Chert petrology and geochemistry at DSDP Leg 62 Holes

Sixty-five chert, porcellanite, and siliceous-chalk samples from Deep Sea Drilling Project Leg 62 were analyzed by petrography, scanning electron microscopy, analysis by energy-dispersive X-rays, X-ray diffraction, X-ray spectroscopy, and semiquantitative emission spectroscopy. Siliceous rocks occur mainly in chalks, but also in pelagic clay and marlstone at Site 464. Overall, chert probably constitutes less than 5% of the sections and occurs in deposits of Eocene to Barremian ages at sub-bottom depths of 10 to 820 meters. Chert nodules and beds are commonly rimmed by quartz porcellanite; opal-CT-rich rocks are minor in Leg 62 sediments 65 to 108 m.y. old and at sub-bottom depths of 65 to 520 meters. Chert ranges from white to black, shades of gray and brown being most common; yellow-brown and red-brown jaspers occur at Site 464.
Seventy-eight percent of the studied cherts contain easily recognizable burrow structures. The youngest chert at Site 463 is a quartz cast of a burrow. Burrow silica maturation is always one step ahead of host-rock silicification. Burrows are commonly loci for initial silicification of the host carbonate. Silicification takes place by volume-f or-volume replacement of carbonate sediment, and more-clay-rich sediment at Site 464. Nannofossils are commonly pseudomorphically replaced by quartz near the edges of chert beds and nodules. Other microfossils, mostly radiolarians and foraminifers, whether in chalk or chert, can be either filled with or replaced by calcite, opal-CT, and (or) quartz.
Chemical micro-environments ultimately control the removal, transport, and precipitation of calcite and silica. Two cherts from Site 465 contain sulfate minerals replaced by quartz. Site 465 was never subaerially exposed after sedimentation began, and the formation of the sulfate minerals and their subsequent replacement probably occurred in the marine environment. Several other cherts with odd textures are described in this paper, including (1) a chert breccia cemented by colloform opal-CT and chalcedony, (2) a transition zone between white porcellanite containing opal-CT and quartz and a burrowed brown chert, consisting of radial aggregates of opal-CT with hollow centers, and (3) a chert that consists of silica-replaced calcite pseudospherules interspersed with streaks and circular masses of dense quartz.
X-ray-diffraction analyses show that when data from all sites are considered there are poorly defined trends indicating that older cherts have better quartz crystallinity than younger ones, and that opal-CT crystallite size increases and opal-CT cf-spacings decrease with depth of occurrence in the sections. In a general way, depth of burial and the presence of calcite promote the ordering in the opal-CT crystal structure which allows its eventual conversion to quartz. Opal-CT in porcellanites converts to quartz after reaching a minimum d-spacing of 4.07 Å. Quartz/opal-CT ratios and quartz crystallinity vary randomly on a fine scale across four chert beds, but quartz crystallinity increases from the edge to the center of a fifth chert bed; this may indicate maturation of the silica.
Twenty-four rocks were analyzed for their major- and minor-element compositions. Many elements in cherts are closely related to major mineral components. The carbonate component is distinguished by high values of CaO, MgO, Mn, Ba, Sr, and (for unknown reasons) Zr. Tuffaceous cherts have high values of K and Al, and commonly Zn, Mo, and Cr. Pure cherts are characterized by high SiO2 and B. High B may be a good indicator of formation of chert in an open marine environment, isolated from volcanic and terrigenous materials.

本研究对深海钻探计划第62航次（Deep Sea Drilling Project Leg 62）采集的65件燧石、瓷岩及硅质白垩样品开展了岩石学、扫描电子显微镜（scanning electron microscopy）、能量色散X射线分析、X射线衍射（X-ray diffraction）、X射线光谱法及半定量发射光谱测试。硅质岩主要赋存于白垩岩层中，同时也见于464站位的远洋黏土与泥灰岩内。整体而言，燧石占地层剖面的比例不足5%，形成于始新世至巴列姆阶时期，埋藏深度为10~820米。燧石结核与燧石层通常被石英质瓷岩包裹；形成于65~108百万年前的第62航次沉积物中，以及埋藏深度65~520米处的富CT型蛋白石（opal-CT）岩石占比较少。燧石颜色多为白色至黑色，以灰色、褐色调最为常见；464站位可见黄褐色与红棕色碧玉。

本次研究的燧石中，78%可见易识别的潜穴构造。463站位最年轻的燧石为潜穴的石英铸型。潜穴内的硅质成熟度始终超前于围岩的硅化作用。潜穴通常是宿主碳酸盐岩初始硅化的位点。硅化作用以体积守恒或体积置换的方式改造碳酸盐沉积物，以及464站位的富黏土沉积物。在燧石层与结核的边缘附近，纳米化石常被石英以假晶置换的方式取代。其他微化石（以放射虫与有孔虫为主），无论赋存于白垩还是燧石中，均可被方解石、CT型蛋白石（opal-CT）及（或）石英充填或置换。

化学微环境最终控制方解石与二氧化硅的迁移、搬运与沉淀。465站位的两块燧石中可见被石英置换的硫酸盐矿物。465站位自沉积作用开始以来从未暴露于地表，硫酸盐矿物的形成及其后续的石英置换作用大概率发生于海洋环境中。本文还描述了数块具有特殊结构的燧石：（1）由胶状CT型蛋白石与玉髓胶结的燧石角砾岩；（2）含CT型蛋白石与石英的白色瓷岩与具潜穴构造的褐色燧石之间的过渡带，该过渡带由具空心结构的CT型蛋白石放射状集合体组成；（3）由二氧化硅置换的方解石假晶球粒构成，且散布有致密石英的条带与团块的燧石。

X射线衍射分析结果显示，综合所有站位的数据可观察到模糊的变化趋势：较古老的燧石具有更高的石英结晶度；CT型蛋白石的晶粒尺寸随埋藏深度增加而增大，其晶面间距随埋藏深度增加而减小。总体而言，埋藏深度与方解石的存在可促进CT型蛋白石晶体结构的有序化，使其最终可转化为石英。瓷岩中的CT型蛋白石在达到最小晶面间距4.07埃后，将转化为石英。燧石层内石英/CT型蛋白石比值与石英结晶度在四个燧石层中呈随机的细微尺度变化，但第五个燧石层的石英结晶度从层边向层中心逐渐升高，这可能指示硅质作用的成熟过程。

本研究对24块岩石的主量与微量元素组成进行了分析。燧石中的多数元素与主要矿物组分密切相关。碳酸盐组分以高含量的CaO、MgO、Mn、Ba、Sr，以及（成因尚不明确的）Zr为特征。凝灰质燧石具有高含量的K与Al，通常还富含Zn、Mo与Cr。纯燧石以高SiO₂与B为特征。高含量的B可作为燧石形成于开放海洋环境的良好指示标志，该环境与火山及陆源物质来源隔绝。