Post-spinel Phases Research Articles

Features of isomorphism of post-spinel phases: result of computer simulation of the composition of inclusions in lower mantle diamonds

The mixing properties of solid solutions of post-spinel phases of the composition CaCr2O4–CaAl2O4, CaCr2O4–CaFe2O4, MgCr2O4–MgAl2O4 and MgCr2O4–MgFe2O4 in the temperature range 1873– 2223 K and pressures 18–25 GPa were studied using the method of semi-empirical modeling. With these PT parameters, the energies of formation of impurity defects of trivalent metal ions (aluminum and iron) in isomorphic sites were estimated. It is shown that (1) the studied binary solid solutions are characterized by complete miscibility, (2) the incorporation of the Fe3+ impurity ion into the post-spinel phases of MgCr2O4 and CaCr2O4 is less favorable in terms of energy than the incorporation of Al3+ ions over the entire range of pressures and temperatures under study. The results obtained were used to interpret the composition of post-spinel phases forming inclusions in lower mantle diamonds.

New high-pressure polymorphs of magnetite, ilmenite, olivine, pyroxenes and feldspars

The new high-pressure shock-produced minerals discovered in meteorites can be considered as the possible components of the deep-seated geospheres. The structure peculiarities of the recently discovered minerals of this genetic type and of the geophysical interest are discussed in the paper. The special attention is addressed on the structure transformation under high pressure of the (Fe,Ti)-oxides and the new HP-polymorphs of ilmenite, post-spinel phases, magnetite, olivine, pyroxene and feldspars. The high-pressure polymorphs of these minerals are thought to be the constituent of the deep Earth’s interior. The crystal structures and composition of new НР-minerals discovered in meteorites and in the rocks near terrestrial impact craters provide the richer information on the complicate petrology of deep-seated geospheres which cannot be obtained on the base of laboratory experiments. On this basis it is also possible to predict their role as potentially important carriers of large ion lithophile elements in the Earth’s mantle and, hence, important contributors to the Earth’s heat budget.

High P-T phase relations of Al-bearing magnetite: Post-spinel phases as indicators for P-T conditions of formation of natural samples

Abstract The phase relations of Al-bearing magnetite were investigated between 6–22 GPa and 1000–1550 °C using a multi-anvil apparatus. This study demonstrates that the spinel-structured phase persists up to ~9–10 GPa at 1100–1400 °C irrespective of the amount of hercynite (FeAl2O4) component present (20, 40, or 60 mol%). At ~10 GPa, the assemblage Fe2(Al,Fe)2O5 + (Al,Fe)2O3 forms and remains stable up to 16–20 GPa and 1200–1550 °C. Fe2(Al,Fe)2O5 adopts the CaFe3O5-type structure with the Cmcm space group. At 18–22 GPa and T >1300 °C the assemblage Fe3(Fe,Al)4O9 + (Al,Fe)2O3 becomes stable. Fe3(Fe,Al)4O9 is isostructural with Fe7O9, having the monoclinic structure of the C2/m space group. At T <1300 °C, Fe3(Fe,Al)4O9 + (Al,Fe)2O3 gives way to the assemblage of a hp-Fe(Fe,Al)2O4 + (Al,Fe)2O3. This hp-Fe(Fe,Al)2O4 phase is unquenchable; a defect-bearing spinel-structured phase was recovered instead, and it contained numerous lamellae parallel to {100} or {113} planes and notably less Al than the initial starting composition. While low-pressure spinel can have a complete solid solution between Fe3+-Al, the post-spinel phases have only very limited Al solubility, with a maximum of ~0.1 cpfu Al in hp-Fe(Fe,Al)2O4, ~0.3 cpfu in Fe2(Fe,Al)2O5, and ~0.4 cpfu in Fe3(Fe,Al)4O9, respectively. As a result, the phase relations of Fe(Fe0.8Al0.2)2O4 can also be applied to bulk compositions richer in Al with the only difference being that larger amounts of an (Al,Fe)2O3 phase are present. Coexisting rhombohedral-structured phases demonstrate that the binary miscibility gap established at low pressure between hematite and corundum is still valid up to 20 GPa. Since iron oxides (e.g., magnetite) with variable Al contents are found in extraterrestrial rocks or as inclusions in diamond, constraints on their high-P-T-fO2 stability might help unravel their formation conditions.

Isomorphism Features of Post-Spinel Phases: Computer Modeling of Inclusion Composition in Lower Mantle Diamonds

Isomorphism Features of Post-Spinel Phases: Computer Modeling of Inclusion Composition in Lower Mantle Diamonds

Erratum to: Post-Spinel Phases in the Earth’s Mantle

Erratum to: Post-Spinel Phases in the Earth’s Mantle

Post-Spinel Phases in the Earth’s Mantle

—The post-spinel phases include compounds with stoichiometry {{A}^{{2 + }}}B_{2}^{{3 + }}{{{text{O}}}_{4}}left( {A_{2}^{{2 + }}{{B}^{{4 + }}}{{{text{O}}}_{4}}} right) and structures of the calcium ferrite CaFe2O4, calcium titanate CaTi2O4, and marokite CaMn2O4 types. The structures with a centered Cmcm (Bbmm) and primitive Pnma (Pmcn) and Pbcm (Pmab) cells are distinguished in this family of topologically related compounds with a “marokite” channel formed by six octahedra. The sites A and B are occupied by various cations, in particular, Cr, Al, Mg, Fe, Ca, Ti, Fe, Na, and Si, which implies the formation of solid solutions of a wide compositional range. In nature, such high-pressure phases were found in meteorites, as inclusions in diamond crystals, and in rocks from some metamorphic complexes. This review provides a characterization of natural mineralogical finds, the results of an experimental study of post-spinel phases of various compositions and their solid solutions, as well as crystal chemical simulation and assessment of likely compositions and the areas of stability of compounds with a “marokite” channel. The discrepancy between the results of individual studies indicates the necessity to clarify the stability parameters and probable isostructural transitions, and, ultimately, to improve the classification of post-spinel phases.

High-Pressure Phase Relations and Crystal Structures of Postspinel Phases in MgV2O4, FeV2O4, and MnCr2O4: Crystal Chemistry of AB2O4 Postspinel Compounds.

We have investigated high-pressure, high-temperature phase transitions of spinel (Sp)-type MgV2O4, FeV2O4, and MnCr2O4. At 1200-1800 °C, MgV2O4 Sp decomposes at 4-7 GPa into a phase assemblage of MgO periclase + corundum (Cor)-type V2O3, and they react at 10-15 GPa to form a phase with a calcium titanite (CT)-type structure. FeV2O4 Sp transforms to CT-type FeV2O4 at 12 GPa via decomposition phases of FeO wüstite + Cor-type V2O3. MnCr2O4 Sp directly transforms to the calcium ferrite (CF)-structured phase at 10 GPa and 1000-1400 °C. Rietveld refinements of CT-type MgV2O4 and FeV2O4 and CF-type MnCr2O4 confirm that both the CT- and CF-type structures have frameworks formed by double chains of edge-shared B3+O6 octahedra (B3+ = V3+ and Cr3+) running parallel to one of orthorhombic cell axes. A relatively large A2+ cation (A2+ = Mg2+, Fe2+, and Mn2+) occupies a tunnel-shaped space formed by corner-sharing of four double chains. Effective coordination numbers calculated from eight neighboring oxygen-A2+ cation distances of CT-type MgV2O4 and FeV2O4 and CF-type MnCr2O4 are 5.50, 5.16, and 7.52, respectively. This implies that the CT- and CF-type structures practically have trigonal prism (six-coordinated) and bicapped trigonal prism (eight-coordinated) sites for the A2+ cations, respectively. A relationship between cation sizes of VIIIA2+ and VIB3+ and crystal structures (CF- and CT-types) of A2+B23+O4 is discussed using the above new data and available previous data of the postspinel phases. We found that CF-type A2+B23+O4 crystallize in wide ionic radius ranges of 0.9-1.4 Å for VIIIA2+ and 0.55-1.1 Å for VIB3+, whereas CT-type phases crystallize in very narrow ionic radius ranges of ∼0.9 Å for VIIIA2+ and 0.6-0.65 Å for VIB3+. This would be attributed to the fact that the tunnel space of CT-type structure is geometrically less flexible due to the smaller coordination number for A2+ cation than that of CF-type.

Phase relations of Fe-Mg spinels including new high-pressure post-spinel phases and implications for natural samples

Phase relations of magnesioferrite-magnetite solid solutions (Mg,Fe 2+ )Fe 2 3+ O 4 were investigated at pressures of 9–23 GPa and temperatures of 1200–1600 °C. Our new results indicate that the assemblage Mg 2 Fe 2 O 5 + Fe 2 O 3 reconstitutes to a hp-MgFe 2 O 4 phase at 20 GPa and 1300–1500 °C. The stability field of hp-MgFe 2 O 4 begins at ~1300 °C and widens to higher temperature. At lower temperature (1200–1300 °C) Mg 2 Fe 2 O 5 + Fe 2 O 3 breaks down to the new phase assemblage Mg 3 Fe 4 O 9 + Fe 2 O 3 with its stability field expanding to higher pressures and temperatures at the expense of hp-MgFe 2 O 4 . The Mg 3 Fe 4 O 9 phase has the same crystal structure that recently reported for Fe 7 O 9 , and thus represents the Mg-end-member. From powder X-ray diffraction, we find that hp-MgFe 2 O 4 has a structure consistent with an orthorhombic unit cell belonging to the Pmcn space group (no. 62). However, it could have undergone a transformation from a different structure during decompression. Experiments conducted with a Mg 0.5 Fe 2+ 0.5 Fe 2 3+ O 4 composition demonstrate that the addition of Fe 2+ significantly changes the topology of the phase relations compared to the MgFe 2 O 4 end-member system. At 10–11 GPa and 1000–1600 °C, Mg 0.5 Fe 0.5 Fe 2 O 4 breaks down to the assemblage MgFeFe 2 O 5 + Fe 2 O 3 , with the phase boundary described by: P (GPa) = 2.0 × 10 − 3 × T (°C) + 8.2. No stability field for the constituent oxides [i.e., (Mg,Fe)O + Fe 2 O 3 ] exists, in contrast to that observed for the MgFe 2 O 4 end-member. The stability of the assemblage MgFe 2+ Fe 2 3+ O 5 + Fe 2 O 3 is limited at higher pressures and appears to pinch out to higher temperatures. At 15–16 GPa and temperatures up to 1350 °C, this assemblage reconstitutes to form a hp-Mg 0.5 Fe 0.5 Fe 2 O 4 phase. However, at higher temperatures a new assemblage of (Mg,Fe) 3 Fe 4 O 9 + Fe 2 O 3 appears. The occurrence of such compositions suggests that solid solution may be complete across the Mg 3 Fe 4 O 9 –Fe 7 O 9 binary. Our results further demonstrate that phase relations even in simple Fe-Mg oxides can become complex at high pressures and temperatures and that phases with various novel stoichiometries (i.e., Mg 3 Fe 4 O 9 ) may become stable. In addition, this study has implications for natural samples by helping to place constraints on the range in pressure and temperature at which a given sample formed. For instance, magnetite or magnesioferrite entrapped as inclusions in diamond could have either have crystallized directly, or formed from precursor phases at depths that exceed the stability of the spinel-structured phases. Evidence for such high-pressure transformations can potentially be found by investigating micro-textures.

