Diagenesis of siliceous particles in ODP Hole 114-699A

A number of neogenic opaline structures, not previously reported in the literature, as well as other neogenic phases are described from four Oligocene to Pliocene biosiliceous sediment samples from Hole 699A. The possible influence of microbes on the formation or the morphology of some of them is discussed. The samples, which are early Pliocene, early to middle Miocene, and late Oligocene (two) in age, were histologically fixed aboard ship upon retrieval.
Investigations of the samples used SEM (with Edax/Tracor) and XRD methods. Diagenesis has affected all four samples, but the most extensive development of neoformed structures occurs in the Miocene and uppermost Oligocene samples, where microbial filaments (0.05 to 10 ?m long), microbial colonies, and siliceous microhemispheroids (0.2 to 0.7 µm diameter) were observed. The latter encrust filaments, diatoms, and detrital grains to varying degrees. Other neoformed structures include (1) flakes formed by coalesced microhemispheroids, some of which are guided by short, stubby filaments, which occur only in the Miocene and uppermost Oligocene samples, and (2) flakes characterized by smooth or microfissured surfaces, which grow on diatom frustules and in pore spaces and have a more widespread distribution. The XRD data indicate possible cristobalite formation in the Miocene and uppermost Oligocene samples; we believe that the neoformed opaline structures (encrusted filaments and microhemispheroids) may represent an early phase of opal-CT. The timing of neoformation of most of these features appears to have been fairly recent, continuing even at the time of sampling.
There appears to be no direct correlation of this incipient, lower Miocene-uppermost Oligocene diagenetic layer and the pore-water chemistry profiles; a massive increase in shear strength in these sediments, however, may indicate some cementation.
Smectite was identified by XRD as the most prominent clay mineral in these generally clay-poor sediments. Honeycombed minerals with filamentous edges, which could correspond to smectite, were observed with SEM in the pore spaces.

本次研究对699A钻孔的4件渐新世（Oligocene）至上新世（Pliocene）生物硅质沉积物样品展开分析，报道了此前文献未记载的多种新生蛋白石结构（neogenic opaline structures）及其他新生矿物相（neogenic phases）。文中探讨了微生物对部分结构的形成过程或形貌特征可能产生的影响。这批样品的时代分别为早上新世、早-中中新世以及晚渐新世（共2件），均在回收后于船上完成了组织学固定。

本次研究采用扫描电子显微镜（Scanning Electron Microscopy, SEM）附带Edax/Tracor能谱仪与X射线衍射（X-ray Diffraction, XRD）技术对样品进行检测。成岩作用（diagenesis）对全部4件样品均造成了影响，但新生结构的发育程度以中新世（Miocene）和最顶部渐新世（uppermost Oligocene）样品最为显著：其中可见微生物丝状体（microbial filaments，长0.05~10 μm）、微生物菌落（microbial colonies）以及硅质微球状体（siliceous microhemispheroids，直径0.2~0.7 μm）。后者以不同程度包覆丝状体、硅藻及碎屑颗粒（detrital grains）。其余新生结构包括两类：①由聚合微球状体（coalesced microhemispheroids）形成的薄片，部分受仅见于中新世和最顶部渐新世样品的短粗丝状体（short, stubby filaments）引导；②表面光滑或存在微裂隙的薄片，生长于硅藻壳（diatom frustules）与孔隙空间（pore spaces）中，分布更为广泛。X射线衍射数据显示，中新世和最顶部渐新世样品中可能存在方英石（cristobalite）；我们认为，此次发现的新生蛋白石结构（包覆丝状体与微球状体）可能代表CT型蛋白石（opal-CT）的早期形成阶段。多数这类新生结构的形成时间相当晚，甚至在采样时仍在持续进行。

这套形成于早中新世-最顶部渐新世的初期成岩地层，似乎与孔隙水化学剖面（pore-water chemistry profiles）无直接关联；但该沉积物的抗剪强度（shear strength）大幅提升，或暗示存在一定程度的胶结作用（cementation）。

X射线衍射分析显示，在这批整体黏土含量较低的沉积物中，蒙脱石（smectite）为优势黏土矿物。扫描电子显微镜下，孔隙空间中可见具有丝状边缘的蜂窝状矿物（honeycombed minerals），其特征与蒙脱石相符。