Dunite--Harzburgite--Chromitite Complexes as Refractory Residue in the Sangun--Yamaguchi Zone, Western Japan

Dunite, harzburgite and chromitite of alpine-type ultramaficcomplexes emplaced in the Paleozoic sediments in the Sangun—Yamaguchizone, western Japan, are massive and almost lacking in layeredstructure. Constituent minerals are more or less deformed andequilibrated at a relatively low temperature, about 700 °Cor lower. Chromian spinels in ultramafic rocks from dunite—harzburgite—chromititecomplexes in the Sangun—Yamaguchi zone are characterizedby the uniformity of the Cr/Cr + Al ratio, around 0.5, regardlessof locality and rock type, which is in contrast to the widevariation of the ratio of chromian spinel from the ordinaryalpine-type dunite—harzburgite complex. Mg/Mg + Fe" ratioof chromian spinel, on the other hand, is variable in parallelto the volume per cent of chromian spinel in ultramafic rocks.Olivine in ultramafic rocks is uniform in chemical composition,from Fo₈₈ to Fo₉₂, except for that in chromitite, of which itis Fo₉₅ to Fo₉₇. Primary chemical compositions (especially the Mg/Mg + Fe" ratio)of the constituent minerals have been modified to some extentby element redistribution at low temperature (700 °C orlower), the degree of modification depending on the volume ratioof the minerals. For example, the Mg/Mg + Fe" ratio of chromianspinel in peridotitic rocks has been lowered substantially,and inversely, that of olivine in chromitite has been raised.Primary Mg/Mg + Fe" ratios of olivine and chromian spinel canbe estimated, assuming a certain value of high temperature,e.g. 1200 °C, and the partition coefficient between olivineand chromian spinel at that temperature. As a result, the Mg/Mg+ Fe" ratio of olivine (0.88 to 0.92), and that of chromianspinel (0.78 to 0.80) were uniform, irrespective of rock typeand locality, at the high-temperature stage. Dunite—harzburgite—chromitite complexes in the Sangun—Yamaguchizone were accumulated as refractory residue after the relativelylarge-scaled partial fusion of some primordial peridotites,which resulted in the chemical uniformity of the residual minerals.They may have initially constituted the lowest part of an ophioliticsuite as ultramafic tectonite and been emplaced as dismemberedportions after the disruption of the parent body.     

Textural and Compositional Relationships of Ca-Amphiboles in Metabasites of the Cape Smith Belt, Northern Qubec: Implications for a Miscibility Gap at Medium Pressure

Textural and chemical relationships in the Ca-amphibole serieshave been examined in progressively metamorphosed mafic schistfrom the Povungnituk Group, northern Qubec. In the hornblendezone, coexisting actinolite and hornblende are characterizedby sharp optical and chemical boundaries. They also displayan epitaxial intergrowth texture, consistent with contemporaneouscrystallization and the presence of a miscibility gap (solvus)between actinolite and hornblende. At higher grade, in the oligoclasezone, actinolite is always rimmed by hornblende. Across theoligoclase isograd, the compositional break between actinoliteand hornblende remains relatively constant in width (6–10wt.% Al₂O₃). Accordingly, the concentric texture and the lackof chemical gradients are interpreted as a local equilibriumat the scale of the interfaces between the two amphiboles. Throughoutthe hornblende zone and the lower part of the oligoclase zone,both amphiboles are characterized by a progressive Mg-enrichment,with no consistent change in edenite and Tschermak components.The hornblende zone can be characterized by a continuous reactionwith Mg-enrichment of reactants (biotite and chlorite) and products(actinolite and hornblende). However, this reaction requiresa mixed-volatile phase with a much higher X_CO2 ({small tilde}0.5) than those calculated from equilibrium curves in the NCMASH—CO₂system. The oligoclase isograd can be represented by a discontinuousoligoclase—hornblende-forming reaction which coincideswith the change from epitaxial to concentric texture at theoligoclase isograd. At the upper limit of the oligoclase zone,gradational boundaries and a continuous solid solution betweenhornblende and actinolite document a disequilibrium texture.Although a miscibility gap can be documented in the hornblendezone, its closure at higher grade is not observed, based onequilibrium crystallization of actinolite and hornblende. Theresults of this study suggest that the crest of the solvus mustbe crossed between the oligoclase and actinolite-out isograds,based on zoned profiles between hornblende rims and relic actinolite. "The term exsolution refers to the process whereby an initiallyhomogeneous solid solution separates into two (or possibly more)distinct crystalline minerals without the addition or removalof material to or from the system. Exsolution generally, althoughnot necessarily, occurs on cooling. The miscibility gap representsa temperature—composition field in which solid solutionbetween the end members decreases gradually from higher to lowertemperatures."     

