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.     

Petrology of Rhyolitic and Mixed Magma Ejecta from the 1875 Eruption of Askja, Iceland

Volcanic activity in Askja central volcano and its fissure swarmin 1875 occurred in response to a crustal rifting episode inIceland, resulting in up to 70 km lateral flow of magma withinthe crust, caldera collapse and a plinian eruption of acid magma(0·2 km³ dense-rock equivalent). Petrologic studies ofthe predominantly rhyolitic and crystal-poor ejecta reveal thata complex array of other liquid compositions was also present,including icelandite (0.75 per cent) and basalt (1.9 per cent),as well as leucocratic xenoliths of trondhjemite type. Mineralgeothermometers indicate that the rhyolite evolved at 990 to1010 °C and 0·5 Kb P_H2O, the icelandite at 1005 to1020 °C and at f_O2 10^–10 atm. and the basalt at 1140to 1170 °C. A petrologic model of Askja in 1875 consists of a density-stratifiedmagma chamber with a rhyolitic upper part and a lower part offerrobasalt, with an intervening layer of icelandite. The modelcalculations show that the icelandite can be derived from ferrobasaltby 50 per cent fractional crystallization, but one-stage fractionalcrystallization models cannot account for generation of theacid magma. Simple partial or complete fusion of the field-associatedtrondhjemite xenoliths cannot produce the acid magma. Instead,a more complex fusion, hybridization and fractional crystallizationmodel is presented, which is consistent with the available petrologicevidence. This model involves large-scale fusion of pre-existingtrondhjemite intrusions or reactivation of previously consolidatedroof-rock in the magma chamber followed by hybridization ofthe acid magma with 7 to 14 per cent basaltic magma. Finally,10 to 11 per cent fractional crystallization of the dacite hybridis required to produce the observed compositional range withinthe rhyolite ejecta. The 1875 explosive eruption was causedby the ascent of tholeiitic basalt magma from depth during crustalrifting. Influx of new basalt magma in 1874–75 triggeredconvective mixing and hybridization in the compositionally zonedmagma chamber.     

The Solubility of Water and Effects of Oxygen Fugacity and Water Content on Crystallization in Mafic Magmas

The solubility of water in a basaltic and in an andesitic melthas been determined in the pressure range from approximately1,000 to 6,000 bars at 1,100? C. The solubility in basalticmelt is 3.1 weight percent at 1,000 bars and 9.4 weight percentat 6,000 bars; in the andesitic melt it is 4.5 weight percentat 1,000 bars and 10.1 weight percent at 5,300 bars. The temperaturesof appearance of the primary, secondary, and tertiary phasesin the basalt have been determined at 1,000 bars water pressureand at the fo₂'s of the magnetite+hematite (MH), magnetite+fayalite+quartz(MFQ) and magnetite+w?stite (MW) buffers. Results are as follows: Buffer Pyroxene Plagioclase Ore mineral MH 1,095?C 1,065?C 1,230?C MFQ 1,040? 1,015? 1,010? MW 1,020? 1,010? 995? A comparison of the solubility of water at 1,100?C and up toapproximately 6,000 bars pressure in several silicate melts,ranging in composition from granitic to gabbroic, indicatesthat the spread of solubility is narrower than has been supposed.The marked effect of f_o2's on the crystallization sequence inthe Columbia River basalt confirms the importance of this factorin determining liquid lines of descent. In experiments withlow f_o2's (MW buffer) and 1,000 bars water pressure, the basaltwas completely liquid at the relatively low temperature of 1,020?C.     

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.     

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

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

Manam Island, Papua New Guinea: Petrology and Geochemistry of a Low-TiO2 Basaltic Island-Arc Volcano

Manam volcano consists of relatively mafic and compositionallysimilar tholeiitic basalts and low-SiO₂ andesites that are characterizedby notably low (mainly 0?3–0?35 weight per cent) TiO₂contents. These rocks provide an ideal opportunity to investigateboth the extent of depletion in their peridotite magma-sourceregions (which are evidently similar in many respects to thehighly depleted sources of boninitic magmas), and the interplayof the high-level processes of magma mixing, crystal fractionation,and upper crustal contamination, in an island-are volcano. Manamrocks have pronounced enrichments in Rb, Ba, K, and Sr relativeto the light rare-earth elements and, especially, to the high-field-strengthelements (Sr/Ti values are exceptionally high). However, thereis no compelling evidence that these enrichments were causedby addition of a hydrous, slab-derived component to the peridotitesource region. Nd and Sr-isotope ratios plot within the oceanicmantle array; ²⁰⁷Pb/²⁰⁴Pb values are only slightly higher thanthose for oceanic rocks; and the absence of hydrous minerals,the early crystallization and modal preponderance of plagioclaseover pyroxene, high estimated quenching temperatures, and lowwater contents in the Manam rocks, are all evidence that themagmas crystallized under markedly water-undersaturated conditions.Unusually anorthite-rich plagioclase phenoerysts in the morediffrentiated rocks may correspond to crystallization underhigher water-vapour pressures, possibly caused by influxes ofgroundwater, or they may be accidental xenocrysts. Fractionationof olivine, clinopyroxene, and spinel (early chromite followedby magnetite) has dominated the evolution of the magma series.However, clear correlations between incompatible trace-elementratios, ⁸⁷Sr/⁸⁶Sr, and 100 Mg/(Mg ? Fe^2?) values are convincingevidence for an accompanying mixing process—either of(1) two basaltic magma types (one more fractionated and lowerin ⁸⁷Sr/⁸⁶Sr than the other), or (2) pristine magmas and contaminantfrom basaltic conduit and reservoir wall rocks. Wall-rock contaminationis the less likely process, and is the more difficult one toidentify, particularly if it accompanied magma mixing.     

Phase Relations on the Actinolite-Pargasite Join

Phase relations along the join Ca₂Mg₄Fe²⁺Si₈O₂₂ (OH)₂ (Actinolite)-NaCa₂Mg_3?2Fe_0?8²⁺AlSi₆Al₂O₂₂(OH)₂ (Pargasite) have been studied at P_H2O = 1 kb andthe oxygen fugacities defined by the iron-wustite(IW) buffer. Actinolite and bornblende are separated by a solvus and thefield of actinolite+hornblende+vapor is present in the regionbetween Ac₈₅Pa₁₅ and Ac₅₅ Pa₄₅ at 680 ?C. Complete miscibilityis achieved at 720 ?C. At temperatures higher than the solvusthere is a continuous solid solution series between the twoend members. The stability field of amphibole solid solutiongradually increases with increasing pargasite content in actinolite.The phase assemblages at temperatures higher than those of asolid solution series between the two end members change withincreasing pargasite content in the bulk composition as follows;Act+Cpx+Qz+V, ActHbl+Cpx+Opx+Qz+V, Hbl+Cpx+Opx+Pl+V and Hbl+Cpx+Pl+Ol+V. In comparison with the Fe-free system, the extent of the miscibilitygap between actinolite and hornblende is reduced by an increasein the Fe²⁺ content. The present study should provide an adequatebasis for the interpretation of actinolite-hornblende pairsin metamorphic rocks.     

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     

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.     

An Experimental Investigation of the Influence of Water and Oxygen Fugacity on Differentiation of MORB at 200 MPa

Crystallization experiments were performed at 200 MPa in thetemperature range 1150–950°C at oxygen fugacitiescorresponding to the quartz–fayalite–magnetite (QFM)and MnO–Mn₃O₄ buffers to assess the role of water andf_O2 on phase relations and differentiation trends in mid-oceanridge basalt (MORB) systems. Starting from a primitive (MgO9·8 wt %) and an evolved MORB (MgO 6·49 wt %),crystallization paths with four different water contents (0·35–4·7wt % H₂O) have been investigated. In primitive MORB, olivineis the liquidus phase followed by plagioclase + clinopyroxene.Amphibole is present only at water-saturated conditions below1000°C, but not all fluid-saturated runs contain amphibole.Magnetite and orthopyroxene are not stable at low f_O2 (QFM buffer).Residual liquids obtained at low f_O2 show a tholeiitic differentiationtrend. The crystallization of magnetite at high f_O2 (MnO–Mn₃O₄buffer) results in a decrease of melt FeO^*/MgO ratio, causinga calc-alkaline differentiation trend. Because the magnetitecrystallization temperature is nearly independent of the H₂Ocontent, in contrast to silicate minerals, the calc-alkalinedifferentiation trend is more pronounced at high water contents.Residual melts at 950°C in a primitive MORB system havecompositions approaching those of oceanic plagiogranites interms of SiO₂ and K₂O, but have Ca/Na ratios and FeO^* contentsthat are too high compared with the natural rocks, implyingthat fractionation processes are necessary to reach typicalcompositions of natural oceanic plagiogranites. KEY WORDS: differentiation; MORB; oxygen fugacity; water activity; oceanic plagiogranite     

