ANDESITES ON MARS: IMPLICATIONS FOR THE ORIGIN OF TERRESTRIAL CONTINENTAL CRUST. Paul D. Lowman Jr., Mail Code 921, NASA Goddard Space Flight Center, Greenbelt MD 20771, USA.

The Alpha Proton X-ray Spectrometer ferentiation" in Mars has been confirmed by (APXS) on the Mars Pathfinder rover the SNC meteorites. Unlike the Earth, this (Rieder et al., 1997) produced analyses in evolutionary path has not involved plate the X-ray mode of 5 rocks with chemical tectonics, beyond the initial crustal fragmen- compositions (62.0+2.7% SiO2 "soil-free tation indicated by the Valles Marineris and rock") corresponding to andesite. The similar features. APXS soil analyses resemble those from the The bulk chemical composition of the Viking landers, providing a field compari- crust of Venus is not known, although ba- son. Furthermore, the Mariner 9 mission in salts have been found by two Soviet landers. 1972 had produced thermal emission spectra However, the unimodal topography of Ve- of suspended dust indicating a composition nus, and the typically continental tectonic averaging 60+10% SiO2, corresponding to style revealed by the Magellan radar "an intermediate igneous rock"(Hanel et al., (Solomon et al., 1992), are consistent with 1972a,) and implying "substantial geo- global differentiation, but as for Mars, with- chemical differentiation of Mars." An iden- out the involvement of plate tectonic proc- tical instrument on a Nimbus satellite had esses. previously produced similar dust spectra Collectively, these discoveries imply that over Africa (Hanel et al., 1972b), which,if silicate planets can undergo early global representative of the surface, support the differentiation by igneous processes not de- similarity between the terrestrial and mar- pending on sea-floor spreading and subduc- tian crusts. Viewed in historical context, and tion. This concept runs counter to prevailing in light of martian geology, the APXS re- views on the origin of the Earth's continen- sults can be considered both accurate and tal crust, which is widely believed to have reasonably representative of the composi- been formed by long-term andesitic volcan- tion of the highland crust of Mars. ism and terrane accretion. There is no doubt An andesitic composition for the martian (Marsh, 1976;Wyllie, 1988) that most Phan- highland crust was predicted as a test of a erozoic andesitic volcanism is subduction- general theory for the origin of continental related, but this is probably not the domi- crust (Lowman, 1989). The Pathfinder data nant process by which continental crust has provide the first tentative confirmation of over geologic time been extracted from the this prediction. The present paper discusses mantle. An intermediate composition for the implications of the Pathfinder results, much of the lower continental crust granu- and other post-1989 findings, for the Earth's lites, and other geologic evidence (Lowman, continental crust. 1984, 1989), points to formation of most The general stages of crustal evolution in continental crust in the earliest Archean Mars (Fig. 1) have been clear for two dec- (Armstrong, 1981), by andesitic volcanism ades (Lowman, 1976). The Pathfinder data accompanying outgassing of the primordial confirm the felsic "first differentiation" and mantle. It is impossible to show that this indicate that, as predicted, magma genera- early crust was global, other than by anal- tion under hydrous conditions (Yoder, 1976; ogy with the global first differentiation on Morse, 1986) produced andesite, in contrast other planets (notably Mars), but the ma- to the basalt and differentiation products fic/sialic crustal dichotomy of the Earth may thereof (e.g., anorthosites) making up the be the eventual result of major impacts cor- lunar highland crust. A basaltic "second dif- responding to the basin-forming impacts on the Moon and Mars (Frey, 1980). Formation Lowman, P.D.,Jr., 1976, J. Geology, 84, of the Moon by a large impact on the pri- 1-16. mordial Earth may also have contributed to Lowman, P.D.,Jr., 1984, Precambrian disruption of the earliest crust. Research, 24, 199-215. This theory for the origin of continental Lowman, P.D.,Jr., 1989, Precambrian crust is a radical but testable one, and has so Research, 44, 171-195. far passed "the most significant" (Lowman, Marsh, B.D., 1976, J. Geology, 84, 27- 1989) test. Further exploration of Mars with 45. in situ analyses will permit additional Morse, S.A., 1986, Earth Planet. Sci. evaluation. Even if disproved, the theory Lett., 81. 118-126. illustrates the value of comparative plane- Rieder, R., et al., 1997, Science, 278, tology in throwing new light on the crustal 1771-1774. evolution of the Earth. Solomon, S.C., et al., 1992, J. Geophysi- References cal Research, 97, 13,199-13,255. Armstrong, R.L., 1981, Phil. Trans. Roy. Wyllie, P.J.,1988, Reviews of Geophys- Soc. London, Ser. A, 301, 443. ics, 26, 370-410. Frey, H., 1980, Precambrian Research, Yoder, H.S.,Jr., 1976, Generation of Ba- 10, 195-216. saltic Magma, National Academy of Sci- Hanel et al., 1972a, Science, 175, 305- ences. 308. Hanel et al., 1972b, Icarus, 17, 423-442.

Fig. 1. Crustal evolution in silicate planets.