TEM Study of a Metal Grain Associated with Stanfieldite, Akaganeite, and Chromite in the Apollo 16 Sample 66095 'Rusty Rock': Insights into the Origin of the Alteration Event

TEM STUDY OF A METAL GRAIN ASSOCIATED WITH STANFIELDITE, AKAGANEITE, AND CHROMITE IN THE APOLLO 16 SAMPLE 66095 ‘RUSTY ROCK’: INSIGHTS INTO THE ORIGIN OF THE ALTERATION EVENT. M. Martínez1, F. E. Brenker1, A. Schreiber2, and C. K. Shearer3, 1Schwiete Cosmochemistry Laboratory, Dept. of Geoscience, Goethe University Frankfurt, Altenhoeferallee 1, 60438 Frankfurt, Germany ([email protected]), 2GFZ Helmholtz-Center for Geosciences, Chemistry and Physics of Earth Materials, Potsdam, Germany, 3Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico, U.S.A. Introduction: The Apollo Program brought back to Earth 2,196 samples from six different landing sites. Analyses of these samples yield to the first models for lunar origin and evolution, which shaped the standard model of lunar formation. The model assumes a global lunar magma ocean (LMO) that solidified uniformly, followed by an overturn of the cumulates (e.g., [1]). Ongoing research using Apollo samples continues to refine our understanding of lunar science more than five decades after their collection. However, as research advances, the standard model faces more challenges, prompting a reevaluation of the Moon's thermochemical history and the distribution of volatiles in its interior. Indigenous lunar volatiles are transported to the surface primarily by basaltic magmatism (e.g., [2]). Therefore, elemental abundances and the volatile content of deposits relating mare basalts (pyroclastic gas-driven deposits) serve to understand the characteristic of volatiles in the lunar interior and their behaviour during distinct volcanic eruptions (e.g., [310]). The Apollo 16 sample 66095, also called ‘rusty rock’, is a volatile-rich, impact-melt rock that contains a wide variety of lithic clasts. It was noted for its distinctive in situ brown stain rust surrounding the iron metal grains when it was collected on the Moon. Previous work has identified oxyhydration in 66095, which has been thought to represent either terrestrial alteration of lunar chlorides and Fe,Ni metal to akageneite, chloride iron-nickel oxide-hydroxide, or indigenous lunar processes (fumarole activity) [1115]. A detailed examination of the alteration mineral assemblage at the submicron scales is still limited [2]. In the present work, we have investigated Fe,Ni metal grains from a 66095 rock chip with its associated material by Transmission Electron Microscopy to gain additional insights into the ongoing debate about the origin of the alteration (terrestrial vs. lunar). Methodology: A 2 mm-sized rock chip of subsample 66095,436 that remained sealed since its return from the Moon was placed in a glass vial in pristine N2 atmosphere. The cup of the vial was punctured to expose the rock chip to the terrestrial atmosphere for 5 months. After that, the rock chip was placed in epoxy and petrographically polished at Goethe University Frankfurt. A total of 3 FIB sections were extracted from three different Fe,Ni metal grains targeting the surrounding material at University of New Mexico using a FEI Helios NanoLab 650 and at GFZ Potsdam using a FEI Helios G4 UC. The first FIB section was investigated using a Thermo Fisher Talos F200X S/TEM TEM in Goethe University Frankfurt operating at 200 kV. Results: The FIB section displays part of a metal grain, ~7.5 µm in its longest dimension, with parallel linear features. The metal consists of kamacite (Fe0.92Ni0.06Co0.01) and is associated with a slightly amorphous phase in the lower part region, filling a fracture-like feature that is 500 nm - 1µm in width. The slightly amorphous phase has been identified as akaganeite with 2.77 wt% Cl. Akaganeite ranges to Niand Cl-richer (14.7 wt% Ni, 4.14 wt% Cl) within a small region (200 nm x 1μm in size) close to a chromite grain that contains some Ti and Zr (Fig. 1). Kamacite and akaganeite are in contact with a rounded, anhedral phosphate grain, ~9 µm in its longest dimension, which is crosscut by a few fractures. STEM-EDS analyses and SAED patterns show that it consists of stanfieldite, a calcium-magnesium phosphate with a composition of Ca7(Fe1.44Ca0.51Mn0.08)(Mg8.82Fe0.18)[(P11.83Si0.06)O4]12 . Although some areas might be polycrystalline, the stanfieldite is mostly a single crystal with significant textural and compositional complexity, such as localized strain and dislocations seen by BF TEM, slight differences in Z contrast seen by DF STEM, nanoinclusions, and porosity. The porosity is abundant, randomly oriented and distributed, ranging from rounded to euhedral morphologies (~20-200 nm in size), sometimes displaying slightly linear arrays, and as elongated nanotubes up to ~200 nm long. The interface between the stanfieldite and the adjacent material is highly irregular, and the adjacent phases are identified as anorthite with a composition close to the Ca-rich endmember (An92.1-99.6Ab0-7.5Or0-0.4) and forsterite (Fo70). Figure 1. STEM-EDS X-ray compositional map of an area in FIB1 showing Si in purple, P in blue, Cl in red, Cr in light blue, Fe in yellow, and Ni in ochre to show the mineral phase distribution. Discussion: Stanfieldite is a rare phosphate found in some pallasites and mesosiderites [16], associated with alteration of Fe,Ni metal. Stanfieldite identified in the lunar sample 66095 might thus be non-lunar in origin, but its complex nanostructures indicate it experienced dissolution and recrystallization by an alteration event that likely occurred on the Moon. The composition of the Fe-Ni metal grain is consistent with other lunar iron metals, but it could also be derived from an impactor, such as a stonyiron meteorite that collided with the Moon. The chromite has a composition consistent with other lunar chromites [17] and distinct from chromites in mesosiderites or pallasites [18]. The akageneite phase reported here has a similar composition to that reported in [3], but contains a region with significantly higher Ni and Cl content. Akaganeite might have formed by terrestrial alteration through oxidation and hydration of lawrencite, but lawrencite is not found in the mineral assemblage. Alternatively but less likely, if the lunar gas phase changed from Cldominant to OH-dominant, akaganeite could have been produced on the Moon. Conclusions: (1) The mineral assemblage found in the rock chip of 66095 studied here has mineral phases from different origins, but lunar processes are involved and recorded in their nanostructures, especially stanfieldite. (2) The sample was kept sealed since its return from the Moon and opened five months prior to the FIB cut. The sample has thus experienced interaction with Earth’s atmosphere, and akageneite may be a product of this interaction. (3) A second sample that remains sealed in N2 atmosphere will be examined without extensive terrestrial exposure. (3) This study illustrates the importance of developing strategies for the return of volatile-bearing samples from the South Pole of the Moon by Artemis. References: [1] Elkins-Tanton, L.T. et al. (2011) EPSL 304, 326-336. [2] Shearer C. K. et al. (2014) GCA 139, 411-433. 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