Phase Relations of Basalts in their Melting Ranges at PH2O = 5 kb. Part II. Melt Compositions

This paper describes the melting relations of three basalts,a Picture Gorge tholeiite, the 1921 Kilauea olivine tholeiite,and the 1801 Hualalai alkali basalt, at 5 kb water pressure,680–1045 °C, at the oxygen fugacities of the quartz-fayalite-magnetite(QFM) and hematite-magnetite (HM) buffer. All melts producedwithin the hornblende stability field are strongly quartzo-feldspathic.All are quartz-normative, including those from the alkali basalt,and all but two of the melts are corundum-normative. Melt compositionshows very little dependence on oxygen fugacity within the hornblendestability field, as MgO and FeO contents are very low. Whenhornblende begins to melt extensively (1000°–1045°C), the TiO₂, FeO, and MgO contents of the melt increaseabruptly. In this range, melts formed on the HM buffer havemuch higher Mg/Fe ratios and lower TiO₂ than melts formed onthe QFM buffer. Melt composition is also quite insensitive to changes in basaltcomposition, within the hornblende stability field. The chiefexception is the Na/Ca ratio, which varies directly with Na/Cain the starting basalt. When projected into the Ab-An-Or-Qzquaternary system, all melts produced follow a rather narrowspiral path through the tetrahedron; they descend from the Ancorner, moving toward Qz at constant Ab/Or, moving toward Oronly when plagioclase± quartz begin to precipitate. The melting behavior of hornblende, plagioclase, and augitein these experiments has been examined closely, with the followingresults: successive partial melts may differ from each otherby compositional increments which are very different in compositionfrom the minerals contributing to the melt in the temperatureinterval under consideration. These increments can almost neverbe expressed solely in terms of members of the one or two mineralsolid solutions from which they are actually derived. In a fewcases the increments cannot be expressed in terms of any reasonablecombination of minerals. This pattern contrasts markedly withthat observed in fractional crystallization, in which the differencebetween successive melts must always correspond to present orpossible phenocryst minerals. The contrast implies that magmaseries generated by any kind of melting process, equilibriumor fractional, should be recognizably different from seriesgenerated by fractional crystallization, if minerals like hornblendeor pyroxene are involved.     

Genesis of Peraluminous Granites II. Mineralogy and Chemistry of the Tourem Complex (North Portugal). Sequential Melting vs. Restite Unmixing

The Tourem granitic complex (North Portugal) consists of quartz-and alkali-feldspar-rich felsic granites, biotite- and plagioclase-richheterogeneous granites, and cordierite-biotite granites, containingnumerous enclaves of orthogneisses and metapelitic schists.Mineralogical, chemical, and experimental data suggest thatall the granites and the orthogneiss enclaves are geneticallyrelated. The felsic granites are characterized by normally zoned plagioclase,absence of cordierite, high SiO₂ and K₂O (72–74 wt.% and5?4–6?4 wt.%, respectively), moderate P₂O₅ and REE (0?22–0?24%and 85?0–95?7 ppm), and low Fe₂O₃^* and Zr contents (1?3–1?5%and 80–90 ppm). These features are consistent with thoseof restite-free melts formed by low extents of melting. Meltingexperiments show that these felsic granites are likely to bederived by melting of a source material similar to the orthogneissenclaves under low water activities (0?5), at relatively hightemperature ( 800?C) and <30% melting. The heterogeneous and cordierite-biotite granites display highcordierite contents (up to 30%) in addition to biotite (5–25%),complexly zoned plagioclase, and high Fe₂O₃ (2?72–6?99%),CaO (0?56–1?95%), Zr (101–213 ppm), and Ce (39?8–98?1ppm) contents, suggesting that the melts contained significantproportions of residual biotite, cordierite, plagioclase, andaccessories. Experimental data indicate that the melts weregenerated under water-undersaturated conditions but by higherextents of melting (30–60% melting) with probably a largeramount of available water compared with the felsic granites. The major and trace element chemical trends of the granites,which do not define single arrays on two-element variation diagrams,and experimental data show that the generation of the Touremanatectic complex cannot be explained by the restite unmixingmodel but could have resulted from sequential low extents ofmelting with efficient melt segregation followed by higher extentsof melting with restite retention.     

Solid solubility of Al2O3 in enstatite at high temperatures and 1–5 kb water pressure

Solid solubility of Al₂O₃ in orthorhombic enstatite by the substitution AlAl=MgSi is, in the range studied, mainly a function of temperature and not strongly pressure-dependent. Even at 1 kb up to 9 wt.-% Al₂O₃ can be substituted at 1200° C. The thermal stability of the orthorhombic pyroxene phase is strongly increased by the incorporation of Al.In crustal rocks the alumina content of orthopyroxene might be used as a geothermometer but not, as sometimes suggested, as a barometer.     

Liquidas phase relationships in the system CaAl2Si2O8-NaAlSi3O8-KAlSi3O8-NaAlSiO4-KAlSiO4 atP(H2O)=5 kb

Liquidus phase equilibria have been determined in the system CaAl₂Si₂O₈-NaAlSi₃O₈-KAlSi₃O₈-NaAlSiO₄-KAlSiO₄ (An-Ab-Or-Ne-Ks) at a pressure of water of 5 kb, for low anorthite contents. The main effects of increasing anorthite content on phase relationships in the system Ab-Or-Ne-Ks include the expansion of the plagioclase stability field towards the potassium-rich part of the system, and an accompanying contraction of the alkali feldspar, leucite, nepheline and kalsilite stability fields; and an increase in liquidus temperatures throughout most of the compositional range. Two quaternary invariant points have been identified in the system, one a reaction point between the fields of alkali feldspar, plagioclase, nepheline and kalsilite at approximately An₄, and the other probably a quaternary eutectic between the fields of alkali feldspar, plagioclase, leucite and kalsilite at approximately An₆. A shallow minimum trough in liquidus temperatures occurs on the two-feldspar surface, and this would be expected to control the paths of liquids cooling under equilibrium conditions. Phase relationships in this quaternary system have been applied to the interpretation of the histories of the potassium-rich rocks of the Roman Volcanic Region, Italy. Differentiation of the phonolitic series in this region may have occurred by two-feldspar fractionation.     

Melting Relations in Part of the System CaO-MgO-Al2O3-SiO2-Na2O-H2O under 5kb Pressure

In the system CaO-MgO-Al₂O₃-SiO₂-Na₂O-H₂O under 5 kb pressurethe invariant equilibrium forsterite-orthopyroxene-Ca-rich clinopyroxene-amphibole-plagioclase-liquid-vapourhas been identified at 960?12 ?C. A similar invariant assemblagewith spinel replacing Ca-rich clinopyroxene exists at 950?8?C. The liquid in the former equilibrium contains 16.5 per cent(wt.) normative quartz and 3 per cent Na₂O; the plagioclaseis more calcic than An₈₇; the pyroxenes contain about 3 percent Al₂O₃ and the amphibole is hypersthene-normative. Two anhydrousthermal maxima, the olivine-Ca-rich clinopyroxene-plagioclaseand the orthopyroxene-Ca-rich clinopyroxene-plagioclase dividezones are not encountered in this system, and nepheline-normativeliquids may crystallize amphibole?olivine?Ca-rich clinopyroxeneto produce quartz-normative residual liquids of andesite-typecomposition. A thermal maximum involving amphibole-olivine-Ca-richclinopyroxene-liquid-vapour exists for liquids containing approximately11 per cent normative nepheline and liquids more undersaturatedthan this will crystallize these phases to produce extremelynephelinitic liquids. Phase diagrams are presented which facilitate the predictionof crystallization sequences and liquid evolution paths forany basic or intermediate composition under the conditions employedhere.     

Experimental studies to 10 kb of the bulk composition tremolite50-tschermakite50+excess H2O

Phase relations for the magnesio-hornblende bulk composition, 2 CaO·4 MgO·Al₂O₃·7 SiO₂+ excess H₂O, have been investigated to 10 kb employing hydrothermal and piston-cylinder techniques. The low-temperature limit of amphibole in this system lies at 519° C, 1,000 bars, 541° C, 2,000 bars, and 718° C, 10 kb. The low-T assemblage consists of an+chl+di+tc(+f), and is related to the adjacent high-T equilibrium assemblage, amph+an+chl+f, by the solid-solid reaction (A): 2 di+tc=tr. Small amounts of aluminum, hypothesized to be preferentially dissolved in the cpx (and in the tc) relative to amph, may account for the broad P-T stability range of the di+tc assemblage in the synthetic work relative to systems involving stoichiometric tr, Ca₂Mg₅Si₈O₂₂(OH)₂, such as are common in natural, Al-poor calc-silicate parageneses. Alternatively, the low-temperature assemblage produced in the experiments may be metastable. For the investigated bulk composition, synthetic tremolitic-cummingtonitic amphibole contains relatively modest amounts of ts, Ca₂Mg₃Al₂ ^IVSi₆-Al₂ ^IVO₂₂(OH)₂; at pressures of 1,000–3,000 bars, solid solution extends from near tremolite only to about cu₁₁tr₆₉ts₂₀, analogous to most analyzed natural magnesio-hornblendic specimens. At 10 kb fluid pressure, the solid solution reaches approximately cu₀₆tr₅₃ts₄₁ for the investigated bulk composition, and appears to be virtually independent of temperature. Amphibole and 14 Å chl react within the amphibole stability field, along curve (B), at about 704° C and 2,000 bars, to produce an, en, fo and f (H=40.9 kcal/ mole); at pressures greater than approximately 7kb, due to the incompatibility of an and fo, the higher temperature assemblage consists of amph, an, en, sp and f. Above P _fluid– T curve (B), the amphibole coexists with an+en+fo+f at low pressures; at higher pressures, the amphibole, which is in equilibrium with an+en+sp+f, is relatively more aluminous. The high-T stability limit of aluminous tr+fo lies approximately 20–25° C below the dehydration curve for stoichiometric tremolite on its own bulk composition. Reaction (C), tr+fo=2 di+5 en+f (H = 39.4 kcal/mole), produces an+di+en+f, the highest temperature subsolidus assemblage investigated for the tr₅₀ts₅₀ bulk composition. Hydrous melt is encountered at temperatures at least as low as 900° C at 10 kb, and at that fluid pressure coexists with amphibole over an interval of more than 60° C. Limited solid solution observed between tr and ts in nature (tr_100-70) is accounted for by the restricted range of amphibole compositions produced in the present study. Such amphiboles, moreover, appear to have both high- and low-temperature stability limits, as demonstrated by the experimental results.Institute of Geophysics and Planetary Physics Publication No. 2811     

Behavior and effects of phosphorus in the system Na2O−K2O−Al2O3−SiO2−P2O5−H2O at 200 MPa(H2O)

The addition of phosphorus to H₂O-saturated and initially subaluminous haplogranitic (Qz–Ab–Or) compositions at 200 MPa(H₂O) promotes expansion of the liquidus field of quartz, a marked decrease of the solidus temperature, increased solubility limits of H₂O in melt at low phosphorus concentrations, and fractionation of melt out of the haplogranite plane (projected along an Or₂₈ isopleth) toward a peralkaline, silica-poor but quartz-saturated minimum composition. The partition coefficient for P₂O₅ between aqueous vapor and melt with an ASI (aluminum saturation index, mol Al/[mol Na+K])=1 is negligible (0.06), and consequently so are the effects of phosphorus on other melt-vapor relations involving major components. Phosphorus becomes more soluble in vapor, however, as the concentration of a NaPO₃ component increases via the fractionation of melt by crystallization of quartz and feldspar. The experimental results here corroborate existing concepts regarding the interaction of phosphorus with alkali aluminosilicate melt: phosphorus has an affinity for alkalis and Al, but not Si. Phosphorus is incorporated into alkali feldspars by the exchange component AlPSi_-2. For subaluminous compositions (ASI=1), the distribution coefficient of phosphorus between alkali feldspar and melt, D[P]Af/m, is 0.3. This value increases to D[P]Af/m=1.0 at a melt ASI value of 1.3. The increase in D[P]Af/m with ASI is expected from the fact that excess Al promotes the AlPSi_-2 exchange. With this experimental data, the P₂O₅ content of feldspars and whole rocks can reveal important facets of crystallization and phosphorus geochemistry in subaluminous to peraluminous granitic systems.     

Phase Relations of Basalts in their Melting Range at PH2O = 5 kb as a Function of Oxygen Fugacity: Part I. Mafic Phases

The phase relations of three basalts, the Picture Gorge tholeiite,the 1921 Kilauea olivine tholeiite, and the 1801 Hualalai alkalibasalt, were studied at 5 kb water pressure, 680–1000°C,at the oxygen fugacities of the quartz-fayalite-magnetite (QFM)and hematite-magnetite (HM) buffers. In the range 680–850 °C, the crystalline assemblageon the QFM buffer is dominantly hornblende+ plagioclase, ±ilmenite, magnetite, sphene, fayalitic olivine, and phlogopiticmica. From 875 to 1000 °C the crystalline assemblage ishornblende+ olivine± augite+ ilmenite± magnetite.A melt phase is present from 700 to 1000 °C; a vapor phasewas present in all charges. The hornblendes formed on the QFM buffer range in compositionfrom common green hornblendes at low temperatures to kaersutitichornblendes at 1000 °C. A1(IV) and Ti increase temperature.AI(VI) passes through a maximum near 825 °C, decreasingboth above and below this temperature. AI(IV) is proportionalto the sum A1(VI)+2Ti. There is a positive linear correlationof approximately 3 : 1 between AI(IV) and the number of cationsin the A-site. The most likely explanation for this correlationat present is that the substitution of AI(VI) or Ti⁺⁴for a divalentcation creates local charge imbalances in the amphibole structurewhich can be compensated only by further A-site substitution.There also appears to be a correlation between the a-cell dimensionof hornblende and the A-site occupancy. Above a thresh holdvalue of approxmately 0.5 cations in A, a increases as A-siteoccupancy increases. Phase relations on the hematite-magnetite buffer are considerablysimpler. The hornblendes show relatively little change in compositionas temperature increases, and in the tholelitic compositionsbreak down at or below 970 °C 35–60 °C above thefirst appearance of augite±olivine. The melting of hornblendeis incongruent in all cases. The Fe-Ti oxides are pseudo-brookiteand titanohematite; at 1000 °C these oxides make up 10 percent by weight of the assemblage and contain most of the Tio₂and FeO in the charge. The patterns of hornblende variation observed in this studycompare closely with those reported in a wide range of experimentaland field data. The appearance of high-TiO₂ kaersutitic hornblendesin the tholeities at 1000° C, P_H2O= 5 kb on the QFM bufferimplies that the restricted occurence of kaersutite in nature(where it is associated only with mafic to intermediate alkalicrocks) is controlled by volatile content (H₂O_,F₂)rather thanby differences in condensed bulk composition.     

Subsolidus and Partial Melting Reactions in the Quartz-excess CaO+MgO+Al2O3+SiO2+H2O System under Water-excess and Water-deficient Conditions to 10 kb: Some Implications for the Origin of Peraluminous Melts from Mafic Rocks

Experimental results up to 10 kb pressure are presented on thestability of amphibole in the quartz-excess CaO+MgO+Al₂O₃ (CMASH)system under H₂O)-excess and H₂O deficient conditions. Amphiboleis stable above the solidus under H₂O-excess conditions whereasunder H₂O-deficient conditions dehydration melting of amphibole-bearingassemblages defines the solidus. The successive appearance ofamphibole, talc, and zoisite with increasing pressure considerablymodifies the plagioclase-pyroxene-garnet-kyanite reactions documentedexperimentally in the CaO+MgO+Al₂O₃+SiO₂ system for gabbro-granulite-eclogitetransitions. Although both clino pyroxene and cordierite (withanorthite+orthopyroxene+quartz) may melt eutectically at oneatmosphere to form diopside-normative and corundum-normativemelts respectively, at higher pressures under H₂O-excess conditionsthe peritectic melting of mafic rock compositions produces corundum-normativeliquids together with either clinopyroxene or amphibole. Dehydrationmelting produces melts which are not corundum-normative. Thesedata are used to discuss the origins and evolution of contrastingbasalt-andesite-dacite-rhyolite volcanic suites and graniticplutons, many of whose silicic variants are corundum-normativein character, such as the Toba luff ignimbrites, Indonesia (Beddoc-Stephenset al., 1983) and I-type granite minimum melts (White &Chappell, 1977). In contrast, it is proposed that for the Cascadesbasalt-andesite-dacite-rhyolite suite the ortho pyroxene-plagioclase-quartzthermal divide was maintained up to rhyolite compositions, therebyprohibiting the derivation of corundum-normative rocks fromdiopside-normative parent magmas. The deduced reaction relations between pyroxenes, amphibole,plagioclase, quartz, and liquid are used to explain the absenceor extreme scarcity of hydrous phases in some hydrous magmas.These phase relations can also explain the development of laterplagioclase overgrowths on resorbed plagioclase cores in graniticintrusives, and the general absence of resorption and overgrowthsin chemically equivalent extrusive rocks. A theoretical analysis of the partial melting of forsterite-bearingassemblages in the CaO+MgO+Al₂O₃+SiO₂+H₂O system shows thatunder H₂O-excess conditions partial melting may generate corundum-normative(but low SiO₂) melts from a peridotite source at shallow depths.     

Water-Saturated and -Undersaturated Melting of Metaluminous and Peraluminous Crustal Compositions at 10 kb: Evidence for the Origin of Silicic Magmas in the Taupo Volcanic Zone, New Zealand, and Other Occurrences

The melting relations of two proposed crustal source compositionsfor rhyolitic magmas of the Taupo Volcanic Zone (TVZ), New Zealand,have been studied in a piston-cylinder apparatus at 10 kb totalpressure and a range of water activities generated by H₂O-CO₂vapour. Starting materials were glasses of intermediate composition(65 wt.% Si0₂ representing a metaluminous ‘I-type’dacite and a peraluminous ‘S-type’ greywacke. Crystallizationexperiments were carried out over the temperature range 675to 975?C, with aH₂O values of approximately 1?0, 0?75, 0?5,and 0?25. Talc-pyrex furnace assemblies imposed oxygen fugacitiesclose to quartz-fayalite-magnetite buffer conditions. Assemblages in both compositions remain saturated with quartzand plagioclase through 675–700?C at high aH₂O, 725–750?Cat aH₂O0?5, and 800–875?C at aH₂O0?25, corresponding to<60–70% melting. Concentrations of refractory mineralcomponents (Fe, Mg, Mn, P, Ti) in liquids increase throughoutthis melting interval with increasing temperature and decreasingaH₂O. Biotite and hornblende are the only mafic phases presentnear the solidus in the dacite, compared with biotite, garnet,gedritic orthoamphibole, and tschermakitic clinoamphibole inthe greywacke. Near-solidus melting reactions are of the type:biotite + quartz + plagioclase = amphibole ? garnet, potentiallyreleasing H₂O for dehydration melting in the greywacke, butproducing larger amounts of hornblende and releasing littleH₂O in the dacite. At aH₂O0?25 and temperatures 825–850?C,amphibole dehydration produces anhydrous mineral phases typicalof granulite fades assemblages (clinopyroxene, orthopyroxene,plagioclase?quartz in the dacite; garnet, orthopyroxene, plagioclase?quartzin the greywacke) coexisting with melt proportions as low as40%. Hornblendce-saturated liquids in the dacite are weaklyperaluminous (0?3–1?6 wt.% normative C—within therange of peraluminous TVZ rhyolites), whereas, at aH₂O0?25 andtemperatures 925?C, metaluminous partial melt compositions (upto 1?8 wt.% normative Di) coexist with plagioclase, orthopyroxene,and clinopyroxene. At all water activities, partial melts ofthe greywacke are uniformly more peraluminous (1?5–2?6wt.% normative C), reflecting their saturation in the componentsof more aluminous mafic minerals, particularly garnet and Al-richorthopyroxene. A metaluminous source for the predominantly Di-normativeTVZ rhyolites is therefore indicated. With decreasing aH₂O the stability fields of plagioclase andquartz expand, whereas that of biotite contracts. These changesare reflected in the proportions of normative salic componentsin partial melts of both the dacite and greywacke. At high aH₂O,partial melts are rich in An and Ab and poor in Or (trondhjemitic-tonalitic);with decreasing aH₂O they become notably poorer in An and richerin Or (granodioritic-granitic). These systematic variationsin salic components observed in experimental metaluminous tostrongly peraluminous melts demonstrate that a wide varietyof granitoid magmas may be produced from similar source rocksdepending upon P-T-aH₂O conditions attending partial melting.Some peraluminous granitoids, notably trondhjemitic leucosomesin migmatites, and sodic granodiorites and granites emplacedat deep crustal levels, have bulk compositions similar to nearsolidus melt compositions in both the dacite and greywacke,indicating possible derivation by anatexis without the involvementof a significant restite component.     

Melting relations of basalt-andesile-rhyolite-H2O and a pelagic red clay at 30 kb

An olivine basalt, a tonalite (andesite), a granite (rhyolite), and a red clay (pelagic sediment) were reacted, with known quantities of water in sealed noble metal capsules, in a piston-cylinder apparatus at 30 kb pressure. For the pelagic sediment, with H₂O+=7.8% and no additional water, the liquidus temperature is 1240°C, the primary phases are garnet and kyanite. The subsolidus phase assemblage is phengite mica+garnet+clinopyroxene+coesite+kyanite. With 5 wt.% water added, the liquidus temperatures and primary phases for the calc-alkaline rocks are 1280°-1180°-1080°, garnet+clinopyroxene, garnet, and quartz respectively. Garnet and clinopyroxene occur throughout the melting interval of the olivine tholeiite for all water contents. Garnet is joined by clinopyroxene 80° below the andesite plus 5% H₂O liquidus, quartz is joined by clinopyroxene 180° below the rhyolite plus 5% H₂O liquidus. The subsolidus phase assemblage is garnet+clinopyroxene+coesite+minor kyanite for all the calc-alkaline compositions. We conclude that calc-alkaline andesites and rhyolites are not equilibrium partial melting pruducts of subducted oceanic crust consisting of olivine tholeiite basalt and siliceous sediments. Partial melting in subduction zones produces broadly acid and intermediate liquids, but these liquids lie off the calc-alkaline basalt-andesite-rhyolite join and must undergo modification at lower pressures to produce calcalkaline magmas erupted in overlying island arcs.     

Crystal chemistry of selenates with mineral-like structures. VI. Hydrogen bonds in the crystal structure of [(H5O2)(H3O)(H2O)][(UO2)(SeO4)2]

The crystal structure of a new compound, [(H₅O₂)(H₃O)(H₂O)][(UO₂)(SeO₄)₂] (monoclinic, P2₁/n a = 8.3105(15), b = 11.0799(14), c = 13.227(2) Å, β = 103.880(13)°, V = 1182.4(3) ų), has been solved by direct methods and refined to R ₁ = 0.036. The structure is based on [(UO₂)(SeO₄)₂]^2? sheet complexes formed by corner-shared UO₇ pentagonal bipyramids and SeO₄ tetrahedrons. The sheets are parallel to the ( $ \bar 1 The crystal structure of a new compound, [(H₅O₂)(H₃O)(H₂O)][(UO₂)(SeO₄)₂] (monoclinic, P2₁/n a = 8.3105(15), b = 11.0799(14), c = 13.227(2) ?, β = 103.880(13)°, V = 1182.4(3) ?³), has been solved by direct methods and refined to R ₁ = 0.036. The structure is based on [(UO₂)(SeO₄)₂]²⁻ sheet complexes formed by corner-shared UO₇ pentagonal bipyramids and SeO₄ tetrahedrons. The sheets are parallel to the (01) plane. Oxonium ions and water molecules forming [(H₃O)·(H₂O)·(H₅O₂)]²⁺ complexes are interlayer. Among minerals, the existence of (H₅O₂)⁺ has been unambiguously confirmed only in rhomboclase, (H₅O₂)⁺[Fe₂(SO₄)₂(H₂O)₂]. Original Russian Text ? S.V. Krivovichev, 2008, published in Zapiski Rossiiskogo Mineralogicheskogo Obshchestva, 2008, No. 2, pp. 123–130.     

Equilibrium Compositions of Coexisting Garnet and Orthopyroxene: Experimental Determinations in the System FeO-MgO-Al2O3-SiO2, and Applications

We have determined the Fe-Mg fractionation between coexistinggarnet and orthopyroxene at 20–45 kb, 975–1400?C,and the effect of iron on alumina solubility in orthopyroxeneat 25 kb, 1200?C, and 20 kb, 975?C in the FMAS system. The equilibriumcompositions were constrained by experiments with crystallinestarting mixtures of garnet and orthopyroxene of known initialcompositions in graphite capsules. All iron was assumed to beFe²⁺. A mixture of PbO with about 55 mol per cent PbF₂ provedvery effective as a flux. The experimental results do not suggest any significant dependenceof K_D on Fe/Mg ratio at T 1000?C. The lnK_D vs. l/T data havebeen treated in terms of both linear and non-linear thermodynamicfunctional forms, and combined with the garnet mixing modelof Ganguly & Saxena (1984) to develop geothermometric expressionsrelating temperature to K_D and Ca and Mn concentrations in garnet. The effect of Fe is similar to that of Ca and Cr³⁺ in reducingthe alumina solubility in orthopyroxene in equilibrium withgarnet relative to that in the MAS system. Thus, the directapplication of the alumina solubility data in the MAS systemto natural assemblages could lead to significant overestimationof pressure, probably by about 5 kb for the relatively commongarnetlherzolites with about 25 mol per cent Ca+Fe²⁺ in garnetand about 1 wt. per cent Al₂O₃ in orthopyroxene.     

Experimental Study of the Composition Join NaAlSiO4-CaCO3-H2O and the Genesis of Alkalic Rock--Carbonatite Complexes

Phase equilibrium studies have been carried out on the compositionjoin NaAlSiO₄-CaCO₃-H₂O with 25 wt per cent H₂O at 1 kb pressurein the temperature range 600–960 °C. Liquid, in equilibriumwith crystalline phases and a sodic, aqueous vapor phase persistsacross the join down to temperatures of about 600 °C. Fractionalcrystallization of a carbonated nepheline-rich liquid in equilibriumwith vapor is capable of generating successively the crystallineassemblages (1) nepheline, (2) melilite+nepheline, (3) hydroxyhaüyne+melilite,(4) cancrinite+melilite, and (5) calcite+cancrinite+melilite.Late-stage liquid fractions are rich in CaCO₃, whereas the vaporphase is enriched in Na. The experimental assemblages are strikinglysimilar to rocks in alkalic rock-carbonatite complexes in generaland in the Oka, Quebec, complex in particular. The successionof assemblages at Oka and at other melilite rock-bearing complexesmay be interpreted as the products of fractionation of a carbonatednephelinite magma by analogy with the experimental results.The sodium-bearing vapor phase of the experiments may be analogousto the fenitizing agent of some carbonatite complexes.     

The Distribution of H2O between Cordierite and Granitic Melt: H2O Incorporation in Cordierite and its Application to High-grade Metamorphism and Crustal Anatexis

Experiments defining the distribution of H₂O [D_w = wt % H₂O(melt)/wt% H₂O(crd)]) between granitic melt and coexisting cordieriteover a range of melt H₂O contents from saturated (i.e. coexistingcordierite + melt + vapour) to highly undersaturated (cordierite+ melt) have been conducted at 3–7 kbar and 800–1000°C.H₂O contents in cordierites and granitic melts were determinedusing secondary ion mass spectrometry (SIMS). For H₂O vapour-saturatedconditions D_w ranges from 4·3 to 7 and increases withrising temperature. When the system is volatile undersaturatedD_w decreases to minimum values of 2·6–5·0at moderate to low cordierite H₂O contents (0·6–1·1wt %). At very low aH₂O, cordierite contains less than 0·2–0·3wt % H₂O and D_w increases sharply. The D_w results are consistentwith melt H₂O solubility models in which aH₂O is proportionalto X_w² (where X_w is the mole fraction of H₂O in eight-oxygenunit melt) at X_w     

The influence of Al2O3 on the H2O content in periclase and ferropericlase at 25 GPa

In this paper I present results of IR spectroscopic measurements of water solubility in Al-bearing periclase and ferropericlase (Mg# = 88) synthesized at 25 GPa and 1400–2000 °C. The IR spectra of their crystals show narrow absorption peaks at 3299, 3308, and 3474 cm^?1. The calculated H₂O contents are 11–25 ppm in periclase (Al₂O₃ = 0.9–1.2 wt.%) and 14–79 ppm in ferropericlase (Al₂O₃ = 0.9–2.9 wt.%). Ferropericlase contains more H₂O and Al₂O₃ than periclase at 1800–2000 °C. I suggest that addition of Al₂O₃ does not influence the solubility of water in ferropericlase but can favor the additional incorporation of Fe₂O₃ into the structure. The incorporation of Fe³⁺ into ferropericlase increases water solubility as a result of iron reduction to Fe²⁺. It is shown that water has limited solubility in ferropericlase from mantle peridotite; therefore, ferropericlase cannot be considered an important hydrogen-bearing mineral in the lower mantle.     

Experimental investigation of zoisite–clinozoisite phase equilibria in the system CaO–Fe2O3–Al2O3–SiO2–H2O

The system Ca₂Al₃Si₃O₁₁(O/OH)-Ca₂Al₂FeSi₃O₁₁(O/OH), with emphasis on the Al-rich portion, was investigated by synthesis experiments at 0.5 and 2.0 GPa, 500-800 °C, using the technique of producing overgrowths on natural seed crystals. Electron microprobe analyses of overgrowths up to >100 µm wide have located the phase transition from clinozoisite to zoisite as a function of P-T-X_ps and a miscibility gap in the clinozoisite solid solution. The experiments confirm a narrow, steep zoisite-clinozoisite two-phase loop in T-X_ps section. Maximum and minimum iron contents in coexisting zoisite and clinozoisite are given by X_ps^zo (max) = 1.9*10 ^{- 4} T+ 3.1*10 ^{- 2} P - 5.36*10 ^{- 2}{\rm X}_{{\rm ps}}^{{\rm zo}} {\rm (max) = 1}{\rm .9*10}^{ - 4} T{\rm + 3}{\rm .1*10}^{ - 2} P - {\rm 5}{\rm .36*10}^{ - 2} and X_ps^czo (min) = (4.6 * 10 ^{- 4} - 4 * 10 ^{- 5} P)T + 3.82 * 10 ^{- 2} P - 8.76 * 10 ^{- 2}{\rm X}_{{\rm ps}}^{{\rm czo}} {\rm (min)} = {\rm (4}{\rm .6} * {\rm 10}^{ - {\rm 4}} - 4 * {\rm 10}^{ - {\rm 5}} P{\rm )}T + {\rm 3}{\rm .82} * {\rm 10}^{ - {\rm 2}} P - {\rm 8}{\rm .76} * {\rm 10}^{ - {\rm 2}} (P in GPa, T in °C). The iron-free end member reaction clinozoisite = zoisite has equilibrium temperatures of 185ᇆ °C at 0.5 GPa and 0ᇆ °C at 2.0 GPa, with (H_r⁰=2.8ǃ.3 kJ/mol and (S_r⁰=4.5ǃ.4 J/mol2K. At 0.5 GPa, two clinozoisite modifications exist, which have compositions of clinozoisite I ~0.15 to 0.25 X_ps and clinozoisite II >0.55 X_ps. The upper thermal stability of clinozoisite I at 0.5 GPa lies slightly above 600 °C, whereas Fe-rich clinozoisite II is stable at 650 °C. The schematic phase relations between epidote minerals, grossular-andradite solid solutions and other phases in the system CaO-Al₂O₃-Fe₂O₃-SiO₂-H₂O are shown.     

