Melting in the FeOSiO2 system to deep lower-mantle pressures: Implications for subducted Banded Iron Formations
Introduction
BIFs are distinctive sedimentary rocks characterized by alternating layers of iron oxides and silica. They occur widely in Archean to Paleoproterozoic terrains all over the world with exposures reaching 1200 m in thickness and several hundred kilometers in extent (see Klein, 2005 for a review). The oldest BIF is found in the 3.8-Ga Isua supracrustal belt, West Greenland. According to Klein (2005), BIFs have been one of the major crustal components of Archean to Paleoproterozoic cratons formed at ∼3.5 to ∼1.8 Ga. BIF deposition rates have peaked at ∼2.5 Ga when the Hamersley Group in Western Australia was formed. Although the detailed mechanism of BIF formation remains unclear, it is robust that they have been formed as a consequence of oxidation of ferrous iron dissolved in seawater. The formation of BIFs stopped at 1.8 Ga, and afterwards has been limited to occasional appearances between 0.8 and 0.6 Ga. The amount of preserved BIFs on the present-day Earth is estimated to be about tons (Isley, 1995). However, these preservations on cratons are very likely only a small part of the total BIFs ever generated, because at least any BIF generated on oceanic crust should have been conveyed into the deep interior of the Earth through at subduction (Hopkins et al., 2008) or non-plate-tectonic delamination processes (Harris and Bédard, 2014).
Dobson and Brodholt (2005) argued that these subducted BIFs accumulate above the CMB and form the ULVZs. The BIFs are likely to be a mixture of FeO and SiO2 near the base of the mantle where the oxidation state is near the iron-wüstite buffer. Dobson and Brodholt (2005) approximated the BIFs to be pure FeO, whose melting temperature was originally reported to be >5000 K at the CMB (Knittle and Jeanloz, 1991), and discussed the possibility that the subducted BIFs remain in a solid state in the mantle. However, the more recent work by Fischer and Campbell (2010), based on measurements up to 77 GPa and the Lindemann's prediction, found that FeO undergoes melting at temperatures above 3690 K at the CMB. This estimate is much lower than the estimates by Knittle and Jeanloz (1991) and Seagle et al. (2008) but consistent with Shen et al. (1993). More importantly, the subducted BIFs are indeed a mixture of FeO and SiO2, and thus melting occurs at the eutectic temperature in the FeOSiO2 binary system, which must be lower than the melting temperature of the FeO end-member.
A melting phase diagram in the FeOSiO2 binary system so far has only been investigated at pressures below 17 GPa, using a multi-anvil apparatus (Ohtani, 1979, Kato et al., 1984). These previous studies show that FeO wüstite and SiO2 stishovite form a eutectic system above 17 GPa.
In this study, we performed melting experiments on Fe2SiO4 between 26 and 131 GPa in a laser-heated DAC. The solidus temperature of Fe2SiO4, corresponding to the eutectic temperature in the FeOSiO2 binary system, was determined from the textural and compositional analyses of DAC samples recovered after pressure release. Our results suggest that the subducted BIFs underwent partial melting near the base of the mantle, which is likely to have contributed to the enrichment in iron above the CMB.
Section snippets
Experimental procedures
Melting experiments were carried out at high pressure by using laser-heated DAC techniques. Synthetic Fe2SiO4 fayalite was used as a starting material. This composition was chosen because the eutectic melt composition is expected to be FeO-rich rather than SiO2-rich from the melting temperatures of FeO and SiO2 end-members. The chemical composition and the homogeneity of the starting material were confirmed by electron microprobe analysis. Either Ar or SiO2 glass was used both as a thermal
Results
We have carried out a total of eleven separate runs in a pressure range from 26 to 131 GPa (Table 1). In experiments performed at relatively high temperatures (runs #1, 2, 5, 11), an FeO-rich homogeneous area was observed at the center of a heated spot (Fig. 1a, b). Such an FeO-rich part was round and exhibited a non-stoichiometric chemical composition. We therefore interpreted it to be a quenched partial melt. Note that the enrichment in FeO in a hot area is not produced by the Soret effect (
Comparison with melting curves of FeO and SiO2
The melting curve of FeO has been repeatedly examined at high pressures (Knittle and Jeanloz, 1991, Shen et al., 1993, Seagle et al., 2008, Fischer and Campbell, 2010). The most recent work by Fischer and Campbell (2010) determined the melting temperature of FeO up to 77 GPa in a laser-heated DAC, employing the discontinuity in the temperature vs. emissivity relationship as a melting criterion, which likely reflects an abrupt change in the sample's optical property associated with melting. The
Conclusion
We have conducted melting experiments on Fe2SiO4 between 26 and 131 GPa, in order to examine the fate of subducted BIFs at the base of the mantle. The results demonstrate that the solidus (eutectic) temperature in the FeOSiO2 binary system is at CMB pressures, which is likely lower than the CMB temperature during the Archean and Paleoproterozoic periods, when the subduction of BIFs occurred. The BIFs, therefore, should have undergone partial melting near the CMB. Present experiments
Acknowledgments
We thank T. Imai for the preparation of Fe2SiO4 fayalite. Discussion with J. Hernlund was helpful. Comments from two anonymous referees helped improve the manuscript. XRD measurements were performed at SPring-8 (proposal no. 2014A0080).
References (47)
- et al.
Phase transition and density of subducted MORB crust in the lower mantle
Earth Planet. Sci. Lett.
(2005) - et al.
Small scale heterogeneity in the mid-lower mantle beneath the circum-Pacific area
Phys. Earth Planet. Inter.
(2010) - et al.
Shock-induced superheating and melting curves of geophysically important minerals
Phys. Earth Planet. Inter.
(2004) - et al.
Precise determination of post-stishovite phase transition boundary and implications for seismic heterogeneities in the mid-lower mantle
Phys. Earth Planet. Inter.
(2010) - et al.
Melting and thermal expansion in the FeFeO system at high pressure
Earth Planet. Sci. Lett.
(2008) - et al.
The Soret diffusion in laser-heated diamond-anvil cell
Phys. Earth Planet. Inter.
(2010) Flow in rocks modelled as multiphase continua: application to polymineralic rocks
J. Struct. Geol.
(1998)- et al.
Pressure calibration of diamond anvil Raman gauge to 310 GPa
J. Appl. Phys.
(2006) - et al.
Phase diagram and equation of state of Al-bearing seifertite at lowermost mantle conditions
Am. Mineral.
(2014) - et al.
Compositional mantle layering revealed by slab stagnation at ∼1000-km depth
Sci. Adv.
(2015)
Iron formation: the sedimentary product of a complex interplay among mantle, tectonic, oceanic, and biospheric processes
Econ. Geol.
Thermodynamics of the MgOFeOSiO2 system up to 140 GPa: application to the crystallization of Earth's magma ocean
J. Geophys. Res., Solid Earth
Stratification of the top of the core due to chemical interactions with the mantle
J. Geophys. Res., Solid Earth
High-pressure elasticity of stishovite and the /mnm ⇔ Pnnm phase transition
J. Geophys. Res.
The system iron-oxygen. II. Equilibrium and thermodynamics of liquid oxide and other phases
J. Am. Chem. Soc.
Melting of lead under high pressure studied using second-scale time-resolved X-ray diffraction
Phys. Rev. B
Subducted banded iron formations as a source of ultralow-velocity zones at the core–mantle boundary
Nature
Toward an internally consistent pressure scale
Proc. Natl. Acad. Sci. USA
High-pressure melting of wüstite
Am. Mineral.
