CO 2 -induced small water solubility in olivine and implications for properties of the shallow mantle

Yang, Xiaozhi; Liu, Dingding; Xia, Qing

doi:10.1016/j.epsl.2014.06.025

Cited by 40 publications

(36 citation statements)

References 88 publications

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“…Petrologic constraints suggest that the source mantle H 2 O content for oceanic basalts is between 50 and 200 ppm [Michael, 1988;Saal et al, 2002;Simons et al, 2002;Workman and Hart, 2005]. In addition, experimental storage capacity data suggest that upper mantle peridotite can hold up to 700 ppm at 8 GPa [Ardia et al, 2012], though CO 2 may decrease this capacity by a factor of 2 [Yang et al, 2014]. Estimated bulk H 2 O contents from the DK model remain below these constraints throughout the mantle based on the SEO3 (QFM) anhydrous model, while the UHO model predicts H 2 O contents within the independently constrained ranges until a depth of 210 km, where H 2 O contents reach over 200 ppm and rise to almost 400 ppm by 300 km.…”

Section: 1002/2014gc005709mentioning

confidence: 99%

The electrical structure of the central Pacific upper mantle constrained by the NoMelt experiment

Sarafian

Evans

Collins

et al. 2015

Geochem Geophys Geosyst

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The NoMelt experiment imaged the mantle beneath 70 Ma Pacific seafloor with the aim of understanding the transition from the lithosphere to the underlying convecting asthenosphere. Seafloor magnetotelluric data from four stations were analyzed using 2-D regularized inverse modeling. The preferred electrical model for the region contains an 80 km thick resistive (>10 3 Xm) lithosphere with a less resistive (50 Xm) underlying asthenosphere. The preferred model is isotropic and lacks a highly conductive (10 Xm) layer under the resistive lithosphere that would be indicative of partial melt. We first examine temperature profiles that are consistent with the observed conductivity profile. Our profile is consistent with a mantle adiabat ranging from 0.3 to 0.5 C/km. A choice of the higher adiabatic gradient means that the observed conductivity can be explained solely by temperature. In contrast, a 0.3 C/km adiabat requires an additional mechanism to explain the observed conductivity profile. Of the plausible mechanisms, H 2 O, in the form of hydrogen dissolved in olivine, is the most likely explanation for this additional conductivity. Our profile is consistent with a mostly dry lithosphere to 80 km depth, with bulk H 2 O contents increasing to between 25 and 400 ppm by weight in the asthenosphere with specific values dependent on the choice of laboratory data set of hydrous olivine conductivity and the value of mantle oxygen fugacity. The estimated H 2 O contents support the theory that the rheological lithosphere is a result of dehydration during melting at a mid-ocean ridge with the asthenosphere remaining partially hydrated and weakened as a result.

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Section: 1002/2014gc005709mentioning

confidence: 99%

The electrical structure of the central Pacific upper mantle constrained by the NoMelt experiment

Sarafian

Evans

Collins

et al. 2015

Geochem Geophys Geosyst

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“…Yet, our rehydration experiments exceed the water contents in untreated crystals, which might indicate an excessive hydration. Another issue is the possible effect that CO 2 and other volatiles might have on the water activity (e.g., Kovács et al 2012;Yang et al 2014) and thus on the rehydration of hydrogen-associated defects during the hydrothermal experiments. However, there are also arguments against insufficient as well as excessive rehydration.…”

Section: Reconstruction Of Initial Water Contents In Clinopyroxenesmentioning

confidence: 99%

“…Another issue regarding excessive hydration of the clinopyroxene crystals is the effect of CO 2 on the activity of water. It has been shown that other volatiles such as CO 2 or halogens within a magmatic system reduce the incorporation of hydrogen in NAMs (e.g., Stalder et al 2008;Kovács et al 2012;Yang et al 2014). As such, the water contents in olivines annealed in a H 2 O + CO 2 fluid phase, for example, were about half of those treated in pure H 2 O (Yang et al 2014).…”

Section: Reconstruction Of Initial Water Contents In Clinopyroxenesmentioning

confidence: 99%

“…It has been shown that other volatiles such as CO 2 or halogens within a magmatic system reduce the incorporation of hydrogen in NAMs (e.g., Stalder et al 2008;Kovács et al 2012;Yang et al 2014). As such, the water contents in olivines annealed in a H 2 O + CO 2 fluid phase, for example, were about half of those treated in pure H 2 O (Yang et al 2014). However, these studies concentrated on the solubility of hydrogen in NAMs, and the conducted experiments were done at pressures of several gigapascals and temperatures exceeding 1000 °C.…”

Section: Reconstruction Of Initial Water Contents In Clinopyroxenesmentioning

confidence: 99%

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Experimental hydration of natural volcanic clinopyroxene phenocrysts under hydrothermal pressures (0.5–3 kbar)

