Abstract:
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.