Distribution of the Period Four Transition Elements among Olivine, Calcic Clinopyroxene and Mafic Silicate Liquid: Experimental Results

The distribution of Ti⁴⁺, V³⁺, Cr³⁺, Mn²⁺, Fe(total), Co²⁺ andNi²⁺ among synthetic olivine, calcic clinopyroxene and maficsilicate liquid has been studied between 1125 and 1250 ?C underanhydrous conditions at 1 bar total pressure. The distributionof iron and magnesium among the three phases was concluded tobe independent of temperature and may be described by the twoequations Titanium and vanadiumdid not enter olivine in significant amounts. The mean valueof the ratio (wt. per cent TiO₂ in Cpx)/(wt. per cent TiO₂ inL) was 0.29?0.04 for assemblages in which the liquid had botholivine and hypersthene in the norm but the ratio was greaterif the liquid was nepheline normative. Vanadium was concentratedin the pyroxene in some experiments and in the liquid in others,but it was not possible to conclude whether the change in distributionbehavior was due to varying temperature or changing liquid composition.Equilibrium partitioning of chromium was not achieved but theresults indicate that Cr₂O₃ was most strongly enriched in clinopyroxeneand showed a slight preference for olivine over the liquid.The divalent transition elements were each enriched in olivinerelative to clinopyroxene and the degree of enrichment increasedin the order predicted by crystal field theory. The mean (wt.per cent oxide in 01)/(wt. per cent oxide in Cpx) ratios were2.0 for MnO, 2.4 for FeO, 3.9 for CoO and 5.6 for NiO. Manganesewas enriched in olivine relative to the liquid and in the liquidrelative to the clinopyroxene. Cobalt and nickel were more concentratedin the crystalline phases than in the liquid but the degreeof enrichment was markedly less in the experiments in whichthe liquids were more mafic.     

The role of H2O during crystallization of primitive arc magmas under uppermost mantle conditions and genesis of igneous pyroxenites: an experimental study

Exposed, subduction-related magmatic arcs commonly include sections of ultramafic plutonic rocks that are composed of dunite, wehrlite, and pyroxenite. In this experimental study we examined the effects of variable H₂O concentration on the phase proportions and compositions of igneous pyroxenites and related ultramafic plutonic rocks. Igneous crystallization experiments simulated natural, arc magma compositions at 1.2 GPa, corresponding to conditions of the arc lower crust. Increasing H₂O concentration in the liquid changes the crystallization sequence. Low H₂O concentration in the liquid stabilizes plagioclase earlier than garnet and amphibole while derivative liquids remain quartz normative. Higher H₂O contents (>3%) suppress plagioclase and lead to crystallization of amphibole and garnet thereby producing derivative corundum normative andesite liquids. The experiments show that alumina in the liquid correlates positively with Al in pyroxene, as long as no major aluminous phase crystallizes. Extrapolation of this correlation to natural pyroxenites in the Talkeetna and Kohistan arc sections indicates that clinopyroxenes with low Ca-Tschermaks component represent near-liquidus phases of primitive, Si-rich hydrous magmas. Density calculations on the residual solid assemblages indicate that ultramafic plutonic rocks are always denser than upper mantle rocks in the order of 0.05 to 0.20 g/cm³. The combination of high pressure and high H₂O concentration in the liquid suppresses plagioclase crystallization, so that ultramafic plutonic rocks form over a significant proportion of the crystallization interval (up to 50% crystallization of ultramafic rocks from initial, mantle-derived liquids). This suggests that in subduction-related magmatic arcs the seismic Moho might be shallower than the petrologic crust/mantle transition. It is therefore possible that calculations based on seismic data have overestimated the normative plagioclase content (e.g., SiO₂, Al₂O₃) of igneous crust in arcs.     

