Near infrared spectra of white mica in the Belt Supergroup and implications for metamorphism
Near infrared spectra of white mica in the Belt Supergroup and implications for metamorphism
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DOI：
10.2138/am.2010.3281
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发表时间：
2010-07
影响因子：
3.1
通讯作者：
E. Duke;Reed S. Lewis
中科院分区：
文献类型：
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作者：
E. Duke;Reed S. Lewis
用便携式可见光和近红外反射光谱仪测定了带状超群及其伴生变质岩中1036个样品在2200 nm附近的白色云母(伊利石和白云母)Al-OH吸收带的波长。Al-OH波长从黑云母带样品的2225 nm减小到硅线石带样品的2194 nm，这与白云母的总Al含量从每11O原子~2.0个原子增加到~2.8个原子相对应。这些观察结果表明：(1)Al-OH振动的频率受控于铝钙长石交换反应[IVSi+VI(Mg，Fe2+)=IVal+Val]；(2)随着变质程度的增加，该反应向更富Al的成分发展。在这种情况下，Al-OH波长提供了成分变化和变质程度的间接监测。研究区大部分变质程度在黑云母带或以下，但低品位岩石中的Al-OH波长决定了与埋藏深度和后来的构造位移相关的系统模式。在高品位地区，波长一般从石榴石等轴线到硅线石带减小；然而，局部出现异常，尚不清楚这些异常是由于主体成分和矿物组合的差异，还是指向实际的变质或构造不连续。这些发现表明，反射光谱可以提供关于含白云母岩石的变质强度的有价值的信息，特别是在缺乏变质程度常规指示的低级别层序中。此外，这种信息可以通过现场便携式光谱仪获得，而且有可能从机载和空间成像光谱仪获得可比较的结果。
Abstract The wavelength of the white mica (illite and muscovite) Al-OH absorption band near 2200 nm in 1036 samples from the Belt Supergroup and associated metasedimentary rocks was determined with a portable visible and near infrared reflectance spectrometer. The Al-OH wavelength decreases from 2225 nm in sub-biotite-zone samples to 2194 nm in sillimanite-zone samples; this decrease corresponds with an increase in total Al content of white mica from ~2.0 to ~2.8 atoms per 11 O atoms. These observations indicate that: (1) the frequency of the Al-OH vibration is controlled by the aluminoceladonite exchange reaction [IVSi+VI(Mg,Fe2+) = IVAl+VIAl], and (2) the reaction proceeds toward more Al-rich composition with increasing metamorphic grade. In these circumstances, Al-OH wavelength provides an indirect monitor of compositional variation and metamorphic grade. Metamorphic grade in most of the study area is in the biotite zone or lower, yet Al-OH wavelengths in low-grade rocks define systematic patterns that correlate with depth of burial and later structural displacements. In higher grade areas, wavelengths generally decrease from the garnet isograd through the sillimanite zone; however, anomalies occur locally, and it is not clear whether these result from differences in bulk composition and mineral assemblage or whether they point to actual metamorphic or structural discontinuities. These findings indicate that reflectance spectroscopy can yield valuable information on metamorphic intensity in rocks containing white mica, particularly in low-grade sequences where conventional indicators of metamorphic grade are lacking. Furthermore, this information can be obtained with field-portable spectrometers and the potential exists to obtain comparable results from airborne and spaceborne imaging spectrometers