Journal of Petrology, Volume 39, Issue 5: May 1 1998.

Genesis of Evolved Ocean Island Magmas by Deep- and Shallow-Level Basement Recycling, Socorro Island, Mexico: Constraints from Th and other Isotope Signatures

WENDY A. BOHRSON¹ AND MARY R. REID²

¹DEPARTMENT OF GEOLOGICAL SCIENCES AND INSTITUTE FOR CRUSTAL STUDIES, UNIVERSITY OF CALIFORNIA AT SANTA BARBARA, SANTA BARBARA, CA 93106, USA ²DEPARTMENT OF EARTH AND SPACE SCIENCES, UNIVERSITY OF CALIFORNIA AT LOS ANGELES, LOS ANGELES,CA 90095, USA

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.

Keywords: assimilation; silicic magma; Socorro Island; U-Th disequilibria; volcanic basement

Pages 995-1008