| Popis: |
Lead isotopic data of alkaline volcanic rocks and carbonatites from the Kaiserstuhl are presented in combination with Sr isotopic ratios. As a whole the initial lead ratios cover the range typical for other Tertiary to Recent alkaline rock provinces in Europe: 206 Pb 204 Pb = 18.9–19.7 , 207 Pb 204 Pb = 15.61–15.70 and 208 Pb 204 Pb = 38.7–39.7 . Especially in the 208 Pb 204 Pb vs. 206 Pb 204 Pb diagram the data reveal a linear array which is interpreted in terms of mixing of two mantle components. In detail the data comprise three distinct groups which are correlated with different magmatic rock types: unfractionated primary mantle melts (olivine nephelinites, olivine melilitites), fractionated alkaline rocks (tephrites, phonolites), and a group with carbonatites, bergalites and hauynophyres. These results are interpreted to reflect either derivation from different mantle sources, or the data trace a mixing line between two mantle reservoirs, one of them being more enriched. Contamination with crustal lead is unlikely: it may have affected the tephritic magma series, but not the carbonatitic rocks. The uppermost part of the updomed mantle beneath the Kaiserstuhl, which is known by petrographic evidence to be partially metasomatized, probably is the site of the enriched reservoir. A “plume”-like structure within the deeper mantle is suggested as the initial source of this mechanism. |
