Geochemical Effects of Small Packet Crystallization in Large Magma Chambers--Further Resolution of the Highly Compatible Element Paradox

This is a study of the effect of solidifying a magma body bypartial crystallization of a series of small packets of liquid,mixing the residual liquid into the main body of liquid beforerepeating the process. It confirms the major conclusions ofearlier workers and demonstrates that the dominant geochemicaleffect of the small packet process is to sustain the relativeconcentrations of the compatible elements in the residual liquidsfrom partial crystallization. Formal introduction of integratedpartial crystallization ivithin the small packets of liquidenhances these effects. Incorporation of such a crystallizationmodel into a refilled, tapped and fractionated magma body enhancesthe effects still more. The process affords a way to explainthe 'anomalously' high compatible element concentrations inerupted liquids which have nevertheless bun subject to substantiallowpressure crystallization. It may also have a bearing on theratios of extremely compatible elements whose concentrationsin the upper mantle are high and relatively undifferentiatedrelative to chondrites. KEY WORDS: nickel; platinum group elements; in situ crystallization; boundary layer; integrated crystallization ^*email oharamj{at}cardiff.ac.uk     

Trace Element Geochemical Effects of Integrated Melt Extraction and 'Shaped' Melting Regimes

The mixing (integration) of liquids obtained as different massfractions of partial melting from source material of the samebulk composition, travelling along different mantle flow-linesthrough a melting regime, can result in deficiencies in therelative concentrations of those incompatible elements whosebulk distribution coefficients are numerically approximatelyequal to the average mass fraction of melt extracted from thetotal source material involved in the provision of the mixedmelts. These deficiencies can be very substantial, exceeding50% of the concentration which would have been expected to bepresent in the liquid if that same average mass fraction ofmelt had been extracted from the whole melting regime by simpleequilibrium or accumulated perfect fractional partial melting.The size of the deficit varies with the shape and plan-formof the melting region, and can be greatly reduced by subsequentperfect fractional crystallization of that liquid. Discriminationis increased between all elements whose distribution coefficientsare numerically smaller than the average mass fraction of partialmelt extracted from the whole region. These effects can leadto steepening of chondrite-normalized REE patterns and to apparentselective light rare earth enrichment in liquid and source. KEY WORDS: melt integration; shaped melting regimes; trace elements; numerical modelling     

Minor Phases as Carriers of Trace Elements in Non-Modal Crystal-Liquid Separation Processes II: Illustrations and Bearing on Behaviour of REE, U, Th and the PGE in Igneous Processes

Minor phases which strongly concentrate selected trace elements,termed here ‘carrier-phases’, release relativelylarge amounts of those elements to the liquid phase when theyare eliminated during partial melting and glean relatively largeamounts of those elements when they first appear during progressivecrystallization. It is characteristic of such relationshipsthat concentrations of the selected trace elements in the bulkresidues of partial melting will rise to a peak somewhat beforethe last of the carrier-phase is eliminated during progressivemelting. In the liquids produced during equilibrium partialmelting a corresponding peak in the concentration of the traceelement occurs at the point where the carrier-phase is eliminated;the corresponding peak in trace element concentration in theliquids produced by accumulated perfect fractional melting isfound somewhat above that point. These peaks become more sharplyaccentuated as the distribution coefficient of the trace elementinto the carrier-phase increases. The highest trace elementconcentration in a partial melt liquid product is found in thesmall drop of liquid produced during perfect fractional meltingat the point where the carrier-phase is eliminated. Still higherconcentrations may be found in the first cumulates containingthe carrier-phase which precipitate during perfect fractionalcrystallization but the corresponding liquids do not containexceptionally high concentrations. Under favourable conditionsa large proportion of the available mass of a trace elementin a magmatic system may be transferred from the solid to theliquid phases or vice versa with only a small change in themass fraction of liquid in and energy content of the system.Within that range, separation of otherwise very similarly behavedtrace elements becomes possible. Further complexities ariseand the opportunities for separation increase when two carrier-phasescompete with differing success for the same group of trace elements. KEY WORDS: platinum; uranium; chromite; sulphide; distribution coefficient     

A trace-element geochemical model for imperfect fractional crystallization associated with the development of crystal zoning

Although many petrological studies of volcanic rocks have suggested that crystallization proceeds within magma bodies, highly compatible trace elements do not display the marked variations and extreme depletions predicted to result from perfect fractional crystallization. Imperfect crystal-liquid separation is a key process in explaining this paradox. The presence of suspended crystals greatly affects variations in highly compatible elements, and has been quantitatively modeled by assuming perfect equilibrium between the suspended crystals and the liquid (equilibrium crystallization and imperfect separation; ECIS); however, volcanic rocks generally contain zoned phenocrysts that reflect the absence of solid-state equilibration. The present study develops a mass-balance model for zoned crystallization and imperfect separation (ZCIS). The ZCIS process is more efficient than the conventional ECIS process in generating depleted compatible elements. These two end-member models are able to explain the compositional range of igneous rocks that experienced imperfect fractional crystallization under natural conditions. The predicted compositional regions in bivariate trace-element diagrams successfully account for the sizes and shapes of the regions defined by whole-rock and melt-inclusion data from the Bishop Tuff, CA, USA.     

Rare-earth elements,Co, Sc and Hf in the Steens Mountain basalts

The concentrations of the rare-earth elements (REE) in 52 of 70 consecutive lava flows from Steens Mountain, Oregon, are reported. The concentrations of Co, Sc and Hf were measured in 17 of the flows. Logarithmic partitioning theory is used to correlate the concentrations of the trace elements with the major element and mineralogical compositions of the samples.Production of the basalts by partial melting requires a parent material that has concentrations of the REE that are several times those of chondrites. Those samples enriched in alumina have positive Eu anomalies compared to chondrites and those depleted in alumina have negative Eu anomalies. Production of these samples by partial melting is unlikely because the Eu anomalies would require that plagioclase was stable under the conditions of melting.All of the samples can be related to a single parent magma by fractional crystallization processes. The concentrations of the trace elements are controlled by the addition or removal of plagioclase and the removal of clinopyroxene. Samples with high concentrations of alumina are interpreted as plagioclase cumulates while those with low concentrations of alumina are residual liquids produced by crystallization of plagioclase. During fractional crystallization, the concentrations of the light REE are increased selectively in the residual liquid. The material that must be crystallized and removed during fractional crystallization has characteristics that are similar to those reported for gabbros.     

Phase Relationships of Hydrous Alkalic Magmas at High Pressures: Production of Nepheline Hawaiitic to Mugearitic Liquids by Amphibole-Dominated Fractional Crystallization Within the Lithospheric Mantle

Experimental melting studies were conducted on a nepheline mugearitecomposition to pressures of 31 kbar in the presence of 0–30%added water. A temperature maximum in the near-liquidus stabilityof amphibole (with olivine) was found for a water content of3·5 wt % at a pressure of 14 kbar. This is interpretedto have petrogenetic significance for the derivation of nephelinemugearite magmas from nepheline hawaiite by amphibole-dominatedfractional crystallization at depth within the lithosphericmantle. Synthetic liquids at progressively lower temperaturesrange to nepheline benmoreite compositions very similar to thoseof natural xenolith-bearing high-pressure lavas elsewhere, andsupport the hypothesis that continued fractional crystallizationcould lead to high-pressure phonolite liquids. Independent experimentaldata for a basanite composition modeled on a lava from the sameigneous province (the Newer Basalts of Victoria) permit theinference that primary asthenospheric basanite magmas undergopolybaric fractional crystallization during ascent, and mayevolve to liquids ranging from nepheline hawaiite to phonoliteupon encountering cooler lithospheric mantle at depths of 42–50km. Such a model is consistent with the presence in some evolvedalkalic lavas of both lithospheric peridotite xenoliths indicativeof similar depths and of megacryst suites that probably representdisrupted pegmatitic segregations precipitated from precursoralkalic magmas in conduit systems within lithospheric mantle. KEY WORDS: experiment; high pressure; alkalic magmas; amphibole; nepheline mugearite; basanite; lithosphere     

Differentiation of Ferro-Basaltic Magmas under Conditions Open and Closed to Oxygen: Implications for the Skaergaard Intrusion and Other Natural Systems

Because processes such as fractional crystallization and crystallizationunder conditions closed to oxygen are difficult to simulatein the laboratory there is a need for quantitative models ofmagma crystallization behaviour. Comparison of experimentaldata on an iron-rich basaltic composition with predictions ofthe MELTS free energy minimization algorithm shows that althoughliquidus temperatures and silicate mineral equilibria are predictedrelatively well, the saturation of Fe–Ti oxides is notWe have used the same experimental data to construct an alternativecrystallization model based on known equilibrium phase relations,mineral–melt partitioning of major elements, and massbalance constraints. The model is used to explore the consequencesof equilibrium and fractional crystallization in systems openand closed to oxygen. Liquid lines of descent for perfect equilibriumand perfect fractional crystallization are predicted to be verysimilar. In a system open to oxygen the model predicts thatmagnetite saturation leads to strongly decreasing iron and increasingsilica contents of residual liquids, whereas systems closedto oxygen crystallize less abundant magnetite, leading to aless pronounced iron depletion in the liquid. Predicted bulksolid compositions and variations of f_o2, with falling temperatureagree well with those observed or inferred from the cumulatesof the Skaergaard intrusion, but none of the predicted liquidlines of descent are consistent with the extreme iron enrichmentproposed for this intrusion based on mass balance calculations.Compositional factors such as water and phosphorus are not thoughtto be the source of the discrepancy as the cumulates of theBasistoppen sill (which closely resemble those of Skaergaard)may be used to calculate a liquid line of descent in agreementwith that predicted by the model for fractional crystallizationclosed to oxygen. A comparison of the predicted T-f_o2, pathsand liquid lines of descent with those inferred from naturalsystems suggests that volcanic centres such as Iceland and Hawaiievolve under conditions open to oxygen, whereas evidence fromplutonic environments (e.g. Skaergaard and Kiglapait layeredintrusions) suggests that they evolved under conditions moreclosed to oxygen. The compositional evolution of the melt phasein volcanic and plutonic systems may therefore be different,although the results of this study suggest that magnetite saturationwill limit Fe enrichment in all environments to <20wt% FeO^*,consistent with enrichments reported for volcanic glasses. KEY WORDS: Skaergaard; ferro-basalt; iron enrichment; oxygen fugacity ^*Bayerisches Geoinstitut, Universitt Bayreuth, D-95440 Bayreuth, Germany. Telephone: $ 49-921-553718. Fax $ 49-921-553769. e-mail: mike.toplis{at}uni-bayreuth.de     

金川岩浆铜镍硫化物矿床中的镍钴分布规律及其控制因素

幔源岩浆形成与演化过程中镍(Ni)、钴(Co)具有相似的地球化学行为。金川岩浆铜镍硫化物矿床以Ni、铜(Cu)为主要矿种, Co为伴生, Ni、Co在金川矿床中的空间分布规律同步变化, 然而其Ni/Co比值(36.7)远高于地幔值(18.2)。这表明在金川矿床形成过程中Ni-Co发生了共生分离, 但Ni-Co分布特征尚不清楚、其控制因素尚不明确。本文对该矿床中主要矿石矿物的Ni、Co含量及分布进行了系统总结, 并与脉石矿物进行对比。结果表明矿石矿物镍黄铁矿是最重要的含Ni、Co矿物相, 其Ni、Co含量均远高于磁黄铁矿、黄铜矿及脉石矿物。对于脉石矿物, Ni在橄榄石、磁铁矿、铬铁矿内的含量依次降低, 在斜方辉石与单斜辉石中含量最低。Co则在铬铁矿、橄榄石内含量依次降低, 在斜方辉石、单斜辉石、磁铁矿中含量最低。在硫化物熔离过程中, Ni在硫化物熔体内相容性更强, 更加倾向于进入硫化物熔体, 使Ni显著富集于硫化物熔体内, 而Co则相对富集于硅酸盐熔体内, 由此导致Ni-Co解耦。硫化物冷却结晶过程中, Ni、Co倾向于进入最早结晶的单硫化物固溶体(MSS), 并在随后分解作用中集中进入镍黄铁矿内, 使镍黄铁矿成为金川矿床中最重要的含Ni-Co矿物相, 并使Ni、Co在金川矿床中具有相似的空间分布规律。在硅酸盐熔体结晶分异过程中, Ni在橄榄石中的相容性最强, Co在铬铁矿中相容性最强, 因此Ni倾向于进入橄榄石, 而Co倾向于进入铬铁矿, 由此导致Ni-Co发生解耦。硫化物熔离、橄榄石堆晶均会造成残余熔体Ni亏损程度高于Co, 且Ni在斜方辉石与单斜辉石中相容性高于Co将导致残余熔体随冷却结晶Ni/Co比值逐渐降低, 因此在粒间硅酸盐矿物结晶过程中Ni、Co倾向于共生。脉石矿物亚固相下与硫化物熔体反应对于Ni-Co共生分离的影响则与结晶作用完全相反。镍黄铁矿和磁黄铁矿出溶于硫化物矿浆结晶早期形成的MSS, 在不同岩/矿石类型内Ni、Co含量同步变化, 表明镍黄铁矿和磁黄铁矿成分可以用来指示岩浆铜镍硫化物矿床成矿过程中硫化物熔体成分的演变。

     

Petrogenesis of Migmatites in Maine, USA: Possible Source of Peraluminous Leucogranite in Plutons?

In Maine, Siluro-Devonian turbidites were metamorphosed underhigh-T–low-P facies series conditions during deformationwithin a Devonian crustal-scale shear zone system, defined bykilometer-scale straight belts of apparent flattening strainthat anastomose around lozenges of apparent constrictional strain.At upper amphibolite facies grade, metapelites are partiallymelted, the onset of which is recorded by a migmatite front.The resulting migmatites are stromatic or heterogeneous, andsmaller-volume granites form sheets or cylinders according tothe structural zone in which they occur, suggesting that migmatitesand granites record syntectonic melt flow through the deformingcrust. Common leucogranite of the nearby coeval Phillips pluton,which was emplaced syntectonically, was sourced from crustalrocks with geochemical characteristics similar to those of thehost Siluro-Devonian succession. Migmatites have melt-depletedcompositions relative to metapelites. Leucosomes are peraluminousand represent the cumulate products of fractional crystallizationand variable loss of evolved fractionated liquid. Among theheterogeneous migmatites are schlieric granites, the geochemistryof which suggests melt accumulation before fractional crystallizationand loss of the evolved liquid. Smaller-volume granites areperaluminous with a range of chemistries that reflect variableentrainment of residual plagioclase and biotite, accumulationof products of fractional crystallization and loss of most ofthe evolved liquid. Common leucogranite of the Phillips plutonand larger granites in the migmatites have compositions thatsuggest crystallization of evolved liquids derived by fractionalcrystallization of primary muscovite dehydration melts. We inferthat the leucogranite represents the crystallized fugitive liquidfrom a migmatite source similar to that exposed nearby. Watertransported through the shear zone system dissolved in meltwas exsolved at the wet solidus to cause retrogression in sub-solidusrocks and retrograde muscovite growth in migmatites. KEY WORDS: anatexis of pelite; Maine; migmatite; peraluminous granite; plutons     

The Liquid Line of Descent of Anhydrous, Mantle-Derived, Tholeiitic Liquids by Fractional and Equilibrium Crystallization--an Experimental Study at 1{middle dot}0 GPa

Two series of anhydrous experiments have been performed in anend-loaded piston cylinder apparatus on a primitive, mantle-derivedtholeiitic basalt at 1·0 GPa pressure and temperaturesin the range 1060–1330°C. The experimental data provideconstraints on phase equilibria, and solid and liquid compositionsalong the liquid line of descent of primary basaltic magmasdifferentiating in storage reservoirs located at the base ofthe continental crust. The first series are equilibrium crystallizationexperiments on a single basaltic bulk composition; the secondseries are fractionation experiments where near-perfect fractionalcrystallization was approached in a stepwise manner using 30°Ctemperature steps and starting compositions corresponding tothe liquid composition of the previous, higher-temperature glasscomposition. Liquids in the fractional crystallization experimentsevolve with progressive SiO₂ increase from basalts to dacites,whereas the liquids in the equilibrium crystallization experimentsremain basaltic and display only a moderate SiO₂ increase accompaniedby more pronounced Al₂O₃ enrichment. The principal phase equilibriacontrols responsible for these contrasting trends are suppressionof the peritectic olivine + liquid = opx reaction and earlierplagioclase saturation in the fractionation experiments comparedwith the equilibrium experiments. Both crystallization processeslead to the formation of large volumes of ultramafic cumulatesrelated to the suppression of plagioclase crystallization relativeto pyroxenes at high pressures. This is in contrast to low-pressurefractionation of tholeiitic liquids, where early plagioclasesaturation leads to the production of troctolites followed by(olivine-) gabbros at an early stage of differentiation. KEY WORDS: liquid line of descent; tholeiitic magmas; equilibrium crystallization; fractional crystallization     

Some Thermal Constraints on Crustal Assimilation during Fractionation of Hydrous, Mantle-derived Magmas with Examples from Central Alpine Batholiths

We provide a model for the fractional crystallization of hydrousmantle-derived magma to form calc-alkaline plutons, based uponmass balance for geological examples of fractionation sequencesin the lower continental crust. This is complemented by a thermalmodel for the heat budget obtained from a projected phase diagramand thermodynamic data. Fractional crystallization (FC) andassimilation–fractional crystallization (AFC) paths havebeen calculated with these models and the mass ratio of assimilationto crystallization as a function of parent magma type and temperature,crustal rock fertility and temperature, and mechanism of assimilation,have been determined. When these results are combined with F(melt fraction) and r (ratio of mass assimilated/crystallized)values evaluated from geochemical data then new information,not available with the methods separately, can be deduced. Thisincludes when and at what depth and temperature in the crustthe assimilation took place, as well as the likely parent magmatype and temperature of the assimilant. Our results are presentedin simple graphical fashion to facilitate future studies thatexamine the evolution of individual calc-alkaline plutons andthe mechanisms of crustal contamination, and to improve meltmodels involving hydrous magma in volcanic arcs and in the lowercontinental crust KEY WORDS: assimilation; hydrous mantle magma; thermal models; fractional crystallization; magma mixing; Alpine batholiths; Adamello; Bergell     

The Blue Tier Batholith,Northeastern Tasmania

The Blue Tier Batholith is one of a number of high-level, essentially postkinematic, composite granitoid bodies occurring at the southern end of the Tasman orogenic belt of Eastern Australia.An integrated study of the structure, texture, and geochemistry of the batholith suggests that it has a cumulate-like character. In particular, the trace element (Ba, Rb, Sr) data, when constrained by textural features of the granitiods, indicate that the batholith formed by fractional crystallization of a single magma which underwent crystallization in situ by progressive nucleation and solidification from the roof, walls, and floor inwards. Progressive changes in liquids (cumulate) mineralogy during crystallization led to the observed sequence of early biotite and/or hornblende granodiorites followed by biotite adamellites and late muscovite biotite granites. Progressive in situ crystallization led in some instances to gradational boundaries between granitoid types whereas periodic tectonic distrubances caused the rest magma to reintrude earlier crystallizates in places: thus emplacement and crystallization sequences are parallel. The ultimate product of fractional crystallization was a water-saturated melt, enriched in incompatible elements, whose crystallization resulted in significant tin mineralization.The chemistry of the rocks comprising the batholith is in many respects analogous to that of basic cumulate rocks, although an origin by outward growth of crystals and expulsion of interstitial melt, coupled with convective mixing, rather than by crystal settling, is favoured for the granitoid suite. It is suggested that the Blue Tier Batholith is not an isolated example of a granitoid body with cumulate-like character, but that such bodies may be more common than is recognized.     

Tholeiitic and alkali basalts from the Mid-Atlantic Ridge at 43 ° N

Tholeiitic basalts dredged from the Mid-Atlantic Ridge (MAR) axis at 43 ° N are enriched in incompatible trace elements compared to the ‘ normal’ incompatible element depleted tholeiites found from 49 ° N to 59 ° N and south of 33 ° N on the MAR. The most primitive 43 ° N glasses have MgO/FeO*= 1.2 and coexist with olivine (Fo_90–91) and chrome-rich spinel. The tholeiitic basalts from the MAR 43 ° N are distinct from the strongly incompatible trace element depleted tholeiities found elsewhere in the Atlantic, and have trace element features typical of island tholeiities and MAR axis tholeiites from 45 ° N. Petrographic, major, and compatible trace element trends of the axial valley tholeiites at 43 ° N are consistent with shallow-level fractionation; in particular, evolution from primitive liquids with forsteritic olivine plus chrome spinel as liquidus phases to fractionated liquids with plagioclase plus clinopyroxene as major crystallizing phases. However, each dredge haul has distinctive incompatible trace element abundances. These trace element characteristics require a hetrogeneous mantle or complex processes such as open system fractional crystallization and magma mixing. Alkali basalts (～5% normative nepheline) were dredged from a prominent fracture zone at 43 ° N. Typical of alkali basalts they are strongly enriched (compared to tholeiites) in incompatible elements. Their highly fractionated rare-earth element (REE) abundances require residual garnet during partial melting. The 43 ° N tholeiites and alkali basalts could be derived from a garnet peridotite source with REE contents equal to 2 × chondrites by ～5% and 1% melting, respectively. Alternatively, they could be derived from a moderately light REE enriched source by ～25% and 9.5% melting, respectively.     

Geochemical and Sm–Nd isotopic study of titanite from granitoid rocks of the eastern Dharwar craton,southern India

Titanite occurs as an accessory phase in a variety of igneous rocks, and is known to concentrate geologically important elements such as U, Th, rare earth element (REE), Y and Nb. The differences in the abundances of the REEs contained in titanite from granitoid rocks could reflect its response to changes in petrogenetic variables such as temperature of crystallization, pressure, composition, etc. Widespread migmatization in the granodiorite gneisses occurring to the east of Kolar and Ramagiri schist belts of the eastern Dharwar craton resulted in the enrichment of the REEs in titanite relative to their respective host rocks. A compositional influence on the partitioning of REEs between titanite and the host rock/magma is also noticed. The relative enrichment of REEs in titanite from quartz monzodiorite is lower than that found in the granodioritic gneiss. Depletion of REE and HFSE (high field-strength elements) abundances in granitic magmas that have equilibrated with titanite during fractional crystallization or partial melting has been modelled. As little as 1% of titanite present in residual phases during partial melting or in residual melts during fractional crystallization can significantly lower the abundances of trace elements such as Nb, Y, Zr and REE which implies the significance of this accessory mineral as a controlling factor in trace element distribution in granitoid rocks. Sm–Nd isotope studies on titanite, hornblende and whole rock yield isochron ages comparable to the precise U–Pb titanite ages, invoking the usefulness of Sm–Nd isochron ages involving minerals like titanite.     

相容元素——不相容元素协变图在岩石成因研究中的意义

笔者根据前人提出的各种岩浆作用模式,如分离结晶作用、部分熔融作用、岩浆混合作用等,通过纯数学推导,提出一套判别岩浆岩形成机制的相容元素—不相容元素协变图。 从几何图形的讨论,指出相容元素i（Di〉2）和不相容元素j的CTi—CTi和IogCTi—logCTi图解上可以区分不同成因的岩石,包括分离结晶作用、平衡结晶作用、实比和非实比分离熔融作用,实比和非实比分批熔融作用以及岩浆混合作用。不同成因岩石在协变图上其成分点的分布有明显差别,这在解释山西临县紫金山碱性环状杂岩体的成因中获得满意的效果,从而确认该者体是多次分批部分熔融作用的产物。     

Equilibrium and Fractional Crystallization Experiments at 0{middle dot}7 GPa; the Effect of Pressure on Phase Relations and Liquid Compositions of Tholeiitic Magmas

Two series of anhydrous experiments have been performed in anend-loaded piston cylinder apparatus on a primitive, mantle-derivedtholeiitic basalt at 0·7 GPa pressure and temperaturesin the range 1060–1270°C. The first series are equilibriumcrystallization experiments on a single basaltic bulk composition;the second series are fractionation experiments where near-perfectfractional crystallization was approached in a stepwise mannerusing 30°C temperature increments and starting compositionscorresponding to that of the previous, higher temperature glass.At 0·7 GPa liquidus temperatures are lowered and thestability of olivine and plagioclase is enhanced with respectto clinopyroxene compared with phase equilibria of the samecomposition at 1·0 GPa. The residual solid assemblagesof fractional crystallization experiments at 0·7 GPaevolve from dunites, followed by wehrlites, gabbronorites, andgabbros, to diorites and ilmenite-bearing diorites. In equilibriumcrystallization experiments at 0·7 GPa dunites are followedby plagioclase-bearing websterites and gabbronorites. In contrastto low-pressure fractionation of tholeiitic liquids (1 bar–0·5GPa), where early plagioclase saturation leads to the productionof troctolites followed by (olivine) gabbros at an early stageof differentiation, pyroxene still crystallizes before or withplagioclase at 0·7 GPa. The liquids formed by fractionalcrystallization at 0·7 GPa evolve through limited silicaincrease with rather strong iron enrichment following the typicaltholeiitic differentiation path from basalts to ferro-basalts.Silica enrichment and a decrease in absolute iron and titaniumconcentrations are observed in the last fractionation step afterilmenite starts to crystallize, resulting in the productionof an andesitic liquid. Liquids generated by equilibrium crystallizationexperiments at 0·7 GPa evolve through constant SiO₂ increaseand only limited FeO enrichment as a consequence of spinel crystallizationand closed-system behaviour. Empirical calculations of the (dry)liquid densities along the liquid lines of descent at 0·7and 1·0 GPa reveal that only differentiation at the baseof the crust (1·0 GPa) results in liquids that can ascendthrough the crust and that will ultimately form granitoid plutonicand/or dacitic to rhyodacitic sub-volcanic to volcanic complexes;at 0·7 GPa the liquid density increases with increasingdifferentiation as a result of pronounced Fe enrichment, renderingit rather unlikely that such differentiated melt will reachshallow crustal levels. KEY WORDS: tholeiitic magmas; experimental petrology; equilibrium crystallization; fractional crystallization     

