Phase Transformations - Implications for Mantle Structure

Weidner and Wang (1999) Phase transformations implications for mantle structure The Transition Zone Complex Interactions among Minerals in the Transition Zone • Mg-Si-O • Mg-Fe-Si-O • Mg-Fe-Ca-Al-Si-O (this study) Pyrolite Image removed due to copyright considerations. Please see: Weidner, D. J., and Y. Wang. “Phase transformations: Implications for mantle structure. In Earth’s deep interior: mineral physics and tomography from the atomic to the global scale.” Geophysical Monograph 117 (2000): 215-235. Physical Properties of Minerals Image removed due to copyright considerations. Please see: Weidner, D. J., and Y. Wang. “Phase transformations: Implications for mantle structure. In Earth’s deep interior: mineral physics and tomography from the atomic to the global scale.” Geophysical Monograph 117 (2000): 215-235. Physical Properties of Minerals • ~4% variation in Fe results in ~1% variation in physical properties. • Replacing 25% of the (Mg,Si) in Gt with Al results in 1% increase in acoustic velocity. • However, adding Al stabilizes Gt instead of Il. 410 Discontinuity • Olivine → Wadsleyite • Pyroxene → Garnet • Not an isolated system! 410 Discontinuity - Phase relations and element partitioning • With Fe: Ol → Ol+Wd → Wd • High T and low Fe → Deeper and narrower discontinuity • Gradual transition from Px to Gt • Gt takes Fe from Ol or Wd, whereas Px gives Fe to Ol or Wd. → Depth and thickness of 410 may be sensitive to degree of the Px→Gt transition 410 Discontinuity - Phase relations and element partitioning Images removed due to copyright considerations. Please see: Weidner, D. J., and Y. Wang. “Phase transformations: Implications for mantle structure. In Earth’s deep interior: mineral physics and tomography from the atomic to the global scale.” Geophysical Monograph 117 (2000): 215-235. Fe lowers Ca in Px. Al decreases in Gt with P, due to formation of Mj 410 Discontinuity - Effect of temperature A – isothermal (1473 K) Image removed due to copyright considerations. Please see: Weidner, D. J., and Y. Wang. “Phase transformations: Implications for mantle structure. In Earth’s deep interior: mineral physics and tomography from the atomic to the global scale.” Geophysical Monograph 117 (2000): 215-235. B – uniform gradient of 0.7 K/km C – temperature gradient of 0.7 K/km with a T increase from Ol→Wd transition. Latent heat of Ol→Wd transition increases T by 40 K. 410 Discontinuity - Effect of Al Al effect is greater than T-effect for Gt stability. Images removed due to copyright considerations. Please see: Weidner, D. J., and Y. Wang. “Phase transformations: Implications for mantle structure. In Earth’s deep interior: mineral physics and tomography from the atomic to the global scale.” Geophysical Monograph 117 (2000): 215-235. 410 Discontinuity - Effect of T 410 width increases with T Image removed due to copyright considerations. Please see: (1) Constant T Weidner, D. J., and Y. Wang. “Phase transformations: Implications for mantle structure. In Earth’s deep interior: mineral physics and tomography from the atomic to the global scale.” Geophysical Monograph 117 (2000): 215-235. (2) 0.7 K/km gradient (3) Latent heat (Largest effect) (4) Latent heat + 0.7 K/km gradient A. T directly affects depth and width of the Ol→Wd transition. B. T affects the Px→Gt transition and changes Fe partitioning. 410 Discontinuity - Effect of Composition Image removed due to copyright considerations. Please see: Weidner, D. J., and Y. Wang. “Phase transformations: Implications for mantle structure. In Earth’s deep interior: mineral physics and tomography from the atomic to the global scale.” Geophysical Monograph 117 (2000): 215-235. A→B: increase in Fe A→C: decrease in Al (Gt) Less Fe is taken from Ol 410 Discontinuity - Effect of Composition Image removed due to copyright considerations. Please see: Weidner, D. J., and Y. Wang. “Phase transformations: Implications for mantle structure. In Earth’s deep interior: mineral physics and tomography from the atomic to the global scale.” Geophysical Monograph 117 (2000): 215-235. A→B: decrease in Al (Gt) Less Fe is taken from Ol A→C: increase in Fe 410 Discontinuity - Lateral variations in T Image removed due to copyright considerations. Please see: Weidner, D. J., and Y. Wang. “Phase transformations: Implications for mantle structure. In Earth’s deep interior: mineral physics and tomography from the atomic to the global scale.” Geophysical Monograph 117 (2000): 215-235. T-insensitive depth would decrease with Al ↓ or T ↑. 410 Discontinuity - Lateral variations in Al Image removed due to copyright considerations. Please see: Weidner, D. J., and Y. Wang. “Phase transformations: Implications for mantle structure. In Earth’s deep interior: mineral physics and tomography from the atomic to the global scale.” Geophysical Monograph 117 (2000): 215-235. 520 Discontinuity Image removed due to copyright considerations. Please see: Weidner, D. J., and Y. Wang. “Phase transformations: Implications for mantle structure. In Earth’s deep interior: mineral physics and tomography from the atomic to the global scale.” Geophysical Monograph 117 (2000): 215-235. 520 Discontinuity • Formation of Ca-pv • Wd → Rw: 21 km thick • But with Rw more enriched in Fe with Gt, the thickness becomes 12 km. 660 Discontinuity • Post-spinel, post-garnet, post-ilmenite • Rw (54 vol%), Gt (39 vol%), Ca-pv (7 vol%) in pyrolite • Weinder considers Fe-free system for 660. 660 Discontinuity Image removed due to copyright considerations. Please see: Weidner, D. J., and Y. Wang. “Phase transformations: Implications for mantle structure. In Earth’s deep interior: mineral physics and tomography from the atomic to the global scale.” Geophysical Monograph 117 (2000): 215-235. • If all Al in Pyrolite goes in Pv → 6.3 mol% Al2O3 • > 12 mol% Al2O3, pure Al2O3 appears 660 Discontinuity • Image removed due to copyright considerations. Please see: Weidner, D. J., and Y. Wang. “Phase transformations: Implications for mantle structure. In Earth’s deep interior: mineral physics and tomography from the atomic to the global scale.” Geophysical Monograph 117 (2000): 215-235. Along 2100 K, Rw+Gt(Py50) → Gt(Py25)+Mw → Gt+Pv+Mw → Pv+Mw • Along 1900 K Rw+Gt(Py50) → Pv+Mw+Gt(25