岩浆过程对玄武岩铁氧化状态和氧逸度的影响: 以雷琼地区晚新生代玄武岩为例

使用幔源岩浆的铁氧化状态估算岩浆fO₂是获得地幔氧逸度及岩浆过程对氧逸度影响的有效途径.使用湿化学法对雷琼地区晚新生代玄武岩的主量元素成分进行了分析,并结合一些前人的工作,发现雷琼地区玄武岩的Fe3+/∑Fe比值可分为两组: 一组比值为0.20~0.35,称之正常样品; 另一组比值为0.45~0.66,称之异常样品.采用氧逸度与Fe₂O₃/FeO之间关系的经验公式对这些玄武岩岩浆的氧逸度进行了估算,正常样品的氧逸度为FMQ+1.1~FMQ+2.4,略高于前人由地幔捕虏体获得的本区地幔氧逸度(FMQ-0.943~FMQ+1.235),这种差异可能是由于计算方法引起的; 而异常样品形成时的氧逸度范围为FMQ+3.3~FMQ+4.8,比地幔源区氧逸度高出2.0log单位以上,不能用计算方法的差异来解释.通过岩浆过程对fO₂影响的讨论,认为分离结晶、岩浆脱气以及熔体上升过程中与岩浆通道物质之间的氧交换都不能解释异常样品的高Fe3+/∑Fe比值和氧逸度.许多异常样品采自近火山口,岩浆冷却速度可以引起fO₂的重要变化,因为缓慢淬火过程中离子的局部结构调整可以使Fe2+氧化为更小的Fe3+.由此可知,缓慢冷却会使喷出地表的岩浆铁的氧化状态明显提高,所以使用玄武质熔岩估计其源区氧化状态时,应选择远离火山口的熔岩. 

Effect of Magmatic Processes on Fe Oxidation State and of Basaltic Melts:A Case Study for Late Cenozoic Basalts from the Leiqiong Area,South China

The major element composition of late Cenozoic basalts from Leiqiong area,South China is analyzed by wet chemical method. Coupled with some previous data,Fe3+/∑Fe ratios of these basalts can be divided into two groups,0.20-0.35 (named as normal samples) and 0.45-0.66 (named as abnormal samples). The oxygen fugacity in the basalt formation is calculated by the empirical equation which linked fO₂ to Fe₂O₃/FeO. The relative fO₂ of normal samples are about FMQ+1.1 to FMQ+2.4,slight higher than that of the mantle xenoliths found by previous researchers (FMQ-0.943 to FMQ+ 1.235). The difference may be attributed to the calculation methods,but the relative fO₂ of abnormal samples are around FMQ+3.3 to FMQ+4.8,2log units higher than that of their source region,and that difference can not be explained in the same way as above. Through discussing the effect of magmatic processes on fO₂,it indicates fractional crystallization,degassing and oxygen exchange between ascending melts and crust material and lithospheric material can not result in so large fO₂ and high Fe3+/∑Fe ratio in abnormal samples. Many abnormal samples collected from craters and the vicinity of craters are always related to unusual high Fe3+/∑Fe ratios. Thus we believe that the local structure resetting in lavas due to slowly cooling rate,which will oxidize Fe^2+ to a smaller Fe3+ ion for size consideration during the densification process,has led to elevated Fe3+/∑Fe ratios of the basaltic melts. One important conclusion is: to estimate the intrinsic fO₂ of the source region from which the basalts derived by Fe oxidation state of these basalts,sampling should be cautious and samples far away from the craters should be selected.