HR: 1330h
AN: T32A-0917    [PDF]
TI: Across-Arc Variability of Outputs From the Mariana Subduction Factory: Melt Inclusions From the $14\deg$35'N Cross-Chain
AU: * Kohut, E J
EM: kohute@geo.orst.edu
AF: Dept. of Geosciences Oregon State University, 104 Wilkinson Hall, Corvallis, OR 97331 United States
AU: Stern, R J
EM: rjstern@utdallas.edu
AF: Geosciences Department University of Texas at Dallas, 2601 N. Floyd Rd., Richardson, TX 75083 United States
AU: Kent, A J
EM: kentad@science.oregonstate.edu
AF: Dept. of Geosciences Oregon State University, 104 Wilkinson Hall, Corvallis, OR 97331 United States
AU: Nielsen, R L
EM: nielsenr@geo.orst.edu
AF: Dept. of Geosciences Oregon State University, 104 Wilkinson Hall, Corvallis, OR 97331 United States
AU: Bloomer, S
EM: sherman.bloomer@orst.edu
AF: College of Science Oregon State University, Kidder Hall, Corvallis, OR 97331 United States
AB: Olivine- and feldspar- hosted melt inclusions (MI) in lavas dredged from the cross-chain along $14\deg$ 35' N in the southern Mariana arc provide a unique means of examining the cross-arc variability of outputs from the Subduction Factory. Unlike in the shorter Guguan cross-chain near $17\deg$20'N (Stern {\it et al.} this session), in the $14\deg$35'N cross-chain lavas with phenocrysts that host primitive MI (Mg\#s up to 78) have erupted both midway between the arc and back-arc and at the magmatic front. These lavas and MI presumably derived by melting of the mantle wedge and thus sample asthenosphere as it is modified during movement towards the magmatic front and at the front itself. In addition, lavas similar to back-arc basin basalts (BABB), presumably derived from shallow mantle melting, have erupted between the midpoint seamount and the arc. Melt inclusions from these BABB-like lavas then provide an opportunity to sample the cross-arc variability of the upper mantle as well. All melt inclusion compositions from the cross-chain and arc front display trace element signatures in spider diagrams characteristic of arcs and evidence of a subduction component. The latter is shown in part as an increase of Th/Yb relative to MORB and OIB at a Nb/Yb ratio of $\sim$1. The subduction signature is less pronounced in BABB-like melts, which suggests mixing between arc-like magmas produced largely by flux melting and those derived from pressure-release melting due to upwelling. Primitive MI from the lavas at both the midpoint and arc show evidence for components derived from the mantle wedge and sediments, as well as a LILE-enriched component. The sediment component dominates in some MI from the arc front, with Pb/Ce up to 0.44 at $\sim$5.7 ppm Pb, while the LILE enriched component is more prominent in some MI from the midpoint seamount. The most LILE enriched MI from the midpoint seamount contain $\sim$0.7-1.5 wt% K$_{2}$O, $\sim$28 ppm Rb, and $\sim$153-163 ppm Ba at Mg\#s of 64-66, and $\sim$1000 ppm Cl. The highest degrees of melting, based on trace-element systematics (e.g. Zr/Y vs. Y; Nb vs. Yb), are recorded in MI from the primitive arc front lavas. HFSE are more depleted in the primitive arc MI relative to the primitive midpoint MI. The BABB-like MI are less depleted in HFSE than the primitive midpoint and arc MI, however these are still depleted relative to MI from the Marianas Trough back-arc spreading axis. These data show that arc melts are composed of mantle wedge components mixed with a sediment-derived component (either sediment melt or sediment-derived fluid) and a LILE enriched component. These magmas may also mix with melts derived from shallow pressure release melting due to upwelling. We also suggest that some of the HFSE depletion observed in arc lavas can be attributed to prior melting in the back-arc and along a cross-arc chain. This HFSE depletion is present in the upper mantle as well as deeper in the mantle wedge. We are currently gathering volatile data via FTIR with which we will be better able to assess to role of fluxing in producing the various components and the origins of fluids themselves.
DE: 1030 Geochemical cycles (0330)
DE: 1065 Trace elements (3670)
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 3640 Igneous petrology
DE: 8150 Plate boundary--general (3040)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting