240-10 Lawrencite ((Fe,Ni)Cl2) in Apollo Mg-Suite Troctolite 76535 and Volatile Processing on the Moon

Session: From Atoms to Asteroids and Habitable Planets: Coordinated Analysis of Planetary Samples and Their Terrestrial Analogues

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“Rusty” samples from the Moon, such as Apollo sample 66095, bear enrichments in Cl and have been used to interrogate the Moon’s volatile inventory. These samples contain volatile-bearing phases like akaganeite (FeO(OH,Cl)) and lawrencite, and bear a weathered appearance. These phases are proposed to form in lunar samples in several ways: (1) exogenous delivery of volatiles to the Moon, potentially by a cometary impactor; (2) through magmatic processes; (3) via fumarolic activity, where these elements are mobilized by a magmatic or impactor-induced vapor that allows the elements to be re-precipitated in the samples in question; or (4) for akaganeite as terrestrial alteration of primary lunar lawrencite. Determining how these formation models may explain observations of lunar rust and chloride phases is important for understanding the Moon’s volatile inventory and volatile cycling.

Here, we present identification and characterization of magmatic lawrencite in an olivine-hosted melt inclusion in Apollo 17 troctolite 76535. As an Mg-suite rock, 76535 is generally inferred to be a pristine cumulate, with minimal impact processing or alteration. It consists primarily of calcic plagioclase and magnesian olivine, with minor pyroxene and chromite. Large (up to 500 μm) holocrystalline melt inclusions are preserved in the olivine, representing pockets of melt trapped during crystallization that subsequently crystallized due to slow cooling. Within one such melt inclusion, denoted 76535,218-174a, we identified Fe-Ni metal with enrichments in Cl up to 6 wt.%. Coordinated analyses have confirmed the presence of lawrencite and akaganeite using electron backscatter diffraction and transmission electron microscopy.

Our observations reveal the lawrencite to be enriched in Ni and depleted in Fe, relative to adjacent Fe-Ni metal, distinct from lawrencite and akaganeite characterized in Apollo 16 rocks. These observations reveal that the lawrencite in 76535 is magmatic in origin and indicate that Cl liberated from the melt during crystallization likely reacted with the Fe-Ni metal to form the Ni-rich lawrencite. Work is ongoing to determine which Cl-bearing phase was liberated from the melt or if the Cl in the melt reacted directly with the Fe-Ni metal to form lawrencite. Void space/relict vesicles around the Fe-Ni metal may indicate that Cl was liberated as either a halide melt or Cl₂ fluid from the melt, demonstrating the Cl-enriched nature of the magma that formed 76535.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6174

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