Volatiles in a Subduction-Related Primitive Basaltic Cinder Cone: Investigating Volcan Jorullo, Mexico

To investigate volatiles in primitive subduction-related basaltic magmas, we have analyzed volatile (H₂O, CO₂, Cl, S) concentrations in olivine-hosted melt inclusions from the 1759-1774 eruption of Jorullo volcano in the central Trans-Mexican Volcanic Belt (TMVB). Jorullo's earliest lavas are primitive (9.3 wt% MgO, Fo_86-90 olivine), and lava compositions evolved over time as a result of crystal fractionation (Luhr and Carmichael, 1985, CMP). Tephra samples were collected from a 5-meter-thick proximal ash fall sequence. Olivine crystals from the base of the section are fractured and contain clusters and chains of Cr-spinel inclusions in addition to melt inclusions. Samples from the top of the section have more euhedral crystals with solitary Cr-spinel inclusions and fewer melt inclusions. Melt inclusions from the basal tephra have variable H₂O (<1-4.8 wt%) and CO₂ (34-770 ppm), corresponding to crystallization pressures of <100 bars to 3.7 kbars. This indicates that olivine crystallized over a wide range of depths, trapping variably degassed melts during magma ascent. Melt inclusions from the upper sample have lower H₂O (0.2-1.4 wt%) and no detectable CO₂, suggesting shallow crystallization of degassed magma toward the end of the violent-Strombolian-style eruptions. For Jorullo melt inclusions, the maximum H₂O contents (4-5 wt%), which should most closely represent primary values, compare with values of ∼4 wt% H₂O at nearby Paricutin (Luhr, 2001, CMP) and 1.3-5.2 wt% in cinder cones in the Chichinautzin volcanic field to the east (Cervantes and Wallace, 2003, Geology). Relatively high Ba/Nb in Jorullo lavas shows that the underlying mantle wedge in this region of the TMVB has been enriched by a subduction-derived component. Ratios of H₂O to incompatible trace elements follow similar patterns to those observed for Chichinautzin cinder cones, and thus further demonstrate a link between H₂O and trace element enrichment in the mantle wedge. The high H₂O in Jorullo basaltic magma is surprising given the low B content of whole rock samples (Hochstaedter et al., 1996, GCA). The latter has been attributed to subduction of young, relatively hot lithosphere beneath the TMVB, which should result in significant slab devolatilization beneath the forearc region. Our results show that despite early loss of B from subducted materials in such "hot" subduction zones, large amounts of H₂O continue to be released from the slab further down dip, resulting in flux melting of the mantle wedge beneath the volcanic arc.