Conditions for the origin of oxidized carbonate-silicate melts: Implications for mantle metasomatism and diamond formation

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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Diamond formation in an electric field under deep Earth conditions

Science Advances ◽

10.1126/sciadv.abb4644 ◽

2021 ◽

Vol 7 (4) ◽

pp. eabb4644

Author(s):

Yuri N. Palyanov ◽

Yuri M. Borzdov ◽

Alexander G. Sokol ◽

Yuliya V. Bataleva ◽

Igor N. Kupriyanov ◽

...

Keyword(s):

Electric Fields ◽

Lithospheric Mantle ◽

Isotope Fractionation ◽

Diamond Formation ◽

Mantle Minerals ◽

Electrochemical Processes ◽

Mantle Mineral ◽

Deep Earth ◽

Diamond Crystallization ◽

Global Carbon

Most natural diamonds are formed in Earth’s lithospheric mantle; however, the exact mechanisms behind their genesis remain debated. Given the occurrence of electrochemical processes in Earth’s mantle and the high electrical conductivity of mantle melts and fluids, we have developed a model whereby localized electric fields play a central role in diamond formation. Here, we experimentally demonstrate a diamond crystallization mechanism that operates under lithospheric mantle pressure-temperature conditions (6.3 and 7.5 gigapascals; 1300° to 1600°C) through the action of an electric potential applied across carbonate or carbonate-silicate melts. In this process, the carbonate-rich melt acts as both the carbon source and the crystallization medium for diamond, which forms in assemblage with mantle minerals near the cathode. Our results clearly demonstrate that electric fields should be considered a key additional factor influencing diamond crystallization, mantle mineral–forming processes, carbon isotope fractionation, and the global carbon cycle.

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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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