Oxygen isotope fractionation in mantle minerals

The increment method is adopted to calculate oxygen isotope fractionation factors for mantle minerals, particularly for the polymorphic phases of MgSiO₃ and Mg₂SiO₄. The results predict the following sequence of¹⁸O-enrichment:pyroxene (Mg, Fe, Ca)₂Si₂O₆>olivine (Mg, Fe)₂SiO₄ > spinel (Mg, Fe)₂SiO₄> ilmenite (Mg, Fe, Ca) SiO₃>perovskite (Mg, Fe, Ca) SiO₃. The calculated fractionations for the calcite-perovskite (CaTiO₃) System are in excellent agreement with the experimental calibrations. If there would be complete isotopic equilibration in the mantle, the spinel-structured silicates in the transition zone are predicted to be enriched in¹⁸O relative to the perovskite-structured silicates in the lower mantle but depleted in¹⁸O relative to olivines and pyroxenes in the upper mantle. The oxygen isotope layering of the mantle might result from differences in the chemical composition and crystal structure of mineral phases at different mantle depths. Assuming isotopic equilibrium on a whole earth scale, the chemical structure of the Earth’s interior can be described by the following sequence of¹⁸O-enrichment:upper crust>lower crust>upper mantle>transition zone>lower mantle>core. Project supported by the National Natural Science Foundation of China and the Chinese Academy of Sciences.     

Spinel disproportionation at high pressure: calorimetric determination of enthalpy of formation of Mg2SnO4 and Co2SnO4 and some implications for silicates

The enthalpies of formation from the oxides of Mg₂SnO₄ and Co₂SnO₄ were found by oxide melt solution calorimetry to be +1.13 ± 0.48 kcal/mol and ?2.31 ± 0.28 kcal/mol, respectively. Using these data, the slopes, ?P/?T, for disproportionation of these spinels to the component oxides at high pressure were calculated to be +30.4 ± 4.2 bar/K for Mg₂SnO₄ and ?10.3 ± 2.4 bar/K for Co₂SnO₄, in general agreement with the data of Jackson et al. (1974a,b). Using thermochemical data for the formation of olivines, for olivine-spinel transitions and for the transformation of quartz to stishovite, we calculate pressures for the disproportionation of silicate spinels to be in the range 150–200 kbar. Calculated slopes ?P/?T for the disproportionation reactions are ?10.7, ?24.9, ?11.2, and +7.6 bar/K for Mg₂SiO₄, Fe₂SiO₄, Co₂SiO₄, and Ni₂SiO₄. The large negative slope calculated for Fe₂SiO₄ results from a surprisingly large positive slope reported for the olivine-spinel transition in that compound (Akimoto et al., 1969). Further consideration of the systematic trends in the thermodynamics of spinel formation from the oxides suggests that the silicate spinels should have entropies of formation close to zero, resulting in values of ?P/?T which are zero or at most only slightly negative. This confirms the conclusion of Jackson, Liebermann, and Ringwood that values of ?P/?T for spinel disproportionation are unlikely to be more negative than ?10 bar/K and may well be slightly positive. Reaction of spinels to form other post-spinel phases, particularly ilmenite and perovskite, are discussed in terms of available thermochemical data.     

Influence of oxygen pressure on defect concentrations in olivine with a fixed cationic ratio

The variation of the point-defect concentrations with oxygen partial pressure, P_O2, and temperature are derived for forsterite, Mg₂SiO₄, and Mg-rich olivine (Mg,Fe)₂SiO₄ assuming the cation/cation ratios are fixed. These dependencies differ from the open-system situation in which matter is easily transferable between forsterite or olivine and other solid phases. The details depend on whether the cation/cation ratio is exactly stoichiometric, or, if non-stoichiometric, the nature of the majority defects at precise oxygen stoichiometry. One generality which emerges is that regardless of the cation/cation stoichiometry the dominant defects in forsterite at low P_O2 are free electrons, Mg and Si interstitials; at high P_O2 the majority defects are holes, Mg and Si vacancies. In Mg-rich olivine the same defect species exist at the extremes of oxygen partial pressure with the exception of trivalent Fe on Mg sites replacing holes. At low P_O2, therefore, both behave as n-type conductors. The models also suggest that in the P_O2 range around precise oxygen stoichiometry the electrical conductivity in both materials can be a complex function of P_O2.     

Elasticity of single crystal pyrope and implications for garnet solid solution series

The elastic moduli of a synthetic single crystal of pyrope (Mg₃Al₂Si₃O₁₂) have been determined using a technique based on Brillouin scattering. These results are used in an evaluation of the effect of composition on the elastic properties of silicate garnet solid solution series (Mg, Fe, Mn, Ca)₃ (Al, Fe, Cr)₂ Si₃O₁₂. In the pyralspites (Mg FeMn aluminum garnets), for which a large amount of data is available, this analysis indicates that the bulk modulus K is independent of the Fe²⁺/Mg²⁺ ratio, which is similar to the behavior observed in olivines and pyroxenes. However, the shear modulus μ of the garnets increases by 10% from the Mg to the Fe end member, in contrast to the decrease of μ with Fe content which is observed in olivines and pyroxenes. This contrasting behavior is most probably related to the oxygen coordination of the cation site occupied by Mg²⁺ and Fe²⁺ in these different minerals.     

Orthorhombic perovskite phases observed in olivine,pyroxene and garnet at high pressures and temperatures

Ferromagnesian silicate olivines, pyroxenes and garnets with Mg/(Mg + Fe)?0.3 (molar) have been found to transform to high-pressure phases characterized by the orthorhombic perovskite structure when compressed to pressures above 250 kbar in a diamond-anvil press and heated to temperatures above 1,000°C with a YAG laser. The zero-pressure density of the perovskite phase of (Mg,Fe)SiO₃ is about 3–4% greater than that of the close-packed oxides, rocksalt plus stishovite. For (Mg,Fe)₂SiO₄ compounds, the perovskite plus rocksalt phase assemblage is 2–3% denser than the mixed oxides. The experimental synthesis of such high-density perovskite phases in olivine, pyroxene and garnet compounds suggests that (Mg,Fe)SiO₃-perovskite is the dominant mineral phase in the earth's lower mantle.     

High-pressure decompositions in cobalt and nickel silicates

High-pressure stability relations in cobalt and nickel silicates have been studied over the pressure range 130–330 kbar employing a double-staged split-sphere-type high-pressure apparatus.γ-Co₂SiO₄ and γ-Ni₂SiO₄ decompose directly into their constituent oxide mixtures (rocksalt plus stishovite) 175 kbar and 280 kbar, respectively. The result that γ-Ni₂SiO₄ has a wider stability field in pressure than γ-Co₂SiO₄, is consistent with simple crystal-field theory.The experimental precision is high enough to show that the decomposition boundary of γ-Co₂SiO₄ has a positive slope (dP/dT > 0) and a preliminary determination of the boundary curve is P(kbar) = 0.065 T (°C) + 110.No positive evidence for the existence of high-pressure forms of CoSiO₃ and NiSiO₃ has been obtained in these quenching experiments, and they finally decompose into constituent oxide mixtures as in the cases of orthosilicates.     

Calorimetric study of the stability of high pressure phases in the systems CoOSiO2 and “FeO”SiO2, and calculation of phase diagrams in MOSiO2 systems

High temperature calorimetric measurements of the enthalpies of solution in molten if2 PbO · B₂O₃ of α- and γ-Fe₂SiO₄ and α-, β-, and γ-Co₂SiO₄ permit the calculation of phase relations at high pressure and temperature. The reported triple point involving α-, β-, and γ-Co₂SiO₄ is confirmed to represent stable equilibrium. The curvature in the α?β phase boundary in Co₂SiO₄ and of an α?γ boundary in Fe₂SiO₄ at high temperature is explained in part by the effects of compressibility and thermal expansion, but better agreement with the observed phase diagram is obtained when one considers the effect of small amounts of cation disorder in the spinel and/or modified spinel phases. The calculated ΔH⁰ and ΔS⁰ values for the α?β, α?γ, and β?γ transitions show that enthalpy and en changes both vary strongly in the series Mg, Fe, Co, and Ni, and are of equal importance in determining the stability relations. The disproportionation of Fe₂SiO₄ and Co₂SiO₄ spinel to rocksalt plus stishovite is calculated to occur in the 170–190 kbar region; cation disorder and/or changes in wüstite stoichiometry can affect the P?T slope. The calorimetric data for CoSiO₃ and FeSiO₃ are in good agreement with the observed phase boundary for pyroxene formation from olivine and quartz. The decomposition of pyroxene to spinel and stishovite at pressures near the coesite-stishovite transition is predicted in both iron and cobalt systems. The use of calorimetric data, obtained from small samples of high pressure phases, is very useful in predicting equilibrium phase diagrams in the 50–300 kbar range.     

