The solubility of hydrogen in Fe 2 O 3 ‐doped rutile and TiO 2 (II) at 1100°C has been experimentally determined. H incorporation in rutile is coupled to substitution of Fe 3+ onto the octahedral Ti 4+ site. In contrast, TiO 2 (II) contains no structurally‐incorporated hydrogen. The dominant Fe 3+ incorporation mechanism in both phases is unrelated to H content, and involves substitution of Fe 3+ onto octahedral Ti 4+ sites, charge‐balanced by oxygen vacancies. Substitution of Fe 2 O 3 into TiO 2 (II) stabilizes the structure to much lower pressures than in the pure TiO 2 system. Results indicate that Al‐bearing stishovite could act as an important carrier of water in subducting oceanic crust, but that formation of the post‐stishovite phase with the α‐PbO 2 structure would represent a significant dehydration event at the base of the lower mantle.
High pressure/temperature annealing experiments are used to determine diffusivities of H+ and D+ in non-stoichiometric spinel, a low-pressure analogue for nominally anhydrous minerals in Earth’s mantle. Data are fitted to the following Arrhenius law: Diffusivity (m2/s) = 4 ± 1 × 10−12 exp(−54 ± 2 kJ mol−1/RT). At low temperatures, H+ and D+ diffusion in non-stoichiometric spinel is charge balanced by flux of O vacancies, with infrared data consistent with protonation of both octahedral and tetrahedral O–O edges in non-stoichiometric spinel, and additional fine structure due to Mg–Al mixing and/or coupling of structurally incorporated H+ with cation vacancies. Absence of changes in the fine structure of O–H absorption bands indicates that H+ can become locally coupled and uncoupled to other defects during bulk diffusion. As such, proton conductivity in spinel group minerals, arising from faster flux of uncoupled H+, can only be calculated from H+ mobility data if the extent of defect coupling is constrained.
Abstract The growth and recycling of continental crust has resulted in the chemical and thermal modification of Earth’s mantle, hydrosphere, atmosphere, and biosphere for ∼4.0 b.y. However, knowledge of the protolith that gave rise to the first continents and whether the environment of formation was a subduction zone still remains unknown. Here, tonalite melts are formed in high P-T experiments in which primitive oceanic plateau starting material is used as an analogue for Eoarchean (3.6–4.0 Ga) oceanic crust generated at early spreading centers. The tonalites are produced at 1.6–2.2 GPa and 900–950 °C and are mixed with slab-derived aqueous fluids to generate melts that have compositions identical to that of Eoarchean continental crust. Our data support the idea that the first continents formed at ca. 4 Ga and subsequently, through the subduction and partial melting of ∼30–45-km-thick Eoarchean oceanic crust, modified Earth’s mantle and Eoarchean environments and ecosystems.
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