Weis

Stalder

Skogby

2016

American Mineralogist

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Water is a key parameter in mantle rheology, magma genesis, magma evolution, and resulting eruption styles, because it controls the density and the viscosity, as well as the melting and crystallization behavior of a melt. The water content in nominally anhydrous minerals (NAMs) such as clinopyroxene recently has been used as a proxy for magmatic water contents. NAMs, however, may dehydrate during magma degassing and eruption. We performed rehydration experiments on potentially degassed clinopyroxene phenocrysts from various volcanic settings. The experiments were conducted in hydrogen gas at 1 atm or hydrothermal pressures ranging from 0.5 to 3 kbar to test the incorporation of water into natural clinopyroxene under water fugacities similar to those in a volcanic system. Our results show a dependence of the water content in the clinopyroxene crystals with pressure as the phenocrysts begin to dehydrate upon lower water fugacities in the experiments. Water loss or gain in a crystal occurs according to the relatively fast redox-reaction OH -+ Fe 2+ ↔ O 2-+ Fe 3+ + ½ H 2 , which was confirmed by Mössbauer spectroscopy. The kinetics of this redox-process are independent of pressure and thus water fugacity. Water contents in rehydrated clinopyroxene crystals can be related to magmatic water contents at various levels in a volcanic system. Our results thus show that the water content in erupted clinopyroxene phenocrysts cannot be taken for granted to be representative of magmatic water contents prior to magma degassing. The conducted experiments indicate the simultaneous dehydration of clinopyroxene along with magma ascent and degassing. Rehydration experiments under hydrothermal pressures, however, may be able to reconstruct clinopyroxene water contents at crystallization prior to dehydration.

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“…Gaetani et al (2014) differed in composition, and the partitioning of C Total H 2 O amongst the three different substitution mechanisms was not reported, this hypothesis cannot yet be tested. Yang et al (2014) also used different ratios of H 2 O and CO 2 , finding a difference of a factor of 2 between values of C H 2 O in olivines from experiments with pure H 2 O and those with H 2 O-CO 2 , but changing X CO2 from 0.22-0.50 apparently had no effect on the water solubility. This observation disagrees with those of Matveev et al (2001) and Gaetani et al (2014).…”

Section: +mentioning

confidence: 99%

The responses of the four main substitution mechanisms of H in olivine to H2O activity at 1050 °C and 3 GPa

Tollan

Smith

O’Neill

et al. 2017

Prog. in Earth and Planet. Sci.

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The water solubility in olivine C H 2 O ð Þ has been investigated at 1050°C and 3 GPa as a function of water activity a H 2 O ð Þ at subsolidus conditions in the piston-cylinder apparatus, with a H 2 O varied using H 2 O-NaCl fluids. Four sets of experiments were conducted to constrain the effect of a H 2 O on the four main substitution mechanisms. The experiments were designed to grow olivine in situ and thus achieve global equilibrium (G-type), as opposed to hydroxylating olivine with a pre-existing point-defect structure and impurity content (M-type). Olivine grains from the experiments were analysed with polarised and unpolarised FTIR spectroscopy, and where necessary, the spectra have been deconvoluted to quantify the contribution of each substitution mechanism. Olivine buffered with magnesiowüstite produced absorbance bands at high wavenumbers ranging from 3566 to 3612 cm −1 . About 50% of the total absorbance was found parallel to the a-axis, 30% parallel to the b-axis and 20% parallel to the c-axis. The total absorbance and hence water concentration in olivine follows the relationship of C H 2 O ∝a H 2 O 2 , indicating that the investigated defect must involve four H atoms substituting for one Si atom (labelled as [Si]). Forsterite buffered with enstatite produced an absorbance band exclusively aligned parallel the c-axis at 3160 cm −1 . The band position, polarisation and observed C H 2 O ∝a H 2 O are consistent with two H substituting for one Mg (labelled as [Mg]). Ti-doped, enstatite-buffered olivine displays absorption bands, and polarisation typical of Ti-clinohumite point defects where two H on the Si-site are charge-balanced by one Ti on a Mg-site (labelled as [Ti]). This is further supported by C H 2 O ∝a H 2 O and a 1:1 relationship of molar H 2 O and TiO 2 in these experiments. Sc-doped, enstatitebuffered experiments display a main absorption band at 3355 cm −1 with C H 2 O ∝a H 2 O 0:5 and a positive correlation of Sc and H, indicating the coupled substitution of a trivalent cation plus a H for two Mg (labelled as [triv]). Our data demonstrate that extreme care has to be taken when inferences from experiments conducted at a H 2 O ¼ 1 are applied to the mantle, where in most cases, a low a H 2 O persists. In particular, the higher exponent of the [Si] substitution mechanism means that the contribution of this hydrous defect to total water content will decrease more rapidly with decreasing a H 2 O than the contributions of the other substitution mechanisms. The experiments confirm previous results that the [Mg] mechanism holds an almost negligible amount of water under nearly all T-PfO 2 -fH 2 O conditions that may be anticipated in nature. However, the small amounts of H 2 O we find in substituting by this mechanism are similar in the experiments on forsterite doped with either Sc or Ti to those in the undoped forsterite at equivalent a H 2 O (all buffered by enstatite), confirming the assumption that, thermodynamically, C H 2 O substituting by each mechanism does not depend on the w...

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CO 2 -induced small water solubility in olivine and implications for properties of the shallow mantle

Cited by 40 publications

References 88 publications

The electrical structure of the central Pacific upper mantle constrained by the NoMelt experiment

The electrical structure of the central Pacific upper mantle constrained by the NoMelt experiment

Experimental hydration of natural volcanic clinopyroxene phenocrysts under hydrothermal pressures (0.5–3 kbar)

The responses of the four main substitution mechanisms of H in olivine to H2O activity at 1050 °C and 3 GPa

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