Microscopic observation of titanomagnetite grains during palaeointensity experiments of volcanic rocks
Microscopic observation of titanomagnetite grains during palaeointensity experiments of volcanic rocks
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火山岩古强度实验中钛磁铁矿颗粒的显微观察
DOI：
10.1093/gji/ggt387
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发表时间：
2014
期刊：
影响因子：
--
通讯作者：
Y.
中科院分区：
文献类型：
--
作者：
Tanaka;H.;Yamamoto;Y.
在Königsberger Thellier Thellier空气实验中，在反射光下显微观察到不同氧化程度的钛磁铁矿（Tmt）颗粒，其中一些部分磁化。反射光显微镜观察表明，在1300 ℃时，均质Tmt的棕色变为蓝色。根据扫描电子显微镜观察，这种假蓝色是由表面上的亚微米级凹凸不平的条纹引起的，这是在最后的加热步骤之后进行的。Tmt颗粒的磁赤铁矿化部分的典型灰色到蓝色在300 °C时变成褐色，表明钛磁赤铁矿转化为磁铁矿和钛铁矿（Ilm）或赤铁矿（Hem）的混合物。虽然这些观察结果来自样品表面上的Tmt晶粒，但样品内的氧化必须进行类似的处理，因为Tmt晶粒的表面变化与Arai图上数据点的行为高度相关。在实验结束时，通过反射光显微镜和扫描电子显微镜评估在610 °C下在空气中加热10至500分钟的增加时间后Tmt的变化。在随后的加热过程中，Tmt颗粒中逐渐出现斑驳的斑块。然而，即使在最后的500分钟加热之后，也没有观察到新的Ilm薄层的形成。总之，在实验室加热过程中，Tmt在10600 °C的空气中的变化可能不是由于典型的高温氧化形成的网格型Ilm薄片。低于400 °C时，工艺应更接近低温氧化。另一方面，Tmt颗粒的磁性化部分在300 °C时瞬间反转，并且在610 °C加热时很快出现具有Hem薄片的网格型结构。
Titanomagnetite (Tmt) grains, some partially maghemitized, of various oxidation levels were microscopically observed under reflected light as a function of temperature step in a Königsberger Thellier Thellier experiment in air. The reflected light microscopy indicated that the brownish colour of homogeneous Tmt turned blue at ∼300 °C. This false blue colour was caused by submicron scale rugged stripes on the surface, according to scanning electron microscope observations, which was made after the final heating step. The typical grey-to-bluish colour of maghemitized parts of Tmt grains turned to a brownish colour at ∼300 °C, indicating inversion of titanomaghemite to a mixture of magnetite and ilmenite (Ilm) or haematite (Hem). Although these observations were from Tmt grains on the sample surface, oxidation must have proceeded similarly within samples because the surface changes in the Tmt grains were highly correlated with behaviour of data points on Arai plots. Alterations in Tmt after heating at 610 °C in air for increasing times from 10 to 500 min were evaluated by reflected light microscopy and scanning electron microscopy at the end of the experiment. Mottled patches gradually emerged in the Tmt grains during subsequent heatings. However, the formation of new Ilm lamellae was not observed, even after the final 500 min heating. In conclusion, the alteration of Tmt during laboratory heating in air at ∼600 °C is likely not due to the typical high-temperature oxidation that forms trellis-type Ilm lamellae. Below ∼400 °C, the process should be closer to low-temperature oxidation. On the other hand, maghemitized parts of Tmt grains invert instantaneously at 300 °C, and a trellis-type structure with Hem lamellae soon emerges when heated at 610 °C.