Paleomagnetism of ash‐flow tuffs: Microanalytical recognition of TRM components
Paleomagnetism of ash‐flow tuffs: Microanalytical recognition of TRM components
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灰流凝灰岩的古地磁：TRM 成分的微分析识别
DOI：
10.1029/jb085ib03p01487
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发表时间：
1980
影响因子：
--
通讯作者：
J. Geissman
中科院分区：
文献类型：
--
作者：
J. Geissman
本文介绍了一种采用低温磁强计和渐进交变磁场退磁的微区分析技术，并用于研究单个组分的自然剩磁（groundmass，crystals，和岩屑和浮石碎片），并确定是否亚阻塞温度流动或冻结相关的运动发生在单位的规模范围内，从体积的个别显微标本（直径1-2 mm）与标准古地磁岩心样品的直径相当。玻璃斑岩显微标本（α95′s = 4.9° ~ 12.4°）的紧密组合方向以及这些方向与标准样品测得的位置平均方向的密切对应表明，这些单元的磁性成分没有受到低温运动的干扰。来自冷却单元失透部分的显微标本的折射率明显低于玻璃斑岩材料，屈服方向要么更分散于其位置平均值（α95大于15°），要么均匀地在一个方向上偏离平均值（高达35°）。反玻璃化作用可能是一个显着的贡献者分散的微观标本的方向是postblocking温度随机化或系统的微构造运动。火山灰流凝灰岩冷却单元的底部和末端附近的玻璃斑岩可能是每个冷却单元中古油气田的最准确的指示物。
A microanalytical technique, employing a cryogenic magnetometer and progressive alternating field demagnetization, is described and used to investigate the natural remanent magnetization of individual components (groundmass, crystals, and lithic and pumice fragments) in seven ash-flow tuff lithologies and to determine if subblocking temperature flow-or compaction-related movements took place in the units on scales ranging from the volume of the individual microspecimen (1–2 mm in diameter) to that of a standard paleomagnetic core specimen. Closely grouped directions from microspecimens of vitrophyre (α95′s = 4.9° - 12.4° ) and close correspondence of these directions with their site mean directions determined from standard samples suggest that the magnetic components of these units have not been disturbed by movements at low temperatures. Microspecimens from devitrified portions of the cooling units are of significantly lower coercivity than vitrophyre material and yield directions either more scattered about their site mean (α95 greater than 15° ) or uniformly displaced in one direction (up to 35°) away from the mean. Devitrification may be as significant a contributor to the scatter of microspecimen directions as are postblocking temperature randomizing or systematic microtectonic movements. Vitrophyres near the base and extremities of ash-flow tuff cooling units are probably the most accurate indicators of the paleofield in each cooling unit.