Physicochemical factors of diamond and graphite formation in carbonatite melts

on experimental grounds

Litvin Yu.A.*, Spivak A.V.*, Solopova N.A.**, Litvin V.Yu.*, Bobrov A.V.**

*Institute of Experimental Mineralogy, Chernogolovka, Russia; **Moscow State University, Moscow, Russia.

ššššššššššššššš Here the purpose is experimental kinetic study of diamond and graphite nucleation and growth in multi-component K-Na-Mg-Ca-carbonatite melt with dissolved carbon under conditions of diamond PT stability. Phenomenal peculiarity of the carbonatite-carbon melt-solution, while oversaturated by elemental carbon in respect to diamond, is nucleation and growth of thermodynamically unstable graphite phase jointly with diamond (similarly to other diamond-forming «melt-carbon solution» systems including metallic [1], silicate [2], silicate-carbonate [3], etc.). Hence the elucidation of the physico-chemical stimulation and mechanism of joint formation of the carbon polymorphs in mantle carbonatite melts has to be understood not only for making better control over diamond crystal growth in carbonatite-carbon growth medium but for revealing the reasons for co-existence of diamond and graphite under the Earth’s mantle conditions as well as syngenetic formation of mantle-derived diamond with primarily included graphite therein. Mechanism of graphite formation in carbonatite melts may be extended over the conditions of graphite PT stability as well but for as long as pressure-induced congruent melting of carbonate solvent of carbon is attained and the couple of solid carbon and carbonate-carbon melt-solution works as a strong self-buffering system [4].

Earlier experimental study of multi-component carbonatite-carbon systems [5] revealed that diamond formation in their melts is very efficient under high pressure. The following investigations made it evident that carbonatite melts are effective solvents for graphite and diamond [6], kinetic of diamond growth is sensitive to change of PT parameters [7], and carbon dissolved in carbonatite melt is of elemental atomic and/or cluster form [8]. š

Present experimental investigation of diamond nucleation and growth is carried out with the use of starting carbonatite compositions, wt. %: K₂CO₃ 35.0, Na₂CO₃ 10.0, MgCO₃ 25.0, CaCO₃ 30.0, which is a Fe-free version of carbonatite composition studied earlier [7]: K₂CO₃ 27.21, Na₂CO₃ 2.89, MgCO₃ 17.36, CaCO₃ 26.91, FeCO₃ 25.63. To a large degree both the carbonatite compositions are chemical replicas of the carbonatite end member for multi-component carbonate-silicate compositions of primary carbonatite inclusions in Botswanian diamonds [10]; graphite is used as a starting carbon material. The experiments were carried out at fixed temperature of 1800^oC and variable pressure within the 7.0 – 8.5 GPa range for time duration of 5 – 30 min. Quantity of spontaneously formed diamond crystals in the volume unit of the sample after quenching and solidification of the growth melt was taken as the conventional indicator of nucleation density for diamond phase («survived nucleation centers»). It was found that the nucleation density is distinctly lowered from 1.8ž10³ nucleižcm^-3 to 1.1ž10³ nucleižcm^-3 for 30 min. duration while pressure decreases from 8.5 to 7.25 GPa. At the same time the linear size of diamond crystals ranges from 40 µm to 160 µm; the maximal size is achievable at the lowest pressure. The normal growth rate for octahedral (111) face of diamond crystal at 7.25 GPa is measured as changeable from 10 µm/min after 5 min to 6 µm/min after 10 min and to 2.3 µm/min after 30 min of duration. The effect of lowering of normal rate of diamond growth arises from difference in density of starting graphite and diamond. This is due to time-sensitive local pressure depression in the experimental samples while the less dense starting graphite re-crystallizes into more dense diamond product (effect of «volume loss» in high-pressure experiment). In case if diamond is used as a starting material the time-dependent diamond growth rate is close to the linear one. š

It is symptomatic that re-crystallized graphite crystals came into being at lowest pressures when the density of diamond nucleation is minimal. This is a specific «signal» of some carbon over-saturation heterogeneity under experimental conditions and pointed out that the field of diamond spontaneous nucleation and crystallization is close to termination with further pressure decrease. The PT points of termination of diamond nucleation establish a boundary line for the field of diamond nucleation inside of the PT-region of diamond stability. Effect of diamond nucleation demonstrates that carbon solutions in carbonatite melts are highly over-saturated in respect to diamond phase (by physico-chemical term, a «labile» over-saturated melt-solution). The resulting data of this study as well as relevant experimental data [6] testify that šdegree of labile carbon over-saturation to diamond that is responsible for diamond nucleation has regularly decreased with pressure lowering until the critical limit is attained in a boundary line.

The boundary line is a demarcation one between the higher pressure regions of «labile» and the lower pressure region of «metastable» carbon over-saturation in respect to diamond. Metastable carbon over-saturation in respect to diamond is responsible for seeded growth of diamond only (possibility of diamond nucleation is suppressed here). But, it is also responsible for spontaneous nucleation and growth of single crystalline graphite (the process accompanies the seeded growth of diamond from the growth medium of metastably over-saturated carbonatite melt-carbon solution). The PT region of the metastably over-saturated solutions is limited by the graphite-diamond equilibrium line for which diamond seed growth and unstable graphite crystallization are suppressed because carbonate melts reaches the state of carbon saturation in respect to both carbon phases – diamond and graphite (solubility value for diamond and graphite becomes equal as for thermodynamically stable phases). šBut, graphite can crystallize at lower pressures as thermodynamically stable phase. Crystallization of unstable graphite phase under PT conditions of diamond stability may be estimated as the example of realization of Ostwald’s rule [10]. The facts of nucleation and growth of unstable single-crystalline graphite under mantle conditions are illustrated by syngenetic inclusions of graphite in diamond [11]. On this basis the assumption that graphite associated with diamond is indicator of «metastable» formation of diamond may be estimated as mistaken.

Moreover, thermodynamically unstable graphite may be a primordial mineral of the Earth’s mantle. The thermodynamics-based carbon phase diagram includes equilibrium graphite-diamond boundary [12] but it is not capable to ascertain that both graphite and diamond phases are kinetically capable to exist firmly as thermodynamically unstable phases far off the equilibrium boundary;š these facts have been determined experimentally [13]. This means that the direct transition of graphite to diamond and vice versa is not practically realizable in the one-component carbon system at the most depths of the Earth’s mantle. But the equilibrium graphite-diamond line controls re-crystallization of graphite to diamond and vice versa in the system «carbon solvent – carbon» [14]. For the re-crystallization, it is important that solubility of thermodynamically unstable solid phase exceeds solubility of the stable one.

Hence formation of thermodynamically stable diamond and unstable graphite in carbonatite melt-carbon solution is under control of the next physico-chemical factors: (1) pressure-induced high carbon solubility in congruent carbonatite melts and formation of carbonatite-carbon melt-solution, (2) initiation of over-saturated carbon solution in respect to diamond because of carbon solubility difference of unstable graphite and diamond if graphite is carbon source or šif temperature gradient exists over experimental sample in the case when diamond is a carbon source, (3) formation of labile carbon over-saturationš for diamond spontaneous nucleation, (4) formation of metastable carbon over-saturation for seeded diamond growth and accompanying unstable graphite nucleation (following the Ostwald’s rule), (5) carbon over-saturation degree is progressively dependent on pressure increase (at fixed temperature), (6) carbon over-saturation degree is regressively dependent on temperature increase (at fixed pressure), (7) kinetics of diamond and graphite nucleation and crystallization is progressively dependent on the degree of carbon over-saturation in carbonatite melt.š šš

ššššššššššššššš This study is supported by the INTAS project 05-1000008-7938, RFBR grant 08-05-00110-a and the RF President grant 4122007.5.

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