The role of mantle ultrapotassic fluids in diamond formation

Abstract

Analysis of data on micro-and nano-inclusions in mantle-derived and metamorphic diamonds shows that, to a first approximation, diamond-forming medium can be considered as a specific ultrapotassic, carbonate/chloride/silicate/water fluid. In the present work, the processes and mechanisms of diamond crystallization were experimentally studied at 7.5 GPa, within the temperature range of 1,400 -1,800°C, with different compositions of melts and fluids in the KCl/K 2CO3/H2O/C system. It has been established that, at constant pressure, temperature, and run duration, the mechanisms of diamond nucleation, degree of graphite-to-diamond transformation, and formation of metastable graphite are governed chiefly by the composition of the fluids and melts. The experimental data suggest that the evolution of the composition of deep-seated ultrapotassic fluids/melts is a crucial factor of diamond formation in mantle and ultrahigh-pressure metamorphic processes. high-pressure experimentA n important stage in solving problems of diamond genesis is constructing a clear physical model of diamond formation (1). One of the key parameters to be modeled is the composition of the diamond crystallization medium, with a fluid being largely considered as a crucial agent in diamond formation. It is generally agreed that the mantle media of diamond crystallization were volatile-saturated melts or fluids (1-7). Carbonbearing fluids and carbonates could be sources of carbon, and the medium could have been saturated with it by redox reactions. Recent studies have been aimed at searching and examining fluid and fluid-containing inclusions in genetically different diamonds. Also, experiments have been performed to model the processes of diamond crystallization in the media compositionally corresponding to the inclusions. It is the combination of these two approaches that permits us to gain deeper insight into diamond genesis and to better understand the specific character of processes of deep-seated mineral formation.Studies of mineral inclusions in diamonds have determined the main types of diamondiferous parageneses: ultramaffic, eclogitic (4,8,9), and intermediate websterite (10) and calc-silicate types (11). Some mineral inclusions, e.g., intergrowths of phlogopite with pyroxene (8, 12, 13) and phlogopite with calcite (14), are unambiguously evidence that K and H 2 O were present in the diamond-forming medium. It is pertinent to note that, as early as 30 years ago, neutron activation analysis of diamonds without visible inclusions, taken from three South African deposits, revealed a mixture of mostly K and of other components, which was interpreted as evidence of the presence of entrapped melt (15). Principally, more recent information has accrued from micro-and nano-inclusions in mantle-derived and metamorphic diamonds. Analysis of the composition of the inclusions in fibrous or cloudy mantle diamonds shows that, at the time of entrapment, the inclusion matter was a specific fluid whose composition belongs to one of the three main types: (i) ...

Melting of carbonated pelites at 8–13 GPa: generating K-rich carbonatites for mantle metasomatism

“…The absence of any evidence for an accumulation of this kind of melts at the base of the lithosphere, where temperatures would be low enough for complete crystallization of the carbonatite melts, suggests that a redox reaction with the reduced mantle (Frost and McCammon 2008;Rohrbach et al 2009) immobilizes these carbonatites. In fact, diamond crystallization has been shown to be favored by the catalytic behaviour of K 2 CO 3 -rich melt and fluid (Taniguchi et al 1996;Palyanov et al 2007;Klein-BenDavid et al 2007).…”

Mantle–slab interaction and redox mechanism of diamond formation

“…A similar effect will be produced by H 2 O-CO 2 fluids (40-42). It was experimentally shown that the addition of these components in the system significantly increases the diamondforming ability of mantle fluids/melts (43,44), acting in the oxidized conditions behind the redox front. Under reducing conditions, ahead of the redox front, where the diamond crystallized from Fe-C melt only, the presence of H 2 O will lead to the inhibitory effect on the diamond-forming process (45).…”

The K₂CO₃ fusion curve revisited: New experiments at pressures up to 12 GPa

“…Thus, the molten K 2 CO 3 may also have high electrical conductivity, which can also contribute to high mantle conductivity. In addition, analyses of microinclusions in diamond (e.g., Sobolev et al, 1998;Hwang et al, 2006) and diamond-formation experiments (e.g., Pal'yanov et al, 1999Pal'yanov et al, , 2007 indicate that the composition of the diamond-forming medium can be roughly considered as ultrapotassic carbonate fluid. Considering the low melting temperature of K 2 CO 3 , we suggest that the existence of K 2 CO 3 -rich liquid may contribute to the formation of diamond in the mantle.…”