RE-OS ISOTOPE SYSTEMATICS IN THE SIERRA DEL CHICHINAUTZIN VOLCANIC FIELD, TRANS-MEXICAN VOLCANIC BELT

The Trans-Mexican Volcanic Belt (TMVB) is believed to be associated with the subduction of the Cocos and Rivera plates, although the relationship is complex with a large variation in distance from trench to volcanic front (~150km in the west and ~400km in the east) and the likely influence of extensional tectonics. Previous studies have led to significant debate regarding the relative roles of subduction and crustal assimilation in generating enriched geochemical signatures in TMVB volcanic rocks. The Re-Os isotope system is highly sensitive to crustal assimilation, and recent Re-Os studies in the western TMVB have suggested an important role for lower-crustal assimilation in the petrogenesis of these basalts [1, 2]. In contrast, a similar study of samples from Popocatepetl in the eastern TMVB concluded that radiogenic isotope signatures are due to subduction-related source enrichment [3]. Our study focuses on high MgO basalts from a series of young cinder cones in the Sierra Chichinautzin volcanic field (SCVF) in the eastern TMVB, in order to investigate the relative roles of subduction-related source metasomatism, and lower and upper crustal assimilation in the petrogenesis of TMVB volcanics.

Five high MgO samples (8-10wt%) exhibit a negative correlation between Os concentration and 187Os/188Os, with Os ranging from 0.01 to 0.15 ppb and 187Os/188Os from 0.1329-0.2607. All of the samples, including those with high Os abundance, have 187Os/188OS ratios more radiogenic than the depleted MORB mantle, suggesting that the mantle source region may have been moderately enriched by radiogenic slab fluids. The negative correlation between 187Os/188Os and Os is consistent with two component mixing between a moderately enriched mantle source and either a crustal assimilant or a strongly fluid-metasomatized mantle source. Ongoing analyses of potential local upper crustal assimilants will aid in distinguishing between these models.

[1] J.C. Lassiter, J.C. & J.F. Luhr, J.F. (2001), Geochemistry, Geophysics, Geosystems -– G-Cubed 3 , 2, (2001). 2000GC000116.

[2] J. Chesley, J., L. Ferrari, L., A. Gomez-Tuena, A., K. Righter, K. & J. Ruiz, J. (2002), Earth Planet. Sci. Lett.,EPSL 195, (2002) 211-221.

[3] S. Alves, S., C. Allegre, C., F. Capmas, F. & P. Schiano, P. (2002), Earth Planet. Sci. Lett.,EPSL 198, (2002) 355-369.