Modal mineralogy of CV3 chondrites by X-ray diffraction (PSD-XRD)
Modal mineralogy of CV3 chondrites by X-ray diffraction (PSD-XRD)
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DOI：
10.1016/j.gca.2010.06.014
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发表时间：
2010-09
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
使用位置敏感探测器X射线衍射（PSD-XRD），我们确定了Vigarano，Efremovka，Mokoia，Grosnaja，Kaba和阿连德中丰度大于1wt%的所有相的完整模式矿物学。还原的CV 3样品由（体积%）：橄榄石（83-85%）;顽火辉石（6.5-8.1%）;钙长石（1.1-1.2%）;磁铁矿（1.4-1.8%）;硫化物（2.4-5.1%）; Fe、Ni金属（2-2.2%）组成。氧化样品由以下成分组成：橄榄石（76.3-83.9%）;顽火辉石（4.8-7.8%）;钙长石（1.1-1.7%）;磁铁矿（0.3-6.1%）;硫化物（2.9-8.1%）;铁、镍金属（0.2-1.1%）;氧化铁（0-2.7%）和页硅酸盐（1.9-4.2%）。当我们的模态数据用于计算与文献数据相比的本体化学时，观察到接近1：1的相关性。PSD-XRD数据表明，在所有CV样品中，橄榄石组合物可以跨越几乎整个Fe-Mg固溶体系列，并且这些样品含有比通常报道的更富Fe（> Fa 60）的细粒橄榄石组分（4-13%）。模式矿物学表明，被分类为氧化和还原的CV 3样品之间存在矿物学差异，但这些亚类最明显的区别是金属的相对丰度和硫化物的Ni含量，而不是磁铁矿的丰度。模态矿物学观察到的最显着的差异是相对缺乏页硅酸盐还原CV，基本上逃脱了水蚀变。铁橄榄石、铁橄榄石和磁铁矿通常被认为是次生蚀变产物。这些矿物的丰度在氧化和还原样品中重叠，并在相对氧化的环境中积极支持常见的形成条件。铁橄榄石、铁橄榄石和磁铁矿的丰度与岩石类型无关，如果将这些丰度用作蚀变的替代，则阿连德将是蚀变最小的CV -与所有先前的数据相反。这意味着，在母体上的热变质作用与富铁次生矿物的形成是解耦的。低温流体辅助变质作用也不能很容易地解释铁橄榄石，铁橄榄石和磁铁矿的起源，因为减少CV似乎在很大程度上不受此过程的影响。母体模型需要一个无水低温机制的次生蚀变。另一种可能性是，这些相在最终CV母体的增生之前形成。
Using position sensitive detector X-ray diffraction (PSD-XRD) we determine a complete modal mineralogy for all phases present in abundances greater than 1wt% in Vigarano, Efremovka, Mokoia, Grosnaja, Kaba and Allende. Reduced CV3 samples are comprised of (vol%): olivine (83–85%); enstatite (6.5–8.1%); anorthite (1.1–1.2%); magnetite (1.4–1.8%); sulphide (2.4–5.1%); Fe, Ni metal (2–2.2%). The oxidized samples are comprised of: olivine (76.3–83.9%); enstatite (4.8–7.8%); anorthite (1.1–1.7%); magnetite (0.3–6.1%); sulphide (2.9–8.1%); Fe, Ni metal (0.2–1.1%); Fe-oxide (0–2.7%) and phyllosilicate (1.9–4.2%). When our modal data is used to calculate a bulk chemistry that is compared to literature data a near 1:1 correlation is observed. PSD-XRD data indicates that olivine compositions may span almost the entire Fe–Mg solid solution series in all CV samples and that these contain a component (4–13%) of fine-grained olivine that is more Fe-rich (>Fa60) than is typically reported. Modal mineralogy shows that there are mineralogic differences between CV3 samples classified as oxidized and reduced but that these sub-classes are most clearly distinguished by the relative abundance of metal and Ni content of sulphide, rather than abundance of magnetite. The most significant difference in modal mineralogy observed is the relative absence of phyllosilicate in reduced CV that essentially escaped aqueous alteration. Fayalite, ferrous olivine and magnetite are typically considered secondary alteration products. The abundances of these minerals overlap in oxidized and reduced samples and correlate positively supporting common conditions of formation in a relatively oxidizing environment. The abundances of fayalite, ferrous olivine and magnetite show no relationship to petrographic type and if these abundances were used as a proxy for alteration, Allende would be the least altered CV – contrary to all previous data. The implication is that thermal metamorphism on the parent body was de-coupled from formation of Fe-rich secondary minerals. Low temperature fluid-assisted metamorphism can also not easily explain the origin of fayalite, ferrous olivine and magnetite, since the reduced CVs appear to be largely unaffected by this process. Parent body models require an anhydrous low-temperature mechanism of secondary alteration. The alternative is that these phases formed prior to accretion of the final CV parent body.