Oxidation During Magmatic Differentiation, Finnmarka Complex, Oslo Area, Norway: Part 1, The Opaque Oxides

Microprobe analyses of the oxide mineral assemblages from monzonitic,granodioritic, and granitic units of the Finnmarka igneous complex,Oslo area, Norway, disclose a remarkable trend of Mn enrichmentin ilmenite with differentiation. Ilmenite-pyrophanite solidsolutions containing up to 63 mole per cent MnTiO₃ have developedunder apparently unusually oxidizing conditions, a conclusiondeduced in part from the fact that Fe²⁺ is more readily oxidizedthan Mn²⁺ and was thus preferentially extracted. Several differentmineral reactions may have been involved in this process. Geothermometricapplications of oxide Mn contents must be made with caution. Additional mineralogic complexities, e.g. ilmenite lamellaein magnetite, magnetite lamellae in ilmenite, and, in the granite,the breakdown of ilmenite to rutile+hematite, all provide insightinto the conditions of emplacement and cooling history of thecomplex.     

Melting and Thermal Reconstitution of Pelitic Xenoliths, Wehr Volcano, East Eifel, West Germany

Pelitic xenoliths derived from amphibolite grade basement rocksoccur within a Pleistocene, trachytic, pyroclastic unit of theWehr volcano, East Eifel, West Germany: With increasing temperatureand/or prolonged heating at high temperature, quartz-plagioclaseand micaceous layers of the xenoliths have undergone meltingto form buchites and thermal reconstitution by dehydration reactions,melting and crystallization to form restites respectively. Thexenoliths provide detailed evidence of melting, high temperaturedecomposition of minerals, nucleation and growth of new phasesand P-T-f_o2 conditions of contact metamorphism of basement rocksby the Wehr magma. Melting begins at quartz-oligoclase (An_17·3Ab_82·3Or_0·4-An_20·0Ab_78·1Or_1·9)grain boundaries in quartz-plagioclase rich layers and the amountof melting is controlled by H₂O and alkalis released duringdehydroxylation/oxidation of associated micas. Initially, glasscompositions are heterogeneous, but with increasing degreesof melting they become more homogeneous and are similar to S-typegranitic minimum melts with SiO₂ between 71 and 77 wt. per cent;A/(CNK) ratios of 1·2–1·4; Na₂O < 2·95and normative corundum contents of 1·9–4·0per cent. Near micas plagioclase melts by preferential dissolutionof the NaAlSi₃O₈ component accompanied by a simultaneous increasein CaAl²Si²O⁸ (up to 20 mol. per cent An higher than the bulkplagioclase composition) at the melting edge. With increasingtemperature the end product of fractional melting is the formationand persistence of refractory bytownite (An_78–80) in thosexenoliths where extensive melting has taken place. Initial stage decomposition of muscovite involves dehydroxylation(H₂O and alkali loss). At higher temperatures muscovite breaksdown to mullite, sillimanite, corundum, sanidine and a peraluminousmelt. Mullite (40–43 mol. per cent SiO₂) and sillimanite(49 mol. per cent SiO₂) are Fe₂O₃ and TiO₂ rich (up to 6·1–0·84and 3·6–0·24 wt. per cent respectively).Al-rich mullite (up to 77 wt. per cent Al₂O₃) occurs with corundumwhich has high Fe₂O₃ and TiO₂ (up to 6·9 and 2·1wt. per cent respectively). Annealing at high temperatures andreducing conditions results in the exsolution of mullite fromsillimanite and ilmenite from corundum. Glass resulting fromthe melting of muscovite in the presence of quartz is peraluminous(A/(CNK) = 1·3) with SiO² contents of 66–69 percent and normative corundum of 4 per cent. Sanidine (An_1·9Ab_26·0Or_72·1-An_1·3Ab_15·9Or_82·9)crystallized from the melt. Dehydroxylation and oxidation of biotite results in a decreaseof K₂O from 8·6 to less than 1 wt. per cent and oxidetotals (less H₂O + contents) from 96·5 to 88·6,exsolution of Al-magnetite, and a decrease in the Fe/(Fe + Mg)ratio from 0·41 to 0·17. Partial melting of biotitein the presence of quartz/plagioclase to pleonaste, Al-Ti magnetite,sanidine(An_2·0Ab_34·9Or63·1) and melt takesplace at higher temperatures. Glass in the vicinity of meltedbiotite is pale brown and highly peraluminous (A/CNK = 2·1)with up to 6 wt. per cent MgO+FeO_{(total iroq)} and up to 10 percent normative corundum. Near liquidus biotite with higher Al₂O₃and TiO₂ than partially melted biotite crystallized from themelt. Ti-rich biotites (up to 6 wt. per cent TiO₂) occur withinthe restite layers of thermally reconstituted xenoliths. Meltingof Ti-rich biotite and sillimanite in contact with the siliceousmelt of the buchite parts of xenoliths resulted in the formationof cordierite (100 Mg/(Mg+Fe+Mn) = 76·5–69·4),Al-Ti magnetite and sanidine, and development of cordierite/quartzintergrowths into the buchite melt. Growth of sanidine enclosedrelic Ca-plagioclase to form patchy intergrowths in the restitelayers. Cordierite (100 Mg/(Mg+Fe+Mn) = 64–69), quartz,sillimanite, mullite, magnetite and ilmenite, crystallized fromthe peraluminous buchite melt. Green-brown spinels of the pleonaste-magnetite series have awide compositional variation of (mol. per cent) FeAl₂O₄—66·6–45·0;MgAl₂O₄—53·0–18·7; Fe₃O₄—6·9–28·1;MnAl₂O₄—1·2–1·5; Fe₂TiO₄—0·6–6·2.Rims are generally enriched in the Fe₃O₄ component as a resultof oxidation. Compositions of ilmenite and magnetite (single,homogeneous and composite grains) are highly variable and resultfrom varying degrees of high temperature oxidation that is associatedwith dehydroxylation of micas and melting. Oxidation mainlyresults in increasing Fe³⁺, Al and decreasing Ti⁴⁺, Fe²⁺ inilmenite, and increasing Fe²⁺, Ti⁴⁺ and decreasing Fe³⁺ in associatedmagnetite. A higher degree of oxidation is reached with exsolutionof rutile from ilmenite and formation of titanhematite and withexsolution of pleonaste from magnetite. Ti-Al rich magnetite(5·1–7·5 and 8·5–13·5wt. per cent respectively) and ilmenite crystallized from meltsin buchitic parts of the xenoliths. Chemical and mineralogic evidence indicates that even with extensivemelting the primary compositions of individual layers in thexenoliths remained unmodified. Apparently the xenoliths didnot remain long enough at high temperatures for desilicationand enrichment in Al₂O₃, TiO₂, FeO, Fe₂O₃, and MgO that resultsby removal of a ‘granitic’ melt, and/or by interactionwith the magma, to occur. T °C-f_o2 values calculated from unoxidized magnetite/ilmenitegive temperatures ranging from 615–710°C for contactmetamorphism and the beginning of melting, and between 873 and1054°C for the crystallization of oxides and mullite/sillimanitefrom high temperature peraluminous melts. f_o2 values of metamorphismand melting were between the Ni-NiO and Fe₂O₃-Fe₃O₄ buffer curves.The relative abundance of xenolith types, geophysical evidenceand contact metamorphic mineralogy indicates that the xenolithswere derived from depths corresponding to between 2–3kb P_load = P_fluid. The xenoliths were erupted during the latestphreatomagmatic eruption from the Wehr volcano which resultedin vesiculation of melts in partially molten xenoliths causingfragmentation and disorientation of solid restite layers.     

Sillimanite and Ilmenite from High-grade Metamorphic Rocks of Antarctica and Other Areas

Sillimanite from a variety of high-grade metamorphic rocks containsfrom 0.13 to 1.82 weight per cent Fe₂O₃ and less than 0.1 weightper cent TiO₂. The iron is trivalent and substitutes for Alonly. Ilmenite associated with the sillimanite contains no morethan 0.4 weight per cent Al₂O₃, SiO₂, CaO, and MnO; and MgOdoes not exceed 1.6 weight per cent. It ranges in compositionfrom Ilm₉₉Hem₁ to Ilm₈₅Hem₁₅. A least squares fit of precision unit cell data on 10 analyzedsillimanites gives the following cell dimensions for iron-freesillimanite: a = 7.4830 Á, b = 7.6708 Á, c = 5.7694Á and V = 331.15 Á³. The projected increase incell volume with substitution of 10 mole per cent Fe₂SiO₃ is1.66 per cent. A regular increase in the Fe₂O₃ content of sillimanite withincreasing Fe₂O₃ content of associated ilmenite in 15 of 21samples analyzed suggests that sillimanite and ilmenite crystallizedin equilibrium in the 15 samples. The compositions of the tensillimanite-ilmenite pairs analyzed by the author fit the followingempirical curve (sol;(X_Fe2O3)Il = 1.110 x 10^–3. This regularincrease in Fe₂O₃ contents fits a model of Fe³⁺ substitutionfor Al on two independent sites in sillimanite and a coupledsubstitution of for Fe²⁺ Ti on two sites in ilmenite. Sillimaniteand ilmenite are behaving as ideal solutions over the compositionalrange 0 < X_Fe2SIO3 < 0.013 in sillimanite and 0 < X_Fe2O3< 0.15 in ilmenite. Equations have been derived for expressing the variation inFe₂O₃ content of sillimanite associated with quartz and ilmeniteor hematite as a function of pressure, temperature, and Fe₂O₃content of the oxide minerals. For example, the Fe₂O₃ contentof a sillimanite with 1.5 mole per cent Fe₂SiO₃ coexisting withTi-free hematite is calculated to decrease 11 per cent witha 5 kb increase in pressure. The rate of increase with temperatureof the Fe₂O₃ content of sillimanite is greater in hematite-bearingassemblages than in ilmenite-bearing assemblages.     

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.     

Petrology of the Blue Mountain Complex, Marlborough, New Zealand

Blue Mountain is a central-type alkali ultrabasic-gabbro ringcomplex (1?1?5 km) introducing Upper Jurassic sediments, Marlborough,New Zealand. The ultrabasic-gabbroic rocks contain lenses ofkaersutite pegmatite and sodic syenite pegmatite and are intrudedby ring dykes of titanaugite-ilmenite gabbro and lamprophyre.The margin of the intrusion is defined by a ring dyke of alkaligabbro. The plutonic rocks are cut by a swarm of hornblende-biotite-richlamprophyre dykes. Thermal metamorphism has converted the sedimentsto a hornfels ranging in grade from the albite-epidote hornfelsfacies to the upper limit of the hornblende hornfels facies. The rocks are nepheline normative and consist of olivine (Fo_82-74),endiopside (Ca₄₅Mg₄₈Fe₇-Ca₃₆Mg₅₅Fe₉), titanaugite (Ca₄₀Mg₅₀Fe₁₀-Ca₄₄Mg₃₉Fe₁₇),plagioclase (An_73-18), and ilmenitetitaniferous magnetite, withvarious amounts of titaniferous hornblende and titanbiotite.There is a complete gradation between end-iopside and titanaugitewith the coupled substitution R_y^+z+Si(Ti⁺⁴+Fe⁺³)+Al⁺³ and asympathetic increase in CaAl₂SiO₆ (0?2-10?2 percent) and CaTiAl₂O₆(2?1-8?1 per cent) with fractionation. Endiopside shows a small,progressive Mg enrichment along a trend subparallel to the CaMgSi₂O₆-Mg₂Si₂O₆boundary, and titanaugite is enriched in Ca and Fe⁺²+Fe⁺³ withdifferentiation. Oscillatory zoning between endiopside and titanaugiteis common. Exsolved ilmenite needles occur in the most Fe-richtitanaugites. The amphiboles show the trend: titaniferous hornblende(1?0–5?7 per cent TiO₂)kaersutite (6?4 per cent TiO₂)Fe-richhastingsite (18?0–19?1 per cent FeO as total Fe). Biotiteis high in TiO₂ (6?6–7?8 per cent). Ilmenite and titaniferousmagnetite (3?5–10?6 per cent TiO₂) are typically homogeneousgrains; their composition can be expressed in terms of R⁺²RO₃:R⁺²O:R₂⁺³O₄. The intrusion of igneous rocks was probably controlled by subterraneanring fracturing. Subsidence of the country rock within the ringfracture provided space for periodic injections of magma froma lower reservoir up the initial ring fracture to form the BlueMountain rocks at a higher level. Downward movement of the floorof the intrusion during crystallization caused inward slumpingof the cumulates which affected the textural, mineralogical,and chemical evolution of the rocks in different parts of theintrusion. The order of mineral fractionation is reflected by the chemicalvariation in the in situ ultrabasic-gabbroic rocks and the successiveintrusions of titanaugite-ilmenite gabbro and lamprophyre ringdykes, marginal alkali gabbro and lamprophyre dyke swarm. Aninitial decrease, then increase in SiO₂; a steady decrease inMgO, CaO, Ni, and Cr: an initial increase, then decrease inFeO+Fe₂O₃, TiO₂, MnO, and V; almost linear increase in Al₂O₃and late stage increase in alkalis and P₂O₃, implies fractionationof olivine and endiopside, followed by titanaugite and Fe-Tioxides, followed by plagioclase, hornblende, biotite, and apatite.Reversals in the composition of cumulus olivine and endiopsideand Solidification Index, indicate that the ultrabasic-gabbroicsequence is composed of four main injections of magma. The ultrabasic rocks crystallized under conditions of high PH₂Oand fairly high, constant P_O2; P_H2 and P_O2 increased duringthe formation of the gabbroic rocks until fracturing of thechamber roof occurred. The abundance of euhedral amphibole inthe latter injection phases suggests that amphibole accumulatedfrom a hydrous SiO₂ undersaturated magma when an increase inP_O2, stabilized its crystallization. Plutonic complexes similar to Blue Mountain are found withinand beneath the volcanic piles of many oceanic islands, e.g.Canaries, Reunion, and Tahiti, and those intruding thick sedimentarysequences, as at Blue Mountain, e.g. the pipe-like intrusionsof the Monteregian Hills, Quebec.     

Petrology of the Blue Mountain Complex, Marlborough, New Zealand

Blue Mountain is a central-type alkali ultrabasic-gabbro ringcomplex (lxl7middot;5 km) introducing Upper Jurassic sediments,Marlborough, New Zealand. The ultrabasic-gabbroic rocks containlenses of kaersutite pegmatite and sodic syenite pegmatite andare intruded by ring dykes of titanaugite-ilmenite gabbro andlamprophyre. The margin of the intrusion is defined by a ringdyke of alkali gabbro. The plutonic rocks are cut by a swarmof hornblendebiotite-rich lamprophyre dykes. Thermal metamorphismhas converted the sediments to a hornfels ranging in grade fromthe albite-epidote hornfels facies to the upper limit of thehornblende hornfels facies. The rocks are nepheline normative and consist of olivine (Fo_82–74),endiopside (Ca₄₅Mg₄₈Fe₇–Ca₃₆Mg₅₅Fe₉), titanaugite (Ca₄₀Mg₅₀Fe₁₀–Ca₄₄Mg₃₉Fe₁₇),plagioclase (An_73–18), and ilmenitetitaniferous magnetite,with various amounts of titaniferous hornblende and titanbiotite.There is a complete gradation between endiopside and titanaugitewith the coupled substitution R_y⁺²+Si;;(Ti⁺⁴+Fe⁺³+Al⁺³ and asympathetic increase in CaAl₂SiO₆ (0·2–10·2percent) and CaTiAl₂O₆ (2·1–8·1 per cent)with fractionation. Endiopside shows a small, progressive Mgenrichment along a trend subparallel to the CaMgSi₂O₆–Mg₂Si₂O₆boundary, and titanaugite is enriched in Ca and Fe⁺²+Fe⁺³ withdifferentiation. Oscillatory zoning between endiopside and titanaugiteis common. Exsolved ilmenite needles occur in the most Fe-richtitanaugites. The amphiboles show the trend: titaniferous hornblende(1·0–57middot;7 per cent TiO₂) kaersutite (6·4per cent TiO2) Fe-rich hastingsite (18·0–19·1per cent FeO as total Fe). Biotite is high in TiO₂ (6·6–7·8per cent). Ilmenite and titaniferous magnetite (3·5–10·6per cent TiO₂) are typically homogeneous grains; their compositioncan be expressed in terms of R⁺²RO₃:R⁺²O:R₂⁺³O₄. The intrusion of igneous rocks was probably controlled by subterraneanring fracturing. Subsidence of the country rock within the ringfracture provided space for periodic injections of magma froma lower reservoir up the initial ring fracture to form the BlueMountain rocks at a higher level. Downward movement of the floorof the intrusion during crystallization caused inward slumpingof the cumulates which affected the textural, mineralogical,and chemical evolution of the rocks in different parts of theintrusion. The order of mineral fractionation is reflected by the chemicalvariation in the in situ ultrabasic-gabbroic rocks and the successiveintrusions of titanaugite-ilmenite gabbro and lamprophyre ringdykes, marginal alkali gabbro and lamprophyre dyke swarm. Aninitial decrease, then increase in SiO2; a steady decrease inMgO, CaO, Ni, and Cr: an initial increase, then decrease inFeO+Fe₂O₃, TiO₂, MnO, and V; almost linear increase in A1₂O₃and late stage increase in alkalis and P₂O₃, implies fractionationof olivine and endiopside, followed by titanaugite and Fe-Tioxides, followed by plagioclase, hornblende, biotite, and apatite.Reversals in the composition of cumulus olivine and endiopsideand Solidification Index, indicate that the ultrabasic-gabbroicsequence is composed of four main injections of magma. The ultrabasic rocks crystallized under conditions of high P_H2Oand fairly high, constant     

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.     

Phase Equilibrium Constraints on Intensive Crystallization Parameters of the Ilimaussaq Complex, South Greenland

The 1·13 Ga Ilímaussaq intrusive complex, SouthGreenland, is composed of various types of alkali granite andsilica-undersaturated alkaline to agpaitic nepheline syenitesrelated to three subsequently intruded magma batches. Mineralchemistry indicates continuous fractionation trends within eachrock type, but with distinct differences among them. The last,peralkaline magma batch is the most fractionated in terms ofX_Fe^{mafic mineral}, feldspar composition and mineral assemblage.This indicates that an evolving magma chamber at depth discontinuouslyreleased more highly fractionated alkaline melts. Fluid inclusionsin some sodalites record a pressure drop from 3·5 to1 kbar indicating that crystallization started during magmaascent and continued in the high-level magma chamber. On thebasis of phase equilibria and preliminary fluid inclusion data,crystallization temperature drops from >1000°C (augitesyenite liquidus) to <500°C (lujavrite solidus) and silicaactivity decreases from     

Reactions Leading to the Disappearance of Pumpellyite in Low-grade Metamorphic Rocks of the Sanbagawa Metamorphic Belt in Central Shikoku, Japan

The major mineral assemblages of the metabasites of the Omoiji-Nagasawaarea in central Shikoku are hematite+epidote+chlorite+actinolite,riebeckitic actinolite+epidote+chlorite, epidote+chlorite+actinolite,and pumpellyite+epidote+chlorite+actinolite. The constituentminerals are often heterogeneous and assemblages in the fieldof a thin section sometimes do not obey the phase rule, butif grains apparently in non-equilibrium with others are excludedand domains of chemical equilibrium are appropriately chosenthe assemblages approximately obey the phase rule. The stability of hematite, pumpellyite, and epidote associatedwith chlorite and actinolite can be dealt with in terms of aternary system with appropriate excess phases. By fixing theFe²⁺/(Fe²⁺ +Mg) ratio of chlorite, it is dealt with in termsof stability relations in the system Ca₂Al₃Si₃O₁₂(OH)–Ca₂AlFe₂Si₃O₁₂(OH)with excess chlorite, actinolite, quartz, and controlled P_H2O.The maximum and minimum Fe³⁺ contents of epidote in this modelsystem are determined by hematite+epidote+chlorite+actinoliteand pumpellyite+epidote+chlorite+actinolite assemblages. Themaximum Fe³⁺ of the three phase assemblage epidote+chlorite+actinoliteis insensitive to temperature, but the minimum Fe³⁺ contentof epidote is sensitive to temperature and can be used to definethe metamorphic grade by a continuous quantity related to temperature.The phase relations expected for the model system are in goodagreement with the parageneses of the Sanbagawa terrain in centralShikoku and offer an explanation to the rule of Miyashiro &Seki (1958a) that the compositional range of epidote enlargeswith increasing temperature. The model also makes it possibleto estimate semi-quantitatively the temperature range in whichthe assemblage pumpellyite+epidote+chlorite+actinolite is stable.The possible maximum range is about 120 ?C, but the assemblageis stable in metabasite only for about 90 ?C. The higher temperaturelimit of the pumpellyite-actinolite facies defined by the disappearanceof pumpellyite in metabasite corresponds to the temperatureat which epidote with Fe³⁺/(Fe³⁺ +Al) = 0.10 0.15 coexistswith pumpellyite, actinolite, and chlorite. The compositions of epidotes in the metabasites of the Omoiji-Nagasawaarea cluster around Fe³⁺/(Fe³⁺ +Al) = 0.33. The grade of thisarea is close to the lower temperature stability limit of thepumpellyite+epidote+chlorite+actinolite assemblage.     

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.     

Transition from the Zeolite to Prehnite-Pumpellyite Facies in the Karmutsen Metabasites, Vancouver Island, British Columbia

The upper Triassic Karmutsen metabasites from northeast VancouverIsland, B.C., are thermally metamorphosed by the intrusion ofthe Coast Range Batholith. The amygdaloidal metabasites developedin the outer portion of the contact aureole show a progressivemetamorphism from zeolite to prehnite-pumpellyite facies. Thesize of an equilibrium domain is extremely small for these metabasites,and the individual amygdule assemblages are assumed to be inequilibrium. Two major calcite-free assemblages (+chlorite+quartz)are characteristic: (i) laumontite+pumpellyite+epidote in thezeolite facies and (ii) prehnite+pumpellyite+epidote in theprehnite-pumpellyite facies. The assemblages and compositionsof Ca-Al silicates are chemographically and theoretically interpretedon the basis of the predicted P-T grid for the model basalticsystem, CaO-MgO-A1₂O₃-Fe₂O₃-SiO₂-H₂O. The results indicate:(1) local equilibrium has been approached in mineral assemblagesand compositions; (2) the X_Fe³⁺ values in the coexisting Ca-Alsilicates decrease from epidote, through pumpellyite to prehnite;(3) with increasing metamorphic grade, the Fe³⁺ contents ofepidotes in reaction assemblages decrease in the zeolite facies,then increase in the prehnite-pumpellyite facies rocks. Suchvariations in the assemblages and mineral compositions are controlledby a sequence of continuous and discontinuous reactions, andallow delineation of T-X_Fe³⁺ relations at constant pressure.The transition from the zeolite to prehnite-pumpellyite faciesof the Karmutsen metabasites is defined by a discontinuous reaction:0·18 laumontite+pumpellyite+0·15 quartz = 1·31prehnite+ 0·78 epidote+0·2 chlorite+ 1·72H₂O, where the X_Fe³⁺ values of prehnite, pumpellyite and epidoteare 0·03, 0·10 and 0·18, respectively.These values together with available thermodynamic data andour preliminary experimental data are used to calculate theP-T condition for the discontinuous reaction as P = 1·1±0·5 kb and T = 190±30°C. The effectsof pressure on the upper stability of the zeolite facies assemblagesare discussed utilizing T-X_Fe³⁺ diagrams. The stability of thelaumontite-bearing assemblages for the zeolite facies metamorphismof basaltic rocks may be defined by either continuous or discontinuousreactions depending on the imposed metamorphic field gradient.Hence, the zeolite and prehnite-pumpellyite facies transitionboundary is multivariant.     

