(Table 1) U and Th isotope analyses of pyrite and anhydrite of ODP Site 158-957@en

Hydrothermal circulation at oceanic spreading ridges causes sea water to penetrate to depths of 2 to 3 km in the oceanic crust where it is heated to ~400 °C before venting at spectacular 'black smokers'. These hydrothermal systems exert a strong influence on ocean chemistry (Edmond et al., 1979, doi:10.1016/0012-821X(79)90061-X), yet their structure, longevity and magnitude remain largely unresolved (Elderfield and Schultz., 1996, doi:10.1146/annurev.earth.24.1.191). The active Transatlantic Geotraverse (TAG) deposit, at 26° N on the Mid-Atlantic Ridge, is one of the largest, oldest and most intensively studied of the massive sulphide mounds that accumulate beneath black-smoker fields. Here we report ages of sulphides and anhydrites from the recently drilled (Humphris et al., 1995, doi:10.1038/377713a0) TAG substrate structures -determined from 234U-230Th systematics analysed by thermal ionization mass spectrometry. The new precise ages combined with existing data (Lalou et al., 1993, doi:10.1029/92JB01898; 1998, doi:10.2973/odp.proc.sr.158.214.1998) show that the oldest material (11,000 to 37,000 years old) forms a layer across the centre of the deposit with younger material (2,300–7,800 years old) both above and below. This stratigraphy confirms that much of the sulphide and anhydrite are precipitated within the mound by mixing of entrained sea water with hydrothermal fluid (James and Elderfield, 1996, doi:10.1130/0091-7613(1996)024<1147:COOFFA>2.3.CO;2). The age distribution is consistent with episodic activity of the hydrothermal system recurring at intervals of up to 2,000 years.大洋扩张脊处的热液循环会使海水穿透至大洋地壳2~3千米深度，在此处海水被加热至约400℃，随后以壮观的“黑烟囱（black smokers）”形式喷溢而出。此类热液系统对海洋化学具有显著影响（Edmond等，1979，doi:10.1016/0012-821X(79)90061-X），但其构造、存续时长与活动规模仍未得到充分解析（Elderfield与Schultz，1996，doi:10.1146/annurev.earth.24.1.191）。位于大西洋中脊（Mid-Atlantic Ridge）26°N海域的活跃跨大西洋地质巡航（Transatlantic Geotraverse, TAG）热液矿床，是黑烟囱场下堆积的大型硫化物丘体中规模最大、形成年代最久且研究程度最高的矿床之一。本文报道了近期钻探（Humphris等，1995，doi:10.1038/377713a0）获得的TAG基底构造中硫化物与硬石膏（anhydrites）的年龄数据，该年龄通过热电离质谱（thermal ionization mass spectrometry）分析得到的234U-230Th同位素体系测定得到。本次获得的高精度年龄数据结合已有资料（Lalou等，1993，doi:10.1029/92JB01898；1998，doi:10.2973/odp.proc.sr.158.214.1998）显示，最古老的样品（年龄为11000~37000年）在矿床中心形成一层连续分布，而上下两层均为较年轻的样品（年龄为2300~7800年）。该地层学证据证实，丘体内大部分硫化物与硬石膏是由裹挟的海水与热液流体混合沉淀形成的（James与Elderfield，1996，doi:10.1130/0091-7613(1996)024<1147:COOFFA>2.3.CO;2）。该年龄分布与热液系统以最长2000年为间隔的间歇性活动特征相符。