(Table 1) Apparent ages from 40Ar/39Ar heating steps for DSDP Hole 72-516F basalts

Submarine basalts are difficult to date accurately by the potassium-argon method. Dalrymple and Moore (1968) and Dymond (1970), for example, showed that, when the conventional K-Ar method is used, pillow lavas may contain excess 40Ar. Use of the 40Ar/39Ar step-heating method has not overcome the problem, as had been hoped, and has produced some conflicting results. Ozima and Saito (1973) concluded that the excess 40Ar is retained only in high temperature sites, but Seidemann (1978) found that it could be released at all temperatures. Furthermore, addition of potassium, from seawater, to the rock after it has solidified can result in low ages (Seidemann, 1977), the opposite effect to that of excess 40Ar. Thus, apparent ages may be either greater or less than the age of extrusion. Because of this discouraging record, the present study was approached pragmatically, to investigate whether self-consistent results can be obtained by the 40Ar/39Ar step-heating method.

海底玄武岩难以通过钾氩定年法（potassium-argon method）进行精确定年。例如，Dalrymple与Moore（1968）及Dymond（1970）的研究证实，采用传统钾氩定年法时，枕状熔岩（pillow lavas）可能含有过剩氩-40（excess ⁴⁰Ar）。尽管学界曾期望氩-40/氩-39阶步加热法（⁴⁰Ar/³⁹Ar step-heating method）能够解决这一难题，但该方法非但未能如愿，反而得出了若干相互矛盾的结果。Ozima与Saito（1973）提出过剩氩-40仅留存于高温位点，而Seidemann（1978）则发现其可在所有温度区间内被释放。此外，岩石固结成岩后经海水引入的钾元素会导致定年结果偏低（Seidemann, 1977），这一效应与过剩氩-40的影响恰好相反。因此，测得的视年龄可能大于或小于火山喷发的真实年龄。鉴于上述不尽如人意的研究进展，本研究采取务实的研究路径，旨在探究氩-40/氩-39阶步加热法能否获得自洽的定年结果。