Magnetic Characteristics of Sedimentary Rocks from the Tsengwen-chi and Erhjen-chi Sections in Southwestern Taiwan

臺灣西南部曾文溪及二仁溪剖面沈積岩的磁學特性

洪崇勝(Chorng-Shern Horng) ； Carlo Laj ； 李德貴(Teh-Quei Lee) ； 陳汝勤(Ju-Chin Chen)

《Terrestrial, Atmospheric and Oceanic Sciences》 3卷4期 (1992/12) Pp. 519-532

摘要

本研究是以四種基本磁性參數(κ, SIRM, Hcr, MDF(下標 SIRM))及兩種熱磁方法尋求曾文溪及二仁溪剖面中具有磁鐵礦、磁硫鐵礦及等軸磁硫鐵礦（暫譯）之岩石磁學特性。儘管岩石所含的磁性礦物相異，但等溫殘磁磁化的獲取行為卻非常相似。由於其磁性礦物之含量與粒徑不同，因而使帶有等軸磁硫鐵礦、磁硫鐵礦及磁鐵礦之岩石各具有高、中、低的相對磁性參數值(κ, SIRM, Hcr, MDF(下標 SIRM))。在氮氣中的熱磁分析進一步顯示每種磁性礦物有獨特的熱磁曲線，並且在加溫時等軸磁硫鐵礦可轉變成磁硫鐵礦及磁鐵礦；磁硫鐵礦變成磁鐵礦；而磁鐵礦則變成赤鐵礦。此外，具有等軸磁硫鐵礦之岩石在加熱至320°之前，其磁感率呈明顯的下降；而具有磁硫鐵礦及磁鐵礦之岩石在加熱至400℃之前，其磁感率各自有“輕微”和“幾乎不受影響”之變化效應。

並列摘要

Four basic magnetic parameters (κ, SIRM, Hcr, MDF(subscript SIRM))and two thermal magnetic analyses were investigated in this study to characterize the magnetic properties of magnetite-, pyrrhotite- and greigite-bearing rocks of the Tsengwen-chi and Erhjen-chi sections. These rocks have very similar behaviors in the acquisition of isothermal remanent magnetization (IRM) in spite of bearing with different magnetic minerals. Because of the difference in magnetic mineral concentration and grain size, greigite-, pyrrhotite-, and magnetite-bearing rocks have relatively high, intermediate and low parameter values (κ, SIRM, Hcr, MDF(subscript SIRM)), respectively. Thermomagnetic analysis in a nitrogen atmosphere has further revealed that each magnetic mineral has its own diagnostic magnetization curves and that greigite could have transformed to pyrrhotite and magnetite, pyrrhotite to magnetite, and magnetite to hematite. Bulk magnetic susceptibility has also shown remarkable drop in greigite-bearing rocks below 320℃, while there are minor and almost noneffective susceptibility changes in pyrrhotite- and magnetite-bearing rocks below 400℃, respectively.