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Space weathering effects in the lunar regolith alter the surfaces of ilmenite grains resulting in “reduced” rims that contain Ti3+ and nanophase Fe0 [1, 2]. The Ti/Fe valence distribution as a function of depth of the rims is largely unknown. We used scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS) to characterize the nm-scale Ti/Fe valence distribution across the ilmenite rims, to gain new insights into the formation of the rims and lunar regolith processes.
The space weathered ilmenite rims are typically 50 to 150 nm wide and have nanophase Fe0 concentrated within the uppermost 50 nm of the rims. STEM-EELS and x-ray mapping reveal that these Fe metal grains are surrounded by Fe-depleted and Ti-rich regions which could result from either reduced Ti oxides or Fe-depleted ilmenite. The highest Ti3+ concentration is observed in these regions (Ti3+/Titot=0.50~0.60) and decreases with depth. Ti3+ is still present at depths of ~100 nm (Ti3+/Titot=0.3) although nanophase Fe0 is not observed. High resolution images shows that some platelet microstructures (10 nm x1 nm) are aligned parallel to the c axis of the ilmenite matrix. These platelets are likely exsolved Ti-oxides which would account for the Ti valence variation in the Fe0 precipitate-free zone. The mechanisms accounting for the different correlations between Ti and Fe valence variations with depths of the ilmenite rims are under investigation.
[1] Keller L. P. et al., (1995) LPSC XXVI, 729. [2] Christoffersen, R. et al., (1996) Meteoritics 31, 835.
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