HR: 1340h
AN: V43D-1634    [Abstracts]
TI: Back-arc compositional variability in the Mariana Trough as a potential function of subduction zone geometry
AU: * Kohut, E J
EM: ekoh@udel.edu
AF: University of Delaware, Dept. of Geological Sciences, Newark, DE 19716,
AU: Kent, A
EM: kentad@geo.oregonstate.edu
AF: Oregon State University, Dept. of Geosciences, Corvallis, OR 97331,
AU: Stern, R J
EM: rjstern@utdallas.edu
AF: University of Texas - Dallas, Geosciences Dept., Richardson, TX 75080,
AB: Compositional variability of back-arc magmas may be attributed in part to differing amounts of added slab component. Such variability would presumably correlate with distance to the magmatic front and height above the subducting slab. However, compositions of glasses and melt inclusions sampled from the southern Mariana Trough back-arc (13.3° - 14° N) do not show any linear correlations with distance to the magmatic front (MF). Instead, notable increases of trace element signatures for an added subduction component only occur in magmas located within 41 km of the arc. At these locations, Ba/La > 20 and Ba/Nb >13, while elsewhere along the back-arc Ba/La< 5 and Ba/Nb < 10. K₂O contents along the back-arc range from 0.09 to 0.26 wt%, but reach 0.37 wt% in glass sampled 33 km from the MF. However, the latter melts are more evolved than most in the Mariana Trough, with 54-56 wt% SiO₂. Cl concentrations range from 150 to 300 ppm along the most of the back-arc, however glasses sampled within 41 km of MF have Cl contents up to 1318 ppm. Although water contents vary from 0.6-1.34 wt% along the back-arc, there are no strong linkages with distance to the arc or added subduction component. These data suggest that compositional variability in the Mariana Trough is not strongly controlled by subduction processes except in portions of the back-arc spreading center located within ~40 km of the magmatic front. However, the geometry of the subducting slab and proximity to the end of the back-arc spreading center must also be considered. The MF of the Marianas is very arcuate in comparison to the more linear back-arc and as a result the southern end of MF curves towards, and potentially intersects, the Mariana Trough. The compositional variation described here may thus reflect the spatial characteristics of the Mariana subduction zone.
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting