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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 (56^6ø) 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.

Item Type:	Article
Authors/Creators:	Johnson, ER and Wallace, PJ and Delgado Granados, H and Manea, VC and Kent, AJR and Bindeman, IN and Donegan, CS
Journal or Publication Title:	Journal of Petrology
DOI / ID Number:	https://doi.org/10.1093/petrology/egp051
Additional Information:	The definitive publisher-authenticated version is available online at: www.oxfordjournals.org
Item Statistics:	View statistics for this item

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