Morphologic and Mineralogic Transitions From Opal-A to Opal-CT in Low-Temperature Siliceous Sinter Diagenesis, Taupo Volcanic Zone, New Zealand
Morphologic and Mineralogic Transitions From Opal-A to Opal-CT in Low-Temperature Siliceous Sinter Diagenesis, Taupo Volcanic Zone, New Zealand
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新西兰陶波火山区低温硅质烧结成岩作用中从 Opal-A 到 Opal-CT 的形态和矿物学转变
DOI：
10.1306/011704740561
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发表时间：
2004
影响因子：
2
通讯作者：
K. Campbell
中科院分区：
文献类型：
--
作者：
B. Lynne;K. Campbell
摘要新西兰硅质温泉沉积物（烧结物）中蛋白石-A到蛋白石-CT的二氧化硅相变伴随着微米尺度上反复出现的结构变化。认识到这些变化是必不可少的提取古水文，古环境，古生物的签名，从古代热液系统，并认识到成岩叠印的主要沉积信号。我们研究了新西兰陶波火山区Orakei Korako和Te Kopia地热田低温（< 35° C）栅栏微相的39个硅质烧结物样品，这些样品处于三种不同的沉积后环境（扰动最小、热影响、风化）。易碎到硬化的样品是从现代的、目前正在硅化的微生物垫中收集的，这些微生物垫位于近中性的碱氯化物泉的流出物中，以烧结3,500年的历史。通过X射线粉末衍射和扫描电子显微镜跟踪了从非晶蛋白石-A到次晶蛋白石-CT的渐进矿物学变化。一个成岩序列与两种形态的途径，球形和光滑（聚合物和单体沉积，分别），被确定在微米尺度。这两种途径最终导致形成典型的，蛋白石-CT叶片lepispheres。沿着球形路径，初始形态重构包括在蛋白石-A球体（直径< 3.0 μ m）中形成圆孔（直径< 0.1至1.0 μ m）。这些变化与蛋白石-A的X射线散射宽带的最大强度从4.0 A到4.09 A（蛋白石-CT的特征位置）的偏移相关。在球形和光滑二氧化硅通道中，六方晶片晶和复合蛋白石-A/-CT散射宽带的发展表明了早期过渡成岩阶段，该散射宽带的峰值比较尖锐，中心位于4.09 A。一个仍然尖锐的峰值，复合蛋白石-CT/-A的X射线衍射带如下，它已经开发了一个初期鳞石英肩;它被发现在样品中，显示六角血小板和初期的“模糊”蛋白石-CT lepispheres。在这个后期的过渡阶段，斑片状的替换结构在宏观尺度上是可见的。只有出现大量发育良好的乳白色-CT叶片状鳞球，才是典型的尖峰（4.09 A），乳白色-CT X射线迹线明显，具有界限清楚的鳞石英肩。这种成岩过程可以产生多孔或玻璃质织物，这取决于二氧化硅填充物降低孔隙度和增加存款密度的程度。这项研究提供了一个纹理矿物学的背景下，了解在地热环境中的微生物群落的硅化及其随后的成岩修改。这些变化在地热活动停止后很长一段时间仍会持续，几乎没有或根本没有埋藏，并且发生在给定存款的层位或局部斑块中。此外，沉积后条件，如热叠加或风化，影响硅质烧结岩的成岩作用，提高其成熟率。
ABSTRACT The opal-A to opal-CT silica phase transformation in New Zealand's siliceous hot-spring deposits (sinter) is accompanied by recurring textural changes at the micron scale. Recognition of these changes is essential to extract paleohydrological, paleoenvironmental, and paleobiological signatures from ancient hydrothermal systems, and to recognize diagenetic overprints upon primary depositional signals. We examined 39 samples of siliceous sinter from the low-temperature (< 35° C) palisade microfacies at Orakei Korako and Te Kopia geothermal fields, Taupo Volcanic Zone, New Zealand, in three different postdepositional environments (least-disturbed, heat-affected, weathered). Friable to indurated samples were collected from modern, presently silicifying microbial mats in the outflow of nearly neutral alkali chloride springs, to sinter 3,500 years old. The progressive mineralogical change from noncrystalline opal-A to paracrystalline opal-CT was traced by X-ray powder diffraction and scanning electron microscopy. One diagenetic sequence with two morphological pathways, spherical and smooth (polymeric and monomeric deposition, respectively), was identified at the micron scale. Both pathways ultimately lead to formation of typical, opal-CT bladed lepispheres. Along the spherical pathway, initial morphological restructuring includes formation of circular holes (< 0.1 to 1.0 µm in diameter) in opal-A spheres (< 3.0 µm in diameter). These changes correlate with a shift in the maximum intensity of the opal-A X-ray scattering broadband, from 4.0 A to 4.09 A, the characteristic position for opal-CT. An early transitional diagenetic stage is indicated in both sphere and smooth silica pathways by development of hexagonal platelets and a composite opal-A/-CT scattering broadband that is sharp-peaked in comparison, and centered at 4.09 A. A still sharper-peaked, composite opal-CT/-A X-ray diffraction band follows, which has developed an incipient tridymite shoulder; it is found in samples that display both hexagonal platelets and incipient "fuzzy" opal-CT lepispheres. At this late transitional stage, patchy replacement textures are visible at the macroscale. Only with the appearance of numerous, well-developed, opal-CT bladed lepispheres is a typical, sharp-peaked (4.09 A), opal-CT X-ray trace evident, with a well-defined tridymite shoulder. This diagenetic process can produce either porous or vitreous fabrics, depending on the degree to which infill of silica has reduced porosity and increased density of the deposit. This study provides a textural-mineralogical context for understanding silicification of microbial communities in geothermal settings and their subsequent diagenetic modifications. These modifications continue long after geothermal activity has ceased, with little or no burial, and occur in horizons or localized patches of a given deposit. Moreover, postdepositional conditions, such as heat overprinting or weathering, influence the diagenesis of siliceous sinter by increasing its maturation rate.