Lunar Silicic Magma Genesis: Insights from Rhyolite-MELTS Modeling
Abstract
Lunar silicic volcanic landforms are rare and are predominantly distributed along the boundary of the thermally and chemically anomalous geologic province, the Procellarum KREEP (potassium (K), rare earth elements (REE), and phosphorus (P)) Terrane (PKT). The presence of silicic volcanic landforms on the Moon is enigmatic given that the key ingredients (i.e., water and plate tectonics) required for producing large-scale silicic melts were produced is essential to understanding the thermal evolution of the PKT and the Moon as whole. While a few grains of silicic materials were returned by the Apollo missions, none of the Apollo or Luna missions visited a silicic target, thus our knowledge of these terrains in limited to remote sensing observations, thereby motivating the need for petrological modeling, which provide constraints on the P-T-x evolution of these constructs and by extension provide insights into the thermal history and differentiation of the Moon. Therefore, we conducted rhyolite-MELTS [1] models to mimic fractional crystallization of KREEP basalts and compared the resulting product to that of returned lunar silicic fragments.
Based on our rhyolite-MELTS model [1], >70% fractional crystallization of KREEP basalts produce silicic melts with SiO2 > 68 wt% and total alkali (Na2O + K2O) > 6 wt%, consistent with returned Apollo samples [2]. Previous studies testing the fractional crystallization hypothesis showed that protoliths of alkali gabbronorite composition produce only 5% silicic melt, whereas protoliths of monzogabbro composition produce 40% melt of intermediate SiO2 wt%, which are not representative of the Apollo sample suite [3]. Results from our rhyolite-MELTS models show that fractional crystallization of KREEP basalts produce up to 30% melt with silicic and alkali compositions comparable to returned Apollo samples. We have demonstrated that KREEP basalts were likely the parent magmas for silicic melts on the Moon rather than alkali gabbronorite or monzogabbro identified by previous work [3]. References: [1] Gualda G.A.R. et al. (2012) J. Petrology, 53, 875-890 [2] Seddio S.M. et al. (2013) Am. Min., 98, 1697-1713 [3] Gullikson A.L. et al. (2016) Am. Min., 101, 2312-2321- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2022
- Bibcode:
- 2022AGUFM.U25B0497R