14. Chemical Composition of Prehistoric Lavas at Makaopuhi Crater, Kilauea Volcano, and Periodic Change in Alkali Content of Hawaiian Tholeiitic Lavas

Abstract

The K2O/Na2Oratio and, to a lesser degree, Na2O+K2O of prehistoric tholeiitic lavas exposed on the wall of Makaopuhi crater, Kilauea volcano, show periodic change with time. Similar variation is found in the lower member of Waianae volcano, Oahu Island. Marked increase in K2O/Na2O ratio of the historic lavas of Kilauea as compared with that of early historic and many prehistoric lavas of the same volcano may correspond to a part of such periodic variations in K2O/Na2O ratio as represented by the above two examples. As one possible mechanism for such increase in K2O/Na2O ratio, fractional crystallization of K2O-poor plagioclase is proposed.|キラウエア火山マカオプヒ火口壁に露出する,一連の先史時代のソレアイト質溶岩を採集し,30個の標本についてNa2OとK2O,そのうち3個については更にSiO2,他の3個については13成分の定位を行つた.その結果,K2O/Na2O比が時間と共に顕著な周期的変化を示すことが明らかになつた.Na2O+K2Oもほぼ同様の変化を示す.同じような変化はオアフ島ワイアナイ火山においても見られる.キラウエア火山の最近の溶岩のK2O/Na2O比は,同火山の20世紀以前の溶岩のそれに比べ著しく高いが,これは,上に述べた周期的変化の2例と同じ現象かも知れない.これを説明する1つの可能性として,K2Oに乏しい斜長石がマグマから晶出分離するモデルを提唱した