Crystal size distributions (CSD) of periclase in contact metamorphic
dolomite marbles are presented for two profiles near the Cima Uzza
summit in the southern Adamello Massif (Italy). The database was
combined with geochemical and petrological information to deduce the
controls on the periclase-forming reaction. The contact metamorphic
dolomite marbles are exposed at the contact of mafic intrusive rocks and
are partially surrounded by them. Brucite is retrograde and pseudomorphs
spherical periclase crystals. Prograde periclase growth is the
consequence of limited infiltration of water-rich fluid at T near 605C.
Stable isotope data show depletion in (13)C and (18)O over a narrow
region (40 cm) near the magmatic contact, whereas the periclase-forming
reaction front extends up to 4 m from the contact. CSD analyses along
the two profiles show that the median grain size of the periclase
crystals does not change, but that there is a progressively greater
distribution of grain sizes, including a greater proportion of larger
grains, with increasing distance from the contact. A qualitative model,
based on the textural and geochemical data, attributes these variations
in grain size to changing reaction affinities along a kinetically
dispersed infiltration front. This study highlights the need to invoke
disequilibrium processes for metamorphic mineral growth and expands the
use of CSDs to systems of mineral formation driven by fluid
infiltration.