Solidification History of the Kitdl?t Lens: Immiscible Metal and Sulphide Liquids from a Basaltic Dyke on Disko, Central West Greenland

A unique troilite-iron-cohenite-rich lens was found on a ledgein the sediment-contaminated Kitdlit dyke on Disko. The lensdisplays a mineralogical diversity and extreme liquid evolutionnot previously described from any single magmatic sulphide ormetal liquid system. It was deposited at 1200 ?C and fo2 = 10–¹³as two immiscible liquids–a sulphide liquid and a C-richmetal liquid—which solidified from about 1100? to 300?C as a closed system in a thermal gradient. The lower iron-rich half of the lens formed by crystallizationof iron, cohenite, troilite, schreibersite and w?stite (in thatorder), with trace amounts of the phases found in the upperhalf of the lens. The approximate crystallization order of theupper troilite-rich half of the lens was: iron, cohenite, troilite,chromite, w?stite, fayalite (pseudomorphosed), high-T ‘chalcopyrite’,high-T ‘heazlewoodite’ (beta-(Ni, Fe, Cu, Co)_3+?S₂)and, finally, lead minerals. The latter comprise native lead,galena, altaite (PbTe), shandite (Ni3Pb2S2) and an unidentifiedphase. Additional immiscible liquids were formed during solidification.Oxysulphide liquid (roughly FeO with a minor FeS-component)exsolved from metal and sulphide liquid in equilibrium withiron and cohenite below 1100 ?C. Later, FeO-rich silicate liquid,and Pb-rich liquid with Ni, Cu, S and Te, exsolved from thesulphide liquid. Segregation of oxysulphide and Pb-rich liquidmay occur during core formation in planetoids, and there arethus important cosmochemical implications.     

P-T-X Relationships Deduced from Corona Textures in Sapphirine--Spinel--Quartz Assemblages from Paderu, Southern India

The sapphirine (Sa)-spinel (Sp)-quartz (Qz)-bearing rocks fromPaderu occur as lenticular enclaves within the Precambrian khondalite-charnockiteterrane of southern India. In addition these rocks contain orthopyroxene(Opx), sillimanite (Sill), garnet (Gt), cordierite (Cd), biotite,potash feldspar (Kf), plagioclase, and symplectites of Cd-Kf-Qz-Opx.The symplectites may have formed from the breakdown of osumilite.Grain contacts of sapphirine and spinel with quartz are rarelyobserved and the incompatibility with quartz during later stagesis displayed by the development of several types of polymineralicreaction coronas. The coronas in the different rock types A,B, etc. are (minerals listed from core to rim of corona): (A-1) sapphirine-bearing rock type without spinel: Sa-Sill-Opx,Sa-Sill-Cd, Sa-Cd-Opx (A-2) sapphirine and spinel-bearing: Sp-Sa-Sill-Opx-Qz, Sp-Sa-Sill,Sp-Sa-Opx, Sp-Sill-Opx, Sp-Sa-Sill-Gt-Qz, Sa-Sill-Opx, Sp-Sa-Sill-Opx,Sa-Sill-Opx-Gt, Sp-Sa-Opx-Gt, Sp-Sa-Sill-Gt; and (B) spinel-bearingbut sapphirine free: Sp-Sill-Opx, Sp-Sill-Gt, Sp-Cd. Commonlythe coronas in the rock type A 2 and B also contain ilmeno-hematite?corundumin the core in association with spinel. These rock types alsoprovide textural evidence for later crystallization of Cd, Cd+ Sa, and Gt + Qz from Opx+Sill?Qz and Gt+Sill+Qz. Sapphirine is aluminous (near 7(Mg, Fe²⁺)O?9(Al, Fe³⁺)₂O₃?3SiO₂)and contains up to 12?2 wt. per cent iron as FeO. Orthopyroxeneis also aluminous, containing up to 10?4 wt. per cent Al₂O₃.Sapphirine and spinel have relatively high contents of Fe₂O₃.X_Mg in the Fe-Mg minerals increases from rock type B to A2 toA1. A sequence of reactions has been deduced from coronas and otherreaction textures, and from the phase compatibility relationsin the FeO-MgO-Al₂O₃-SiO₂-H₂O system. The P-T-X relationshipsfrom geothermobarometry and petrogenetic grids, viz. µ_Fe2O3vs. µ_FeO and µ_H2O vs. µ_Fe2O3, suggest: (1)a retrograde, mildly decompressive trajectory from 900?60?C/65?0?7kb (core) to 760?50?C/5 ? 0?6 kb (rim); and (2) the observedmineralogy of the coronas and reactions deduced from them aredependent on the relative FeO, Fe₂O₃, and H₂O contents of therocks (µ_FeO3, µ_Fe2O3), and µ_H2O).     

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.     

Major Addition of Magma at the Pyroxenite Marker in the Western Bushveld Complex, South Africa

A major, but gradual, reversal in the cryptic variation patternof the plagioclase and pyroxenes, of 13 mol% anorthite and 10mol% Mg/ (Mg + Fe) respectively, is documented in the Main Zoneof the western Bush veld Complex. These changes are accompaniedby a decrease in initial ⁸⁷Sr/⁸⁶Sr ratio from > 0.708 to< 0.707. The Pyroxenite Marker, a distinctive orthopyroxenitelayer, occurs close to the top of this reversed differentiationsequence. This is attributed to addition of less differentiatedmagma. On the basis of a mass balance calculation of the initial⁸⁷Sr/ ⁸⁶Sr ratios, it is estimated that the volume of magmaadded was comparable to that of the resident magma. Increases in the Fe₂O₃, TiO₂, Al₂O₃, and Na₂O contents of thepyroxenes above the level of magma addition indicate that thenew magma had a lower silica activity and higher fO₂ than theresident magma. Quantification of the trace element and REEcontent of the two magmas is hampered by the very low proportionof trapped intercumulus component in these adcumulate rocks.However, semi-quantitative modelling indicates that the traceand REE signatures of the two magmas were similar, with moderateLREE enrichment and flat HREE profiles. The new magma had aslightly higher La/ Sm ratio than the resident magma, consistentwith its more alkaline nature. The new magma was probably added gradually, while 100–150m of cumulates formed. It probably intruded at an intermediatelevel within an existing stratified magma chamber, where itcooled and crystallized, and composite packets of liquid pluscrystals plunged to the base of the chamber. The cores of plagioclasegrains formed during this mixing interval show a wider rangeof compositions than in other sections, and plagioclase primocrystsfrom both magmas may be preserved within single samples. Therefore,although intimate physical mixing of packets of unknown sizeof the two magmas occurred, re-equilibration of the major oxidecomposition of the plagioclase primocrysts was not achieved.However, the data and calculations based on diffusion ratesindicate that partial Sr isotopic resetting of plagioclase mayhave occurred.     

Primary Igneous Graphite in Ultramafic Xenoliths: II Isotopic Composition of the Carbonaceous Phases Present in Xenoliths and Host Lava at Tissemt (Egg?r?, Alegerian Sahara)

