Thallium isotope composition in pyrites@en

This paper presents the first study of Tl isotopes in early diagenetic pyrite. Measurements from two sections deposited during the Toarcian Ocean Anoxic Event (T-OAE, ~183 Ma) are compared with data from Late Neogene (<10 Ma) pyrite samples from ODP legs 165 and 167 that were deposited in relatively oxic marine environments. The Tl isotope compositions of Late Neogene pyrites are all significantly heavier than seawater, which most likely indicates that Tl in diagenetic pyrite is partially sourced from ferromanganese oxy-hydroxides that are known to display relatively heavy Tl isotope signatures. One of the T-OAE sections from Peniche in Portugal displays pyrite thallium isotope compositions indistinguishable from Late Neogene samples, whereas samples from Yorkshire in the UK are depleted in the heavy isotope of Tl. These lighter compositions are best explained by the lack of ferromanganese precipitation at the sediment-water interface due to the sulfidic (euxinic) conditions thought to be prevalent in the Cleveland Basin where the Yorkshire section was deposited. The heavier signatures in the Peniche samples appear to result from an oxic water column that enabled precipitation of ferromanganese oxy-hydroxides at the sediment-water interface. […]本文首次针对早成岩期黄铁矿中的铊（Tl）同位素展开研究。我们对托阿尔阶海洋缺氧事件（Toarcian Ocean Anoxic Event, T-OAE，约1.83 Ma）时期沉积的两处剖面的测试数据，与取自大洋钻探计划（Ocean Drilling Program, ODP）165、167航次的晚新近纪（<10 Ma）黄铁矿样品数据进行了对比——后者沉积于相对氧化的海洋环境中。晚新近纪黄铁矿的铊同位素组成均显著重于海水，这一结果大概率表明，成岩黄铁矿中的铊部分来源于铁锰氢氧化物（ferromanganese oxy-hydroxides）；现有研究表明这类物质本身具有相对偏重的铊同位素特征。葡萄牙佩尼切地区的一处T-OAE剖面，其黄铁矿铊同位素组成与晚新近纪样品几乎无差别；而英国约克郡的样品则存在铊重同位素亏损的现象。上述偏轻的同位素组成可通过沉积物-水界面（sediment-water interface）缺乏铁锰沉淀得到合理解释：约克郡剖面所在的克利夫兰盆地当时普遍被认为处于硫化（euxinic）水体环境中，该环境会抑制铁锰氢氧化物的沉淀。佩尼切样品的偏重同位素信号，则源于当时的氧化水柱环境——该环境使得铁锰氢氧化物能够在沉积物-水界面形成沉淀。……