High-pressure
melting relations of diamond-forming carbonatites:
formation of syngenetic peridotitic
and eclogitic minerals (experiments at 7.0 and 8.5 GPa)
Bobrov A.V.*, Spivak A.V.**, Divaev F.K.***, Dymshits A.M.*, Litvin Yu.A.**
*
A substantial body
of experimental results on diamond synthesis in carbonate media has been gained
in recent years. There are primary inclusions in diamond crystals containing
fluid-carbonate material [1], which can be modeled by the system K2O–Na2O–CaO–MgO–FeO–CO2
and was considered as a medium of diamond formation under high pressure [2].
Diamond was successfully synthesized in multicomponent
carbonate-silicate melts at 5–7 GPa and 1200–1500œC
[3]. The discovery of diamond in carbonate-silicate rocks of the Chagatai Complex [4] motivated our interest in the
experimental study of phase relations in these rocks [5]. Our experiments
allowed us to simulate the initial (mantle) silicate assemblage of the
carbonate-silicate rocks of the Chagatai Complex.
This assemblage was similar to that of high-Ca eclogite
and grospidite from kimberlite
pipes, which provided an explanation for diamond discoveries in these rocks. The association of carbonatite and kimberlite that
was described in the Slave Craton (
The discovery of
diamondiferous carbonatites provides additional
support to the theory of diamond genesis in the Earth's mantle, according to
which diamonds were formed in strongly compressed carbonatite
melts [8]. The carbonatite nature of the parental
media in which spontaneous nucleation and growth of diamond occurred under PT-conditions
of its thermodynamic stability obtained convincing experimental support through
the simulation of diamond crystallization in melts chemically analogous to
natural parent media both in simplified model systems and real multicomponent compositions [8]. Under such conditions the
matter of primary fluid-mineral inclusions transforms to the state of carbonatite melts with an essential content of alkali silicate
components.
The aim of this
study was to study experimentally diamond nucleation and simulate the
high-pressure mineral assemblages in peridotite-carbonate
and eclogite-carbonate systems with dissolved carbon.
We simulated crystallization of diamonds in melts with variable compositions of
model peridotite [60 wt% olivine (Ol),
16 wt% orthopyroxene (Opx),
12 wt% clinopyroxene (Cpx),
12 wt% garnet (Grt)] and eclogite
[50 wt% Grt, 50 wt% Cpx].
with carbonate of dolomite composition (CaCO3žMgCO3), K2CO3,
and multi-component K-Na-Ca-Mg-Fe-carbonatites.
Carbonate-silicate melts in all experiments performed at PT-conditions of
diamond stability are completely soluble. Concentration barriers of nucleation
(CBN) [9, 10] were estimated at a pressure of 8.5 GPa
for variable concentrations of silicate and carbonate components in parental
melts: 25, 30, and 30 wt.% of peridotite components
and 35, 45, and 35 wt.% of eclogite components, for
CaCO3žMgCO3, K2CO3, and model carbonatite, respectively. At higher silicate
concentrations in carbonate-silicate melts, diamond grows only on seed being
accompanied by thermodynamically unstable graphite phase.
The appearance of peridotite minerals syngenetic to
diamond in the studied diamond-forming melts was established in special run
series at P = 7.0 GPa and T = 1200–1800œó for the composition of peridotite30–carbonate70
(wt.%). Ol is a liquidus
phase in the system with (CaCO3žMgCO3); at T < 1700œó, an association of Cpx + Ol + carbonate-silicate melt (L) is stable; at 1600œó, Grt is added. The
prevalence of Cpx over other silicates was
established for this system; none of the runs demonstrated the presence of Opx. It is assumed that in CaCO3-rich systems Opx enters into reaction like 2MgSiO3 + CaCO3
→ CaMgSi2O6 + MgCO3 and practically is
not presented as a proper phase. In the system of peridotite–alkali
carbonate (K2CO3) the following assemblages are
established: Opx(Ol) + L
(1800œC); Opx + X phase [11] + L (1500œC); Opx + Ol + carbonate + L
(1300œC), Opx + Ol + Si-wadeite + carbonate (1200œó). Crystallization of melts with model multi-component (K-Na-Ca-Mg-Fe) carbonatite proceeds with the following change of mineral parageneses as the temperature decreases: Ol + L → Ol + Cpx (Grt) + L → Ol + Cpx (Grt)
+ carbonate. In principle, the appearance of Opx is
possible in this system, but only if initial peridotite
is enriched in this component, and alkali carbonate (K2CO3)
essentially prevails over CaCO3 among carbonate phases. In the eclogite-carbonate system, for the composition of [Grt50Cpx50]35Carb65
(wt%), Grt and Cpx were
obtained as liquidus phases depending on the starting
composition. Melt crystallization proceeds by the following scheme with a
temperature decrease: L – Cpx+L (Grt+L)
– Grt+Cpx+L (Grt+Carb+L) – Grt+Cpx+Carb+L – Grt+CPx+Carb. It
should be specially emphasized that the compositions of the minerals
synthesized in experiments were quite similar to the compositions of starting
phases. Some features which are typical for inclusions in diamonds, such as Na2O
admixture in garnet (up to 0.5 wt.%) and K2O admixture in clinopyroxene (up to 1 wt.%) were established in run
products. Incorporation of Na in Grt and K in CPx provides evidence not only for high pressure, but also
demonstrates and important role of alkaline components in silicate-carbonante (carbonatite) melts
during the diamond formation [12, 13].
The results
obtained correspond to the carbonatite
(carbonate-silicate) model of diamond genesis and point on theoretical
possibility of crystallization of peridotitic and eclogitic silicate minerals syngenetic
to diamond in silicate-carbonate melts under PT-conditions of diamond
stability. The definite set of mineral inclusions in diamond is determined by
the chemistry of carbonate-silicate systems including carbonate composition.
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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