HR: 1340h
AN: T13B-1363    [Abstracts]
TI: Geochemistry of basalt from the Ayu Trough, equatorial western Pacific
AU: * Park, S
EM: shpark21@snu.ac.kr
AF: School of Earth and Environmental Sciencies, Seoul National University, Sillim-dong, Gwanak-gu, Seoul, 151-742 Korea, Republic of
AU: Lee, S
EM: smlee@snu.ac.kr
AF: School of Earth and Environmental Sciencies, Seoul National University, Sillim-dong, Gwanak-gu, Seoul, 151-742 Korea, Republic of
AU: Arculus, R
EM: Richard.Arculus@anu.edu.au
AF: Department of Earth and Marine Sciencies, The Australian National University, Geology Building 047, Canberra, ACT 0200 Australia
AB: We analyzed dredge rock samples collected from the Ayu Trough, a divergent margin located between the Philippine Sea and Caroline plates in the equatorial western Pacific. The region may be divided into three sections from north to south on the basis of geochemistry and axial morphology. The recovered Ayu Trough basalt shows a large geochemical variation from N-MORB to relatively enriched-MORB. The N-MORBs were mainly derived from the southern and northern sections, while the E-MORBs were sampled from the middle section ($2\deg$20'N - $4\deg$N). The variations in major and trace elements between the N-MORB and E-MORB are too large to be explained by melting of uniformly depleted source mantle alone. Instead, mantle heterogeneity where both depleted mantle and enriched end-member exist may explain the overall geochemistry. The composition of depleted component in the basalt appears to be close to that of the depleted mantle source, whereas the enriched end-member may be close to that of EM. Sr-Nd-Pb isotope data also supported the mantle heterogeneity argument. We infer that the enriched component has different mineralogy with respect to the depleted component and suggest garnet-pyroxenite vein in the mantle as the source of the enrichment. According to this argument, the samples from the middle section then may have originated from pyroxenite-rich part of the mantle. It appears that such enriched parts are unevenly distributed under the Ayu Trough. In addition, variations in the degree of melting may have also contributed to the variability of incompatible elements especially in the middle section. From one of our dredges that targeted the axial volcanic ridge in the middle section, we found two distinct populations of samples where one has much lower degree of melting than the other. Although the exact cause of the contrasting degree of melting is unclear, it is possible that the samples with low degree of melting were produced after the spreading has halted or when the trough was slowing down, which then led to the emplacement of melt from the lower part of melting regime.
DE: 1744 Tectonophysics
DE: 1749 Volcanology, geochemistry, and petrology
DE: 1020 Composition of the crust
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting