HR: 1340h
AN: B13A-0194    [Abstracts]
TI: Variability of Southern Valu Fa Ridge Magmatic Systems
AU: * Goddard, C I
EM: cgoddard@coas.oregonstate.edu
AF: COAS, Oregon State University, Corvallis, OR 97331-5503 United States
AU: Christie, D M
EM: dchristie@coas.oregonstate.edu
AF: COAS, Oregon State University, Corvallis, OR 97331-5503 United States
AU: Arculus, R
EM: Richard.Arculus@anu.edu.au
AF: Dept. Earth and Marine Sci., Australian National Univ., Canberra, ACT 0200 Australia
AB: Valu Fa Ridge (VFR), which encompasses the southernmost segments of the East Lau Spreading Center (ELSC), is an important end-member in the spectrum of back-arc spreading centers because it is strongly affected in all aspects of the spreading process by inputs from the nearby active Tofua (Tonga) volcanic arc, and because its magma systems are rapidly evolving as the VFR propagates to the south. New lava samples collected by the TELVE Expedition of the R/V Southern Surveyor (Australia) from the four southernmost VFR segments have greatly increased the number and spatial distribution of fresh volcanic glass samples, quadrupling the availability of "primitive" (MgO $>$ 6 wt.%) glass samples and encompassing significant along- and across-axis geochemical variability. These new data provide an opportunity to evaluate both the evolution of crustal magmatic processes relative to southward rift propagation and variability in mantle source inputs relative to the active volcanic arc. The four sampled segments of VFR are separated by left-stepping overlapping offsets that differ from their mid-ocean ridge counterparts in their longer, more parallel overlapping limbs and in the absence of an overlap basin. The TELVE glasses display considerable major element variability and the VFR is unusual among well-developed spreading centers in its strongly bimodal volcanism, in the abundance of evolved lavas and in the coexistence of two distinct liquid lines of descent. Dacites and rhyolites (SiO2 ~67-75 wt.%) are relatively abundant close to segment ends or discontinuities along the southernmost three ridge segments, but rare from the northernmost sampled segment and from off-axis seamounts. A "Daly Gap" from ~60-67 wt.% SiO2 and 1.75-0.75 wt.% MgO is present along the northern segments but absent near the southern propagating rift tip. Primitive (MgO $>$ 6wt.%) glasses were recovered from all VFR segments and on 5 of the 9 sampled seamounts. Both high-silica glasses and FeTi basalt glasses appear to represent the culmination of simple crystal fractionation trends derived from similar tholeiitic parental compositions that bifurcate between 5 and 6 wt. % MgO. Lavas from the off-axis seamounts, both east and west of VFR, are systematically more mafic than the on-axis samples and most have higher K2O contents. A single boninitic glass recovered from seamount K-11 is similar in composition to melts reported in a xenolith from the same seamount by Kamenetsky et al. (EPSL, 151, 205, 1997). At its southern end, the VFR is propagating into old arc crust of unknown age. If the high silica lavas are excluded, along-axis major element trends define a typical rift propagation pattern. High-silica lavas occur sporadically with no apparent relationship to the petrologic trends defined by the more mafic lavas, suggesting that magmas only fractionate to high SiO2 compositions under ideal, localized conditions. Of particular interest is the possibility that the high-Si trend is initiated and/or facilitated by changes in oxygen fugacity, perhaps initiated by assimilation of hydrated pre-existing crustal material. Preliminary trace element data for the southern VFR segments show the characteristic Nb and Ta depletions associated with volcanic arc sources, confirming the strong input from the volcanic arc to the back-arc mantle source region. More complete spatial coverage is expected to help define the extent and, perhaps the pathways, of this mantle input.
DE: 9355 Pacific Ocean
DE: 3655 Major element composition
DE: 3670 Minor and trace element composition
DE: 3035 Midocean ridge processes
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting