usually generated. Unlike every other aspect of petrology and structural geology this facet of petrostructural research in granite plutons has not yet benefited significantly from the development of modem techniques. We contend that low field magnetic susceptibility measurements can contribute to the rapid and effective field mapping and petrographic characterization of certain types of granitoid plutons, particularly those of the ilmenite series, rather than the magnetite series granitoids [Ishihara, 1977; Takahashi et INow at G6oscience de l'Environement, URA 132 CNRSPaper number 9ZIB01590. 0148-0227/93/9ZIB-01590505.00.al., 1980]. The present approach, which therefore constitutes a basis for more detailed tectonophysical studies of graniteiris, naturally has strong potential to be a tool for geophysical exploration of a large fraction of the continental crust.Magnetic susceptibility measurement in low magnetic field (a few 10 -4 T) offers the promise of a simple and inexpensive technique. Two principal aspects can be explored with this technique, namely, structural aspects using the anisotropy of magnetic susceptib...
The magnetic fabric of ferromagnetic granitic rocks results from both the shape preferred orientation of individual magnetite grains and their distribution anisotropy through magnetic interactions between neighbouring grains. Measurement of the low‐field magnetic anisotropy of single multi‐domain magnetite grains shows a linear correlation between their magnetic anisotropy degree and their aspect ratio. Interactions between two elongated grains were studied experimentally using two types of grain arrangement: an “aligned” configuration and a “side‐by‐side” configuration. For a distance between the grain centers equal to approximately twice the average grain size, the magnetic susceptibility and its anisotropy are enhanced in both configurations, and the direction of kmax, the easiest magnetization axis, is stable in the “aligned” configuration, whereas it rotates toward an orthogonal direction in the “side‐by‐side” configuration. Depending on the distribution of the interacting magnetite grains, magnetic interactions may therefore either increase the whole‐rock anisotropy magnitude, or reduce it as in the given example of the granitic rocks from Madagascar.
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