Other
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmdi33b..05k&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #DI33B-05
Other
[5410] Planetary Sciences: Solid Surface Planets / Composition, [5480] Planetary Sciences: Solid Surface Planets / Volcanism, [6225] Planetary Sciences: Solar System Objects / Mars, [6295] Planetary Sciences: Solar System Objects / Venus
Scientific paper
Models of the thermal evolution and adiabatic decompression melting on Mars and Venus commonly assume that the mantle compositions of these planets are the same as Earth. However, subtle differences in composition may nevertheless have important effects on the expected magma production rates. Studies of the martian meteorites suggest that the martian mantle differs from Earth in several ways: it is enriched in alkalis (Na and K), enriched in iron (lower Mg#), and has a lower oxygen fugacity (close to the iron-wustite buffer). These differences all affect the magmatic history. Both the higher Na and lower Mg# act to lower the solidus temperature. The two effects have roughly equal importance, and together may lower the dry solidus temperature of the primitive martian mantle by up to 50 K at 3 GPa relative to Earth. Together, these two effects may have significantly enhanced the crustal production rate on early Mars. Because Na is incompatible, it partitions into the crust during volcanism and thus both its abundance in the mantle and its affect on the magma production rate decline over time. On the other hand, the Mg# of the mantle does not change significantly with time, and its affect on the solidus should continue to be important to pressure release melting in mantle plumes on present-day Mars. Although the change in the present-day volcanism rate is small in absolute terms, it nevertheless may be the difference between a planet that is still active and one that is magmatically dead at present. The lower Mg# also acts to decrease the mantle viscosity, although this is partially offset by the low oxygen fugacity. In turn, the lower viscosity implies more vigorous convection and higher magma production on early Mars. Due to self-regulation of mantle temperature and viscosity, the importance of this effect will decrease with time. Because Na and K are relatively volatile, lower abundances of these elements may have accreted on Venus than on either Earth or Mars. This would raise the solidus on Venus relative to Earth and also implies less radioactive heating of the mantle. Both effects would contribute to a lower magma production rate than on Earth. Because argon-40 is a decay product of potassium-40, the abundance of Ar40 in the atmosphere has been used as a tracer for the time-integrated volcanism rates on the terrestrial planets. The Venus atmosphere contains only about half as much Ar40 per unit mass as Earth, and the implied lower volcanism rate may be at least partially explained by differences in planetary Na and K abundances.
Filiberto Justin
Kiefer Walter Scott
Li Qian
Sandu Constantin
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