The Mg 3 Al 2 Si 3 O 12-Na 2 MgSi 5 O 12 system at pressures of 7.0 and 8.5 GPa and a temperature of 1300–1800° C: Phase relationships and crystallization of Na-bearing majoritic garnet (original) (raw)
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Contributions to Mineralogy and Petrology, 2008
The pseudo-binary system Mg 3 Al 2 Si 3 O 12 -Na 2 MgSi 5 O 12 modelling the sodium-bearing garnet solid solutions has been studied at 7 and 8.5 GPa and 1,500-1,950°C. The Na-bearing garnet is a liquidus phase of the system up to 60 mol% Na 2 MgSi 5 O 12 (NaGrt). At higher content of NaGrt in the system, enstatite (up to *80 mol%) and then coesite are observed as liquidus phases. Our experiments provided evidence for a stable sodium incorporation in garnet (0.3-0.6 wt% Na 2 O) and its control by temperature and pressure. The highest sodium contents were obtained in experiments at P = 8.5 GPa. Near the liquidus (T = 1,840°C), the equilibrium concentration of Na 2 O in garnet is 0.7-0.8 wt% (*6 mol% Na 2 MgSi 5 O 12 ). With the temperature decrease, Na concentration in Grt increases, and the maximal Na 2 MgSi 5 O 12 content of *12 mol% (1.52 wt% Na 2 O) is gained at the solidus of the system (T = 1,760°C). The data obtained show that most of natural diamonds, with inclusions of Nabearing garnets usually containing \0.4 wt% Na 2 O, could be formed from sodium-rich melts at pressures lower than 7 GPa. Majoritic garnets with higher sodium concentrations ([1 wt% Na 2 O) may crystallize at a pressure range of 7.0-8.5 GPa. However the upper pressure limit for the formation of naturally occurring Na-bearing garnets is restricted by the eclogite/garnetite bulk composition.
Geochemistry International, 2009
Results of experimental study at 7.0-8.5 GPa and 1300-1900@C of the systems pyrope Mg 3 Al 2 Si 3 O 12 ( Prp )-Na 2 MgSi 5 O 12 ( Na Grt modeling solid solutions of Na-bearing garnets, Prp -jadeite NaAlSi 2 O 6 ( Jd ) in a simplified mode demonstrating melting relations of Na-rich eclogite, and Prp -Na 2 CO 3 are presented. Prp -Na 2 MgSi 5 O 12 join is a pseudobinary that results from the decomposition of Na Grt on to coesite and Na-pyroxene. Synthesized garnets are characterized by Na admixture (>0.32 wt % Na 2 O). Maximal Na 2 O concentrations (1.5 wt % Na 2 O) are reached on the solidus of the system at 8.5 GPa. Clear correlation between Na and Si was established in synthesized garnets; this provides evidence for heterovalent isomorphism of the Mg + Al Na + Si type with the appearance of Na 2 MgSi 5 O 12 component as a mechanism of such garnet formation. The Prp -Jd join is also pseudobinary that results from the formation of two series of solid solutions:
Doklady Earth Sciences, 2010
The solubility of alkalis in the structures of high pressure minerals, as well as their abundance under the conditions of the upper mantle and transition zone, attracts considerable interest from experimentalists and mineralogists. Sodium admixture in garnets was originally discovered by Sobolev and Lavrent'ev [1] in the study of a representative collection of this mineral from inclusions in diamonds, xenoliths of diamondif erous eclogites, and high pressure metamorphic com plexes. By now garnets with significant sodium con centrations (>1 wt % Na 2 O) have been found in many diamond provinces of the world including South Africa, Brazil, Guinea, and Yakutia (see, for example, review [2]). Excessive (relatively to 3 f.u.) silicon con centration is a characteristic feature of these garnets that allows us to attribute them to Na bearing major itic garnets.
