Phase transformations between garnet and perovskite phases in the Earth’s mantle: A theoretical study (original) (raw)

Phase relations and equation-of-state of aluminous Mg-silicate perovskite and implications for Earth's lower mantle

Earth and Planetary Science Letters, 2004

We have investigated the effect of Al 3 + on the room-temperature compressibility of perovskite for stoichiometric compositions along the MgSiO 3 -AlO 1.5 join with up to 25 mol% AlO 1.5 . Aluminous Mg-perovskite was synthesized from glass starting materials, and was observed to remain a stable phase in the range of f 30 -100 GPa at temperatures of f 2000 to 2600 K. Lattice parameters for orthorhombic (Pbnm) perovskite were determined using in situ X-ray diffraction at SPring8, Japan. Addition of Al 3 + into the perovskite structure increases orthorhombic distortion and unit cell volume at ambient conditions (V 0 ). Compression causes anisotropic decreases in axial length, with the a axis more compressive than the b and c axes by about 25% and 3%, respectively. The magnitude of orthorhombic distortion increases with pressure, but aluminous perovskite remains stable to pressures of at least 100 GPa. Our results show that substitution of Al 3 + causes a mild increase in compressibility, with the bulk modulus (K 0 ) decreasing at a rate of À67 F 35 GPa/X Al . This decrease in K 0 is consistent with recent theoretical calculations if essentially all Al 3 + substitutes equally into the six-and eight-fold sites by charge-coupled substitution with Mg 2 + and Si 4 + . In contrast, the large increase in compressibility reported in some studies with addition of even minor amounts of Al is consistent with substitution of Al 3 + into six-fold sites via an oxygen-vacancy forming substitution reaction. Schematic phase relations within the ternary MgSiO 3 -AlO 1.5 -SiO 2 indicate that a stability field of ternary defect Mgperovskite should be stable at uppermost lower mantle conditions. Extension of phase relations into the quaternary MgSiO 3 -AlO 1.5 -FeO 1.5 -SiO 2 based on recent experimental results indicates the existence of a complex polyhedral volume of Mgperovskite solid solutions comprised of a mixture of charge-coupled and oxygen-vacancy Al 3 + and Fe 3 + substitutions. Primitive mantle with about 5 mol% AlO 1.5 and an Fe 3+ /(Fe 3+ +Fe 2+ ) ratio of f 0.5 is expected to be comprised of ferropericlase coexisiting with Mg-perovskite that has a considerable component of Al 3 + and Fe 3 + defect substitutions at conditions of the uppermost lower mantle. Increased pressure may favor charge-coupled substitution reactions over vacancy forming reactions, such that a region could exist in the lower mantle with a gradient in substitution mechanisms. In this case, we expect the physical and transport properties of Mg-perovskite to change with depth, with a softer, probably more hydrated, 0012-821X/$ -see front matter D address: M.J.Walter@bristol.ac.uk (M.J. Walter). www.elsevier.com/locate/epsl Earth and Planetary Science Letters 222 (2004) 501 -516 defect dominated Mg-perovskite at the top of the lower mantle, grading into a stiffer, dehydrated, charge-coupled substitution dominated Mg-perovskite at greater depth. D

Subsolidus phase relations and perovskite compressibility in the system MgO–AlO1.5–SiO2 with implications for Earth's lower mantle

Earth and Planetary Science Letters, 2006

Experimentally determined phase relations in the system MgO-AlO 1.5 -SiO 2 at pressures relevant to the upper part of the lower mantle indicate that Mg-silicate perovskite incorporates aluminum into its structure almost exclusively by a charge-coupled reaction. MgSiO 3 -rich bulk compositions along the joins MgSiO 3 -MgAlO 2.5 and MgSiO 3 -MgAl 2 O 4 crystallize assemblages of perovskite coexisting with periclase. MgO-saturated perovskites along these joins have ambient unit cell volumes consistent with those measured and calculated for aluminous perovskite along the charge-coupled substitution join, MgSiO 3 -AlO 1.5 . The compressibility of aluminous perovskite along the MgO-saturated joins is not anomalously low as predicted for oxygen-defect perovskites. The bulk moduli, however, are consistent with previous measurements made for aluminous perovskites along the charge-coupled substitution join. These results agree with first-principles calculations showing very limited stability of O-defects in Mg-perovskite at pressures and temperatures corresponding to lower mantle conditions, but are inconsistent with earlier experimental results showing unusually compressive aluminous perovskite. The maximum solubility of alumina in perovskite is ∼25 mol% along the MgSiO 3 -AlO 1.5 join within the ternary MAS-system (i.e. pyrope composition), and the join is apparently binary. Although primitive mantle peridotite compositions are MgO-saturated and fall nearly on the oxygen vacancy join, alumina substitution into perovskite is expected to occur primarily by charge-coupled substitution throughout the lower mantle. The compressibility of aluminous perovskite in primitive mantle is expected to be only a few percent lower than for end member MgSiO 3 perovskite.

