HR: 1340h
AN: V43D-1641    [Abstracts]
TI: Sr-Nd-Hf-Pb Isotopic Constraints on the Role of South China Sea Sediments in Mantle Wedge Metasomatism Beneath the North Luzon Arc
AU: Kuo, T
EM: ing0955888846@yahoo.com.tw
AF: Department of Earth Sciences, National Cheng-Kung University, 1 University Rd., Tainan, 70101, Taiwan
AU: * Yang, H
EM: hjyang@mail.ncku.edu.tw
AF: Department of Earth Sciences, National Cheng-Kung University, 1 University Rd., Tainan, 70101, Taiwan
AU: Lee, D
EM: dclee@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, 128 Academia Road, Sec. 2, Nankang, Taipei, 115, Taiwan
AU: Lai, Y
EM: dreamingtreee@yahoo.com.tw
AF: Department of Earth Sciences, National Cheng-Kung University, 1 University Rd., Tainan, 70101, Taiwan
AB: Recycling sediments into mantle through subduction zones causes mantle heterogeneity and is critical on chemical evolution of the Earth. Because the compositions of subducted sediments vary significantly between subduction zones, there is a need to characterize the sediments from individual subduction zones and evaluate their contributions to mantle wedge metasomatism. This study investigates the role of South China Sea (SCS) sediments in the chemical characteristics of the North Luzon arc (NLA) magmatism. Thirty-five sediment samples (0-35 Ma) recovered by ODP Leg 184 at sites 1148 and 1147 were analyzed for Sr, Nd, Hf, and Pb isotope ratios and trace element abundances. Results were compared to the data of the North Luzon arc lavas to establish models for mantle wedge metasomatism. The NLA lavas deviate from the terrestrial array to higher ¹⁷⁶Hf/¹⁷⁷Hf values at a given ¹⁴³Nd/¹⁴⁴Nd value, consistent with involving subducted sediments in source regions. Since Hf in the subducted slabs cannot be transported to mantle wedges by hydrous fluids, slab-derived siliceous melts are the most probable metasomatic agents. This algorithm leads to three metasomatism models: (I) addition of bulk sediments to depleted mantle, (II) depleted mantle metasomatized by sediment-derived melts, and (III) depleted mantle metasomatized by melts derived from sediments and altered oceanic crust (AOC). Although not considered in model calculations, the contributions of sediment-derived and AOC-derived fluids are also addressed qualitatively. The first model results in mixing curves overlapping with the mantle array in Sr versus Nd and Hf isotope plots, inconsistent with the distributions of the NLA lavas, which deviate from mantle array to lower ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf values. The involvement of sediment-derived melts (Model II) leads to source compositions with larger deviation from the NLA lavas in Sr versus Nd and Hf isotope plots, because sediment-derived melts have higher Sr/Nd and Sr/Hf ratios than their sources. Model III with variable amounts of residual zircons (0.0016-0.005%) provides the best fit to most NLA lavas. The sediment-derived and AOC-derived melts were generated by 15-20% and 2-4% melting, respectively, and these two metasomatic agents contributed sub-equally to sources of the NLA lavas. In all three models, the addition of AOC-derived fluids can improve the model fits to the NLA data. However, their contribution to trace element budgets in the metasomatized mantle should be far less significant compared to that of the slab-derived melts due to their low trace element abundances. These model calculations show that the SCS sediments can be the proper long-term enriched component in the sources of NLA lavas.
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting