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Granulite-facies quartzofeldpathic gneisses metamorphosed 1000m.y. ago are exposed around Molodezhnaya Station (67°40'S,46°E) in East Antarctica. In addition to quartz, K-feldspar,and plagioclase, the fourteen samples studied in detail consistof the assemblages biotite-orthopyroxene-magnetite, biotite-garnet-orthopyroxene-ilmenite±magnetite, biotite-garnet ± ilmenite ± magnetite,biotite-garnet-sillimanite-ilmenite ± rutile, and biotite-garnet-cordierite-ilmenite-(sillimanite-rutile).Garnets are pyrope-almandine (13 to 34 mol per cent pyrope).Biotite (XFe = 0.33 to 0.57) is rich in TiO2 (4 to 6.3 wt percent) and its Al2O3 content depends on the mineral assemblage.Orthopyroxene (XFe = 0.45 to 0.60) contains 1.5 to 3.0 weightper cent Al2O3. By and large, the minerals are chemically homogeneousand compositional variations are systematic, which indicatecrystallization under equilibrium conditions. On the basis ofthe compositions of coexisting garnet-biotite, garnet-cordierite,garnet-plagioclase (with sillimanite), and garnet-plagioclase-orthopyroxene,temperatures and pressures during the granulite-facies metamorphismare estimated to be 700°C ± 30°C and 5.5 ±1 kb. Water pressure apparently was significantly less thantotal pressure. Alteration during events following the granulite-facies metamorphismhas resulted in chemical zoning in garnet, in which grain edgesare more iron-rich than cores, heterogeneous biotite compositions,and anomalous trends involving MnO. Temperatures based on biotiteand garnet-edge compositions range from 410 to 580°C. Differences in the chemical potential (µ) of water andoxygen in the fluid phase can explain compositional variationsamong the three sillimanite-bearing samples and the relativelyiron-rich compositions of garnet and biotite associated withcordierite. Apparently, the water released by the formationof cordierite remained in the rock, forcing µH2O to increaseas cordierite formed. Buffering of fluid phase composition bythe mineral assemblage suggests that water was not removed fromthe Molodezhnaya rocks by flushing with CO2-rich fluids duringmetamorphism, a hypothesis evoked to explain ‘dry’mineral assemblages in other granulite-facies terrains. 相似文献
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GREW EDWARD S.; YATES MARTIN G.; SHEARER CHARLES K.; HAGERTY JUSTIN J.; SHERATON JOHN W.; SANDIFORD MICHAEL 《Journal of Petrology》2006,47(5):859-882
Anatectic veins containing the Be minerals khmaralite and berylliansapphirine as primary phases (or surinamite derived therefrom)are associated with Mg–Al-rich paragneisses at three localitiesin the ultrahigh-temperature Napier complex, Antarctica, a uniqueBe mineralization in the granulite facies. Likely precursorsof the paragneisses are volcaniclastic deposits that were hydrothermallyaltered by heated seawater prior to metamorphism. Regular distributionof Be among minerals in the paragneisses suggests an approachto equilibrium with Be greatly concentrated in sapphirine (25–3430ppm Be) or cordierite (560–930 ppm Be) relative to plagioclaseAn53–66 (14–43 ppm Be) > cores of coarse-grainedorthopyroxene (0·7–29 ppm Be) > coronitic orthopyroxene(0·4–14 ppm Be) sillimanite (0·1–26ppm Be) plagioclase An18–33 (0·6–15 ppmBe) > biotite (0·06–8 ppm Be) > K-feldspar,quartz, garnet (0·05–0·7 ppm Be). Sapphirine-bearingparagneisses have average Be concentrations, 4·9 ±2·4 ppm (13 samples), about twice that of typical pelites,whereas paragneisses lacking sapphirine and primary cordieritehave only 2·9 ± 2·1 ppm Be (12 samples),implying some loss of Be during metamorphism. The likely sourcerocks for the Be-rich