The Subduction Component at Volcan El Jorullo: Young but Subdued

Volcan El Jorullo (1759 to 1774) was produced in one of two historical eruptions within the Michoacan- Guanajuato Volcanic field (MGVF) in the western Trans Mexican Volcanic Belt. The MGVF has experienced widespread Pleistocene volcanism primarily at monogenetic fissures and cones; its exact relationship to the Mexican Subduction zone has been the subject of some discussion in the literature. Here we demonstrate that Jorullo basalts and basaltic andesites have U-series disequilibria characteristic of arc magmatism, and constrain melting and melt transport from a previously slab-fluid enriched source within the past few thousand years. All Jorullo lavas have excess 238U relative to 230Th (a slab-fluid signature found almost exclusive in arc magmas) and excess ^{226}Ra relative to 230Th (a short-lived melting signature common in all young volcanics enhanced by slab derived fluids at arcs). These signatures are well correlated with other chemical indicators of fluid enrichment in the lavas, and are consistent with a recent Jorullo melt inclusion volatile study indicating relatively high water contents inherrited from the magma source (Johnson et al., EOS 85, Fall Meet. Suppl, 2004). ^{226}Ra excesses in Jorullo lavas reach 70%, a value which is similar to but greater than those reported by Reid (IAVCEI, 1987) for Paricutin (the other historical MGVF eruption). This result is consistent with a similar but slightly shorter crustal residence time of the more mafic Jorullo magmas. Jorullo 238U excesses (5-10%) are at the low end of values reported for Paricutin (up to 20%; Condomines et al., 1988). We have previously reported (Rubin et al., EOS 85, Fall Meet. Suppl, 2004) that country rock assimilation is a more widespread occurrence at Jorullo than previously thought (Luhr and Carmichael, CMP 90, 1985); this significantly impacts the trace element and radiogenic isotope (e.g., Sr, Nd, Pb) composition of some Jorullo lavas. Assimilation also disrupts the U-series system in some Jorullo lavas, primarily through relative addition of Th (including its minor isotope 230Th), resulting in subdued disequilibria in both (^{226}Ra/230Th) and (238U/230Th). Nevertheless, the least affected lavas show relatively small 238U excesses, suggesting that U fluxing from the slab fluid component is subdued at Jorullo, an observation that is generally consistent with relatively low fluid mobile element enrichments in calcalkaline MGVF cones attributed to subduction of relative hot, early dewatering lithosphere in the area (Hochstaedter et al., GCA, 1996). Nevertheless it is unclear why Paricutin lavas should show greater 238U deficits, given the same proximity to the arc front of both volcanoes and the greater degree of differentiation/assimilation at Paricutin.