Hf-Nd isotopes in West Philippine Basin basalts; results from International Ocean Discovery Program (IODP) Site U1438 and implications for the early history of the Izu-Bonin-Mariana (IBM) subduction system

Author(s): Yogodzinski, G. M.; Hocking, B.; Bizimis, Michael; Hickey-Vargas, Rosemary; Ishizuka, O.; Bogus, K.; Arculus, R. J.
Author Affiliation(s): Primary:
University of South Carolina, Columbia, SC, United States
Other:
Florida International University, Miami, FL, United States
Geological Survey of Japan, Tsukuba, Japan
International Ocean Discovery Program, College Station, TX, United States
Australian National University, Canberra, A.C.T., Australia
Volume Title: AGU 2015 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2015; American Geophysical Union 2015 fall meeting, San Francisco, CA, Dec. 14-18, 2015. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: Drilling at IODP Site U1438, located immediately west of Kyushu-Palau Ridge (KPR), the site of IBM subduction initiation, penetrated 1460 m of volcaniclastic sedimentary rock and 150 m of underlying basement. Biostratigraphic controls indicate a probable age for the oldest sedimentary rocks at around 55 Ma (51-64 Ma - Arculus et al., Nat Geosci in-press). This is close to the 48-52 Ma time period of IBM subduction initiation, based on studies in the forearc. There, the first products of volcanism are tholeiitic basalts termed FAB (forearc basalt), which are more depleted than average MORB and show subtle indicators of subduction geochemical enrichment (Reagan et al., 2010 - Geochem Geophy Geosy). Shipboard data indicate that Site U1438 basement basalts share many characteristics with FABs, including primitive major elements (high MgO/FeO*) and strongly depleted incompatible element patterns (Ti, Zr, Ti/V and Zr/Y below those of average MORB). Initial results thus indicate that FAB geochemistry may have been produced not only in the forearc, but also in backarc locations (west of the KPR) at the time of subduction initiation. Hf-Nd isotopes for Site 1438 basement basalts show a significant range of compositions from εNd of 7.0 to 9.5 and εHf of 14.5 to 19.8 (present-day values). The data define a well-correlated and steep array in Hf-Nd isotope space. Relatively radiogenic Hf compared to Nd indicates an Indian Ocean-type MORB source, but the dominant signature, with εHf >16.5, is more radiogenic than most Indian MORB. The pattern through time is from more-to-less radiogenic and more variable Hf-Nd isotopes within the basement section. This pattern culminates in basaltic andesite sills, which intrude the lower parts of the sedimentary section. The sills have the least radiogenic compositions measured so far (εNd ≈6.6, εHf ≈13.8), and are similar to those of boninites of the IBM forearc and modern IBM arc and reararc rocks. The pattern within the basement suggests modest enrichment of a depleted Indian MORB source over time.
Year of Publication: 2015
Research Program: IODP Integrated Ocean Drilling Program
IODP2 International Ocean Discovery Program
Key Words: 07 Marine Geology and Oceanography; Basalts; IODP Site U1438; Igneous rocks; International Ocean Discovery Program; Izu-Bonin Arc; Mariana Islands; Marine sediments; Micronesia; Mid-ocean ridge basalts; North Pacific; Northwest Pacific; Oceania; Pacific Ocean; Philippine Basin; Philippine Sea; Sediments; Volcanic rocks; West Pacific
Record ID: 2016066766
Copyright Information: GeoRef, Copyright 2017 American Geosciences Institute. Reference includes data supplied by, and/or abstract, Copyright, American Geophysical Union, Washington, DC, United States

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