,
Longevity of fluorine-bearing tremolite on Venus
Icarus, 165 (2): 340--348 (2003)
Аннотация
There is a general belief that hydrous minerals cannot exist on Venus
under current surface conditions. This view was challenged when Johnson
and Fegley (2000, Icarus 146, 301-306) showed that tremolite (Ca2Mg5Si8O22(OH)(2)),
a hydrous mineral, is stable against thermal decomposition at current
Venus surface temperatures, e.g., 50% decomposition in 4 Ga at 740
K. To further explore hydrous mineral thermal stability on Venus,
we experimentally determined the thermal decomposition kinetics of
fluorine-bearing tremolite. Fluor-tremolite is thermodynamically
more stable than OH-tremolite and should decompose more slowly. However
how much slower was unknown. We measured the decomposition rate of
fluorine-bearing tremolite and show that its decomposition is several
times to greater than ten times slower than that of OH-tremolite.
We also show that F-bearing tremolite is depleted in fluorine after
decomposition and that fluorine is lost as a volatile species such
as HF gas. If tremolite ever formed on Venus, it would probably also
contain fluorine. The exceptional stability of F-bearing tremolite
strengthens our conclusions that if hydrous minerals ever formed
on Venus, they could still be there today. (C) 2003 Elsevier Inc.
All rights reserved.
