Diffusional fractionation of helium isotopes in silicate melts

Abstract Chlorine is a minor element present in obsidians in quantities greater than in average igneous rocks. The chlorine concentration in obsidians is generally low, of the order of tenths of wt %, but it exhibits an appreciable differentiation among geological sources. Despite these characteristics, chlorine has rarely been taken into consideration as a possible indicator of obsidian provenance and it does not appear in the chemical analytical tables accompanying the geochemical characterisation of obsidian samples. In this work, after an overview of chlorine geochemistry and cycle, we present thirty-one new electron microprobe (EPMA) analyses, including Cl, of geologic obsidians sampled from the four sources of the Central Mediterranean, exploited in prehistoric times (Monte Arci, Palmarola, Lipari and Pantelleria). The results are compared with 175 new EPMA analyses, including Cl, of archaeological obsidians already characterised in previous work and of known provenance. As such it was possible to ascertain that each source has a characteristic chlorine concentration, showing the utility of its use in the studies of obsidian provenance. Furthermore, given that the solubility of chlorine in silicate melts is correlated to its alkali content, in particular sodium, we assessed the efficacy of simple binary graphs Cl vs Na2O to better constrain the provenance of the obsidian samples.

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Comment on “Compositional and temperature effects on sulfur speciation and solubility in silicate melts” by Nash et al. [Earth Planet. Sci. Lett. 507 (2019) 187–198]

Earth and Planetary Science Letters ◽

10.1016/j.epsl.2021.116843 ◽

2021 ◽

Vol 560 ◽

pp. 116843

Author(s):

Hugh St.C. O'Neill

Keyword(s):

Temperature Effects ◽

Silicate Melts ◽

Earth Planet ◽

Sulfur Speciation

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Behavior and properties of water in silicate melts under deep mantle conditions

Scientific Reports ◽

10.1038/s41598-021-90124-7 ◽

2021 ◽

Vol 11 (1) ◽

Author(s):

Bijaya B. Karki ◽

Dipta B. Ghosh ◽

Shun-ichiro Karato

Keyword(s):

Electrical Conductivity ◽

Molar Volume ◽

Water Solution ◽

Pure Water ◽

Electrical Conductance ◽

Water Structure ◽

Bulk Water ◽

Silicate Melts ◽

Pressure Regime ◽

Low Pressures

AbstractWater (H2O) as one of the most abundant fluids present in Earth plays crucial role in the generation and transport of magmas in the interior. Though hydrous silicate melts have been studied extensively, the experimental data are confined to relatively low pressures and the computational results are still rare. Moreover, these studies imply large differences in the way water influences the physical properties of silicate magmas, such as density and electrical conductivity. Here, we investigate the equation of state, speciation, and transport properties of water dissolved in Mg1−xFexSiO3 and Mg2(1−x)Fe2xSiO4 melts (for x = 0 and 0.25) as well as in its bulk (pure) fluid state over the entire mantle pressure regime at 2000–4000 K using first-principles molecular dynamics. The simulation results allow us to constrain the partial molar volume of the water component in melts along with the molar volume of pure water. The predicted volume of silicate melt + water solution is negative at low pressures and becomes almost zero above 15 GPa. Consequently, the hydrous component tends to lower the melt density to similar extent over much of the mantle pressure regime irrespective of composition. Our results also show that hydrogen diffuses fast in silicate melts and enhances the melt electrical conductivity in a way that differs from electrical conduction in the bulk water. The speciation of the water component varies considerably from the bulk water structure as well. Water is dissolved in melts mostly as hydroxyls at low pressure and as –O–H–O–, –O–H–O–H– and other extended species with increasing pressure. On the other hand, the pure water behaves as a molecular fluid below 15 GPa, gradually becoming a dissociated fluid with further compression. On the basis of modeled density and conductivity results, we suggest that partial melts containing a few percent of water may be gravitationally trapped both above and below the upper mantle-transition region. Moreover, such hydrous melts can give rise to detectable electrical conductance by means of electromagnetic sounding observations.

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Path-integral calculation of the fourth virial coefficient of helium isotopes

The Journal of Chemical Physics ◽

10.1063/5.0043446 ◽

2021 ◽

Vol 154 (10) ◽

pp. 104107

Author(s):

Giovanni Garberoglio ◽

Allan H. Harvey

Keyword(s):

Path Integral ◽

Virial Coefficient ◽

Helium Isotopes

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Activities in ternary silicate melts

Transactions of the Faraday Society ◽

10.1039/tf9565201312 ◽

1956 ◽

Vol 52 ◽

pp. 1312 ◽

Cited By ~ 42

Author(s):

F. D. Richardson

Keyword(s):

Silicate Melts

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Helium Isotopes Quantum Sieving through Graphtriyne Membranes

Nanomaterials ◽

10.3390/nano11010073 ◽

2020 ◽

Vol 11 (1) ◽

pp. 73

Author(s):

Marta I. Hernández ◽

Massimiliano Bartolomei ◽

José Campos-Martínez

Keyword(s):

Low Temperatures ◽

Quantum Tunneling ◽

Transition State Theory ◽

Selective Adsorption ◽

State Theory ◽

Helium Isotopes ◽

Quantum Calculations ◽

Approximate Methods ◽

Helium Atoms ◽

Quantum Behavior

We report accurate quantum calculations of the sieving of Helium atoms by two-dimensional (2D) graphtriyne layers with a new interaction potential. Thermal rate constants and permeances in an ample temperature range are computed and compared for both Helium isotopes. With a pore larger than graphdiyne, the most common member of the γ-graphyne family, it could be expected that the appearance of quantum effects were more limited. We find, however, a strong quantum behavior that can be attributed to the presence of selective adsorption resonances, with a pronounced effect in the low temperature regime. This effect leads to the appearance of some selectivity at very low temperatures and the possibility for the heavier isotope to cross the membrane more efficiently than the lighter, contrarily to what happened with graphdiyne membranes, where the sieving at low energy is predominantly ruled by quantum tunneling. The use of more approximate methods could be not advisable in these situations and prototypical transition state theory treatments might lead to large errors.

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