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Alteration of undisturbed igneous material used for argon dating work often results in inaccurate estimates of the crystallization age. A new quantitative technique to detect alteration has been developed (see Baksi, this volume, Chapter 15), utilizing the 36Ar levels observed in rocks and minerals. The method is applied to data in the literature for rocks linked to hotspot activity.

For subaerial rocks, argon dating results are critically examined for the Deccan Traps, India. The duration of volcanic activity and its coincidence in time with the K-T boundary are shown to be uncertain. The bulk of dated seafloor material (recovered from the Atlantic, Indian, and Pacific oceans) proves to be altered. Ages determined using large (hundreds of milligram) samples are generally unreliable, due to inclusion of altered phases. Such analyses include studies suggesting an age of ca. 43 Ma for the bend in the Hawaiian-Emperor chain. More recent attempts, using much smaller sub-samples (∼10 mg) that have been acid leached to remove alteration products, are generally of higher reliability. Plagioclase separates sometimes yield reliable results. However, many whole-rock basalts from the ocean floor yield ages that are, at best, minimum estimates of the time of crystallization. Most “rates of motions,” calculated from hotspot track ages, are shown to be invalid.

Seafloor rocks are recovered at considerable expense but often are not suitable for dating by the argon methods. Most are severely altered by prolonged contact with sea-water. A method is recommended for testing silicate phases prior to attempts at argon dating. This involves a quantitative determination of the 36Ar content of the material at hand; dating phases without pretreatment—leaching with HNO3 for material containing ferromagnesian phases, and HF for feldspars—is strongly discouraged.

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