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本文对发育在澳大利亚悉尼附近的Long Reef Beach中新世古土壤剖面进行了系统的岩石磁学研究,测量了磁化率、饱和磁化强度、饱和等温剩磁、非磁滞剩磁等常温磁学参数和磁滞回线,并对所有样品进行了热磁分析.实验结果表明:全新世软土层主要磁性矿物为MD颗粒磁铁矿,磁性矿物含量与黄土高原黄土层相当.中新世老成土层随地层深度增加主要磁性矿物由磁铁矿转变为磁赤铁矿,随着磁铁矿向磁赤铁矿的转化,开始出现赤铁矿;磁性矿物粒径分布较广,以PSD颗粒为主,其次为SD颗粒,同时含有少量MD颗粒;磁性矿物含量高于黄土高原强发育古土壤层.中新世红土矿层主要磁性矿物为赤铁矿,同时含有少量磁赤铁矿和针铁矿,属于铁的富集层,赤铁矿以SD颗粒为主,含少量PSD和MD颗粒.Long Reef Beach中新世古土壤形成时期,对应着一种全球性高温多雨气候,地表化学风化作用十分强烈.丰富的降水,导致中新世老成土层发生淋溶作用,磁铁矿在向下淋溶迁移过程中逐渐氧化为磁赤铁矿和赤铁矿,铁氧化物最终在红土矿层淀积,磁赤铁矿经高温压实作用再结晶转化为赤铁矿.磁性矿物转化过程可概括为磁铁矿—磁赤铁矿化的磁铁矿—磁赤铁矿—赤铁矿,其中部分磁赤铁矿具有热稳定性,在空气(氩气)环境中加热到700℃未发生转化.
In this paper, we conducted a systematic rock magnetism study of the Miocene palaeogeography in Long Reef Beach, near Sydney, Australia, and measured magnetic susceptibility, saturation magnetization, saturation isothermal remanence and non-hysteresis remanence at room temperature Hysteresis loop and thermal analysis of all the samples.The experimental results show that the main magnetic minerals in the Holocene soft layer are MD magnetite and the content of magnetic minerals is similar to that of loess layer in the Loess Plateau.The Miocene With the increase of formation depth, the main magnetic minerals changed from magnetite to maghemite. With the conversion of magnetite to maghemite, hematite began to appear. The distribution of magnetic minerals was broader, with PSD particles as the main , Followed by SD particles and a small amount of MD particles at the same time.The content of magnetic minerals is higher than that of the strongly developed paleosol layer in the Loess Plateau.The major magnetic minerals of the lateritic Miocene are the hematite and a small amount of maghemite and goethite, Belongs to the enrichment layer of iron, and the hematite is mainly composed of SD grains, which contains a small amount of PSD and MD grains.Long Reef Beach corresponds to a global high temperature and rainy climate with very strong chemical weathering on the surface Rich precipitation led to leaching of the Miocene mature soil. Magnetite gradually oxidized to maghemite and hematite during the downward leaching and migration process, and the iron oxide was eventually deposited on the lateritic ore bed , The maghemite is recrystallized into hematite by high temperature compaction.Magnetic mineral transformation process can be summarized as magnetite-maghemite-maghemite-maghemite-hematite, some of which Maghemite is thermally stable and does not convert to 700 ° C in an air (argon) environment.