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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 2017: 2.31

SCImago Journal Rank (SJR) 2017: 1.440
Source Normalized Impact per Paper (SNIP) 2017: 1.059

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1945-3027
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Volume 100, Issue 8-9

Issues

Decrease of hydrogen incorporation in forsterite from CO2-H2O-rich kimberlitic liquid

Virginie Baptiste
  • Géosciences Montpellier, Université Montpellier & CNRS, CC 60, Place E. Bataillon, 34095 Montpellier cedex 5, France
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/ Sylvie Demouchy
  • Corresponding author
  • Géosciences Montpellier, Université Montpellier & CNRS, CC 60, Place E. Bataillon, 34095 Montpellier cedex 5, France
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/ Shantanu Keshav
  • Géosciences Montpellier, Université Montpellier & CNRS, CC 60, Place E. Bataillon, 34095 Montpellier cedex 5, France
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/ Fleurice Parat
  • Géosciences Montpellier, Université Montpellier & CNRS, CC 60, Place E. Bataillon, 34095 Montpellier cedex 5, France
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/ Nathalie Bolfan-Casanova / Pierre Condamine / Patrick Cordier
  • Unité Matériaux et Transformation, Université Lille 1 & CNRS, UMR 8207, Villeneuve d’Ascq, France
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Published Online: 2015-08-12 | DOI: https://doi.org/10.2138/am-2015-5200

Abstract

To test if hydrogen incorporation by ionic diffusion can occur between a volatile-rich kimberlitic liquid and forsterite, results of high-pressure and high-temperature experiments using a piston-cylinder apparatus at 1200-1300 °C and 1 GPa for durations of 1 min, 5 h, and 23 h, are reported here. Kimberlitic liquid in the system CaO-MgO-Al2O3-SiO2-CO2-H2O and synthetic forsterite single crystals were chosen as a first simplification of the complex natural kimberlite composition. Unpolarized Fourier transform infrared spectroscopy was used to quantify the concentrations of OH in the crystallographically oriented forsterite. Scanning electron microscopy, electron backscattered diffraction, electron microprobe analyses, and transmission electron microscopy were performed to identify the run products. After 5 and 23 h, a forsterite overgrowth crystallized with the same orientation as the initial forsterite single crystal. The kimberlitic liquid has crystallized as micrometer-scale euhedral forsterite neocrystals with random crystallographic orientations, as well as a nanoscale aluminous phase and a calcic phase. Despite theoretical water-saturation of the system and long duration, none of the initial forsterite single crystals display signs of hydration such as hydrogen diffusion profile from the border toward the center of the crystal. Most likely, the presence of CO2 in the system has lowered the H2O fugacity to such an extent that there is no significant hydration of the starting forsterite single crystal or its overgrowth. Also, the presence of CO2 enhances rapid forsterite crystal growth. Forsterite growth rate is around 2 × 108 mm3/h at 1250 °C. These experimental results suggest a deep mantle origin of the high OH content found in natural mantle-derived xenoliths transported in kimberlites, as reported from the Kaapvaal craton. In agreement with previous studies, it also points out to the fact that significant hydration must take place in a CO2-poor environment.

Keywords: Kimberlite; water; hydrogen; diffusion; olivine; point defect

About the article

Received: 2014-09-24

Accepted: 2015-03-03

Published Online: 2015-08-12

Published in Print: 2015-08-01


Citation Information: American Mineralogist, Volume 100, Issue 8-9, Pages 1912–1920, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am-2015-5200.

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© 2015 by Walter de Gruyter Berlin/Boston.

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