Dehydration-melting of Biotite Gneiss and Quartz Amphibolite from 3 to 15 kbar

We performed vapor-absent melting and crystallization experimentson two bulk compositions that model metamorphic rocks containinga single hydrous phase: a biotite gneiss [37% bio (mg-number55), 34% qtz, 27% plg (An₃₈), 2% ilm] and a quartz amphibolite[54% hbl (mg-number 60), 24% qtz, 20% plg (An₃₈), 2% ilm]. Experimentswere performed at 3 and 5 kbar in internally heated pressurevessels (IHPV), and at 7, 10, 125 and 15 kbar in piston cylinderapparatus (PC), from the vapor-absent solidi to (at least) thetemperature at which the hydrous mineral disappeared. Dehydration-meltingbegins at similar temperatures in both bulk compositions, rangingfrom T850C at P = 3 kbar T930C at P = 15 kbar. The hydrousmineral disappears 50C above the solidus in both systems, exceptin IHPV experiments at f(O₂) above Ni–NiO, in which biotitestability extends up to atleast 80C above the solidus. At theT at which the hydrous minerals disappear the biotite gneissproduces 2–3 times more melt than the quartz amphibolite(50–60 wt% vs 20–30 wt%). In both systems, variationsin melt productivity with P are controlled by three competingfactors: (1) the positive d P/dT slopes of the solidi, (2) decreasingH₂O activity with increasing P at constant H₂O content, and(3) Na₂O activity, which increases with P concomitantly withbreakdown of plagioclase. Melt productivities at T = 920–950Care maximized at intermediate pressures (7 kbar). The biotitegneiss produces strongly peraluminous granitic melts (SiO₂>70wt%) and residual assemblages of quartz norite (P>125 kbar)or garnet pyroxenite (P>125 kbar). The quartz amphiboliteproduces strongly peraluminous granodioritic melts (SiO₂>70wt%) that coexist with clinopyroxene + orthopyroxene + plagioclase+ quartz at P>10 kbar)garnet. The results of coupled meltingand crystallization experiments on the quartz amphibolite suggestthat strongly peraluminous granitoid rocks (e.g. cordierite-bearingand two-mica granites) can be derived from melting of Al-poorprotoliths. KEY WORDS: dehydration-melting; biotite gneiss; amphibolite; felsic magmas ^*Corresponding author     

Petrology of Biotite-Cordierite-Garnet Gneiss of the McCullough Range, Nevada I. Evidence for Proterozoic Low-Pressure Fluid-Absent Granulite Grade Metamorphism in the Southern Cordillera

Proterozoic migmatitic paragneisses exposed in the McCulloughRange, southern Nevada, consist of cordierite+almanditic garnet+biotite+sillimanite+plagioclase+K-feldspar+quartz+ilmenite+hercynite.This assemblage is indicative of a low-pressure fades seriesat hornblende-granulite grade. Textures record a single metamorphicevent involving crystallization of cordierite at the expenseof biotite and sillimanite. Thermobarometry utilizing cation exchange between garnet, biotite,cordierite, hercynite, and plagioclase yields a preferred temperaturerange of 590–750?C and a pressure range of 3–4 kb.Equilibrium among biotite, sillimanite, quartz, garnet, andK-feldspar records a_H2O between 0?03 and 0?26. The low a_H2Otogetherwith low f_O2 (QFM) and optical properties of cordierite indicatemetamorphism under fluid-absent conditions. Preserved mineralcompositions are not consistent with equilibrium with a meltphase. Earlier limited partial melting was apparently extensiveenough to cause desiccation of the pelitic assemblage. The relatively low pressures attending high-grade metamorphismof the McCullough Range paragneisses allies this terrane withbiotite-cordierite-garnet granulites in other orogenic belts.aosure pressures and temperatures require a transient apparentthermal gradient ofat least 50?C/km during part of this Proterozoicevent in the southern Cordillera. ^*Present address: Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024-1567     

The Solubility of Alumina in Orthopyroxene Coexisting with Garnet in FeO-MgO--Al2O3--SiO2 and CaO--FeO--MgO--Al2O3--SiO2

The pressure-temperature-compositional (P-T-X) dependence ofthe solubility of Al₂O₃ in orthopyroxene coexisting with garnethas been experimentally determined in the P-T range 5–30kilobars and 800–1200 ?C in the system FeO—MgO—Al₂O₃—SiO₂(FMAS). These results have been extended into the CaO—FeO—MgO—Al₂O₃—SiO₂(CFMAS) system in a further set of experiments designed to determinethe effect of the calcium content of garnet on the Al₂O₃ contentsof coexisting orthopyroxene at near-constant Mg/(Mg + Fe). Startingmaterials were mainly glasses of differing Mg/(Mg + Fe) or Ca/(Ca+ Mg + Fe) values, seeded with garnet and orthopyroxene of knowncomposition, but mineral mixes were also used to demonstratereversible equilibrium. Experiments were performed in a piston-cylinderapparatus using a talc/pyrex medium. Measured orthopyroxene and corrected garnet compositions werefitted by multiple and stepwise regression techniques to anequilibrium relation in the FMAS system, yielding best-fit,model-dependent parameters G^o_y= –5436 + 2.45T cal mol^–1,and W^M1_FeA1= –920 cal mol^–1. The volume change ofreaction, V^o, the entropy change, S^o₉₇₀ and the enthalpy changeH^o_1,970, were calculated from the MAS system data of Perkinset al. (1981) and available heat capacity data for the phases.Data from CFMAS experiments were fitted to an expanded equilibriumrelation to give an estimate of the term W^ga_CaMg = 1900 ? 400cal/mole cation, using the other parametric values already obtainedin FMAS. The experimental data allow the development of a arnet-orthopyroxenegeobarometer applicable in FMAS and CFMAS: where This geobarometer is applicable to both pelitic and metabasicgranulites containing garnet orthopyroxene, and to garnet peridoditeand garnet pyroxenite assemblages found as xenoliths in diatremesor in peridotite massifs. It is limited, however, by the necessityof an independent temperature estimate, by errors associatedwith analysis of low Al₂O₃ contents in orthopyroxenes in high-pressureor low-temperature parageneses, and by uncertainties in thecomposition of garnet in equilibrium with orthopyroxene. Ananalysis of errors associated with this formulation of the geobarometersuggests that it is subject to great uncertainty at low pressuresand for Fe-rich compositions. The results of application ofthis geobarometer to natural assemblages are presented in acompanion paper.     

Reaction Textures in a Suite of Spinel Granulites from the Eastern Ghats Belt, India: Evidence for Polymetamorphism, a Partial Petrogenetic Grid in the System KFMASH and the Roles of ZnO and Fe2O3

Spinel granulites, with or without sapphirine, occur as lensesin garnetiferous quartzofeldspathic gneisses (leptynites) nearGokavaram in the Eastern Ghats Belt, India. Spinel granulitesare mineralogically heterogeneous and six mineral associationsoccur in closely spaced domains. These are (I) spinel–quartz–cordierite,(II) spinel–quartz–cordierite–garnet–orthopyroxene–sillimanite,(III) spinel–cordierite–orthopyroxene–sillimanite,(IV) spinel–quartz–sapphirine–sillimanite–garnet,(V) spinel–quartz-sapphirine–garnet and (IV) rhombohedral(Fe–Ti) oxide–cordierite–orthopyroxene–sillimanite.Common to all the associations are a porphyroblastic garnet(containing an internal schistosify defined by biotite, sillimaniteand quartz), perthite and plagioclase. Spinel contains variableamounts of exsolved magnetite and is distinctly Zn rich in thesapphirine-absent associations. X_Mg in the coexisting phasesdecreases in the order cordierite–biotite–sapphirine–orthopyroxene–spinel–garnet–(Fe–Ti)oxides. Textural criteria and compositional characteristicsof the phases document several retrograde mineral reactionswhich occurred subsequent to prograde dehydration melting reactionsinvolving biotite, sillimanite, quartz, plagioclase and spinel.The following retrograde mineral reactions are deduced: (1)spinel + quartz cordierite, (2) spinel + quartz garnet + sillimanite,(3) garnet + quartz cordierite + orthopyroxene, (4) garnet+ quartz + sillimanite cordierite, (5) spinel + cordierite orthopyroxene + sillimanite, (6) spinel + sillimanite + quartz sapphirine, (7) spinel + sapphirine + quartz garnet + sillimanite,and (8) spinel + quartz sapphirine + garnet. A partial petrogeneticgrid for the system FeO–MgO–Al₂O₃–SiO₂–K₂O–H₂Oat high fo₂, has been constructed and the effects of ZnO andFe₂O₃ on this grid have been explored Combining available experimentaland natural occurrence data, the high fo₂ invariant points inthe partial grid have been located in P–T space. Geothermobarometricdata and consideration of the deduced mineral reactions in thepetrogenetic grid show that the spinel granulites evolved throughan anticlockwise P–T trajectory reaching peak metamorphicconditions >9 kbar and 950C, followed by near-isobaric cooling(dT/dP = 150C/kbar). This was superimposed by an event of near-isothermaldecompression (dT/dP = 15C/kbar). The studied spinel granulites,therefore, preserve relic prograde mineral associations andreaction textures despite being metamorphosed at very high temperatures,and bear evidence of polymetamorphism. KEY WORDS: spinel granulite; Eastern Ghats; India; polymetamorphism; geothermometry; geobarometry Corresponding author     

Melting of a Hydrous Mantle: I. Phase Relations of Natural Peridotite at High Pressures and Temperatures with Controlled Activities of Water, Carbon Dioxide, and Hydrogen

Four natural peridotite nodules ranging from chemically depletedto Fe-rich, alkaline and calcic (SiO₂ = 43.7–45.7 wt.per cent, A1₂O₃ = 1.6O–8.21 wt. per cent, CaO = 0.70–8.12wt. per cent, alk = 0.10–0.90 wt. per cent and Mg/(Mg+Fe²⁺)= 0.94–0.85) have been investigated in the hypersolidusregion from 800? to 1250?C with variable activities of H₂O,CO₂, and H₂. The vapor-saturated peridotite solidi are 50–200?Cbelow those previously published. The temperature of the beginningof melting of peridotite decreases markedly with decreasingMg/(Mg+SFe) of the starting material at constant CaO/Al₂O₃.Conversely, lowering CaO/Al₂O₃ reduces the temperature at constantMg/(Mg+Fe) of the starting material. Temperature differencesbetween the solidi up to 200?C are observed. All solidi displaya temperature minimum reflecting the appearance of garnet. Thisminimum shifts to lower pressure with decreasing Mg/(Mg + Fe)of the starting material. The temperature of the beginning ofmelting decreases isobarically as approximately a linear functionof the mol fraction of H₂O in the vapor (X_H2O^v). The data alsoshow that some CO₂ may dissolve in silicate melts formed bypartial melting of peridotite. Amphibole (pargasitic hornblende) is a hypersolidus mineralin all compositions, although its P/T stability field dependson bulk rock chemistry. The upper pressure stability of amphiboleis marked by the appearance of garnet. The vapor-saturated (H₂O) liquidus curve for one peridotiteis between 1250? and 1300?C between 10 and 30 kb. Olivine, spinel,and orthopyroxene are either liquidus phases or co-exist immediatelybelow the temperature of the peridotite liquidus. The data suggest considerable mineralogical heterogeneity inthe oceanic upper mantle because the oceanic geotherm passesthrough the P/T band covering the appearance of garnet in variousperidotites. The variable depth to the low-velocity zone is explained byvariable aHjo conditions in the upper mantle and possibly alsoby variations in the composition of the peridotite itself. Itis suggested that komatiite in Precambrian terrane could formby direct melting of hydrous peridotite. Such melting requiresabout 1250?C compared with 1600?C which is required for drymelting. The genesis of kimberlite can be related to partial meltingof peridotite under conditions of X_H2O^v = 0.5–0.25 (X_CO2^v= 0.5–0.75). Such activities of H₂O result in meltingat depths ranging between 125 and 175 km in the mantle. Thisrange is within the minimum depth generally accepted for theformation of kimberlite.     

Experimental and Thermodynamic Constraints on the Sulphur Yield of Peralkaline and Metaluminous Silicic Flood Eruptions

Many basaltic flood provinces are characterized by the existenceof voluminous amounts of silicic magmas, yet the role of thesilicic component in sulphur emissions associated with trapactivity remains poorly known. We have performed experimentsand theoretical calculations to address this issue. The meltsulphur content and fluid/melt partitioning at saturation witheither sulphide or sulphate or both have been experimentallydetermined in three peralkaline rhyolites, which are a majorcomponent of some flood provinces. Experiments were performedat 150 MPa, 800–900°C, fO₂ in the range NNO –2 to NNO + 3 and under water-rich conditions. The sulphur contentis strongly dependent on the peralkalinity of the melt, in additionto fO₂, and reaches 1000 ppm at NNO + 1 in the most stronglyperalkaline composition at 800°C. At all values of fO₂,peralkaline melts can carry 5–20 times more sulphur thantheir metaluminous equivalents. Mildly peralkaline compositionsshow little variation in fluid/melt sulphur partitioning withchanging fO₂ (D_S 270). In the most peralkaline melt, D_S risessharply at fO₂ > NNO + 1 to values of >500. The partitioncoefficient increases steadily for S_bulk between 1 and 6 wt% but remains about constant for S_bulk between 0·5 and1 wt %. At bulk sulphur contents lower than 4 wt %, a temperatureincrease from 800 to 900°C decreases D_S by 10%. These results,along with (1) thermodynamic calculations on the behaviour ofsulphur during the crystallization of basalt and partial meltingof the crust and (2) recent experimental constraints on sulphursolubility in metaluminous rhyolites, show that basalt fractionationcan produce rhyolitic magmas having much more sulphur than rhyolitesderived from crustal anatexis. In particular, hot and dry metaluminoussilicic magmas produced by melting of dehydrated lower crustare virtually devoid of sulphur. In contrast, peralkaline rhyolitesformed by crystal fractionation of alkali basalt can concentrateup to 90% of the original sulphur content of the parental magmas,especially when the basalt is CO₂-rich. On this basis, we estimatethe amounts of sulphur potentially released to the atmosphereby the silicic component of flood eruptive sequences. The peralkalineEthiopian and Deccan rhyolites could have produced 10¹⁷ and10¹⁸ g of S, respectively, which are comparable amounts to publishedestimates for the basaltic activity of each province. In contrast,despite similar erupted volumes, the metaluminous Paraná–Etendekasilicic eruptives could have injected only 4·6 x 10¹⁵g of S in the atmosphere. Peralkaline flood sequences may thushave greater environmental effects than those of metaluminousaffinity, in agreement with evidence available from mass extinctionsand oceanic anoxic events. KEY WORDS: silicic flood eruptions; sulphur; experiment; Ethiopia; Deccan     

An Experimental Study of the Influence of Oxygen Fugacity on Fe-Ti Oxide Stability, Phase Relations, and Mineral--Melt Equilibria in Ferro-Basaltic Systems

Equilibrium crystallization experiments at atmospheric pressureand over a range of oxygen fugacity (fO₂) have been carriedout on a ferro-basaltic composition similar to liquids proposedto have been parental to much of the exposed portion of theSkaergaard intrusion. Before Fe-Ti oxide saturation the liquidline of descent is little affected by fO₂. However, the appearancetemperatures of the magnetite-ulvspinel solid solution (Mt)and the ilmenite-haematite solid solution (Ilm) depend stronglyon fO₂. Above the fayalite-magnetite-quartz (FMQ) buffer Mtis the first oxide phase to appear on the liquidus, but belowthe FMQ buffer Ilm is the first oxide to crystallize. The appearancetemperature of Mt is 1100C at FMQ and the Mt liquidus slopeis 30C/log fO₂ unit between FMQ–;2 and FMQJ+1. The Ilmliquidus is at 1100C between FMQ and FMQ–2, but movesto lower temperature at higher fO₂ where Mt is the first oxidephase. The results indicate that the ferric iron content ofMt-saturated melts varies linearly with inverse temperature,and that Ilm saturation is closely related to melt TiO₂ content.Mt saturation produces an immediate enrichment of SiO₂ and depletionin FeO^* in the melt phase, whereas Ilm saturation produces similarenrichment in SiO², but inn enrichment may continue for 10Cbelow the ilmenite liquidus. The experimental liquids reacha maximum of 18 wt% FeO^*, at 48 wt% SiO₂ for ilmenite-saturatedmelts at low f_O2, more differentiated melts having lower ironand higher silica. Cotectic proportions, derived from mass balancecalculations, are in good agreement with data from natural samplesand other experimental studies. Olivine resorption is inferredat all fO₂, with the onset of resorption occurring 10C higherthan the appearance of magnetite. The effect of fO₂ on silicatemineral compositions, and partitioning of elements between coexistingmineral-melt pairs, is small. Thermodynamic considerations suggestthat variations of Fe-Mg partitioning between the iron-richolivines, pyroxenes and melts produced in this study may beexplained by known non-idealities of Fe-Mg mixing in the crystallinephases, rather than nonidealities in the coexisting melts. Theseexperiments also provide insights into many features commonto natural tholeiitic series of volcanic and plutonic rocks,and provide experimental data required for modelling of fractionalcrystallization and crystallization closed to oxygen, processeswhich are not easily investigated experimentally. KEY WORDS: ferro-basalt; Fe-Ti oxides; oxygen fugacity; Skaergaard intrusion; iron enrichment ^*Corresponding author. Present address: Bayerisches Geoinstitut, Univerritt Bayreuth, D-95440 Bayreuth, Germany     

Crystallization of Chromite and Chromium Solubility in Basaltic Melts

The equilibrium between chromite and melt has been determinedon four basalts at temperatures of 1200–1400?C over arange of oxygen fugacity (f_o2) and pressures of 1 atm and 10kb. The Cr content of chromite-saturated melts at 1300?C and1 atm ranges from 0?05 wt.% Cr₂O₃ at a log f_o2= –3 to1?4 wt.% at a log f_o2=–12?8. The Cr²⁺/Cr³⁺ of melt increaseswith decreasing f_o2 and is estimated by assuming a constantpartitioning of Cr³⁺ between chromite and melt at constant temperature.The estimated values of Cr²⁺/Cr³⁺ in the melt are at f_o2 valuesof 4–5 orders of magnitude lower than the equivalent Fe²⁺/Fe³⁺values. The Cr/(Cr+Al) of chromite coexisting with melt at constanttemperature changes little with variation of f_o2 below log f_o2=–6.Five experiments at 10 kb indicate that Cr₂O₃ dissolved in themelt is slightly higher and the Cr/(Cr + Al) of coexisting chromiteis slightly lower than experiments at 1 atm pressure. Thus variationin total pressure cannot explain the large variations of Cr/(Cr+ Al) that are common to mid-ocean ridge basalt (MORB) chromite. Experiments on a MORB at 1 atm at f_o2 values close to fayalite-magnetite-quartz(FMQ) buffer showed that the Al₂O₃ content of melt is highlysensitive to the crystallization or melting of plagioclase,and consequently coexisting chromite shows a large change inCr/(Cr + Al). It would appear, therefore, that mixing of a MORBmagma containing plagioclase with a hotter MORB magma undersaturatedin plagioclase may give rise to the large range of Cr/(Cr +Al) observed in some MORB chromite.     

The Formation of Chrysoberyl in Metamorphosed Pegmatites

Mineral assemblages in pegmatite samples from Kolsva, Swedenand Marikov, Czechoslovakia show that chrysoberyl is alwaysaccompanied by quartz, and is a breakdown product of primarypegmatitic beryl. Textures and the mineral-forming process forthe Kolsva pegmatite are explained by the reactions beryl +K-feldspar + H+ = chrysoberyl + quartz + SiO_{2, aq} + K⁺ + H₂Oor alternatively beryl —K—feldspar + H₂O = chrysoberyl+ quartz + melt. Mineral assemblages from mica-rich parts ofthe pegmatite include sillimanite—K—feldspar, muscovite—K—feldspar—sillimanite,and annite—magnetite—spinel—sillimanite—garnet.Details about the composition and the textural relationshipsof these minerals are given; they indicate a post-pegmatiticmetamorphic event at P—T conditions near to the anatecticregime. The samples from Marikov show textures, which are explainedby the reactions beryl + albite + H⁺ = chrysoberyl + quartz+ Na⁺ + H₂O or alternatively beryl + albite + H₂O = chrysoberyl+ quartz + melt. Breakdown of muscovite produces sillimaniteaccording to the reactions beryl + albite + muscovite + H⁺ =chrysoberyl + quartz + sillimanite + Na⁺ + K⁺ + H₂O or alternativelyberyl + albite + muscovite + H₂O = chrysoberyl + quartz + sillimanite+ melt. Similar reaction textures and mineral assemblages were foundin other chrysoberyl-bearing pegmatites (Maroankora, Madagascar;Helsinki, Finland; Haddam, Greenfield, Greenwood, U.S.A.). Hydrothermal experiments located the reaction beryl + alkalifeldspar + H₂O = chrysoberyl + phenakite + melt at P—Tconditions between the K—feldspar—quartz—H₂Osolidus and the K—feldspar—albite—quartz-H₂Osolidus. It is concluded that the formation of Al-rich minerals likechrysoberyl and sillimanite in pegmatites is due to a post-pegmatiticevent at high P—T conditions. The question as to whichof the alternative set of reactions is more likely, the ionicequilibria or the anatectic chrysoberyl formation, must be leftopen. The previous hypothesis of a desilification of a pegmatitewhich intruded into SiO₂-poor country rocks, or of the assimilationof Al₂O₃-rich country rocks, cannot explain the mineral assemblagesof the two pegmatites.     

Experimentally Determined Conditions in the Fish Canyon Tuff, Colorado, Magma Chamber

The Fish Canyon Tuff, Colorado, forms one of the largest (3000km³ known silicic eruptions in Earth history. The tuff is ahomogeneous quartz latite consisting of 40% phenocrysts (plagioclase,sanidine, biotite, hornblende, quartz, magnetite, apatite, sphene,and ilmenite) in equilibrium with a highly evolved rhyoliticmelt now represented by the matrix glass. Melt inclusions trappedin hornblende and quartz phenocrysts are identical to the newlyanalyzed matrix glass composition indicating that hornblendeand quartz crystallized from a highly evolved magma that subsequentlyexperienced little change. This study presents experimentalphase equilibrium data which are used to deduce the conditions(P, T, f_O2, f_H2O, etc.) in the Fish Canyon magma chamber priorto eruption. These new data indicate that sanidine and quartzare not liquidus phases until 780?C temperatures are achieved,consistent with Fe-Ti oxide geothermometry which implies thatthe magmatic temperature prior to eruption was 760?30?C. NaturalFe-Ti oxide pairs also suggest that log f_O2 was -12.4 (intermediatebetween the Ni-NiO and MnO-Mn₃O₄ oxygen buffers) in the magmachamber. This f_O2.102 is supported by the experimentally determinedvariations in hornblende and melt Mg-numbers as functions off_O2 A new geobarometer based on the aluminum content of hornblendesin equilibrium with the magmatic assemblage hornblende, biotite,plagioclase, quartz, sanidine, sphene, ilmenite or magnetite,and melt is calibrated experimentally, and yields pressuresaccurate to ?0.5 kb. Total pressure in the Fish Canyon magmachamber is inferred to have been 2.4 kb (equivalent to a depthof 7.9 km) based on the Al-content of natural Fish Canyon hornblendesand this new calibration. This depth is much shallower thanhas been proposed previously for the Fish Canyon Tuff. Variationsin experimental glass (melt) composition indicate that the magmawas water-undersaturated prior to eruption. X_H2O in the fluidphase that may have coexisted with the Fish Canyon magma isestimated to have been 0.5 by comparing the An-content of naturalplagioclases to experimental plagioclases synthesized at differentX_H2O and P_totals. This ratio corresponds to about 5 wt.% waterin the melt at depth. The matrix glass chemistry is reproducedexperimentally under these conditions: 760?C, 2.4 kb, X_H2O=0.5,and log f_o2=NNO+2 log units. The fugacity of SO₂ (91 b) is calculatedfrom the coexistence of pyrrhotite and magnetite. Maximum CO₂fugacity (2520 b) is inferred assuming the magma was volatilesaturated at 2.4 kb.     

Ferric Iron in CaTiO3 Perovskite as an Oxygen Barometer for Kimberlitic Magmas I: Experimental Calibration

A method to estimate the oxygen fugacity (fO₂) during the crystallizationof kimberlites is developed using the Fe content of CaTiO₃ perovskite(Pv), a common groundmass phase in these rocks. With increasingfO₂, more Fe exists in the kimberlitic liquid as Fe³⁺, and thuspartitions into Pv. Experiments to study the partitioning ofFe between Pv and kimberlite liquid were conducted at 100 kPaon simple and complex anhydrous kimberlite bulk compositionsfrom 1130 to 1300°C over a range of fO₂ from NNO –5 to NNO + 4 (where NNO is the nickel–nickel oxide buffer),and at Nb and rare earth element (REE) contents in the startingmaterials of 0–5 wt % and 1500 ppm, respectively. Thepartitioning of Fe between Pv and kimberlite liquid is influencedmostly by fO₂, although the presence of Nb increases the partitionof Fe³⁺ into perovskite at a given T and fO₂. Multiple linearregression (MLR) of all the experimental data produces a relationshipthat describes the variation of Fe and Nb in Pv with fO₂ relativeto the NNO buffer:

(uncertaintiesat 2, and Nb and Fe as cations per three oxygens). Over therange of conditions of our experiments, this relationship showsno temperature (T) dependence, is not affected by the bulk Fecontent of the kimberlite starting material and reproduces experimentaldata to within 1 log fO₂ unit. KEY WORDS: kimberlites; oxygen fugacity; perovskite; ferric iron; magma     

Melting of a Hydrous Mantle: I. Phase Relations of Natural Peridotite at High Pressures and Temperatures with Controlled Activities of Water, Carbon Dioxide, and Hydrogen

Four natural peridotite nodules ranging from chemically depletedto Fe-rich, alkaline and calcic (SiO₂=43?7–45?7 wt. percent, Al₂O3=1?6O–8?21 wt. per cent, CaO=0?70–8?12wt. per cent,alk=0?10–0?90 wt. per cent and Mg/(Mg+Fe²⁺)=0?94–0?85)have been investigated in the hypersolidus region from 800?to 1250?C with variable activities of H₂O, CO₂, and H₂. Thevapor-saturated peridotite solidi are 50–200?C below thosepreviously published. The temperature of the beginning of meltingof peridotite decreases markedly with decreasing Mg/(Mg+Fe)of the starting material at constant CaO/Al₂O₃. Conversely,lowering CaO/Al₂O₃ reduces the temperature at constant Mg/(Mg+Fe)of the starting material. Temperature differences between thesolidi up to 200?C are observed. All solidi display a temperatureminimum reflecting the appearance of garnet. This minimum shiftsto lower pressure with decreasing Mg/(Mg+Fe) of the startingmaterial. The temperature of the beginning of melting decreasesisobarically as approximately a linear function of the mol fractionof H₂O in the vapor (X_H2O). The data also show that some CO₂may dissolve in silicate melts formed by partial melting ofperidotite. Amphibole (pargasitic hornblende) is a hypersolidus mineralin all compositions, although its P/T stability field dependson bulk rock chemistry. The upper pressure stability of amphiboleis marked by the appearance of garnet. The vapor-saturated (H₂O) liquidus curve for one peridotiteis between 1250? and 1300?C between 10 and 30 kb. Olivine, spinel,and orthopyroxene are either liquidus phases or coexist immediatelybelow the temperature of the peridotite liquidus. The data suggest considerable mineralogical heterogeneity inthe oceanic upper mantle because the oceanic geotherm passesthrough the P/T band covering the appearance of garnet in variousperidotites. The variable depth to the low-velocity zone is explained byvariable a_H2O conditions in the upper mantle and possibly alsoby variations in the composition of the peridotite itself. It is suggested that komatiite in Precambrian terrane couldform by direct melting of hydrous peridotite. Such melting requiresabout 1250?C compared with 1600?C which is required for drymelting. The genesis of kimberlite can be related to partial meltingof peridotite under conditions of (). Such activities of H₂Oresult in melting at depths ranging between 125 and 175 km inthe mantle. This range is within the minimum depth generallyaccepted for the formation of kimberlite.     

Experimental Interaction of Granitic and Basaltic Magmas and Implications for Mafic Enclaves

We have performed time series experiments for periods rangingfrom 3 min to 44 h on the interaction of granite melt and partiallymolten basalt at 920C and 10 kbar, in the presence of 5 wt.%water. With time, the assemblage of the basalt domain changesfrom predominantly amphibole+plagioclase to clinopyroxene+garnet;the melt fraction increases from {small tilde}2•5 to 40%;and between the two domains, the melt compositions progressivelyequilibrate. Initially in each run, melts of the basalt domainhave uniform plateau concentrations for SiO₂, Al₂O₃, CaO, MgO,and FeO because the activities of these components are regulatedby the mineral assemblage, but at advanced stages of reaction,no such control is evident. We have derived analytical expressionsto describe and simulate the diffusion profiles. The concentrationprofiles for SiO₂, Al₂O₃, CaO, and Na₂O in the granite, emanatingfrom the basalt–granite interface, have been used to estimateeffective diffusivities. The values from the shorter runs arecompared with those of the experiment of longest duration forwhich we assumed finite couples in our calculations. In thediffusion calculations for K₂O the difference in melt fractionbetween the two domains is accounted for. The resulting values(in cm²/s) are: D_Na2O=6 10^–7, D_K2O=3 10^–7, D_MgO=9 10^–8, D_CaO=(4–6) 10^–8, and D_SiO2 and D_Al2O3=(3–0•6) 10^–8. They are in reasonable agreement with values fromother studies. On the basis of our experiments we calculatethat mafic enclaves of magmatic origin should equilibrate toa large degree with their host magma in slowly cooling non-convectinggranitic plutons. Enclaves approaching complete re-equilibrationretain distinctly higher modal amounts of mafic minerals. Theydo not compositionally resemble binary magma mixtures, but aremore like host magma with accumulated crystals. We show thatthe modal differences between enclave and host are indicativeof the temperature of homogenization and that, in principle,this temperature can be deduced from equilibrium phase diagrams. ^* Present address: Mineralogisch-Petrologisches Institut, Universitt Gttingen, Goldschmidtstrasse 1, 3400 Gttingen, Germany     

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.     

Thermodynamic data of lawsonite and zoisite in the system CaO–Al2O3–SiO2–H2O based on experimental phase equilibria and calorimetric work

The enthalpy of drop-solution in molten 2PbO·B₂O₃ of synthetic and natural lawsonite, CaAl₂(Si₂O₇)(OH)₂·H₂O, was measured by high-temperature oxide melt calorimetry. The enthalpy of formation determined for the synthetic material is (_fH^Oxides=-168.7Dž.4 kJ mol^-1, or (_fH⁰₂₉₈=-4,872.5dž.0 kJ mol^-1. These values are in reasonable agreement with previously published data, although previous calorimetric work yielded slightly more exothermic data and optimisation methods resulted in slightly less exothermic values. The equilibrium conditions for the dehydration of lawsonite to zoisite, kyanite and quartz/coesite at pressures and temperatures up to 5 GPa and 850 °C were determined by piston cylinder experiments. These results, other recent phase equilibrium data, and new calorimetric and thermophysical data for lawsonite and zoisite, Ca₂Al₃(SiO₄)(Si₂O₇)O(OH), were used to constrain a mathematical programming analysis of the thermodynamic data for these two minerals in the chemical system CaO-Al₂O₃-SiO₂-H₂O (CASH). The following data for lawsonite and zoisite were obtained: (_fH⁰₂₉₈ (lawsonite)=-4,865.68 kJ mol^-1 , S⁰₂₉₈ (lawsonite)=229.27 J K^-1 mol^-1 , (_fH⁰₂₉₈ (zoisite)=-6,888.99 kJ mol^-1 , S⁰₂₉₈ (zoisite)=297.71 J K^-1 mol^-1 . Additionally, a recalculation of the bulk modulus of lawsonite yielded K=120.7 GPa, which is in good agreement with recent experimental work.     

Distribution of Ferric Iron in some Upper-Mantle Assemblages

The distribution of ferric iron among the phases of upper-mantlerocks, as a function of pressure (P), temperature (T) and bulkcomposition, has been studied using ⁵⁷Fe Mssbauer spectroscopyto determine the Fe³⁺/Fe ratios of mineral separates from 35peridotite and pyroxenite samples. The whole-rock Fe³⁺ complementof a peridotite is typically shared approximately evenly amongthe major anhydrous phases (spinel and/or garnet, orthopyroxeneand clinopyroxene), with the important exception of olivine,which contains negligible Fe³⁺. Whole-rock Fe³⁺ contents areindependent of the T and P of equilibration of the rock, butshow a well-defined simple inverse correlation with the degreeof depletion in a basaltic component. Fe³⁺ in spinel and inboth pyroxenes from the spinel Iherzolite facies shows a positivecorrelation with temperature, presumably owing to the decreasein the modal abundance of spinel. In garnet peridotites, theFe³⁺ in garnet increases markedly with increasing T and P, whereasthat in clinopyroxene remains approximately constant. The complexnature of the partitioning of Fe³⁺ between mantle phases resultsin complicated patterns of the activities of the Fe³⁺ -bearingcomponents, and thus in calculated equilibrium f_O2, which showlittle correlation with whole-rock Fe³⁺ or degree of depletion.Whether Fe³⁺ is taken into account or ignored in calculatingmineral formulae for geothermobarometry can have major effectson the resulting calculated T and P. For Fe-Mg exchange geothermometers,large errors must occur when applied to samples more oxidizedor reduced than the experimental calibrations, whose f_O2 conditionsare largely unknown. Two-pyroxene thermometry is more immuneto this problem, and probably provides the most reliable P—Testimates. Accordingly, the convergence of P—T valuesderived for a given garnet peridotite assemblage may not necessarilybe indicative of mineral equilibrium. The prospects for thecalculation of accurate Fe³⁺ contents from electron microprobeanalyses by assuming stoichiometry are good for spinel, uncertainfor garnet, and distinctly poor for pyroxenes. KEY WORDS: mantle; oxidation; partitioning; peridotite; thermobarometry ^*Corresponding author. Present address: School of Earth and Ocean Sciences, University of Victoria, P.O. Box 1700, Victoria, B.C., V8W 2Y2, Canada     

The Stability of Andradite, Hedenbergite, and Related Minerals in the System Ca--Fe--Si--O--H

Phase relations for the bulk compositions 3CaO·2FeO_x·3SiO₂+excessH₂O and CaO·FeO_x·2SiO₂+excess H₂O were determinedusing conventional hydrothermal techniques with solid phaseoxygen buffers to control fO₂. Andradite, Ca₃Fe³⁺₂Si₃O₁₂, synthesized above 550 °C hasan average unit cell edge, a_o, of 12.055±0.001 Å,and an index of refraction, n, of 1.887±0.003. Belowthis temperature, a_o increases whereas n decreases, indicatingthe formation of a member of the andradite-hydroandradite solidsolution. At 2000 bars P_fluid andradite is stable above an f_O2of 10¹⁵ bar at 800 °C and 10-³² bar at 400 °C. At lowerf_O2 andradite+fluid gives way at successively lower temperaturesto the condensed assemblages magnetite+wollastonite, kirschsteinite(CaFe²⁺SiO₄)+ wollastonite and kirschsteinite+xonotlite (Ca₆Si₆O₁₇(OH)₂). Synthetic hedenbergite, CaFe²⁺Si₂O₆, has average unit cell dimensionsof a_o = 9.857± 0.004 Å, b_o = 9.033±0.002Å, c_o = 5.254±0.002 Å and ß = 104.82°±0.03°,and refractive indices of n = 1.731±0.003 and n = 1.755±0.005.At 2000 bars P_fiuid, hedenbergite is stable below an f_O2 of10-¹³ bar at 800 °C and 10-²⁸ bar at 400 °C. Above thesef_O2 values, hedenbergite+O₂ breaks down to andradite+magnetite+quartz. The mineral pair andradite +hedenbergite thus limit the f_O2range possible for their joint formation under equilibrium conditions. The hydration of wollastonite to xonotlite occurs at much lowertemperatures than previous experimental work indicated. A tentativehigh temperature limit for this reaction is set at 185°±15°C and 5000±25 bars and 210°±15 °Cand 2000±20 bars. Inasmuch as the growth of xonotlitefrom wollastonite + H₂O was never accomplished, this high temperaturelimit does not represent an equilibrium univariant curve. Nine phases were encountered in the study of andradite and hedenbergite.They are andradite, hedenbergite, magnetite, wollastonite, kirschsteinite,xonotlite, quartz, ilvaite, and vapor (fluid). An invariantpoint analysis using the method of Schreinemakers shows thetopologic relations of the reactions involved. The resultinggrid can be used to interpret natural occurrences.     

Melting of a Hydrous Mantle: II. Geochemistry of Crystals and Liquids Formed by Anatexis of Mantle Peridotite at High Pressures and High Temperatures as a Function of Controlled Activities of Water, Hydrogen, and Carbon Dioxide

The system peridotite-H₂O–CO₂ serves as a simplified modelfor the phase relations of mantle peridotite involving morethan one volatile component. Run products obtained in a studyof phase relations of four mantle peridotites in the presenceof H₂O- and (H₂O+CO₂)- bearing vapors and with controlled hydrogenfugacity (f_H2) at high pressures and temperatures have beensubjected to a detailed chemical investigation, principallyby the electron microprobe. Mg/(Mg+Fe) of all phases generally increases with increasingtemperature and with increasing Mg/(Mg+Fe) of the starting material.This ratio appears to decrease with increasing pressure forolivine, and for amphibole coexisting with garnet. Decreasingf_H2 from that of IW buffer to that of MH buffer decreases Mg/(Mg+Fe)of the partial melt from approximately 0-85 to approximately0.50, whereas the Fo content of coexisting olivine increasesslightly less than 3 per cent and the Mg/(Mg+Fe) of clinopyroxeneincreases about 4 per cent. However, the variations in Fo contentof olivines are within those observed in olivines from naturalmantle peridotite. The chemistry of other silicate mineralsdoes not significantly reflect variations of f_H2. Consequently,the peridotite mineralogy and/or chemistry is not a good indicatorfor the f_H2 conditions during crystallization. All crystalline phases, except amphibole, and to some extentgarnet, show increasing Cr content with increasing temperatureand increasing Cr content of the starting material, resultingin a positive correlation with Mg/(Mg+Fe). Partial melts aredepleted in Cr₂O₃ relative to the crystalline phases. High Mg/Mg+Fe)and Cr₂O₃ are thus expected in crystal residues after partialmelting. The absolute values depend on degree of melting andthe composition of the parent peridotite. Liquids formed by anatexis of mantle peridotite are andesiticunder conditions of X_H2O^v > 0.6 to at least 25 kb total pressureand to more than 200?C above the peridotite solidus. This observationsupports numerous suggestions that andesite genesis in islandarcs may result from partial melting of underlying peridotitemantle. In contrast to basaltic rocks, the absence of amphibole(paragasitic hornblende) does not affect the silica-saturatednature of the liquids. Increasing K₂O content of the startingmaterial (up to 1 wt. per cent K₂O) results in increasing potassiumcontent of the amphibole (1 wt. per cent K₂O) as well as theappearance of phlogopite. The liquid under these conditionsis relatively K₂0-poor (less than 1 wt. per cent K₂O). Partial melts are olivine normative with X_H2O 0.5, and initialliquids contain normative ol and ne at X_H2O 0.4. The alkalinityof these liquids increases with decreasing X_H2O below valuesof 0.5. The (ol+opx)-normative liquids resemble oceanic basaltswhereas (ol+ne)-normative liquids resemble olivine nepheliniteand melilite basalt. Low aHlo and high a_Co2 conditions may bethose under which kimberlites and related rocks are formed inthe mantle.     

