Subduction-related Volatile Recycling and Magma Generation beneath Central Mexico: Insights from Melt Inclusions, Oxygen Isotopes and Geodynamic Models

Abstract

The subduction-related Michoaca¤n^Guanajuato Volcanic Field (MGVF) in central Mexico contains 900 cinder cones and numerous larger shield volcanoes of Late Pliocene to Holocene age.We present data for major, trace and volatile (H2O, CO2, S, Cl) elements in olivine-hosted melt inclusions from eight calc-alkaline cinder cones with primitive magma characteristics and one more evolved alkali basalt tuff ring.The samples span a region extending from the volcanic front to 175 km behind the front. Relationships between H2O and incompatible trace elements are used to estimate magmatic H2O contents for 269 additional volcanic centers across the MGVF and central Mexico. The results show that magmatic H2O remains high (3^575 wt %) for large distances (150 km) behind the front. Chlorine and S concentrations are strongly correlated with melt H2O and are also high across most of the arc (700^1350 ppm Cl, 1500^2000 ppm S).The alkali basalt, located far behind the front (175 km), has much lower volatile contents (515wt%H2O, 200 ppm Cl, 500 ppm S), and is compositionally similar to other melts erupted in this region. Oxygen isotope ratios of olivine phenocrysts (56^6o) from the calc-alkaline samples are higher than for typical mantle-derived magmas but do not vary systematically across the arc. Calc-alkaline samples have high large ion lithophile element concentrations relative to Nb andTa, as is typical of subduction-related magmas, but alkali basalt samples far behind the front have high Nb and Ta and lack enrichments in fluidmobile elements. Modeling based on volatiles and trace elements suggests that the calc-alkaline magmas were generated by 6^15% partial melting of a variably depleted mantle wedge that was fluxed with H2O-rich components from the subducted slab. In contrast, the alkali basalts formed by small degrees of decompression melting of an ocean island basalt source that had not been fluxed by slabderived components. Based on high d18Oolivine values and trace element characteristics, the H2O-rich subduction components added to the mantle wedge beneath the MGVF are likely to be mixtures of oceanic crust derived fluids and sediment melts. Integrating these results with new 2-D thermo-mechanical models of the subduction zone beneath the MGVF, we demonstrate that the present-day plate configuration beneath theMGVFcauses fluids to be released beneath the forearc and volcanic front, and that sediment melts can be produced beneath the volcanic front by the waning stages of fluid released from the oceanic crust percolating through already dehydrated sediments. Down-dragging of serpentine- and chloritebearing peridotite in the lowermost mantle wedge probably plays a role in fluid transport from the forearc to beneath the arc. H2O-rich magmas located more than 50 km behind the volcanic front can be explained by mantle hydration related to a shallower slab geometry that existed at 3 Ma. Rollback of the slab over the last 2 Myr has resulted in strong mantle advection that forms low-H2O, high- Nb alkali basaltic magmas by decompression melting far behind the present-day volcanic front.