Compositional variation in pyrochlores of Amba Dongar carbonatite complex,Gujarat

Pyrochlore is interpreted as a primary magmatic mineral that appeared in early carbonatite phases at Amba Dongar. Later intrusive phases incorporated this early pyrochlore and caused compositional modification, particularly around the rims, in response to changing magma chemistry. Consequently pyrochlore compositions show wide ranges in Nb, Ta, Ca, Ba, Ti and Ce. The final carbonatite phase was ankeritic and rich in hydrothermal fluids, giving rise to extreme compositional zoning and introduction of diverse elements (Si, U, Sr, Th, Fe), in the contained pyrochlore. Enrichment in radioelements such as U lead to metamictization, alteration and A-site vacancy.     

Comparison of numerical methods for simulating strongly nonlinear and heterogeneous reactive transport problems—the MoMaS benchmark case

Although multicomponent reactive transport modeling is gaining wider application in various geoscience fields, it continues to present significant mathematical and computational challenges. There is a need to solve and compare the solutions to complex benchmark problems, using a variety of codes, because such intercomparisons can reveal promising numerical solution approaches and increase confidence in the application of reactive transport codes. In this contribution, the results and performance of five current reactive transport codes are compared for the 1D and 2D subproblems of the so-called easy test case of the MoMaS benchmark (Carrayrou et al., Comput Geosci, 2009, this issue). This benchmark presents a simple fictitious reactive transport problem that highlights the main numerical difficulties encountered in real reactive transport problems. As a group, the codes include iterative and noniterative operator splitting and global implicit solution approaches. The 1D easy advective and 1D easy diffusive scenarios were solved using all codes, and, in general, there was a good agreement, with solution discrepancies limited to regions with rapid concentration changes. Computational demands were typically consistent with what was expected for the various solution approaches. The differences between solutions given by the three codes solving the 2D problem are more important. The very high computing effort required by the 2D problem illustrates the importance of parallel computations. The most important outcome of the benchmark exercise is that all codes are able to generate comparable results for problems of significant complexity and computational difficulty.     

A hydrogeochemical survey of Kilimanjaro (Tanzania): implications for water sources and ages

Kilimanjaro, Tanzania, the highest mountain in Africa, has undergone extensive hydrologic changes over the past century in an area where water resources are critical. A hydrochemical and isotopic synoptic sampling program in January 2006 is used to characterize hydrogeology, hydrology, and water quality of the area. Samples were collected from the summit and southern side of Kilimanjaro and the Moshi region (Tanzania). Sample sources included four glaciers, seven groundwater wells, 12 rivers, 10 springs, precipitation, and a lake. Analyses included major ion chemistry, stable isotopes of water (¹⁸O and D); in addition, seven samples were analyzed for tritium. The samples generally have good water quality with the exception of three samples with elevated fluoride concentrations (>3 mg/L) and elevated nitrate concentrations (>2.5 mg/L NO₃ as N). There is a strong elevation control on stable isotopes, with an apparent elevation effect of – 0.1 ‰ δ¹⁸O per 100 m rise in elevation (R ² = 0.79). The results, including the tritium values, show that the hydrogeologic system is comprised of both local and regional flow systems, and that regional rivers are receiving significant inflow from shallow groundwater, and at very high elevations the hydrologic system is derived from groundwater, precipitation, and glacial melt water.     

Back analysis of landslide susceptibility zonation mapping for the 2005 Kashmir earthquake: an assessment of the reliability of susceptibility zoning maps

The October 2005 earthquake triggered several thousand landslides in the Lesser Himalaya of Kashmir in northern Pakistan and India. Analyses of ASTER satellite imagery from 2001 were compared with a study undertaken in 2005; the results show the extent and nature of pre- and co-/post-seismic landsliding. Within a designated study area of ~2,250 km², the number of landslides increased from 369 in 2001 to 2,252 in October 2005. Assuming a balanced baseline landsliding frequency over the 4 years, most of the new landslides were likely triggered by the 2005 earthquake and its aftershocks. These landslides mainly happened in specific geologic formations, along faults, rivers and roads, and in shrubland/grassland and agricultural land. Preliminary results from repeat photographs from 2005 and 2006 after the snowmelt season reveal that much of the ongoing landsliding occurred along rivers and roads, and the extensive earthquake-induced fissuring. Although the susceptibility zoning success rate for 2001 was low, many of the co-/post-seismic landsliding in 2005 occurred in areas that had been defined as being potentially dangerous on the 2001 map. While most of the area in 2001 was (very) highly susceptible to future landsliding, most of the area in 2005 was only moderate to low susceptible, that is, most of the landsliding in 2005 actually occurred in the potentially dangerous areas on the 2001 map. This study supports the view that although susceptibility zoning maps represent a powerful tool in natural hazard management, caution is needed when developing and using such maps.