The Range of Spinel Compositions in Terrestrial Mafic and Ultramafic Rocks

Compositional fields for spinels from a wide variety of mafic–ultramaficigneous rock types and tectonic environments have been determinedfrom a global database of over 26 000 analyses. These fieldsare defined using contoured data density plots based on thespinel prism, and plots of T iO₂ vs ferric iron, for mantlexenoliths, ophiolitic rocks, continental layered intrusions,alkalic and lamprophyric rocks, tholeiitic basalts, Alaskanultramafic complexes and komatiites. Several trends appear regularlyin the various environments: a trend of widely variable Cr/(Cr+ Al) at low Fe²⁺/(Mg + Fe²⁺) (the Cr–Al trend); increasingFe³⁺, Fe²⁺/(Mg + Fe²⁺) and T iO₂ at constant Cr/(Cr + Al) (Fe–Ti trend); a trend found primarily in kimberlites, similar toFe–T i but at constant Fe²⁺/(Mg + Fe²⁺); and an unusualtrend of increasing Al found only in layered intrusions. TheCr–Al and Fe–T i trends are both found to varyingdegrees in tholeiitic basalts. The Cr–Al trend is prevalentin rocks that have equilibrated over a range of pressures, whereasthe Fe–T i trend is dominantly due to low-pressure fractionation.The most Cr-rich chromites found in nature occur in boninites,diamond-bearing kimberlites, some komatiites and ophioliticchromitites. Exceptionally reduced chromites are found in somekomatiites and in ophiolitic chromitites. Detrital chromitesfrom the Witwatersrand conglomerates are of komatiitic provenance. KEY WORDS: basalt; chromite; kimberlite; ophiolite; spinel     

Pumpellyite-Actinolite Facies Schists of the Taveyanne Formation near Lo?che, Valais, Switzerland

Electron microprobe analyses are presented for new-formed mineralsfrom a small exposure of semi-schistose Taveyanne Formationof the pumpellyite-actinolite facies near Lo?che, Valais. Comparisonsare drawn with minerals of other low-grade metamorphic areas,especially in southern New Zealand. Sphene shows considerablesubstitution of Ca(Al,Fe)SiO₄(OH) for CaTiSiO₅. Epidotes aresharply divided into early pistacitic (Ps = 0.28–0.37)and later clinozoisitic varieties (Ps = 0.11–0.19). Pumpellyitesrange from pumpellyite-(Fe) to pumpellyite-(Al) and are generallyless Fe-rich than those of zeolite and prehnite-pumpellyitefacies. Pumpellyite inclusions in albitized plagioclase areparticularly low in Mg. Actinolites are low in A1₂O₃, TiO₂,and Na₂O, essentially identical compositions being nucleatedon detrital augite, hornblende, and in the matrix. Phengitesare also extremely low in Na₂O and TiO₂. Chlorites are ripidolites.Albitized clastic plagioclase has the composition An0.7–1.6and albite in clinozoisite-calcite-albite-phengite-chloriteveins An2.1–2.3. Calcites carry minor Mn > Fe ? Mg.New-formed iron oxides are absent, whereas pyrrhotite and minorpyrite occur in one rock, buffering fs₂ and indicating low fo₂. Ratios Mg: Fe^* (Fe^* = total Fe) in coexisting chlorites andA1, Na-poor actinolites vary sympathetically both in the Lo?cheand southern New Zealand rocks here considered, giving K_D =(Mg/Fe^*) _actlnolIte/(Mg/Fe^*)_chlorle = 1.72. Mg/Fe^* ratios inpumpellyites tend to vary sympathetically with those of coexistingchlorites and actinolites but are more variable. Substitutionof (Fe, Mg)Si for A1₂ in phengitic micas and chlorites variessympathetically in the same suites between mafic volcanic andmore pelitic extremes. Various minor elements also behave ina consistent fashion, indicating an encouraging tendency towardsequilibrium. Variable (though small) A1₂O₃ contents of actinolite,Fe: Al ratios in epidotes and pumpellyites, and Mg: Fe^* ratiosin phengites, even within a single grain, are evidence of short-rangedisequilibrium; metamorphic equilibration is evidently easierbetween some crystal structures and structural sites than betweenothers. In phase rule analysis of assemblages in such rocks it is commonlynecessary to treat Fe²O₃, FeO, and MgO as separate componentsand it may also be necessary to regard CO² as an inert componentand/or to interpret observed assemblages as of low variance.The presence of the Ca-Al silicates and sphene indicates verylow X_co2 in the metamorphic fluids in all rocks examined exceptan albite-chlorite-calcite-quartz-anatase assemblage. But higherAn in albites than in isofacial and in greenschist facies rocksof southern New Zealand can be ascribed to significantly higherXco₂ at Lo?che, especially in the veins, than in New Zealand. Pumpellyite and epidotes of the pumpellyite-actinolite faciestend to be lower in Fe and richer in Al than those of lowergrade facies. Important reactions include those of the formpumpellyite-(Fe³⁺)+chlorite+quartz+H₂=pumpellyite-(Al)+actinolite,and pumpellyite+chlorite+quartz- ‘epidote’+actinolite+water.Careful selection of pumpellyite and chlorite compositions isrequired for experimental and chemographic analysis of pumpellyitestability. In the absence of critical data, temperatures ofabout 250–350? and pressures of several kilobars are provisionallysuggested for the Lo?che metamorphism.     

The Crossite Content of Ca-Amphibole as a Guide to Pressure of Metamorphism

A correlation between the crossite component (NaM₄) in Ca-amphiboleand pressure of metamorphism has long been recognized (Shido& Miyashiro, 1959), but only recently has the reaction beenidentified which buffers this aspect of amphibole composition(Brown, 1974): Ca-amphibole+iron oxide+albite+chloriteI+H₂O (±stilp,qtz) = crossite+epidote (±muscovite, qtz). The exact stoichiometry of the reaction depends on compositionalvariables in the minerals, especially Fe²⁺/Mg and Fe³⁺/Al. Ca-amphiboleshould have fixed NaM₄, at any given T and P, where it coexistswith iron oxide, albite, and chlorite. Comparison of Ca-amphibole composition with mineral assemblage,in rocks from Otago, N.Z., and elsewhere, supports this hypothesis.In any terrane NaM₄ is nearly constant at a particular metamorphicgrade where amphibole exists in the buffering assemblage, butvaries widely outside of this assemblage. Variations in Fe²⁺/Mgand Fe³⁺/Al in the amphibole have relatively little effect onNaM₄, but in high pressure amphiboles NaM₄ varies inverselywith Al^iv. Ca-amphiboles from high pressure areas have substantially moreNaM₄ (Otago, 0.6 of 2.0) than those from lower pressure areas(Sierra contact aureoles, 0.1). These relations suggest thatin the buffering assemblage, the NaM₄ content of Ca-amphiboleshould be a useful relative barometer for low to medium grademetamorphic rocks.     

Fe--Al Oxides: Phase Relationships below 1,000?C

The subsolidus phase relationships of magnetite, hercynite,hematite, corundum, wostite, and iron are described. The phaseswere synthesized from chemical mixtures. Reactions and solidsolution between them were induced under controlled conditionsof composition, temperature, total vapor pressure, and partialpressure of oxygen. Reaction rates are slow, so that the experimentslasted from 1 to 40 days, and quenching is completely successful. A solvus was determined which limits solid solution along themagnetitc-hercynite join at temperatures below 860^o?15^oC. Compositionsof the spinel solid solutions were determined by measuring theshift of the (440) reflection, using a powder X-ray diffractometer.The calibration curve, 20 vs. composition, was made from measurementsof spinel solid solutions synthesized in the one-phase region.The cell edge a_o changes from 8–391?0.002 A (magnetic,Fe⁺²Fe₂⁺²O₄OJ to 8.150?0.004 (hercynite, Fe⁺²Al₂O₄)by a_o?8.391–0.00190x- 0.5X²10^-5 (X is mol per cent FeAl₂O₄ in solid solution). In the system Fe-Al₂-O₃-O there are five univariant assemblages: 1. Hematite-corundum+magnetite +V (vapor) 2. Corundum+magnetite+hercynite+V 3. Magnetite+hercynite+w?stite+V 4. Hercynite+wilstite+iron+V 5. Hercynite+iron+corundum+V The lines were located by determining the composition of themagnetite, hercynite, hematite, and corundum solid solutionsfor each assemblage. The diagrams provide a basis for the discussion of the paragenesisof the oxide minerals. The progressive metamorphism of lateritedeposits can be represented by (1) laterites and bauxites: hematiteH+hydratedaluminum oxides; (2) diasporites: hematite+diaspore+corundum,with magnetite as a rare accessory; (3) emery: corundum+magnetite,with hematite as an accessory. The path of these mineral changeson the diagrams shows the decrease in oxygen content of thesolids with decrease in the partial pressure of oxygen and relatesthe aluminum content of the magnetite to temperature. The occurrences of hercynite are discussed. It is a rare mineralbecause it requires unusual conditions to grow, i.e. relativelylow oxygen pressure and an extremely Fe-Al-rich environment.     

Chemistry of Kornerupine and Associated Minerals, a Wet Chemical, Ion Microprobe, and X-Ray Study Emphasizing Li, Be, B and F Contents

Kornerupine and associated minerals in 31 samples of high-graderocks relatively rich in Al and Mg were analysed by wet chemistry,ion microprobe mass analyser, electron microprobe and X-raypowder diffraction. For 11 samples of kornerupine and threesamples of biotite (F only) analysed by both wet chemical andion microprobe methods, the best agreement was obtained forB₂O₃, whereas the ion microprobe Li₂O values were systematicallysomewhat higher than the wet chemical values. The wet chemicalmethods give Li₂O=0–0?19 wt.%; BeO=0–0?032 wt.%;B₂O₃=0–4?01 wt.%; and F=0?07–0?77 wt.% in kornerupine,whereas ion microprobe analyses on other kornerupines give valuesup to 0?35 wt.% Li₂O, O066 wt.% BeO, and 4?72 wt.% B₂O₃. Thesum B+Al+Fe³⁺+Cr is close to 6?9 atoms per 22 (O, OH, F) or21?5 (O) in kornerupine. In general, Li/Fe ratios decrease as follows: kornerupine ?sapphirinebiotite> Crd (Na<0?03 per 18 oxygens)>tourmaline, garnet,orthopyroxene. However, for cordierite with Na>004, Li/Fedecreases as follows: cordierite>kornerupine. Sapphirineand sillimanite are the only associated minerals to incorporatesignificant boron (0?1–0?85 wt.% B₂O₃) and then only whenthe single site for B in kornerupine is approaching capacity.Sillimanite B₂O₃ contents increase regularly with kornerupineF. Fractionation of fluorine increases as follows: kornerupine<biotite<tourmaline,and K^krn-BtD=(F/OH)^Krn/(F/(OH)^Bt (assuming ideal anion composition)increases with biotite Ti. Kornerupine B₂O₃ content is a measureof B₂O₃ activity in associated metamorphic fluid, whereas sillimaniteB₂O₃ content increases with temperature, exceeding 0?4 wt.%whenT=900?C at very low water activities. New data on 11 kornerupines and literature data indicate thatthe unit cell parameters a, c, and V decrease with increasingB content and b, c, and V increase with increasing Fe³⁺ content.In Fe³⁺-poor kornerupines, b increases with Mg and with (Mg+ Fe²⁺) but the effect of Mg on b via the substitution ^VIMg+^IVSi=^VIAl+^IVAloverwhelms the effect of Fe²⁺=Mg substitution.     

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.     

攀枝花岩体钛铁矿成分特征及其成因意义

峨眉大火成岩省是全球最大的钒钛磁铁矿床聚集区,攀枝花岩体是其中的典型代表。根据岩性特点,攀枝花岩体主体可划分为上、中、下三个岩相带,其中中部岩相带和下部岩相带岩性旋回非常发育,每个旋回从下向上铁钛氧化物和暗色硅酸盐矿物逐渐减少,块状铁钛氧化物矿石或磁铁矿辉长岩都出现在每个旋回的底部和下部。然而,尽管钛铁矿固相线以下固溶体出溶远弱于磁铁矿,从而能更好地保留成因信息,但其成分变化的成因意义没有受到足够重视。本次研究发现作为主要金属氧化物之一的钛铁矿的成分不仅在不同岩性中有明显差异,同时,中、下部岩相带的各岩性旋回中钛铁矿成分也具有周期性变化。例如,块状矿石中钛铁矿具有最高的MgO和TiO2及最低的FeO、Fe2O3和MnO,而辉长岩中钛铁矿则具有相反的成分特征。同时,钛铁矿的MgO含量与磁铁矿的MgO含量及橄榄石的Fo牌号具有显著的正相关关系。这种规律性变化说明每个旋回可以代表一次比较明显的岩浆补充,每次新岩浆补充后,钛铁矿和磁铁矿及橄榄石都是结晶较早的矿物。与Skaergaard岩体相比,攀枝花岩体钛铁矿的MgO含量较高,表明攀枝花岩体分离结晶过程中铁钛氧化物结晶较早;与挪威Tellnes斜长岩套铁钛矿床中的钛铁矿相比,攀枝花岩体的钛铁矿不仅具有较高的MgO和FeO,还具有极高的TiO2和MnO,但Fe2O3却很低,说明地幔柱背景下形成的钛铁矿与斜长岩套中钛铁矿的成分有显著的区别。     

Effects of P, f(O2) and Mg/Fe Ratio on Dehydration Melting of Model Metagreywackes

We present results of dehydration melting experiments [3–15kbar, 810–950C f(O₂) QFM (quartz-fayalite-magetite)and Ni-NiO] on two Fe-rich mixtures of biotite (37%), plagioclaseAn₃₈ (27%), quartz (34%) and ilmenite (2%), which differ onlyin their biotite compositions (mg-number 23 and 0.4). Dehydrationmelting of metagreywackes of constant modal composition generatesa wide range of melt fractions, melt compositions and residualassemblages, through the combined effects of pressure, Fe/Mgratio and f(O₂). Crystallization of garnet is the chief controlon melting behavior, and is limited by two reactions: (1) thebreakdown of garnet + quartz to orthopyroxene + plagioclaseat low P, and (2) the oxidation of garnet to magnetite + anorthite+ quartz (enstatite), which is sensitive to both f(O₂) andP. Because of these reactions, melting of Mg-rich metagreywackesis rather insensitive to f(O₂) but strongly sensitive to P;the converse is true for Fe-rich metagreywackes. Garnet crystallizationrequires that plagioclase break down incongruently, liberatingalbite. This increases the Na₂O content of the melts and enhancesmelt production. Thus, melting of metagreywacke in a reducingdeep-crustal environment (with garnet stable) would producemore, and more sodic, melt than would garnet-absent meltingof the same source material in a relatively oxidizing, shallow-crustalenvironment. KEY WORDS: anatexis; metasediments; gneisses; granites; garnet ^*Corresponding author. Telephone: 706-542-2394; fax: 706-542-2425; e-mail: alpatino{at}uga.cc.uga.edu