Postcaldera (≤ 180 ka) peralkaline trachytes and rhyolites from Socorro Island, Mexico, have relatively restricted ranges of (230Th)/(232Th)o (0.99-1.14) and (238U)/(232Th) (0.91-0.98). Most of the samples exhibit 238U-230Th disequilibria, with initial (230Th) enrichments of up to 21%. In general, (230Th)/(232Th)o values are lower in the rhyolites than in the trachytes. (230Th)/(232Th)o of postcaldera alkalic basalts may also have a relatively restricted range (0.94-0.96), although ratios as low as 0.84 and as high as 1.04 are permitted by uncertainties in some of the ages. These values are also lower than those of the trachytes. Sr and Nd isotopic ratios of the basaltic and silicic suites have fairly restricted ranges that collectively overlap. Previous work suggests that trachytic magmas form by partial melting of basaltic basement, but closed-system partial melting of a source with Th isotope ratios like those of the postcaldera basalts would probably produce trachytic melts with lower rather than higher (230Th)/(232Th)o. Open-system processes affecting melts from such basement are also ruled out because of the lack of a reasonable contaminant that could increase the (230Th)/(232Th) of the trachytic magmas. Postcaldera trachytic magmas probably derive from basaltic basement characterized by Th isotope ratios similar to those of the silicic rocks, and moderate degrees of melting of such material can account for the Th/U of the trachytes. Differences in (230Th)/(232Th)o between the trachytes and rhyolites delimit maximum residence times for rhyolitic magmas of 40-50 kyr, but it is likely that residence times are shorter. This coupled with correlations between Sr and Nd isotopes and indices of differentiation within the silicic suite suggest that rhyolites are related to trachytes by assimilation-fractional crystallization. The most likely assimilant is basement that formed during the silicic phases of magmatism on Socorro. Our results indicate that volcanic basement may be an important component in the genesis of evolved ocean island magmas.
ASJC Scopus subject areas
- Geochemistry and Petrology