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Shergottite Formation and Implications for Present-Day Mantle Convection on Mars

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Shergottite Formation and Implications for Present-Day Mantle Convection on Mars Meteoritics & Planetary Science 39, Nr 12, 1815–1832 (2003) Abstract available online at http://meteoritics.org Melting in the martian mantle: Shergottite formation and implications for present-day mantle convection on Mars Walter S. KIEFER Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, Texas 77058, USA E-mail: [email protected] (Received 23 April 2003; revision accepted 23 December 2003) Abstract–Radiometric age dating of the shergottite meteorites and cratering studies of lava flows in Tharsis and Elysium both demonstrate that volcanic activity has occurred on Mars in the geologically recent past. This implies that adiabatic decompression melting and upwelling convective flow in the mantle remains important on Mars at present. I present a series of numerical simulations of mantle convection and magma generation on Mars. These models test the effects of the total radioactive heating budget and of the partitioning of radioactivity between crust and mantle on the production of magma. In these models, melting is restricted to the heads of hot mantle plumes that rise from the core-mantle boundary, consistent with the spatially localized distribution of recent volcanism on Mars. For magma production to occur on present-day Mars, the minimum average radioactive heating rate in the martian mantle is 1.6 × 10−12 W/kg, which corresponds to 39% of the Wänke and Dreibus (1994) radioactivity abundance. If the mantle heating rate is lower than this, the mean mantle temperature is low, and the mantle plumes experience large amounts of cooling as they rise from the base of the mantle to the surface and are, thus, unable to melt. Models with mantle radioactive heating rates of 1.8 to 2.1 × 10−12 W/kg can satisfy both the present-day volcanic resurfacing rate on Mars and the typical melt fraction observed in the shergottites. This corresponds to 43–50% of the Wänke and Dreibus radioactivity remaining in the mantle, which is geochemically reasonable for a 50 km thick crust formed by about 10% partial melting. Plausible changes to either the assumed solidus temperature or to the assumed core-mantle boundary temperature would require a larger amount of mantle radioactivity to permit present-day magmatism. These heating rates are slightly higher than inferred for the nakhlite source region and significantly higher than inferred from depleted shergottites such as QUE 94201. The geophysical estimate of mantle radioactivity inferred here is a global average value, while values inferred from the martian meteorites are for particular points in the martian mantle. Evidently, the martian mantle has several isotopically distinct compositions, possibly including a radioactively enriched source that has not yet been sampled by the martian meteorites. The minimum mantle heating rate corresponds to a minimum thermal Rayleigh number of 2 × 106, implying that mantle convection remains moderately vigorous on present-day Mars. The basic convective pattern on Mars appears to have been stable for most of martian history, which has prevented the mantle flow from destroying the isotopic heterogeneity. INTRODUCTION improved our knowledge of young surface ages on Mars. At Olympus Mons and in Elysium Planitia, cratering studies Several lines of evidence indicate that volcanic activity indicate that some lava flows formed in the last 10–30 Myr has occurred on Mars in the geologically recent past. The (Hartmann and Neukum 2001). These cratering ages shergottites are a class of igneous meteorite, the geochemistry incorporate the current uncertainty in the cratering flux rate at of which indicates a martian origin (e.g., McSween and Mars. Treiman 1998). Many of the shergottites have radiometric The existence of young volcanism on Mars implies that ages of 180 Myr, although some are older (Nyquist et al. adiabatic decompression melting and, hence, upwelling 2001). The Mars Orbital Camera on Mars Global Surveyor convective flow in the mantle remains important on Mars at permits imaging of very small craters and has greatly present. In this study, I model mantle convection and melt 1815 © Meteoritical Society, 2003. Printed in USA. 1816 W. S. Kiefer production on present-day Mars. In particular, I test the on model results. Recently, Schott et al. (2001) considered sensitivity of the models to different choices of total magma production in a mantle convection simulation for radioactive heating rate and of the partitioning of radioactive Mars. Their model considered a very weak lithosphere and elements between the mantle and crust. I also test the effects did not permit differentiation of radioactivity into the crust. of varying the lithospheric thickness, the core-mantle These conditions are only relevant to the very earliest stage of boundary temperature, and the choice of solidus temperature. martian history. Moreover, the melting relationships used in The primary observational constraint on the models is the Schott’s study were experimentally constrained only up to requirement that successful models are able to generate at 1.5 GPa. In contrast, the simulations described here consider least some melting at present. This binary test (melt either conditions appropriate for present-day Mars: a thick, strong occurs or does not occur) provides a strong constraint on the lithosphere; up to 90% of the total radioactivity differentiated minimum amount of radioactive heating that remains in the into the crust; and melting relationships that are mantle of Mars. Additional tests are provided by the spatial experimentally constrained up to 9 GPa. distribution of melting, the total melt production rate, and the mean melt fraction. The principal conclusion of this study is A CONCEPTUAL MODEL FOR THARSIS that the minimum average radioactive heating rate in the martian mantle at present is 1.6 × 10−12 W/kg, which Geologically recent volcanic activity has occurred in 2 corresponds to 39% of the total radioactivity of the Wänke general regions of Mars: Tharsis and Elysium (Scott and and Dreibus (1994) composition model. This heating rate sets Tanaka 1986; Greeley and Guest 1987). The Tharsis rise is the minimum temperature and, thus, the maximum viscosity 4500 km across and reaches an elevation of 10 km. Three of the martian mantle. The minimum present-day thermal large shield volcanos, Arsia Mons, Pavonis Mons, and Rayleigh number is 2 × 106, implying that mantle convection Ascraeus Mons, occupy the crest of Tharsis. Olympus Mons, on Mars remains at least moderately vigorous at present. the largest volcano in the solar system, is located on the Previous studies of mantle convection and the thermal northwest flank of Tharsis, and Alba Patera is on the north evolution of Mars have taken 2 different approaches. Studies side of Tharsis. In addition, Tharsis also contains 7 smaller of magma production on Mars have usually been performed volcanos and vast lava plains (Hodges and Moore 1994). The using parameterized convection models (Spohn 1991; Breuer Elysium rise, in the other hemisphere, is a small-scale version et al. 1993; Weizman et al. 2001; Hauck and Phillips 2002). of Tharsis. Elysium is 1500 km across and reaches an Parameterized convection models assume a functional elevation of 4 km. It includes 3 large shield volcanos and relationship between the vigor of convection, as measured by extensive regional lava plains (Hodges and Moore 1994; the Rayleigh number, and the efficiency of heat loss from the Mouginis-Mark and Yoshioka 1998). mantle. This allows the thermal evolution of a planet to be Many investigators have treated Tharsis primarily as a calculated by solving a system of ordinary differential region of volcanically thickened crust (e.g., Solomon and equations. An important caveat is that such models are one- Head 1982; Phillips et al. 2001; Zhong 2002). Obviously, this dimensional (vertical only) and, thus, yield only a solution for must be true in part, but generation of the required volume of the average temperature as a function of depth. Because magma implies a hot mantle, which would contribute to the melting is limited to the highest temperatures in the mantle, observed topographic uplift. The evidence cited in the parameterized convection calculations cannot be used to Introduction for young volcanism on Mars indicates that directly calculate magmatism on Mars. Previous studies have convective upwelling and adiabatic decompression melting made arbitrary assumptions either about melting efficiency or continues to be important, even on present-day Mars. about the magnitude of lateral temperature variations to There are 2 fundamental types of energy source for estimate melting in the parameterized convection framework. driving convective flow in the mantle of Mars: bottom heating In contrast to these earlier investigations, the current study and internal heating (Davies 1999; Schubert et al. 2001). explicitly solves for mantle temperature as a function of both Bottom heating is heat that enters the base of the mantle from horizontal and vertical location in the mantle. Thus, my the core due to cooling of the core. Bottom heating produces results can be used directly with experimentally determined a thin thermal boundary layer in the lowermost mantle just melting laws to calculate the expected magma production on above the core-mantle boundary and, consequently, produces Mars. narrow, rising plumes in the mantle. Such plumes are thought The second class of studies simulate mantle flow in either to cause terrestrial hotspots such as Hawaii and Iceland (e.g., 2 or 3 dimensions and solve the complete set of partial Morgan 1972; Watson and McKenzie 1991; Ito et al. 1999). differential equations that govern mantle convection Internal heating is due to radioactive decay within the mantle (Schubert et al. 1990; Harder and Christensen 1996; Harder of Mars and to the release of specific heat as the mantle cools. 1998, 2000; Breuer et al. 1997, 1998; Reese et al. 1998; Internal heating is not associated with a deep thermal Wüllner and Harder 1998; Zhong and Zuber 2001).
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