Rubidium and potassium relationship in some volcanoes of central Italy
1968; Elsevier BV; Volume: 3; Issue: 3 Linguagem: Inglês
10.1016/0009-2541(68)90019-3
ISSN1872-6836
AutoresM. Barbieri, M. Fornaseri, Adelia Penta,
Tópico(s)earthquake and tectonic studies
ResumoThe rubidium and potassium content of extrusive and pyroclastic rocks of Alban Hills, Vico and Cimino Volcanoes as well as of their minerals such as leucite, sanidine, biotite and zeolites have been investigated. The extrusive rocks of Alban Hills (Colli Albani) have a K/Rb ratio of 174; the rocks of Vico Volcano have a ratio of 141 and those of Cimino Volcano a ratio of 188. The pyroclastic rocks of Alban Hills show a K/Rb ratio of 104, and those of Vico Volcano a ratio of 78. Both are significantly lower as compared with respective rocks. The relative enrichment of Rb observed in pyroclastics is explained by terms of the behaviour of the elements during the magmatic crystallization, and during weathering and other secondary alteration processes affecting the pyroclastic rocks. The leucite phenocrysts of extrusive rocks show a rather variable Rb content (from 1,160 to 3,000 p.p.m.), but a K/Rb ratio in any case lower than the K/Rb ratio of the groundmass. Sanidine phenocrysts likewise show a rather variable Rb content (from 285 to 620 p.p.m.), but a K/Rb ratio in any case higher than the corresponding ratio of the groundmass. It follows that the early separation of leucite or of sanidine can control the K/Rb ratio of the residual melt. The behaviour of rubidium during the weathering and other secondary alteration processes affecting the extrusive and pyroclastic rocks is here discussed. Analcimization gives rise to an indirect enrichment of rubidium: alteration of feldspars and feldspathoids leading to illitic or to amorphous clay minerals produces a depletion of rubidium. The influence of the structural type of the host mineral and of the secondary alteration processes on the K/Rb ratio is emphasized.
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