Geochemistry and ages at DSDP Hole 19-192A

Conventional K-Ar and 40Ar/39Ar age data on altered basalts from DSDP Hole 192A on Meiji Guyot, Emperor Seamount chain, indicate a minimum age for the volcano of 61.9 ± 5.0 m.y. The K-Ar data are consistent with the early Maestrichtian date of the overlying sediments, but do not provide either a positive or negative test of the hypothesis that Meiji is older than Emperor volcanoes to the south. The most prominent alteration affecting the age measurements is potassium metasomatism, particularly of feldspar phenocrysts. The K-Ar apparent ages of feldspar separates from the Meiji basalts show that more than half of the potassium metasomatism occurred within the last 25 m.y. or so, and that if the potassium replacement rate has been constant, then the alteration of the Meiji basalts did not begin for 10 to 20 m.y. after the volcano formed.

对天皇海山链（Emperor Seamount chain）明治海山（Meiji Guyot）处深海钻探计划（Deep Sea Drilling Project, DSDP）192A钻孔中的蚀变玄武岩开展的常规钾氩（K-Ar）与氩氩（40Ar/39Ar）定年数据显示，该火山的最小年龄为61.9±5.0百万年。所得钾氩定年数据与上覆沉积物的马斯特里赫特阶早期年代相符，但既无法为“明治海山比南侧的天皇火山更为古老”这一假说提供正面佐证，也无法给出反面检验结果。对年龄测定影响最为显著的蚀变作用为钾交代作用，尤以长石斑晶的钾交代最为突出。对明治海山玄武岩分离得到的长石组分的钾氩视年龄分析表明，超过半数的钾交代作用发生于距今约25百万年以内；若钾置换速率保持恒定，则明治海山玄武岩的蚀变作用是在火山形成后的10至20百万年间才开始发生的。