High-resolution 40Ar/39Ar dating of the oldest oceanic basement basalts in the western Pacific basin

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doi: 10.1029/2003GC000574
Author(s): Koppers, Anthony A. P.; Staudigel, Hubert; Duncan, Robert A.
Author Affiliation(s): Primary:
University of California at San Diego, Institute of Geophysics and Planetary Physics, San Diego, CA, United States
Oregon State University, United States
Volume Title: Geochemistry, Geophysics, Geosystems - G<sup>3</sup>
Source: Geochemistry, Geophysics, Geosystems - G>3`, 4(11), 20p. Publisher: American Geophysical Union and The Geochemical Society, United States. ISSN: 1525-2027
Note: In English. Accessed on Feb. 2, 2004. 53 refs.; illus., incl. 4 tables, sketch map
Summary: We report new 40Ar/39Ar ages for the oldest Pacific oceanic floor at Ocean Drilling Program Site 801C in the Pigafetta basin and Site 1149D close to the Izu-Bonin subduction zone in the Nadezhda basin. These ages were determined by applying high-resolution incremental heating experiments (including 15-30 heating steps) to better resolve the primary argon signal from interfering alteration signatures in these low-potassium ocean crust basalts. Combined with previous results from Pringle [1992] for Site 801B and 801C, we arrive at a multistage history for the formation of the Pigafetta ocean crust. The oldest part of the Pacific plate was formed at the spreading ridges at 167.4±1.4/3.4 Ma (n = 2, 2σ internal/absolute error), offering an important calibration point on the Geological Reversal Timescale (GRTS) since it represents the old end of the Mesozoic magnetic anomalies. This mid-ocean ridge basalt sequence, however, is overlain by more tholeiites and alkali basalts that were formed 7.3±1.5 Myr later around 160.1±0.6 Ma (n = 7, 2σ internal error). The older age group is confirmed independently by radiolarian ages ranging from Late Bajocian to Middle Bathonian (167-173 Ma [Bartolini and Larson, 2001]) and by profound differences in the structural characteristics of this basement section [Pockalny and Larson, 2003]. Thin layers comprising hydrothermal deposits separate these sequences, which in addition to the difference in isotopic age show distinct major and trace element compositions. This indicates that key volcanic and hydrothermal activity took place 400-600 km away from the spreading ridges, on the basis of a Jurassic ∼66 km/Myr half spreading rate in the Pacific. It remains unclear if these processes were active continuously after the initial formation of the Pacific oceanic crust, but all our observations seem to point to an episodic history. Site 1149D gives another important calibration point on the GRTS of 127.0±1.5/3.6 Ma (n = 1, 2σ internal/absolute error) for anomaly M12 that is slightly younger when compared to current timescale compilations (134.2±2.1 Ma [Gradstein et al., 1995]). This might suggest that the dated basalt from Site 1149D does not represent the age of the ocean crust formed at its ridge axis; it may also be part of the Early Cretaceous intraplate events that have produced dolerite sills in the Pacific crust at Sites 800 and 802 around 114-126 Ma.
Year of Publication: 2003
Research Program: ODP Ocean Drilling Program
Key Words: 03 Geochronology; Absolute age; Ar/Ar; Basalts; Bathonian; Cretaceous; Crust; Dates; Igneous rocks; Intraplate processes; Izu-Bonin Arc; Jurassic; Leg 129; Leg 185; Lower Cretaceous; Magnetic anomalies; Mesozoic; Middle Jurassic; Nadezhda Basin; North Pacific; Northwest Pacific; ODP Site 1149; ODP Site 800; ODP Site 802; Ocean Drilling Program; Oceanic crust; Pacific Ocean; Volcanic rocks; West Pacific
Record ID: 2004067082
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data supplied by, and/or abstract, Copyright, American Geophysical Union, Washington, DC, United States

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