melts were biotitic rocks lacking theBe sinks sapphirine and cordierite. These gneisses were probablyless competent than the sapphirine-bearing gneisses, so themelts were drawn to the latter and collected in spaces openedduring deformation and boudinage of the more competent paragneisses.Fractionation of the melts concentrated Be to the extent thatBe minerals could crystallize. The final result was Be-mineralizedanatectic veins hosted by relatively Be-rich sapphirine-bearingparagneisses. KEY WORDS: Antarctica; beryllium; granulite facies; microprobe; sapphirine 相似文献
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GREW EDWARD S.; PERTSEV NIKOLAY N.; YATES MARTIN G.; CHRISTY ANDREW G.; MARQUEZ NICHOLAS; CHERNOSKY JOSEPH V. 《Journal of Petrology》1994,35(5):1275-1293
Sapphirine occurs with humite-group minerals and forsteritein Precambrian amphibole-facies rocks at Kuhi-lal, SW PamirMountains, Tajikistan, a locality also for talc+kyanite magnesiohornblendewhiteschist. Most of these sapphirine-bearing rocks are graphiticand sulfidic (pyrite and pyrrhotite) and contain enstatite,clinohumite or chondrodite, spinel, rutile, gedrite, and phlogopite.A phlogopite schist has the assemblage with XFe = Fe/(Fe+Mg)increasing as follows: chlorite (0-003)<phlogopite (0.004–0.005)sapphirine (0.004–0.006) enstatite (0-006)forsterite (0-006–0-007)<spinel (0-014). This assemblage includes the incompatiblepair sapphirine+forsterite, but there is no textural evidencefor reaction. In one rock with clinohumite, XFe increases asfollows: clinohumite (0-002) <sapphirine (0-003) <enstatite(0-004–0-006) <spinel (0-010). Ion microprobe and wet-chemicalanalyses give 0-57–0-73 wt.% F in phlogopite and 0-27wt.% F in chlorite in the phlogopite schist; 0-04, 1.5–1.9,and 4.4 wt.% F in forsterite, clinohumite, and chondrodite,respectively; and 0-0-09 wt.% BeO and 0-05–0-21 wt.% B2O3in sapphirine. Stabilization of sapphirine+clinohumite or sapphirine+chondroditeinstead of sapphirine+phlogopite is possible at high F contentsin K-poor rocks, but minor element contents appear to be toolow to stabilize sapphirine as an additional phase with forsterite+enstatite+spinel.Although sapphirine+forsterite is metastable relative to spinel+enstatitein experiments conducted at aH2O=1 in the MgO-Al2O3-SiO2-H2Osystem, it might be stabilized at aH2O0.5, P4 kbar, T650–700C.Textures in the Kuhi-lal whiteschists suggest a polymetamorphicevolution in which the rocks were originally metamorphosed atT650C, P 7 kbar, conditions under which sapphirine+clinohumiteand sapphirine+chondrodite are inferred to have formed, andsubsequently affected by a later event at lower P, similar T,and lower aH2O. The latter conditions were favorable for sapphirine+forsteriteto form in a rock originally containing chlorite+forsterite+spinel+enstatite. 相似文献
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Sillimanite and Ilmenite from High-grade Metamorphic Rocks of Antarctica and Other Areas 总被引：1，自引：0，他引：1
Sillimanite from a variety of high-grade metamorphic rocks containsfrom 0.13 to 1.82 weight per cent Fe2O3 and less than 0.1 weightper cent TiO2. The iron is trivalent and substitutes for Alonly. Ilmenite associated with the sillimanite contains no morethan 0.4 weight per cent Al2O3, SiO2, CaO, and MnO; and MgOdoes not exceed 1.6 weight per cent. It ranges in compositionfrom Ilm99Hem1 to Ilm85Hem15. A least squares fit of precision unit cell data on 10 analyzedsillimanites gives the following cell dimensions for iron-freesillimanite: a = 7.4830 Á, b = 7.6708 Á, c = 5.7694Á and V = 331.15 Á3. The projected increase incell volume with substitution of 10 mole per cent Fe2SiO3 is1.66 per