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American Mineralogist; April 2007; v. 92; no. 4; p. 453-459; DOI: 10.2138/am.2007.2224
© 2007 Mineralogical Society of America
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New constraints on metamorphic history of Adirondack diopsides (New York, U.S.A.): Al and {delta}18O profiles

G. Desbois1, J. Ingrin1,*, N.T. Kita2, J.W. Valley2 and E. Deloule3

1 Minéralogie, LMTG-CNRS-OMP—Université Paul Sabatier, 14 Avenue Edouard Belin, F-31400 Toulouse, France
2 Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, U.S.A.
3 CRPG-CNRS, BP20, F-54501 Vandoeuvre Cedex, France

Correspondence: * E-mail: ingrin{at}lmtg.obs-mip.fr

Detailed electron- and ion-microprobe analysis of diopsides extracted from a block of marble collected in the Mt. Marcy anorthosite massif (Adirondack Highlands, New York) shows that single crystals have preserved Al-Si zoning features from early contact metamorphism later modified by regional metamorphism. Diopsides show Al and Si complementary zoning from core to rim; average Al concentration in rims is constant (0.11 pfu), but values in the cores vary with crystals from 0.06 to 0.14 pfu. A {delta}18O profile measured with the ion microprobe shows no zoning. Modeling of the Al-diffusion profiles affected by regional metamorphism of the Grenville orogeny shows that original Al zoning was sharp and corresponds to a rapid change of crystal-growth conditions during early contact metamorphism: (1) diopside cores crystallized with variable Al contents due to restricted fluid circulation or differing sedimentary composition, (2) then pervasive fluid infiltration crystallized the homogeneous rim of diopside, and (3) core to rim Al zoning was later smoothed by granulite-facies metamorphism. Homogeneity of {delta}18O in core and mantle (near rim) of diopsides suggests that the crystals were isotopically homogeneous prior to and during regional metamorphism. Values of Al-Si diffusion coefficient deduced from modeling are in agreement with low-temperature extrapolation of experimental data.

Key Words: Diopside • zoning • granulite • diffusion • ion probe • oxygen isotopes • Adirondack






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