Radiometric age determinations for basements of ODP Sites 123-765 and 123-766@en

The results of experiments in 40Ar/39Ar age dating using fresh basement material from Sites 765 and 766 of Leg 123 of the Ocean Drilling Program are inconsistent and cannot be used to constrain the basement age of the Argo Abyssal Plain in the Indian Ocean. However, a celadonite sample, which was precipitated during a low-temperature alteration event that affected the basement at Site 765, yielded a K-Ar age of 155.3 ±3.4 Ma. Celadonites, which have been dated using Rb-Sr methods for basement in the Atlantic Ocean (Staudigel et al., 1981, doi:10.1016/0012-821X(81)90186-2) and by K-Ar methods for the Troodos Ophiolite (Staudigel et al., 1986, doi:10.1130/0091-7613(1986)14<72:AASAOC>2.0.CO;2), and for sediments from the Pacific Ocean (Peterson et al., 1986, doi:10.2973/dsdp.proc.92.132.1986) yield ages that are up to 15 Ma younger than the age for the formation of basement. Thus, the celadonite age is retained as a reliable minimum age for basement at Site 765. This radiometric age is inconsistent with biostratigraphic ages, which indicate a maximum of late Berriasian (approximately 140 Ma) for Site 765, but is consistent with geophysical interpretations of marine magnetic anomalies and with the early north-south seafloor spreading history of the Argo Abyssal Plain region of the Indian Ocean.

本研究针对大洋钻探计划（Ocean Drilling Program）第123航次765、766站位采集的新鲜基底物质开展氩-40/氩-39（40Ar/39Ar）年代测年实验，所得结果存在矛盾，无法为印度洋阿尔戈深海平原的基底年代提供有效约束。不过，针对765站位基底在低温蚀变事件中沉淀的一块绿鳞石（celadonite）样品开展测试，测得其钾-氩（K-Ar）年龄为155.3±3.4 百万年（Ma）。此前已有多项研究采用铷-锶（Rb-Sr）方法对大西洋基底中的绿鳞石进行定年（Staudigel等，1981，doi:10.1016/0012-821X(81)90186-2），采用钾-氩方法对特罗多斯蛇绿岩套（Troodos Ophiolite）中的绿鳞石定年（Staudigel等，1986，doi:10.1130/0091-7613(1986)14<72:AASAOC>2.0.CO;2），并采用类似方法对太平洋沉积物中的绿鳞石定年（Peterson等，1986，doi:10.2973/dsdp.proc.92.132.1986），结果显示此类定年结果比基底形成年龄年轻最多15 Ma。因此，该绿鳞石年龄可作为765站位基底的可靠最小年代约束。该放射性同位素年代结果与生物地层年代存在矛盾：生物地层学研究显示765站位的最大年代为贝里阿斯阶晚期（约140 Ma），但该结果与海洋磁异常的地球物理解释以及印度洋阿尔戈深海平原区域早期南北向海底扩张历史相一致。