HR: 11:20h
AN: V52A-05 INVITED    [Abstracts]
TI: The Composition of Water-Rich Components in the Sources of Back Arc and Arc Magmas
AU: * Kelley, K A
EM: kelley@gso.uri.edu
AF: Grad. Sch. of Oceanography, Univ. of Rhode Island, Narragansett, RI 02882, United States
AU: Hauri, E H
EM: hauri@dtm.ciw.edu
AF: Dept. of Terrestrial Magnetism, Carnegie Inst. of Washington, Washington, DC 20015, United States
AB: Arc and back-arc basin basalts are distinguished by geochemical signatures that are a complex function of the composition of crustal inputs form the subducting plate and the internal "factory" processes that extract materials from the slab and distribute them across the mantle wedge. Here, we use a global data set of basaltic melts from back arcs and arcs to examine relationships between four elements thought to be primary constituents of slab- derived materials: H₂O, Na₂O, K₂O, and Cl. We use the methods of Kelley et al. (2006), coupled with recent constraints on mantle/melt D_H2O and D_Cl (Hauri et al., 2006) to constrain concentrations of these elements in the mantle source, which compare directly with compositional models of the mantle and the slab-derived component beneath the Mariana trough (Stolper & Newman, 1994). The Mariana trough samples define linear trends in Na₂O/H₂O (1), K₂O/H₂O (0.2), and Cl/H₂O (0.03) consistent with mixing between these two components. Four other back-arc basins (Sumisu, Central Lau, Manus, and E. Scotia) also fall along this trend in Na₂O/H₂O and K₂O/H₂O, suggesting that slab additions to most back-arc mantle sources have common major element characteristics. The Valu Fa Ridge segment of the southern Lau basin, however, trends away from other back arcs towards lower Na₂O/H₂O (0.1) and K₂O/H₂O (0.08). Such lower ratios are generally characteristic of arc sources in the Marianas and other arcs (e.g., Na₂O/H₂O=0.5-0.05), but also appear to be distinct for specific arc volcanoes. In back arcs, Cl may also be affected by late-stage magmatic assimilation of seawater, which drives up Cl/H₂O and obscures primary trends, although some central Lau basin samples do point towards a mixing array coincident with the Mariana trough. Mariana arc sources have higher H₂O and Cl concentrations than the Mariana trough, and indicate overall lower, but volcano-specific, Cl/H₂O (0.02-0.01). In terms of these major-element constituents, arc volcanoes sample a broad diversity of slab-derived materials, whereas global back arc sources are infiltrated by a comparatively homogeneous H₂O-rich component from the slab.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1030 Geochemical cycles (0330)
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1038 Mantle processes (3621)
DE: 1065 Major and trace element geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting