Oxygen isotopes internal equilibrium in magmatic epidote-bearing granitoids

OXYGEN ISOTOPES INTERNAL EQUILIBRIUM IN MAGMATIC EPIDOTE-BEARING GRANITOIDS

VALDEREZ P. FERREIRA¹, JOHN W. VALLEY², ALCIDES N. SIAL¹ AND MICHAEL J. SPICUZZA²

¹NEG-LABISE, Department of Geology, UFPE, Cx. Postal 7852, 50732-970 Recife, PE, Brazil.

²Department of Geology and Geophysics, University of Wisconsin, 1215 W. Dayton St., Madison, WI, 53706, USA.

In this study we present oxygen isotope compositions of mineral separates from three metaluminous epidote-bearing granitoid series from northeastern Brazil.

Zircon has an overall d¹⁸O variation of ~ 5º/_^oo, ranging from 5.72º/_^oo to 10.30º/_^oo, but is very homogenous within a single pluton. The highest values of d¹⁸O (zircon) (9.88º/_^oo ± 0.35º/_^oo; n = 13 samples) are for the calc alkalic granitoids, which are slightly, but significantly higher than those for high-K calc-alkalic granitoids (9.10º/_^oo ± 0.41º/_^oo; n = 19 samples) with little overlap. Values for these granitoids greatly differ from the average for the shoshonitic granitoids (7.43º/_^oo ± 0.19º/_^oo; n = 5 samples), and from that for the São Rafael pluton, a high-K calc-alkalic granitoid (5.92º/_^oo ± 0.26º/_^oo; n = 9 samples). These differences repeat for the other analyzed minerals, with little overlap of values. There is a good positive correlation between mineral pairs in the three series, although quartz-epidote and zircon-epidote fractionations are higher than predicted for equilibrium at magmatic temperatures. Measured mineral-mineral fractionations suggest continuous sub-solidus inter-mineral isotope exchange among all minerals except zircon. Calculated d¹⁸O (magma) values using whole rock SiO₂ contents and d¹⁸O (zircon) values are up to 1º/_^oo lower that those calculated using d¹⁸O (quartz); values of d¹⁸O in zircon are interpreted to provide the best evidence of magmatic value. Oxygen isotope fractionation between natural zircon and magmatic epidote is opposite to that predicted from theoretical determinations, as in all analyzed samples d¹⁸O (epidote) < d¹⁸O (zircon). The systematic mineral-epidote fractionations suggest that epidote cooled in a closed system, and is magmatic in origin. — (May 24, 2002).

Publication Dates

• Publication in this collection
  09 Oct 2002
• Date of issue
  Sept 2002