1. Variations in alkali feldspar/melt trace-element partitioning during fractionation of peralkalic quartz trachyte-rhyolite suites John C. White 1, Don F. Parker, and Minghua Ren 2 Baylor University Waco, Texas 76798 1 Current address: Elizabeth City State University, Elizabeth City, NC 27909 2 Current address: University of Texas at El Paso, El Paso, TX 79968

3. Rb ln D(Rb) = -2.2122 + 0.0273(SiO 2 ) – 0.6396(A.I.) R 2 = 0.90 p < 0.0001 64.8 to 76% SiO 2 0.97 to 1.75 A.I.

4. Rb(M) ln D(Rb) = -2.1370 + 0.0354(Or) – 0.3071(A.I.) R 2 = 0.85 p < 0.0001 28.3 to 47.7 mol% Or 0.97 to 1.95 A.I.

5. Eu ln D(Eu) = 20.3729 – 0.2119(SiO 2 ) – 4.0374(A.I.) – 1.1188(Na 2 O/K 2 O) R 2 = 0.91 p = 0.0001 ~66 to 74% SiO 2 0.99 to 1.75 A.I.

6. Eu(A) ln D(Eu) = 5.2871 – 4.8100(A.I.) + 0.0830(Na 2 O/K 2 O) R 2 = 0.85 p < 0.0001 0.90 to 1.95 A.I.

7. HFSE ln D(Zr) = -13.4268 + 0.1554(SiO 2 ) – 0.3997(A.I.) ln D(Nb) = -14.9113 + 0.1793(SiO 2 ) – 0.6432(A.I.) R 2 = 0.84(Zr), 0.91(Nb) P < 0.0001

9. Pantelleria: Major-Element FC Model Opl361i (64.86% SiO2) to Sic172 (70.88% SiO2) 66.1% (86.3%) Alkali Feldspar (AF-20-13) 4.4% (5.7%) Sodic Hedenbergite (CPX-20-41) 2.2% (2.9%) Fayalite (OL-20-21) 3.5% (4.5%) Cossyrite (Aenigmatite, COS-22-23) 0.3% (0.4%) Ilmenite (IL-31-22) 0.2% (0.2%) Apatite (#32, Mahood and Stimac, 1990) 23.3% Daughter Magma (76.6% Crystallization) SSR = 0.005

10. Trace-Element Partitioning Models F = C O Zr / C L Zr (Allegre et al., 1977) F = C Opl361i Zr / C Sic172 Zr = 480 ppm / 2314 ppm = 0.207 Major-element model: F = 0.233 (76.6% crystallization) Allegre et al. (1977) model: F = 0.207 (79.3% crystallization)

12. FC Model (Greenland, 1970) dC = (D – 1) d[ln F] ln (C L /C O ) = (D – 1) ln F C L /C O = F (D – 1) D = a + bF dC = (a + bF – 1) d[ln F] ln C L /C O = (a – 1) ln F + b(F – 1)

13. Data: White et al. (2003), Civetta et al. (1998), Mahood and Stimac (1990)

15. Concluding Remarks In addition to being strongly incompatible, trace- elements used as fractionation indices should also have nearly constant partition coefficients. Other trace-elements (e.g., Rb and Eu) may demonstrate considerable variability in partitioning behavior. The Greenland (1970) fractional crystallization model, which can incorporate variable D’s, may be a more realistic model of FC processes.