HR: 1330h
AN: V32D-1053 [PDF]
TI: Multiple Lava Types Erupted Over Short Spatial and Temporal Scales at the Southern East Pacific Rise:
$17\deg$23'-$17\deg$35'S
AU: * Bergmanis, E C
EM: bergmani@hawaii.edu
AF: SOEST, University of Hawaii at Manoa, 1680 East-West Rd., Honolulu, HI 96822 United States
AU: Sinton, J M
EM: sinton@hawaii.edu
AF: SOEST, University of Hawaii at Manoa, 1680 East-West Rd., Honolulu, HI 96822 United States
AU: Rubin, K H
EM: krubin@hawaii.edu
AF: SOEST, University of Hawaii at Manoa, 1680 East-West Rd., Honolulu, HI 96822 United States
AB:
Major, trace element and U-series disequilibria data reveal both short- and long-term compositional heterogeneity in lavas
from the southern EPR $17\deg$23'-$17\deg$35'S.
Major-element compositional variation in the $\sim$18km-long Aldo-Kihi flow (MgO 7.7-8.4 wt %) is consistent with
shallow-level fractional crystallization. ($^{230}$Th)/($^{238}$U) disequilibria for samples from the Aldo-Kihi flow cover a
narrow range of 1.85$\pm$0.15. However, the range of Th/U ratios (2.61-2.84) and ($^{226}$Ra)/($^{230}$Th) disequilibria
(2.26-2.65) are very broad for a single eruptive unit and cannot be a result of shallow-level fractional crystallization of a
single magma composition. MgO contents, Th/U ratios, and ($^{226}$Ra)/($^{230}$Th) disequilibria for Aldo-Kihi samples all
peak at $\sim$$17\deg$30.6'S and generally are greater for samples south of $17\deg$30'S implying that more than one magmatic
component has contributed to this lava flow. This along-axis compositional diversity is hard to reconcile with propagation
of a dike originating from a small area, and suggests vertical eruption from a magma chamber that is compositionally zoned
along-axis. Along this $\sim$22km-long section of the southern EPR, major, trace element and U-series disequilibria data
indicate that at least six compositionally distinct lava types are present within 1km of the axis. ($^{210}$Pb)/($^{226}$Ra)
deficits of $\sim$5 % for the Aldo-Kihi lava and two other compositionally distinct units suggest they are substantially
younger than $\sim$100 yrs. Although geologic relationships demonstrate that Aldo-Kihi is the youngest of these, the range of
($^{210}$Pb)/($^{230}$Ra) disequilibria for the individual units overlap making relative age determinations impossible using
that criteria alone. Regional trends in major-element data show that twenty-three other samples older than Aldo-Kihi contain
$>$8.0 wt % MgO; all but two occur south of $17\deg$28.4'S suggesting that the location of hottest sub-axial magma is a
long-lived feature of this region. The distribution of rock types in this area requires a complex history of mantle melting,
recharge, cooling, and eruption that varies considerably over along-axis distances of $\sim$22 kilometers and between
successive eruptions.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 3035 Midocean ridge processes
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting