HR: 1340h
AN: P23B-0196    [Abstracts]
TI: The Martian Surface is old and so are Shergottites
AU: Bouvier, A
EM: abouvier@ens-lyon.fr
AF: ENS Lyon, LST, LYON, 69007 France
AU: * Blichert-Toft, J
EM: jblicher@ens-lyon.fr
AF: ENS Lyon, LST, LYON, 69007 France
AU: Vervoort, J D
EM: vervoort@wsu.edu
AF: WSU, Dept of Geology, Pullman, WA 99164 United States
AU: Albarede, F
EM: albarede@ens-lyon.fr
AF: ENS Lyon, LST, LYON, 69007 France
AB: We report new Sm-Nd, Lu-Hf, and Pb-Pb mineral and whole-rock (WR) isotope data for the basaltic shergottite (BS) Zagami (Zag), as well as Pb-Pb WR isotope data for the BS Los Angeles (LA). The isotopic analyses were carried out by MC-ICP-MS at ENSL. The Sm-Nd and Lu-Hf data for Zag yield internal isochron ages of 155±9 Ma (MSWD=0.45) and 185±36 Ma (MSWD=1.2), respectively. While these young ages overlap with earlier Rb-Sr, Sm-Nd, and U-Pb ages (2), the Pb-Pb age does not. Our Pb isotope data on Zag and LA lie on the same Pb-Pb array as previous analyses of BS by (1), which, if interpreted as an isochron, indicate an age of ~4 Ga. The range of δ¹⁸O (3.9-5.2 permil) observed in shergottites (3, 4) is too broad to be accounted for by igneous processes only and attests to low-T interaction with fluids. The Martian surface appears to be covered with sulfates, while essentially lacking carbonates (5, 6), implying that the surface of Mars was once covered with acidic water bodies of unknown depths (7). An important observation is that apatite is a common phase in Zag and LA, as in all the shergottites (8), and explains why most of the REE, Th, U, and some fraction of Pb can be removed by leaching (9). The main inventory of Pb, however, resides in maskelynite. The Pb isotope data on shergottites, in conjunction with the existing body of geochemical and geophysical evidence, have important implications for the history of the Martian surface and lithosphere. A fundamental problem with the young crystallization ages for the Martian meteorites has been that these ages are difficult to reconcile with the large ¹⁸²W and ¹⁴²Nd isotopic anomalies present in these meteorites. On one hand, the anomalies from the extinct radionuclides appear to require a static, non-convecting mantle, whereas widespread volcanism on Mars as young as 150 Ma seems to require an actively convecting mantle. We suggest, based on the Pb isotope systematics of shergottites, that the Martian surface is very old and formed mostly over the first one billion years of the planet's history, thus eliminating the above paradox. We further interpret the young shergottite Rb-Sr, Sm-Nd, and Lu-Hf ages to be the result of the resetting of these isotopic systems by acidic groundwater percolation through the Martian crust, ending approximately 150-300 My ago. We argue that throughout much of Martian history, large acid lakes of regional extent collected and mixed groundwaters and redistributed ¹⁴²Nd and ¹⁸²W between rocks of different ages, some of them nearly as old as the planet itself and carrying strong isotopic anomalies. From an interpretation of satellite images, it has been argued (10) that over the planet's first billion years of evolution, one third of its surface was covered by bodies of standing water and ice floodwaters derived from a subpermafrost aquifer. The last pools of liquid water occupying various spots on the Martian surface may have disappeared either by evaporation or by retreating into a permafrost layer now buried beneath thick wind-blown deposits. 1. Chen and Wasserburg, GCA 50, 955 (1986). 2. Nyquist et al., Space Sci. Rev. 96, 105 (2001). 3. Clayton and Mayeda, GCA 60, 1999 (1996). 4. Franchi et al., Phil. Trans. R. Soc. London, 359, 2019 (2001). 5. Squyres et al., Science 306, 1698 (2004). 6. Gendrin et al., Science 307, 1587 (2005). 7. Fairen et al., Nature 431, 423 (2004). 8. McCoy et al., GCA 63, 1249 (1999). 9. Dreibus et al., LPS 27, 323 (1996). 10. Clifford, Icarus 154, 40 (2001).
DE: 1028 Composition of meteorites (3662, 6240)
DE: 1039 Alteration and weathering processes (3617)
DE: 1040 Radiogenic isotope geochemistry
DE: 1125 Chemical and biological geochronology
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: Fall Meeting 2005