HR: 12:05h
AN: V52A-08    [Abstracts]
TI: Cross-arc Variations in Lava Chemistry in the Tonga Arc-Lau Back Arc System, 19- 23°S
AU: * Michael, P J
EM: pjm@utulsa.edu
AF: The Univ. of Tulsa, 600 S. College Ave., Tulsa, OK 74104, United States
AU: Bezos, A
EM: antoine.bezos@univ-nantes.fr
AF: Faculte des Sciences Univ. Nantes, rue de la Houssiniere, BP 92208 Cedex 3, Nantes, 44322, France
AU: Langmuir, C H
EM: langmuir@eps.harvard.edu
AF: Harvard University, 20 Oxford St., Cambridge, MA 02138, United States
AU: Escrig, S
EM: escrig@eps.harvard.edu
AF: Harvard University, 20 Oxford St., Cambridge, MA 02138, United States
AU: Matzen, A K
EM: amatzen@caltech.edu
AF: Caltech, 1200 E. California Blvd., Pasadena, CA 91125, United States
AU: Asimow, P
EM: asimow@gps.caltech.edu
AF: Caltech, 1200 E. California Blvd., Pasadena, CA 91125, United States
AU: Arculus, R
EM: Richard.Arculus@anu.edu.au
AF: Australia Nat'l. Univ., Bldg 47 Daley Rd., Canberra, ACT 0200, Australia
AB: The Tonga arc system from 19°-23°S consists of the active Tofua arc, the Eastern Lau Spreading Center (ELSC; a back arc spreading center), and numerous seamounts between them. We use the excellent sampling of ELSC and 34 nearby seamounts, along with sparser published analyses of Tofua arc, to examine the spatial relations of chemistry and melting in this subduction system. The spatial constraints can be used to better understand the nature and mechanism of enrichment that is caused by subduction. Geochemistry along the axis of ELSC is related to its distance to the Tofua arc, which decreases continuously from 100 km in the north to 40 km in the south. The subduction influence (e.g., fluid mobile elements) along ELSC increases in several sharp gradients towards the south as ELSC gets closer to the arc. The six different tectonic segments of ELSC display mixing relationships in trace element ratio-ratio diagrams (e.g., Ba/La vs Th/La) in which one end member is a subduction component that is distinctive for each segment (Escrig et al., this meeting). We explore whether the distinctive subduction components of each ELSC segment are reflected by the Tofua arc that is adjacent to that segment, and by the intervening seamounts. Relationships between the arc, back arc and seamounts are different in the north and the south. In the south where the arc-back arc distance is smaller, the Tofua arc volcanic rocks share the distinctive trace element characteristics of their corresponding ELSC segment, and extend the mixing trajectories to higher, more arc-like values. Seamounts that are located between Tofua arc and ELSC also share the distinctive trace element characteristics of the local arc + back-arc, and are intermediate in their trace element ratios. These observations are consistent with the model of Langmuir et al., (2006) in which magmas of back arc spreading centers form from two components: a dry side similar to mid-ocean ridges and a wet (trenchward) side that produces hydrous melts. We suggest that Tofua arc formed completely from the wet side. Seamounts have a small input from the dry side and a greater input from the wet side compared to ELSC, consistent with their location. Our data suggest that the contribution from the wet side varies in composition along the length of the axis, and that the distinctiveness at each latitude is maintained across the arc from Tofua to ELSC. In the north, the relationships between arc, back-arc and seamounts are more complex. Segment NELSC-1 (Bezos et al., in prep and this meeting) displays two distinctive mixing trends toward different subduction components. Neither of the subduction components of NELSC-1 matches the Tofua arc at this latitude: they are both too low in Ba and Pb relative to H₂O. This reflects the great distance between Tofua arc and NELSC-1. The seamounts chemistry seems to reflect their proximity to ELSC or Tofua arc. Seamounts that are close to NELSC-1 fall on mixing arrays of NELSC-1. Seamounts that are closer to Tofua arc lie on a mixing arrays between Tofua arc and NELSC-1. A seamount at the northern end of NELSC-1 has more chemical affinity to the ILSC that is offset to the NW from the northern tip of ELSC. It seems that the subduction components associated with Tofua have a limited range of influence toward the back arc.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 3001 Back-arc basin processes
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting