Late Devonian Diamondiferous Kimberlite and Alkaline Picrite (Proto-kimberlite?) Magmatism in the Arkhangelsk Region, NW Russia

Widespread penecontemporaneous igneous activity affected NWRussia (the Kola Peninsula and adjoining areas to the SE aroundArkhangelsk) during the Late Devonian (360–380 Ma). Magmatismvaries from tholeiitic basalts, erupted in the axial regionsof former Middle Proterozoic (Riphean) rifts, to strongly alkalinerock-types on and marginal to Archaean cratons. NNE of Arkhangelskkimberlites, olivine lamproites and alkaline picrites were emplaced;all these rock-types are diamondiferous to varying extents.Higher TiO₂ (and also total Fe) distinguish predominantly mica-poorEastern Group kimberlites (TiO₂ = 2·4–3·1wt %) and spatially associated alkaline picrites (TiO₂ = 3·2–3·7wt %) from nearby micaceous Western Group kimberlites (TiO₂= 0·8–1·1 wt %). Each rock-type also hasdistinctive rare earth element (REE) patterns, and     

Selected Phase Relationships in the System Al-Mn-Fe-Si-O-H: A Model for Garnet Equilibria

The bulk compositions 3FeO_x.Al₂O₃.3SiO₂ $ excess H₂O and 3MnO.Al₂O₃.3SiO₂$ excess H₂O were investigated employing conventional hydrothermaltechniques. Almandine and spessartine were synthesized and stabilityrelationships determined in terms of temperature, fluid pressure,and oxygen fugacity. Synthetic almandine has unit cell edge, a₀ = 11.528 0.001 index of refraction, N_D = 1.829 0.003. No systematic variationsof these values with respect to temperature, fluid pressure,and oxygen fugacity were observed. Spessartine, synthesizedat high temperatures, has average values of a₀ = 11.614 0.001 and N_D = 1.799 0.003. However, below about 600 C a₀ graduallyincreases to 11.635 0.001 and N_D decreases to 1.772 0.003with decreasing temperature, irrespective of fluid pressureand oxygen fugacity. These changes appear to reflect the productionof hydrospessartine below about 600 C. The stability of almandine strongly depends on the oxygen fugacity.It is stable up to the vicinity of oxygen fugacities definedby the fayalite–magnetite$quartz buffer; the low f_o2,range has not been determined, but lies at oxygen fugacitiesless than those defined by the ironquartz–fayalite buffer.The stability field of almandine$fluid is bounded by the followingP_fluid-T values. At low oxidation states, the low temperature hydrous assemblageof equivalent composition consists of quartz$iron chlorite ($magnetite)$fluidand the high temperature equivalent assemblage consists of fayalite$ironcordierite$hercynite₈₈$fluid. Where f_O2 approximates or is inexcess of that defined by the fayalite–magnetite$quartzbuffer the low temperature hydrous assemblages consist of quartz$ironchlorite$magnetite$fluid, iron chlorite$pyrophyllite$magnetite$fluid,magnetite$mullite$pyrophyllite$fluid, and hematite$mullite$pyrophyllite$fluid;the anhydrous equivalent assemblages consist of quartz$hercynite₈₈,$magnetite₈₈$fluid, quartz$mullite$magnetite$fluid, and quartz$mullite$hematite$fluid,both in order of increasing oxygen fugacity. The stability of spessartine, in contrast to that of almandine,is essentially independent of oxygen fugacity at least up tothat defined by the magnetite-hematite buffer. Spessartine isstable up to the highest temperature, 930 C, employed in thisinvestigation at P_fluid = 500 bars. However, it decomposes toa hydrous assemblage consisting of quartz$manganese chlorite$fluidat the following P_fluid-T values: 414 5 C and 3000 bars;405 5C and 2000 bars; 386 10 C and 1000 bars; 3645C and 500 bars. Garnets are rare constituents of igneous rocks; those whichdo occur are predominantly spessartine-rich, and are virtuallyconfined to felsic magmas. Garnets are absent from mafic igneousrocks because the thermal stability ranges of iron-rich membersare below the solidus. The near absence of almandine in contactmetamorphosed pelitic rocks may reflect a relatively high oxidationstate in the aureoles rather than inappropriate P-T conditions.It is argued that the compositions of pyralspite garnets inpelitic schists are subject to various physical and chemicalfactors, including f_O2. With appropriate provisions, the Mn/Feratios of garnet coexisting with chlorite and quartz might beused as a temperature indicator. The rarity of spessartine in igneous and metamorphic rocks apparentlystems from the departure of rock bulk composition from Mn-richvalues rather than from the absence of appropriate physicalconditions.     

Lawsonite-Omphacite-Bearing Metabasites of the Pam Peninsula, NE New Caledonia: Evidence for Disrupted Blueschist- to Eclogite-Facies Conditions

The Diahot terrane of NE New Caledonia contains an interbeddedsequence of Cretaceous to Eocene metasediments, felsic and maficmetavolcanics that experienced c. 40 Ma high-P/T metamorphism.Metabasaltic assemblages define two prograde events (M₁ andM₂) and a tectonically disrupted crustal profile that extendsfrom lawsonite–blueschist conditions in the SW to paragonite–eclogiteconditions in the NE. Weakly deformed metabasalts from lowest-gradeparts of the Diahot terrane contain M₁ omphacite, chlorite,lawsonite and glaucophane-bearing assemblages that partiallypseudomorph igneous plagioclase and augite, and reflect P =0·7–1·0 GPa and T = 350–400°C.M₁ assemblages are enveloped by a steeply SW-dipping S₂ foliationthat becomes progressively more intense towards the NE overa distance of c. 15 km. S₂ assemblages are divided into fourzones: (1) lawsonite–omphacite; (2) lawsonite–clinozoisite–spessartine;(3) clinozoisite–hornblende–almandine; (4) almandine–omphacite.S₂ assemblages reflect a P–T gradient that spans the exposed15 km of the Diahot terrane from P = 0·8–1·0GPa and T = 350–400°C (Zone 1) to P = 1·6–1·7GPa and T = 550–600°C (Zone 4). The systematic mineralogicalchanges reflect parts of a P–T array between 1·0and 1·7 GPa that was extensively disrupted by tectonicthinning during exhumation. KEY WORDS: blueschist; eclogite; New Caledonia; CNFMASH; pseudosection     

Liquidus Phase Relations in the System CaO-MgO-Al2O3-SiO2 at 2{middle dot}0 GPa: Applications to Basalt Fractionation, Eclogites, and Igneous Sapphirine

To model magmatic crystallization processes for mafic to intermediatecompositions at high pressure, liquidus phase relations in theforsterite–anorthite–diopside–silica (FADS)tetrahedron within the CaO–MgO–Al₂O₃–SiO₂system have been determined at 2·0 GPa. Compositionsof five liquidus invariant points have been determined and theapproximate compositions of five others have been inferred.These involve primary phase volumes for forsterite (fo), enstatite(en), diopside (di), high quartz (qz), spinel (sp), sapphirine(sa), garnet (gt), anorthite (an), and corundum (cor). The determined(with wt % coefficients) and inferred reactions (without coefficients)that define each isobaric invariant point are as follows: 23 en + 68 di + 9 sp = 84 liq + 16 fo 37 di + 63 sa = 47 liq + 40 sp + 13 en 100 gt = 21 liq + 27 sa + 55 en + 18 di 1 di + 59 en + 41 an = 43 liq + 57 gt 18 di + 21 qz + 15 en + 47 an = 100 liq di + an + gt = liq + sa an + gt = liq + sa + en sa + an + di = liq + sp sa + an = liq + cor + sp di + cor = liq + an + sp. These phase relations provide a diverse range of constraintson igneous processes at pressures near 2 GPa. They show thatfractional crystallization of a model basalt gives a residualliquid strongly enriched in SiO₂, strongly depleted in MgO,and mildly enriched in Al₂O₃. Such a trend is consistent withthe calc-alkaline fractionation trend observed at subductionzones, but is in disagreement with suggestions that fractionationof tholeiitic basalt in this pressure range yields an alkalicbasalt. Both trends may occur for natural basalts dependingon the Na₂O content of the parental magma. Also, the data showthat the minimum pressure for the formation of cumulate eclogitesand garnet pyroxenites is about 1·8–1·9GPa. The lower limit of pressure at which sapphirine can crystallizefrom a liquid in the FADS tetrahedron is estimated to be 1·1–1·5GPa and the upper limit is >3 GPa. Sapphirine crystallizesfrom magmas intermediate in composition between basalt and andesite.Probable igneous sapphirine in mafic associations is rare, butit occurs as part of a pyroxenite xenolith from Delegate, Australia,that we suggest is a cumulate assemblage and in a sapphirinenorite at Wilson Lake, Labrador, Canada. KEY WORDS: basalt; eclogite; sapphirine; fractional crystallization     

