HR: 1330h
AN: V12C-0601 [PDF]
TI: Along-Axis Geochemical Variations in Basaltic Glasses From the Incipient Rift Adjacent to the East
Pacific Rise at 2$^{o}$40'N
AU: * Hanna, H D
EM: hdh3@duke.edu
AF: Duke University, Division of Earth and Ocean Sciences
Box 90229, Durham, NC 27708 United States
AU: Klein, E M
EM: ek4@duke.edu
AF: Duke University, Division of Earth and Ocean Sciences
Box 90229, Durham, NC 27708 United States
AU: Smith, D K
EM: dsmith@whoi.edu
AF: WHOI, Dept. of Geology and Geophyics, Woods Hole, MA 02543 United States
AU: Zhu, W
EM: wzhu@whoi.edu
AF: WHOI, Dept. of Geology and Geophyics, Woods Hole, MA 02543 United States
AB:
The east-west-trending Incipient Rift (IR) is located adjacent to the East Pacific Rise (EPR) at 2$^{o}$40'N in the
equatorial Pacific. This slow-spreading ($\sim$0-30 mm/yr), magmatic rift forms the boundary between the Cocos plate and
Galapagos microplate (Lonsdale, 1988) and connects to the East Pacific Rise (EPR) through a magmatically active linking
ridge.
This study examines glass compositions from 41 dredges and one wax core collected along a 60-km section of the IR, including
two dredges from an on-axis volcano, as well as four dredges and three wax cores from the adjacent EPR. This represents the
most thorough sampling of the IR to date, with previous sampling limited to 11 wax cores collected during the 1999 Hess Deep
expedition and a single dredge from an earlier cruise.
To first order, IR samples are similar in major and trace element compositions to basalts erupted at the adjacent EPR. IR
glasses, in general, become more evolved with increasing distance from the EPR (Mg\# 67-41). The samples are generally
depleted ((La/Sm)$_{N}$: 0.38-0.86), with the most depleted samples occurring within $\sim$7 km of the EPR. Samples $>$7 km
from the EPR are less depleted than the near-EPR samples and have a relatively constant range of (La/Sm)$_{N}$ (0.61-0.73).
Trace element ratios indicative of source heterogeneity (e.g. Zr/Hf) and garnet signature in the melting residue
((Sm/Yb)$_{N}$) are also lower in the near-EPR samples than those $>$7 km from the EPR. Fe$_{8.0}$ values, an indicator of
pressure of melting, range from 9.2-10.4 and increase away from the EPR for $\sim$10 km. At distances $>$10 km, Fe$_{8.0}$
values are variable (10.4-11.6) exhibiting no clear spatial trend. Samples from the volcano $\sim$13 km from the EPR are the
least depleted ((La/Sm)$_{N}$: 0.85-0.86); have the strongest garnet signature; and are most primitive among the IR samples.
The data from the IR will be examined in the context of geophysical and geochemical modeling results on the nature of
melting and melt focusing beneath the adjacent EPR, as well as compared to other propagating rifts, triple junctions, and
ultra-slow spreading centers.
DE: 3035 Midocean ridge processes
DE: 3655 Major element composition
DE: 3670 Minor and trace element composition
DE: 9355 Pacific Ocean
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting