  The aim of this work is to investigate contrasting spectral trends observed
in carbonaceous chondrites by simulating space weathering effects on a subset
of minerals found in these meteorites. We use pulsed laser irradiation to
simulate micrometeorite impacts on aqueously altered minerals and observe their
spectral and physical evolution as a function of irradiation time. Irradiation
of the mineral lizardite, a Mg-phyllosilicate, produces little reddening and
darkening, but a pronounced reduction in band depths. Irradiation of an Fe-rich
aqueously altered mineral assemblage composed of cronstedtite, pyrite and
siderite, produces significant darkening and band depth suppression. The
spectral slopes of the Fe-rich assemblage initially redden then become bluer
with increasing irradiation time. Analyses of the Fe-rich assemblage using
scanning and transmission electron microscopy reveal the presence of micron
sized carbon-rich particles that contain notable fractions of nitrogen and
oxygen. Radiative transfer modeling of the Fe-rich assemblage suggests that
npFe0 particles result in the initial spectral reddening of the samples, but
the increasing production of micron sized carbon particles results in the
subsequent spectral bluing. The presence of npFe0 and possibly cronstedtite
likely promotes the synthesis of these organic-like compounds. These
experiments indicate that space weathering processes may enable organic
synthesis reactions on the surfaces of volatile-rich asteroids. Furthermore,
Mg- and Fe-rich aqueously altered minerals are dominant at different phases of
the alteration process. Thus, the contrasting spectral slope evolution between
the Fe- and Mg-rich samples in these experiments may indicate that space
weathering trends of volatile-rich asteroids have a compositional dependency
that could be used to determine the aqueous histories of asteroid parent
bodies.