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.     

Chromite in Komatiites, II. Modification during Greenschist to Mid-Amphibolite Facies Metamorphism

Chromite compositions in komatiites are influenced by metamorphicprocesses, particularly above 500°C. Metamorphosed chromiteis substantially more iron rich than igneous precursors, asa result of Mg–Fe exchange with silicates and carbonates.Chromite metamorphosed to amphibolite facies is enriched inZn and Fe, and depleted in Ni, relative to lower metamorphicgrades. Relative proportions of the trivalent ions Cr³⁺, Al³⁺and Fe³⁺ are not greatly modified by metamorphism up to loweramphibolite facies, although minor Fe³⁺ depletion occurs duringtalc–carbonate alteration at low temperature. SignificantAl is lost from chromite cores above 550°C, as a resultof equilibration with fluids in equilibrium with chlorite. ElevatedZn content in chromite is restricted to rocks with low (metamorphic)Mg/Fe ratios, and is the result of introduction of Zn duringlow-temperature alteration, with further concentration and homogenizationduring prograde metamorphism. Cobalt and Mn also behave similarly,except where carbonate minerals are predominant in the metamorphicassemblage. Chromite at amphibolite facies is typically extensivelyreplaced by magnetite. This is the result of incomplete metamorphicreaction between chromite and chlorite-bearing silicate assemblages.Magnetite compositions at the inner chromite–magnetiteboundary are indicators of metamorphic grade. KEY WORDS: chromite; komatiite; spinel; metamorphism; Zn     

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     

Low-temperature Compatibility Relations of Cordierite in Haplopelites of the System K2O--MgO--Al2O3--SiO2--H2O

The equilibrium temperatures of the reaction muscovite+chlorite+quartz= cordierite+phlogopite+H₂O (1) in the pure system K₂O—MgO—Al₂O₂—SiO₂—H₂Owere found to be 495±10°C at 1 kb P_H2O; 525±10°Cat 2 kb; 610±15°C at 5 kb; 635±10°C at6 kb. From intersection of this curve with the lower temperaturestability limit of cordierite close to 645°C, 6.5 kb P_H2O,a reaction cordieritc+muscovite = phlogopite+aluminum silicate+quartz+H₂O(2) is generated which has a negative slope and passes throughthe points 645°C, 6.5 kb PH₂O and 700°C, 5 kb PH₂O.On the high-pressure side of this reaction curve cordieriteis restricted to K₂O—poor bulk compositions. Application of the experimentally determined phase relationsto more complex natural pelitic rocks suggests that reaction(1) represents maximum temperatures for the disappearance ofchlorite from pelitic assemblages containing muscovite and quartz,whereas reaction (2) gives maximum water pressures for the disappearanceof cordierite from these rocks.     

Synthesis and Stability Relations of Mg--Al Pumpellyite, Ca4Al5MgSi6O21(OH)7

Hydrothermal synthesis and investigations of stability relationsof Mg—Al pumpellyite were conducted using high-pressurecold-seal apparatus over the temperature range 250–600°C and 2–8 kb P_fluid. Mg—Al pumpellyite Ca₄Al₅MgSi₆O₂₁(OH)₇was synthesized from partially crystalline gel mixtures of stoichiometriccomposition at 275–410 °C, 6–9 kb P_fluid, andruns of 7–90 days. Pure monomineralic synthetic Mg—Alpumpellyite has refractive index n_ß = 1.624 (2) andcell dimensions = 8.825 (8) Á, b = 5.875 (5) Á,c = 19.10 (1) Á, and ß = 97.39 (7)°. The high temperature assemblage of the equivalent bulk compositionconsists of clinozoisite, hydrogrossular/grossular, aluminousseptechlorite/chlorite, quartz, and H₂O. Hydrogrossular wassynthesized in the presence of quartz at 8 kb from 400–500°C, and hydrogrossular + quartz are unstable with respectto grossular + H₂O at 400 °C and 8 kb P_fluid. At 8 kb P_fluid,aluminous septechlorite forms at temperatures below 500 °Cwhereas aluminous 14 Á chlorite crystallizes at 500–600°C. The equilibrium relations of Mg—Al pumpellyite were determinedusing subequal mixtures of synthetic Mg—Al pumpellyiteand its high temperature assemblage. The reaction 9 Mg—Alpumpellyite = 9 clinozoisite + 6 grossular + 2 chlorite + 4quartz + 19 H₂O occurs at temperatures of 390 °C at 8 kb,368 °C at 5 kb, and near 325 °C at 2 kb P_fluid. Thereversal data yield an approximate value of –3141 joules/mole°K for the standard entropy of formation for the syntheticMg—Al pumpellyite. The Schreinemakers' relations for pumpellyite, prehnite, clinozoisite,tremolite, grossular, and amesite in the presence of excessquartz and fluid were constructed in the pseudo-ternary systemCaO–Al₂O₃–MgO(SiO₂–H₂O). The results, togetherwith reconnaissance experiments on the reaction 4 Mg—Alpumpellyite + 2 quartz = 8 prehnite + aluminous septechlorite+ 2 H₂O, locate the invariant point [TR] at approximately 5.7kb P_fluid and 375 °C. The results of the present study arenot compatible with previous experimental data on the invariantpoint [GR]. The P–T oriented phase relations are used to interpretsome natural parageneses developed in low-grade metabasalticrocks recrystallized under conditions of low _co2. The high-temperaturestability relations of Mg—Al pumpellyite are useful todenote the onset of greenschist facies metamorphism in rocksof basaltic composition.     

A Natural Example of Metastable Reactions Involving Garnet and Sillimanite

The broad objective of this work is to develop models of reactionthat can be used to understand dynamic processes. The kineticsof reactions can be significant factors affecting heat and masstransfer, and the kinetic inhibition of equilibration enablesus to trace the history of natural samples. Samples were selectedfrom a metamorphic aureole for study because they show measurabledisequilibrium, and the approximate P—T path and timeof metamorphism are known. Detailed analysis of compositional variations of crystals andreaction textures suggests that biotite was being produced bythe breakdown of garnet at a faster rate than it was consumedby reaction with sillimanite. Some of the compositional variationsof biotite and cordierite can be explained by the process ofintergranular diffusion; however, the co-existence of metastablebiotite and sillimanite and the widespread distribution of Fe-richbiotite in the sample argue for a partial equilibrium modelfor which reaction rate was controlled by surface kinetic processes. Quantitative modelling was accomplished by breaking the overallreaction into three component reactions. The rate of one ofthese component reactions through time can be estimated fromgarnet zoning profiles, and the relative abundance of productphases is used then to estimate the absolute rates of the othercomponent reactions. A plausible interpretation of the datais that rate of dissolution of sillimanite was the rate-controllingprocess, and took place at a rate of 10^–7 to 10^–8cm/year in this sample. Analysis of this sample illustrates possible significant effectsof the kinetics of reaction in dry rocks in the deep crust.Even though biotite would have been unstable at final equilibriumand the overall reaction did not involve significant gain orloss of water, the particular kinetic path under taken resultedin production of biotite and consumption of water at the stageof reaction preserved in this sample.     

Analysis of Equilibrium in Garnet-Biotite-Sillimanite Gneisses from Quebec

An analysis is presented of equilibrium in six specimens ofgarnet—biotite—sillimanite—plagioclase—potashfeldspar—quartz ... gneiss from a metamorphic terrainin south-western Quebec. A nearly uniform Ti content of biotite may be accounted forby an equilibrium (a) involving biotite, sillimanite, quartz,garnet, potash feldspar, and H₂O. The nature of the distributionof Fe and Mg between garnet and biotite may be accounted forby another equilibrium (b) involving the same mineral suite,or by a simple exchange equilibrium (c) involving only garnetand biotite. The distribution of Mn between garnet and biotiteis accounted for by an exchange equilibrium (d). A nearly uniformvalue of the ratio Ca content of plagioclase/Ca content of garnetmay be accounted for by an equilibrium (e) involving plagioclase,garnet, sillimanite, and quartz. A proposed equilibrium (f)involving biotite, quartz, ilmenite, potash feldspar, sillimanite,and H₂O conflicts with equilibrium (a) and was evidently notestablished in the gneisses. The factors governing the Ca contentof biotite remain largely unknown. Some of these equilibria form potential indicators of relativegeologic temperature, pressure, and chemical potential of H₂O.     

Biotites as Indicators of Fluorine Fugacities in Late-Stage Magmatic Fluids: the Gardar Province of South Greenland

The major element chemistry and fluorine contents of biotitesfrom the Gardar alkaline province of South Greenland providean insight into the F contents of late-stage fluids associatedwith the magmatism. Biotites were taken from composite intrusionsranging from alkali gabbro to syenites, nepheline syenites andalkali granites. In each complex they show a large range ofFe/(Fe+Mg) (from 0•2 to 1) and exhibit strong Fe-F avoidance.There is considerable variation in F for any value of Fe/(Fe+Mg)but for each centre maximum F values plotted against Fe/(Fe+Mg)define a nearly straight line of characteristic slope towardslow or zero F at pure annite. Micas in the SiO₂ undersaturatedcentres have higher F contents than those from oversaturatedcentres. Cl is low (0•69 wt%) except in the Kûngnâtintrusion, where it reaches 1•4 wt%. Phase equilibriumand textural considerations suggest that most or all biotitesgrew subsolidus in a pervasive deuteric fluid. ¹⁸O values suggestthat these fluids were largely magmatic in character and thatextensive reactions with envelope fluids did not occur. Metasomaticresetting of F in biotites in early intrusive units in the aureoleof later units can be demonstrated. Experimental data of Munoz (Mineralogical Society of America,Reviews in Mineralogy 13, 469–494, 1984) were used tocalculate families of curves showing variation in F with Fe/(Fe+Mg)for biotites in equilibrium with fluids of fixed fugacity ratio,f(HF)/f(H₂O), at fixed P and T. The resulting curves cut sharplyacross the maximum fluorine lines observed in the natural examples.As it seems highly unlikely that changes in fluid compositionand T, acting together, could produce the observed linear relationship,we conclude that the partitioning of F between fluid and micain the plutonic environment is not well modelled by the experiments.Possible explanations are short-range order (SRO) of Fe andMg on octahedral sites in biotite (Mason, Canadian Mineralogist30, 343–354, 1992) or effects resulting from differentF speciation in alkaline fluids. If perfect Fe-F avoidance isassumed, Fe-Mg SRO can increase maximum F content. The F levelsseen in the silica-saturated centres are broadly consistentwith a cooling-rate related control. It is possible that thehigher F in biotites in the undersaturated centres reflectsdifferent speciation in the fluid rather than higher F contents,with strongly bound SiF₄° complexes more common. KEY WORDS: biotites; Gardar; fluorine; alkaline rocks; fluids     

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.     

Petrogenesis of the Eastern Bushveld Complex: Crystallization of the Middle Critical Zone

The bronzite—chromite-anorthite assemblage of the F—unit(Cameron & Emerson, 1959) from the Critical Zone of theBushveld Igneous Complex, was examined with the aid of an electrolyticcell designed after Sato (1971). The resultant fO₂-T data reveala last equilibration at an fO₂ value of 10^{11·82 ±·40} atm and at a temperature of 1091 ± 35 °C.These fO₂-T data when compared with: (1) a one atmosphere quenching—technique solidus determinationof 1110 ± 5 °C, (2) the Bushveld plagioclase compositional trends (Cameron,1970), (3) Bushveld petrofabric examinations (Cameron, 1969) (4) phase equilibria in the system CaO–MgO–FeO–CaAl₂Si₂O₈–SiO₂(Roeder & Osborn, 1966), (5) phase equilibria in the system CaAl₂Si₂O₈–NaAlSi₃O₈–SiO₂–MgO–Fe–O₂–H₂O–CO₂(Eggler, 1974), all support the idea that the Eastern Bushveld magma was notappreciably differentiating in the middle Critical Zone betweenF and the L Horizons, an accumulation of nearly 220 meters.     

Geochemical Constraints from Zoned Hydrothermal Tourmalines on Fluid Evolution and Sn Mineralization: an Example from Fault Breccias at Roche, SW England

Hydrothermal fluid evolution north of the St Austell granite,southwest England, has been studied through geochemical analysisof tourmaline from a fault breccia of <2 cm width withinmassive quartz–tourmaline rocks at Roche. Brecciated tourmalinegrains have overgrowths of <400 µm width [Fe/(Fe +Mg) = 0·31–0·99] with four chemically distinctzones (1–4, towards the margins). Variations in overgrowthcomposition were caused by episodic mixing between Mg-, Al-richmagmatic hydrothermal fluids (dominant in zone 1), with an increasingcomponent of more oxidizing, Fe-rich formation waters (zones2 and 4). More oxidizing conditions are supported by high Sncontents in zone 2 (<0·35 wt %), with Sn probablypresent as Sn⁴⁺ rather than Sn²⁺, the usual form in hydrothermalfluids. From X-ray maps, zones 1 and 3 occur exclusively asovergrowths on pre-existing grains, indicating that overgrowthformation was kinetically favoured over tourmaline nucleation.In zones 2 and 4, nucleation and growth occurred, possibly asa result of supersaturation with respect to tourmaline duringincreased mixing with formation waters. Tourmaline is associatedwith the main episode of mineralization in many important mineraldeposits, often unaffected by alteration. This method of studyinghydrothermal fluid evolution may therefore have uses in exploration,particularly for tourmaline-breccia-hosted ores in Cu-porphyrydeposits. KEY WORDS: breccia; Cornwall; hydrothermal; tin; tourmaline     

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

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

Editorial 2003

2002 was a highly successful year. The most recent (2001) ISIImpact Factors for journal citations confirm that Journal ofPetrology remains the premier journal in the field of petrology(Journal of Petrology—3.283; Earth & Planetary ScienceLetters—2.700; Geochimica Cosmochimica Acta—2.614;Contributions to Mineralogy & Petrology     

Thermal Stability of Assemblages in the Cu--Fe--S System

The phase relations in the Cu-Fe-S system were determined from700 C to approximately 200 C in most portions of the systemand below 100 C in restricted areas. Approximate solid solutionlimits for bornite, chalcopyrite, and pyrrhotite were determinedat elevated temperatures. At low temperatures emphasis was placedon establishing the stable assemblages and less on determiningthe compositions of coexisting phases. At 700 C two extensiveternary solid solutions dominate the phase relations in thissystem. One of these solid solutions (bornite) includes thecompositions Cu₂S, Cu₁₈S, and Cu₅FeS₄and the other (chalcopyrite)lies with in the area bounded by the compositions CuFeS₂ CuFe₂S₃,and CU₃Fe₄S₄. The two fields are separated by approximately10 weight per cent copper at 700 C. The chalcopyrite volume,as seen in a trigonal prism representing temperature and composition,is intersected by a miscibility gap below approximately 600C.Below this temperature the two one-phase volumes are referredto as chalcopyrite and cubanite. Chalcopyrite is tetragonalat low temperature but isometric above approximately 550C.The temperature of the transformation is a function of composition.Cubanite is isometric above 252C, tetragonal from 252 to atleast 213C, and orthorhombic at lower temperature. The temperatureof the second transformation is unknown because the tetragonal-to-orthorhombictransformation has not been achieved in the laboratory. Borniteand pyrite become stable together at 568C and coexist downto 228C. Covellite appears with lowering temperature at 507C,and idaite at 501C. Idaite—pyrite and idaite—borniteare stable assemblages below 501 C. The composition of bornitecoexisting with idaite changes gradually towards digenite withdecreasing temperature, thus permitting the change from thebornite—pyrite tie-line to the digenite—chalcopyritetie-line at 228C. Other major tie-line changes are bornite—ironto pyrrhotite—copper below 475C and cubanite—pyriteto chalcopyrite—pyrrhotite below 334C. A new syntheticphase, x-bornite, which has a composition close to bornite (Cu₅FeS₄)but contains about 04 weight per cent more sulfur, forms whensulfur-rich bornite synthesized at high temperature is annealedbetween 62 and 140C. Optically this new phase is very similarto bornite, and their X-ray powder diffraction patterns aregiven for comparison. o The determined phase relations are applicable to numerous deposits.The tie-line changes involving bornitepyrite reacting to producedigenitechalcopyrite below 228 C and cubanite (isometric)pyritegoing to chalcopyritepyrrhotite below 334 C are of considerablegeological interest. The rates of these reactions are sufficientlyslow to allow the higher temperature assemblages to be observedin some ores. The cubic—tetragonal inversion in chalcopyriteis often deduced in ores by inversion twins. However, twinningis also commonly produced through deformation. Geological applicationof the inversion therefore depends on correct interpretationof the twinning. Because of the considerable solubility of copperin pyrrhotite the pyrrhotite—pyrite solvus of the pureFe—S system cannot be applied indiscriminately to oresthat also contain chalcopyrite or cubanite, or both. The newx-bornite phase was identified with the natural ‘anomalousbornites’, which when heated exsolve chalcopyrite and,depending on their composition, also digenite. The experimental results indicate that the mineral commonlyidentified as chalcopyrrhotite is in reality tetragonal or evenisometric cubanite. Experimental evidence could not be obtainedfor the existence of a phase of Cu₂Fe₄S₇ or Cu₂Fe₄S₇ composition,the older formulae given foor valleriite. The thermal breakdownof natural material supports the idea that valleriite is a low-temperaturepolymorph of chalcopyrite. The relatively uncommon occurrenceof idaite in comparison to covellite is attributed to the greaterdifficulty in nucleating idaite. The possibility of stable coexistenceof chalcocite and pyrite was investigated but was found to beprohibited by tie-lines between bornite and digenite even aslow as 100 C.     

Diffusion Gradients in an Eclogite Xenolith from the Roberts Victor Kimberlite Pipe: 1. Mechanism and Evolution of Garnet Exsolution in Al2O3-rich Clinopyroxene

Xenolith JJG41 is from the Roberts Victor kimberlite and isa bimineralic eclogite which is striking for its Al- and Ca-richclinopyroxene crystals showing garnet exsolution lamellae. Thedevelopment of the exsolution has been interpreted as a resultof a slow cooling at depth from near-solidus conditions (c.1400?C) towards normal mantle lithosphere temperatures (Harte& Gurney, 1975). The clinopyroxene retains marked compositionalgradients adjacent to the garnet lamellae and the present paperis concerned with the generation and preservation of these diffusiongradients within a rock from the Earth's mantle In order to understand the mechanism of exsolution reactiona re-examination has been made of the microtexture and chemistryof the garnet lamellae in relation to the compositional gradientsin adjacent clinopyroxene. Three sets of garnet lamellae, whichappear to have crystallized in sequence, may be recognised:type A, large lamellae, nucleated first and closest to the transformationtemperature; type B of intermediate size and age; and type C,small lamellae, nucleated last and with the greatest overstepof the transformation temperature. The major compositional zoning in JJG41 clinopyroxene, a decreaseof Al as Si and Mg increase, is consistent with the garnet growthreaction 2Diop+Al₂Si^–1Mg^–1=2Gros, 1Py.Ca, unlikemost of the elements, shows very flat composition profiles,but with a higher concentration than the initial Ca contentof the unexsolved clinopyroxene. Garnet lamellae are individuallyhomogeneous, but Ca contents vary between lamellae as a functionof lamellae size. In contrast the Fe/Mg distribution coefficientsat interfaces between garnet and clinopyroxene are relativelyconstant irrespective of garnet size. The redistribution of the principal cations—Ca, Fe, Mg,Al, Si—between and within the clinopyrox ene and garnet,during garnet exsolution and cooling, obviously proceeded differentlyfor different elements. Two principal stages in the coolinghistory may be identified: (1) The growth of the sets of garnetlamellae controlled by Al₂Mg^–1Si^–1 redistributionin clinopyroxene. This redistribution was both part of the nettransfer reaction creating garnet, and an exchange reactionin clinopyroxene essential for the diffusional transport ofAl to the growing garnet. Al diffusion in clinopyroxene wasprobably the rate-limiting step, and all other cations, Ca andFe as well as Mg and Si, were mobile during this stage. (2)The occurrence, after the cessation of garnet growth, of diffusionof Fe, Mg and Ca in garnet and interdiffusion of Fe-Mg in clinopyroxene.This resulted in the setting of the KD Fe-Mg at the Cpx-Gt interfacesto a roughly constant value equivalent to approximately 1000?C,which is taken to be the final (‘freezing-in’) temperaturefor redistribution of any elements. During this post garnet-growthstage Ca also became homogenized within individual garnet lamellae,but there is no evidence of Ca equilibration with the clinopyroxene.Under the P-T conditions operating, the initial clinopyroxenecomposition probably resulted in a maximum (M2 site fully occupied)Ca content in clinopyroxene during the stage of garnet growth,and this was maintained during the post-growth stage.     

Amphiboles of a Metagabbro--Amphibolite Sequence, Hidaka Metamorphic Belt, Hokkaido

The western part of the Hidaka Metamorphic Belt, Hokkaido, consistsof primary pyroxene gabbro and lesser amounts of olivine gabbrothat have been dynamically metamorphosed to metagabbro—gabbroicamphibolite-amphibolite-epidote amphibolite during uplift andshearing about 23 m.y. ago. Textures and the presence of relic and recrystallized amphiboleand plagioclase in the same rock indicate incomplete reactionand non attainment of equilibrium during recrystallization. EPMA and bulk analyses of 165 amphiboles indicate a continuousoverall compositional range from actinolite to dark green hornblende(with 100 mg/(Mg+Fe²⁺+Fe³⁺Mn) ratios varying from 89.5 to 32.0)marked by increasing Al, Fe, Ti, and Na. A compositional gapis usually present between relic and recrystallized amphibolesin any one rock which becomes more prominent with increasingshearing. In addition to host rock chemical control, amphibole compositionis largely dependent on the An content of coexisting plagioclase.Locally epidote and sphene exert a strong influence on bothamphibole and plagioclase compositions. Amphibole Ti and Mncontents decrease with shearing and Fe enrichment of the hostrocks largely as a result of the incoming of rutile, sphene,and Fe-Ti oxides. Analysis of host rock oxidation ratio andamphibole compositions indicates that the rocks essentiallybehaved as closed systems to oxygen during metamorphism. Al^1V-Al^IV, Al^IV-Fe³⁺, and Al^IV-(Na, K)^A are the main substitutionsin the amphiboles. Within any one rock the recrystallized amphibolesare enriched in Al, Fe, Ti, and Na relative to the relice amphiboles.Increasing metamorphism results in a progressive change of amphiboles(recrystallized) to more Fe and Si (rather than Al) rich compositionsreflecting the trend towards greenschist where Fe-actinolite(+Mg chlorite) would be stable. Differentiation of the amphiboles is within the limits of SiAlreplacement and the compositional limits of the early stagereaction rim and replacement amphiboles in the host olivineand pyroxene metagabbros.     

