| 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.
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