The Electronic Structure of Structurally Strained Mn3O4Postspinel and the Relationship with Mn3O4Spinel

We report the electronic structure of Mn3O4 postspinel for the first time. In contrast with Mn3O4 spinel, Mn3O4 postspinel adopts a CaMn2O4-type structure with a built-in strain. We conducted both optical measurements and ab initio calculations, and systematically studied the electronic band structures of both the postspinel and spinel Mn3O4 phases. The theoretical electronic structure of Mn3O4 postspinel is consistent with the optical absorption spectra, and both Mn3O4 postspinel and spinel display characteristic band-splitting of the conduction band. The band gap of the postspinel phase is 0.9–1.3 eV smaller than that of the spinel phase. This difference can be explained by the lowering of Mn3+ 3d eg level related to the built-in strain of the postspinel structure. The Mn 3d \(t_{2g}\) and O 2p form antibonding orbitals situated at the conduction band with higher energy.

Structural and chemical characterization of Mg[(Cr,Mg)(Si,Mg)]O4, a new post-spinel phase with sixfold-coordinated silicon

The crystal structure and chemical composition of a crystal of Mg(Mg,Cr,Si) 2 O 4 post-spinel phase synthesized in the model system MgCr 2 O 4 –Mg 2 SiO 4 at 16 GPa and 1600 °C have been investigated. The compound was found to crystallize with a distorted orthorhombic calcium-titanate (CaTi 2 O 4 ) structure type, space group Cmc 2 1 , with lattice parameters a = 2.8482(1), b = 9.4592(5), c = 9.6353(5) A, V = 259.59(1) A 3 , and Z = 4. The structure was refined to R 1 = 0.018 using 345 independent reflections. The loss of the inversion center is due to the ordering of cations at the octahedral sites: Cr is mainly hosted at the M1 site, whereas Si at the M2 site. Such an ordered distribution induces a distortion thus provoking a change in coordination of Mg, which becomes sevenfold-coordinated instead of the usual eightfold coordination observed in post-spinel phases. Electron microprobe analysis gave the Mg[(Cr 0.792 Mg 0.208 )(Si 0.603 Mg 0.397 )]O 4 stoichiometry for the studied phase. The successful synthesis of this phase can provide new constraints on thermobarometry of wadsleyite/ringwoodite-bearing assemblages.

A MICROINCLUSION OF LOWER-MANTLE ROCK AND OTHER MINERAL AND NITROGEN LOWER-MANTLE INCLUSIONS IN A DIAMOND

A microcrystalline polymineral aggregate was identified as an inclusion in a diamond from the Rio Soriso area, Mato Grosso State, Brazil. It is composed of iron carbides, Fe-rich periclase (with exsolved magnesioferrite), and two orthorhombic, postspinel phases — Mg-Cr-Fe and Ca-Cr oxides, which are new mineral phases. This association was formed during several stages and may be considered as a rock microfragment from the lower mantle. In addition to the carbide-oxide fragment, other phases were identified as inclusions in the diamond, such as parascandolaite KMgF3 (that is stable at pressures of up to 50 GPa) in association with orthorhombic MgO, and porous dolomite occurring together with nanoinclusions of calcite, apatite, spinel, Fe-sulfide, and an assemblage of periclase plus wustite. These minerals belong to the carbonatitic association and, along with syngenetic nanoinclusions of fluid nitrogen, represent the media of diamond formation. The assemblage of periclase and wustite within the carbonatitic matrix points to the origin of diamond from a carbonatitic environment within the lower mantle under pressure conditions of ≥ 85–86 GPa (~1,900 km depth).

High-pressure high-temperature transitions in MgCr2O4 and crystal structures of new Mg2Cr2O5 and post-spinel MgCr2O4 phases with implications for ultrahigh-pressure chromitites in ophiolites

