HR: 08:00h
AN: V51G-01 INVITED    [Abstracts]
TI: Water and Melting in Back-arc Basins: New perspectives from the Eastern Lau Spreading Center
AU: * Langmuir, C H
EM: langmuir@eps.harvard.edu
AF: Harvard University, 20 Oxford St., Cambridge, MA 02138,
AU: Bezos, A
EM: antoine.bezos@univ-nantes.fr
AF: Harvard University, 20 Oxford St., Cambridge, MA 02138,
AU: Bezos, A
EM: antoine.bezos@univ-nantes.fr
AF: Laboratoire de Planetologie et Geodynamique, Universite de Nantes, Nantes, 44322, France
AU: Escrig, S
EM: escrig@eps.harvard.edu
AF: Harvard University, 20 Oxford St., Cambridge, MA 02138,
AU: Michael, P J
EM: pjm@utulsa.edu
AF: University of Tulsa, 600 S. Ave., Tulsa, OK 74104,
AB: Since the work of Stolper and Newman (EPSL, 1994) it has been well recognized that water and extent of melting correlate positively in back-arc basin basalts. Quantification of this effect has been used to determine the effect of water content in the source on extent of melting. The slope of the relationship δF/δH₂O_o is linear, and varies from one back-arc basin to another. MELTS and other modeling (Hirschmann et al., J. Petrol., 1999; Gaetani and Grove, Contrib. Mineral. Pet. 1998; Geophys. Mon., 2003; Kelley et al., JGR, 2006) has led to the suggestion that the slope varies regularly with mantle temperature, and that water has a much larger effect on melting at higher compared to lower temperatures. This modeling has been done in the context of isothermal, isobaric addition of water. For back-arcs worldwide, a critical aspect of the data is that more hydrous basalts have very low Fe contents, even when corrected appropriately for hydrous fractionation. This leads to clear negative correlations between Fe and H₂O corrected back to mantle values at Fo₉₀. The 3 wt.% variations in Fe content are not compatible with isobaric models, and require very different melting conditions for hydrous basalts as compared to anhydrous back-arc basalts. Back-arc basin basalts also plot on the global correlations of axial depth and Na_8.0, and this relationship has been used to estimate mantle temperatures in back-arc basins, which on this basis extend to very high values. New data on major elements, trace elements and water from the Eastern Lau Spreading Center (ELSC), along with a re-evaluation of global back-arc data and modeling of mantle melting in the context of a polybaric spreading center environment (Langmuir et al., Geophys. Mon., 2006) provide new perspectives on these issues. The ELSC1 segment has a lower δF/δH₂O_o than both the Mariana and Manus Basins, which would suggest the lowest temperature. However, its extent of melting inferred from its "F" intercept (on a plot of F vs. water in the source) is similar to the Marianas, suggesting a similar temperature, and its Na contents are as low as Manus, suggesting a high temperature. These inconsistent results can be understood from quantitative models and a more realistic melting process beneath back-arc spreading centers. δF/δH₂O_o does not change with mantle temperature. In the back-arc environment, there are two independent halves of the melting regime, the "dry side" and the "wet side." The dry side undergoes polybaric fractional melting like other ocean ridges. The wet side (somehow) produces low pressure equilibrium hydrous melts with high water and low Fe contents. Mixing between the two creates the back-arc arrays. Large variations of Fe and Ti that anti-correlate linearly with water reflect this two component mixing in the back-arc. Both Ti and Na are mobile in the back-arc mantle, and source depletion and enrichment is an essential factor for evaluation of mantle temperature variations. Despite the low Na contents in the Lau Basin does not appear to be particularly hot, and instead is derived from a depleted mantle with low Na contents at only modestly elevated potential temperatures of 1400 degrees.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 3619 Magma genesis and partial melting (1037)
DE: 3621 Mantle processes (1038)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting