HR: 1340h
AN: T53A-1395    [Abstracts]
TI: Volatile (H₂O, CO₂, Cl, S) contents of magmas in the Izu-Bonin back and rear arc
AU: * Kent, A J
EM: adam.kent@geo.oregonstate.edu
AF: Oregon State University, 104 Wilkinson Hall, Corvallis, OR 97331 United States
AU: Gill, J B
EM: jgill@pmc.ucsc.edu
AF: University of California at Santa Cruz, Earth Sciences, Santa Cruz, CA 95064 United States
AB: In back arc environments mantle melting may be promoted by mantle upwelling and decompression as well as by addition of volatile-rich slab components that lower the mantle solidus. Distinguishing between these two modes has important ramifications for models of subduction zone dynamics.
We report results of an ongoing study of volatile contents of primitive submarine lavas from the Izu-Bonin rear arc region (Latitude 30°30' to 32°30' N). A diversity of lava types is evident in this region, and we have studied seven representative samples. From east to west these include samples from the active rift zone that lies immediately behind the volcanic front, backarc knolls adjacent to the rift, and seamount chains that extend west into the Shinkoku basin. Few samples are glassy and so we have concentrated on olivine-hosted melt inclusions, measuring volatile and major element contents by SIMS and EMPA, and will determine trace elements by LA-ICP-MS. In most cases melt inclusions contain appreciable CO₂, which suggests that minimal water has been lost from these melts via low pressure degassing.
Magma from each portion of the rift have distinctive water contents. Inclusions from the active rift zone have water contents that are typically < 1 wt.%, consistent with a greater degree of decompression and upwelling. Back arc knolls have highly variable water contents, from < 0.5 wt.% up to > 3 wt.%. Western and east seamounts have water contents typically between 1 and 2.5 wt.%. The range of water contents we observe is similar to that from the Mariana trough further south. Overall our results are consistent with existing models that suggest a role for slab-derived fluids across the arc and rear arc region, but suggest that decompression melting may also be locally important. Further trace element studies on these samples are underway.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1038 Mantle processes (3621)
DE: 3619 Magma genesis and partial melting (1037)
DE: 3621 Mantle processes (1038)
SC: Tectonophysics [T]
MN: Fall Meeting 2005