An Intrusive Origin for Lunar Mascons: Magma Ascent Theory, Gravitational Signatures, and Tests for Grail

Introduction: The origin of the large gravity anomalies associated with lunar impact basins, or " mascons " , has been a matter of debate since their discovery at the dawn of the age of lunar exploration. More recently, studies of basin structure have led to a prevailing model for producing the anomalies: super-isostatic uplift of the crust-mantle boundary, frozen in at the time of the basin impact [e.g., 1]. However, recent studies show that the immense amount of heat delivered to the surface of the Moon by basin-forming impacts [e.g., 2] makes it difficult to support su-perisostatic topography in the immediate post-impact era [3]. Thus, it is more likely that the super-isostatic load was emplaced after the central basin lithosphere had thickened (cooled) enough to support it [3]. Insights from intrusion theory: Studies of intrusive volcanism suggest a role for intrusive bodies as the hidden component of the broader volcanic system that is expressed at the surface as mare volcanism. For example, a compilation of magma emplacement and volcanic output gives ratios of intrusive to extrusive volumes in the ranges 5:1 for oceanic settings and 10:1 for continental ones [4]. While caution should be used in applying a rule of thumb derived from conventional terrestrial situations to the quite different tectonic (both small-and large-scale) and magmatic environment of the Moon, the existence of at least comparable volumes of magma at depth beneath basin-filling mare is quite plausible through this finding. Further, the em-placement of mare at the surface creates a flexural stress trap for magma in the mid-lithosphere [e.g., 5]. Tests for intrusive mascon formation from GRAIL: Test 1: annular mascon signals. Mid-lithosphere intrusions can contribute to gravity anomalies via its own gravity signal and that of the surface. Caculations of the topographic effects of a sub-surface sill-like intrusion with diameter comparable to the mascon gravity signal (Fig. 1) show a central uplift with a flanking trough. If the basin fills with basalts to a more or less flat surface, the trough creates a basis for an annular signal. Strictly speaking, if the surface deformation were the same as the sill dimensions, there would be a nearly disk-like signal, but stiffness of material above acts as a plate that filters the response, so instead a strongly annular signal may be generated. Such a signal would not be expected from super-isostatic uplift, providing a way to distinguish these mascon models. Test …