Experimental investigation of the origin of majoritic garnet inclusions in diamonds

 Multianvil experiments were carried out at 10–15 GPa and 1600–1700 °C to match the compositions of majoritic garnet inclusions from diamonds, and to determine the compositions of other phases potentially coexisting with these inclusions in the source. Most experiments produced coexisting majoritic garnet, diopsidic clinopyroxene, one or more (Mg,Fe)₂SiO₄ polymorphs, and quenched carbonatic melt. The experimental garnets had relatively high Ca and Fe contents similar to the observed Ca and Fe contents of the inclusions. The resulting Si contents confirmed that the depth of origin of the inclusion with the highest Si content did not exceed 410 km, thus none of the majoritic garnet inclusions found so far originated in the transition zone (410–660 km). The evidence from inclusions and experiments is consistent with the presence of an eclogite layer occurring globally between 200 and 410 km. Compositional variations observed among more than 100 majoritic garnet inclusions with their Si content, which is a measure of pressure and depth, are consistent with the origin of the eclogite layer by crystal fractionation in a magma ocean. The compositions of olivine coexisting with majoritic garnet in the experimental products had the average Fe/(Fe + Mg) ratios between 0.16 and 0.28. Inclusions with such high Fe contents have not been found; the Fe/(Fe + Mg) ratio of the olivine inclusions in diamonds usually varies between 0.05 and 0.09. Hence, the mantle between 200 and 410 km may not contain olivine. In the absence of olivine, the discontinuity at 410 km is most likely a chemical boundary between the 200-km-thick eclogite layer and a more mafic transition zone. Received: 15 March 2001 / Accepted: 14 September 2001     

Pressure--Temperature--Time Paths of Regional Metamorphism I. Heat Transfer during the Evolution of Regions of Thickened Continental Crust

The development of regional metamorphism in areas of thickenedcontinental crust is investigated in terms of the major controlson regional-scale thermal regimes. These are: the total radiogenicheat supply within the thickened crust, the supply of heat fromthe mantle, the thermal conductivity of the medium and the lengthand time scales of erosion of the continental crust. The orogenicepisode is regarded as consisting of a relatively rapid phaseof crustal thickening, during which little temperature changeoccurs in individual rocks, followed by a lengthier phase oferosion, at the end of which the crust is at its original thickness.The principal features of pressure—temperature—time(PTt) paths followed by rocks in this environment are a periodof thermal relaxation, during which the temperature rises towardsthe higher geotherm that would be supported by the thickenedcrust, followed by a period of cooling as the rock approachesthe cold land surface. The temperature increase that occursis governed by the degree of thickening of the crust, its conductivityand the time that elapses before the rock is exhumed sufficientlyto be affected by the proximity of the cold upper boundary.For much of the parameter range considered, the heating phaseencompasses a considerable portion of the exhumation (decompression)part of the PTt path. In addition to the detailed calculationof PTt paths we present an idealized model of the thickeningand exhumation process, which may be used to make simple calculationsof the amount of heating to be expected during a given thickeningand exhumation episode and of the depth at which a rock willstart to cool on its ascent path. An important feature of thesePTt paths is that most of them lie within 50 °C of the maximumtemperature attained for one third or more of the total durationof their burial and uplift, and for a geologically plausiblerange of erosion rates the rocks do not begin to cool untilthey have completed 20 to 40 per cent of the total uplift theyexperience. Considerable melting of the continental crust isa likely consequence of thickening of crust with an averagecontinental geotherm. A companion paper discusses these resultsin the context of attempts to use metamorphic petrology datato give information on tectonic processes.