加拿大Sudbury陨石冲击角砾岩中锆石显微构造与相变

关于加拿大Sudbury构造的成因,现在的学者大多认同其形成于陨石撞击。为了找到更加强有力的证据,文中对取自Sudbury冲击角砾岩中的锆石进行了研究。Sudbury冲击角砾岩是Sudbury构造的特殊现象,采用扫描电镜、结合拉曼光谱方法从角砾岩的锆石中发现了显微构造和相变的证据。所观察到的显微构造呈一系列面状构造(planar microstructure),在3个方向上平行展布,并相互剪切。Sudbury角砾岩锆石的典型Raman谱线中,除锆石的几个峰外,在640cm-1和817cm-1也可见两个峰,并可分别与铁橄榄石和一种ZrO2相匹配。该ZrO2相的谱线也不同于典型的斜锆石,却与López等合成的一种含有0.5%(质量分数)TiO2的ZrO2相非常相似。这些证据显示,一些锆石的颗粒边缘发生了如下反应:ZrSiO4+2FeOZrO2+Fe2SiO4,并已变成了一种ZrO2相和铁橄榄石(fayalite)。这些显微构造和相变特征都能够说明Sudbury角砾岩受到过高压冲击,并进一步支持Sudbury构造的陨石撞击成因。

Shock-metamorphosed zircons in the fragments of the Sudbury breccias,Ontario,Canada

It is widely accepted that the Sudbury structure formed by large bolide impact.To find more supporting evidences we have used electron-beam scanning microscopy and Raman micro-spectroscopy to document the microstructures and phase transformation in zircons from the fragments of the Sudbury breccias which are among the most prominent features of the Sudbury structure.In addition to the important peaks for zircon,two additioanl Raman peaks at 640 cm~(-1) and 817 cm~(-1) are shown.These two additional peaks match with those from a synthetic ZrO_2 polymorph containing 0.5 wt% TiO_2 and fayalite,respectively.Our data show that planar microstructures in three directions and decomposition of zircon to form a ZrO_2 polymorph plus fayalite(ZrSiO_4+2FeO(=)ZrO_2+Fe_2SiO_4)are common in the Sudbury zircons we have examined.These features support an impact origin for the Sudbury breccias and thereby the Sudbury structure at large.