Compositional change of majoritic garnet in a MORB composition from 7 to 17 GPa and 1400 to 1600°C

doi:10.1016/0031-9201(96)03149-4

Physics of the Earth and Planetary Interiors

Volume 96, Issues 2–3, August 1996, Pages 171-179

https://doi.org/10.1016/0031-9201(96)03149-4 Get rights and content

Abstract

Experimental phase relations of an average normal mid-ocean ridge basalt (N-MORB) composition from 7 GPa to 17 GPa and 1400 to 1600°C reveal the compositional variation of garnet in equilibrium with pyroxene. The Na₂O content of majoritic garnet in equilibrium with pyroxene and stishovite increases up to 2.5 wt.% with increasing pressure and temperature. Na enters the garnet structure via the substitution (Mg,Fe,Ca) + Al = Na + (Si,Ti). The pyroxene-garnet partition coefficient (D^cp-ga) for na decreases with increasing pressure. However, clinopyroxene coexisting with garnet becomes more Na rich as pressure increases. These relationships are independent of the FeO content in garnet. Our experiments confirm that rare majoritic and Na-rich garnet inclusions in diamonds have formed at transition zone pressure, in lithologies of basaltic bulk composition.

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Cited by (88)

High pressure-temperature phase relations of basaltic crust up to mid-mantle conditions
2022, Earth and Planetary Science Letters
A substantial amount of subducted basaltic crusts may exist in the lower mantle. It features distinct chemical composition from the peridotitic mantle and plays important roles in the chemical and dynamic evolution of Earth's interior. However, the chemical composition of mineral phases present in basaltic crust in the lower mantle is still poorly constrained. Here, we determined phase relations of normal mid-ocean ridge basalt (MORB) up to 52 GPa at 2000 K by multi-anvil press. Throughout our experiments, the mineral assemblages consist of five phases including bridgmanite, stishovite, calcium perovskite (davemaoite), calcium ferrite and new hexagonal aluminous phases. The density of MORB calculated from our phase assemblages and previously published thermoelastic data is 2-3% higher than that of the average mantle, which is consistent with literature. The new hexagonal aluminous phase is a host of potassium but only occupies 1-2 vol.% due to limited abundance of potassium in normal MORB. This indicates that the new hexagonal aluminous phase doesn't affect the elastic properties of basalt. Electron energy-loss spectra of recovered basaltic Al-rich bridgmanite show significant enrichment of ferrous iron (75-85%) compared with peridotitic Al-poor bridgmanite (∼30%), which is against previous studies showing that ferric iron ratio in bridgmanite increases with Al content. Ferric iron exhibits strong partitioning into the new hexagonal aluminous phase (41-64%), whereas bridgmanite (14-28%) and calcium ferrite phase (5-27%) remain Fe²⁺-enriched. The oxygen vacancy component of MgAlO_2.5 in bridgmanite is ∼11% up to 40 GPa, which is much higher than that in peridotitic bridgmanite (2-3%), possibly producing a viscosity contrast in the mid-mantle that would explain slab stagnation and plume thinning between 660 km and 1000 km depth. The presence of ferrous iron-rich bridgmanite in the deep lower mantle may contribute to seismic features of large low-shear-velocity provinces.
Phase relations and melting of nominally ‘dry’ residual eclogites with variable CaO/Na<inf>2</inf>O from 3 to 5 GPa and 1250 to 1500 °C; implications for refertilisation of upwelling heterogeneous mantle
2018, Lithos
This study investigates the phase and melting relations of nominally ‘dry’ residual eclogites (Res2 and Res3), with varying bulk CaO/Na₂O ratios (4 and 12, respectively), from ~160 (5 GPa) to ~90 km (3 GPa) depth. Garnet, clinopyroxene and minor quartz/coesite are subsolidus phases in both compositions. In contrast to Res2, in Res3, the proportions of garnet always exceeding those of clinopyroxene. This also leads to higher modal quartz/coesite in Res3 relative to Res2.
In modelling melting along a near-adiabatic upwelling path with a mantle potential temperature of ~1360 °C, at 5 GPa, near-solidus andesitic Res3 partial melts are much less siliceous and sodic, and are more calcic and magnesian than the incipient dacitic melts of Res2. Continuously self-fluxed melting increases considerably from 4 to 3 GPa due to the increased breakdown of Ca-Eskolaite solid solution component in clinopyroxene along the adiabat. This causes a steepening of the solidus, but more-so for Res2 than for Res3. At 3 GPa, the near exhaustion of residual clinopyroxene causes higher melt productivity for Res3 (~60%) than for Res2 (~30%), despite both melts being of basaltic-andesite composition. Resulting Res3 melts are therefore significantly more calcic and magnesian, and less sodic than those of Res2 melts.
As Res3 undergoes a higher degree of melting relative to Res2 during adiabatic ascent, Res3 eclogitic residues become significantly more refractory; with relatively higher Mg# and grossular in garnet, higher Mg# and Ca-tschermaks, and lower jadeite components of clinopyroxene, and higher garnet/clinopyroxene ratios than eclogitic Res2 residuals.
In upwelling heterogenous mantle domains, the siliceous eclogitic melts formed within a body of eclogite will react with encapsulating mantle peridotite, effectively refertilising it and producing hybrid pyroxene- and garnet-rich rocks. Subsequent melting of these sources may lead to compositionaly diverse primitive mantle-derived magmas, with high Ca/Al and low Na/Ca signatures indicators of preferential melting of a heterogeneous mantle, previously refertilised by recycled Ca-rich oceanic crustal material, and primitive magmas with low Ca/Al and high Na/Ca derived from melting of mantle with a ‘normal recycled crustal material signature’. Thus, compositional magma diversity may directly reflect precursor compositions of the mantle source region.
The depth of sub-lithospheric diamond formation and the redistribution of carbon in the deep mantle
2017, Earth and Planetary Science Letters
Most diamonds form in the Earth's lithosphere but a small proportion contain Si-rich majoritic garnet inclusions that indicate formation in the deeper mantle. The compositions of syngenetic garnet inclusions can potential yield information on both the depth and mantle lithology in which the diamonds formed. Pressure dependent changes in garnet compositions have been calibrated using the results of experiments conducted in a multi-anvil apparatus at pressures between 6 and 16 GPa and temperatures of 1000 to 1400 °C. Using the results of these experiments a barometer was formulated based on an empirical parameterisation of the two major majoritic substitutions, referred to as majorite (Maj; ${Al}^{3 +} = {Mg}^{2 +} + {Si}^{4 +}$ ), and Na-majorite (Na-Maj; ${Mg}^{2 +} + {Al}^{3 +} = {Na}^{+} + {Si}^{4 +}$ ). Moreover, previously published experimental garnet compositions from basaltic, kimberlite, komatiite and peridotite bulk compositions were included in the calibration, which consequently covers pressures from 6 to 20 GPa and temperatures from 900 to 2100 °C. Experimental pressures are reproduced over these conditions with a standard deviation of 0.86 GPa.
The barometer is used to determine equilibration pressures of approximately 500 reported garnet inclusions in diamonds from a range of localities. As the majority of these inclusions are proposed to be syngenetic this allows a detailed picture of diamond formation depths and associated source rocks to be established using inclusion chemistry. Geographic differences in diamond source rocks are mapped within the sub-lithospheric mantle to over 500 km depth. Continuous diamond formation occurs over this depth range within lithologies with eclogitic affinities but also in lithologies that appear transitional between eclogitic and peridotitic bulk compositions, with an affinity to pyroxenites. The geographic differences between eclogitic and pyroxenitic diamond source rocks are rationalised in terms of diamond formation within downwelling and upwelling regimes respectively. Macroscopic diamond formation in rocks with pyroxenite compositions are likely facilitated in the deep mantle by higher average oxidation states and low mineral $H_{2} O$ solubility compared to the surrounding mantle, which aid the mobility of C–O–H volatile species. The apparent lack of inclusions with a peridotite affinity may result from generally low oxygen fugacities in such lithologies, which reduces carbon mobility, and the lack of a suitable oxidising agent to allow diamonds to form from CH₄. This glimpse of deep carbon cycle processes implies that heterogeneities in the carbon content, redox state and chemical composition of the mantle may be strongly coupled.
Tissintite, (Ca,Na, ⊕)AlSi<inf>2</inf>O<inf>6</inf>, a highly-defective, shock-induced, high-pressure clinopyroxene in the Tissint martian meteorite
2015, Earth and Planetary Science Letters
Tissintite is a new vacancy-rich, high-pressure clinopyroxene, with a composition essentially equivalent to plagioclase. It was discovered in maskelynite (shocked plagioclase) and is commonly observed included within, or in contact with, shock-melt pockets in the Tissint meteorite, a depleted olivine-phyric shergottite fall from Mars. The simple composition of tissintite (An58–69) and its precursor plagioclase (An59–69) together with the limited occurrence, both spatially (only in maskelynite less than ∼25 μm of a shock melt pocket) and in terms of bulk composition, make tissintite a “goldilocks” phase. It formed during a shock event severe enough to allow nucleation and growth of vacancy-rich clinopyroxene from a melt of not too calcic and not too sodic plagioclase composition that was neither too hot nor too cold. With experimental calibration, these limitations on occurrence can be used to place strong constraints on the thermal history of a shock event. The kinetics for nucleation and growth of tissintite are probably slower for more-sodic plagioclase precursors, so tissintite is most likely to occur in depleted olivine-phyric shergottites like Tissint and other highly shocked meteorites and lunar and terrestrial rocks that consistently contained calcic plagioclase precursors in the appropriate compositional range for a shock of given intensity.
Tissintite, $({Ca}_{0.45} {Na}_{0.31} □_{0.24}) ({Al}_{0.97} {Fe}_{0.03} {Mg}_{0.01}) ({Si}_{1.80} {Al}_{0.20}) O_{6}$ , is a $C 2 / c$ clinopyroxene, containing 42–60 mol% of the Ca-Eskola component, by far the highest known. The cell parameters are $a = 9.21$ (17) Å, $b = 9.09$ (4) Å, $c = 5.20$ (2) Å, $β = 109.6$ (9)°, $V = 410$ (8) Å³, $Z = 4$ . The density is 3.32 g/cm³ and we estimate a cell volume for the Ca-Eskola end-member pyroxene of $411 \pm 13 Å^{3}$ , which is consistent with a previous estimate and, therefore, supports the importance of this component in clinopyroxenes from ultra-high pressure metamorphic rocks from the Earth's upper mantle. At least in $C 2 / c$ clinopyroxenes as sodic as tissintite, the a- and b-cell parameters as a function of vacancy concentration intersect at ∼0.3 vacancies pfu, much lower than the Ca-Eskola end-member (0.5), an inversion of anisotropy suggesting an elastic instability that drives clinopyroxene toward a disordered trigonal structure closely related to that of wadeite; it may mark the boundary beyond which the breakdown of vacancy-rich clinopyroxene to a wadeite-structured phase + stishovite becomes stable, although this was not observed in Tissint.
Experimental evidence of bulk chemistry constraint on SiO<inf>2</inf> solubility in clinopyroxene at high-pressure conditions
2015, Lithos
Citation Excerpt :
The supersilicic clinopyroxene is stabilised at pressures higher than 3 GPa but is unstable at lower pressures (e.g., Smyth, 1980). Despite a large amount of experimental data (Gasparik, 1986; Konzett et al., 2008; Okamoto and Maruyama, 2004; Ono and Yasuda, 1996; Wood and Henderson, 1978), the thermodynamic properties of supersilicic clinopyroxene remain ambiguous, and the SiO2 solubility in clinopyroxene has yet to be exactly formulated as a function of intensive variables such as pressure, temperature and composition. Therefore, it is of urgent need to perform a series of experiments under ultrahigh-pressure metamorphic conditions in order to reveal the thermodynamic behaviours, especially the solid solution relation of supersilicic clinopyroxene.
We have experimentally confirmed that the solubility of SiO₂ in clinopyroxene at ultrahigh-pressure metamorphic conditions is buffered by coesite and kyanite. The present findings were derived from high-pressure experiments on metapelite glass, powdered andesite and eclogite glass under anhydrous conditions. The metapelite glass and powdered andesite were recrystallised in boron nitride capsules at 8 GPa and 1100–1500 °C. The eclogite glass was heated in an AuPd capsule, both ends of which were welded, at 3 GPa and 1000 °C.
Clinopyroxene nucleated from metapelite glass, the bulk composition of which is saturated in both SiO₂ and Al₂SiO₅ components plotting within the Jd (Na,K)(Al,Cr)(Si,Ti)₂O₆−Qtz (Si,Ti)O₂−Grt M₃(Al,Cr)₂(Si,Ti)₃O₁₂−Als (Al,Cr)₂(Si,Ti)O₅ tetrahedron (M = Fe, Mn, Mg, Ni, Zn, Ca), coexists with garnet, coesite and kyanite. The average excess silica content of the clinopyroxene ranges from 23.4 to 35.4 mol%. In contrast, an andesite experiment saturated in SiO₂ but undersaturated in Al₂SiO₅ within the Jd–Qtz–Aug M(Si,Ti)O₃–Grt tetrahedron produced clinopyroxene, garnet and coesite but no kyanite. The average excess silica in the clinopyroxene was 9.7–15.5 mol%, which is comparable to previous experimental data. Experiment on the eclogite glass with similar composition to andesite yielded clinopyroxene, garnet and coesite. An average excess silica content in clinopyroxene counts 6.4 mol%, which is much lower than that obtained from the andesite.
The SiO₂ content of clinopyroxene coexisting with garnet, coesite and kyanite is much higher than that of clinopyroxene coexisting with garnet and coesite without kyanite. Although the temperature dependence is unclear, the SiO₂ solubility increases with pressure and Fe/(Fe+Mg). Clinopyroxene forms the solid solution series Jd–Es □_0.5M_0.5Al(Si,Ti)₂O₆ and Aug–Es, rather than Jd–Ts MAl₂(Si,Ti)O₆ and Es–Ts joins. Our experimental data suggest the probable existence of octahedral Si which may accompany the M2 vacancies in clinopyroxene.
Partitioning of trace elements between Na-bearing majoritic garnet and melt at 8.5GPa and 1500-1900°C
2014, Lithos
New experimental data on trace element partitioning between Na-bearing majoritic garnet and melt at P = 8.5 GPa and T = 1500–1900 °C applicable to partial melting of Na-rich eclogite are presented. We have found that Na-bearing garnet is a liquidus phase of the system at 1850–1650 °C being accompanied by enstatitic pyroxene at lower temperatures. With decreasing temperature, Na concentration in garnet increases up to > 1 wt.% Na₂O due to progressive incorporation of a Na majorite component (Na₂MgSi₅O₁₂). Most of the studied trace elements are incompatible, except for Er, Tm, Yb (in some runs), Lu, and Sc (in all runs), which are distributed into garnet. The main characteristic of the trace-element partitioning in our experiments is a different behaviour of the LREE (La, Ce, Pr) in comparison with MREE and HREE (Nd, Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, and Lu). In particular, a significant increase of D values for LREE with the increase of Na₂O concentration in garnet is observed. As predicted from lattice strain, partitioning coefficients for REEs entering the X site of garnet exhibit a near-parabolic dependence on ionic radius. The results of the study are applied to the formation of inclusions of Na-bearing majoritic garnets in diamonds, and equilibrium melts significantly enriched in LREEs.

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Research paperCompositional change of majoritic garnet in a MORB composition from 7 to 17 GPa and 1400 to 1600°C

Abstract

Phys. Earth Planet. Inter.

Earth Planet. Sci. Lett.

Phys. Earth Planet. Inter.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Phys. Earth Planet. Inter.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Tectonophysics

Olivine-modified spinel-spinel transitions in the system Mg2SiO4Fe2SiO4: calorimetric measurements, thermochemical calculation, and geophysical application

J. Geophys. Res.

Transition region of the Earth's upper mantle

Nature

Subducting slabs in the mantle transition zone

J. Geophys. Res.

Transformation of enstatite-diopside-jadeite pyroxenes to garnet

Contrib. Mineral. Petrol.

Research paper
Compositional change of majoritic garnet in a MORB composition from 7 to 17 GPa and 1400 to 1600°C

Olivine-modified spinel-spinel transitions in the system Mg₂SiO₄Fe₂SiO₄: calorimetric measurements, thermochemical calculation, and geophysical application