Basement basalts from IODP Site 1438, Amami-Sankaku Basin; implications for sources and melting processes during subduction initiation in the Izu-Bonin-Mariana system

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Author(s): McCarthy, A. J.; Hickey-Vargas, R.; Yogodzinski, G. M.; Ishizuka, O.; Hocking, B.; Bizimis, M.; Savov, I. P.; Kusano, Y.; Arculus, R. J.
Author Affiliation(s): Primary:
University of Lausanne, Lausanne, Switzerland
Florida International University, United States
University of South Carolina Columbia, United States
Geological Survey of Japan, Tsukuba, Japan
University of South Carolina, United States
University of Leeds, United Kingdom
Australian National University, Australia
Volume Title: AGU 2016 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2016; American Geophysical Union 2016 fall meeting, San Francisco, CA, Dec. 12-16, 2016. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: IODP Expedition 351 Site 1438 is located in the Amami-Sankaku Basin, just west of the Kyushu-Palau Ridge (KPR), a remnant of the early Izu-Bonin-Mariana (IBM) volcanic arc. 150 meters of basement basalt were drilled beneath 1460 m of volcaniclastic sediments and sedimentary rock. The age range inferred for these basalts is 51-52 Ma, close to the 48-52 Ma age of basalts associated with subduction initiation in the IBM forearc (forearc basalts or FABs). Site 1438 basement basalts form several distinct subunits, all relatively mafic (MgO = 6-14 %; Mg# = 51-83). Non-fluid-mobile incompatible trace element patterns are profoundly depleted. Sm/Nd (0.34-0.43) and Lu/Hf (0.18-0.37) reach values higher than most normal MORBs while La/Yb (0.31-0.98) and Ti/V (15.8-27.0) are lower. These features are shared with basalts drilled just west of the KPR at ODP Site 1201 and DSDP Site 447, and many FABs. Abundances of fluid-mobile incompatible elements vary together and are correlated with subunits defined by flow margins and rock physical properties, suggesting control by post-eruptive seawater alteration rather than varying inputs of subduction fluids. Hf-Nd isotopes for Site 1438 basement basalts range from (present-day) εNd of 7.0 to 9.5 and εHf of 14.5 to 19.8 in a well-correlated array. Their more radiogenic Hf-isotope character could indicate an indian-type MORB source, however, basalts with εHf >16.5, are more radiogenic than many Indian MORB. Pb isotope data will help distinguish differing mantle source domains and origins for fluid-mobile elements. Overall, the combined geochemical data indicate that the mantle source of basement basalts in drill sites west of the KPR (1438, 1201, 447) are closely similar to those for FAB, and that as a group, these rocks are more depleted than more than 90% of global MORB. Our interpretation is that both IBM forearc basalts and basalts from drill sites immediately west of the KPR formed by melting of the same uniquely depleted mantle source during subduction initiation. Melting may have been promoted by rapid decompression and by flux melting with a solute-poor hydrous subduction fluid. These basalts were erupted over a broad area in an extensional setting, which later narrowed as subduction and the subduction-related IBM volcanic arc became established.
Year of Publication: 2016
Research Program: IODP Integrated Ocean Drilling Program
IODP2 International Ocean Discovery Program
Key Words: 07 Marine Geology and Oceanography; Cores; Expedition 351; IODP Site U1438; International Ocean Discovery Program; Izu-Bonin Arc; Marine sediments; North Pacific; Northwest Pacific; Pacific Ocean; Sediments; West Pacific
Coordinates: N272300 N272301 E1341907 E1341905
Record ID: 2017062824
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