Late-stage effusive volcanism on Mercury: Evidence from Mansurian impact basins Jack Wright1, David A. Rothery1, Matt R. Balme1 and Susan J. Conway2

1The Open University, Milton Keynes, MK7 6AA, UK Email: [email protected] 2LPG Nantes - UMR CNRS 6112 Université de Nantes, France

Introduction Young, post-impact volcanism Fig. 1. Time systems of Mercury and ? Widespread Volcanism ? conventional absolute model ages. We are looking for geological evidence for volcanism in Mansurian impact basins Tolstojan Tolstojan

[1] (>100 km). This will tell us about how plains volcanism ended on Mercury Calorian

during an era of global cooling and contraction [2]. Within the Mansurian, we Pre- Hermean predict that older, larger basins host more volcanism than younger, smaller ones. Kuiperian MANSURIAN System We expect there came a time when impacts could no longer liberate magma from Mercury's interior, marking the end of effusive volcanism. Time before 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 present (Ga) Colour Boundaries Pyroclastics & Edifice Shield volcano talk tomorrow in Waterway 1 @ 10.00 am! 18°E 20°E 22°E 54°E 56°E 58°E 52°E 56°E 60°E 120°E 122°E 124°E 123°E A B C A B D 1000 34°S ± 800 30°N ± 600 ± 2020 kmkm 400

8°S ± ± 200 14°S 34°S 0

C 0 5 10 15 20 25 30 relative relative elevation m / 34°S 26°N ± A distance along profile / km A'

10°S 2020 kmkm 200200 kmkm 100100 kmkm

36°S 123°E 16°S -5 km Mercury global topography +4 km 100100 kmkm 100100 kmkm Fig. 6. A: Unnamed basin with pyroclastic deposit (arrow). B: ! monochrome, and C: enhanced colour views of candidate shield volcano Fig. 2. Often, peak-rings confine a distinct colour unit. A: Derain, with 60°N ! 60°N [3]. D: x10 vert. exag. profile of another candidate shield on Mercury [3]. ! Ahmad Baba ! pyroclastics. B: Nabokov. C: Rachmaninoff, with pyroclastics. Rustaveli ! ! ! ! ! ! ! ! Strindberg The smooth infills of many basins have Sharp colour boundaries within basin fills ! superposing pyroclastic deposits. The same Rachmaninoff ! 30°N ! ! 30°N suggest lava flow units, rather than impact ! volcanic plumbing system could have Vivaldi ! melt. ! ! ! delivered the earlier smooth plains material. ! ! ! 0° Derain ! 0° ! Nabokov Steichen ! ! Ghost Craters ! Infill Thickness Unnamed ! ! 76°E 78°E 30°S ! 30°S ! ! 88°W 86°W 84°W 80°E 82°E 84°E 86°E

! ! ! ! A B 16°N Fig. 3. Steichen. One of ! !

! 54°N the most heavily 60°S ! ! 60°S ± ± degraded craters in this 12°S ± study. Smooth plains 0° 90°E 180° 270°E 0° 14°N deformed by wrinkle Fig. 4. Mansurian basins >100 km in diameter (black dots, n=43). 52°N ridges cover the basin 140°W 132°W 124°W floor. Arrows indicate Relative Age Dating 12°N ghost craters. 100100 kmkm 100100 kmkm 14°S 58°N ± Superposition relationships show the order in which Fig. 7. A: Vivaldi, which has one of the lowest d/D ratios in the study. 5050 kmkm some basins formed. We can B: Rustaveli, with deeply buried peak elements.

'Ghost' craters have been buried by smooth plains. 54°N use them to calibrate a Some basins have deeply buried peak materials, This requires a delay between the original impact degradation scheme for suggesting volcanic flooding. Volcanic infilling will 200200 kmkm and plains formation for craters to accumulate on within the Mansurian. decrease the depth (d) of a basin without changing the unflooded floor, ruling out burial by impact melt. Fig. 5. Secondary craters (arrows) from Ahmad Baba (upper-right) its diameter (D). Mansurian basins with unusually We find ghost craters to be rare within our basins. superpose Strindberg (lower-left) ejecta and smooth infill. low d/D ratios are likely to host a thick volcanic unit. Future Work We will compile each of these lines of evidence for post-impact volcanism and assess if there is a relationship to basin size and age. If there is no apparent relationship, then cooling and contraction cannot be the only controlling factors. We can plot the locations of post-impact volcanism against crustal thickness [4] to see if volcanism preferentially occurs in areas with thin crust. Post-impact volcanism may also be controlled by a heterogeneous distribution of heat-producing elements within Mercury, upwellings in the mantle or even the local depth of the core-mantle boundary.

References Acknowledgements [1] Kinczyk M. J. et al. (2016) LPS XLVII, #1573. [2] Byrne P. K. et al. (2016) Geophys. Res. Lett., 43, J. Wright is funded by UK Science and Technology 7408-7416. [3] Wright J. et al. (2017) LPS XLVIII, #1871. [4] Smith D. E. et al. (2012) Science, 336, Facilities Council training grant ST/N50421X/1. J. Wright is 214-217. grateful to the UK Remote Sensing and Photogrammetry Society for a travel bursary to attend LPSC 2017.