Study On Meteoritic Shocked Genesis Of Pure Metallic Iron

Fe element is an important valence variation element in the solar system,its valence state can imply the redox condition of the system,and its form of occurrence in extraterrestrial samples can efficiently indicate geological evolution of the planetary bodies.Pure metallic iron as the main product of the space weathering has an important significance for understanding the space weathering on the surface of celestial bodies?especially the airless bodies?.At present,the presence of metallic iron has been detected in Apollo lunar soil particles,lunar rocks,lunar meteorites and Martian meteorites.The formation of these metallic iron particles was considered to relate to the extreme reduction conditions caused by meteorite impact and other factors.This paper is based on the previous research,mainly focus on the pure metallic iron in ordinary chondrites and HED clan of meteorites to do research.By means of the Scanning electron microscope?Electron microprobe?Laser Raman Spectrometers and Transmission Electron Microscope,the samples of ordinary chondrite?GRV 022115?and HED meteorites?NWA 11592?were analyzed,so as to determine the form of occurrence and the process of elemental migration for pure metallic iron in each sample.Finally,the following understanding on the formation mechanism of pure metallic iron were obtained:?1?Grove mountains 022115?GRV 022115?ordinary chondrite has an extreme shocked characteristic.Majorite,magnesiowüstite and ringwoodite high-pressure mineral assemblage was observed in the matrix of shock-induced melt vein in GRV 022115,indicating the equilibrium temperature and pressure conditions of the vein over 1800?and20-23 GPa.The pure metallic iron particles associating with the high pressure clinoenstatite?HP-CEn?,silica glass and vesicles appear in pyroxene glass at the edge of one big host rock fragment in a wide melt vein,implying the decomposition of pyroxene.According to the quantitative analysis of the environment in which pure metallic iron exist,the Fe element in the pure metallic iron is derived from the host rock pyroxene.Combining with other phases,the following reaction can be concluded:?Mg,Fe?SiO₃?Mg²⁺_{?in HP-CEn?}+Fe+SiO₂+1/2O₂?According to the formation condition of this reaction,high temperature and quenching are necessary for the formation of pure metallic iron.?2?Howardite-Eucrite-Diogenite?HED?clan of meteorites are generally considered to be derived from the Vesta asteroid,and mainly composed of pyroxene and plagioclase.Here we analyzed five HED meteorites,the Fe/Mn values of pyroxene indicating these samples come from the Vesta.According to the composition data of the exsolved pyroxene in those samples,the temperature of thermal metamorphism on Vesta is ranging from 700?to 1000?.?3?Northwest Africa 11592?NWA 11592?is a basaltic Eucrite meteorite,thin melt veins and one wider melt pocket were observed in this sample.The melt pocket is characterized by the quenching texture:pure metallic iron particles?²00nm?and the needle-like plagioclase embedded by abundant Nano-scaled hercynite grains.Based on the element features in the melt pocket,the reaction can be inferred as:Low-Ca pyroxene+plagioclase?pure metallic iron+hercynite+special phase.the crystallization of hercynite indicating the temperature of this reaction is ranging from 1310?to 1750?.The relatively larger size of those pure iron particles in NWA 11592 suggesting the quenching duration is important for the growth of pure metallic iron grains during shock process.?4?Ordinary chondrites correspond to the S-type asteroid,while the HED clan of meteorites were derived from Vesta asteroid.Because the Vesta underwent the process of magmatic differentiation and thermal metamorphism,compared with the S-type asteroid,Vesta has less possibility to produce the pure metallic iron under the same meteorite or micrometeorite impact conditions.This is consistent with the actual observed reflectance spectra of bodies.