Crustal evolution and deformation in a non-plate-tectonic Archaean Earth: comparisons with Venus
The fate of subducted basaltic crust in the Earth's lower mantle
Nature
Low heat flow inferred from >4 Gyr zircons suggests Hadean plate boundary interactions
Nature
Hydrothermal plumes and the delivery of iron to banded iron formation
J. Geol.
Cited by (16)
High-pressure fluid-phase equilibria: Experimental methods, developments and systems investigated (2013–2016)
2024, Fluid Phase EquilibriaMelting curve of iron to 290 GPa determined in a resistance-heated diamond-anvil cell
2019, Earth and Planetary Science LettersModeling viscosity of (Mg,Fe)O at lowermost mantle conditions
2019, Physics of the Earth and Planetary InteriorsCitation Excerpt :Third, we assume that the effective Tm values for magnesiowüstite and ferropericlase vary linearly between these two end–members. This is a reasonable first-order approximation, given (1) the large spread in reported high pressure melting temperatures of FeO (Knittle and Jeanloz, 1991b; Shen et al., 1993; Fischer and Campbell, 2010; Komabayashi, 2014; Kato et al., 2016) and MgO (see Fat'yanov and Asimow, 2014 for a review); (2) the large spread in the pressure dependence of the solidus and liquidus curves for the (Mg,Fe)O solid solution (Zerr and Boehler, 1994; Zhang and Fei, 2008; Du and Lee, 2014; Deng and Lee, 2017; Fu et al., 2018); and (3) the lack of empirical evidence that the solidus or liquidus temperature of a solid solution is a better approximation for the effective melting temperature used in the homologous temperature scaling than a linear interpolation of the end-member melting temperatures. where P is pressure in GPa and T is temperature in K.
Seismic scatterers in the mid-lower mantle beneath Tonga-Fiji
2018, Physics of the Earth and Planetary InteriorsCitation Excerpt :The presence of fluid supplied by dehydration in the oceanic crust could also be the cause of heterogeneity (e.g., Ohtani and Litasov, 2006). Kato et al. (2016) suggested that almost pure SiO2 segregated from subducted Banded Iron Formations may be the origin of the mid-lower mantle scatterers. We analyzed deep and intermediate-depth earthquakes beneath Tonga and Fiji by array-processing the seismograms that were recorded at regional seismograph networks in the US (UW array and USArray), in Alaska, and in Japan (Hi-Net).
Surface modification of SiO<inf>2</inf> coated ZnO nanoparticles for multifunctional cotton fabrics
2017, Journal of Colloid and Interface ScienceCitation Excerpt :Overall, the synthesis of core/shell structured material has the objective of acquiring a new composite material having synergetic or complementary behaviors between the core and shell materials. Numerous have been reported on the synthesis of nanocomposites e.g. TiO2 [3–6], ZnO [7–9], SiO2 [10,11], Ag [4,12,13], Au [14–16], FeO [17], Fe2O3 [18,19], etc. SiO2 is one of the most studied shell candidate because of its relative ease in preparation, great ecological soundness and compatibility with other different materials [20] which spurred us to prepare the core/shell structured composite of ZnO and SiO2 and anticipated that would accomplish novel properties coming about because of the synergic interaction of these two chemical components.
Pressure-dependent compatibility of iron in garnet: Insights into the origin of ferropicritic melt
2017, Geochimica et Cosmochimica ActaCitation Excerpt :Alternatives suggest that the chemical interaction of the Earth’s iron-poor silicate mantle with the liquid outer core strongly affects the deep mantle’s iron (and perhaps iron-loving elements) content and can create an iron-rich source for mantle plumes that originate at the core–mantle boundary (Knittle and Jeanloz, 1989; Brandon et al., 1998; Humayun et al., 2004; Herzberg et al., 2013). In addition, recycled dense, iron-rich crustal materials (e.g., banded iron formations) could have contributed to the formation of chemical heterogeneities that are composed of potentially iron-rich regions at the base of the mantle (Kaneshima and Helffrich, 1999; Dobson and Brodholt, 2005; Nebel et al., 2010; Kato et al., 2016). As a whole, the average iron content of Archean basalts is higher than post-Archean basalts and modern MORBs (Fig. 1a).