Geochimica et Cosmochimica Acta, 2008
Phase relations on the diopside (Di)-hedenbergite (Hd)-jadeite (Jd) system modeling mineral associations of natural eclogites were studied for the compositions (mol %) Di 70 Jd 30 , Di 50 Jd 50 , Di 30 Jd 70 , Di 20 Hd 80 , and Di 40 Hd 10 Jd 50 using a toroidal anvil-with-hole (7 GPa) and a Kawai-type 6-8 multianvil apparatus (12-24 GPa). We established that Di, Hd, and Jd form complete series of solid solutions at 7 GPa, and melting temperatures of pure Di (1980°C) and Jd (1870°C) for that pressure were estimated experimentally. The melting temperature for the Di 50 Jd 50 composition at 15.5 GPa is 2270°C. The appearance of garnet is clearly dependent on initial clinopyroxene composition: at 1600°C the first garnet crystals are observed at 13.5 GPa in the jadeite-rich part of the system (Di 30 Jd 70 ), whereas diopside-rich starting material (Di 70 Jd 30 ) produces garnet only above 17 GPa. The proportion of garnet increases rapidly above 18 GPa as pyroxene dissolves in the garnet structure and pyroxene-free garnetites are produced from diopside-rich starting materials. In all experiments, garnet coexists with stishovite (St). At a pressure above 18 GPa, pyroxene is completely replaced by an assemblage of majorite (Maj) + St + CaSiO 3perovskite (Ca-Pv) in Ca-rich systems, whereas Maj is associated with almost pure Jd up to a pressure of 21.5 GPa. Above $22 GPa, Maj, and St are associated with NaAlSiO 4 with calcium ferrite structure (Cf). We established that an Hd component also spreads the range of pyroxene stability up to 20 GPa. In the Di 70 Jd 30 system at 24 GPa an assemblage of Maj + Ca-Pv + MgSiO 3 with ilmenite structure (Mg-Il) was obtained. The experimentally established correlation between Na, Si, and Al contents in Maj and pressure in Grt(Maj)-pyroxene assemblages, may be the basis for a ''majorite" geobarometer. The results of our experiments are applicable to the upper mantle and the transition zone of the Earth (400-670 km), and demonstrate a wide range of transformations from eclogite to perovskite-bearing garnetite. In addition, the mineral associations obtained from the experiments allowed us to simulate parageneses of inclusions in diamonds formed under the conditions of the transition zone and the lower mantle.
Lithos, 2014
Trace and rare-earth elements Garnet/melt partitioning coefficients High-pressure experiment Eclogite New experimental data on trace element partitioning between Na-bearing majoritic garnet and melt at P = 8.5 GPa and T = 1500-1900°C applicable to partial melting of Na-rich eclogite are presented. We have found that Na-bearing garnet is a liquidus phase of the system at 1850-1650°C being accompanied by enstatitic pyroxene at lower temperatures. With decreasing temperature, Na concentration in garnet increases up to N 1 wt.% Na 2 O due to progressive incorporation of a Na majorite component (Na 2 MgSi 5 O 12 ). Most of the studied trace elements are incompatible, except for Er, Tm, Yb (in some runs), Lu, and Sc (in all runs), which are distributed into garnet. The main characteristic of the trace-element partitioning in our experiments is a different behaviour of the LREE (La, Ce, Pr) in comparison with MREE and HREE (Nd, Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, and Lu). In particular, a significant increase of D values for LREE with the increase of Na 2 O concentration in garnet is observed. As predicted from lattice strain, partitioning coefficients for REEs entering the X site of garnet exhibit a near-parabolic dependence on ionic radius. The results of the study are applied to the formation of inclusions of Na-bearing majoritic garnets in diamonds, and equilibrium melts significantly enriched in LREEs.
Phase relations in the system (Mg,Ca)3Al2Si3O12-Na2MgSi5O12 at 7.0 and 8.5 GPa and 1400–1900°C
Geochemistry International, 2014
The CaO-MgO-Al 2 O 3 -SiO 2 -Na 2 O multicomponent system was experimentally studied at 7.0 and 8.5 GPa using an anvil with hole toroidal high pressure apparatus to examine two binary joins: pyropegrossular and grossular-Na majorite. These and literature data were employed to simulate the liquidus sur face of the pyrope-grossular-Na majorite system. The liquidus surface of garnet of predominantly pyrope composition is dominant in the diagram, and the garnet contains much of the Na 2 MgSi 5 O 12 end member. Melting was observed in this region at temperatures above 1900°C, and the solidus of the system occurs at temperatures below 1550°C. The pyrope-grossular system shows a miscibility gap at 50-65 mol % of the pyrope component and two series of garnet solid solutions. The dominant phase at grossular and Na majorite mixing is pyroxene, and garnet crystallizes within a fairly narrow field in the grossular rich region. All garnets synthesized in the systems have elevated Si and Na concentrations and belong to the majorite series, for which a uniform mechanism of isomorphism (Mg, Ca) + Al = Si + Na was proved.
