澳大利亚中部Alice Spring红色表土磁学特征分析

炎热干旱的澳大利亚中部是现代正在形成红色地表的区域。围绕中部Alice Spring地区采集了一系列红色地表样品。这些红色样品按照岩性大致可以分为三类: 砂土、沙丘砂和岩石。通过对这些样品详细的磁学测量, 发现这三类样品存在明显磁学特征差异, 红色砂土和沙丘砂样品磁化率较岩石样品高出许多(砂土和沙丘砂磁化率均值为93.82×10-8m3/kg), 红色岩石样品磁化率值最低(均值为23.2×10-8m3/kg); 三者主要磁性矿物也不尽相同; 磁颗粒均以超顺磁(SP)颗粒为主, 而单畴(SD)颗粒含量少。研究区百分比频率磁化率χfd % 均值为8.28 %, 较黄土高原西部表土(兰州为3.5 %)高出许多。该区现代年降水量约300mm, 比兰州(约330mm)还低, 成土导致百分比频率磁化率增高却比兰州明显高出几倍。这可能反映了黄土高原表土频率磁化率增高与降水量密切相关; 而在澳洲中部可能与持续高温成土条件和作用的时间存在更密切的关系。澳洲中部不论岩石还是表土均以红色为主。磁学实验表明, 砂土和沙丘砂样品均以磁铁矿为主, 三类样品普遍含一定量磁赤铁矿和赤铁矿的贡献, 与地表红色外貌相符。说明澳洲红色地表过程很可能是黑色磁铁矿颗粒表面被长期氧化条件下形成的红色磁赤铁矿/赤铁矿所包裹的现象, 使得澳洲中部广大地区岩石露头和地表沉积物, 普遍形成一层红色染色层。

MAGNETIC ANALYSIS OF RED TOPSOIL IN ALICE SPRING,CENTRAL AUSTRALIA

Central Australia is universally characterized by the red surface. Red samples(n=44)on the surface were collected around Alice Spring(23°42'177"~25°20'534"S, 130°44'411"~133°53'098"E) which is located in the southern hemisphere subtropical regions. These samples can be divided into three groups: sandy soil samples(n=20), dune sand samples(n=15)and bed rock samples(n=9). Magnetic analysis shows that the three type samples are different in magnetic characteristics: the susceptibility (χ) of sandy soil and dune sand samples are higher than that of the bed rock samples, magnetic particle of all samples are dominated by super paramagnetic(SP), and the Single domain(SD)is lower. The mean of frequency-dependent magnetic susceptibility(χfd %)is 8.28 %, and it is higher than that at Lanzhou(3.5 %)which lies in the Chinese loess plateau, but the annual rainfall of Alice Spring are 300mm, lower than that in Lanzhou(330mm). This is very likely that χfd % enhancing on the Loess Plateau is controlled by rainfall, but in central Australia, it is determined by high temperature. Sandy soil and dune sand samples have massive content of magnetite, and maghemite and hematite is widespread in all samples, which is consistent with the red appearance contributed by hematite and maghemite. It suggests that after long time oxidation, the magnetite surface was oxidized to hematite and maghemite.