Magnetic Properties and Ferromagnetic Mineralogy of Oceanic Basalts
Magnetic Properties and Ferromagnetic Mineralogy of Oceanic Basalts
复制标题
大洋玄武岩的磁性和铁磁矿物学
DOI：
--
复制
发表时间：
1993
期刊：
影响因子：
--
通讯作者：
T. Furuta
中科院分区：
文献类型：
--
作者：
T. Furuta
摘要
研究了大洋玄武岩岩样的岩石磁学性质，并结合这些岩样的古地磁特征，研究了45航次以后的大洋玄武岩钻探和一次挖泥作业采集的玄武岩岩样。电子探针(EPMA)分析表明，原生钛磁铁矿的化学成分多种多样，尤其是Al_2O_3、MgO、MnO、V_2O_3等次要成分。钛磁铁矿原生相化学成分的变化似乎受控于结晶过程中温度、氧逸度和挥发压力的变化以及岩浆中残留液体的化学作用。发现了总的结晶趋势，即由于温度和氧逸度的降低，后期结晶的钛磁铁矿具有较高的超尖晶石成分(较高的氧化钛和较低的氧化铁)。通过对比主成分元素的变化，确定了微量成分元素是钛磁铁矿结晶序列的良好指示剂。后期，Al_2O_3、MgO(尖晶石组分)和V_2O_3减少，而MnO增加，少数几种元素出现逆晶化趋势。这一逆转趋势可能表明，在一个结晶序列中，岩浆中存在控制钛磁铁矿化学的环境条件的不连续变化。一次化学变化与二次氧化变化趋势不同。利用改进的EPMA技术，对大洋玄武岩中钛磁铁矿的氧化过程进行了详细的研究，改进后的EPMA技术可直接测定物质中氧的绝对含量。定量测氧结果表明，钛磁铁矿的氧化过程不是亚铁离子随氧离子向铁离子转变的简单过程，而是与钛磁铁矿晶格中铁离子的迁移有关。这与最近对大洋玄武岩中钛磁铁矿氧化过程的研究结果一致。
SUMMARY
Rockmagnetic properties of a number of oceanic basalt samples collected by the DSDP/ODP drillings after Leg 45 as well as one dredge haul were investigated in conjunction with palaeomagnetic characteristics of the same samples. the electron-probe micro-analysis (EPMA) study indicated that chemical composition of the primary titanomagnetites has a variety, particularly in minor constituent elements such as Al2O3, MgO, MnO, and V2O3. the variation in chemical composition in the primary phase of titanomagnetites seems to be controlled by changes in temperature, oxygen fugacity and volatile pressure during the crystallization as well as chemistry of residual liquid hi magma. General crystallization trend is found, that is, the later crystallized titanomagnetites have higher ulvospinel component (higher titanium oxide and lower iron oxide) due to decrease in temperature and oxygen fugacity. It is established that minor constituent elements are good indicators of crystallization sequence of titanomagnetites in the contrast of variation in major constituent elements. In the later stage, Al2O3 and MgO (spinel component) and V2O3 decrease, whereas MnO increases, although there is a reversed crystallization trend with a few elements in several cases. This reversed trend may suggest that there are discontinuous changes of environmental condition in magma to control titanomagnetite chemistry during a crystallization sequence. the primary variation of chemistry shows different trends of the secondary variation due to oxidation. Oxidation process of titanomagnetites contained in oceanic basalts was examined in detail by means of an improved EPMA technique being able to directly determine the absolote contents of oxygen in materials. the result of the quantitative oxygen measurement shows that the oxidation process of titanomagnetites is not a simple process by changes of ferrous ions to ferric ones with acrretion of oxygen but associated with migration of iron ions of the titanomagnetite lattice. This is consistent with the results of recent studies on oxidation processes of titanomagnetites in oceanic basalts.