Southeastern Section–55th Annual Meeting (23–24 March 2006)

Paper No. 9
Presentation Time: 11:00 AM


WHITE, John Charles, Department of Earth Sciences, Eastern Kentucky University, Roark 103, 521 Lancaster Ave, Richmond, KY 40475, REN, Minghua, Dept. of Geological Sciences, Univ of Texas at El Paso, El Paso, TX 79968, PARKER, Don F., Dept. of Geology, Baylor Univ, Waco, TX 76798-7354 and ANTHONY, Elizabeth Y., Department of Geological Sciences, Univ of Texas at El Paso, 500 W. University Ave, El Paso, TX 79968,

            Recent studies have demonstrated the utility and reliability of QUIlF equilibria in constraining intensive crystallization parameters (T-P-fO2-aSiO2) in peralkaline igneous rocks with assemblages of fayalite-clinopyroxene-ilmenite.  Briefly: temperature is constrained by clinopyroxene-fayalite equilibria; the addition of silica activity (or presence of quartz) can constrain pressure, or vice versa; and the addition of ilmenite allows the calculation of oxygen fugacity.  However, many highly-evolved pantellerites have assemblages of clinopyroxene-ilmenite-aenigmatite or clinopyroxene-aenigmatite; the lack of fayalite precludes the use of QUIlF equilibria to determine T- fO2 in these rocks.  We demonstrate that temperature and oxygen fugacity in highly-evolved pantelleritic melts can be reasonably constrained, given a set of assumptions (fixed P, aSiO2 = 1.0 relative to quartz, and aNa2Si2O5 = 1.0), by the following equilibria:

            1) Aen + O2 = Ilm + 2 Hem + 2 SiO2 + Na2Si2O5,

            2) 2 SiO2 + 2 Na2Si2O5 + 2 Fa + O2 = 4 Aeg,

            3) 2 Aeg = Hem + 2 SiO2 + Na2Si2O5,

            4) Aen + Na2Si2O5 + O2 = 4 Aeg + Ilm, and

            5) 2 Fs (cpx) + O2 = 4 SiO2 + 2 Hem.

Reaction 1 defines a univariant curve in T-fO2 space for assemblages with aenigmatite-ilmenite.  Reaction 2 defines the fayalite-out reaction, and the intersection of this curve with 1 provides a maximum T.  Reaction 3 defines the rhombohedral oxide-out reaction, and provides a minimum T for assemblages with ilmenite and a maximum T for assemblages without ilmenite.  Reaction 4 is the upper boundary of the “no-oxide” field, and provides a minimum T for assemblages without oxides.  Reaction 5 is the displaced AHQ buffer, one of the QUIlF reactions.  The intersections of curves defined by reactions 1 and 5 may be able to provide a reasonable estimate of T- fO2 for assemblages of aenigmatite-ilmenite-clinopyroxene.  Since the free energy data for aenigmatite and aegirine used in these calculations are from estimates, rather than experiment, curves for equilibria 1-4 should be drawn with a very broad brush!  However, results from these techniques are consistent with both experimental data and results from other techniques applied to suites from Pantelleria, Italy, and Eburru, Kenya.