Experimental study
of formation of Na-rich majoritic garnet
in the context of
diamond deep-mantle genesis
Dymshits A.M.*, Bobrov A.V.*,
Litvin Yu.A.**
* Moscow State University,
Moscow, Russia, **Institute of Experimental Mineralogy, Chernogolovka, Russia
The present work summarizes results
of experimental study of silicate Prp (Mg3Al2Si3O12)
– NaGrt (Na2MgSi5O12) and Prp – Jd (NaAlSi2O6)
as well as silicate-carbonate Prp – Carb (Na2CO3) systems
responsible for formation of sodium bearing garnet solid solution under PT conditions
of diamond stability.
The particular interest to
Na-rich majoritic garnet arises from that it has been usually found as primary inclusion
into mantle-derived diamonds [1-6] and is known as mineral of rare ultra-deep
xenoliths in kimberlites [7]. Of paramount importance is that the only samples
recognized so far as being from the asthenosphere and transition zone are inclusions
of majoritic garnets [8]. šAll this
strongly suggests on possibility of the majoritic garnet formation in all of
the Earth’s mantle environments: upper mantle, transition zone and lower mantle
that is reflected in a model experiment-based PT-phase diagram for the Earth’s
mantle material up to 25 GPa [9].
Our earlier experimental studies
[10] have showed a good correlation between pressure, temperature and garnet
composition, especially for Si and Na concentrations. Finding of Na-rich
majoritic garnets with specific for majoritic garnet features of extreme high
Si (up to 3.5 f.u.) and Na (up to 1.08 wt. %) content [1, 11] suggests for
ultra-high pressure genesis of diamonds which host the majoritic inclusions.
Experimental criterion of
syngenesis of diamonds and their inclusions [12] gives plausible arguments for
physico-chemical nature of the mantle diamond-parent medium. It is also strongly
applicable to the deep-mantle formation of diamond with syngenetic majoritic
garnet inclusions. Eventually, the experimental solution has to lead to construction
of “syngenesis” phase diagram for the majoritic garnet-bearing diamond-parent
medium. šThe experimental syngenesis
criterion was successfully used on examples of diamond-forming melts of peridotite-carbonate-carbon
and eclogite-carbonate-carbon systems at 7.0 GPa [13-15]. Experimental results demonstrate
syngenetic formation of diamond with silicate peridotitic olivine (Ol), othopyroxene
(Opx), clinopyroxene (Cpx), garnet (Grt) and eclogitic Cpx and Grt minerals. Excess
in Si and Na content for eclogitic garnets was identified. The experimental
data are in agreement with the carbonate-silicate (carbonatite) model of
diamond genesis [12] and relevant mineralogical data.
Hence experimental study of diamond-bearing
systems potential for crystallization of sodium rich majoritic garnets is of
essential interest for the problem of ultra-high pressure diamond genesis. New experiments
have been carried out at 7.0 and 8,5 GPa for investigation of melting phase relations
and compositional ranges of formation of Na-bearing majoritic garnets in silicate
pyrope - Na2MgSi5O12 and pyrope – jadeite
joins as well as in silicate-carbonate pyrope - Na2CO3 join
under high pressure and temperature with the use of toroidal anvil-with-hole high-pressure
apparatus. Carbonate inclusions of CaCO3 and CaMg(CO3)2
composition in syngenetic relation with diamond host and silicate
minerals are first reported [16] as the deepest carbonate samples of the
Earth’s mantle. This fact is not contradicts to version of multi-component
K-Na-Mg-Ca-Fe-carbonate-silicate parent melt for diamond and carbonate minerals
formation. The version is also supported by experimental conclusions [9]. If
diamond-parent melt exists in transition zone – lower mantle conditions it has
to include Na-carbonate component that is undoubtedly indicated by syngenetic
formation of majoritic garnet and diamond.
In the pseudo-binary Mg3Al2Si3O12
– Na2MgSi5O12 system Na-bearing garnet is a
liquidus phase up to 60 mol. % NaGrt. At higher content of NaGrt in the system
(>80 mol. %), enstatite (En) and coesite (Cs) are observed as liquidus
phases. Our experiments provided evidence for persistent sodium incorporation
in Grt (0.3-0.6 wt. % Na2O) under temperature and pressure control.
The highest sodium contents were obtained in experiments at P = 8.5 GPa. Near
the liquidus (T = 1840˚C), the equilibrium concentration of Na-component
in Grt is 5 mol. % Na2MgSi5O12. With the
temperature decrease, Na concentration in Grt increases, and the maximal Na2MgSi5O12
content of 12 mol. % (1.52 wt. % Na2O) is gained at solidus condition
of the system (T = 1760˚C). Grossular-containing starting materials also
produces Na-garnet (up to 1 wt% Na2O) accompanied by pyroxene and
Al-rich phases (kyanite, corundum, and spinel).
The Mg3Al2Si3O12
– NaAlSi2O6 system should be also considered as
pseudo-binary, because Na is incorporated in garnet as Na2MgSi5O12
[17] and pyroxene forms jadeite-enstatite (En) solid solution with Eskola (Esk)
Mg0.5AlSi2O6 component. Main phases obtained
in experiments were garnet, pyroxene, kyanite (sometimes corundum) and quenched
melt. Liquidus garnet appeared at a temperature < 1800˚C in a wide
range of starting compositions and had a stable Na2O admixture (up
to 0.8 wt. % at 8.5 GPa and up to 0.6 wt. % at 7GPa) and elevated Si
concentration (up to 3.128 f.u.). At near-eutectic temperatures (~1500˚C)
garnet becomes progressively enriched in Na2MgSi5O12
and majorite Mg4Si4O12. Garnets crystallizing
from near-eutectic starting materials (Prp20Jd80) are the
most sodium-rich. This fact indicates the influence of melt alkalinity on
the formation of Na-bearing majoritic garnets. It is of interest that the model
system is connected with pyroxene. Significant concentration of Esk molecule
(up to 20 mol. %) and jadeite rich pyroxene and its connection with kyanite may
explain formation of natural kyanite eclogite [16] in the frames of magmatic
model. Kyanite crystallizes as accessory mineral of < 5% content in both pyroxene
and garnet stability fields.
In pyrope–Na2CO3
system garnet as a solid solution of pyrope, NaGrt and majorite (Maj) was
formed in the range of 15−100 mol% Prp. The highest sodium concentration
in garnet (0.8 wt% Na2O) was registered at 1200˚C. Starting compositions
with <15 mol% Prp produce carbonate and pyroxene as liquidus phases.
The results obtained
demonstrate that Na is incorporated in garnet as Na2MgSi5O12component
independently on the starting composition of the system. Thus, mechanism of the
formation of Na-bearing majoritic garnets suggested on mineralogical grounds earlier
[18] and experimentally simulated [10] is confirmed. Crystallization of
Na-bearing garnets is mainly controlled by temperature, pressure and
composition of the system. Increase of Na concentration in garnet at constant
PT-parameters may result only from the increase of melt alkalinity. Concentration
of Na in carbonate-silicate system is higher then in only silicate one that is
indicative for the role of carbonate component in garnet crystallization. Alkalinity
also have positive impact on Na-rich majoritic garnet growth that is in good agreement
with existence of carbonatite inclusions enriched in H2O, CO2,
K2O in diamonds under high internal pressure (4-7 GPa) [19, 20]. The
increase of pressure causes the growth of melting temperature and regular
increase of Na content in garnet.
The study is supported by the INTAS
grant 05-1000008-7936 “Diamond and graphite in carbonate magma”, the RFBR grant
08-05-00110 and šthe RF President grant NSh-5367.2008.5.
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