The shocking state of apatite and merrillite in shergottite Northwest Africa 5298 and extreme nanoscale chlorine isotope variability revealed by atom probe tomography
The shocking state of apatite and merrillite in shergottite Northwest Africa 5298 and extreme nanoscale chlorine isotope variability revealed by atom probe tomography
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DOI：
10.1016/j.gca.2020.11.007
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发表时间：
2021-01
影响因子：
5
通讯作者：
James Darling;L. White;T. Kizovski;A. Černok;D. Moser;K. Tait;J. Dunlop;B. Langelier;J. O. Douglas;X. Zhao;Ian A. Franchi;M. Anand
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文献类型：
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作者：
James Darling;L. White;T. Kizovski;A. Černok;D. Moser;K. Tait;J. Dunlop;B. Langelier;J. O. Douglas;X. Zhao;Ian A. Franchi;M. Anand
钙磷矿物的元素和氯同位素组成是火星挥发性清单以及该星球内生岩浆和热液历史的关键记录器。大多数火星陨石都有明确的证据表明外源撞击引起的变形和变质作用，但这些冲击变质过程对磷酸钙中所含氯同位素记录的影响尚未得到评估。在这里，我们测试了一次冲击变质旋回对高度冲击、富含雪戈特西北部非洲(NWA)5298的磷灰石中氯同位素体系的影响。详细的纳米结构(EBSD、拉曼和透射电子显微镜)数据揭示了广泛的分布激波特征。这主要是强烈塑性变形、再结晶和/或冲击熔化的结果。这些冲击特征与化学不均一性直接相关，包括与冲击熔体和富铁矿脉有关的横切微尺度富氯特征。NWA5298磷灰石的纳米氯同位素测量揭示了δ37Cl值的范围(−3到1‰；2σ不确定性<；0.9‰)，几乎与以前所有玄武岩钙钛矿的测量结果一样大，并且测量的δ37Cl值可以很容易地与目标磷灰石的不同纳米结构状态联系起来。高空间分辨率原子探针断层扫描(APT)数据显示，富氯和富缺陷纳米尺度的边界具有高度负的δ37Cl值(−15 ± 8‰的平均值)。我们的结果表明，冲击变质作用会对磷酸钙的化学和氯同位素记录产生重大影响，这主要是由于冲击熔化和再结晶过程中氯的动员所致。尽管如此，低应变磷灰石结构域已被EBSD鉴定，并产生平均δ37Cl值为−0.3NWA0.3 ± 0.6NWA0.6‰，这被认为是对NWA5298一次氯同位素组成的最佳估计。结合纳米结构、微尺度化学和纳米尺度的APT同位素方法，能够更好地分离和识别岩浆和近地表过程的内源挥发性元素记录以及与地震有关的外源效应。
The elemental and chlorine isotope compositions of calcium-phosphate minerals are key recorders of the volatile inventory of Mars, as well as the planet’s endogenous magmatic and hydrothermal history. Most martian meteorites have clear evidence for exogenous impact-generated deformation and metamorphism, yet the effects of these shock metamorphic processes on chlorine isotopic records contained within calcium phosphates have not been evaluated. Here we test the effects of a single shock metamorphic cycle on chlorine isotope systematics in apatite from the highly shocked, enriched shergottite Northwest Africa (NWA) 5298. Detailed nanostructural (EBSD, Raman and TEM) data reveals a wide range of distributed shock features. These are principally the result of intensive plastic deformation, recrystallization and/or impact melting. These shock features are directly linked with chemical heterogeneities, including crosscutting microscale chlorine-enriched features that are associated with shock melt and iron-rich veins. NanoSIMS chlorine isotope measurements of NWA 5298 apatite reveal a range of δ37Cl values (−3 to 1‰; 2σ uncertainties <0.9‰) that is almost as large as all previous measurements of basaltic shergottites, and the measured δ37Cl values can be readily linked with different nanostructural states of targeted apatite. High spatial resolution atom probe tomography (APT) data reveal that chlorine-enriched and defect-rich nanoscale boundaries have highly negative δ37Cl values (mean of −15 ± 8‰). Our results show that shock metamorphism can have significant effects on chemical and chlorine isotopic records in calcium phosphates, principally as a result of chlorine mobilization during shock melting and recrystallization. Despite this, low-strain apatite domains have been identified by EBSD, and yield a mean δ37Cl value of −0.3 ± 0.6‰ that is taken as the best estimate of the primary chlorine isotopic composition of NWA 5298. The combined nanostructural, microscale-chemical and nanoscale APT isotopic approach gives the ability to better isolate and identify endogenous volatile-element records of magmatic and near-surface processes as well as exogenous, shock-related effects.