¹³C/¹²C ratios have been analysed in graphite adcumulate crystalsassociated with the olivine and orthopyroxene of a suite ofultramafic xenoliths in the alkali basalts of Tissemt (Egg?r?,Algerian Sahara). The ¹³C of these graphites varies from –24?6to – 14?4 per milk with internal variations of up to 1?6per mille within a given sample. Carbonates, linked to the volcanicphase and precipitated at medium temperatures have ¹³C valuesfrom –0?8 to + 3?5 and ¹⁸O values from 21?6 to 31?5 Thehost lava has a high carbon content of 675 p.p.m. with a global¹³C of –4?2. It can be separated into three parts: a majorpeak is obtained between 670 and 985 ?C and also liberated byslow acid attack with a ¹³C of up to +7?3 and representing CO₂trapped in the natrolite; a high temperature peak (above 1160?C with 47 p.p.m at –25?8) will represent carbon trappedin silicates and there is an additional carbon not well determinedextracted from 300 ?C to high temperature which represent 215p.p.m. at –23?0 with a possible fraction as low as –68?0.The graphite is interpreted as the result of the precipitationof carbon from an interstitial liquid previously outgassed thensupersaturated by the ongoing crystallization process. The volcanicevent can be considered to a first approximation as remobilizingthe carbon from the xenoliths by oxidation to CO₂, subsequentpartial reduction upon cooling and precipitation of carbonateand trapping of residual CO₂ within zeolite. Only volatilereducedspecies can be significantly outgassed so that the total ¹³Cof –4?2 is probably close to (or a little higher than)the original ¹³C.     

Experimental Phase Relations of Water- and Sulfur-Saturated Arc Magmas and the 1982 Eruptions of El Chich?n Volcano

The equilibrium phase relations of two volcanic rocks from thesubduction-related Mexican Volcanic Belt have been determinedwith an argon-pressurized internally heated vessel. One rockis the trachyandesite erupted from El Chich?n Volcano in 1982;the other is a primitive basalt erupted from Jorullo Volcanoin 1759. A simplified synthetic equivalent to the trachyandesitewas also investigated in lesser detail. All charges were saturatedwith hydrous vapor and a sulfur-bearing mineral. Temperatureranged from 800 to 1000?C, pressure from 1 to 4 kb, and f_o2was controlled by four different solid oxygen buffers in a doublegold capsule configuration: fayalite-magnetite-quartz (FMQ),Ni-NiO (NNO), manganosite-hausmanite (MNH), and magnetite-hematite(MTH). Pyrrhotite was the only sulfur-bearing mineral observed in chargesbuffered under FMQ and NNO, whereas anhydrite crystallized underthe more oxidizing MNH or MTH; both of these observations areconsistent with those of earlier workers. With increasing temperatureand pressure, SiO₂ and K₂O decreased in the experimental melts,whereas Al₂O₃ and CaO increased. Sulfur solubility in silicatemelts was low (<0?1 wt% equivalent SO^t₃) for pyrrhotite-saturatedcharges, but significantly greater (to 1?3 wt.% SO^t₃) when anhydritewas present. Sulfur solubility in anhydrite-saturated meltsshowed strong positive dependence on both temperature and P_vapor. Sulfur amounted to some 2?5 wt.% (SO^t₃) of the total ejectaduring the 1982 El Chich?n eruptions, and the original magmaticsulfur content was in the range 1?25–2–5 wt% SO^t₃.Extrapolations of experimental temperature and pressure dependencesfor sulfur solubility indicate that such concentrations couldbe contained in a hydrous, oxidized, basaltic parent melt generatedunder Benioff zone conditions. During ascent through the uppermantle and crust, the sulfur solubility limit of the melt wouldcontinuously decrease; in response, most of the sulfur wouldbe transferred from the melt to anhydrite crystals and a separategas phase. Trachyandesite pumices erupted from El Chich?n in1982 contained both pyrrhotite and anhydrite at a temperatureof 800?C. The composition of the natural pyrrhotite yieldedan f_o2 estimate 1 log unit above the NNO buffer. Based on compositionalvariations in the experimental melts with temperature and pressure,the composition of the matrix glass in the 1982 pumices indicatesequilibration of the magmatic liquid at about P_total=P_vapor=2kb just before eruption. At that time, sulfur in El Chich?ntrachyandesite was about equally partitioned between anhydritemicrophenocrysts and some 20 vol.% gas phase in which H₂S wasprobably the dominant sulfur-bearing species. The melt thencontained only 0?05 wt.% SO^t₃, consistent with experimentalsolubility limits at 800?C and P_vapor=2 kb.     

