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Na-majorite (Na2MgSi5O12) as a potential concentrator of Na in the deep upper mantle and transition zone: solid solutions and phase relations at 7-20 GPa

Bobrov A.V.*, Dymshits A.M.*, Litvin Yu.A.**, Litasov K.D.***, Shatskiy A.F.***, Ohtani E.***

* Geological Faculty, Moscow State University, Moscow, Russia; ** Institute of Experimental Mineralogy, Chernogolovka, Russia; *** Tohoku University, Sendai, Japan

archi3@yandex.ru

 

Majoritic garnets were found as inclusions in diamonds from several localities worldwide, including South Africa, Guinea, Canada, Brazil, Russia, and China (see (Stachel, 2001) for review). They belong to both the peridotitic and eclogitic suites. However majoritic garnets with Si atoms pfu >3.07 are almost exclusively eclogitic. These majorites show high Na with Na2O wt% ranging up to ~1.4. Recently Harte and Cayzer (2007) have documented evidence of exsolution textures involving clinopyroxene and majoritic garnet which indicate original majoritic garnets with up to 2.16 wt% Na2O. Most of the Na present in majoritic garnet is accommodated via the pressure dependent exchange reaction Na+ + Si4+ = Mg2+ + Al3+ (Na for Mg in the X site and Si for Al in the Y site). Such mechanism of sodium incorporation in majoritic garnet supports the idea of a presence of the Na2MgSi5O12 end-member in garnet solid solution (Bobrov et al., 2008).

As is evident from the experimental study of the model systems pyrope (Mg3Al2Si3O12)-Na-majorite (Na2MgSi5O12), pyrope-jadeite (NaAlSi2O6), and pyrope-Na2CO3 at P = 7.0 and 8.5 GPa and T = 1100-1900ºC (Bobrov et al., 2009), magmatic crystallization of Na-bearing majoritic garnet and its composition are controlled by several factors, among which are melt composition, pressure, and temperature. Experiments confirmed the compatibility of Na-bearing majoritic garnet with sodium-rich silicate and carbonate-silicate melts. In all systems, an increase of pressure leads to the growth of sodium content in garnet. In addition, in all studied systems, a decrease of temperature relatively to liquidus values resulted in the successive increase of sodium content, so that the highest sodium concentrations were observed at the solidus of the systems.

Pure Na-garnet was not synthesized so far, although T. Gasparik (1989) originally proposed the stability of garnet phase with the composition of Na2MgSi5O12 at pressures above 15 GPa. Pyroxene with the composition of Na(Mg0.5Si0.5)Si2O6 (NaPx) was first obtained in (Angel et al., 1988) at P = 16 GPa and T = 1600ºC; it contained both tetrahedral- and octahedral-coordinated silicon. This experimental study is aimed on synthesis of garnet phase Na2MgSi5O12 and establishing of pyroxene/garnet phase transition boundaries in P−T coordinates.

Experiments were performed in the Tohoku University (Sendai, Japan) at P = 13.0−19.5 GPa and T = 15002100ºC on high-pressure Kawai-type apparatus. Cell assemblage with the sample was compressed by eight cubic anvils truncated with triangular faces with 3.5 and 5.0 mm edge lengths depending on assembling. Octahedral cell assemblage with edge lengths of 9.9 and 11.4 mm was manufactured from ZrO2-based ceramics. Pyrophyllite gaskets with widths of 3 and 4 mm were used as deformed hardening closing compressing volume. Heating of the sample was performed by tubular LaCrO3 heater. The sample was loaded into platinum and rhenium capsules isolated from heater by MgO insulator. Gel of the Na2MgSi5O12 composition dried at 900ºC was used as starting material. Temperature was controlled by W97Re3−W75Re25 thermocouple. Press loading was calibrated according to (Litasov, Ohtani, 2005) and additionally confirmed using phase transitions in standard materials (MgSiO3 Mg2SiO4) placed inside the heater together with the sample.

Depending on P−T parameters the main phases obtained in experiments are Na-pyroxene and Na-garnet. Their concentration in run products is ≥ 90 vol %. The values of d-spacing and intensity of corresponding peaks used for phase diagnostics are in good consistence with the data available for Na-pyroxene and pyropic garnet (RRUFF, http://rruff.info/pyrope/display=default/R080060).

Na-pyroxene forms relatively large idiomorphic crystals of prismatic habit with a size of up to 50 m in fine-granular stishovite mass. Electron microprobe analyses of this mineral (wt %, SiO2 74.64; MgO 10.09; Na2O 15.33; total 100.06) demonstrate its closeness to ideal stoichiometric composition Na0.995Mg0.503Si2.500O6.000. Na-garnet crystals of isometric shape with sizes of up to 50 m are overgrown by smaller (up to 5 m) grains from edges forming fine-granular interstitial aggregate together with stishovite. The largest garnets (up to 100 m) were obtained in the long-time run (ES-238, 1440 min). Their compositions (wt %, SiO2 75.32; MgO 10.11; Na2O 15.08; total 100.51; formula Na1.944Mg1.003Si5.013O12.000) are quite close to ideal stoichiometric composition. In some runs (1810 and 1811) incorporation of a small admixture of majoritic component (Mg4Si4O12) is assumed judging from higher magnesium concentration in some grains (up to 1.273 f. u. Mg) and lower content of Si (up to 4.911 f. u.) relatively to 5.000 per formula unit.

The results of performed experiments allowed us to plot P−T phase diagram demonstrating the fields of Na-pyroxene and Na-garnet stability. The phase boundary is described by the equation P (GPa) = 0.0050(2)∙T + 7.5(4) and has quite steep slope, because the first appearance of garnet is observed at a pressure of 16 GPa and a temperature of 1500ºC; garnet is formed at 1900ºC with increase of pressure up to 17.5 GPa.

Note that the garnet stability was followed up to 19.5 GPa (2100ºC). The stability of this phase at higher P−T is not confirmed yet. However, by the analogy with pyrope and majorite, we may assume its decomposition at pressures above 20−22 GPa with the formation of MgSiO3 perovskite and new sodium-rich phases.

In spite of significant progress in experimental studies, physicochemical behavior of sodic components in minerals under high temperatures and pressures is studied insufficiently. Currently new results on synthesis of sodium-bearing compounds with silicon in octahedral coordination were obtained that is reliable evidence for their ultrahigh-pressure formation. Among them are (see (Yang et al., 2009) for review) Na2Si(Si2O7), Na1.8Ca1.1Si6O14, Na6Si3(Si9O27), and Na8Si(Si6O18), Na2Mg4+xFe2−2x3+Si6+xO20, (K,Na)0.9(Mg,Fe)2(Mg,Fe,Al,Si)6O12, as well as solid solutions with the composition of Na(MgxSixAl1−2x)Si2O6 (0≤ x ≤0.5). Pyroxene NaMg0.5Si2.5O6 (Angel et al., 1988) and garnet first synthesized in this study should hold the certain place among the mentioned phases by far.

Na-bearing majoritic garnet should play a key role in discussion about high-pressure phases, which may be potential sodium concentrators in the lower parts of the upper mantle and transition zone. According to the existing concepts (for example, pyrolite model of Ringwood, 1991), the portion of garnet within the depth range from 410 to 660 km may exceed 50 vol %. Calculations show that at a bulk concentration of ~0.4 wt % Na2O, sodium content in garnet resulting from incorporation of Na2MgSi5O12 end-member in it will not exceed 0.8−0.9 wt % Na2O. If we accept the model of layered structure of the mantle, according to which eclogite (Anderson, 1979) being transformed to granitite at a pressure of >18 GPa (Gasparik, 1989) predominates at the base of the upper mantle and in the transition zone, garnet of these rocks will contain from ~1 to ~5 wt % Na2O depending on the bulk eclogite (granitite) composition. It is established experimentally that Na2MgSi5O12 solubility is quite significant and exceeds 30 mol % in pyrope-grossular garnet. According to the preliminary data obtained during our study of the system pyropeNa2MgSi5O12 at 1120 GPa, the content of Na-component in such garnets exceeds 40 mol %. Although the limits of sodium solubility in garnets and character of solid solutions in the pyropeNa2MgSi5O12 join require additional study, the ability of garnet phase to accumulate significant concentrations of sodium under the conditions of the lower parts of the upper mantle and transition zone is beyond question.

This study was financially supported by RFBR grant 09-05-00027.

 

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