We determined phase relations in MgCr2O4 at 12–28 GPa and 1000–1600 °C using a multi-anvil apparatus. At 12–15 GPa, spinel-type MgCr2O4 (magnesiochromite) first decomposes into a mixture of new Mg2Cr2O5 phase + corundum-type Cr2O3 at 1100–1600 °C, but it dissociates first into MgO periclase + corundum-type Cr2O3 at l000 °C. At about 17–19 GPa, the mixture of Mg2Cr2O5 phase + corundum-type Cr2O3 transforms to a single MgCr2O4 phase. Structure refinements using synchrotron X-ray powder diffraction data indicated that the high-pressure MgCr2O4 phase has a CaTi2O4-type structure ( Cmcm ), and that the basic structure of the Mg2Cr2O5 phase is the same as that of recently found modified ludwigite-type Mg2Al2O5 and Fe2Cr2O5 ( Pbam ). The phase relations in this study may suggest that natural chromitites in the Luobusa ophiolite regarded as the deep-mantle origin were derived from the mantle shallower than the depths corresponding to pressure of 12–15 GPa because of absence of the assemblage of (Mg,Fe)2Cr2O5 + Cr2O3 in the chromitites.

High-pressure transformation in the cobalt spinel ferrites

We report high pressure angle-dispersive x-ray diffraction measurements on CoxFe3−xO4 (x=1, 1.5, 1.75) spinels at room temperature up to 34GPa. The three samples show a similar structural phase transformation from the cubic spinel structure to an analogous post-spinel phase at around 20GPa. Spinel and post-spinel phases coexist in a wide pressure range (~20–25GPa) and the transformation is irreversible. The equation of state of the three cubic spinel ferrites was determined and our results agree with the data obtained in related oxide spinels showing the role of the pressure-transmitting medium for the accurate determination of the equation of state.Measurements releasing pressure revealed that the post-spinel phase is stable down to 4GPa when it decomposes yielding a new phase with poor crystallinity. Later compression does not recover either the spinel or the post-spinel phases. This phase transformation induced by pressure explains the irreversible lost of the ferrimagnetic behavior reported in these spinels.

High-pressure phase transitions in FeCr2O4 and structure analysis of new post-spinel FeCr2O4 and Fe2Cr2O5 phases with meteoritical and petrological implications

We determined phase relations in FeCr 2 O 4 at 12–28 GPa and 800–1600 °C using a multi-anvil apparatus. At 12–16 GPa, FeCr 2 O 4 spinel (chromite) first dissociates into two phases: a new Fe 2 Cr 2 O 5 phase + Cr 2 O 3 with the corundum structure. At 17–18 GPa, the two phases combine into CaFe 2 O 4 -type and CaTi 2 O 4 -type FeCr 2 O 4 below and above 1300 °C, respectively. Structure refinements using synchrotron X-ray powder diffraction data confirmed the CaTi 2 O 4 -structured FeCr 2 O 4 ( Cmcm ), and indicated that the Fe 2 Cr 2 O 5 phase is isostructural to a modified ludwigite-type Mg 2 Al 2 O 5 ( Pbam ). In situ high-pressure high-temperature X-ray diffraction experiments showed that CaFe 2 O 4 -type FeCr 2 O 4 is unquenchable and is converted into another FeCr 2 O 4 phase on decompression. Structural analysis based on synchrotron X-ray powder diffraction data with transmission electron microscopic observation clarified that the recovered FeCr 2 O 4 phase has a new structure related to CaFe 2 O 4 -type. The high-pressure phase relations in FeCr 2 O 4 reveal that natural FeCr 2 O 4 -rich phases of CaFe 2 O 4 - and CaTi 2 O 4 -type structures found in the shocked Suizhou meteorite were formed above about 18 GPa at temperature below and above 1300 °C, respectively. The phase relations also suggest that the natural chromitites in the Luobusa ophiolite previously interpreted as formed in the deep-mantle were formed at pressure below 12–16 GPa.

