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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian


IMPACT FACTOR 2017: 2.645

CiteScore 2018: 2.55

SCImago Journal Rank (SJR) 2018: 1.355
Source Normalized Impact per Paper (SNIP) 2018: 1.103

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1945-3027
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Volume 98, Issue 10

Issues

Effects of fluid and melt density and structure on high-pressure and high-temperature experimental studies of hydrogen isotope partitioning between coexisting melt and aqueous fluid

Bjorn Mysen
  • Corresponding author
  • Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington, D.C. 20015, U.S.A.
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Published Online: 2015-03-07 | DOI: https://doi.org/10.2138/am.2013.4449

Abstract

Hydrogen isotope partitioning (as H2O and D2O) between silicate-saturated aqueous fluid and water-saturated aluminosilicate melt has been determined with vibrational spectroscopy (Raman and infrared) in situ with the samples at high temperature and pressure by using a hydrothermal diamondanvil cell (HDAC) for sample containment. To assess the effects of pressure and, therefore, different silicate speciation in fluids and melts, on the D/H partitioning behavior, two pressure/temperature experimental trajectories (450-800 °C/155-754 MPa, and 450-800 °C/562-1271 MPa) were used. In these temperature and pressure ranges, the fluid/melt partition coefficients are temperature (and pressure) dependent with the average enthalpy change, △H = -6.6 ± 15 kJ/mol and -10.3 ± 1.1 kJ/mol for H2O and D2O, respectively. The △H-values for the lower-pressure trajectory (and, therefore, lower fluid density) were 15-20% higher than for the higher-pressure (and higher fluid density) trajectory. The (D/H) ratios of fluids and melts, (D/H)fluid and (D/H)melt, are also temperature dependent with a small negative DH for (D/H)fluid (average: -2.4 ± 0.8 kJ/mol) and a positive △H-value for (D/H)melt (2.3 ± 1.4 kJ/mol). The (D,H) exchange equilibrium between fluid and melt is also temperature (and pressure) dependent so that for the low-density P/T trajectory, the △H = -4.2 ± 0.6 kJ/mol, whereas for the higher-density trajectory, △H = -5.4 ± 0.7 kJ/mol. The difference between the H2O and D2O fluid/melt partition coefficients and the temperature- and pressure-dependent D/H fractionation behavior in and between hydrous silicate melts and silicate-saturated aqueous fluid in part is because pressure increases with increasing temperature in the HDAC experiments and the volume difference between fluid and melt differ for H2O and D2O. In addition, the silicate speciation in fluids and melts are temperature and pressure dependent, which also leads to significantly temperature- and pressuredependent D/H fractionation within and between silicate melts and fluids at high temperature and pressure. In the Earth’s deep crust and upper mantle, hydrogen isotope partitioning between condensed phases and aqueous fluid can differ substantially from that between condensed phases and pure H2O because the aqueous fluid in the Earth’s interior is a concentrated silicate solution wherein the silicate speciation affects the isotope partitioning.

Keywords : Hydrogen isotopes; vibrational spectroscopy; aqueous fluid; hydrous melt; structure; pressure; temperature

About the article

Received: 2012-12-21

Accepted: 2013-05-07

Published Online: 2015-03-07

Published in Print: 2013-10-01


Citation Information: American Mineralogist, Volume 98, Issue 10, Pages 1754–1764, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am.2013.4449.

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© 2015 by Walter de Gruyter Berlin/Boston.

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