Explosive volcanic eruptions on Mercury: Eruption conditions, magma volatile content, and implications for interior volatile abundances Laura Kerber a, ⁎, James W. Head a , Sean C. Solomon b , Scott L. Murchie c , David T. Blewett c , Lionel Wilson d a Geological Sciences Department, Brown University, 324 Brook Street, Box 1846, Providence, RI 02912, USA b Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington, DC 20015, USA c The Johns Hopkins University Applied Physics Laboratory,11100 Johns Hopkins Road, Laurel, MD 20723, USA d Environmental Science Department, Lancaster University, Lancaster LA1 4YQ, UK abstract article info Article history: Accepted 27 April 2009 Available online 29 May 2009 Editor: T. Spohn Keywords: Mercury pyroclastic volcanism volatile accretion interior MESSENGER Images obtained by the MESSENGER spacecraft have revealed evidence for pyroclastic volcanism on Mercury. Because of the importance of this inference for understanding the interior volatile inventory of Mercury, we focus on one of the best examples determined to date: a shield-volcano-like feature just inside the south- western rim of the Caloris impact basin characterized by a near-central, irregularly shaped depression sur- rounded by a bright deposit interpreted to have a pyroclastic origin. This candidate pyroclastic deposit has a mean radius of ~24 km, greater in size than the third largest lunar pyroclastic deposit when scaled to lunar gravity conditions. From the extent of the candidate pyroclastic deposit, we characterize the eruption param- eters of the event that emplaced it, including vent speed and candidate volatile content. The minimum vent speed is ~ 300 m/s, and the volatile content required to emplace the pyroclasts to this distance is hundreds to several thousands of parts per million (ppm) of the volatiles typically associated with pyroclastic eruptions on other bodies (e.g., CO, CO 2 ,H 2 O, SO 2 ,H 2 S). For comparison, measurements of the exsolution of volatiles (H 2 O, CO 2 , S) from basaltic eruptive episodes at Kilauea volcano, Hawaii, indicate values of ~1300–6500 ppm for the terrestrial mantle source. Evidence for the presence of significant amounts of volatiles in partial melts derived from the interior of Mercury is an unexpected result and provides a new constraint on models for the planet's formation and early evolution. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The planet Mercury is generally thought to be deficient in interior volatiles compared with the other terrestrial planets (e.g., Boynton et al., 2007). Numerical simulations of planetary accretion indicate that Mercury is likely to be dominated by material formed in the inner solar nebula (Wetherill, 1994), where temperatures were compara- tively high and volatile species remained in the gas phase through- out the time interval when nebular gas was present (e.g., Boss, 1998; Chambers, 2005). Several of the scenarios proposed to account for Mercury's anomalously high bulk density (and inferred high ratio of metal to silicate) involve one or more episodes of further heating, either by the nebula itself (Cameron, 1985; Fegley and Cameron, 1987) or as a result of collision with another large object (Wetherill, 1988; Benz et al., 1988, 2007). Such heating should further deplete volatile species in Mercury's interior (Boynton et al., 2007). Moderately vola- tile alkali metals are known to be important surface-derived species in Mercury's exosphere (Potter and Morgan, 1985, 1986), and polar deposits postulated to consist of water ice have been documented by Earth-based radar on the floors of permanently shadowed impact craters near Mercury's poles (Harmon and Slade, 1992; Slade et al., 1992); these volatiles may be derived dominantly from meteoritic and cometary sources (e.g., Moses et al., 1999; Leblanc and Johnson, 2003), however, and need not constrain interior volatile abundances. An important new constraint on interior volatile abundances on Mercury comes from imaging conducted during the first flyby of Mercury by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft (Solomon et al., 2008). MES- SENGER images of the 1550-km-diameter Caloris basin reveal several irregular depressions surrounded by bright, relatively red deposits (compared to Mercury as a whole). Detailed assessment of morpho- logic characteristics indicates that these features are broad, low shield volcanoes (Head et al., 2008, 2009a-this issue). By analogy with similar features on the Moon, the haloes of diffuse-bordered, high-reflectance material surrounding several of these irregular depressions are inter- preted as pyroclastic deposits (Head et al., 2008, 2009a-this issue), which are the product of explosive volcanic eruptions driven by the exsolution of magmatic volatiles during ascent of the magma from the mantle or lower crust (e.g., Wilson and Head, 1981). These volcanic features therefore provide evidence for volatiles in Mercury's mantle or lower crust at the time of magma genesis. In this paper we offer a detailed rationale for the identification of pyroclastic deposits on Mercury, some inferences on eruption Earth and Planetary Science Letters 285 (2009) 263–271 ⁎ Corresponding author. Tel.: +1 401 863 3841; fax: +1 401 863 3978. E-mail address: Laura_Kerber@brown.edu (L. Kerber). 0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2009.04.037 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl