Mineral chemistry and in situ UPb geochronology of the mare basalt Northwest Africa 10597: Implications for low-Ti mare volcanism around 3.0 Ga
Yunhua Wu, Weibiao Hsu
In Press, Journal Pre-proof, Available online 4 November 2019
• NWA 10597 is a newly found unbrecciated low-Ti mare basalt, most likely paired with NWA 4734.
• In situ UPb analyses of baddeleyite and apatite reveal a mutually consistent age of ~ 3.0 Ga, implying the apatite UPb isotopic system was not severely disturbed although NWA 10597 was heavily shocked.
• Rare earth element (REE) distribution feature indicates the REE enrichment of NWA 10597 is inherited from its mantle source.
• One baddeleyite yielded a young age of 2.8 Ga which may indicate a thermal event on the Moon.”
“Unbrecciated mare basalts are rare in the lunar meteorite collection. Found in 2015, Northwest Africa (NWA) 10597 is a medium-grained low-Ti mare basalt with a subophitic texture. The meteorite consists dominantly of mm-sized pyroxene and plagioclase, with minor olivine, spinel, ilmenite, phosphates, silica, and trace Zr-rich minerals, such as baddeleyite, zirconolite and tranquillityite. A portion of plagioclase and silica has been transformed to their high-pressure polymorphs due to shock metamorphism. NWA 10597 has a low TiO2 content (2.9 wt%) but is relatively enriched in rare earth elements (REE) (Laaverage = 65 × CI) with an overall unfractionated pattern except for a negative Eu anomaly. Calculated REE concentrations of parent melts in equilibrium with Mg-rich pyroxene and Ca-rich plagioclase suggest no significant assimilation of REE-rich melts after the onset of pyroxene crystallization. In situ UPb analyses of baddeleyite and apatite reveal a mutually consistent age of ~3.0 Ga, which is also in excellent agreement with that of low-Ti mare basalts NWA 4734 and LaPaz Icefield (LAP) 02205 dated with other independent techniques. The concordance suggests no significant thermal disturbance in the UPb isotopic system of NWA 10597 although it was heavily shocked. NWA 10597 closely resembles NWA 4734 in terms of petrographic texture, mineral chemistry and geochronology, indicating a pairing relationship.”