High-Temperature Phase Transition and Dissociation of (Mg, Fe)SiO3 Perovskite at Lower Mantle Pressures

Science, 1995

To study the crystallography of Earth's lower mantle, techniques for measuring synchrotron x-ray diffraction from a laser-heated diamond anvil cell have been developed. Experiments on samples of (Mg,Fe)Si0 3 show that silicate perovskite maintains its orthorhombic symmetry at 38 gigapascals and 1850 kelvin. Measurements at 65 and 70 gigapascals provide evidence for a temperature-induced orthorhombic-to-cubic phase transition and dissociation to an assemblage of perovskite and mixed oxides. If these phase transitions occur in Earth, they will require a significant change in mineralogical models of the lower mantle.

In situ observations of phase transition between perovskite and CaIrO-type phase in MgSiO and pyrolitic mantle composition

Earth and Planetary Science Letters, 2005

In situ observations of the perovskite-CaIrO 3 phase transition in MgSiO 3 and in pyrolitic compositions were carried out using a laser-heated diamond anvil cell interfaced with a synchrotron radiation source. For pure MgSiO 3 , the phase boundary between the orthorhombic Mg-perovskite and CaIrO 3 -type phases in the temperature range of 1300-3100 K was determined to be P (GPa) = 130 (F 3) + 0.0070 (F 0.0030) Â (T À 2500) (K) using platinum as a pressure calibrant. We confirmed that the CaIrO 3 -type phase remained stable up to pressures of at least 156 GPa and temperatures of 2600 K. The consistency of our results with previous theoretical calculations leads us to conclude that the 2700 km seismic discontinuity at the bottom of the lower mantle can be attributed to a phase transition to the CaIrO 3 -type phase. The phase change from an orthorhombic Mgperovskite to a CaIrO 3 -type bearing assemblage in a pyrolitic mantle composition was also observed at P = 125 GPa, which corresponds to the same mantle depth as the seismic discontinuity. The phase boundary between the orthorhombic Mgperovskite and CaIrO 3 -type bearing assemblage was determined to be P (GPa) = 124 (F 4) + 0.008 (F 0.005) Â (T À 2500) (K) using gold as a pressure calibrant. This transition boundary indicates that the temperature at a depth of 2700 km is about 2600 K, and the adiabatic temperature gradient in the lower mantle is estimated to be 0.31 K/km. The partition coefficients and the effect of some elements on the phase equilibrium between the orthorhombic MgSiO 3 perovskite and CaIrO 3 -type MgSiO 3 were estimated from ab initio calculations. Our experimental and theoretical results indicate that the DW layer consists of a CaIrO 3 -type bearing assemblage which is likely to have significant effect on the chemical and thermal evolution of the Earth's mantle. D

Perovskite Phase Relations in the System CaO-MgO-TiO2-SiO2 and Implications for Deep Mantle Lithologies

Journal of Petrology, 2012

Experiments at 20^97 GPa and 2000 K in the system CaO^MgOT iO 2^S iO 2 constrain phase relations involving Mg-rich and Ca-rich perovskite solid solutions at conditions relevant to the Earth' s deep Transition Zone and lower mantle. Bulk compositions were investigated with molar Ti/(Ti þ Si) up to 0•5 within the quasi-ternary ' perovskite plane' , which is defined by a reciprocal solution among the components MgSiO 3 , MgTiO 3 , CaSiO 3 , and CaTiO 3. Multi-anvil experiments at 20 GPa and 2000 K on bulk compositions within the plane produce akimotoite coexisting with Ca-perovskites that lie close to the CaSiO 3^C aTiO 3 join. Higher-pressure experiments using a laser-heated diamond anvil cell constrain the position of a two-perovskite field that extends into the perovskite plane from the solvus along the MgSiO 3^C aSiO 3 binary join, where limited mutual solubility exists between MgSiO 3 and CaSiO 3 perovskites. On the join MgSiO 3^M gTiO 3 , MgTiO 3 solubility in MgSiO 3 perovskite increases with pressure, with MgSi 0•8 Ti 0•2 O 3 perovskite stable at 50GPa.Limitedreciprocalsolutionat50 GPa. Limited reciprocal solution at 50GPa.Limitedreciprocalsolutionat25 GPa results in an expansive two-perovskite field that occupies much of the Si-rich portion of the perovskite plane. Solution of Ti into Mg-rich and Ca-rich perovskites enhances the solubility of reciprocal Ca and Mg components, respectively. Increase in pressure promotes reciprocal solution, and the two-phase field collapses rapidly with pressure toward the MgSiO 3^C aSiO 3 join. We find that a single-phase, orthorhombic perovskite with near equimolar Ca and Mg is stable in a composition with Ti/ (Ti þ Si) of only 0•05 at 97 GPa, requiring that by this pressure the two-phase field occupies a small area close to the MgSiO 3^C aSiO 3 join. On the basis of experiments at$1500 K, temperature has only a mild effect on the position of the Ca-rich limb of the solvus. Ca(Ti,Si)O 3 mineral inclusions in deep sublithospheric diamonds could not have formed in equilibrium with Mg-perovskite owing to their virtual lack of MgSiO 3 component at pressures of Mg-perovskite stability, but may have equilibrated with Transition Zone MgSiO 3-rich phases at lower pressures; this observation can be extended generally to near-endmember CaSiO 3 inclusions. On an iron-free basis, the average bulk compositions of clinopyroxene^ilmenite and orthopyroxene^ilmenite megacrysts from kimberlites plot in single-perovskite fields at pressures greater than about 45 and 65 GPa, respectively, when projected onto the perovskite plane.We predict that the effect of iron will not be large, and estimate that single-phase perovskites may form at somewhat lower pressures than in the iron-free system. Thus, the origin of pyroxene^ilmenite megacrysts from single-phase perovskite solutions in the lower mantle is plausible on the basis of phase relations, although a lower pressure magmatic origin appears more likely. Deeply subducted Ti-rich lithologies such as ocean-island basalt will crystallize a single perovskite rather than a two-perovskite assemblage beginning at pressures of $80 GPa. Normal mid-ocean ridge basalt and primitive mantle peridotite are expected to remain within a two-phase perovskite field until Mg-perovskite transforms to post-perovskite.