cent. A regular increase in the Fe2O3 content of sillimanite withincreasing Fe2O3 content of associated ilmenite in 15 of 21samples analyzed suggests that sillimanite and ilmenite crystallizedin equilibrium in the 15 samples. The compositions of the tensillimanite-ilmenite pairs analyzed by the author fit the followingempirical curve (sol;(XFe2O3)Il = 1.110 x 10–3. This regularincrease in Fe2O3 contents fits a model of Fe3+ substitutionfor Al on two independent sites in sillimanite and a coupledsubstitution of for Fe2+ Ti on two sites in ilmenite. Sillimaniteand ilmenite are behaving as ideal solutions over the compositionalrange 0 < XFe2SIO3 < 0.013 in sillimanite and 0 < XFe2O3< 0.15 in ilmenite. Equations have been derived for expressing the variation inFe2O3 content of sillimanite associated with quartz and ilmeniteor hematite as a function of pressure, temperature, and Fe2O3content of the oxide minerals. For example, the Fe2O3 contentof a sillimanite with 1.5 mole per cent Fe2SiO3 coexisting withTi-free hematite is calculated to decrease 11 per cent witha 5 kb increase in pressure. The rate of increase with temperatureof the Fe2O3 content of sillimanite is greater in hematite-bearingassemblages than in ilmenite-bearing assemblages. 相似文献
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Chemistry of Kornerupine and Associated Minerals, a Wet Chemical, Ion Microprobe, and X-Ray Study Emphasizing Li, Be, B and F Contents 总被引：5，自引：2，他引：3
GREW E. S.; CHERNOSKY J. V.; WERDING G.; ABRAHAM K.; MARQUEZ N.; HINTHORNE J. R. 《Journal of Petrology》1990,31(5):1025-1070
Kornerupine and associated minerals in 31 samples of high-graderocks relatively rich in Al and Mg were analysed by wet chemistry,ion microprobe mass analyser, electron microprobe and X-raypowder diffraction. For 11 samples of kornerupine and threesamples of biotite (F only) analysed by both wet chemical andion microprobe methods, the best agreement was obtained forB2O3, whereas the ion microprobe Li2O values were systematicallysomewhat higher than the wet chemical values. The wet chemicalmethods give Li2O=0–0?19 wt.%; BeO=0–0?032 wt.%;B2O3=0–4?01 wt.%; and F=0?07–0?77 wt.% in kornerupine,whereas ion microprobe analyses on other kornerupines give valuesup to 0?35 wt.% Li2O, O066 wt.% BeO, and 4?72 wt.% B2O3. Thesum B+Al+Fe3++Cr is close to 6?9 atoms per 22 (O, OH, F) or21?5 (O) in kornerupine. In general, Li/Fe ratios decrease as follows: kornerupine ?sapphirinebiotite> Crd (Na<0?03 per 18 oxygens)>tourmaline, garnet,orthopyroxene. However, for cordierite with Na>004, Li/Fedecreases as follows: cordierite>kornerupine. Sapphirineand sillimanite are the only associated minerals to incorporatesignificant boron (0?1–0?85 wt.% B2O3) and then only whenthe single site for B in kornerupine is approaching capacity.Sillimanite B2O3 contents increase regularly with kornerupineF. Fractionation of fluorine increases as follows: kornerupine<biotite<tourmaline,and Kkrn-BtD=(F/OH)Krn/(F/(OH)Bt (assuming ideal anion composition)increases with biotite Ti. Kornerupine B2O3 content is a measureof B2O3 activity in associated metamorphic fluid, whereas sillimaniteB2O3 content increases with temperature, exceeding 0?4 wt.%whenT=900?C at very low water activities. New data on 11 kornerupines and literature data indicate thatthe unit cell parameters a, c, and V decrease with increasingB content and b, c, and V increase with increasing Fe3+ content.In Fe3+-poor kornerupines, b increases with Mg and with (Mg+ Fe2+) but the effect of Mg on b via the substitution VIMg+IVSi=VIAl+IVAloverwhelms the effect of Fe2+=Mg substitution. 相似文献
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