【作者】 徐梦婧；

【导师】 金振民； 费英伟；

【作者基本信息】 中国地质大学 ， 构造地质学， 2011， 硕士

【摘要】 豆荚状铬铁矿一般被认为是通过熔岩反应形成于上地幔浅部,而金刚石、柯石英等超高压矿物的发现使得豆荚状铬铁矿的深部成因开始被大家所关注。由于大部分超高压矿物是通过人工重砂技术获得的,存在后期混染的可能性,因此,铬铁矿中原位发现的单斜辉石、斜方辉石和柯石英出溶体是深部成因最重要的依据。出溶体一般被认为是某一均匀的矿物固溶体,在外界条件改变时分离形成的,所以,辉石和柯石英出溶体的发现,说明在特定的条件下铬铁矿可以含有一定量的CaO和SiO2。本文将采用西藏罗布莎天然铬铁矿和纯净SiO2为起始材料进行高温高压实验,探讨铬铁矿中柯石英出溶体的形成机制并对铬铁矿成因深度进行限定,并通过围岩地幔橄榄岩中橄榄石的组构特征对其提供进一步的约束条件。橄榄石在不同的环境下具有不同的组构类型,一般认为在地幔浅部低压环境下最易发育A型,地幔深部高压环境下主要发育C型组构,大洋俯冲带发育B型组构,因此,豆荚状铬铁矿围岩地幔橄榄岩的组构特征可以为铬铁矿是否具有深部成因提供一定的约束条件。本文以蛇纹石化程度较低的二辉橄榄岩和方辉橄榄岩为研究对象,采用氧化坠饰法进行位错研究,借助电子背散射衍射技术(EBSD)测量组构。罗布莎地幔橄榄岩矿物定向较弱,但是橄榄石和斜方辉石均可见扭折带、波状消光、晶体弯曲等变形现象。氧化坠饰法揭示,橄榄石主要发育了低温常见的直线型自由位错,局部可见位错弓弯、位错环和位错壁等高温位错。组构测量结果指山,地幔橄榄岩中橄榄石和斜方辉石均具有一定的结晶学优选方位,组构强度M指数分别为0.13-0.29和0.15-0.20。测量结果显示,橄榄石的[100]轴平行于线理方向,而[010]轴垂直于线理方向,为典型的A型组构,反映罗布莎地幔橄榄岩形成于岩石圈地幔环境。通常认为,出溶体是由于氧逸度改变、冷凝或分解反应使得寄住矿物中某些成分溶解度降低而形成的,因此,铬铁矿中辉石和柯石英出溶体的发现,说明在特定的条件下铬铁矿可以含有一定量的CaO和SiO2,但前人研究显示,在5GPa、1500℃时,铬铁矿中SiO2含量仍无明显增加。本文采用多面砧(multi-anvil)高温高压仪进行实验,实验条件为5-15GPa和1000-1600℃。实验结果显示,在5GPa,1000-1200℃,铬铁矿和SiO2反应反应形成铬铁矿、石榴石、氧化铬和斜方辉石,随着压力的增加,斜方辉石消失,反应产物为铬铁矿、石榴石、氧化铬。当温度大于1400℃,压力为5-8GPa时,实验发生熔融,形成富镁和硅的熔体。压力增加至14GPa,铬铁矿发生分解反应,形成一个新的矿物相和氧化铬,实验产物为石榴石、氧化铬和新的矿物相。与铬铁矿相比,新形成的矿物相含有更低的Al2O3(4.70-4.89wt.%)和Cr2O3(52-36-52.76wt.%)以及更高的MgO(22.95-32.76wt.%)和FeO(8.08-18.12wt.%),计算化学式接近于(Fe, Mg)2(Al, Cr)2O5, SEM-EDS分析也显示,新矿物相中(Mg+Fe)/(Cr+Al)比例为1：1,与镁铝尖晶石在高温高压下形成的新相Mg2Al2O5一致,其可能为Mg2Al2O5的类质同像,并具有相同的晶体结构——修正的硼镁铁矿结构。综上所述,在高温高压条件下,尖晶石族中的铝端元和铬端元矿物均可发生分解反应形成氧化物(A12O3和Cr2O3)和新矿物相(Mg2Al2O5和(Fe, Mg)2(Al, Cr)2O5),因此,A2B2O5(A:二价阳离子；B：三价阳离子)可能是除CaFe2O4或CaTi204结构相之外的另一种后尖晶石相。电子探针分析结果显示,铬铁矿中Si02含量随温度和压力的升高略微增加,在14GPa,1600℃,铬铁矿与新的矿物相共存,Si02含量达到最大,而新矿物相中Si02含量降低,为0.29-0.49%。铬铁矿中Si与Cr+Al具有明显的负相关性,而与Mg+Fe无相关性,反映Si和Ca可能4X3+→3Si4++△(Χ:Al, Cr;△:空位)替代机制进入铬铁矿。大部分淬火样品中,铬铁矿的SiO2含量小于0.7%,但在14GPa、1600℃的样品中升高至2.44%,该温压条件下,铬铁矿相变为CF相,可能引起了Si02含量显著增加,达到出溶所需含量。此外,单斜辉石出溶体的发现暗示铬铁矿中同时应含有CaO。含有大量Si02和CaO的铬铁矿在降温降压过程中首先出溶形成单斜辉石和斜方辉石,当Ca和Mg完全消耗掉时,形成柯石英出溶体。高温高压实验研究指出,罗布莎豆荚状铬铁矿可能经历了12-14GPa的高温高压过程(相当于350-400km的深度),但显微组构分析测试显示,围岩中橄榄石发育低温低压条件下常见的A型组构,二者结果不一致,这可能是因为本研究所观测到的组构特征是地幔橄榄岩在后期侵位过程中所形成的,其改造了早期高压组构。更多还原

