Stability of Yoderite in the Absence and in the Presence of Quartz: an Experimental Study in the System MgO-Al2O3-Fe2O3-SiO2-H2O

The water-pressure temperature stability field of yoderite,ideally Mg₂Al_5.6Fe^{3 +} _0.4Si₄O₁₈(OH)₂, was determined at highoxygen fugacities by high-pressure bracketing runs on eightpossible breakdown reactions involving the phases chlorite,kyanite, talc, staurolite, pyrope, enstatite, boron-free kornerupine,cordierite, quartz, and invariably an excess of hematite. Yoderitewas found to be stable over the surprisingly large PT rangefrom 6 to 25 kbar water pressure and 590 to 795 C. It is thusa high-pressure mineral covering the upper amphibolite and portionsof the eclogite facies. In the presence of quartz its upperpressure stability is reduced to some 15 kbar, and its uppertemperature stability to 715 C. Two of the yoderite-producingreactions are anomalous as they show dehydration in the directiontowards lower temperatures. Importantly, this is also true forthe reaction kyanite + talc + hematite+H₂O=yoderite+quartz whichis responsible for the only yoderite occurrence in nature atMautia Hill, Tanzania. Preliminary thermodynamic calculationsindicate that—owing to this unusual dehydration behavior—thestability field for the assemblage yoderite+quartz disappearsfor water activities lower than 0.5. The rarity of yoderitein natural rocks, which is in contrast to its large PT stabilityfield, must be explained on chemical rather than on physicalgrounds. Yoderite can only occur in whiteschist-type bulk compositionsrich in MgO, Al₂O₃, SiO₂, and containing some iron, but poorin alkalis and CaO. Oxygen fugacities must be unusually highto keep Fe trivalent, and—at least for rocks with excessquartz—the water activity must be high as well. In anenvironment of this kind, yoderite formation in the Mautia Hillwhiteschist may have occurred even at constant total pressureand temperature simply by an influx of hydrous fluid duringthe late stages of metamorphism under amphibolite facies conditions.     

Metastability of the 10- Phase in the System MgO-SiO2-H2O (MSH). What about Hydrous MSH Phases in Subduction Zones?

The so-called 10- phase of the MgO-SiO₂-H₂O (MSH) system wassynthesized with 100% yields from a 3: 4 MgO/SiO₂ gel at fluidpressure of 50 kb and 500 C, but only during runs of up to1 h and with total H₂O contents near 50 wt. % in the sealedcapsules. The water contents determined in the run productsindicate a chemical composition Mg₃Si₄O₁₀(OH)₂0.65 H₂O, whichis different from the composition reported by Bauer & Sclar(1981), which had 1 H2O molecule per talc formula. The 10-phase has a = 5.293(3) , b = 9.194(3) , c = 10.044(3) , ß=96.10(8)its calculated density is p_calc.= 2.672 g/cm³; the refractiveindices measured are n_x = 1.554. and n_x = 1.574. The compatibilityof mean refractive index, calculated density, and chemical compositionfalls within the category ‘excellent’ of Mandarino(1979). Runs of longer durations and with total H₂O again near 50 wt.%, seeded with the less hydrous phase talc with the same Mg/Siratio, show that the 10- phase breaks down in the range 30–70kb, 200–700 C to form more talc. Thus it is not the stablehydrous high-pressure equivalent of talc as reported by Yamamoto& Akimoto (1977), but a metastable phase, which cannot playany role as a natural mineral of the mantle or of subductionzones. If the tie-line forsterite-H₂O is stable in the MSH system,as it seems to be for the conditions of our experiments, mostof the hydrous, silica-poor, new high-pressure MSH phases maybe without significance for the Earth, except perhaps phaseA with as yet an ambiguous composition.     

Corundum-Fuchsite Rocks in Greenstone Belts of Southern Africa: Petrology, Geochemistry, and Possible Origin

Unusual corundum-fuchsite rocks with Al₂O₃ contents up to 89per cent and Cr₂O₃ values up to 2.8 per cent were investigatedfrom two localities in Zimbabwe and Transvaal. They form lenseswithin volcano-sedimentary series of different metamorphic gradesand are closely associated with metamorphic ultramafics. In Zimbabwe the corundum contains up to 3.8 wt. per cent Cr₂O₃,and fuchsite up to 3.7 per cent Cr₂O₃. Coexisting minerals areandalusite (2 per cent Cr₂O₃), chlorite (3.2 per cent Cr₂O₃),complex margarite solid solutions, tourmaline (4.9 per centCr₂O₃), dispore (1.1 per cent Cr₂O₃), and rutile (1.9 per centCr₂O₃); gersdorffite (NiAsS), wehrlite (BiTe), and native bismuthare occasional opaque accessories. The minerals from the Transvaallocality are generally poorer in Cr₂O₃. Important parageneticdifferences are the lack of diaspore, tourmaline and margarite,the occurrence of kyanite (0.9 per cent Cr₂O₃) instead of andalusite,exsolution bodies of complex CrFeAl-oxides in rutile, and theappearance of biotite and plagioclase. Both biotite and fuchsitemay be rich in Ba. Critical mineral assemblages indicate that the Zimbabwe rockswere metamorphosed at temperatures not greatly exceeding 400°C and at pressures below 3.5 kb, those from Transvaal near600 °C at or above 5 kb. The textures suggest that the extremeAl-enrichment did not occur during metamorphism but essentiallyprior to it or at least in its early stages. Major and minor element analyses of the rocks from both localitiesshow that they are strongly enriched in the elements Al, Cr,B, V, and As, and locally also in K, Rb, Ni, Sb, Bi, and Te,whereas they are depleted in Si, Mg, Fe, Mn, Na, Ca, S, Cu,Zn, Ga, Sr, and Y. During their formation a strong Al/Ga fractionationmust have taken place leading to exceptionally low Ga/Al ratios. Three modes of primary origin are discussed. (1) Formation ofa low-iron bauxite in a reducing Archaean atmosphere is consideredunlikely, ly on geochemical grounds (very high B-contents; aberrantCr/Ni ratios; low Ga and Y), partly because similar rocks arefound in a non-Archaean formation of New Zealand. (2) Metasomatismin connection with early metamorphic serpentini-zation of theultramafic country rock does not seem impossible but would haveto be utterly different from the commonly observed rodingitizationand other metasomatic zones surrounding serpentinites. (3) Amodel is proposed for premetamorphic postvolcanic exhalativealteration of ultramafic komatiitic lavas, during which theelements B, K, Rb, As, Sb, Bi, Te were deposited from the solutions,while Al, Cr, Ni, and V were concentrated as immobile remaindersof the original rock, and Mg, Si, Fe, and Ca were largely dissolvedand transported away. The mineralogy of these alteration productsmay have been governed by aluminous sulphate minerals like alunite,KAl₃[SO₄]₂ (OH)₆, which, during subsequent regional metamorphism,broke down to form, with the remaining silica, fuchsite andAl₂SiO₅, and without silica, diaspore and corundum, while sulphatewas carried away by the metamorphic solutions.     

Low-Temperature, Low-Pressure Metamorphism of Mn-rich Rocks in the Lienne Syncline, Venn--Stavelot Massif (Belgian Ardennes), and the Role of Carpholite

Low-grade Mn-rich metamorphic rocks of the Lienne syncline (westernpart of the Venn–Stavelot Massif, Belgian Ardennes) havebeen re-examined to evaluate the petrological significance ofcarpholite proper, Mn^2$ Al₂[Si₂O₆](OH)₄. Metamorphic P–Tconditions of these rocks are estimated to be {small tilde}300C1–2 kbar, which is in accordance with the exclusive occurrenceof carpholite in low-P rocks such as hydrothermal environmentselsewhere. Carpholite of the Lienne syncline exclusively occursin quartz-rich segregations. Its composition is close to end-member.Thermodynamic calculations confirm that carpholite is a stablephase at low-pressure–low-temperature conditions, in contrastto ferro- and magnesiocarpholite, which are high-pressure minerals.No information is available on the high-P behaviour of carpholite.The occurrence of carpholite is partly closely associated withspessartine-bearing country rocks, or carpholite is alteredto assemblages with spessartine, sudoite, chlorite, muscoviteand paragonite. Spessartine in these rocks contains minor amountsof hydrogarnet component {(H/4)/[Si$(H/4)] = 0.03–0.06}.The presence of carpholite-spessartine assemblages in theselow-P rocks is in contrast to high-pressure metamorphic rocksfrom other areas, where parageneses such as fem/magnesiocarpholite–chloritoidor magnesiocarpholite–chlorite–kyanite occur. Theappearance of carpholite–garnet assemblages in low-P Mn-richrocks can be explained by contrasting phase relations becauseof a high Mn–Mg partition coefficient between the mineralsunder consideration. In rhodo-chrosite-bearing veins in theLienne syncline, nearly complete replacement of carpholite byspessartine and chlorite is due to the continuous reaction carpholite$ rhodochrosite $ quartz = spessartine $ chlorite $ H₂O $ CO₂,which defines a very low X_co, in the temperature range underconsideration. It is suggested that spessartine (possibly containingsome hydrogarnet component), during prograde metamorphism atlow pressure, becomes stable at a temperature of {small tilde}300C KEY WORDS: carpholite; spessartine; sudoite; Venn–Stavelot Massif; Ardemes ^*Corresponding author. Fax: x49/531/3918131. e-mail: t.theye{at}tu.bs.de     

Compositions and Structural States of Anhydrous Mg-Cordierites: A Re-investigation of the Central Part of the System MgO-Al2O3-SiO2

The central portion of the system MgO–Al₂O₃–SiO₂has been studied with the aim of determining the range of solidsolution, as well as the stability limits of the various structuralstates of the ternary compound cordierite. The previously suggestedlimited solid solution between cordierite of the composition2MgO? 2Al₂O₃? 5SiO₂ (2: 2: 5) and SiO₂ is now believed to existonly metastably. Between 800? and 1,300? C the composition ofcordierite was found to be invariably 2MgO. 2Al₂O₃ 5SiO2. Above1,300?C, however, there is evidence for the existence of limitedsolid solution in cordierite (2: 2: 5) toward a theoreticalcompound ‘Mg-beryl’ (3: 1: 6). The existence ofcordierite solid solution at liquidus temperatures has an importantbearing on the melting relations of many compositions withinthe system. Because of this solid solution the courses of crystallizationof melts consisting of normative cordierite (2: 2: 5) and smallamounts of MgSiO3, for example, have to follow parts of theboundary curve between the cordierite and spinel fields withthese two phases coprecipitating over a limited range of temperatures.The dividing line between compositions which complete theircrystallization at the ternary eutectic forsterite+protoenstatite+cordierite+liquid,1,364? ?3? C, and those which complete their crystallizationat the ternary eutectic protoenstatite +cordierite+tridymite+liquid,1, 355??3? C was formerly considered to be the join MgSiO3-cordierite(2: 2: 5). Because of solid solution in cordierite coexistingwith liquid this dividing line is displaced slightly in thedirection toward more siliceous bulk compositions. Furthermore,the temperature maximum along the boundary curve cordierite+protoenstatite+liquid cannot lie at the intersection of this boundary curvewith the join MgSiO3–2: 2: 5, but with the tie line MgSiO3-cordieritess.The position of this temperature maximum thus moves closer tothe ternary eutectic protoenstatite+cordierite+tridymite+liquid.Temperatures and compositions of some of the invariant pointsin the system have been redeter-mined.     

A Talc--Phengite Assemblage in Piemontite Schist from Brezovica, Serbia, Yugoslavia

Talc occurs in direct contact with phengite in a manganiferousschist containing piemontite, spessartine, quartz, chlorite,hematite, braunite, and occasional phlogopite. The resultingtalc-phengite tie line in the AKF plot is a novelty for bothnatural rocks and the synthetic model system K₂O-MgO-Al₂O₃-SiO₂-H₂Owhich contains about 95 per cent of the components making upthe four phyllosilicates present in the schist. The remainingcomponents are CuO, MnO, and minor Fe₂O₃, CoO, NiO in the chlorite,talc, and phlogopite, as well as Fe₂O₃ in the phengite. Two possibilities for the origin of the talc-phengite assemblageare discussed: 1. The presence of the components CuO etc. in the phyllosilicatesprovides additional degrees of freedom or causes shifts of reactioncurves in the synthetic model system thus creating a hypotheticalnew invariant point at intermediate pressures allowing a talc-phengitefield. 2. The rock was formed under very high water pressures whichpermit the coexistence of talc and phengite even in the puresynthetic system according to a theoretical prediction of phaserelations from limited experimental data available.     

Phase Relations in Pelitic and Psammitic Gneisses of the Sillimanite--Potash Feldspar and Cordierite-Potash Feldspar Zones in the Moldanubicum of the Lam--Bodenmais Area, Bavaria

Bulk chemical and mineral analyses were carried out on a progressiveseries of low-pressure metamorphic pelites and psammites ofthe Bavarian Forest. The variation of rock compositions in thelower grade (=sillimanite-K-feldspar) zone with coexisting biotite+sillimanite(+K-feldspar+quartz) is essentially the same as that in thehigher grade (=cordierite-K-feldspar) zone with coexisting biotite+cordierite±sillimanite(+K-feldspar+quartz), so that nearly isochemical conditionscan be assumed for the metamorphic processes. The two metamorphiczones are related to each other through the multivariant reaction: biotite+sillimanite+quartz = cordierite+K-feldspar+H²O, but analyses of coexisting biotites and cordierites indicatethat metamorphism continues to increase even within the cordierite-K-feldsparzone. This increase is signalized through a continuous shiftof the 3-phase AFM field cordierite-biotite-sillimanite fromMg-rich to more Fe-rich compositions according to the abovereaction. At the highest grade detected the coexistence of biotite+sillimanitein the presence of quartz+K-feldspar is discontinued in favourof cordierite+garnet. Comparison with other metamorphic areas exhibiting the sameAFM assemblages leads to the tenative conclusion that the shiftdetected here is mainly due to increasing temperatures of metamorphism,whereas increasing pressures would shift the 3-phase AFM fieldin the opposite direction, that is towards more Mg-rich compositions.Thus the position of the biotite-sillimanite-cordierite fieldwithin the AFM plot can be used as indicator of metamorphicconditions in seemingly similar cordierite-sillimanite-biotite-quartz-K-feldspargneisses of variable provenance. Assuming water pressure toequal total pressure the conditions that lead to the cordierite-potashfeldspar zone studied here are estimated as 2-3 kb, 650-700°C.