Petrological and geochemical evidence for diagenesis of inoceramid bivalve shells in the Plentzia Formation (Upper Cretaceous, Basque–Cantabrian Region, northern Spain)
Petrological and geochemical evidence for diagenesis of inoceramid bivalve shells in the Plentzia Formation (Upper Cretaceous, Basque–Cantabrian Region, northern Spain)
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Plentzia 组（西班牙北部巴斯克-坎塔布连地区上白垩统）中 inoceramid 双壳类贝壳成岩作用的岩石学和地球化学证据
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发表时间：
1996
期刊：
影响因子：
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通讯作者：
F. Garcı́a
中科院分区：
文献类型：
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作者：
J. Elorza;F. Garcı́a
摘要对巴斯克-坎塔布里亚地区普伦齐亚组上部(塞诺曼期中期至桑托期下部)的Inocerid双壳类贝壳进行了后沉积、成岩蚀变试验。从野外关系、岩相学和阴极发光(CL)观察来看，壳体的粗棱柱状钙质介质体似乎受化学成岩作用的影响最大，但柱状微结构没有结构变化，除了壳体边缘的石英辉长岩边缘的一些选择性硅化作用和充填可能的有机细丝通道的小的正面体方解石晶体(直径1-2μm)。由于整个岩石中的MnO含量相对较高，CL为鲜艳的红黄色，分别为0.28%和0.03%。贝壳棱镜测得的δ18O同位素范围为−1.76‰~−4.70‰[Pee Dee Belemnite]，平均值为−3.03±0.69‰(PDB)。成岩蚀变最明显的证据来自于全壳地球化学和微探针分析。事实上，成岩示踪剂的分布从内到外表面可以检测到互补的分带性，同时稀土元素在δ1 8O值较低的壳体中也得到了富集。泥灰岩和贝壳的REE均值对比，进一步证实了贝壳碳酸盐的化学成岩蚀变作用。
Abstract Inoceramid bivalve shells from the upper part of the Plentzia Formation (middle Cenomanian to lower Santonian) of the Basque-Cantabrian Region were tested for post-depositional, diagenetic alteration. From field relations, petrography and cathodoluminescence (CL) observations, the coarsely prismatic calcitic ostracum of the shells seem to be most affected by chemical diagenetic processes, but without textural changes in the prismatic microstructure, with the exception of some selective silicification by quartzine-lutecite rims at the margin of the shells and small euhedral calcite crystals (1-2 μ m across) infilling possible organic filament ducts. CL is a bright red-yellowish colour owing to a relatively high MnO content, 0.28% vs. 0.03% (wt) in the whole rock. The δ 1 8 O isotopic values obtained from the shell prisms range from −1.76‰ to −4.70‰ [Pee Dee Belemnite (PDB)], with a mean value of −3.03±0.69‰ (PDB). The clearest evidence of diagenetic alteration is inferred from both whole-shell geochemistry and microprobe analysis. In fact, a complementary zonation in the distribution of ‘diagenetic tracers’ can be detected from internal to external surfaces of inoceramid shells, together with a rare earth element (REE) enrichment in the shells which have lighter δ 1 8 O values. Comparison between REE mean values of the marly host rock and those of inoceramid shells reinforces the idea of chemical diagenetic alteration of shell carbonate.