P53A-2100
Widespread Plains Volcanism on Mercury Ended by 3.6 Ga

Friday, 18 December 2015
Poster Hall (Moscone South)
Paul K Byrne1,2, Lillian Rose Ostrach3, Caleb Fassett4, Clark R Chapman5, Alexander J Evans6, Christian Klimczak1,7, Maria E Banks8,9, James W Head III10 and Sean C Solomon1,11, (1)Carnegie Institution of Washington, Department of Terrestrial Magnetism, Washington, DC, United States, (2)North Carolina State University Raleigh, Marine, Earth, and Atmospheric Sciences, Raleigh, NC, United States, (3)NASA Goddard Space Flight Center, Greenbelt, MD, United States, (4)Mount Holyoke College, Department of Astronomy, South Hadley, MA, United States, (5)Southwest Research Institute, Boulder, CO, United States, (6)Lamont -Doherty Earth Observatory, Palisades, NY, United States, (7)University of Georgia, Department of Geology, Athens, GA, United States, (8)Smithsonian Institution, National Air and Space Museum, Center for Earth and Planetary Studies, Washington, DC, United States, (9)Planetary Science Institute Tucson, Tucson, AZ, United States, (10)Brown University, Providence, RI, United States, (11)Lamont-Doherty Earth Observatory, Palisades, NY, United States
Abstract:
The largest volcanic plains deposits on Mercury are situated in its northern hemisphere and include the extensive northern smooth plains and the Caloris interior plains. Crater size–frequency analyses have shown that both deposits were emplaced around 3.8 Ga, for any of the published model production function (MPF) chronologies for impact crater formation on Mercury. The largest volcanic deposit in the southern hemisphere, the Rembrandt interior plains, has a model age of ~3.7 Ga. To test the hypothesis that all major volcanic smooth plains on Mercury were emplaced at about the same time, we determined crater size–frequency distributions for nine additional deposits (see Table 1). The diameters of craters that superpose the smooth plains at each site were measured with CraterTools, yielding crater areal densities in terms of N(10), the number of craters ≥10 km in diameter per 106 km2 area (Table 1). Our crater density measurements span N(10) values of 29–146, a range that encompasses corresponding values for the larger areas of smooth plains. With CraterStats, we fit our data (for craters ≥4 km in diameter) to the MPF chronologies of Le Feuvre and Wieczorek. For porous scaling, the model ages of all nine sites span a narrow window (Table 1). Non-porous scaling fails to match the crater size–frequency distributions. We show that widespread plains volcanism, likely the primary process by which Mercury’s crust developed, had ended by 3.6 Ga. Younger volcanic deposits have been identified on the planet, but only within impact structures and at volumes much less than the smallest deposit considered here. Superposition relations between shortening landforms and craters on Mercury indicate that global contraction in response to interior cooling was underway by ~3.6 Ga. The cessation of widespread plains volcanism on Mercury may therefore reflect the onset of a stress state within the planet’s lithosphere that inhibited magma ascent. Conversely, mantle thermochemical evolution models indicate that magma generation may have been voluminous only until ~3.5 Ga. Whatever the cause, the main building phase of Mercury’s crust ended within the first 20% of the age of the planet, with only small-scale, explosive volcanism enduring beyond that time.