Low Temperature Alteration of the Magnetic Minerals in Ocean Floor Basalts
Low Temperature Alteration of the Magnetic Minerals in Ocean Floor Basalts
复制标题
洋底玄武岩中磁性矿物的低温蚀变
DOI：
10.1029/me002p0169
复制
发表时间：
2013
影响因子：
2.8
通讯作者：
U. Bleil
中科院分区：
文献类型：
--
作者：
N. Petersen;P. Eisenach;U. Bleil
对大西洋、太平洋和地中海等12个不同洋区的玄武岩样品进行了海底风化作用与磁性的相关性研究。与前人的研究结果一致，海底玄武岩的自然磁化作用是由平均体积分数为1%的小颗粒钛磁铁矿携带的，这些钛磁铁矿在海底条件下被大量氧化，从而形成缺乏阳离子的尖晶石，即所谓的钛磁铁矿。这种氧化在较低的温度下进行，很可能是在底层水温。大洋钛磁铁矿的主要成分只在很小的范围内变化。从本文研究的样品中推断出其主要成分为1.50Fe3+0.77Ti0.58Al0.07Mg0.06Mn0.02O4。钛磁铁矿的显微探针分析结合居里温度的测量表明，大洋条件下钛磁铁矿氧化的主要机制是铁的迁移：铁从尖晶石晶格中迁移出来，或被结合到周围的粘土矿物中，或被海水运往其他地方。Fe-的迁移过程受到尖晶石晶格中剩余Fe2+的数量的限制；对于每个离开晶体的Fe2+，另一个Fe2+被转化为Fe3+。铁的迁移导致钛磁铁矿的Fe/Ti比值在氧化过程中发生变化。通过对分离出的钛磁铁矿的Fe_2O_3/Fe_2O_3+FeO的分析，确定了居里温度T_c与氧化参数z之间的关系。这一关联式与Readman和O‘Reilly(1972)的相应关系式不一致。洋底玄武岩的居里温度随洋壳年龄的增加而升高。虽然数据中有相当大的离散性，但Tc似乎随着年龄的对数而成比例地增加。大西洋海底以枕状玄武岩为主的上部600米相对于磁性矿物或多或少地发生了均匀的变化。但在单一岩石单元内，氧化速率随深度略有下降，大洋玄武岩的饱和磁化强度首先随低温氧化程度的增加而降低，直至氧化参数z=0.60。超过这个z值，为了进一步增加氧化，它又增加了。这种磁化强度的变化可以用一个简单的模型定性地解释，该模型假设Fe首先从八面体尖晶石晶格位置迁移，然后从四面体晶格位置迁移。中生代海洋磁异常的高幅度可能是这一过程造成的。侏罗纪磁性静止带可以用完全氧化的钛磁铁矿转变为非磁性矿物相来解释，海底玄武岩的自然剩余磁化方向不受海底蚀变的影响。
The correlation between submarine weathering and magnetic properties of ocean floor basalt has been studied on samples from 12 different oceanic sites: 10 sites in the Atlantic, 1 in the Pacific and 1 in the Mediterranean sea. In agreement with earlier studies it has been found that the natural magnetization of ocean floor basalts is carried by small grains of titanomagnetites with average volume content of 1%.These titanomagnetites are oxidized grandually under suboceanic conditions and form thereby cation deficient spinels, the so‐called titanomaghemites. This oxidation proceeds at low temperatures, most likely at bottom water temperature.The primary composition of oceanic titanomagnetites varies only within narrow limits. From the samples studied in this paper a primary composition of1.50Fe3+0.77Ti0.58Al0.07Mg0.06Mn0.02O4is deduced. Microprobe analyses of titanomagnetites in combination with Curie temperature measurements indicate that the dominating mechanism of titanomagnetite oxidation under oceanic conditions is iron‐migration: Fe migrates out of the spinel lattice and is either incorporated in the surrounding clay minerals, or transported elsewhere by sea water. The process of Fe‐migration is limited by the amount of Fe2+left in the spinel lattice; for each Fe‐ion leaving the crystal another Fe2+is converted to Fe3+. Fe‐migration causes the Fe/Ti ratio of titanomagnetite to change during oxidation.A correlation between the Curie temperature Tcand the oxidation parameter z has been determined from the analysis of Fe2O3/Fe2O3+ FeO of separated titanomagnetites. This correlation does not agree with the corresponding one derived from the data of Readman and O'Reilly (1972).The Curie temperature of ocean floor basalts increases with increasing age of the oceanic crust. Although there is considerable scatter in the data, it seems that Tcincreases proportionally with the logarithm of age. The upper 600 m of the predominantly pillow basalts of the Atlantic ocean floor are altered more or less uniformly with respect to the magnetic minerals. However within single litho‐logical units there is a slight decrease of oxidation rate with depth.The saturation magnetization of oceanic basalts first decreases with increasing low temperature oxidation until the oxidation parameter reaches z = 0.60. Beyond this z‐value, for further increasing oxidation, it increases again. This variation of magnetization can be explained qualitatively by a simple model assuming Fe‐migration first from the octahedral spinel lattice sites and then from the tetrahedral sites. The high amplitudes of the mesozoic marine magnetic anomalies could possibly be caused by this process. The jurassic magnetic quiet zone could then be explained by inversion of the completely oxidized titanomagnetites to non‐magnetic mineral phases.The direction of the natural remanent magnetization of ocean floor basalts is not changed by sea floor alteration.