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John C. White 1 , Don F. Parker, and Minghua Ren 2 Baylor University Waco, Texas 76798

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

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John C. White 1 , Don F. Parker, and Minghua Ren 2 Baylor University Waco, Texas 76798

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  1. Variations in alkali feldspar/melt trace-element partitioning during fractionation of peralkalic quartz trachyte-rhyolite suites John C. White1, Don F. Parker, and Minghua Ren2 Baylor University Waco, Texas 76798 1Current address: Elizabeth City State University, Elizabeth City, NC 27909 2Current address: University of Texas at El Paso, El Paso, TX 79968

  2. Rb • ln D(Rb) = -2.2122 + 0.0273(SiO2) – 0.6396(A.I.) • R2 = 0.90 • p < 0.0001 • 64.8 to 76% SiO2 • 0.97 to 1.75 A.I.

  3. Rb(M) • ln D(Rb) = -2.1370 + 0.0354(Or) – 0.3071(A.I.) • R2 = 0.85 • p < 0.0001 • 28.3 to 47.7 mol% Or • 0.97 to 1.95 A.I.

  4. Eu • ln D(Eu) = 20.3729 – 0.2119(SiO2) – 4.0374(A.I.) – 1.1188(Na2O/K2O) • R2 = 0.91 • p = 0.0001 • ~66 to 74% SiO2 • 0.99 to 1.75 A.I.

  5. Eu(A) • ln D(Eu) = 5.2871 – 4.8100(A.I.) + 0.0830(Na2O/K2O) • R2 = 0.85 • p < 0.0001 • 0.90 to 1.95 A.I.

  6. HFSE • ln D(Zr) = -13.4268 + 0.1554(SiO2) – 0.3997(A.I.) • ln D(Nb) = -14.9113 + 0.1793(SiO2) – 0.6432(A.I.) • R2 = 0.84(Zr), 0.91(Nb) • P < 0.0001

  7. 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

  8. Trace-Element Partitioning Models • F = COZr / CLZr (Allegre et al., 1977) • F = COpl361iZr / CSic172Zr = 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)

  9. FC Model (Greenland, 1970) • dC = (D – 1) d[ln F] • ln (CL/CO) = (D – 1) ln F • CL/CO = F (D – 1) • D = a + bF • dC = (a + bF – 1) d[ln F] • ln CL/CO = (a – 1) ln F + b(F – 1)

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

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

  12. 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.

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