High-pressure phase transformations in the system of MgSiO3-FeSiO3

Two synthetic pyroxenes (FeSiO₃, MgSiO₃) and five natural pyroxenes with compositions of about Fs₈₀En₂₀, Fs₆₀En₄₀, Fs₅₀En₅₀, Fs₄₀En₆₀, and Fs₂₀En₈₀ have been subjected to pressures up to250 ± 50kbars at a temperature of about1500 ± 200°C in a diamond anvil cell heated by an infrared laser beam. After quenching and unloading X-ray data analysis indicates that (1) those with Mg less than 50% undergo the following reactions: 2(Mg,Fe)SiO₃ (pyroxene) → (Mg,Fe)₂SiO₄ (spinel) + SiO₂ (stishovite) → 2(Mg,Fe)O (magnesiowu¨stite) + SiO₂ (stishovite) with increase of pressure, and (2) those with Mg higher than 60%, undergo the following reactions: 2(Mg,Fe)SiO₃ (pyroxene) → (Mg,Fe)₂SiO₄ (spinel) + SiO₂ (stishovite) → 2(Mg,Fe)SiO₃ (hexagonal phase) → 2(Mg,Fe)O (magnesiowu¨stite) + SiO₂ (stishovite) with increase of pressure.     

The structure and sharpness of (Mg,Fe)2SiO4 phase transformations in the transition zone

To calculate accurately the pressure interval and mineral proportions (i.e. yields) across the olivine to wadsleyite and wadsleyite to ringwoodite transformations requires a detailed knowledge of the non-ideality of Fe-Mg mixing in these (Mg,Fe)₂SiO₄ solid solutions. In order to constrain the activity-composition relations that describe non-ideal mixing, Fe-Mg partitioning experiments have been conducted between magnesiowüstite and (Mg,Fe)₂SiO₄ olivine, wadsleyite and ringwoodite as a function of pressure at 1400°C. Using known activity-composition relations for magnesiowüstite the corresponding relations for the three polymorphs were determined from the partitioning data. In all experiments the presence of metallic iron ensured redox conditions compatible with the Earth’s transition zone. The non-ideality of the (Mg,Fe)₂SiO₄ solid solutions was found to decrease in the order W^wadsleyite_FeMg>W^ringwoodite_FeMg>W^olivine_FeMg. These partitioning data were used, along with published phase equilibria measurements for the Mg₂SiO₄ and Fe₂SiO₄ end-member transformations, to produce an internally consistent thermodynamic model for the Mg₂SiO₄-Fe₂SiO₄ system at 1400°C. Using this model the pressure interval of the olivine to wadsleyite transformation is calculated to be significantly smaller than previous determinations. By combining these results with Fe-Mg partitioning data for garnet, the widths of transition zone phase transformations in a peridotite composition were calculated. The olivine to wadsleyite transformation at 1400°C in dry peridotite was found to occur over a pressure interval equivalent to approximately 6 km depth and the mineral yields were found to vary almost linearly with depth across the transformation. This transformation is likely to be even sharper at higher temperatures or could be significantly broader in wet mantle or in regions with a significant vertical component of mantle flow. The entire range of estimated widths for the 410 km discontinuity (4-35 km) could, therefore, be explained by the olivine to wadsleyite transformation in a peridotite composition over a range of quite plausible mantle temperatures and H₂O contents. The wadsleyite to ringwoodite transformation in peridotite mantle was calculated to take place over an interval of 20 km at 1400°C. This transformation yield was also found to be near linear.     

Ni2SiO4-enthalphy of the olivine-spinel transition by solution calorimetry at 713°C

The high pressure spinel polymorph of Ni₂SiO₄ persists metastably at 713°C and atmospheric pressure. The enthalpy of the olivine-spinel transition was obtained by measuring the heats of solution of both polymorphs in a molten oxide solvent, 2PbO · B₂O₃, at that temperature. For Ni₂SiO₄(ol)→Ni₂SiO₄, ΔH₉₈₆⁰ = +1.4 ± 0.7kcal/mol. The heat content increments, H₉₈₆ ? H₂₉₇, were found to be: olivine, 25.73 ± 0.42kcal/mol, and spinel, 25.39 ± 0.20kcal/mol. The measured enthalpy of the transformation is consistent with the low slope of the phase boundary, ?P/?T = ～ 12b/deg, observed by Akimoto and others. The entropy of the olivine-spinel transition in Ni₂SiO₄ is accordingly about a factor of three smaller in magnitude (ΔS = ～ ?1cal/deg mol) than that for Co₂SiO₄,Fe₂SiO₄,Mg₂SiO₄or Mg₂GeO₄ (ΔS = ?3to?3.5cal/deg mol).     

High-pressure phase transformations in baddeleyite and zircon,with geophysical implications

Phase transformations in baddeleyite (ZrO₂) and zircon (ZrSiO₄) have been investigated in the pressure range between 100 and 300 kbar at about 1000°C in a diamond-anvil press coupled with laser heating. Baddeleyite has been found to transform to an orthorhombic cotunnite-type structure at pressures greater than 100 kbar, and is the first oxide known to adopt this structure. The lattice parameters of the cotunnite-type ZrO₂ at room temperature and atmospheric pressure area = 3.328 ± 0.001 ,b = 5.565 ± 0.002 , andc = 6.503 ± 0.003A? withZ = 4 , and its volume is 14.3% smaller than baddeleyite and 7.6% smaller than the fluorite-type ZrO₂. It is suggested that all the polymorphic structures of ZrO₂ are possible high-pressure models for the post-rutile phase of SiO₂. The polyhedral coordination in these model structures varies from 7 to “9”, compared with 6 for stishovite. If SiO₂ were to adopt any of these structures in the deep mantle, Birch's hypothesis of a mixed-oxide lower mantle may still be viable, but the primary coordination of silicon would be greater than 6. Zircon has been found to transform to a scheelite-type structure at about 120 kbar as noted earlier. The scheelite-type ZrSiO₄ was found to decompose further into a mixture of ZrO₂ (cotunnite-type) plus SiO₂ (stishovite) in the pressure range 200–250 kbar. As implied by the transitions in zircon, the large cations of U and Th in the earth's deep mantle are most likely to occur in dioxides with structures such as the cotunnite-type, rather than to occur in silicates.     

High-pressure X-ray diffraction studies on β- and γ-Mg2SiO4

Pressure effects on the lattice parameters of β- and γ-Mg₂SiO₄ have been measured at room temperature and at pressures up to 100 kbar using a multi-anvil high-pressure X-ray diffraction apparatus. The volume changes (ΔV/V₀) at 90 kbar are 5.4 · 10^?2 and 4.2 · 10^?2 for β- and γ-Mg₂SiO₄, respectively. Isothermal bulk moduli at zero pressure have been calculated from least-square fits of the data to straight lines. They turn out to be 1.66 ± 0.4 and 2.13 ± 0.1 Mbar for β- and γ-Mg₂SiO₄, respectively. The α → γ transition obeys Wang's linear V_φ?ρ relation but the α → β transition does not.     

Reaction microstructures in corundum‐ and kyanite‐bearing mafic mylonites from the Takahama metamorphic rocks,western Kyushu,Southwest Japan

High‐grade mylonites occur in the Takahama metamorphic rocks, a member of the high‐pressure low‐temperature type Nagasaki Metamorphic Rocks, western Kyushu, Japan. Mafic layers within the mylonites retain reaction microstructures consisting of margarite aggregates armoring both corundum and kyanite. The following retrograde reaction well accounts for the microstructures in the CaO–Al₂O₃–SiO₂–H₂O system: 3Al₂O₃ + 2Al₂SiO₅ + 2Ca₂Al₃Si₃O₁₂(OH) + 3H₂O = 2Ca₂Al₈Si₄O₂₀(OH)₄ (corundum + kyanite + clinozoisite + fluid = margarite). Mass balance analyses and chemical potential modeling reveal that the chemical potential gradients present between kyanite and corundum have likely driven the transport of the CaO and SiO₂ components. The mylonitization is considered to take place chronologically after peak metamorphism and before the above reaction, based on the following features: approximately constant thickness of the margarite aggregates, random orientation of margarite, and local modification of garnet composition at a boudin neck that formed during mylonitization. The estimated peak temperature of 640°C and the pressure–temperature conditions of the above reaction indicate that the mylonitization took place at temperature between 530 and 640°C at pressures higher than 1.2 GPa, approximately equivalent to the depth of the lower crust of island arcs.     

Static compression of hydrous silicate melt and the effect of water on planetary differentiation

High pressure experiments using the sink/float method have bracketed the density of hydrous iron-rich ultrabasic silicate melt from 1.35 to 10.0 GPa at temperatures from 1400 to 1860 °C. The silicate melt composition was a 50–50 mixture of natural komatiite and synthetic fayalite. Water was added in the form of brucite Mg(OH)₂ and was present in the experimental run products at 2 wt.% and 5 wt.% levels as confirmed by microprobe analyses of total oxygen. Buoyancy marker spheres were olivines and garnets of known composition and density. The density of the silicate melt with 5 wt.% water at 2 GPa and 1500 °C is 0.192 g cm^? 3 less than the anhydrous form of this melt at the same P and T. This density difference gives a partial molar volume of water in silicate melt of ～ 7 cm³ mol^? 1, which is similar to previous studies at high pressure. The komatiite–fayalite liquids with 0 and 2 wt.% H₂O, have extrapolated density crossovers with equilibrium liquidus olivine at 8 and 9 GPa respectively, but there is no crossover for the liquid with 5 wt.% H₂O. These results are consistent with the hypothesis that dense hydrous melts could be gravitationally stable atop the 410 km discontinuity in the Earth. The results also support the notion that equilibrium liquidus olivine could float in an FeO-rich hydrous martian magma ocean. Extrapolation of the data suggests that FeO-rich hydrous melt could be negatively buoyant in the Earth's D″-region or atop the core–mantle-boundary (CMB), although experiments at higher pressure are needed to confirm this prediction.     

Disproportionation of Ni2SiO4 to stishovite plus bunsenite at high pressures and temperatures

Samples of Ni₂SiO₄ in both olivine and spinel phases have been compressed to pressures above 140 kbar in a diamond-anvil cell and heated to temperatures of 1400–1800°C using a continuous YAG laser. After quenching and releasing pressure, X-ray diffraction examination indicates that the samples disproportionate to a mixture of stishovite (SiO₂) and bunsenite (NiO) at pressures between 140 and 190 kbar. The exact disproportionation pressure is not certain due to transient increases in pressure during the local and rapid heating. However, thermodynamic calculations suggest that the transition pressure is about 192 ± 4 kbar at 1545°C and that the equation of the spinel-mixed oxides phase boundary isP(kbar) = 121 + (0.046 ± 0.020) T (°C).     

High-pressure disproportionation of Co2SiO4 spinel and implications for Mg2SiO4 spinel

Co₂SiO₄ spinel has been found to disproportionate into its isochemically mixed oxides with rocksalt and rutile structures at pressures between 170 and 190 kbar and temperatures between 1400 and 1800°C in a diamond-anvil press. The exact disproportionation pressure is not certain due to transient increases in pressure during the local and rapid heating by a continuous YAG laser. The slope of the phase boundary between the spinel phase and the mixed oxides is calculated to be?33 ± 20bar/deg. This negative slope is consistent with the observed anomalously large entropy of CoO (relative to its isostructural oxides) in entropy vs.(MV)^?1/2 systematics, whereM is the formula weight andV the molar volume. The sign of the slope for a phase boundary in the disproportionation of spinel depends on the values of entropy of the rocksalt oxides as well as the inverse character exhibited in the spinel phases. The normal entropy of MgO suggests that the phase boundary for the disproportionation of Mg₂SiO₄ spinel has positive slope.     

Oxygen self-diffusion in forsterite: Implications for the high-temperature creep mechanism

The diffusivity of¹⁸O in forsterite Mg₂SiO₄ has been measured in the temperature range 1150–1600°C. The activation energy of oxygen self-diffusion in this silicate is found to equal0.32 ± 0.04MJ/mol(77 ± 10kcal/mol), and there is no dependence of the diffusivity upon the oxygen partial pressure surrounding the samples. The diffusion profiles were analysed either with an ion probe or by means of the¹⁸O(p, α)¹⁵N nuclear reaction. The latter method made use of a resonance in the nuclear cross-section in the case of diffusion profiles shorter than 100 nm (1000Å); for diffusion profiles up to 4 μm the same reaction was used, but in a non-resonant mode. New data on creep in forsterite and natural olivine are also given, including the influence of the oxygen partial pressurep_O2 which is zero for forsterite and proportional to(p_O2)¹⁶ for natural olivine. From this set of data we infer the possible relationship between diffusion and creep for these materials. This relationship may be more complicated than that predicted by simple climb mechanism.     

