Mineralogical, Geochemical and Chronological Study of the lunar fragmental breccia Pakepake_005

Pakepake_005 is a lunar fragmental breccia recovered from the Taklamakan Desert, Xinjiang, China. It exhibits a clastic breccia texture, in which mineral fragments and subordinate lithic clasts are cemented by matrix and impact glass. The dominant phases are plagioclase and pyroxene, whereas olivine is less abundant but widely distributed. Minor to accessory phases include ilmenite, chromite, troilite, phosphates, silica, baddeleyite, armalcolite, and Fe–Ni metal. Lithic clasts comprise impact-melt, plutonic, and basaltic components, as well as symplectites produced by breakdown of pyroxene.

Pyroxene clasts are predominantly subhedral to anhedral and range from ~0.1 to 1 mm in size. A subset exhibits fine clinopyroxene–orthopyroxene exsolution lamellae, with Mg# spanning 15.3–71.4 and locally well-developed Fe–Mg zoning. In contrast, some Fe-rich pyroxenes lack exsolution, are compositionally homogeneous, commonly fractured, and have Mg# values of 6.6–45.2. Some Fe-rich pyroxenes underwent breakdown reactions to form symplectites consisting of augite (Mg# = 30.7–35.5), fayalitic olivine, and quartz, accompanied by minor ilmenite and phosphate minerals. Mg-rich pyroxenes also lack exsolution, are comparatively homogeneous, and have Mg# values of 50.7–74.8.

Pyroxene compositions define two distinct populations on the Fe/(Fe+Mg)–Ti/(Ti+Cr) diagram, indicating multiple sources. The first group shows a positive correlation between Fe/(Fe+Mg) and Ti/(Ti+Cr), consistent with pyroxenes from very low-Ti (VLT) lunar basalts [1]. The second group is characterized by higher Mg# together with relatively elevated Ti/(Ti+Cr), consistent with magnesian pyroxenes crystallized from a more primitive melt. CI-chondrite-normalized REE patterns [2] further indicate that those pyroxenes record at least two sources.

In situ SHRIMP U–Pb geochronology of phosphates and baddeleyite from different components constrains two major events recorded by Pakepake_005. Phosphates hosted in the matrix and impact-derived lithic clasts yield an impact age of 3923 Ma, consistent with the Imbrium basin forming event around ~3.9 Ga[3]. In contrast, phosphates in symplectites and baddeleyite from a VLT clast yield an age of 3486 Ma, documenting a VLT magmatic episode. Taken together, these petrographic, mineral-chemical, and chronological constraints suggest that Pakepake_005 was sourced from an Imbrium-ejecta–related VLT basaltic unit, broadly analogous to basaltic materials exposed in the northern Mare Imbrium region (e.g., east of the Chang’e-3 landing site), where remote-sensing data indicate VLT compositions and yield model eruption ages of ~3.5 Ga for the associated basaltic unit [4].

Acknowledgments: This study was financially supported by National Key R&D Program of China from Ministry of Science and Technology of the People’s Republic of China grant no. 2022YFF0704905, the National Natural Science Foundation of China (NSFC) grant no. 42241107 and the Open Project for Innovative Platform of Meteoritical Research, Shanghai Science and Technology Museum.

[1] Robinson K. L. et al. (2012). Meteoritics & Planetary Science 47: 387–399.[2] Anders E. and Grevesse N. (1989). Geochimica et Cosmochimica Acta 53: 197–214.[3] Nemchin A. A. et al. (2021). Geochemistry 81: 125683.[4] Ji J. et al. (2022). Science Bulletin 67: 1544–1548.