P-V-T equation of state of (Mg,Fe)SiO3 perovskite: constraints on composition of the lower mantle

Physics of The Earth and Planetary Interiors, 1994

Unit-cell volumes (V) of Mg11Fe~SiO3 perovskite (x = 0.0 and 0.1) have been measured along several isobaric paths up to P = 11 GPa and T= 1300 K using a DIA-type, cubic anvil high-pressure apparatus (SAM-85). With a combination of X-ray diffraction during heating cycles and Raman spectroscopy on recovered samples, pressure and temperature conditions were determined under which the P-V-T behavior of the perovskite remains reversible. At 1 bar, perovskites of both compositions begin to transform to amorphous phases at T 400 K, accompanied by an irreversible cell volume contraction. Electron microprobe and analytical electron microscopy studies revealed that the iron-rich perovskite decomposed into at least two phases, which were Fe and Si enriched, respectively. At pressures above 4 GPa, the P-V-T behavior of MgSiO3 perovskite remained reversible up to about 1200 K, whereas the Mg09Fe015i03 exhibited an irreversible behavior on heating. Such irreversible behavior makes equation-of-state data on Fe-rich samples dubious. Thermal expansivities (ay) of MgSiO3 perovskite were measured directly as a function of pressure. Overall, our results indicate a weak pressure dependence in a~,for MgSiO3. Analyses on the P-V-T data using various thermal equations of state yielded consistent results on thermoelastic properties. The temperature derivative of the bulk modulus, (oK~/~T)~, is -0.023(±0.011)GPa K 1 for MgSiO 3 perovskite. Using these new results, we examine the constraints imposed by av and (8K/OT)~on the Fe/(Mg + Fe) and (Mg + Fe)/Si ratios for the lower mantle. For a temperature of 1800 K at the foot of an adiabat (zero depth), these results indicate an overall iron content of Fe/(Mg + Fe) = 0.12(1) for a lower mantle composed of perovskite and magnesiowüstite. Although the (Mg + Fe)/Si ratio is very sensitive to the thermoelastic parameters of the perovskite and it is tentatively constrained between 1.4 and 2.0, these results indicate that it is unlikely for the Jower mantle to have a perovskite stoichiometry.

Fe-Mg partitioning between (Mg, Fe)SiO3post-perovskite, perovskite, and magnesiowüstite in the Earth's lower mantle

Geophysical Research Letters, 2005

We report here new data on pressure dependence of Fe-Mg partitioning between (Mg, Fe)SiO 3 perovskite (Pv) and magnesiowüstite (Mw), K Pv/Mw , and (Mg, Fe)SiO 3 postperovskite (PPv) and Mw, K PPv/Mw , up to 123.6 GPa at 1600 K measured by synchrotron X-ray diffraction method and analytical transmission electron microscopy (ATEM). We observed a high FeO content in PPv coexisting with Mw [K PPv/Mw = (FeO/MgO) PPv /(FeO/MgO) Mw = 0.30] compared to that in Pv [K Pv/Mw = (FeO/MgO) Pv /(FeO/ MgO) Mw = 0.12] observed from 23.0 to 95.4 GPa. K Pv/Mw keeps a constant value of 0.12 up to the PPv phase boundary. Our results also support the possibility that a metallic phase may form in the lower mantle. The assemblage of PPv and Mw is 1.5-1.7% denser than the Pv bearing assemblage, which results in a gravitational stabilization of the lowermost mantle.