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Native Sn-Pb droplets in a zeolitic amygdale (Isle of Mull, Inner Hebrides) |
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| Authors: | Vesselin M Dekov Ulf Hålenius George D Kamenov Lars Eriksson Mark Schmidt |
| Institution: | a Department of Geology and Paleontology, University of Sofia, 15 Tzar Osvoboditel Blvd., 1000 Sofia, Bulgaria b Department of Mineralogy, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden c Laboratory for Isotope Geology, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden d Department of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville, FL 32611, USA e Abteilung Ionenstrahlanalytik, Forschungszentrum Dresden-Rossendorf, Bautzner Landstrasse 128, D-01328 Dresden, Germany f Department of Structural Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden g Institute of Materials Research, Cockcroft Building, University of Salford, Salford, M5 4WT, UK h Leibniz-Institut für Meereswissenschaften, IFM-GEOMAR, Wischhofstr. 1-3, D-24124 Kiel, Germany i Institute of Earth Sciences, University of Kiel, Ludewig-Meyn-Str. 10, D-24118 Kiel, Germany |
| Abstract: | Despite the particular scientific interest in the elements with high affinity to S and O2, but found in zero-valence state in nature, the origin of these native minerals has been little explored and remains obscure. Here we describe unique Sn-Pb droplets found in a closed analcime-calcite amygdale collected from a basaltic unit cropping out at Carsaig Bay (Isle of Mull, Inner Hebrides). The droplets consist of intimate intergrowths of nearly pure Sn0 and Pb0 domains in proportion 88:12 and are enveloped in a thin, brownish film of organic composition. The occurrence of the Sn-Pb droplets in a closed amygdale, their relationship with the host analcime + calcite and their Pb isotope composition (which does not match any known anthropogenic Pb source) rule out the possibility of anthropogenic contamination and support the natural origin of the Sn-Pb alloy.The variable isotope (Pb, Sr, Nd) compositions in different members of the host basaltic sequence suggest that a parent basaltic magma was modified by crustal assimilation and post-emplacement alteration processes. Considering all possible explanations, it appears that the most likely source of Pb for the Sn-Pb alloy is a discrete basaltic unit with an isotopic composition comparable to the Antrim basalts (Northern Ireland). The amygdale phases, on the other hand, show isotopic evidence for incorporation of elements from both local basaltic and sedimentary units. The apparent isotopic disequilibrium between Sn-Pb droplets and amygdale phases indicates a complex, multi-stage fluid evolution. The occurrence of Sn-Pb droplets in organic capsules suggests that the droplets and the enveloping organic substances are co-precipitates. This implies that the transportation and deposition of Sn and Pb might have occurred through organometallic compounds. We assume interaction of seawater fluids carrying metals leached from basaltic rocks with hydrocarbons from sedimentary units as a prerequisite for the formation of the organometallic complexes. The zeolites lining the basaltic vesicles might have destabilized the migrating organo-Sn and Pb compounds causing their breakdown and precipitation of Sn-Pb alloy. |
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