The Effect of Evolving Gas Distribution on Shallow Lunar Magmatic Intrusion Density: Implications for Gravity Anomalies

Introduction: The GRAIL (Gravity Recovery and Interior Laboratory [1]) mission has enabled unprecedented views of the lunar crust and interior structure. Indeed, the data solutions now resolve gravity anomalies arising from lateral heterogeneities within the crust globally at scales < 10 km[2,3]. Intrusive magmatic features such as dikes and sills are predicted to be prevalent within the lunar crust [4] and gravity data present a unique way of investigating their presence. Recently, GRAIL data were used to interpret the presence of large, ancient dikes within the lunar crust [5]; however, as yet, the detection of smaller scale dikes and sills has proven difficult due to their moderate spatial dimensions and uncertain density contrast. Recent morphologic studies [6, 7] have supported the hypothesis that members of the class of lunar craters known as floor-fractured craters (FFCs) were formed by the intrusion and evolution of a shallow magmatic body beneath the crater. The presence of a large magmatic intrusion beneath these craters should be easily visible in the observed crater Bouguer gravity anomaly. Using the FFC Alphonsus as an example, we examine the observed Bouguer anomaly and compare this to idealized predictions of the Bouguer anomaly, which are based on predicted intrusion thickness from crater morphology [6, 7]. We also examine processes within the magmatic intrusion which could alter the intrusion density, and hence affect the Bouguer anomaly. Specifically, we examine how volatiles within the intrusion and intrusion degassing affect the overall intrusion density. Comparison of Predicted and Observed Gravity Signature: The crater Alphonsus, D = 119 km, (13.4°S, 2.8°W) is a class 5 FFC [6], possessing a flat floor profile and numerous dark-halo craters [8] identified as pyroclastic deposits [9] located along the crater floor fractures (Fig. 1a). Using LOLA (Lunar Orbiter Laser Altimeter) data, we measure the crater depth as ~2.7 km, compared with 4.4 km depth suggested by the empirically derived crater depth v. diameter relationships developed by Pike (1980) [10]. We thus infer a maximum intrusion thickness of 1.69 km, ~1.7 km, beneath the crater [6]. We assume a lunar magma density of 3000 kg/m 3 [11] and a lunar crustal density of 2550 kg/m 3 [12]. Assuming the sill morphology can be modeled approximately as a Bouguer plate, an intrusion of constant thickness and large extent will generate a positive Bouguer anomaly of ~30 mGal.