HR: 17:15h
AN: V34A-06    [Abstracts]
TI: Geochemistry of Seamounts Between the East Lau Spreading Center and Tonga Arc
AU: * Michael, P J
EM: pjm@utulsa.edu
AF: The University of Tulsa, 600 S. College Ave., Tulsa, OK 74104 United States
AU: Langmuir, C H
EM: langmuir@eps.harvard.edu
AF: Harvard University, 20 Oxford Street, Cambridge, MA 02138 United States
AU: Bezos, A
EM: bezos@fas.harvard.edu
AF: Harvard University, 20 Oxford Street, Cambridge, MA 02138 United States
AU: Escrig, S
EM: escrig@fas.harvard.edu
AF: Harvard University, 20 Oxford Street, Cambridge, MA 02138 United States
AU: Matzen, A K
EM: andrew-matzen@utulsa.edu
AF: The University of Tulsa, 600 S. College Ave., Tulsa, OK 74104 United States
AU: Matzen, A K
EM: andrew-matzen@utulsa.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: Woods, S
EM: swoods@fas.harvard.edu
AF: Harvard University, 20 Oxford Street, Cambridge, MA 02138 United States
AU: Gier, E
EM: egier@ldeo.columbia.edu
AF: Harvard University, 20 Oxford Street, Cambridge, MA 02138 United States
AU: Goddard, C
EM: cgoddard@coas.oregonstate.edu
AF: Oregon State University, 104 COAS Admin. Bldg., Corvallis, 97331
AU: Arculus, R
EM: Richard.Arculus@anu.edu.au
AF: Australian National University, Building 47 Daley Rd., Canberra, ACT 0200 Australia
AB: Seamounts that are located between the Tonga (i.e., Tofua) arc and Eastern Lau Spreading Center (ELSC) provide an excellent opportunity to examine spatial relationships of chemistry and melting in the back arc environment. The spatial constraints can be used to better understand the nature of the enrichment that is caused by subduction. Geochemical trends along the axis of ELSC appear to be related to its distance to the Tofua arc, which increases continuously from 40 km in the south to 100 km in the north (see Bezos et al., this meeting). The intervening seamounts allow us to test how geochemical variations are related to regional differences in the wedge and to proximity to the arc. Thirty-four seamounts were sampled (24 during cruise KM0417; 10 during cruise TELVE) at distances of 0 to 30 km from ELSC: i.e. considerably closer to ELSC than to Tofua arc. Some seamounts are arranged in chains while many are not. Seamounts range from 1 to 5 km in diameter and up to 1000 m in height. Lava freshness suggested a limited range of ages for individual seamounts, but a larger range of ages for the seamounts as a group. Seamount lavas are often more primitive and more phyric than lavas from nearby ELSC, (especially in the south) but there is overlap. Both Mg# and depth remain relatively constant for seamounts from north to south, unlike for ELSC where both depth and Mg# generally decrease to the south. Clinopyroxene and olivine were dominant phases, as expected for arc-related magmas where feldspar crystallization is delayed by high magmatic H₂O. Chemical trends also suggest that plagioclase crystallized after clinopyroxene. High H₂O concentrations measured in glasses confirm that virtually all magmas were saturated with (and degassed) H₂O during eruption, consistent with high vesicularity. Ratios of H₂O to the major fluid mobile elements (e.g., K, Na, Ba) suggest that the enrichment was transported in a hydrous fluid. The seamounts display gradients of decreasing moderately incompatible elements (e.g., Na,Ti, Zr) and increasing fluid mobile elements (e.g., Ba, Rb, K) toward the south that are similar to those of ELSC and are also related to proximity to the Tofua arc. The seamounts' geochemical characteristics are exaggerated compared to ELSC: they are typically more depleted in Na, Zr and Ti and more enriched in Ba, Rb and K than ELSC at the same latitude, or at the same distance from the arc. Moreover, their degree of depletion or enrichment increases with proximity to Tofua arc (i.e., distance from ELSC). The greater depletion in Na, Ti and Zr in the seamounts may be caused by either greater extents of melting or by a more refractory (but more H₂O and LIL-enriched) source. Increased melting could be related to higher H₂O contents in the mantle sources closer to the arc. Detailed trace element systematics show that the seamounts also show sympathetic latitudinal variations of the discrete enriched components (Bezos et al., this meeting) that are observed along ELSC.
DE: 3001 Back-arc basin processes
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3619 Magma genesis and partial melting (1037)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005