Coupling mechanisms in double sandbar systems. Part 2: impact on alongshore variability of inner‐bar rip channels

Double sandbar systems are common morphological features along sandy, wave‐dominated, micro‐ to meso‐tidal coastlines. In the companion paper, we demonstrated how various alongshore inner‐bar rip‐channel patterns can develop through morphological coupling to an alongshore‐variable outer bar. The simulated coupling patterns are, however, scarcely observed in the field. Instead, inner‐bar rip channels more often possess remarkably smaller and more variable alongshore length scales, suggesting that coupling mechanisms do not play a substantial role in the overall double‐sandbar dynamics. Here we use a numerical model to show that the relative importance of self‐organization and morphological coupling changes in favour of the latter with an increase in waterdepth variability along the outer‐bar crest. Furthermore, we find that the typical alongshore variability in inner‐bar rip‐channel scale is indicative of a mixture of self‐organization and morphological coupling rather than self‐organization alone. Morphological coupling may thus be more important to understanding and predicting the evolution of inner‐bar rip channels than previously envisaged. Copyright © 2010 John Wiley and Sons, Ltd.     

Wollastonite at Nuliyam,Kerala, southern India: a reassessment of CO2-infiltration and charnockite formation at a classic locality

The occurrence of a charnockitised felsic gneiss adjacent to a marble/calc-silicate horizon at Nuliyam, southern India, has been cited in recent literature as a classic example of the dehydration of crustal rocks resulting from the advective infiltration of CO₂-rich fluids generated from a local carbonate source. Petrographic study of the Nuliyam calc-silicate, however, reveals it to consist of abundant wollastonite and scapolite and contain locally discordant veins rich in wollastonite. At the pressure—temperature conditions proposed for charnockite formation in recent studies, 5 kbar and 725°C, this wollastonite-bearing mineral assemblage was stable in the presence of a fluid phase only if X _CO2 was near 0.25 and could not have coexisted with the fluid causing biotite breakdown and charnockite development in adjacent rocks (X _CO2>0.85). The stable coexistence of wollastonite and scapolite prohibits the calc-silicate from being a source for fluid driving charnockitisation at the required P-T conditions. Textural observations such as the limited replacement of wollastonite by calcite+quartz symplectites and mosaics, are consistent with late fluid infiltration into the calc-silicate. The extensive isotopic, chemical and mineral abundance data of Jackson and Santosh (1992) are re-interpreted and integrated with these observations to develop a model involving the infiltration of an externally derived CO₂-rich fluid during high-temperature decompression. Increased charnockite development next to the calc-silicate has arisen because the calc-silicate acted as a relatively unreactive and impermeable barrier to fluid transport and caused fluid ponding beneath antiformal closures. The Nuliyam charnockite/calc-silicate locality is an example of a structural trap in a metamorphic setting rather than a site where charnockite formation can be attributed to local fluid sources.     

A robust algorithm for ellipse-based discrete element modelling of granular materials

With recent research indicating the importance of the rolling mechanism of deformation in granular systems consisting of perfectly round particles, it has become popular to use ellipse-shaped particles in the Discrete Element Method (DEM) numerical model. Inherent in this technique is the need for accurately computing ellipse to ellipse intersection, in order to properly detect contact formation and compute relative contact velocities. However, the commonly used algorithms for computing ellipse-ellipse intersection are generally poorly conditioned and can be inaccurate. An alternate method for computing ellipse-ellipse intersection is developed and presented which results in a well-conditioned, stable and accurate contact detection method. These modification are incorporated into the general DEM algorithm.     

Mantle(?) xenoliths in the carbonatites of northern Transbaikalia

The composition and nature of high-Cr minerals in lithic clasts from the carbonatites of the Veseloe occurrence, northern Transbaikalia, were considered. In order to determine their source, the Cr-bearing phases were compared with chromite, magnetite, and rutile from ultrabasic rocks, mantle xenoliths, and eclogites. It was suggested that the xenoclasts studied were formed at great depths, whereas the carbonatites were directly derived from the mantle rather than formed by the crustal differentiation of a silicate-carbonate melt.     

Solubility of alumina in orthopyroxene plus spinel as a geobarometer in complex systems. Applications to spinel-bearing alpine-type peridotites

The addition of Fe and Cr to the simple system MgO-SiO₂-Al₂O₃ markedly affects the activities of phases involved in the equilibrium