Optical and near infrared spectra of ringwoodite to 21.5 GPa: Implications for radiative heat transport in the mantle
Optical and near infrared spectra of ringwoodite to 21.5 GPa: Implications for radiative heat transport in the mantle
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21.5 GPa 尖角橄榄石的光学和近红外光谱：对地幔辐射热传输的影响
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发表时间：
2005
期刊：
影响因子：
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通讯作者：
J. Smyth
中科院分区：
文献类型：
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作者：
H. Keppler;J. Smyth
摘要 测量了成分为 (Mg0.90Fe0.10)2SiO4 的尖柱橄榄石单晶的高压光学和近红外光谱，压力为 21.5 GPa。环境压力下的光谱显示在 12 265 cm-1 处有一个强谱带，在 8678 cm-1 和 17 482 cm-1 处有两个肩峰。 12 265 cm-1 和 8678 cm-1 处的能带是由于八面体 Fe2+ 的自旋允许晶体场跃迁所致，而 17 482 cm-1 处的能带很可能是由于 Fe2+ → Fe3+ 电荷转移所致。由于配体到金属的电荷转移而产生的吸收边发生在接近 30 000 cm-1 处。随着压力的增加，晶体场和电荷转移带都转移到更高的频率。虽然这对于晶体场带来说是预期的，但对于间隔电荷转移带来说，这种蓝移是令人惊讶的。此外，晶体场和电荷转移带都不会随着压力而显着变宽或增强。这些结果对地幔中的辐射传热具有重大意义。人们普遍认为辐射传热在地幔中被阻挡，因为人们相信红移和电荷转移带强度随压力的增加将有效地使地幔矿物在整个近红外和可见光范围内不透明。我们的结果表明，对于具有适合地幔的 Mg/Fe 比例的尖角橄榄石，不会发生这种效应。恰恰相反，尖晶橄榄石中光子的平均自由程实际上随着压力的增加而增加，因为吸收带远离黑体辐射的最大值。
Abstract High pressure optical and near infrared spectra of a single crystal of ringwoodite with composition (Mg0.90Fe0.10)2SiO4 were measured to 21.5 GPa. The spectrum at ambient pressure shows a strong band at 12 265 cm-1 with two shoulders at 8678 cm-1 and 17 482 cm-1. The bands at 12 265 cm-1 and at 8678 cm-1 are due to spin -allowed crystal field transitions of octahedral Fe2+, while the band at 17 482 cm-1 is most likely due to Fe2+ → Fe3+ charge transfer. The absorption edge due to ligand-to-metal charge transfer occurs close to 30 000 cm-1. With increasing pressure, both the crystal field and the charge transfer bands shift to higher frequencies. Whereas this is expected for the crystal field bands, this blue shift is surprising for an intervalence charge transfer band. Moreover, neither the crystal field nor the charge transfer bands broaden or intensify significantly with pressure. These results have major implications for radiative heat transfer in the Earth's mantle. It has commonly been assumed that radiative heat transfer is blocked in the mantle, because it was believed that the red shift and the increased intensity of charge transfer bands with pressure would effectively make mantle minerals opaque throughout the near infrared and visible range. Our results demonstrate that this effect does not occur for ringwoodite with a Mg/Fe ratio realistic for the Earth's mantle. Quite to the contrary, the mean free path of photons in ringwoodite actually increases with pressure, because the absorption bands move away from the maximum of the blackbody radiation.