Journal of Petrology Advance Access originally published online on February 27, 2009
Journal of Petrology 2009 50(3):451-465; doi:10.1093/petrology/egp007
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Crystal Size Distribution of Periclase in Contact Metamorphic Dolomite Marbles from the Southern Adamello Massif, Italy
1Institut De Mineralogie Et Geochimie, Universite De Lausanne, Batiment L'anthropole, CH-1015 Lausanne, Switzerland
2Department of Geology, University of Iowa, Iowa City, IA 52242, USA
3Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112, USA
RECEIVED SEPTEMBER 10, 2007; ACCEPTED JANUARY 21, 2009
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Abstract |
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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 605°C. Stable isotope data show depletion in 13C and 18O 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.
KEY WORDS: X-ray µC-tomography; contact metamorphism; fluid infiltration; crystal size distribution; 3D; stable isotopes; Adamello Massif
*Corresponding author. Present address: Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, D-44780 Bochum, Germany. Telephone: +49-234-322 4141. E-mail: Thomas.H.Mueller{at}rub.de
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