CO2-CO fluid inclusions in a composite peridotite xenolith: implications for upper mantle oxygen fugacity

AbstractThe occurrence of phlogopite and amphibole in mantle ultramafic rocks is widely accepted as the modal effect of metasomatism in the upper mantle. However, their simultaneous formation during metasomatic events and the related sub-solidus equilibrium with the peridotite has not been extensively studied. In this work, we discuss the geochemical conditions at which the pargasite-phlogopite assemblage becomes stable, through the investigation of two mantle xenoliths from Mount Leura (Victoria State, Australia) that bear phlogopite and the phlogopite + amphibole (pargasite) pair disseminated in a harzburgite matrix. Combining a mineralogical study and thermodynamic modelling, we predict that the P–T locus of the equilibrium reaction pargasite + forsterite = Na-phlogopite + 2 diopside + spinel, over the range 1.3–3.0 GPa/540–1500 K, yields a negative Clapeyron slope of -0.003 GPa K–1 (on average). The intersection of the P–T locus of supposed equilibrium with the new mantle geotherm calculated in this work allowed us to state that the Mount Leura xenoliths achieved equilibrium at 2.3 GPa /1190 K, that represents a plausible depth of ~ 70 km. Metasomatic K-Na-OH rich fluids stabilize hydrous phases. This has been modelled by the following equilibrium equation: 2 (K,Na)-phlogopite + forsterite = 7/2 enstatite + spinel + fluid (components: Na2O,K2O,H2O). Using quantum-mechanics, semi-empirical potentials, lattice dynamics and observed thermo-elastic data, we concluded that K-Na-OH rich fluids are not effective metasomatic agents to convey alkali species across the upper mantle, as the fluids are highly reactive with the ultramafic system and favour the rapid formation of phlogopite and amphibole. In addition, oxygen fugacity estimates of the Mount Leura mantle xenoliths [Δ(FMQ) = –1.97 ± 0.35; –1.83 ± 0.36] indicate a more reducing mantle environment than what is expected from the occurrence of phlogopite and amphibole in spinel-bearing peridotites. This is accounted for by our model of full molecular dissociation of the fluid and incorporation of the O-H-K-Na species into (OH)-K-Na-bearing mineral phases (phlogopite and amphibole), that leads to a peridotite metasomatized ambient characterized by reduced oxygen fugacity.

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Oxygen fugacity during tin ore deposition from primary fluid inclusions in cassiterite

Ore Geology Reviews ◽

10.1016/j.oregeorev.2021.104451 ◽

2021 ◽

pp. 104451

Author(s):

Christian Schmidt ◽

Matthias Gottschalk ◽

Rongqing Zhang ◽

Jianjun Lu

Keyword(s):

Oxygen Fugacity ◽

Fluid Inclusions ◽

Primary Fluid ◽

Ore Deposition

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Fluid inclusions in upper mantle xenoliths from Northern Patagonia, Argentina: Evidence for an upper mantle diapir

Mineralogy and Petrology ◽

10.1007/bf01173707 ◽

1997 ◽

Vol 60 (3-4) ◽

pp. 145-164 ◽

Cited By ~ 7

Author(s):

M. E. Varela ◽

E. A. Bjerg ◽

R. Clocchiatti ◽

C. H. Labudia ◽

G. Kurat

Keyword(s):

Fluid Inclusions ◽

Upper Mantle ◽

Mantle Xenoliths ◽

Northern Patagonia ◽

Mantle Diapir

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Moissanite and its bearing on the oxygen fugacity of the deeper regimes of the Earth's upper mantle

International Kimberlite Conference Extended Abstracts: 1998 ◽

10.29173/ikc2945 ◽

2019 ◽

Keyword(s):

Oxygen Fugacity ◽

Upper Mantle

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An Overview of the Experimental Studies on the Electrical Conductivity of Major Minerals in the Upper Mantle and Transition Zone

Materials ◽

10.3390/ma13020408 ◽

2020 ◽

Vol 13 (2) ◽

pp. 408 ◽

Cited By ~ 1

Author(s):

Lidong Dai ◽

Haiying Hu ◽

Jianjun Jiang ◽

Wenqing Sun ◽

Heping Li ◽

...

Keyword(s):

Electrical Conductivity ◽

Transition Zone ◽

Oxygen Fugacity ◽

Upper Mantle ◽

Activation Enthalpy ◽

Small Polaron ◽

Experimental Studies ◽

Transport Processes ◽

Structural Water ◽

Nominally Anhydrous Minerals

In this paper, we present the recent progress in the experimental studies of the electrical conductivity of dominant nominally anhydrous minerals in the upper mantle and mantle transition zone of Earth, namely, olivine, pyroxene, garnet, wadsleyite and ringwoodite. The main influence factors, such as temperature, pressure, water content, oxygen fugacity, and anisotropy are discussed in detail. The dominant conduction mechanisms of Fe-bearing silicate minerals involve the iron-related small polaron with a relatively large activation enthalpy and the hydrogen-related defect with lower activation enthalpy. Specifically, we mainly focus on the variation of oxygen fugacity on the electrical conductivity of anhydrous and hydrous mantle minerals, which exhibit clearly different charge transport processes. In representative temperature and pressure environments, the hydrogen of nominally anhydrous minerals can tremendously enhance the electrical conductivity of the upper mantle and transition zone, and the influence of trace structural water (or hydrogen) is substantial. In combination with the geophysical data of magnetotelluric surveys, the laboratory-based electrical conductivity measurements can provide significant constraints to the water distribution in Earth’s interior.

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The stability of carbonate under upper-mantle conditions as a function of temperature and oxygen fugacity

European Journal of Mineralogy ◽

10.1127/ejm/7/4/0883 ◽

1995 ◽

Vol 7 (4) ◽

pp. 883-892 ◽

Cited By ~ 6

Author(s):

John A. Dalton ◽

Bernard J. Wood

Keyword(s):

Oxygen Fugacity ◽

Upper Mantle ◽

The Stability

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The effects of solid-solid phase equilibria on the oxygen fugacity of the upper mantle

American Mineralogist ◽

10.2138/am-2020-7162 ◽

2020 ◽

Vol 105 (10) ◽

pp. 1445-1471

Author(s):

Edward M. Stolper ◽

Oliver Shorttle ◽

Paula M. Antoshechkina ◽

Paul D. Asimow

Keyword(s):

Phase Equilibria ◽

Oxygen Fugacity ◽

Upper Mantle ◽

Phase Changes ◽

Primitive Mantle ◽

Spinel Lherzolite ◽

Garnet Lherzolite ◽

Constant Composition ◽

Al Content ◽

Mantle Sources

Abstract Decades of study have documented several orders of magnitude variation in the oxygen fugacity (fO2) of terrestrial magmas and of mantle peridotites. This variability has commonly been attributed either to differences in the redox state of multivalent elements (e.g., Fe3+/Fe2+) in mantle sources or to processes acting on melts after segregation from their sources (e.g., crystallization or degassing). We show here that the phase equilibria of plagioclase, spinel, and garnet lherzolites of constant bulk composition (including whole-rock Fe3+/Fe2+) can also lead to systematic variations in fO2 in the shallowest ~100 km of the mantle. Two different thermodynamic models were used to calculate fO2 vs. pressure and temperature for a representative, slightly depleted peridotite of constant composition (including total oxygen). Under subsolidus conditions, increasing pressure in the plagioclase-lherzolite facies from 1 bar up to the disappearance of plagioclase at the lower pressure limit of the spinel-lherzolite facies leads to an fO2 decrease (normalized to a metastable plagioclase-free peridotite of the same composition at the same pressure and temperature) of ~1.25 orders of magnitude. The spinel-lherzolite facies defines a minimum in fO2 and increasing pressure in this facies has little influence on fO2 (normalized to a metastable spinel-free peridotite of the same composition at the same pressure and temperature) up to the appearance of garnet in the stable assemblage. Increasing pressure across the garnet-lherzolite facies leads to increases in fO2 (normalized to a metastable garnet-free peridotite of the same composition at the same pressure and temperature) of ~1 order of magnitude from the low values of the spinel-lherzolite facies. These changes in normalized fO2 reflect primarily the indirect effects of reactions involving aluminous phases in the peridotite that either produce or consume pyroxene with increasing pressure: Reactions that produce pyroxene with increasing pressure (e.g., forsterite + anorthite ⇄ Mg-Tschermak + diopside in plagioclase lherzolite) lead to dilution of Fe3+-bearing components in pyroxene and therefore to decreases in normalized fO2, whereas pyroxene-consuming reactions (e.g., in the garnet stability field) lead initially to enrichment of Fe3+-bearing components in pyroxene and to increases in normalized fO2 (although this is counteracted to some degree by progressive partitioning of Fe3+ from the pyroxene into the garnet with increasing pressure). Thus, the variations in normalized fO2 inferred from thermodynamic modeling of upper mantle peridotite of constant composition are primarily passive consequences of the same phase changes that produce the transitions from plagioclase → spinel → garnet lherzolite and the variations in Al content in pyroxenes within each of these facies. Because these variations are largely driven by phase changes among Al-rich phases, they are predicted to diminish with the decrease in bulk Al content that results from melt extraction from peridotite, and this is consistent with our calculations. Observed variations in FMQ-normalized fO2 of primitive mantle-derived basalts and peridotites within and across different tectonic environments probably mostly reflect variations in the chemical compositions (e.g., Fe3+/Fe2+ or bulk O2 content) of their sources (e.g., produced by subduction of oxidizing fluids, sediments, and altered oceanic crust or of reducing organic material; by equilibration with graphite- or diamond-saturated fluids; or by the effects of partial melting). However, we conclude that in nature the predicted effects of pressure- and temperature-dependent phase equilibria on the fO2 of peridotites of constant composition are likely to be superimposed on variations in fO2 that reflect differences in the whole-rock Fe3+/Fe2+ ratios of peridotites and therefore that the effects of phase equilibria should also be considered in efforts to understand observed variations in the oxygen fugacities of magmas and their mantle sources.

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