Multicomponent magnetization in paleomagnetic records of reversals from continental sediments in Bolivia
Multicomponent magnetization in paleomagnetic records of reversals from continental sediments in Bolivia
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DOI：
10.1016/0012-821x(92)90166-s
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发表时间：
1992-06
影响因子：
5.3
通讯作者：
X. Quidelleur;J. Valet;N. Thouveny
中科院分区：
文献类型：
--
作者：
X. Quidelleur;J. Valet;N. Thouveny
在玻利维亚高原的三个大陆段进行了一项关于地磁逆转的古地磁研究。第一部分(Viscachani)显示了一个独特的极性区间，其磁化强度由磁铁矿携带。在另外两个剖面(阿斯特卡拉和伊尔帕维)，磁化作用的特征是以磁性硫化物(主要是磁黄铁矿)和磁铁矿为主的复杂磁性矿物学。在真空下进行的热退磁比在空气中更有效地隔离了硫化物的低温组分和磁铁矿的高温组分携带的剩磁方向。对比了两种矿物的磁化方向，特别是地磁反转的记录。在阿斯特卡拉，两个组件都显示出相同的极性模式。然而，LT分量的过渡方向位于磁铁矿记录的极性突变下方约50 cm处。这样的观察结果与最近对西西里上新世海洋泥炭的研究非常相似[1]。这些特征与模拟的结果是一致的，该模拟假设剩磁的非常早期的获取是由LT分量携带的，并且磁铁矿发生了沉积后的重新定向。在最后一节(伊尔巴维)中，观测到的LT和HT分量具有相反的极性。虽然磁性参数表明硫化物的存在，但我们不能排除LT组分由于粗大的磁铁矿颗粒而具有二次成因的假设。如果我们假设硫化物的假说是正确的，这一观察结果强调了确定硫化物矿物来源的困难。
A paleomagnetic study of geomagnetic reversals has been attempted from three continental sections of the altiplano in Bolivia. The first section (Viscachani) shows a unique polarity interval with a magnetization carried by magnetite. In the two other sections (Achocalla and Irpavi) the magnetization is characterized by a complex magnetic mineralogy dominated by magnetic sulphides (mostly pyrrhotite) and magnetite. Thermal demagnetization performed under vacuum was found to be much more efficient than in air in isolating the directions of remanence carried by a low temperature (LT) component due to sulphides and a high temperature (HT) component due to magnetite. The directions of magnetization carried by the two minerals have been compared, with particular attention being paid to records of geomagnetic reversals. At Achocalla, the same polarity pattern is revealed by both components. However, the transitional directions of the LT component are located about 50 cm below the abrupt polarity change recorded by magnetite. Such observations are very similar to those from a recent study of Pliocene marine marls in Sicily [1]. These characteristics are compatible with the results of a simulation which assumed that a very early acquisition of the remanence was carried by the LT component and that a post-depositional reorientation of the magnetite occurred. In the last section (Irpavi), the observed LT and HT components have opposite polarities. Although the magnetic parameters indicate the presence of sulphides, we cannot rule out the hypothesis that the LT component has a secondary origin due to coarse magnetite grains. If we assume that the hypothesis of sulphides is correct, this observation emphasizes the difficulties in establishing the origin of sulphide minerals.