Stability Relations of the Ferruginous Biotite, Annite

Annite, KFe₃AISi₃O₁₀(OH)₂ a member of the iron biotites andthe ferrous analogue of phlogopite, has been synthesized andits phase relations have been determined as functions of temperature,fugacity of oxygen (fo₂), and total pressure (P_totalPH₂O+PH₂).A method for controlling fo₂at high total pressures is described,and data for the ‘oxygen buffers’ used are given.Buffers range from quartz+iron+fayalite assemblages (low fo₂)to magnetite-hematite assemblages (high fo₂). Optical propertiesand unit-cell dimensions of synthetic annites depend on theconditions of synthesis. By recalculating published analyses of natural iron-rich biotitesit can be shown that one cannot assume a constant hydrogen contentfor such biotites. Oxidation may have occurred by drying at115?C. Octahedral occupancy therefore cannot be calculated fromsuch data. Phase relations of annite are presented in 2,070 and 1,035 barsections. Depending on fo₂-T values annite was found to decomposeto one of the following assemblages: hematite+ sanidine, magnetite+sanidine,fayalite+leucite+kalsilite, iron+sanidine. All decompositionsare dehydration and redox reactions and are sensitive to changesin fH₂0 and fo₂ (or fH₂0 and fH₂). At 2, 070 bars total pressureannite+magnetite+sanidine can coexist between 425?C and 825?C, depending upon the magnitude of fo₂. In the presence of quartz the stability field of annite is morerestricted. Phase equilibria in the system KAlSiO₄–SiO₂–Fe–O₂–H₂have been summarized schematically. Wherever possible, thermodynamic extrapolations are made totest the internal consistency of the data. Enthalpies of formationare calculated for both annite and phlogopite. Ranges of fo₂values in nature as well as mechanisms for changes in fo₂ areinvestigated. It is useful to distinguish between assemblageswhich are internally buffered with respect to fo₂changes andthose which are not buffered. The applications of individualreactions involving annite to specific geologic problems arediscussed with respect to igneous, metamorphic, and sedimentaryrocks.     

Phonolitic Diatremes within the Dunedin Volcano, South Island, New Zealand

The Port Chalmers Breccia is a vent-filling, clastic volcanicunit exposed within the Miocene Dunedin Volcano of South Island,New Zealand. Clasts (up to in excess of 1 m but generally <20cm) are supported in ash and fine lapilli of phonolitic (ne-benmoreiteor tephro-phonolite) composition and the dominant clast type(55 to almost 100%) is also phonolitic. Less abundant lithologiesinclude ne-normative basalt (basanite), hawaiite, mugeariteand trachyandesite, syenites and microsyenites, coarse-grainedmafic (gabbros) and ultramafic rocks (pyroxenites, hornblendites),schists and sediments. The breccias were emplaced as diatremesassociated with localized, but highly explosive, eruptive eventsin which mantle-derived CO₂ was an important component. Thesyenitic and ultramafic clasts could represent intrusive suitesproduced by crystal fractionation acting on parental ne-benmoreitemagmas that may themselves have been derived by crystal fractionationfrom basanitic precursors. An alternative variation on thismodel is that the parental ne-benmoreites were generated throughpartial melting of an alkalic igneous underplate. Sr, Nd andPb isotopic compositions are strikingly similar to those ofintraplate igneous rocks, ranging in age from 100 to less than10 Ma, from elsewhere in the South Island, and New Zealand'ssub-Antarctic islands, the south Tasman Sea and the Ross Searegion. This regional, HIMU-influenced, isotopic signature isbelieved to be derived from within the lithospheric mantle. KEY WORDS: phonolite; diatreme; nepheline syenite; Dunedin Volcano; alkalic rocks; fractional crystallization     

Phase Equilibria in the Pyroxene Quadrilateral

Experimental phase equilibrium data on compositions of coexistingpyroxenes in the quadrilateral enstatite-diopside-ferrosilite-hedenbergitehave been used to model pyroxene solid solutions and to formulatepyroxene geothermometers. Each pyroxene is treated as a solidsolution of four quad-components using the Kohler formulation where G_ij^* is the excess free energy of mixing in a binary solutioncalculated with binary mole fractions (e.g. X_i^o = X_i/(X_i+X_j))and X_i is the mole fraction in a multicomponent solution. Thefit to the experimental data is achieved by minimizing the totalGibbs free energy of the assemblage. The following set of thermochemicaldata and simple mixture parameters (W_ij) are found to be bestsuited. Standard (T = 298?15 K) enthalpy and entropy of formationfrom elements for fictive orthohedenbergite are –1416?8kJ and 84?88 J K^–1 mol ^–1 respectively. The heatcapacity is given by 114?67+17?09E-3T–31?40E5T^–2.The W_ij data are: Opx: W₁₂ = W₂₁ = 25 W₁₃ = (13?1–0-015T),W₃₁ = (3?37–0?005T), W₂₃ = 20, W₃₂ = 16, W₂₄ = 5, W₄₂= 7, W₃₄ = 15, W₄₃ = 15; Cpx: W₁₂ = (25?484+0?0812P), W₂₁ =(31?216–0?0061P),W₃₁ = W₁₃ = 0W₁₄ = (93?3–0?045T), W₄₁ = (–20?0+0?028T),W₂₃ = 24, W₃₂ = 15, W₂₄ = 12, W₄₂ = 12, W₃₄ = (16?941+0?00592P),W₄₃ = (20?697–0?00235P). Coexisting pyroxene compositionshave been computed in the temperature range of 700 to 1400?C. Two geothermometers have been constructed, one based on atomicfraction of iron (Fe/(Fe + Mg)) in orthopyroxene and the Fe-Mgdistribution coefficient and the other, based on wollastonitecontent of clinopyroxene. The two scales yield different temperatureswhen applied to the same rock. In igneous pyroxenes, the Catransfer ceased at 150 to 200?C above the closure temperatureof the Fe-Mg ion-exchange. In metamorphic rocks an oppositeeffect seems to have prevailed.     

P-T Conditions and the Influence of Graphite on Pelitic Phase Relations in the Ballachulish Aureole, Scotland

Pressure-temperature conditions of pelites in the Ballachulishaureole, Scotland, have been determined from a calibrated petrogeneticgrid and from published geothermometers and geobarometers. Tocalibrate the mineral reactions in the grid, thermodynamic datafor appropriate end members of Ms, Chi, Qtz, And, Sil, Ky, Crn,Crd, Kfs, and Bt were derived from experimental data. This approachwas hampered by the unknown compositions of many of the mineralsused in the experiments, and by apparent inconsistency betweenthe experiments. A best compromise grid that satisfies mostof the data was obtained, which is applicable to the Ballachulishand other contact aureoles. In this grid, the first developmentof sillimanite is constrained to lie between the Richardsonet al. (1969) and Holdaway (1971) andalusite-sillimanite boundaries. A pressure estimate of 3.0 + 0.5 kb is obtained from the calibratedgrid, within 0.3 kb of estimates from geobarometry and fromtwo other independent petrological studies. Temperatures rangedfrom 560?20?C at the first development of cordierite in theassemblage Ms+Qtz+Chl+Crd+Bt to 750–800?C in Grt+Crd+Hyassemblages in pelitic screens within the igneous complex. In graphitic slates, in contrast to non-graphitic pelites, anentire andalusite-bearing subzone is developed, and initialcordierite development occurs further from the igneous contacts.The presence of graphite lowered a_H2o in the slaters, expandingthe stability field of the andalusite-bearing assemblage And+Qtz+Bt+Ms+Crdrelative to the assemblage Kfs+Qtz+Bt+Ms+Crd developed in non-graphiticunits. Initial development of cordierite in the assemblage Ms+Qtz+Chl+Crd+Btwas also promoted by reduced a_H2o in graphitic slates. The regular sequence and spacing of mineral zones in the aureolesuggests that gross equilibrium was attained during contactmetamorphism, even though the thermal metamorphic pulse is estimatedto have been less than 0.2 Ma (Buntebarth, in press). Thereis no evidence for reaction overstepping in cordierite-producingreactions.     

Mineralogical Evidence for Fluid-Rock Interaction Accompanying Prograde Contact Metamorphism of Siliceous Dolomites: Alta Stock Aureole, Utah, USA

Contact metamorphism of siliceous dolomite in the southern partof the metamorphic aureole of the Alta stock (Utah, USA) producedthe prograde isograd sequence: talc (Tc), tremolite (Tr), forsterite(Fo), and periclase (Per). Calcite (Cc)–dolomite (Do)geothermometry and phase equilibria define a general progradeT–X(CO₂) path of decreasing X(CO₂) with rising temperaturefor the dolomite. High-variance assemblages typify the aureole.Per + Cc and Fo + Cc + Do characterize the inner aureole (Perand Fo zones), and Tr + Do + Cc and Tc + Do + Cc are widespreadin the outer aureole (Tr and Tc zones). Low-variance assemblagesare rare and the thickness of reaction zones (coexisting reactantand product minerals) at the isogradic reaction fronts are narrow(tens of metres or less). The mineral assemblages, calculatedprogress of isograd reactions, and the prograde T–X(CO₂)path all indicate that massive dolomite was infiltrated by significantfluxes of water-rich fluids during prograde metamorphism, andthat the fluid flow was down-temperature and laterally awayfrom the igneous contact. Fluid infiltration continued throughat least the initial retrograde cooling of the periclase zone.Down-T fluid flow is also consistent with the results of Cc–Dogeothermometry and patterns of ¹⁸O depletion in this area. Theclose spatial association of reacted and unreacted chert nodulesin both the tremolite and talc zones plus the formation of tremoliteby two reactions indicate that the outer aureole varied in X(CO₂),and imply that fluid flow in the outer aureole was heterogeneous.The occurrence of dolomite-rich and periclase (brucite)-absent,high-     

Petrogenesis and 40Ar/39Ar Geochronology of the Brandberg Complex, Namibia: Evidence for a Major Mantle Contribution in Metaluminous and Peralkaline Granites

Anorogenic granites of the Brandberg igneous complex in NW Namibiaformed during early Cretaceous rifting and continental break-upof Gondwana. A metaluminous series [SiO₂ = 62–77 wt %,molar (Na + K)/Al = 0·8–0·95] includes anearly monzonite body, major biotite–hornblende granite,late biotite granite segregations and peripheral dykes of trachydacite.Volumetrically minor peralkaline granites of the Amis complex[SiO₂ = 72–77 wt %, (Na + K)/Al = 1·0–1·5]intrude the main granite and adjacent country rocks. Comparedwith the metaluminous main granite, these are in part highlyenriched in Zr, Nb, Y, U and Th. Initial Nd and Sr isotope ratiosof the metaluminous suite are     

Petrology of Franciscan Metabasites Along the Jadeite-Glaucophane Type Facies Series, Cazadero, California

The Cazadero blueschist allochthon lies within the Central MelangeBelt of the Franciscan assemblage in the northern Coast Rangeof California. Mineral compositions and assemblages of morethan 200 blueschists from Ward Creek were investigated. Theresults delineate lawsonite-, pumpellyite-, and epidote-zones.The lawsonite and pumpellyite zones are equivalent to the TypeII metabasites of Coleman & Lee (1963) and are characterizedby well-preserved igneous textures, relict augite, and pillowstructures, whereas epidote zone rocks are equivalent to theType III strongly deformed and schistose metabasites. Chlorite,phengite, aragonite, sphene, and minor quartz and albite areubiquitous. The lawsonite zone metabasites contain lawsonite ( < 3 wt.per cent Fe₂O₃), riebeckite-crossite, chlorite, and Ca-Na-pyroxene;some rocks have two distinct clinopyroxenes separated by a compositionalgap. The clinopyroxene of the lowest grade metabasites containsvery low X_jd. In pumpellyite zone metabasites, the most commonassemblages contain Pm + Cpx + Gl + Chl and some samples withhigher Al₂O₃ and/or Fe₂O₃ have Pm + Lw + Cpx + Chl, Actinolitejoins the above assemblage in the upper pumpellyite zone wherethe actinolite-glaucophane compositional gap is well defined.The epidote zone metabasites are characterized by the assemblagesEp + Cpx + two amphiboles + Chl, Lw + Pm + Act + Chl, and Ep+ Pm + two amphiboles + Chl depending on the Fe₂O₃ content ofthe rock. In the upper epidote zone, winchite appears, Fe-freelawsonite is stable, pumpellyite disappears and omphacite containsvery low Ac component. Therefore, the common assemblages areEp + winchite + Lw, and Lw + Omp + winchite. With further increasein metamorphic grade, epidote becomes Al-rich and lawsoniteis no longer stable. Hence Ep + winchite + omphacite ? garnetis characteristic. Mineral assemblages and paragenetic sequences delineate threediscontinuous reactions: (1) pumpellyite-in; (2) actinolite-in;and (3) epidote-in reactions. Using the temperatures estimatedby Taylor & Coleman (1968) and phase equilibria for Ca-Na-pyroxenes,the P–T positions of these reactions and the metamorphicgradient are located. All three metabasite zones occur withinthe aragonite stability field and are bounded by the maximumpressure curve of Ab = Jd + Qz and the maximum stabilities ofpumpellyite and lawsonite. The lawsonite zone appears to bestable at T below 200?C with a pressure range of 4–6?5kb; the pumpellyite zone between 200 and 290?C and the epidotezone above 290?C with pressure variation between 6?5 and 9 kb.The metamorphic field gradient appears to have a convex naturetowards higher pressure. A speculative model of underplatingseamounts is used to explain such feature.     

A Transitional Basalt-Pantellerite Sequence of Fractional Crystallization, the Boina Centre (Afar Rift, Ethiopia)

Volcanological and petrological evidence, ⁸⁷Sr/⁸⁶Sr data, thelinear correlation between pairs of residual elements (e.g.Th, U, Zr, Hf, La, Ce, Ta) indicate that the rock series frommildly alkaline (transitional) basalt to pantellerite eruptedin recent Quaternary times at the Boina volcanic centre, canbe entirely explained in terms of fractional crystallizationat shallow depth. The fractionation process has been reconstructedby referrin to variation diagrams of major and trace elementsreported as a function of the fraction (f) of the initial compositionformed by the residual liquid, evaluated from the distributionof residual elements and by estimating the composition of theparent magma. The main crystal phases involved in the differentiationare, in the order of appearance: olivine, plagioclase, clinopyroxene,Fe-Ti oxides, alkali feldspar. Crystallization of Fe-Ti oxidesoccurs only at an advanced stage of fractionation in iron richliquids (ferrobasalts). The transition to the peralkaline field(near f=0?20) occurs without passing through a ‘true’trachytic (low-silica) salic stage and is determined by the‘plagioclase effect’. Fractionation within the peralkalinefield is dominated by alkali feldspar. Evidence is given fora ‘low-temperature zone’ of the oversaturated mildlyperalkaline system running along a line of constant alkali-ratio.Po₂ variations are recorded during the evolution of the Boinaseries as suggested by petrological data and distribution curvesof total iron, Fe⁺⁺/Fe⁺⁺⁺and europium. Po₂ values calculatedfrom europium distribution in feldspar and whole rocks agreewith published Po₂ mineralogical calculations in pantelleritesfrom other localities. A crucial stage is recognized near thetransition to the peralkaline field, with a sudden Po₂ dropduring the crystallization of the oxides probably provokingthe precipitation of apatite, followed by a rapid Po₂ increaseat f=0?30. This limited oxygen unbuffered zone is importantin the basalt-pantellerite evolution, as it determines markedchemical variations in a restricted crystallization interval.It is suggested that such a crucial stage occurs also in theother known pantellerite series, such as Pantelleria. It mayalso account for the scarcity of rocks frequently reported atthis stage (‘Daly gap’). Data obtained from Boina and comparisons with other volcanicseries indicate that many peralkaline rhyolites are geneticallyrelated to transitional basalts and that their nature is mainlycontrolled by the composition of the parent basaltic magma.The association is characteristically found in zones of extensionof both continental and oceanic environments. The views of Coombs(1963) are confirmed and the rocks series from transitionalbasalts to comendites and/or pantellerites should be distinguishedfrom the alkalic (undersaturated) igneous rock suites producedby differentiation of alkali basalts.     

Open-System Magma Chamber Evolution: an Energy-constrained Geochemical Model Incorporating the Effects of Concurrent Eruption, Recharge, Variable Assimilation and Fractional Crystallization (EC-E'RA{chi}FC)

Significant petrogenetic processes governing the geochemicalevolution of magma bodies include magma Recharge (includingformation of ‘quenched inclusions’ or enclaves),heating and concomitant partial melting of country rock withpossible ‘contamination’ of the evolving magma body(Assimilation), and formation and separation of cumulates byFractional Crystallization (RAFC). Although the importance ofmodeling such open-system magma chambers subject to energy conservationhas been demonstrated, the effects of concurrent removal ofmagma by eruption and/or variable assimilation (involving imperfectextraction of anatectic melt from wall rock) have not been considered.In this study, we extend the EC-RAFC model to include the effectsof Eruption and variable amounts of assimilation, A. This model,called EC-E'RAFC, tracks the compositions (trace elements andisotopes), temperatures, and masses of magma body liquid (melt),eruptive magma, cumulates and enclaves within a composite magmaticsystem undergoing simultaneous eruption, recharge, assimilationand fractional crystallization. The model is formulated as aset of 4 + t + i + s coupled nonlinear differential equations,where the number of trace elements, radiogenic and stable isotoperatios modeled are t, i and s, respectively. Solution of theEC-E'RAFC equations provides values for the average temperatureof wall rock (T_a), mass of melt within the magma body (M_m),masses of cumulates (M_ct), enclaves (M_en) and wall rock () and the masses of anatectic melt generated () and assimilated (). In addition, t trace element concentrations and i + s isotopic ratios inmelt and eruptive magma (C_m, _m, _m), cumulates (C_ct, _m, _m), enclaves(C_en, , ) and anatectic melt (C_a, , ) as a function of magma temperature (T_m) are also computed. Input parametersinclude the (user-defined) equilibration temperature (T_eq),a factor describing the efficiency of addition of anatecticmelt () from country rock to host magma, the initial temperatureand composition of pristine host melt (, , , ), recharge melt (, , , ) and wall rock (, , , ), distribution coefficients (D_m, D_r, D_a) and their temperaturedependences (H_m, H_r, H_a), latent heats of transition (meltingor crystallization) for wall rock (h_a), pristine magma (h_m)and recharge magma (h_r) as well as the isobaric specific heatcapacity of assimilant (C_p,a), pristine (C_p,m) and recharge(C_p,r) melts. The magma recharge mass and eruptive magma massfunctions, M_r(T_m) and M_e(T_m), respectively, are specified apriori. M_r(T_m) and M_e(T_m) are modeled as either continuous orepisodic (step-like) processes. Melt productivity functions,which prescribe the relationship between melt mass fractionand temperature, are defined for end-member bulk compositionscharacterizing the local geologic site. EC-E'RAFC has potentialfor addressing fundamental questions in igneous petrology suchas: What are intrusive to extrusive ratios (I/E) for particularmagmatic systems, and how does this factor relate to rates ofcrustal growth? How does I/E vary temporally at single, long-livedmagmatic centers? What system characteristics are most profoundlyinfluenced by eruption? What is the quantitative relationshipbetween recharge and assimilation? In cases where the extractionefficiency can be shown to be less than unity, what geologiccriteria are important and can these criteria be linked to fieldobservations? A critical aspect of the energy-constrained approachis that it requires integration of field, geochronological,petrologic, and geochemical data, and, thus, the EC-ERAFC ‘systems’approach provides a means for answering broad questions whileunifying observations from a number of disciplines relevantto the study of igneous rocks. KEY WORDS: assimilation; energy conservation; eruption; open system; recharge     

Volcan Popocatepetl, Mexico. Petrology, Magma Mixing, and Immediate Sources of Volatiles for the 1994-Present Eruption

Volcán Popocatépetl has been the site of voluminousdegassing accompanied by minor eruptive activity from late 1994until the time of writing (August 2002). This contribution presentspetrological investigations of magma erupted in 1997 and 1998,including major-element and volatile (S, Cl, F, and H₂O) datafrom glass inclusions and matrix glasses. Magma erupted fromPopocatépetl is a mixture of dacite (65 wt % SiO₂, two-pyroxenes+ plagioclase + Fe–Ti oxides + apatite, 3 wt % H₂O, P= 1·5 kbar, f_O2 = NNO + 0·5 log units) and basalticandesite (53 wt % SiO₂, olivine + two-pyroxenes, 3 wt % H₂O,P = 1–4 kbar). Magma mixed at 4–6 km depth in proportionsbetween 45:55 and 85:15 wt % silicic:mafic magma. The pre-eruptivevolatile content of the basaltic andesite is 1980 ppm S, 1060ppm Cl, 950 ppm F, and 3·3 wt % H₂O. The pre-eruptivevolatile content of the dacite is 130 ± 50 ppm S, 880± 70 ppm Cl, 570 ± 100 ppm F, and 2·9 ±0·2 wt % H₂O. Degassing from 0·031 km³ of eruptedmagma accounts for only 0·7 wt % of the observed SO₂emission. Circulation of magma in the volcanic conduit in thepresence of a modest bubble phase is a possible mechanism toexplain the high rates of degassing and limited magma productionat Popocatépetl. KEY WORDS: glass inclusions; igneous petrology; Mexico; Popocatépetl; volatiles     

H2O Abundance in Depleted to Moderately Enriched Mid-ocean Ridge Magmas; Part I: Incompatible Behaviour, Implications for Mantle Storage, and Origin of Regional Variations

The H₂O contents and trace-element abundances are presentedfor two well-studied suites of mid-ocean ridge basalt (MORB)glasses from the Northern East Pacific Rise (EPR, 9–11°N)and the South East Indian Ridge (SEIR, 127–129°E).Exactly the same region of the glass samples has been analysedfor these components using microbeam techniques. Our data allowexamination of the fine details of H₂O geochemical behaviourduring MORB genesis. We demonstrate that relative H₂O contents[i.e. H₂O/(another incompatible element)] vary systematicallywith increasing (La/Sm)_N in MORB glasses from both the EPR andSEIR. This indicates that H₂O behaves like other incompatible(in peridotite mineralogies) elements during MORB petrogenesis,and is primarily controlled by solid–melt partitioning.However, the relative H₂O contents of MORB glasses from theSEIR are higher than in glasses from the EPR at a given (La/Sm)_N,demonstrating global variations in the H₂O contents of MORB.Despite regional differences in relative H₂O contents, the incompatiblebehaviour of H₂O is similar in both studied regions. The relativeincompatibility of H₂O varies systematically with increasing(La/Sm)_N: in depleted MORB, H₂O is similar to La whereas inEMORB, H₂O is similar to Ce. Similar patterns of varying relativeincompatibility (to REE) are displayed by Zr, Hf, and P. Ourdata are best explained if H₂O is stored in the mantle in thesame phase with LREE (clinopyroxene?) at sub-solidus. Regionalvariations in relative H₂O contents in EMORB that have moreradiogenic Sr, Nd and Pb isotopes might be explained by differencesin the nature of enriched components recycled via subductionprocesses. However, when EMORB have the same radiogenic isotopecompositions as NMORB within a segment, relative H₂O contentsin EMORB probably reflect local processes that lead to enrichmentin incompatible elements. Regional differences in relative H₂Ocontents of NMORB may reflect either initial variations in theEarth’s mantle or inhomogeneities left after formationof the continental crust. KEY WORDS: glass; geochemistry; H₂O; MORB; petrology     

