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RESEARCH PRODUCT

Amygdaloidal basalts: Isotopic and petrographic evidence for non-diagenetic crustal source of carbonate inclusions

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Even though carbonate amygdules in volcanics are generally assumed to be diagenetic in origin, the authors are of the opinion that almost all carbonate inclusions in the investigated amygdaloidal volcanic rocks from Sicily (Italy), Pindos (Greece) and Bohemia (Czechoslovakia), have a different origin. On the basis of a mineralogical, petrological and geochemical study these “amygdules” are interpreted as being remains ofmagmatic incorporation of carbonate. Therefore, the identification of carbonate globules in volcanics purely on field observations, is inadequate to distinguish true amygdules from carbonate assimilation remnants. A nomenclature of the various types of inclusions is proposed. The isotopic data suggest that the incorporation of carbonatic material by magma could have taken place without any significant modification of the isotopic composition of the carbonate. This study suggests that limited amounts of carbonate could be dissolved in alkali basalt magma; the further addition of moderate amounts of carbonate develops liquid immiscibility. Our inclusions, in agreement with the experimental evidence (Wyllie, 1974), are contrary to the limestone syntexis hypothesis for the generation of feldspathoidal magmas from subalkaline magmas. A possible model of interaction between magma and carbonate is also suggested.