Phase Stability of Post-spinel Compound AMn2O4 (A = Li, Na, or Mg) and Its Application as a Rechargeable Battery Cathode

At high pressures, spinel compounds can transform to CaFe2O4, CaMn2O4, or CaTi2O4 phases, which are regarded as post-spinel phases. Here, first-principles calculations are used to systematically study the stability of post-spinel LiMn2O4, NaMn2O4, and MgMn2O4, as well as their potential application as rechargeable battery cathodes. Thermodynamically, the stability of the post-spinel phase is highly related to the electronic configuration of transition-metal ions. By changing the concentration of Jahn–Teller active Mn3+, the relative stabilities of post-spinel phases can be easily monitored. It provides a practical way to obtain post-spinel compounds with desirable structures. Kinetically, post-spinel phases can be stable under ambient conditions, because of the high barrier that must be overcome to rearrange MnO6 octahedrons. The most spectacular finding in this work is the high cationic mobility in post-spinel compounds. The activation energy barrier of the migration of Mg2+ in CaFe2O4-type MgMn2O4 is 0.4 eV, suggesting that the mobility of Mg2+ in this compound is comparable to that of Li+ in typical Li-ion battery cathodes. To explore the potential application of post-spinel compounds as rechargeable battery cathodes, the voltage profile for the electrochemical insertion/removal of Mg in CaFe2O4-type MgMn2O4 is predicted. Its theoretical energy density is 1.3 times greater than that of typical Li-ion battery cathodes. These outstanding properties make CaFe2O4-type MgMn2O4 an attractive cathode candidate for rechargeable Mg batteries.

Investigations of meta-stable and post-spinel silicon nitrides

Since the discovery of post-spinel Si3N4, its fundamental physical properties are highly required. In this paper, theoretical calculations are performed to investigate the structural and elastic properties of the β-, γ-, wII- and post-spinel Si3N4 polymorphs. The calculated ground-state properties compare well with available experiments. The phase transformations of the β-, γ-, wII- and post-spinel phases are investigated by the famous plane-wave pseudo-potential density functional theory. From the elastic constants obtained, we find that β-, γ- and wII-Si3N4 are stable at 0 GPa and the post-spinel phase is unstable/stable at 0 GPa/160 GPa. When the high-temperature β→γ transformation is bypassed due to kinetic reasons, β-Si3N4 is predicted to undergo a first-order phase transition to a new phase (wII-Si3N4). It is found that the transition pressures of β→wII and γ→post-spinel transitions are 20.8 GPa and 152.5 GPa, respectively. The phase boundary of the γ→post-spinel transition can be described as P=152.3631−6.39×10−3T+2.01062×10−5T2−1.93962×10−9T3. Through the quasi-harmonic approximation, the dependences of heat capacity, entropy, thermal expansion coefficient and the Debye temperature on temperature, are also successfully predicted.

Structural transition of post-spinel phases CaMn2O4, CaFe2O4, and CaTi2O4 under high pressures up to 80 GPa

Three structures of CaMn 2 O 4 , CaFe 2 O 4 , and CaTi 2 O 4 have been proposed as post-spinel phases. Because these structures are very similar, several ambiguities and inconsistencies appear in high-pressure studies, leading to many problems that are yet to be solved. Systematic powder diffraction studies related to these three phases were conducted under high pressure using synchrotron radiation. All three samples have further high-pressure polymorphs. CaMn 2 O 4 transforms to the CaTi 2 O 4 -type structure at about 30 GPa. The MnO 6 octahedron in the lower-pressure structure is distorted by the Jahn-Teller effect. A new phase was observed at pressures above 50 GPa during compression of CaFe 2 O 4 . Rietveld profile fitting analysis of diffraction data at 63.3 GPa demonstrated that the high-pressure structure, with space group Pnam, is produced via a martensitic transformation by displacing atoms in every third layer perpendicular to the c axis. CaTi 2 O 4 also has a new high-pressure polymorph above 39 GPa with space group Bbmm. The most probable post-spinel candidate in the mantle is the CaTi 2 O 4 -type structure. The CaMn 2 O 4 -type structure is only formed at high pressure from spinel phases with atoms susceptible to Jahn-Teller distortion.

