53-4 Constraints on Young, KREEP-rich Lunar Magmatism from 207Pb/206Pb SIMS Geochronology of the Lunar Basaltic Meteorite Northwest Africa 16895

Abstract:

Recovered in 2023, Northwest Africa (NWA) 16895 is a lunar basaltic meteorite dominantly consisting of compositionally-zoned augite and laths of plagioclase and ilmenite. Initially classified as a martian augite basalt, it was reclassified following updated oxygen isotope analyses, in agreement with the high An# in plagioclase (93.0 ± 1.8) , high Fe/Mn ratios in silicates (95 ± 7 in olivine; 60 ± 5 in clinopyroxene), and lack of magnetite despite abundant ilmenite.

A 0.94-gram subsample of NWA 16895 was first characterized by backscatter electron imaging and energy-dispersive X-ray spectroscopy point analyses using a Tescan Vega-3 XMU variable pressure scanning electron microscope at the University of California, Los Angeles (UCLA) to identify minor and accessory phases suitable for geochronology, including baddeleyite (ZrO₂), chlorapatite (Ca₅(PO₄)₃Cl), and Si-Al-K-rich mesostasis.

Pb-isotopic analyses of these phases were then conducted using the Cameca ims-1290 secondary ion mass spectrometer (SIMS) at UCLA. A ^₂₀₇Pb/²⁰⁶Pb isochron age for NWA 16895 was determined using an inverse isochron to plot ²⁰⁷Pb/²⁰⁶Pb against ²⁰⁴Pb/²⁰⁶Pb to constrain the crystallization age and initial Pb-isotopic composition of NWA 16895. The data were reduced to limit terrestrial ²⁰⁴Pb influence, yielding a ²⁰⁷Pb/²⁰⁶Pb inverse isochron age of ~1.96 Ga (n = 11; MSWD = 1.3; p(χ²) = 0.21). The inverse isochron demonstrates a low influence of terrestrial 204Pb contamination and an enriched, KREEP-like initial-Pb value.

Shock petrographic estimates suggest that this age represents the primary crystallization age of NWA 16895 from a basaltic melt with a KREEP-rich component. The increasing sample suite of young, ~2 Ga basaltic lunar samples with variable KREEP compositions–including the Chang’e 5 and 6 basalts and the lunar basaltic meteorite NWA 16286–continue to constrain the timing, modes, and mechanisms of late lunar magmatism and the variable incorporation of KREEP-rich components. These developments continue to improve our understanding of the processes and timescales of prolonged lunar magmatism.

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.