Nanoscale Vibrational Electron Energy Loss Spectroscopy (vibEELS) of Lunar Highland Finest Fraction

Session: Advancing Mineralogy and Spectroscopy Across the Solar System in Honor of MSA Roebling Medalist M. Darby Dyar

Presenting Author:

Kenneth Livi

Authors:

Livi, Kenneth J. T.¹, Ramasse, Quentin², Kepaptsoglou, Demi³, Ramprasad, Tarunika⁴, Cahill, Joshua T.S.⁵, McCanta, Molly Coleen⁶, Dyar, Darby⁷

(1) Materials Characterization & Processing, Johns Hopkins University, Baltimore, MD, USA, (2) SuperSTEM Facility, Daresbury, Daresbury, United Kingdom, (3) SuperSTEM Facility, Daresbury, WA44AD Daresbury, United Kingdom, (4) Material Characterization and Processing, Baltimore, MD, USA, (5) Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA, (6) University Tennessee Knoxville, Knoxville, TN, USA, (7) Planetary Science Institute, Tuscon, AZ, USA,

Abstract:

The original anorthite-dominated rocks of the lunar highlands have been bombarded by solar radiation, cosmic rays, charged particles, comets, meteorites, and micrometeorites for over 4 billion years, resulting in impact products at many scales. The finest fraction of this lunar regolith includes both endogenous and exogenous materials. A majority are the former: micro- to nano-sized crystalline fragments of the original minerals, shocked and amorphized grains by solar and cosmic radiation, vapor-deposited glass, impact splash, volcanic glass spheres, micro- and nano-scale meteorites and extraterrestrial particles. The finest fractions of lunar regolith are challenging to interpret due to grain size, thermal, and space weathering product effects that contribute to bulk spectra. Until recently, spectroscopy on the individual components at scales of causality (nanometer level) was intractable.

 

For the first time, we apply vibEELS to three Apollo 16 samples of differing space weathering maturities (Is/FeO). Apollo 62231 is mature (91), 61141 sub-mature (56), and 61221 immature (8.2). Identified components in the soils include crystalline anorthite, amorphous CaAl₂Si₂O₈ (maskelynite) glassy rims with/without iron nanoparticles (FeNPs), micrometeorites, and glass spheres. Mid/near IR (MNIR) ‘aloof’ spectra were collected from edges of all components. Crystalline anorthite spectra reproduce positions of the five clusters of MNIR absorption peaks -(217, 363, 548, 750, 976-1049 cm^-1) at slightly lower resolution than FTIR. Loss of crystalline structure causes the split peak at ~1100 cm^-1 to broaden, merge and decrease in intensity. Also, the peak at ~550 cm^-1 drops dramatically in intensity. The addition of FeNPs within the amorphous material flattens the spectra, leaving only the 1100 cm^-1 peak. The Christiansen Feature position appears to be affected by crystallinity, glass composition and presence/absence of FeNPs.

 

This project catalogues MNIR spectra of lunar finest fraction components and details in an itemized fashion the effects of space weathering observed in anorthositic substrates at nanometer scales. VibEELS is exquisitely well suited for examination of lunar finest fraction and brings planetary events and materials mixed into this fraction into new focus and perspective. Combining vibEELS data with STEM compositional mapping, high resolution TEM, and electron diffraction can yield a uniquely detailed description of complex regolith mixtures.