Microtextures of authigenic Or‐rich feldspar in the Upper Jurassic Humber Group, UK North Sea
Microtextures of authigenic Or‐rich feldspar in the Upper Jurassic Humber Group, UK North Sea
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英国北海上侏罗统亨伯群自生富奥长石的微观结构
DOI：
10.1046/j.1365-3091.2003.00567.x
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发表时间：
2003
期刊：
影响因子：
3.5
通讯作者：
I. Parsons
中科院分区：
文献类型：
--
作者：
Martin R. Lee;I. Parsons
摘要英国北海Fulmar油田上侏罗统亨伯群中埋藏深度为3.2 - 3.5 km的碎屑碱性长石长满，部分被自生富长石取代。晶内显微结构表明碎屑颗粒有几种不同的来源。过度生长是均匀的非阴极射线发光，偶尔有钡长石丰富的区域。透射电子显微镜显示，它们由亚微米至微米尺寸的亚颗粒的微孔镶嵌物组成，与重晶石、伊利石和黄铁矿相关。亚颗粒有些圆形，但具有近似{110} Adularia习性，并在纳米尺度上显示出微弱的调制显微织构。它们具有三斜对称性，但晶格角仅与单斜对称性略有偏离。这些特征是钾长石在低温下相对快速沉淀的特征。自生富钾长石也部分取代了碎屑颗粒中的微斜长石和条纹钠长石，通常在亚光学尺度上。虽然像成岩钠长石化一样，钾长石置换可能是一种非常常见的成岩反应，但由于大多数工作者使用扫描电子显微镜技术难以成像诊断结构，因此很少报道。亚颗粒显微结构的渗透性可能会显著阻碍长石过度生长用于成岩作用的K/Ar和40 Ar/39 Ar定年，并且碎屑颗粒中亚光学、非晶自生钾长石的存在可能会显著修改碎屑颗粒的表观Ar年龄。在光学无特征的石英过度生长类似的亚晶粒显微结构也说明了。
ABSTRACT Detrital alkali feldspars currently at burial depths of 3·2–3·5 km in the Upper Jurassic Humber Group of the Fulmar oilfield, UK North Sea, are overgrown and have been partially replaced by authigenic Or‐rich feldspar. Intracrystal microtextures suggest several different provenances for the detrital grains. The overgrowths are uniformly non‐cathodoluminescent and have occasional celsian‐rich zones. Transmission electron microscopy shows that they are composed of a microporous mosaic of subµm‐ to µm‐sized sub‐grains associated with barite, illite and pyrite. The subgrains are somewhat rounded but have an approximate {110} Adularia habit and display a faint modulated microtexture on the nanometre scale. They have triclinic symmetry, but the lattice angles depart only slightly from monoclinic symmetry. These features are characteristic of K‐feldspar precipitated relatively rapidly and at low temperature. Authigenic Or‐rich feldspar has also partially replaced microcline and perthitic albite within the detrital grains, often at a suboptical scale. Although, like diagenetic albitization, replacement by K‐feldspar is probably a very common diagenetic reaction, it has rarely been reported owing to difficulties in imaging the diagnostic textures with the scanning electron microscopy techniques used by most workers. The permeability of the subgrain microtexture may significantly hinder the use of feldspar overgrowths for K/Ar and 40Ar/39Ar dating of diagenesis, and the existence of suboptical, replacive authigenic K‐feldspar within detrital grains may significantly modify the apparent Ar ages of detrital grains. Similar subgrain microtextures in optically featureless quartz overgrowths are also illustrated.