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Koppers, Anthony A. P., Staudigel, Hubert, Duncan, Robert A. (2003). High-resolution40Ar/39Ar dating of the oldest oceanic basement basalts in the western Pacific basin. Geochemistry, Geophysics, Geosystems 4. doi: 10.1029/2003GC000574. issn: 1525-2027. |
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[1] We report new Ar-40/(39) Ar 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, 2sigma 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, 2s 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 similar to 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/ 3.6Ma ( n = 1, 2sigma 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. |
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Abstract |
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Abstract |
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Appendix A |
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Table A1 |
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Table 1 |
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Table 2 |
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Table 2 (continued) |
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Table 3 |
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40Ar/39Ar Dating Techniques |
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40Ar/39Ar Dating Techniques (continued) |
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Keywords
Geochemistry, Geochronology, Information Related to Geologic Time, Cenozoic, Marine Geology and Geophysics, Midocean ridge processes |
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Project -- MARGINS Focus Sites -- Subduction Factory: Izu Bonin and Mariana Trench
The Subduction Factory Initiative will proceed by focused investigations combining swath mapping of the incoming plate and fore-arc slope with both active and passive seismic experiments to image accretionary and slab structures, respectively. Heatflow measurements, magnetotelluric investigations and GPS plate and deformation rate estimates will combine with the other geophysical data to constrain the physical operation of the subduction system. Riserless drilling will provide samples of the input material seaward of the trench and output material in the forearc and arc. Riser drilling would permit deeper holes into the altered incoming crust, and riser or on-land drilling into the arc would sample a record of volcanic evolution and fluxes on the upper plate. Boreholes will be exploited to sample fluid outputs from the system. Field and analytical studies of the arc system will focus on the chemical composition and mass fluxes of lavas, melt inclusions and gases. Laboratory studies will provide element partitioning relationships, phase equilibria, and calibrations for rheological and seismological properties. A wide array of in situ observatories and multiple re-occupation campaigns, coupled with a strategy for rapidly responding to major events, round out the data collection strategy. |
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Publisher
American Geophysical Union
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