Cumulate gabbros from the Southwest Indian Ridge, 54°S-7° 16′ E: implications for magmatic processes at a slow spreading ridge

A diverse volcanic and plutonic rock suite was recovered from the center of the 80 km long ridge segment of the Southwest Indian Ridge (54°S, 7°16 E) between the Islas Orcadas and Shaka Fracture Zones. The cumulus nature of the gabbroic rocks in the suite is indicated by phase, modal and cryptic layering, igneous lamination, and low incompatible element abundances. We present a mass-balance model for calculating the proportions and compositions of cumulus phases and crystallized intercumulus liquid from bulk-rock major element compositions. The model is based on the ability to define a compositional array of basaltic liquids and on the assumption that cumulus minerals are initially in equilibrium with trapped liquid. Calculated proportions of trapped liquid range from 3%–15%; values that are characteristic of adcumulates to mesocumulates. Models of postcumulus crystallization indicate significant enrichments of incompatible elements and buffering of compatible elements in residual trapped liquids, thus explaining the high TiO₂ contents observed in magnesian clinopyroxenes. Cumulus phase assemblages and compositions suggest solidification in shallow level magma chambers, but disequilibrium plagioclase compositions suggest some crystallization at greater depth. Furthermore, basalt compositions projected onto the olivine-clinopyroxenequartz pseudoternary suggest magma generation over a range of pressures (from less than 10 to greater than 20 kb) as well as polybaric fractional crystallization. We suggest that the Southwest Indian Ridge is characterized by low magma supply with small batches of melt that either ascend directly to the surface having undergone limited polybaric crystallization or are trapped in shallow crustal magma chambers where they evolve and solidify to form cumulate gabbros. The adcumulus nature of the gabbros investigated here suggests slow cooling rates typical of large intrusions implying relatively large, but ephemeral magma chambers below segments of the Southwest Indian Ridge.     

The Genesis of Intermediate and Silicic Magmas in Deep Crustal Hot Zones

A model for the generation of intermediate and silicic igneousrocks is presented, based on experimental data and numericalmodelling. The model is directed at subduction-related magmatism,but has general applicability to magmas generated in other platetectonic settings, including continental rift zones. In themodel mantle-derived hydrous basalts emplaced as a successionof sills into the lower crust generate a deep crustal hot zone.Numerical modelling of the hot zone shows that melts are generatedfrom two distinct sources; partial crystallization of basaltsills to produce residual H₂O-rich melts; and partial meltingof pre-existing crustal rocks. Incubation times between theinjection of the first sill and generation of residual meltsfrom basalt crystallization are controlled by the initial geotherm,the magma input rate and the emplacement depth. After this incubationperiod, the melt fraction and composition of residual meltsare controlled by the temperature of the crust into which thebasalt is intruded. Heat and H₂O transfer from the crystallizingbasalt promote partial melting of the surrounding crust, whichcan include meta-sedimentary and meta-igneous basement rocksand earlier basalt intrusions. Mixing of residual and crustalpartial melts leads to diversity in isotope and trace elementchemistry. Hot zone melts are H₂O-rich. Consequently, they havelow viscosity and density, and can readily detach from theirsource and ascend rapidly. In the case of adiabatic ascent themagma attains a super-liquidus state, because of the relativeslopes of the adiabat and the liquidus. This leads to resorptionof any entrained crystals or country rock xenoliths. Crystallizationbegins only when the ascending magma intersects its H₂O-saturatedliquidus at shallow depths. Decompression and degassing arethe driving forces behind crystallization, which takes placeat shallow depth on timescales of decades or less. Degassingand crystallization at shallow depth lead to large increasesin viscosity and stalling of the magma to form volcano-feedingmagma chambers and shallow plutons. It is proposed that chemicaldiversity in arc magmas is largely acquired in the lower crust,whereas textural diversity is related to shallow-level crystallization. KEY WORDS: magma genesis; deep hot zone; residual melt; partial melt; adiabatic ascent     

