Diagenesis and Crystal Caskets: Echinoderm Mg Calcite Transformation, Dry Canyon, New Mexico, U.S.A.
Diagenesis and Crystal Caskets: Echinoderm Mg Calcite Transformation, Dry Canyon, New Mexico, U.S.A.
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DOI：
10.1306/2dc40966-0e47-11d7-8643000102c1865d
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发表时间：
2001-09
影响因子：
2
通讯作者：
J. Dickson
中科院分区：
文献类型：
--
作者：
J. Dickson
摘要 活体 Echinus esculentus 骨骼的立体结构由镁方解石组成（间节板含有 6 摩尔 % MgCO3；刺含有 4 摩尔 % MgCO3），并与来自干峡谷 Holder 地层的宾夕法尼亚棘皮动物的立体结构进行了比较。一些干峡谷棘皮动物的孔隙系统（55％体积）在立体体转化（1型）为小骨（经历2型转化的小骨直径为20微米）之前，其孔隙系统（55％体积）被镁铁方解石水泥填充，经历了2型转化的小骨表明离子转移超过了数十微米。粘土基质中的一些（2型）小骨的边缘富含铁并含有重晶石晶体；Fe2+和Ba2+是从周围数百个粘土转移而来的1 型转变是由立体体中的内部水催化的；2 型骨骼镁方解石转变涉及立体体和孔隙填充的镁方解石水泥。可能是由于埋藏引起的温度升高而引发或加速的，经过 1 型转变的立体体成分可以作为棘皮动物骨骼的原始主要元素化学的代表，干卡农（1 型）小骨中存在的少量镁方解石胶结物表明，靠近沉积物-水界面的孔隙水的化学成分以多种方式得以保存，并且表明它们的结构变化可用于解释。绘制成岩反应的分布图。在大多数棘皮动物化石中，立体结构被破坏，即使在那些保留了立体形状的棘皮动物中，其内部结构也不会发生变化，且结构保持完美。
ABSTRACT The stereom of living Echinus esculentus skeletons is composed of Mg calcite (interambulacral plates 6 mole % MgCO3; spines 4 mole % MgCO3) and is compared with the stereom of Pennsylvanian echinoderms from the Holder Formation, Dry Canyon. Some Dry Canyon echinoderms had their pore system (55% volume) filled by magnesian ferroan calcite cement before the stereom transformed (type 1) into small ( 20 µm diameter in ossicles that underwent type 2 transformation indicates ionic transfer over tens of microns. The margins of some (type 2) ossicles set in a clay matrix are Fe-rich and contain barite crystals; the Fe2+ and Ba2+ were transferred from the surrounding clay hundreds of microns into these ossicles. Type 1 transformation was catalyzed by internal water from the stereom; the presence of a ferroan calcite crystal casket confined alteration to the skeletal Mg calcite. Type 2 transformation involved both stereom and pore-filling Mg calcite cement. The unstable dolomite crystals formed by both transformation processes were protected from further change by the crystal caskets in which they are encased. The transformation of Dry Canyon echinoderms was probably triggered, or accelerated, by rising temperature caused by burial. The bulk composition of stereom that has undergone type 1 transformation can be used as a proxy for the original, major-element chemistry of echinoderm skeletons. Minor amounts of Mg calcite cement that occur in Dry Canon (type 1) ossicles indicate the chemistry of porewater close to the sediment-water interface. Fossil echinoderms are preserved in many ways, and it is suggested that their textural variations can be used to map the distribution of diagenetic reactions. The stereom is destroyed in most fossil echinoderms, and even in those where the stereom shape is preserved its internal structure is changed. Echinoderm Mg calcite does not transform with perfect textural preservation.