GEOCHEMICAL JOURNAL, 1992
We have determined experimentally the partition coefficients of elements between silicate minerals and coexisting melts at 16 GPa and 2,000°C in chondritic and CaO-rich silicate systems. Majorite garnet +melt±olivine forms in the MgO-rich chondritic system, while in the CaO-rich system, mer winite coexists with melt ± clinopyroxene. The partition coefficients (D-values) of La, Ce, Sm, Yb, Sc, Zr, Hf and other minor or major elements for majorite garnet, merwinite, and clinopyroxene/melt pairs have been determined through EPMA analysis. D-values of a limited number of elements were also determined for the olivine/melt pair with EPMA. The D(HREE and Sc) for majorite/melt are close to unity. These values are much lower than reported values for pyrope/melt pairs at 2-3 GPa and 1,200-1,500°C and for pyrope megacryst/host rock matrix pairs. The pressure-induced coordination change in Al" at higher pressures above 10 GPa is responsible for the enormous difference in the D values as well as the contrasting temperature conditions: The coupled substitution of ("'M2+, IvSi4+) =(v"IIHREE3+ , 'Al'), important for accommodating HREE3+ into garnets, is severely limited because IvA13+ is unlikely to exist in the majorite/melt sysiem. The mechanism to reduce D(REE and Sc) with in creasing pressure may also be operative between the clinopyroxene/melt pair. Systematic comparison of Onuma diagrams for v'I'M2+ and vII.M3+ in almandine, pyrope, and majorite garnet/melt pairs clearly indicates that high enrichments of HREE in almandine and pyrope garnets are related to charge-balanc ed substitution and the presence of more or less polymerizing silicate melts with IvAl3+. These Onuma diagrams also suggest that the partitioning behaviors of Fe' and Co2+ between garnet and melt are anomalous. This may reflect the tendency for Fe 2+ and Co2+ to prefer six coordinated sites in melts to cubic sites in garnets due to the crystal field effect. The D(REE and Sc) value pattern for the mer winite/Ca-rich melt pair has been found to be quite similar to those for CaTiO3 /melt and CaSi03 (perovskite)/ melt pairs. This is interesting in view of the substructure of merwlnite that mixed Ca 2+ and 02 layers comprise a pseudo-hexagonal dense packing analogous to the perovskite structure. Element partitioning between silicate minerals and melts 359
Thermodynamics of (Fe2+, Mn2+, Mg, Ca)3? Al2Si3O12 garnet: a review and analysis
Mineralogy and Petrology, 1999
The thermodynamic properties of garnets in the system (Fe 2+, Mn 2+, Mg, Ca)3A12Si3012 are reviewed. The thermodynamic properties of the three end-member garnets pyrope, almandine and grossular, including their volume, enthalpy of formation, entropy, compressibility and thermal expansion have been well determined. For spessartine enthalpy of formation and heat capacity at low temperatures are needed. Pyrope's unusual behavior in some of its properties is probably related to the presence of the small, light Mg cation, which has a large anisotropic thermal vibration. The thermodynamic mixing properties of the six binaries are also discussed. Good volume of mixing data exist now for all of the binaries, but much work is still required to determine the enthalpies and third-law vibrational entropies of mixing. It is shown that the magnitude of the positive deviations in the volumes of mixing is related to the volume difference between the two end-member components. It is probable that excess entropies, if present, originate at low temperatures below 200 K. Recent 29Si NMR experiments have demonstrated the presence of short-range ordering (SRO) of Ca and Mg in pyrope-grossular solid solutions. Short-range order will have to be considered in new models describing the entropies of mixing. Its possible presence in all garnet solid solutions needs to be examined. The mixing properties of pyrope-grossular garnets, which are the best known for any garnet binary, can, in part, be described by the Quasi-Chemical approximation, which gives insight into the microscopic interactions which determine the macroscopic thermodynamic mixing properties. Microscopic properties are best investigated by spectroscopic and computational approaches. Hard mode IR measurements on binary solid solutions show that the range of local microscopic structural distortion is reflected in the macroscopic volumes of mixing. The nature of * The contents of this contribution was presented at the IMA Meeting in Toronto in August, 1998. It precedes issues of "Mineralogy and Petrology" containing thematic sets of IMA papers 272 C.A. Geiger strain fields and site relaxation needs to be studied in order to obtain a better understanding of the solid-solution process and energetics in garnet. Critical areas for future experimentation are also addressed.