Diffusion between Olivine and Spinel Application to Geothermometry
Diffusion between Olivine and Spinel: Application to Geothermometry Lehmann, 1983
Olivine inclusions – host Spinel of banded chromitite from New Caledonia SPINEL - Chromite • IV(Fe 2+, Mg 2+)VI(Cr, Al) 3+O 2 4 • 60 wt. % Cr 2 O 3 OLIVINE - Forsterite • VI(Mg 2+, Fe 2+, Cr 2+)IVSi 4+O • 95% Forsterite 4
Factors affecting diffusion SPINEL (Chromite) OLIVINE (Forsterite) Bond Length IVFe-O Ionic Radius IVMg 2+ VIFe = ? IVMg-O = 1. 9783 VICr-O = 1. 9864 VICr 3+ IVFe 3+ Specific Gravity Packing Density = = = Z = 4 V = 289 0. 01384083 1 -O = VIFe -O = 2 VIMg -O = 1 2. 1576 2. 1839 2. 0931 VIMg 2. 1286 2 -O VIMg 2+ 0. 71 0. 76 0. 63 5. 09 VICr 2+ VIFe 2+ = = = Z=8 V = 578 = 0. 86 0. 87 -0. 94 0. 75 -0. 92 3. 22 0. 01384083
Re-equilibration in olivine
Equations for Geothermometry • Mg 22+Si. O 4 + 2 Fe 2+Cr 23+O 4 → Fe 22+Si. O 4 + 2 Mg 2+Cr 23+O 4 OLIVINE SPINEL • Mg 22+Si. O 4 + 2 Fe 2+Al 23+O 4 → Fe 22+Si. O 4 + 2 Mg 2+Al 23+O 4 OLIVINE SPINEL OLIVINE ONLY Mg 2+-Fe 2+ PARTITIONING ! SPINEL
Is that a problem ?
Cr entering Olivine Mg 22+Si. O 4 + 2 Fe 2+Cr 23+O 4 → Cr 22+Si. O 4 + Mg 2+Cr 23+O 4 + Mg 2+Fe 23+O 4 OLIVINE SPINEL OLIVINE Fe 2+ → Fe 3+ Cr 3+ → Cr 2+ SPINEL
The effect of Oxygen Fugacity. . . (1) on diffusion rate ? Yes
The effect of Oxygen Fugacity. . . (2) on chemical composition. . . of olivine and spinel at equilibrium ? The fugacity of oxygen has NO EFFECT on chemical composition as long as no other phase appears ! O 2 + 6 Mg 2 Si. O 4 + 6 Fe. Cr 2 O 4 → 6 Mg. Cr 2 O 4 + 6 Mg. Si. O 3 + Fe 3 O 4 OLIVINE SPINEL OLIVINE ORTHOPYROXENE SPINEL
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