Minerals and geochemistry abundance and occurance, as well as K-Ar determinations for DSDP Site 92-597 basalts

Preliminary studies of hydrothermally altered massive basalts formed at the fast-spreading Mendoza Rise and recovered from DSDP Holes 597B and 597C indicate the presence of three secondary mineral assemblages which formed in the following order: (1) trioctahedral chlorite and talc, (2) goethite and smectite, and (3) calcite and celadonite. The sequential precipitation of these mineral assemblages denotes high water:rock ratios and time-varying conditions of temperature (early >200°C to late <30°C) and state of oxidation (early nonoxidative to late oxidative). A decrease in the relative proportion of oxidative mineral assemblages with depth to 70 m in Site 597 basement indicates a zone of oxidative alteration that became shallower with time as the deeper, more constricted fracture systems were filled by secondary mineralization. In this report we present the first results of the K-Ar dating of celadonite formation age; celadonite formation reflects end-stage hydrothermal alteration in Site 597 basement. Three celadonite dates obtained from Site 597 samples include 13.1 ± 0.3 m.y. from 17 m basement depth (Hole 597B), 19.9 ± 0.4 m.y. from 18 m basement depth (Hole 597C), and 19.3 ± 1.6 m.y. from 60 m basement depth (Hole 597C). The age of host rock crystallization (28.6 m.y.) and the K-Ar dates of celadonite formation establish that hydrothermal alteration in the upper 70 m of Site 597 basement continued for at least 10 m.y. and possibly as long as 16 m.y. after basalt crystallization at the ridge crest. Assuming a half-spreading rate of 55 km/m.y., we calculate that hydrothermal circulation was active in shallow basement at a distance of at least 550 km off ridge crest and possibly as far as 1000 km off ridge crest.对形成于快速扩张门多萨海隆、并由深海钻探计划（Deep Sea Drilling Project，DSDP）597B号与597C号钻孔采集的热液蚀变块状玄武岩开展的初步研究显示，存在三类按形成先后顺序产出的次生矿物组合：（1）三八面体绿泥石与滑石；（2）针铁矿与蒙脱石；（3）方解石与绿鳞石。这些矿物组合的依次沉淀指示了高水岩比值，以及随时间演化的温度条件（早期＞200℃至晚期＜30℃）与氧化状态（早期为非氧化环境，晚期转为氧化环境）。在597号钻孔基底中，随深度增至70米处氧化型矿物组合的相对占比逐渐降低，这表明随着更深且受限制更强的断裂系统被次生矿化作用充填，氧化蚀变带随时间逐渐向浅部迁移。 本报告首次报道绿鳞石形成年龄的钾氩（K-Ar）定年结果；绿鳞石的形成过程对应597号钻孔基底的末期热液蚀变。从597号钻孔样品中获得的三组绿鳞石定年数据分别为：钻孔597B17米基底深度处13.1±0.3百万年，钻孔597C18米基底深度处19.9±0.4百万年，钻孔597C60米基底深度处19.3±1.6百万年。 寄主岩结晶年龄（28.6百万年）与绿鳞石形成的钾氩定年结果共同表明，597号钻孔基底上部70米范围内的热液蚀变作用，在脊峰处玄武岩结晶完成后至少持续了10百万年，最长可能达16百万年。假设半扩张速率为55公里/百万年，经计算可知浅部基底中的热液循环活动范围至少距脊峰550公里，最远可至1000公里。