In situ study of the fractionation of hydrogen isotopes between aluminosilicate melts and coexisting aqueous fluids at high pressure and high temperature – Implications for the δD in magmatic processes

Abstract

The hydrogen isotopic composition of melt inclusions trapped in phenocrysts during their crystallization and growth in a magma may contribute to a better understanding of the water cycle between the atmosphere, the hydrosphere and the lithosphere. Such understanding relies on the knowledge of the hydrogen isotopic fractionation factors between aqueous fluids, silicate melts, and minerals at temperature and pressure conditions relevant to the Earth's interior. Significant D/H fractionation between silicate melts and aqueous fluids was reported at hundreds of MPa and • C by using in situ measurements in hydrothermal diamond anvil cell (HDAC) experiments (Mysen, 2013a, 2013b, Am. Mineral. 98, 376-386 and 1754-1764. However, the available dataset is focused on fluids and melts with D/H ratios close to unity. The relevance of such data for natural processes that involve per mil variations of δD-values may not always be clear. To address such concerns, the effect of the bulk D/H ratio on hydrogen isotope partitioning between water-saturated silicate melts and coexisting silicatesaturated aqueous fluids has been determined in the Na 2 O-Al 2 O 3 -SiO 2 -H 2 O-D 2 O system. To this end, in situ Raman spectroscopic measurements were performed on fluids and melts with bulk D/H ratios from 0.05 to 2.67 by using an externally-heated diamond anvil cell in the 300-800 • C and 200-1500 MPa temperature and pressure range, respectively. In these pressure/temperature ranges, the D/H ratios of fluids in equilibrium with melt barely change with temperature (in average H fluid = 0.47 ± 1.15 kJ/mol). In contrast, the D/H ratios of coexisting melts display strong dependence on temperature (average H melt = 7.18 ± 1.27 kJ/mol). The temperaturedependence of the D/H fractionation factor between melt and fluid (α fluid-melt = D/H fluid /D/H melt ) is comparable in all the experiments and can be written: 1000 · ln(α fluid-melt ) = 263 (±26) · T −2 -126 (±48).Therefore, the α fluid-melt is independent of the bulk D/H ratio of the melt + fluid system. Experimentally determined α fluid-melt using D-enriched fluids, therefore, can be applied to natural systems. It follows that for water-saturated magma strong isotopic fractionation of D and H between water dissolved in magmas and deep aqueous fluids may occur. The δD-values in melt inclusions in phenocrysts in such water-saturated magma will reflect such fluid/melt fractionation effects. A likely result is underestimation of the δD isotopic composition of slab fluids based on δD-values in melt inclusions. The temperature-dependent hydrogen isotope fractionation must be taken into account in the modeling of slab fluid-magma interaction in the mantle wedge.

Hydrogen isotope fractionation and redox‐controlled solution mechanisms in silicate‐COH melt + fluid systems

“…In the latter report, the D/H fractionation factor between fluid and coexisting melt was independent of total D/H ratio of the system. Moreover, the D/H ratio of the starting H 2 O + D 2 O mixture calculated from the Raman area ratios, 2600/3600, of its spectrum (with equation ) was identical to the actual ratio [ Mysen , ; Dalou et al ., ].…”

“…The background subtraction followed the procedure described by Dalou et al . []. Briefly, this was accomplished by recording Raman spectra of the diamond cell away from the sample hole in the Ir gasket but at the same focal plane and the same temperature and pressure as that of the spectroscopic measurements of fluid and melt.…”

Mass transfer in the Earth’s interior: fluid-melt interaction in aluminosilicate–C–O–H–N systems at high pressure and temperature under oxidizing conditions

“…This background signal was eliminated in the FTIR spectra by normalizing to those of an empty diamond cell at the same temperature after the sample was removed. For the Raman spectra, the procedure is more complex, and the diamond background was subtracted by using the procedure of Dalou et al (2015). Briefly, this required recording Raman spectra of the diamond cell away from the sample hole in the Ir gasket but within the same focal plane as the sample measurement at the same temperature and pressure.…”

Intramolecular fractionation of hydrogen isotopes in silicate quenched melts

“…Therefore, these structural features can control the isotopic fractionation of 2 H and 1 H between the melts and the fluids. This theory might explain the large fractionation of 2 H and 1 H between those substances (Mysen, 2013a,b;Dalou et al, 2015).…”

“…Further, the behaviour of Fe isotopes in silicate melts is affected by the effects of melt composition on the Fe environment (Dauphas et al, 2014). (Mysen, 2013a,b;Dalou et al, 2015). Such values are higher than typical a fluid-mineral in the tens of ‰ level (Chacko et al, 2001).…”