par Javoy, Marc;Weis, Dominique 
Référence Earth and planetary science letters, 84, 4, page (415-422)
Publication Publié, 1987-08
Référence Earth and planetary science letters, 84, 4, page (415-422)
Publication Publié, 1987-08
Article révisé par les pairs
| Résumé : | The primary isotopic characteristics of alkaline granites are often obscured by secondary processes enhanced by their unusual chemical compositions. This is true for radiogenic as well as for stable isotopes. For example, the 87Sr 86Sr ratios can vary drastically in closed systems because of very high Rb/Sr ratios and can also be easily modified by direct or indirect interaction with continental crust because of low Sr concentrations. Moreover, the frequent occurrence of the granitic massifs as hypovolcanic complexes increases the probability of interaction with meteoric waters which is a common source of important isotopic variability. The investigation of oxygen isotope systematics in alkaline acidic rocks from various environments shows the 18O content of their quartz to be highly invariable, and the δ18O values to be close to the mantle range of values. This is due to the resistance of quartz to isotopic exchange, which makes it a good tracer of primary isotopic composition. If we eliminate the quartz δ18O values for which interaction with meteoric water is well documented (five samples), the total range of variation (seventeen samples) is from 6.0 to 7.3‰ relative to SMOW. The values can easily be accounted for by, and correspond to, equilibrium with mantle-type material in a temperature range of 1200-800°C. If we consider possible effects of fractional crystallization, this temperature range can probably be reduced to its lower limit which is much more likely for rocks of acidic composition. The present oxygen isotope study strongly supports an origin for alkaline anorogenic granites from mantle-dominated sources. © 1987. |
