Modal mineralogy of CM2 chondrites by X-ray diffraction (PSD-XRD). Part 1: Total phyllosilicate abundance and the degree of aqueous alteration
Modal mineralogy of CM2 chondrites by X-ray diffraction (PSD-XRD). Part 1: Total phyllosilicate abundance and the degree of aqueous alteration
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DOI：
10.1016/j.gca.2009.04.038
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发表时间：
2009-08
影响因子：
5
通讯作者：
K. Howard;G. Benedix;P. Bland;G. Cressey
中科院分区：
文献类型：
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作者：
K. Howard;G. Benedix;P. Bland;G. Cressey
CM碳质陨石是不完全蛇纹化的原始小行星的样本。使用位置敏感探测器X射线衍射（PSD-XRD）和图案剥离技术，我们量化的模式矿物学的CM 2陨石：Mighei，默里，默奇森，Nogoya和冷Bokkeveld。在最丰富的相Mg-蛇纹石（25-33%）和Fe-钙钛矿（43-50%）的组合模态体积（体积%）中存在窄的范围。冷Bokkeveld是异常的，含有更多的镁蛇纹岩（49-59%）比铁cronstedtite（19-27%）。即使包括Cold Bokkeveld，模态总页硅酸盐的范围为73-79%（平均=75%）。总层状硅酸盐丰度提供了水蚀变程度的明确测量，并表明这些陨石都经历了基本相同的水蚀变程度。这反映了CM 2样品中基质的普遍水合作用。在岩石学研究中观察到的球粒的蚀变的明显差异代表了所有组分完全水合的进展中的各个阶段，但在模态矿物学中没有表现出显着差异。对于所有样品，橄榄石（6.9%）和辉石（5%）丰度的范围有限。其余已识别相的模态丰度也显示出有限的范围：方解石（0-1.3%）;石膏（0-1.6%）;磁铁矿（1.1-2.4%）;镍黄铁矿（0-2.1%）和磁黄铁矿（1-3.8%）。正如预期的那样，我们观察到一个强大的负相关性的模态丰度的无水铁镁硅酸盐（橄榄石+辉石）和总页硅酸盐（镁蛇纹石+铁cronstedtite）的想法相一致，页硅酸盐形成的水蚀变的无水成分。镁蛇纹石和铁镁闪石模态丰度的负相关性表明：（a）铁镁闪石通过流体驱动重结晶向镁蛇纹石的矿物学转变;（B）这两种陨石具有不同的橄榄石和辉石初始丰度。观察到的正相关性的相对比例的镁-蛇纹石与总层状硅酸盐丰度的增加反映了日益丰富的镁层状硅酸盐在水蚀变的演变。Fe-cronstedtite是主要的层状硅酸盐，而CM球粒橄榄石是镁橄榄石，并将在水蚀变过程中形成镁-蛇纹石。这意味着基质橄榄石更富铁比球粒橄榄石之前，水蚀变。
CM carbonaceous chondrites are samples of incompletely serpentinized primitive asteroids. Using position sensitive detector X-ray diffraction (PSD-XRD) and a pattern stripping technique, we quantify the modal mineralogy of CM2 chondrites: Mighei; Murray; Murchison; Nogoya and Cold Bokkeveld. There is a narrow range in the combined modal volume (vol%) of the most abundant phases Mg-serpentine (25–33%) and Fe-cronstedtite (43–50%). Cold Bokkeveld is anomalous in containing more Mg-serpentine (49–59%) than Fe-cronstedtite (19–27%). Even including Cold Bokkeveld, the range in modal total phyllosilicate is 73–79% (average=75%). Total phyllosilicate abundance provides a non-ambiguous measure of the degree of aqueous alteration and indicates that these meteorites have all experienced essentially the same degree of aqueous alteration. This reflects pervasive hydration of matrix across CM2 samples. Apparent differences in the alteration of chondrules observed in petrographic studies represent various stages in the progression towards complete hydration of all components but are not manifest in significant differences in modal mineralogy. For all samples there is a limited range in olivine (6.9%) and pyroxene (5%) abundances. Modal abundances of the remaining identified phases also show a limited range: calcite (0–1.3%); gypsum (0–1.6%); magnetite (1.1–2.4%); pentlandite (0–2.1%) and pyrrhotite (1–3.8%). As expected, we observe a strong negative correlation in the modal abundance of anhydrous Fe–Mg silicates (olivine+pyroxene) and total phyllosilicate (Mg-serpentine+Fe-cronstedtite) consistent with the idea that phyllosilicate is forming by aqueous alteration of the anhydrous components. The negative correlation in the modal abundance between Mg–serpentine and Fe-cronstedtite indicates: (a) mineralogic transformation of Fe-cronstedtite to Mg-serpentine by fluid driven recrystallisation or (b) that these meteorites had different initial abundances of olivine and pyroxene. The observed positive correlation in the relative proportion of Mg-serpentine with increasing total phyllosilicate abundance reflects the evolution of increasingly Mg-rich phyllosilicate during aqueous alteration. Fe-cronstedtite is the dominant phyllosilicate, while CM chondrule olivines are forsteritic and will form Mg-serpentine during aqueous alteration. This implies that matrix olivine was more Fe-rich than chondrule olivine prior to aqueous alteration.