Theoretical Prediction of Post‐Spinel Phases of Silicon Nitride

New phases of Si3N4 that may be stable at higher pressure than spinel have been searched using a first‐principles plane‐wave pseudopotential method. The CaTi2O4‐type phase is found to be the prime candidate for the post‐spinel phase among six phases selected on the analogy to high‐pressure oxides. The phase transformation from the spinel is predicted to occur at 210 GPa. All silicon atoms of the new phase are coordinated by six anions, similar to the case of the high‐pressure forms of SiO2 and SiC. Because of its high energy at zero pressure, this new phase may be difficult to quench. The bandgap increases with an increase of pressure when compared in the same polymorph. However, the bandgap and the net charge decrease in the order of β, spinel, and CaTi2O4‐type phases at zero pressure. The theoretical bulk modulus of the CaTi2O4‐type phase is comparable with that of spinel.

Mechanisms and kinetics of the post-spinel transformation in Mg 2SiO 4

Mechanisms and kinetics of the post-spinel transformation in Mg 2SiO 4 were examined at 22.7–28.2 GPa and 860–1200 °C by in situ X-ray diffraction experiments using synchrotron radiation combined with microstructural observations of the recovered samples. The post-spinel phases nucleated on spinel grain boundaries and grew with a lamellar texture. Under large overpressure conditions, reaction rims were formed along spinel grain boundaries at the initial stage of the transformation, whereas under small overpressure conditions, the transformation proceeded without formation of reaction rims. Mg 2SiO 4 spinel metastably dissociated into MgSiO 3 ilmenite and periclase, and stishovite and periclase as intermediate steps in the transformation into the stable assemblage of MgSiO 3 perovskite and periclase. Topotactic relationships were found in the transformation from spinel into ilmenite and periclase. Kinetic parameters in the Avrami rate equation, time taken to 10% completion, and the growth rate were estimated by analysis of the kinetic data obtained by in situ X-ray observations. The empirical activation energy for 10% transformation decreases with increasing pressure because the activation energy for nucleation becomes smaller at larger overpressure conditions. Extrapolations of the 10% transformation to ∼700 °C, which is the lowest temperature expected for the cold slabs at ∼700 km depth, suggest that overpressure of more than ∼1 GPa is needed for the transformation. Because the growth rate is estimated to be large even at low-temperatures of ∼700 °C and overpressures of 1 GPa, the depth of the post-spinel transformation in the cold slabs is possibly controlled by nucleation kinetics.

Seismic moments of earthquakes beneath island arcs, phase changes, and subduction velocities

The analysis of seismic moments of intermediate and deep-focus earthquakes in four island arc regions supports the hypothesis that these shocks are the result of shear stresses caused by phase transitions in the descending slab at depth near 150, 350, and 600 km. Cumulative seismic moments summation M/sub 0/ for earthquakes listed by Gutenberg and Richter were found to exhibit four maxima when they were plotted as a function of depth; the maxima occur within the depth ranges 0 to 100, 100 to 250, 250 to 450, and 450 to 700 km. The uppermost range corresponds to the zone of underthrusting. The seismic moments of earthquakes in the three lower depth ranges ae related to phase transitions in the slab by summation M/sub 0/ = ..mu..LT vertical bar ..delta..V/V vertical bar v/sub sub/, where ..mu.. is the modulus of rigidity, L is the length along strike of the subduction zone, T is the slab thickness, ..delta..V/V is the fractional volume change associated with a particular phase transition, and v/sub sub/ is the subduction velocity. In the range 100 to 250 km the earthquakes result from volume decreases associated with the melting of the oceanic crust. In the depth range 250more » to 450 km the volume change is due mostly to the transformation of olivine to spinel structure with a density increase of about 8%. In the lowest depth range the decrease in volume is due to the change of spinel to postspinel phases with a density increase of about 8%.For any island arc region, up to four independent estimates of v/sub sub/ can be calculated by using Brune's method for the zone of shallow underthrusting and the relationship between summation M/sub 0/ and volume changes for the three deeper zones. As an example, the four estimates of v/sub sub/ for the Tonga arc, in order of increasing depth, are 8.4, 7.9, 7.6, and 8.3 cm/yr.« less