【Abstract】 The finding of micro-diamond, coesite and highly reduced metal phases in chromite from Luobusha podiform chromitite has led to the suggestion that the chromite may originate from deep mantle, which is contrary to the magma mingling and melt-rock reaction origin in subduction zone under low-pressure conditions. Because these ultra-high pressure minerals were obtained from mineral separates, the conclusion is still controversial. So the coesite and clinopyroxene esolution lamellae in chromite are the most important evidence for the deep mantle originIn general, exsolution of a mineral is caused by a decrease in solubility due to a change in oxygen fugacity, cooling or decompression, so the presence of coesite and clinopyroxene esolution lamellae in chromite demonstrates that the Si solubility can be significant under certain conditions. In the paper, we studied the Si solubility in chromite by high pressure and high temperature experiments in natural chtomite and pure SiO2 system, and further investigated the origin of coesite and clinopyroxene exsolution in chromite and the formation depth of Luobusa chromitite. In addition, the fabric characteristic of olivine in hosted mantle peridotite can also constrain the origin.Various deformation fabrics of olivine are observed in different conditions. Previous researches showed A-type fabric and C-type fabric generally develop in uppermost mantle and deep mantle, respectively, whereas B-type fabric develop in subduction zone, so we can use the fabric characteristic of olivine to distinguish the formation depth hosted mantle peridotite. The samples were collected from Luobudsa lherzolite and harzburgite, and dislocations and fabrics in olivine were observed by the oxidation decoration technique and the electron backscatter diffraction (EBSD) technique, respectively. The minerals in both lherzolite and harzburgite are not directional aligment, but olivine and orthopyroxene display kink band, wavy extinction and crystal bending. Analysis of the dislocation configurations of olivine by oxidation decoration technique show that dislocation microstructures are dominant linear type free dislocation which usually forms at low temperature, and also include some high temperature dislocation, such as dislocation bows, dislocation loops and dislocation wall. Olivine and orthopyroxene display clear lattice-preferred orientation (LPO) and the fabric strengths of olivine and orthopyroxene are 0.13-0.29 and 0.15-0.20, respectively. A-type fabric of olivine was identified, in which is conve the olivine [100] axis is subparallel to the shear direction and the (010) plane is parallel to the shear plane. The A-type fabric of olivine is conventional in lithosphere mantle.In general, exsolution of a mineral is caused by a decrease in solubility due to a change in oxygen fugacity, cooling or decompression with hosted rock exhumation, so the presence of coesite and clinopyroxene esolution lamellae in chromite demonstrates that the Si solubility can be significant under certain conditions. But the previous experiments showed that there is no significant increase at pressure lower than 5GPa and temperature lower than 1500℃.In this paper, experiments were conducted at temperatures between 1000-1600℃and at pressures from 5-15GPa using multi-anvil apparatus. Experimental results suggested that the starting material was first transformed into an assemblage consisting of chromite+garnet+eskolaite+orthopyroxene at pressure of 5GPa in the temperature range of 1000-1200℃and orthopyroxene completely reacted with eskolaite to produce garnet and the assemblage transformed into chromite+garnet+eskolaite with increasing pressure. In the pressure range of 5-8GPa and temperatures above 1400℃, Mg-Si-rich melt was observed. At pressure above 14GPa, chromite decomposed into a new phase and eskolaite coexisting with majorite.The new phase has lower A12O3 (4.70-4.89 wt.%) and Cr2O3 (52.36-52.76 wt.%) content and higher MgO (22.95-32.76 wt.%) and FeO (8.08-18.12 wt.%) content in contrast to chromite, and its chemical formula is closed to stoichiometry of (Fe, Mg)2(Al, Cr)2O5. In addition, compositional analysis by SEM-EDS indicate that the new phase has (Mg+Fe)/(Cr+Al) ratio of 1:1 within analytical errors, consisting with the new high-pressure phase of Mg2Al2O5, so the new phase may be isomorphism of Mg2Al2O5 with modified ludwigite structure. The experimental results suggest that Cr end-member spinel also dissociates into a mixture of Cr2O3 eskolaite and Mg2Cr2O5 the same as Mg2Al2O5 phase, which implies A2B2O5(A:divalent cations; B:trivalent cations) phase with modified ludwigite structure may be a kind of candidate for postspinel transitions in the Earth’s mantle.Electron microprobe analyses showed that Si solubility in chromite increased slightly with pressure and temperature increasing, reached the maximum when chromite coexists with the new phase and decreased obviously in the new phase. There is a negative correlation between Si and Cr+Al and no correlation between Si and Mg+Fe and Si4+ substitution in chromite may be controlled by 4X3+→3Si4++△(Χ:Al, Cr;△:vacancy) substitution mechanism. In most products, SiO2 content in chromite is always lower than 0.7 wt.% and increases slightly with pressure and temperature increasing, but it increases strongly to 2.44 wt.% at 14GPa and 1600℃. In the sample of 14GPa and 1600℃, the phase transition of chromite may cause SiO2 content increase. At about 14GPa, chromite transform into CF phase and abundant Si can be incorporated into chromite. With temperature and pressure decreasing, the SiO2, CaO and MgO would be extracted to form clinopyroxene and MgSiO3 exsolution lamellae from the host chromite with a topotaxial relationship at first. When CaO and MgO are consumed totally, coesite would exsolve in the coesite stability field.The fabric of olivine indicates that hosted mantle peridotite formed at low pressure and temperature, whereas the experimental results show the clinopyroxene, MgSiO3 and coesite exsolved at high pressure. The reason may be the one that the later deformation fabric of olivine which formed at low pressure and low temperature replaced early high pressure fabric.更多还原