Stability of almandine in the system FeO-(Fe2O3)-Al2O3-SiO2-(H2O) at elevated pressures

Almandine, although decomposing in the presence of metallic iron into the anhydrous subsolidus assemblage fayalite + ferrocordierite + hercynite solid solution at low pressures, melts incongruently to hercynite_ss + quartz + liquid at 10 kb. At pressures between about 12 and 20 kb the products of incongruent melting are hercynite_ss + liquid only, and at still higher pressures almandine melts congruently. For the intermediate pressures between 2 and 10 kb not investigated a sequence of probable breakdown and melting relations involving the phases ferrocordierite, fayalite, hercynite_ss, quartz, and liquid is derived through Schreinemakers' analyses.The lower temperature stability limit of almandine in the presence of water at low oxygen fugacities and pressures of 15 to 20 kb lies between 550° and 600° C as at low pressures. It is marked, however, by the breakdown to a hydrous assemblage involving chloritoid and the new phase aluminous deerite. Since the anhydrous melting at these pressures occurs between 1300° and 1400° C, the thermal stability range of almandine increases drastically with pressure. Its upper breakdown limit shows in principle a similar behavior as those of other garnet end members.     

Experimental Phase Relations in the System Tonalite-Peridotite-H2O at 15 kb; Implications for Assimilation and Differentiation Processes near the Crust-Mantle Boundary

Experiments at 15 kb in the tonalite-peridotite-H₂O system provideinformation on some of the phase equilibrium factors that mayinfluence reaction and assimilation processes between quartznormativemagmas and ultramafic rocks in the deep crust and upper mantle.Experiments were done with 5 or 10 wt.% H₂O added to powderednatural samples of tonalite, and mixtures of tonalite with 5or 10 wt.% peridotite added (TP5 and TP10, respectively). Theliquidus phase relations of these starting compositions wereinvestigated between 850 and 1100?C at 15 kb, using gold capsulesso that iron loss to the sample containers was not a problemand meaningful glass and mineral analyses could be obtained.Experiments on the tonalite alone show either liquidus garnet,for samples with 5% H₂O added, or liquidus hornblende, for sampleswith 10% H₂O. In contrast, orthopyroxene is the sole liquidusphase, irrespective of water content, in experiments using startingmixtures of 5 or 10 wt.% peridotite added to tonalite. Glassanalyses of partially crystallized tonalite define a crystallizationpath diverging significantly from the calc-alkaline trend towardshigher Ca/(Mg + Fe) in the CaO–(MgO + FeO)–?SiO₂triangle. In contrast, glasses from partially crystallized mixturesof tonalite with 5 or 10 wt.% peridotite added define a liquidtrend close to natural calc-alkaline compositions in terms ofCa/(Mg + Fe). Of more general significance, the proximity ofa field ofliquidus orthopyroxene on the high (Mg + Fe) sideof compositions along the calc-alkaline trend serves to limitthe Mgenrichment of such melts by interaction with ultramaficrocks. Unless heat is added to the system, reaction of tonaliticcomposition melts with ultramafic rocks will produce only slightlyMg-enriched melts: increasing degree of reaction simply resultsin further precipitation of orthopyroxene + garnet ? clinopyroxeneonce melt compositions reach the orthopyroxene field boundary.     

Crystal chemistry of selenates with mineral-like structures: V. Crystal structures of (H3O)2[(UO2)(SeO4)2(H2O)](H2O)2 and (H3O)2[(UO2)(SeO4)2(H2O)](H2O), new compounds with rhomboclase and goldichite topology

The crystal structures of two new compounds (H₃O)₂[(UO₂)(SeO₄)₂(H₂O)](H₂O)₂ (1, orthorhombic, Pnma, a = 14.0328(18), b = 11.6412(13), c = 8.2146(13) Å, V = 134.9(3) ų) and (H₃O)₂[(UO₂)(SeO₄)₂(H₂O)](H₂O) (2, monoclinic, P2₁/c, a = 7.8670(12), b = 7.5357(7), c = 21.386(3) Å, β = 101.484(12)°, V = 1242.5(3) ų) have been solved by direct methods and refined to R ₁ = 0.076 and 0.080, respectively. The structures of both compounds contain sheet complexes [(UO₂)(SeO₄)₂]^2? formed by cornershared [(UO₂)O₄(H₂O)] bipyramids and SeO₄ tetrahedrons. The sheets are parallel to the (100) plane in structure 1 and to (?102) in structure 2. The [(UO₂)(SeO₄)₂(H₂O)]^2? layers are linked by hydrogen bonds via interlayer groups H₂O and H₃O⁺. The sheet topologies in structures 1 and 2 are different and correspond to the topologies of octahedral and tetrahedral complexes in rhomboclase (H₂O₂)⁺[Fe(SO₄)₂(H₂O)₂] and goldichite K[Fe(SO₄)₂(H₂O)₂](H₂O)₂, respectively.