Phase Equilibria in Three Assemblages of Kyanite-Zone Pelitic Schists, Lincoln Mountain Quadrangle, Central Vermont

Petrologic and geologic arguments suggested that a close approachto chemical equilibrium at a single temperature and pressuremight be found in the rocks near Mt. Grant in central Vermont.An area 2,800 feet by 4,000 feet was selected and studied indetail. The major assemblages are: kyanite-chloritoid-chlorite-quartz-muscovite-paragonite-rutile;garnet-chloritoid-chlorite-quartz-muscovite-paragonite-ilmenite;and garnet-chlorite-biotite-quartz-muscovite-albite-ilmenite.All minerals were separated from a primary sample for each ofthese assemblages and purified, complete gravimetric and spectrographicanalyses performed, and optical properties, X-ray properties,and densities measured. Chlorite, garnet, and chloritoid orbiotite were separated from an additional nine samples of theseassemblages and spectrographic and partial gravimetric analysesperformed. Distribution coefficients of each mineral pair for Mg/Fe andMn/Fe+ Mg and for the minor elements are similar in nearly everysample for a given assemblage. Distribution coefficients forgarnet-chlorite differ in the two assemblages which containthis pair; this difference is attributed to the difference inAl-content of the chlorite from the two assemblages. The smallrange of distribution coefficients, despite a wide range inthe relative proportions of the ferromagnesian phases, is convincingevidence that an equilibrium partition of the major and minorcations between the phases in each sample had been attainedand that the equilibration temperature was the same for eachsample. The coexistence of kyanite+muscovite+quartz and comparison ofthe composition of coexistent Ca-free muscovite and paragoniteand of O¹⁸/O¹⁰ ratios for coexistent quartz and magnetite withexperimental values indicate that these rocks formed at P_s 11 kb, T 550? C, and aH₂o low enough (Pe(H₂o) sufficiently lessthan P_s) to depress the pyrophyllite breakdown temperature byabout 40? C. The cations and the ao₂ are equilibrated locally(within each sample), but all the samples have equilibratedto a common O¹⁸/O¹⁶ ratio. A possible explanation is that afluid medium had sufficient oxygen in H₂O to control the O¹⁸/O¹⁶ratio of the rock and its phases, but that the rock system hadsufficient buffer capacity to control the ao₂ of the fluid.     

Kiglapait Geochemistry I: Systematics, Sampling, and Density

The Kiglapait intrusion affords many opportunities for evaluatingplutonic fractionation processes. Estimates of boundary conditionsinclude emplacement of anhydrous high alumina basaltic magmaat 4 kbar and about 1250 °C, initial crystallization nearthe WM buffer, and fractionation to Mg-free ferrosyenite whichcrystallized at 960°C, somewhat above the WM buffer. Thelast ferrosyenites represent fractionation to 0.01 per centof the initial volume. Plagioclase varies from An₆₇ to An₁₀, olivine from Fo₆₉ to Fo₀,and augite from En₇₃ to En₀. The specific gravity of the intrusionis 2.93, varying between 2.87 and 3.18 on smoothed models. Crystals accumulated chiefly at the floor. As they did so, themagma depth decreased as the square root of the volume fractionof liquid. The volume fraction solidified was roughly proportionalto time. Cooling was slower than the t relation because of hotsurroundings on one side. The crystallization time was about10⁶ yr, and the average accumulation rate was about 1 cm/yr.Average crystal concentrations of 3–300 ppm are impliedfor the nucleation zone. The cooling rate corresponds to crystallizationof 2 x 10⁷ kg/yr per km² roof area, about 550 times slower thana lava lake. Calculated liquid densities range from 2.67 to 2.88 g/cm³ athigh temperatures. Feldspar almost surely did not sink in themagma, but nevertheless it accumulated mainly on the floor.Cooperative sinking with mafics as proposed by Coats, combinedwith oscillatory nucleation as proposed by Wager, may accountfor this paradox. Oscillatory nucleation leading to feldspar-supersaturatedliquids is supported by laboratory evidence on the feldspar-likestructure of liquids and the concave-úp plagioclase liquidusin systems involving olivine. Both lines of evidence imply highpolymerization of feldspar-rich liquids, particularly in slowprocesses. Such polymerization can help to explain the genesisof primary Eu anomalies and anorthositic magmas in additionto the floor accumulation of feldspar and rhythmic layering.