| Autor: |
Dominique Weis, Léon Dejonghe, Daniel Demaiffe |
| Rok vydání: |
1998 |
| Předmět: |
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| Zdroj: |
Chemical Geology. 144:63-71 |
| ISSN: |
0009-2541 |
| Popis: |
Anhydrite and calcite pseudomorphs after anhydrite from Palaeozoic formations in Belgium have been analysed for their Sr isotope compositions. All Givetian (3 samples: ∼0.7080) and Visean (6 samples: ∼0.7076) anhydrites have 87 Sr/ 86 Sr ratios close to the seawater value at their deposition time which implies a marine origin for the Sr. In contrast, Frasnian anhydrite from Soumagne has a significantly higher 87 Sr/ 86 Sr ratio (0.71261) than the seawater value during the Frasnian. This indicates an origin other than marine. The high 87 Sr/ 86 Sr may be due either to the presence of a very small amount of material derived from the continental crust and/or to 87 Rb in-situ decay in the surrounding shales. The Sr isotopic compositions of strongly deformed and recrystallised Visean anhydrite layers are identical to those of undeformed anhydrite. Tournaisian (1 sample) and Visean (5 samples) calcite pseudomorphs after anhydrite have 87 Sr/ 86 Sr values in the range of seawater values at their respective ages and variable, but often high, Sr contents (up to 2500 ppm). For these samples, the isotope geochemistry suggests that the sulphate to carbonate replacement was a local phenomenon without significant external input. In contrast, Eifelian (1 sample) and Givetian (1 sample) calcite pseudomorphs after sulphate have higher 87 Sr/ 86 Sr (0.70971 and 0.71184) and significantly lower Sr contents (42 and 71 ppm). This could reflect either significant in-situ 87 Rb decay since the pseudomorph formation or the addition of radiogenic Sr by the solution responsible for the conversion of sulphate to carbonate. This study contributes to the refinement of the Devono–Dinantian 87 Sr/ 86 Sr curve and brings new insight for the origin of Belgian anhydrites and their pseudomorphs. |
| Databáze: |
OpenAIRE |
| Externí odkaz: |
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