关键词: 裂变径迹退火, 地质热历史, 磷灰石裂变径迹分析, 自发裂变径迹, 地质时间, 临清坳陷, 古地温, 投影长度, 退火温度, 表观年龄

Abstract: In this paper an experimental technique for investigating geothermal histories of sedimentary basins by analysis of uranium fission tracks in detrital apatites is presented.This method is based on the fact that ²³⁸U fission tracks in minerals can be annealed by heating over a geological period.In the case of apatite the annealing zone occurs between about 70° and 125℃ for a time span in the order of 10 million years(Myr) which is remarkably conforms to the condition favorable for generation of petroleum.The twenty samples analysed were collected from the cores at various depths of exploratory boreholes in the Linqing sedimentary basin.The apatite fission track age T of these samples have been determined by the external detector method,and the distributions of the projected lengths of fossil tracks in these samples have been measured as well.The fission tracks were observed with an optical microscope at a magnification of 1500X.The precision of this method depends mainly on the counts of the fossil fission tracks and the error of the thermal neutron fluence.The obtained relations between the age T and(mean projected length Pof the fossil track relative to the depth of burial) show that T and P are de-creased with increase of depth of burial in a range of 2100-4000 meter,and thus this zone is the apatite fossil fission track annealing zone.The fact that T and Premain fairly constant in 1600-2100 meter range shows that the primary age of the formations penetrated by the borehole is about 85 Myr.The fact fhat all fossil track projected length distributions have a similar form with a peak shows that no strong palaeotemperature fluctuation occured in the region in which the borehole is located,thus this region was in a fairly stable geological state.For large part of the samples r_si>0.8(r_si is a correlation coefficient between spontaneous and induced fission track densities),which also shows that the region was in a stable geological state.The experimental results show that measuring fossil track projected lengths is more powerful to reveal palaeotemperature and is very simple.