Phase Transitions in the Earth's Mantle and Mantle Mineralogy

Phase transitions in Earth’s Mantle and Mantle Mineralogy Upper Mantle Minerals: Olivine, Orthopyroxene, Clinopyroxene and Garnet ~13.5 GPa: Olivine Æ Wadsrlyite (DE) transition (ONSET TRANSITION ZONE) ~15.5 GPa: Pyroxene component gradually dissolve into garnet structure, resulting in the completion of pyroxene-majorite transformation >20 GPa: High CaO content in majorite is unfavorable at high pressure, leading to the formation of CaSiO3 perovskite ~24 GPa: Division of transition zone and lower mantle. Sharp transition silicate spinel to ferromagnesium silicate perovskite and magnesiowustite >24 GPa: Most of Al2O3 resides in majorite at transition zone pressures, a transformation from Majorite to Al-bearing orthorhombic perovskite completes at pressure higher than that of post-spinel transformation Lower Mantle Minerals: Orthorhobic perovskite, Magnesiowustite, CaSiO3 perovskite ~27 GPa: Transformation of Al and Si rich basalt to perovskite lithology with assemblage of Al-bearing perovskite, CaSiO3, stishovite and Al-phases Upper Mantle: olivine, garnet and pyroxene Transition zone: olivine (a-phase) transforms to wadsleyite (b-phase) then to spinel structure (g-phase) and finally to perovskite + magnesio-wüstite. Transformations occur at P and T conditions similar to 410, 520 and 660 km seismic discontinuities Xenoliths: (mantle fragments brought to surface in lavas) 60% Olivine + 40 % Pyroxene + some garnet Garnet: A3B2(SiO4)3 Majorite FeSiO4, (Mg,Fe)2 SiO4 Germanates (Co-, Ni- and Fe- containing olivines MgSiO4, Olivine (ALPHA) 10 GPa = 300 Km Mantle E-Mg2SiO4 Wadsleyite [beta-spinel] Cr-Mg2SiO4 Chromium doped Forsterite Spinel group AB2O4 [MgAl2O4] (GAMMA) A: divalent Mg, Fe, Ni, Mn, Zn B: trivalent Al, Fe, Cr, Mn (possibly Ti4+ or Pb2+) 410 km discontinuity: Alpha Æ Beta transition responsible for this seismic velocity discontinuity in mantle 660 km discontinuity: divides lower mantle and transition zone (dissociation of ferromagnesium silicate spinel to denser mineral assemblage (20 GPa) Mg2SiO4-Fe2SiO4 Æ MgSiO3-FeSiO3 (Mg,Fe)2SiO4 (Gamma spinel ringwoodite) Æ (Mg,Fe)SiO3 + (Mg,Fe)O Review of High pressure techniques used for phase equilibrium study High pressure experimental study on chemical systems SiO2 Mg2SiO4 Fe2SiO4 MgSiO3 FeSiO3 Mg3Al2Si3O12 Fe3Al2Si3O12 CaSiO3 CaMgSi2O6 Review Experimental results on mantle peridotite compositions and high pressure phase transformations in the systems related to subducted oceanic lithosphere Piston Cylinder Apparatus For phase equilibrium measurements under crust and mantle conditions (up to 130 km) Multianvil Apparatus (MAIN FOCUS OF THIS PAPER) To extend the range of study up to transition zone (750 km) Features: Eight WC cubes with truncated corners separated from one another by compressible pyrophyllite gasket. Sample placed in a furnace assembly that fits into a hole in the center of octahedron formed by corners of truncated WC cube. LHDAC This cover whole mantle range but it suffers from a large thermal gradient, small sample size and achieving equilibrium Olivine One of major content and studied extensively because of its connection to 410 and 660 km Discontinuities. Three polymorphs of Mg2SiO4: Olivine E-phase (Wadselyite) J-Spinel or Ringwoodite Discrepancies in determination of phase boundaries because if difference in pressure scale at high temperature Spinel Æ Perovskite + MgO 22-25 GPa at Room temperature 23 GPa at RT <21 GPa at RT (After temperature correction To the pressure) Quartz Common Crustal Mineral Quartz Æ Coesite 3 GPa Coesite Æ Stishovite 9 GPa Stishovite Æ Dense CaCl2 type structure 50 GPa Post Stishovite phases Æ >50 GPa Discrepancy in DE-coesite triple point from the correction for friction in piston cylinder apparatus Coesite has been found in ultra high pressure metamorphic rock and as mineral inclusion in eclogite diamondÆ host rocks were subjected to pressures equivalent to depth of ~ 80 – 100 km No stishovite in metamorphic/igneous rock Except in shocked rocks and metrorites Its formation requires host rock to be deeper Than 300km Fe2SiO4 Direct transformation from Olivine to J-spinel Phase boundary between 1073 – 1473 K Post spinel transition reported at 17.3 GPa with no apparent temperature dependence Transition Pressure = 2.75 + 0.0025T (C) Phase Relation between Mg2SiO4 and Fe2SiO4 No wadsleyite in Fe2SiO4, therefore (Mg,Fe)2SiO4 wadsleyite solid solution does not extend to Fe-rich region. Max. solubility FeO in wadsleyite structure is 28 mol% at 1600 C Composition of coexisting D and E phases at three pressures Compression and Reversal points and shows that the calibartion of apparatus was very good At 13, 13.45 and 13.9 GPa the composition of D and E phase vary linearly with P and intersect at 14.6 GPa and Mg2SiO4 composition Reversal expt. 12.9, 13.35 GPa from pre-synth. E-phase and olivine Coexisting D-phase has Fe/Fe+Mg = 0.118 and 0.111 Using D and E as start mat. Coexisting b-phase comp. are 0.212 and 0.216 Phase Diagram of Mg2SiO4-Fe2SiO4 at 1600 C (Mg,Fe)SiO3 forms limited solid solution govern by (Mg,Fe)SiO3 Æ Mg.Fe)O + SiO2 Solubility is function of pressure and temp. MA expt.: 0.05 at 1000C to 0.12 at 1750 C at 26 GPa LHDAC expt. : ~28 mol% FeSiO3 can be dissolved into pv at 50 GPa and 1600C Six polymorphs of MgSiO3 perovskite: (1) Protoenstatite: Æ clinoenstatite at 8.1 GPa and 1000C (positive slope) (2) Orthoenstatite: Æ (3) Clinoenstatite: Æ At high pressure, decomposes to two phase region Wadsleyite + Stishovite or spinel + stishovite region Which separate the phase field of pyroxene and Ileminite At 1700 C 17 GPa pyroxene Æ tetragonal Garnet (majorite) Æ perovskite transition not yet determined (> 2000C) Al bearing majorite Æ perovskite has positive slope (4) Non-cubic garnet (5) Ilmenite Æ Perovskite at 24.3 GPa and 1000C (negative slope) (6) Perovskite Phase relations in perovskite (MgSiO3) and Pyrope (Mg3Al2Si3O12): Phase transformations in Pyrope: Pyrope (Mg3Al2Si3O12) Æ Ilmenite at 24 GPa and 1000C Pyrope – Ilmenite Æ Al bearing silicate perovskite + Al2O3 (corundum) at 26 GPa and >1000C Al2O3 solubility in perovskite increases with pressure Orthorhombic perovskite with pyrope composition forms: at 37 GPa Phase Transformation in FeSiO3, Fe3Al2Si3O12, CaSiO3, CaMgSi2O6 Ferrosillite FeSiO3: Perovskite, Majorite and Ilmenite are not stable FeSiO3 Æ Fe2SiO4 (spinel) + SiO2 (Stishovite) at 10 GPa Æ FexO (Wustite) + SiO2 (stishovite) at 17.3 GPa MgSiO3-FeSiO3 phase diagram shows: pyroxene-spinel-stishovite and spinelmagnesiowustite-stishovite loop in Fe-rich region and complex phase relations in Mgrich region Almandine (Fe3Al2Si3O12) Æ wustite + corundum + Stishovite at 21GPa Phase diagram of Mg3Al2Si3O12 - Mg3Al2Si3O12 not experimentally determined -Interpretation from phase relations of end members indicates that solubility of FeO in Al-bearing perovskite is limited - Change of Al2O3 solubility in perovskite at pressure between 26 – 37 GPa Like MgSiO3 - Mg3Al2Si3O12 system, FeSiO3 - Fe3Al2Si3O12 also forms solid solution by a hetrovalent substitution FeSi – AlAl. Solubility of FeSiO3 increases in Fe3Al2Si3O12 with pressure (max. solubility of 40 mol% at 9 GPa and 1000C) Clinopyroxene, garnet, (Ca, Mg) SiO3 and CaSiO3 are major Ca-bearing phases in Earth’s mantle CaSiO3 appears around 17 – 18 GPa depending on CaO content in bulk composition Walstromite CaSiO3 Æ Ca2SiO4 + CaSi2O5 at 10 GPa Æ CaSiO3 perovskite at12 GPa Diopside CaMgSi2O6 is end member of rock forming pyroxene Æ CaSiO3 perovskite and (Ca,Mg)SiO3 majorite at 17-18 GPa and 1300 C or to CaSiO3 perovskite + MgSiO3 Ilmenite below 1300 C or to CaSiO3 perovskite + Mg2SiO4 spinel + SiO2 stishovite at 19 GPa and 1500 C [ Discrepancy is due to uncertainty in determination phase transformation boundaries because of kinetics and in the pressure scale] BULK ROCKS Two competing petrological models: Pyrolite (perdotite): olivine rich (61% by volume) assemblage Four part dunite + one part basalt……chemical composition in Table 2 Piclogite: Clinopyroxene-garnet ricg, olivine bearing rock (< 50% by vol. olivine) From seismic data: 40% olivine + 37% c-pyroxene+13% garnet+10% opyroxene Sound velocity date: 38-50% olivine in upper mantle is reqd. to satify 410 km discontinuity (uncertainty due to temp. effect on velocity contrast) 410 km discontinuity 13-16 GPa in peridotite composition depending on temperature & 13-16 GPa and 1380 C in pyrolitic composition