Petrogenesis of Tertiary Continental Intra-plate Lavas from the Westerwald Region, Germany

Tertiary volcanic rocks from the Westerwald region range frombasanites and alkali basalts to trachytes, whereas lavas fromthe margin of the Vogelsberg volcanic field consist of morealkaline basanites and alkali basalts. Heavy rare earth elementfractionation indicates that the primitive Westerwald magmasprobably represent melts of garnet peridotite. The Vogelsbergmelts formed in the spinel–garnet peridotite transitionregion with residual amphibole for some magmas suggesting meltingof relatively cold mantle. Assimilation of lower-crustal rocksand fractional crystallization altered the composition of lavasfrom the Westerwald and Vogelsberg region significantly. Thecontaminating lower crust beneath the Rhenish Massif has a differentisotopic composition from the lower continental crust beneaththe Hessian Depression and Vogelsberg, implying a compositionalboundary between the two crustal domains. The mantle sourceof the lavas from the Rhenish Massif has higher ²⁰⁶Pb/²⁰⁴Pband ⁸⁷Sr/⁸⁶Sr than the mantle source beneath the Vogelsbergand Hessian Depression. The 30–20 Ma volcanism of theWesterwald apparently had the same mantle source as the QuaternaryEifel lavas, suggesting that the magmas probably formed in apulsing mantle plume with a maximum excess temperature of 100°Cbeneath the Rhenish Massif. The relatively shallow melting ofamphibole-bearing peridotite beneath the Vogelsberg and HessianDepression may indicate an origin from a metasomatized portionof the thermal boundary layer. KEY WORDS: continental rift volcanism; basanites; trachytes; assimilation; fractional crystallization; partial melting     

Trace element and isotopic variations in a zoned pluton and associated volcanic rocks,Unalaska Island,Alaska: A model for fractionation in the Aleutian calcalkaline suite

Trace elements, including rare earth elements (REE), exhibit systematic variations in plutonic rocks from the Captains Bay pluton which is zoned from a narrow gabbroic rim to a core of quartz monzodiorite and granodiorite. The chemical variations parallel those in the associated Aleutian calcalkaline volcanic suite. Concentrations of Rb, Y, Zr and Ba increase as Sr and Ti decrease with progressive differentiation. Intermediate plutonic rocks are slightly enriched in light REE (La/Yb=3.45–9.22), and show increasing light REE fractionation and negative Eu anomalies (Eu/Eu^*=1.03–0.584). Two border-zone gabbros have similar REE patterns but are relatively depleted in total REE and have positive Eu anomalies; indicative of their cumulate nature. Initial ⁸⁷Sr/⁸⁶Sr ratios in 8 samples (0.70299 to 0.70377) are comparable to those of volcanic rocks throughout the arc and suggest a mantle source for the magmas. Oxygen isotopic ratios indicate that many of the intermediate plutonic rocks have undergone oxygen isotopic exchange with large volumes of meteoric water during the late stages of crystallization; however no trace element or Sr isotopic alteration is evident.Major and trace element variations are consistent with a model of inward fractional crystallization of a parental high-alumina basaltic magma at low pressures (6 kb). Least-squares approximations and trace element fractionation calculations suggest that differentiation in the plutonic suite was initially controlled by the removal of calcic plagioclase, lesser pyroxene, olivine and Fe-Ti oxides but that with increasing differentiation and water fugacity the removal of sub-equal amounts of sodic plagioclase and hornblende with lesser Fe-Ti oxides effectively drove residual liquids toward dacitic compositions. Major and trace element compositions of aplites which intrude the pluton are not adequately explained by fractional crystallization. They may represent partial melts derived from the island arc crust. Similarities in Sr isotopes, chemical compositions and differentiation trends between the plutonic series and some Aleutian volcanic suites indicates that shallow-level fractional crystallization is a viable mechanism for generating the Aleutian calcalkaline rock series.LDGO Contribution no. 2964