New shock microstructures in titanite (CaTiSiO5) from the peak ring of the Chicxulub impact structure, Mexico
New shock microstructures in titanite (CaTiSiO5) from the peak ring of the Chicxulub impact structure, Mexico
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DOI：
10.1007/s00410-019-1565-7
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发表时间：
2019-05
影响因子：
3.5
通讯作者：
N. Timms;M. Pearce;T. Erickson;A. Cavosie;A. Rae;J. Wheeler;A. Wittmann;L. Ferrière;M. Poelchau;N. Tomioka;G. Collins;S. Gulick;C. Rasmussen;J. Morgan
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作者：
N. Timms;M. Pearce;T. Erickson;A. Cavosie;A. Rae;J. Wheeler;A. Wittmann;L. Ferrière;M. Poelchau;N. Tomioka;G. Collins;S. Gulick;C. Rasmussen;J. Morgan
磷灰石、斜锆石、独居石、磷钇矿和锆石等辅助矿物地质年代计因其保存超高速撞击过程的诊断显微结构证据的能力而日益受到认可。尽管钛铁矿是一种广泛存在的副相，也是一种U-Pb地质年代计，但迄今为止，人们对钛铁矿对冲击变质作用的响应知之甚少。在这里，我们报告了两个新的机械双模式在冲击花岗岩类的希克苏鲁布的影响结构，墨西哥的钛。国际海洋发现计划孔M0077 A新获得的岩心中的钛铁矿颗粒保留了多组多合成孪晶，最常见的是组合面（K1）= ~，剪切方向（η1）=< 110 >，较少见的是模式K1 = {130}，η1= ~<522 >。在某些晶粒中，{130}形变带与形变孪晶同时形成，表明在冲击变质过程中，位错滑移具有Burgers矢量b =< 341 >。在这里所描述的模式的钛铁矿双胞胎还没有报道内生变形岩石，因此，我们提出这种新确定的双胞胎形式作为冲击变形的结果。孪晶的形成条件尚未经过实验校准，并且在此经验性地受到石英中存在平面变形特征（12 ± 5和~ 17 ± 5 GPa）和锆石中不存在冲击孪晶（< 20 GPa）的限制。虽然钛铁矿双晶形成的下限仍然受到很大的限制，但这些双晶的识别突出了钛铁矿在12至17 GPa的压力范围内作为冲击指标的实用性。鉴于寻找冲击变质作用的诊断指标以识别地球上古代和近代的撞击证据的挑战，这里展示了钛铁矿的显微结构分析，以提供一种新的工具，用于识别岩石中的撞击变形，其中其他撞击证据可能被擦除，改变，或由于一般较低（< 20 GPa）的冲击压力而没有表现出来。
Accessory mineral geochronometers such as apatite, baddeleyite, monazite, xenotime and zircon are increasingly being recognized for their ability to preserve diagnostic microstructural evidence of hypervelocity-impact processes. To date, little is known about the response of titanite to shock metamorphism, even though it is a widespread accessory phase and a U–Pb geochronometer. Here we report two new mechanical twin modes in titanite within shocked granitoid from the Chicxulub impact structure, Mexico. Titanite grains in the newly acquired core from the International Ocean Discovery Program Hole M0077A preserve multiple sets of polysynthetic twins, most commonly with composition planes (K1) = ~, and shear direction (η1) = < 110 > , and less commonly with the modeK1= {130},η1= ~ <522 > . In some grains, {130} deformation bands have formed concurrently with the deformation twins, indicating dislocation slip with Burgers vectorb= < 341 > can be active during impact metamorphism. Titanite twins in the modes described here have not been reported from endogenically deformed rocks; we, therefore, propose this newly identified twin form as a result of shock deformation. Formation conditions of the twins have not been experimentally calibrated, and are here empirically constrained by the presence of planar deformation features in quartz (12 ± 5 and ~ 17 ± 5 GPa) and the absence of shock twins in zircon (< 20 GPa). While the lower threshold of titanite twin formation remains poorly constrained, identification of these twins highlight the utility of titanite as a shock indicator over the pressure range between 12 and 17 GPa. Given the challenges to find diagnostic indicators of shock metamorphism to identify both ancient and recent impact evidence on Earth, microstructural analysis of titanite is here demonstrated to provide a new tool for recognizing impact deformation in rocks where other impact evidence may be erased, altered, or did not manifest due to generally low (< 20 GPa) shock pressure.