40Ar-39Ar dating of rocks drilled at sites 60-458 and 60-459

40Ar-39Ar dating of a high-MgO bronzite andesite from near the top of basement drilled at Site 458 shows the characteristic symptoms of artificially disturbed samples - i.e., an inverse staircase-type age spectrum, approximate linearity on an isochron plot, and concordance between total fusion age and isochron age. From conclusions based on other artificially disturbed samples (Ozima et al., 1979), we suggest that the reference isochron age (33.6 Ma) approximates the age of the sample.
A basalt from deeper in Hole 458 gives an isochron age of 19.1 ± 0.2 Ma, which is slightly younger than the plateau age of 21.4 ± 1.0 Ma. Both ages are, however, considerably younger than the age of fossils in the overlying sediments (30 - 34 Ma). The age discrepancy may be explained if the 40Ar-39Ar age represents the age of secondary minerals, which formed later.
No useful age data were obtained from a basalt sample recovered from Hole 459B.

对458站位钻井基底顶部附近产出的高镁古铜安山岩开展的40Ar-39Ar年代测定法（40Ar-39Ar dating）测试结果，呈现出人工扰动样品的典型特征：即反阶梯型年龄谱、等时线图近似呈线性，且总熔融年龄与等时线年龄一致。结合其他人工扰动样品的相关研究结论（Ozima等，1979），我们认为该参考等时线年龄（33.6 Ma）近似代表了该样品的形成年代。
采自458钻孔深部的玄武岩样品的等时线年龄为19.1 ± 0.2 Ma，略低于其21.4 ± 1.0 Ma的坪年龄。但上述两个年龄均远小于上覆沉积物中化石的测年结果（30~34 Ma）。若40Ar-39Ar年代测定结果代表后期形成的次生矿物的生成年代，则可合理解释这一年龄偏差。
从459B钻孔中回收的玄武岩样品未获得有效年代数据。