Mineralogy, lithology and stable isotope composition of diagenetic siliceous rocks from ODP Leg 129 sites

Ocean Drilling Program Leg 129 recovered chert, porcellanite, and radiolarite from Middle Jurassic to lower Miocene strata from the western Pacific that formed by different processes and within distinct host rocks. These cherts and porcellanites formed by (1) replacement of chalk or limestone, (2) silicification and in-situ silica phase-transformation of bedded clay-bearing biosiliceous deposits, (3) high-temperature silicification adjacent to volcanic flows or sills, and (4) silica phase-transformation of mixed biosiliceous-volcaniclastic sediments.
Petrologic and O-isotopic studies highlight the key importance of permeability and time in controlling the formation of dense cherts and porcellanites. The formation of dense, vitreous cherts apparently requires the local addition and concentration of silica. The influence of permeability is shown by two examples, in which: (1) fragments of originally identical radiolarite that were differentially isolated from pore-water circulation by cement-filled fractures were silicified to different degrees, and (2) by the development of secondary porosity during the opal-CT to quartz inversion within conditions of negligible permeability. The importance of time is shown by the presence of quartz chert below, but not above, a Paleogene hiatus at Site 802, indicating that between 30 and 52 m.y. was required for the formation of quartz chert within calcareous-siliceous sediments.
The oxygen-isotopic composition for all Leg 129 carbonate- and Fe/Mn-oxide-free whole-rock samples of chert and porcellanite range widely from d18O = 27.8 per mil to 39.8 per mil vs. V-SMOW. Opal-CT samples are consistently richer in 18O (34.1 per mil to 39.3 per mil) than quartz subsamples (27.8 per mil to 35.7 per mil). Using the O-isotopic fractionation expression for quartz-water of Knauth and Epstein (1976) and assuming d18Opore water = -1.0 per mil, model temperatures of formation are 7°-26°C for carbonate-replacement quartz cherts, 22°-25°C for bedded quartz cherts, and 32°-34°C for thermal quartz cherts. Large variations in O-isotopic composition exist at the same burial depth between co-existing silica phases in the same sample and within the same phase in adjacent lithologies. For example, quartz has a wide range of isotopic compositions within a single breccia sample; d18O = 33.4 per mil and 28.0 per mil for early and late stages of fracture-filling cementation, and 31.6 per mil and 30.2 per mil for microcrystalline quartz precipitation within enclosed chert and radiolarite fragments. Similarly, opal-CT d101 spacing varies across lithologic or diagenetic boundaries within single samples.
Co-occurring opal-CT and chalcedonic quartz in shallowly buried chert and porcellanite from Sites 800 and 801 have an 8.7 per mil difference in d18O, suggesting that pore waters in the Pigafetta Basin underwent a Tertiary shift to strongly 18O-depleted values due to alteration of underlying Aptian to Albian-Cenomanian volcaniclastic deposits after opal-CT precipitation, but prior to precipitation of microfossil-filling chalcedony.

大洋钻探计划（Ocean Drilling Program）第129航次从西太平洋中侏罗世至早中新世地层中获取了燧石（chert）、瓷岩（porcellanite）与放射虫岩（radiolarite），这些岩石由不同地质作用形成于不同围岩中。这些燧石与瓷岩的形成途径包括：（1）白垩或石灰岩的交代作用；（2）层状含黏土生物硅质沉积的硅化作用与原位硅质相变；（3）火山岩流或岩床附近的高温硅化作用；（4）混合生物硅质-火山碎屑沉积的硅质相变。

岩石学与氧同位素研究凸显了渗透率与时间在控制致密燧石和瓷岩形成过程中的关键作用。致密玻璃质燧石的形成显然需要硅质的局部补给与富集。渗透率的影响可通过两个实例体现：其一，原本均质的放射虫岩岩块因被充填胶结物的裂缝与孔隙水循环的隔绝程度不同，硅化程度存在差异；其二，在渗透率可忽略的条件下，蛋白石-CT向石英转化过程中发育次生孔隙。时间的重要性则通过802站位古近纪沉积间断下方存在石英燧石、上方却无此现象得以证明，这表明在钙硅质沉积物中形成石英燧石需要3000万至5200万年的时间。

本次129航次采集的所有不含碳酸盐与铁/锰氧化物的燧石和瓷岩全岩样品，其相对于维也纳标准平均海洋水（Vienna Standard Mean Ocean Water, V-SMOW）的δ¹⁸O值分布范围为27.8‰至39.8‰。蛋白石-CT样品的δ¹⁸O值（34.1‰至39.3‰）始终高于石英亚样品（27.8‰至35.7‰）。利用克诺特与爱泼斯坦（Knauth and Epstein, 1976）提出的石英-水氧同位素分馏公式，并假设孔隙水δ¹⁸O值为-1.0‰，可计算得到不同成因燧石的形成模拟温度：交代白垩形成的石英燧石为7℃至26℃，层状石英燧石为22℃至25℃，热成因石英燧石为32℃至34℃。同一埋藏深度下，同一样品内共存的不同硅质相之间，以及相邻岩性中同一硅质相内部，氧同位素组成均存在较大差异。例如，单块角砾岩样品中的石英同位素组成跨度较大：裂缝充填胶结作用早、晚阶段的石英δ¹⁸O值分别为33.4‰和28.0‰，包裹于燧石和放射虫岩岩块中的微晶石英沉淀的δ¹⁸O值则为31.6‰和30.2‰。类似地，单一样品内不同岩性或成岩边界处的蛋白石-CT的d101晶面间距也存在变化。

800和801站位浅埋藏燧石和瓷岩中伴生的蛋白石-CT与玉髓石英，其δ¹⁸O值相差8.7‰，这表明皮加费塔海盆的孔隙水经历了一次第三纪偏移：在蛋白石-CT沉淀之后、充填微化石的玉髓沉淀之前，下伏阿普第期至阿尔布-森诺曼期火山碎屑沉积发生蚀变，导致孔隙水的δ¹⁸O值显著降低。