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Title: Magnetic signatures of the orogenic crust of the Patagonian Andes with implication for planetary exploration
Authors: Díaz Michelena, M.
Kilian, R.
Keywords: Orogenic crust;Martian crust;Magnetic carrier;Susceptibility;Ground survey;Königsberg;Plutonic rocks;Green schist;Titano Magnetite;Shear stress;Hydrothermal mineralisation
Issue Date: 10-Nov-2015
Publisher: Elsevier
DOI: 10.1016/j.pepi.2015.08.005
Published version:
Citation: Physics of the Earth and Planetary Interiors 248: 35-54(2015)
Abstract: The Patagonian Andes represent a good scenario of study because they have outcrops of diverse plutonic rocks representative of an orogenic crust on Earth and other planets. Furthermore, metamorphic surface rocks provide a window into deeper crustal lithologies. In such remote areas, satellite and aerial magnetic surveys could provide important geological information concerning exposed and not exposed rocks, but they integrate the magnetic anomalies in areas of kilometres. For the southernmost Andes long wavelength satellite data show clear positive magnetic anomalies (>+100 nT) for the Patagonian Batholith (PB), similar as parts of the older martian crust. This integrated signal covers regions with different ages and cooling histories during magnetic reversals apart from the variability of the rocks. To investigate the complex interplay of distinct magnetic signatures at short scale, we have analysed local magnetic anomalies across this orogen at representative sites by decimeter-scale magnetic ground surveys. As expected, the investigated sites have positive and negative local anomalies. They are related to surface and subsurface rocks, and their different formation and alternation processes including geomagnetic inversions, distinct Curie depths of the magnetic carriers, intracrustal deformation among other factors. Whole rock chemistry (ranging from 45 to >80 wt.% SiO2 and from 1 to 18 wt.% FeOtot.), magnetic characteristics (susceptibilities, magnetic remanence and Königsberger ratios) as well as the composition and texture of the magnetic carriers have been investigated for representative rocks. Rocks of an ultramafic to granodioritic intrusive suite of the western and central PB contain titanomagnetite as major magnetic carrier. Individual magnetic signatures of these plutonic rocks reflect their single versus multidomain status, complex exolution processes with ilmenite lamella formations and the stoichiometric proportions of Cr, Fe and Ti in the oxides. At the eastern margin of the PB the investigated plutons and mafic dykes have been emplaced and equilibrated at 4–6 km depth. They do not contain magnetite but include variable amounts of ferrimagnetic monoclinic C4 pyrrhotite, which was formed along fractures zones by a hydrothermal gold-bearing mineralisation. The intensity of their positive magnetic anomalies (up to +220 nT) is well correlated with the amount of pyrrhotite (1–4 vol.%). In all cases, high resolution ground surveys variations of the magnetic signature down to 20 nT could be used to clearly distinguish different rock types on a decimeter scale. Vector magnetometer MOURA designed for martian exploration was successfully used for these surveys and provides results comparable to a commonly used caesium scalar magnetometer. This is important for near future ground surveys on Mars and the Moon.
Description: Highlights In this work it is analysed the different crustal volumes related to the distinct magnetic surveys from in orbit to on ground prospections in the Pathagonian Batholith (PB) plutonic rock suite. The Patagonian Batholith (PB) plutonic suite is considered a key site because it includes cumulates, gabbros, tonalites and granodiorites with an expected variety in their capability to acquire remanent magnetisation. The magnetic remanence depends on stoichiometric proportions of Cr, Ti, and Fe in titanomagnetites as well as their single versus multi-domain status. Different old rock types show positive and negative anomalies formed during magnetic field inversions while integrating long wavelength Champ satellite magnetic anomalies exhibit only a clear positive anomaly for the PB. Some granites, gneisses and mafic dykes which have been equilibrated within the middle crust do not contain magnetite, but they include monoclinic pyrrhotite formed by a local and regional hydrothermal mineralisation with copper and gold enrichment along fracture zones. The intensity of their positive magnetic anomalies is strongly correlated to the amount of pyrrhotite. This relationship is important for future exploration of ore deposits. High resolution mapping is able to distinguish rocks by its magnetic contrast of >20 nT at distances of ∼20 cm. The vector magnetometer MOURA, designed for the strong physical conditions of Mars exploration, was also successfully used for this mapping. This opens a new prospection field for the near future planetary exploration rovers.
E-ISSN: 1872-7395
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