Geochemistry and Intrusive History of the Ashland Pluton, Klamath Mountains, California and Oregon

The Ashland pluton is a calc-alkaline plutonic complex thatintruded the western Paleozoic and Triassic belt of the KlamathMountains in late Middle Jurassic time. The pluton comprisesa series of compositionally distinct magma pulses. The oldestrocks are hornblende gabbro and two-pyroxene quartz gabbro withinitial ⁸⁷Sr/⁸⁶Sr = 0{dot}7044, ¹⁸O = 8{dot}7%, and REE patternswith chondrite normalized La/Lu = 7. These units were followedby a suite of tonalitic rocks (La_N/Lu_N = 7) and then by a suiteof K₂O- and P₂O₅ rocks of quartz monzodioritic affinity (La_N/Lu_N= 13–21; La_N/Sm_N = 2{dot}4–3{dot}) The quartz monzodioriticrocks were then intruded by biotite granodiorite and granitewith lower REE abundances but more fractionated LREE(La_N/Lu_N= 13–19; La_N/Sm_N = 4{dot}3–6 and they, in turn,were host to dikes and bosses of hornblende diorite. The latestintrusive activity consisted of aplitic and granitic dikes.Combined phase equilibria and mineral composition data, indicateemplacement conditions of approximately P_total = 2{dot}3kb,P_H2O between 1{dot}5 and 2{dot}2 kb, and f_O2 between the nickel-nickeloxide and hematite-magnetite buffers. Successive pulses of magma display increasing SiO₂ togetherwith increasing ¹⁸O and decreasing initial ⁸⁷Sr/⁸⁶Sr. The isotopicdata are consistent with either (1) combined fractional crystallizationof andesitic magma and concurrent assimilation of crustal materialcharacterized by low Sr₁ and high (¹⁸O or, more probably, (2)a series of partial melting events in which sources were successivelyless radiogenic but richer in ¹⁸O Each intrusive stage displaysevidence for some degree of crystal accumulation and/or fractionalcrystallization but neither process adequately accounts fortheir compositional differences. Consequently, each stage appearsto represent a distinct partial melting or assimilation event. The P₂O₅-rich nature of the quartz monzodiorite suite suggestsaccumulation of apatite. However, the suite contains abundantmafic microgranitoid enclaves and most apatite in the suiteis acicular. These observations suggest that magma mixing affectedthe compositional variation of the quartz monzodiorite suite.Mass balance calculations are consistent with a simple mixingprocess in which P₂O₅-rich alkalic basalt magma (representedby the mafic microgranitoid enclaves) was combined with a crystal-poorfelsic magma (represented by the tonalite suite), yielding aquartz monzodioritic magma that then underwent differentiationby crystal fractionation and accumulation.     

Metamorphism in the Adirondacks: II. The Role of Fluids

Quantitative estimates of metamorphic fluid speciation, stableisotopic analyses, and studies of fluid inclusions all documentthe local complexity of fluids in the deep crustal rocks exposedin the Adirondack Mountains, NY. Estimates of the activity ofH₂O in the granulite facies are substantially lower than inthe amphibolite facies gneisses. The onset of low water activitiesin semi-pelitic gneisses generally correlates with migmatitictextures in the uppermost amphibolite facies, suggesting thatpartial melts absorbed H₂O at the peak of metamorphism. In granulitefacies marbles and calc-silicates, conditions varied from extremelyundersaturated in H₂O-CO₂ fluid to fluid saturated, and _H2Oand _CO2 show sharp gradients within single outcrops. Low valuesof f_O2 and f_H2O, or of f_CO2, and f_H2O indicate fluid-absentconditions for some orthogneisses and marbles, which are inferredto have been ‘dry’ rocks before and during granulitefacies recrystallization. Wollastonite is preserved from earlycontact metamorphism and serves as an index mineral for fluid-absentconditions in granulites where _H2O is low. Values off_O2 rangefrom near the hematite + magnetite buffer in metamorphosed ironformation to substantially below the quartz + magnetite + fayalitebuffer in some orthogneisses. The anorthosite suite is moreoxidized than some associated granitic gneisses. Halogens (Fand Cl) substitute extensively for OH in micas and amphiboles,extending their stability, although F₂, Cl₂, HCl, and HF areminor components in any fluid. Oxybiotite-type exchanges involvingO for OH are also important, extending the stability of biotite.Stable isotopic ratios of O and C demonstrate that premetamorphicwhole-rock compositions are commonly preserved whereas mineralcompositions generally reflect equilibration at the peak ofmetamorphism. The Marcy Anorthosite Massif was intruded as ahigh ¹⁸O magma. The combination of mineral equilibria, stable isotope data,and fluid inclusions is used to identify and to distinguishamong pre-orogenic contact metamorphic/hydrothermal events,peak metamorphic events, and retrograde/postmetamorphic events.Polymetamorphism is documented at skarn zones adjacent to anorthosite,where large volumes of hydrothermal fluid were channeled duringearly, shallow contact metamorphism and where conditions werefluid poor during subsequent regional metamorphism. Peak metamorphicevents are inferred to have been caused primarily by magmaticprocesses of intrusion and anatexis. Partial melting has causedlow values of _H2O in many rocks, but in other cases low valuesof _H2O are recorded in orthogneisses derived from H₂O-poor magmas.Isotopic studies show that maximum fluid/rock ratios were <0?land possibly 0?0 for infiltrating fluids at the peak of metamorphismin many localities. No evidence of pervasive, regional infiltrationby a fluid at the peak of metamorphism has been substantiatedin the Adirondacks. Fluid inclusions containing high-densityCO₂ or CO₂ + H₂O represent conditions from after the peak ofmetamorphism and document isobaric cooling, in agreement withestimates from garnet zoning. Fine-scale retrograde veins arecommon and are associated with high-density CO₂-rich fluid inclusions.