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

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

The Sulfide Capacity and the Sulfur Content at Sulfide Saturation of Silicate Melts at 1400{degrees}C and 1 bar

The solubility of sulfur as S^2– has been experimentallydetermined for 19 silicate melt compositions in the system CaO–MgO–Al₂O₃–SiO₂(CMAS)± TiO₂ ± FeO, at 1400°C and 1 bar, using CO–CO₂–SO₂gas mixtures to vary oxygen fugacity (fO₂) and sulfur fugacity(fS₂). For all compositions, the S solubility is confirmed tobe proportional to (fS₂/fO₂)^1/2, allowing the definition ofthe sulfide capacity (C_S) of a silicate melt as C_S = [S](fO₂/fS₂)^1/2.Additional experiments covering over 150 melt compositions,including some with Na and K, were then used to determine C_Sas a function of melt composition at 1400°C. The resultswere fitted to the equation     

Thermodynamic Mixing Models for Muscovite--Paragonite Solutions Based on Solution Calorimetric and Phase Equilibrium Data

Enthalpies of solution have been measured on a series of muscovite—paragonitemicas in 20.1% HF at 50C under isoperibolic conditions. Themolar enthalpy of formation of paragonite at 298 K, for whichno calorimetrically measured value is currently available, hasbeen determined to be –5937.5 (3) kJ. An inversion ofall calorimetric, volumetric and phase equilibrium data hasbeen performed, giving a range of mixing models compatible withmost experimental data. The following expressions of the mixingproperties of 2M₁ micas for enthalpy (H_ex) and volume (V_ex)at pressures up to 10 kbar, forcing excess entropy (S_ex) tobe zero and using a subregular mixing model are favoured: H_ex(kJ) = [10.6+4.45(1–2X_ms)]X_ms(1–X_ms) V_ex(J/bar) = 0.452X_ms(1–X_ms). However, mixing models of higher order with asymmetric negativeS_ex are also possible. KEY WORDS: muscovite; paragonite; solvus; calorimetry; solid solution ^*Corresponding author.     

The Petrology and Origin of the Tertiary Lundy Granite (Bristol Channel, UK)

The British Tertiary Volcanic Province (BTVP) comprises within-platecentral igneous complexes associated with plateau lavas andregional dyke swarms. Lundy is the southernmost complex of theBTVP and comprises granite ({small tilde}90%) emplaced intodeformed Devonian sedimentary rocks within the Hercynian Cornubiangranite province of southwest England. The complex is intrudedby a northwest-southeast trending dyke swarm. In common withother BTVP igneous complexes, Lundy is associated with positivegravity and magnetic anomalies which are interpreted in termsof the presena of an underlying basic intrusion at shallow depth,with a volume exceeding that of the overlying granite. The Lundy intrusion is a coarse-grained megacrystic granitecontaining up to 20% alkali feldspar megacrysts in a coarse-grainedgroundmass composed of alkali feldspar, quartz, lithium-bearingmuscovite, and ‘biotite’ (lithian siderophyllite),with a range of aaxssory minerals. The main granite has a coarse-grained(locally miarolitic) pegmatitic facies and is intruded by thinsheets and veins of fine-grained aplite and microgranite. Themineralogy indicates crystallization of the Lundy granite froma highly fractionated H₂O- and halogen-rich magma at a relativelyshallow crustal level. The main Lundy granite is a peraluminous leucogranite with Na₂O=3–4%,K₂O{small tilde}5%, low TiO₂, MeO, CaO, Zr, and Sr, and highRb and Rb/Sr in comparison with many other peralurninous granites,including those from the Cornubian batholith and the BTVP. Anew Rb-Sr whole-rock isochron for the granite yields an ageof 58?7?1?6 Ma with an initial ⁸⁷Sr/86Sr of 0?715?0?006. _Ndvalues for the granite (–0?9 to –1?9) plot betweencontemporaneous mantle (positive _Nd and Cornubian granites (_Nd=ca.–11). The trace element data (Rb, Y, Nb) show affinities with syn-collisionand within-plate granites. As the Sr isotope data indicate amajor crustal component, and the Nd isotope data suggest bothmantle and crustal components, we propose that the Lundy graniteis derived from a parental magma comprising crustal components(derived from a similar source to that of the Cornubian granitebatholith) and a mantle-derived component (derived from a differentiateof contemporaneous basaltic magma This magma experienced fractionalcrystallization of plagioclase, alkai feldspar, Fe-Mg minerals,and REE-bearing accessory minerals before emplacement, and theLundy granite experienced further in situ fractional crystallization,associateded with crustal contamination by the Devonian shaleafter emplacement.     

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).