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]. All this strongly suggests on possibility of the majoritic garnet formation in all of the Earths mantle environments: upper mantle, transition zone and lower mantle that is reflected in a model experiment-based PT-phase diagram for the Earths 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. The 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 Earths 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 pyropeNa2CO3 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 the RF President grant NSh-5367.2008.5.

 

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