Microstructure, chemistry, and origin of grain rims on ilmenite from the lunar s

2019-10-10 13:11:51

inner fraction ilmenite sublayer rims

责任者: Christoffersen, R.;Keller, L.P.;McKay, D.S. 单位: NASA Johnson Space Center, Houston, TX, USA 来源出处: Meteoritics Planetary Science(Meteorit. Planet. Sci. (USA)),1996/11/,31(6):835-48 摘要: Analytical transmission electron microscope (TEM) observations reveal that ilmenite grains sampled from the sub-10 μm size fraction of Apollo 11 (10084) and Apollo 16 (61221, 67701) soils have rims 10-300 nm thick that are chemically and microstructurally distinct from the host ilmenite. The rims have a thin outer sublayer 10-50 nm thick that contains the ilmenite-incompatible elements Si, Al, Ca and S. This overlies a relatively thicker (50-250 nm) inner sublayer of nanocrystalline Ti-oxide precipitates in a matrix of single-crystal ilmenite that is structurally continuous with the underlying host grain. Microstructural information, as well as data from X-ray spectrometry (EDS) and electron energy loss spectrometry (EELS) analysis of the inner sublayer, suggest that both the inner and outer sublayer assemblages are reduced and that the inner layer is depleted in Fe relative to the underlying ilmenite. The chemistry of the outer sublayer suggests that it is a surface deposit of sputtered or impact-vaporized components from the bulk lunar soil. The inner sublayer is part of the original host grain that has been physically and chemically processed, but not amorphized, by solar ion irradiation and possibly some subsolidus heating. The fact that the deposited outer sublayer is consistently much thinner than the radiation-altered inner sublayer indicates that only a minor fraction of the total rim volume is a product of vapor or sputter deposition. This finding is in contrast to recent descriptions of thick deposited layers on one-third of regolith silicate grains and indicates that ilmenite and silicate rims as a group are different in the fraction of deposited material that they contain 关键词: electron energy loss spectra;lunar rocks;minerals;X-ray spectroscopy;microstructure;chemistry;origin;grain rims;ilmenite;lunar soil finest fraction;transmission electron microscope observations;Apollo 11 soils;Apollo 16 soil;6770;61221;10084;nanocrystalline Ti-oxide precipitates;X-ray spectrometry;electron energy loss spectrometry;surface deposit;sputtered component;impact-vaporized components;inner sublayer;solar ion irradiation;subsolidus heating;regolith;Si;Al;Ca;S;Ti