Disproportionation of kyanite to corundum plus stishovite at high pressure and temperature

Natural kyanite (Al₂SiO₅) has been found to disproportionate into a mixture of its component oxides, corundum and stishovite, at a loading pressure of about 160 kbar and temperature between 1000–1400°C in a diamond-anvil press. The exact transition pressure is not certain due to transient increases in pressure during the local and rapid heating by a continuous YAG laser. The phase boundary, however, has been estimated to be P(kbar) = (138 ～ 174) + 0.011 T (°C) on the basis of the available thermodynamic data. The shock-wave Hugoniot data above 650 kbar for andalusite (Al₂SiO₅) and sillimanite (Al₂SiO₅) as starting materials are consistent with the present results.     

Experimental petrology relating to oversaturated peralkaline volcanics: A review

Experiments specifically devoted to problems of oversaturated peralkaline rocks have been primarily concerned with quartz + feldspar + liquid equilibria, and the determination of the low temperature liquids in the feldspar primary phase region. The results are brought together, and compared with natural compositions, by recalculating and plotting in the system Na₂O-K₂O-Al₂O₃-SiO₂ (molecular). The minimum zone in the peralkaline quartz-feldspar cotectic is the synthetic analogue of most comendites and many pantellerites. Peralkaline trachytes and trachytic pantellerites appear to be the natural equivalents of synthetic low temperature liquids in the feldspar primary phase region, but the more peralkaline liquids cannot be a simple evolutionary series controlled only by feldspar fractionation. Experiments have yet to reveal the relating process (or processes) for the series pantelleritic trachyte to pantellerite. Feldspars separating from low temperature synthetic and natural liquids are usually Or₃₅ ±₅ i.e. equivalent to the composition range of the thermal minimum between the anorthoclase and sanidine solid solution loops in the alkali feldspar join. Such liquids may therefore be envisaged as the locus of compositions in the peralkaline system that are in equilibrium with alkali feldspar at the minimum in the solid solution series. Such feldspar, when it separates from the vast majority of peralkaline liquids is fractionating K₂O and Al₂O₃, making the residual liquids more peralkaline and more sodic. Development of the peralkaline condition in natural liquids is commonly ascribed to the « plagioclase effect », but this creates its own dilemma by seeming to be effective only in liquids which are already distinctively alkaline. Furthermore it can only work in a low pressure regime. Examination of the high pressure melting curves of possible mantle minerals shows that acmitic pyroxenes have the lowest melting, in either hydrous or anhydrous conditions, especially at low partial pressures of oxygen. This provides a simple source control by which liquids will either be intrinsically peralkaline (if the melt volume is small) or inherit the potential for low pressure operation of the « plagioclase effect » (most basic magmas). Alkali transfer is well-attested in solid ? vapour experiments and in natural examples of metasomatic aureoles. The mobility of alkalis (and iron) must figure in any realistic scheme of peralkaline petrogenesis. This points up the need for experiments designed to meet the challenge ofopen system magmatism.     

Experimental investigation of reaction coronas on olivine in Apollo 14 high-grade breccias

Reaction coronas of pyroxene ± ilmenite occur around clasts of olivine in Apollo 14 high-grade metamorphic breccias. In experiments of several months duration, there was no evidence of corona formation at 1000°C, but at 1050°, withfO₂ at or above Ilm-Ru-Fe and below Fe-Fe₁?x O, incipient coronas formed around Fo_50–70 in synthetic 14311 matrix. In addition, withfO₂ controlled by Ilm-Ru-Fe at 1050°C, the olivines reduced to Fo₆₈, En₆₉ + Fe. Reduction of olivine under these conditions is inconsistent with the calculated stability relations and is attributed to uncertainties in the activity coefficient for olivine or pyroxene. The experiments also suggest that vesicularity in the Apollo 14 high-grade breccias may correlate with the amount of glassy material in their unmetamorphosed precursors. The metamorphic event is attributed to burial in a hot ejecta blanket, such as that of the Imbrium event.