Exploring relationships between shock-induced microstructures and H2O and Cl in apatite grains from eucrite meteorites
Exploring relationships between shock-induced microstructures and H2O and Cl in apatite grains from eucrite meteorites
复制标题
DOI：
10.1016/j.gca.2021.03.018
复制
发表时间：
2021-03
影响因子：
5
通讯作者：
T. Barrett;A. Černok;G. Degli-Alessandrini;X. Zhao;M. Anand;I. Franchi;J. Darling
中科院分区：
文献类型：
--
作者：
T. Barrett;A. Černok;G. Degli-Alessandrini;X. Zhao;M. Anand;I. Franchi;J. Darling
陨石中挥发性元素的丰度和同位素组成对于理解行星演化至关重要，因为它们在各种地球化学过程中都很重要。人们对矿物磷灰石一直很感兴趣，它是大多数陨石中的一种次要相，已知含有相当数量的挥发物(高达wt.%的F、Cl和OH)。撞击驱动的冲击变质作用普遍存在于许多陨石中，可能会通过脱挥发和扩散等过程改变挥发分的原始特征。在这项研究中，我们用电子背散射衍射(EBSD)研究了六个榴辉石在广泛的冲击阶段(S1-S5)的磷灰石颗粒的微结构，以探索冲击诱导磷灰石的结晶学特征。用纳米二次离子质谱仪(NanoSIMS)采集了中、高度冲击样品(S3-S5)的新的氯、氢丰度和同位素组成数据。在这项研究中，之前报道的S1和S2榴辉石的挥发性数据与EBSD发现相结合。我们的发现表明，在较高的冲击阶段，磷灰石的微结构变得越来越复杂。在低冲击阶段(S1-S2)，样品表现为磷灰石的角砾化和破碎。S3和S4的样品表现出晶体塑性变形的增加，表现为极图上扩展的增加。在较高的冲击阶段(S4/S5)，存在潜在的再结晶，表现为亚晶界密度的增加。高冲击磷灰石的氯含量和δ37Cl值分别为∼940~1410 ppm和−3.38~+7.70‰，与未受冲击的榴辉石相同。相比之下，H2O丰度的变化更大(从186ppm到∼4010ppm)，然而，测得的水含量仍然落在以前报道的低冲击榴辉石的范围内。测得的δD值范围从−15 7到+163‰，也在已知低震玄武岩的值范围内。这两个同位素体系(δD−122 ± 20‰，δ37Cl + 1.76 ± 0.66‰)的加权平均值与其他内太阳系天体的范围一致。纳米级磷灰石颗粒的NanoSIMS同位素图像显示，其氯丰度在纳米尺度上是不均匀的，其复杂性随着冲击阶段的增加而增加。然而，这种日益增加的复杂性与EBSD中观察到的变形微结构或晶间或晶内尺度上的氯同位素组成无关。这些发现类似于对可变冲击的月球磷灰石的分析，因此，尽管受到强烈的冲击，但磷灰石似乎是一个强大的氯和氢记录器(至少以目前NanoSIMS可以实现的空间分辨率和精度)。
The abundance and isotopic composition of volatile elements in meteorites is critical for understanding planetary evolution, given their importance in a variety of geochemical processes. There has been significant interest in the mineral apatite, which occurs as a minor phase in most meteorites and is known to contain appreciable amounts of volatiles (up to wt. % F, Cl, and OH). Impact-driven shock metamorphism, pervasive within many meteorites, can potentially modify the original signatures of volatiles through processes such as devolatilization and diffusion. In this study, we investigate the microstructures of apatite grains from six eucrites across a broad range of shock stages (S1–S5) using electron backscatter diffraction (EBSD) to explore shock-induced crystallographic features in apatite. New Cl and H abundance and isotopic composition data were collected on moderate to highly shocked samples (S3-S5) by Nano Secondary Ion Mass Spectrometry (NanoSIMS). Previously reported volatile data for S1 and S2 eucrites were integrated with EBSD findings in this study. Our findings indicate that apatite microstructures become increasingly more complex at higher shock stages. At low shock stages (S1–S2) samples display brecciation and fracturing of apatite. Samples in S3 and S4 display increasing crystal plastic deformation indicated by increasing spread in pole figures. At the higher shock stages (S4/S5) there is potential recrystallisation demonstrated by an increased density of subgrain boundaries. The Cl content and δ37Cl values of highly-shocked apatite grains range from ∼940 to 1410 ppm and −3.38 to +7.70‰, respectively, within the range observed in less-shocked eucrites. In contrast, H2O abundances are more variable (from 186 to ∼4010 ppm), however, the measured water content still falls within the range previously reported for low-shock eucrites. The measured δD values range from −157 to +163‰, also within the range of values from known low-shock basaltic eucrites. Weighted averages for both isotopic systems (δD −122 ± 20‰, δ37Cl + 1.76 ± 0.66‰) are consistent with the range displayed in other inner Solar System bodies. NanoSIMS isotope images of apatite grains display heterogeneity in their Cl abundance at the nanoscale which increases in complexity with shock stage. This increasing complexity, however, does not correlate with deformation microstructures observed in EBSD or with the Cl isotopic composition at either an inter-grain or intra-grain scale. These findings are similar to analyses of variably shocked lunar apatite and, therefore, apatite appears to be a robust recorder of Cl and H (at least at spatial resolution and precision currently achievable by NanoSIMS) on airless bodies, despite intensive shock.