Diamond growth in mantle fluids

Hélène Bureau a,⁎, Daniel J. Frost b, Nathalie Bolfan-Casanova c, Clémence Leroy a,
Imène Esteve a, Patrick Cordier d. “Diamond Growth in Mantle Fluids.” Lithos 265: 4–15.
a Institut de Minéralogie, de Physique desMatériaux et de Cosmochimie (IMPMC), Sorbonne Universités – UPMCUniv. Paris 06, CNRSUMR 7590,MuséumNational d'Histoire Naturelle, IRD UR 206, 4 place Jussieu, 75252 Paris Cedex 05, France
b Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany
c Laboratoire Magmas et Volcans, Université Blaise Pascal, Clermont-Ferrand, France
d Unité Matériaux et Transformations, Université Lille 1, 59655 Villeneuve d'Ascq, France

Abstract
In the uppermantle, diamonds can potentially growfromvarious forms ofmedia (solid, gas, fluid)with a range of compositions (e.g. graphite, C–O–H fluids, silicate or carbonate melts). Inclusions trapped in diamonds are one of
the few diagnostic tools that can constrain diamond growth conditions in the Earth's mantle. In this study, inclusion-bearing diamonds have been synthesized to understand the growth conditions of natural diamonds in the upper mantle. Diamonds containing syngenetic inclusions were synthesized in multi-anvil presses employing starting mixtures of carbonates, and silicate compositions in the presence of pure water and saline fluids (H2O–NaCl). Experiments were performed at conditions compatible with the Earth's geotherm (7 GPa, 1300–1400 °C). Results show that within the timescale of the experiments (6 to 30 h) diamond growth occurs if water and carbonates are present in the fluid phase.Water promotes faster diamond growth (up to 14 mm/year at 1400 °C, 7 GPa, 10 g/l NaCl), which is favorable to the inclusion trapping process. At 7 GPa, temperature and fluid composition are the main factors controlling diamond growth. In these experiments, diamonds grew in the presence of two fluids: an aqueous fluid and a hydrous silicate melt. The carbon source for diamond growth must be carbonate (CO3 2 ) dissolved in the melt or carbon dioxide species in the aqueous fluid (CO2aq). The presence of NaCl affects the growth kinetics but is not a prerequisite for inclusion-bearing diamond formation. The presence of small discrete or isolated volumes of water-rich fluids is necessary to grow inclusion-bearing
peridotitic, eclogitic, fibrous, cloudy and coated diamonds, and may also be involved in the growth of ultradeep, ultrahigh-pressure metamorphic diamonds.

http://www.sciencedirect.com/science/article/pii/S0024493716303334