40Ar/39Ar dating and zircon chronochemistry for the Izu-Bonin rear arc, IODP site U1437

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http://abstractsearch.agu.org/meetings/2016/FM/V13C-2853.html
Author(s): Schmitt, Axel K.; Konrad, K.; Andrews, G. D.; Horie, K.; Brown, S. R.; Koppers, Anthony A. P.; Busby, C.; Tamura, Y.
Author Affiliation(s): Primary:
University of Heidelberg, Institute of Earth Sciences, Heidelberg, Germany
Other:
Oregon State University, United States
West Virginia University, United States
National Institute of Polar Research, Japan
University of California at Davis, United States
Japan Agency for Marine-Earth Science and Technology, Japan
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: The scientific objective of IODP Expedition 350 drilling at Site U1437 (31°47.390'N, 139°01.580'E) was to reveal the "missing half of the subduction factory": the rear arc of a long-lived intraoceanic subduction zone. Site U1437 lies in a ∼50 km long and ∼20 km wide volcano-bounded basin, ∼90 km west of the Izu Arc front, and is the only IODP site drilled in the rear arc. The Izu rear arc is dominated by Miocene basaltic to dacitic seamount chains, which strike at a high angle to the arc front. Radiometric dating targeted a single igneous unit (1390 mbsf), and fine to coarse volcaniclastic units for which we present zircon and 40Ar/39Ar (hornblende, plagioclase, and groundmass) age determinations. All zircons analyzed as grain separates were screened for contamination from drill-mud (Andrews et al., 2016) by analyzing trace elements and, where material was available, O and Hf isotope compositions. Igneous Unit 1 is a rhyolite sheet and yielded concordant in-situ and crystal separate U-Pb zircon ages (13.7±0.3 Ma; MSWD = 1.3; n = 40 spots), whereas the 40Ar/39Ar hornblende plateau age (12.9±0.3; MSWD = 1.1; n = 9 steps) is slightly younger, possibly reflecting pre-eruptive zircon crystallization, or alteration of hornblende. U-Pb zircon and 40Ar/39Ar plateau ages from samples above igneous Unit 1 are concordant with biostratigraphic and paleomagnetic ages (available to ∼1300 mbsf), but plagioclase and groundmass samples below 1300 m become younger with depth, hinting at post-depositional alteration. A single zircon from ∼1600 mbsf yielded a U-Pb age of 15.4±1.8 Ma; its trace element composition resembles other igneous zircons from U1437, and is tentatively interpreted as a middle Miocene age for the lowermost lithostratigraphic unit VII. Oxygen and Hf isotopic values of igneous zircon indicate mantle origins, with some influence of assimilation of hydrothermally altered oceanic crust evident in sub-mantle oxygen isotopic compositions. Lessons from site U1437 are that integrated chronochemistry is essential for achieving accurate age models in oceanic drilling. Reference: Andrews, G. D., Schmitt, A. K., Busby, C. J., Brown, S. R., Blum, P., & Harvey, J. (2016). Age and compositional data of zircon from sepiolite drilling mud to identify contamination of ocean drilling samples. G3. DOI: 10.1002/2016GC006397.
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 350; IODP Site U1437; International Ocean Discovery Program; Izu-Bonin Arc; Marine sediments; North Pacific; Northwest Pacific; Pacific Ocean; Sediments; West Pacific
Coordinates: N314723 N314724 E1390136 E1390134
Record ID: 2017062821
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