Ultra‐reduced phases in Apollo 16 regolith: Combined field emission electron probe microanalysis and atom probe tomography of submicron Fe‐Si grains in Apollo 16 sample 61500

Abstract

The lunar regolith contains a variety of chemically reduced phases of interest to planetary scientists and the most common, metallic iron, is generally ascribed to space weathering processes (Lucey et al. 2006). Reports of silicon metal and iron silicides, phases indicative of extremely reducing conditions, in lunar samples are rare (Anand et al. 2004;Spicuzza et al. 2011). Additional examples of Fe-silicides have been identified in a survey of particles from Apollo 16 sample 61501,22. Herein is demonstrated the utility of low keV electron probe microanalysis (EPMA), using the Fe Ll X-ray line, to analyze these submicron phases, and the necessity of accounting for carbon contamination. We document four Fe-Si and Si 0 minerals in lunar regolith return material. The new Fe-Si samples have a composition close to (Fe,Ni) 3 Si, whereas those associated with Si 0 are close to FeSi 2 and Fe 3 Si 7 . Atom probe tomography of (Fe,Ni) 3 Si shows trace levels of C (60 ppma and nanodomains enriched in C, Ni, P, Cr, and Sr). These reduced minerals require orders of magnitude lower oxygen fugacity and more reducing conditions than required to form Fe 0 . Documenting the similarities and differences in these samples is important to constrain their formation processes. These phases potentially formed at high temperatures resulting from a meteorite impact. Whether carbon played a role in achieving the lower oxygen fugacitiesand there is evidence of nearby carbonaceous chondritic material-it remains to be proven that carbon was the necessary component for the unique existence of these Si 0 and iron silicide minerals.

A Low O/Si Ratio on the Surface of Mercury: Evidence for Silicon Smelting?

“…Much of what is known about space weathering products was realized through the study of lunar samples. Based on lunar sample studies, one of the primary metallic byproducts of space weathering is iron metal (Keller & McKay, ; Keller & McKay, ; Noble et al, ; Taylor et al, ), although iron silicides (e.g., hapkeite) and rare metallic Si also make up a minor fraction of metallic space weathering products (Anand et al, ; Gopon et al, ). The iron metal produced through space weathering occurs along a continuum of nanophase iron particles that coarsen through Ostwald ripening to larger particles (Lucey & Noble, ; Noble et al, ; Pieters et al, ; Pieters & Noble, ).…”

“…products (Anand et al, 2004;Gopon et al, 2017). The iron metal produced through space weathering occurs along a continuum of nanophase iron particles that coarsen through Ostwald ripening to larger particles (Lucey & Noble, 2008;Noble et al, 2007;Pieters et al, 2000;Pieters & Noble, 2016).…”

“…Even if all the Fe on Mercury was originally FeO that converted to Fe metal through space weathering, it would only account for 0.25–0.57 wt % O. Although iron silicides and metallic Si make up a minor volume fraction of space weathering products in lunar samples (Anand et al, ; Gopon et al, ), Gopon et al () report that the presence of trace amounts of reduced carbon in the lunar regolith may have aided in the production of iron silicides and metallic Si produced by space weathering. Consequently, the apparently graphite‐rich nature of the Mercurian regolith and the extreme space weathering environment on Mercury both support the possibility that space weathering is a viable process by which oxygen could have been lost from a physicochemical standpoint.…”

Possible records of space weathering on Vesta: Case study in a brecciated eucrite Northwest Africa 1109

“…) and Apollo 16 sample 61500 (Gopon et al. ) indicates that space weathering products are not necessarily nanoscale in grain size. They could be large up to tens of micrometers (Anand et al.…”

“…Besides npFe 0 , recently, Gu et al (2018) reported the presence of silicon oxide nanoparticles (10-25 nm in grain size) in Apollo 15 lunar soil. Although typical space weathering products are on nanometer scale, the discovery of hapkeite (Fe 2 Si) in lunar meteorite Dhofar 280 (Anand et al 2004) and Apollo 16 sample 61500 (Gopon et al 2017) indicates that space weathering products are not necessarily nanoscale in grain size. They could be large up to tens of micrometers (Anand et al 2004).…”