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Lacassie J.P.,SERNAGEOMIN Servicio Nacional de Geologia Y Mineria | Ruiz-Del-Solar J.,University of Chile
Proceedings of the International Joint Conference on Neural Networks | Year: 2010

In this paper we illustrate how self organizing maps (SOM) can be used to assess the influence of natural and anthropogenic factors in an impacted fluvial system. We use the Growing Cell Structures (GCS) algorithm to study a multivariable data set that includes the chemical composition of 90 stream sediments, collected along the Rapel Fluvial System, in Central Chile. The GCS algorithm, an extension of SOM, allows simultaneous adaptation of the position of the map pointers in the input space and the topology of the output space. Therefore it helps to find visual representations of geochemical data that simplify the analysis and yet allow all relevant information to be preserved. The results show that the data can be separated into a limited number of groups with clear and different chemical characteristics. Each group corresponds to stream sediments that are restricted to a specific portion of the fluvial system. In the upper part of the catchment, the Cachapoal River is characterized by elevated Cu-Mo-As-Sb concentrations that reflect input of mining-derived material, coupled with high B values associated with agro-industrial activities. In turn, the high P concentrations of the Tinguiririca River reflect an extensive use of phosphates in its floodplain. Due the influence of the Rapel Lake, the lower part of the catchment is mainly controlled by natural factors such as downstream changes in the lithologycal composition of the bedrock and in the hydrodynamic conditions of the Rapel River. Comparisons are made between the results obtained by using GCS and SOM. Both techniques enable the recognition of the cluster structure and their chemical characteristics. However, GCS requires a simpler map structure with less number of units, which optimizes translating the relevant information to geographical maps and make associations with spatially distributed features such as bedrock geology, urbanization, industrial activities and land use. © 2010 IEEE.


Wehrmann H.,Leibniz Institute of Marine Science | Hoernle K.,Leibniz Institute of Marine Science | Jacques G.,Leibniz Institute of Marine Science | Garbe-Schonberg D.,University of Kiel | And 3 more authors.
International Journal of Earth Sciences | Year: 2014

Here we present the first systematic investigation of volatile geochemistry along the Southern Volcanic Zone (SVZ) of Chile. Holocene olivine-hosted melt inclusions in the most mafic tephras sampled from 16 volcanoes along the volcanic front of the SVZ between 33°S and 43°S were analysed for pre-eruptive sulphur, chlorine, and major element contents. These results are combined with trace element compositions of the host whole rocks. The highest fractionation-corrected gas contents occur in the least-degassed melt inclusions from small monogenetic cones of Los Hornitos, Cabeza de Vaca, and Apagado from both the transitional and the southern-central SVZ, reaching ~3,000 μg/g S and 1,400 μg/g Cl, while the lowest abundances of ~1,100 μg/g S and ~600 μg/g Cl were found in the central SVZ at Volcán Lonquimay, Volcán Llaima, and Volcán Villarrica. Chlorine co-varies with trace element indicators for the degree of melting and/or source enrichment, such that the lowest Cl contents are found in high-degree melts from the most depleted mantle sources. The size of the volcanic edifices correlates inversely with Cl abundances in the melt. This could reflect more extensive degassing during ascent through the complex magma plumbing systems beneath the stratovolcanoes or greater dilution during larger degrees of melting of more depleted sources, or a combination of these factors. Compared to other subduction zones, the SVZ melt inclusions exhibit Cl and S abundances in the same range as most of those from the Central American and those from the Marianas arcs. © 2014, Springer-Verlag Berlin Heidelberg.

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