Minor Phases as Carriers of Trace Elements in Non-Modal Crystal-Liquid Separation Processes I: Basic Relationships

Some trace elements have the property that, although they areincompatible with most mineral phases in magmatic systems, theyare strongly concentrated in certain minor mineral phases. Theseminor phases, termed here ‘carrier-phases’, andtheir associated trace elements include platinum group elementsin base metal sulphide and chromite; chromium and vanadium inmagnetite; uranium group metals in zircon and monazite; andrare earth elements in monazite and xenotime. Carrier-phasesmay form only a small fraction of a source rock undergoing partialmelting and tend to be eliminated from the residue at an intermediatepoint in the partial melting history; conversely, those sameminor carrier-phases tend to precipitate late during fractionalcrystallization of a liquid produced in the above manner, butmay constitute a high proportion of the cumulate then forming.This paper explores the phase equilibria aspects of such processesin a simple system, outlining a nomenclature which is then usedin a mathematical treatment applicable to non-modal meltingand crystallization processes involving several crystal species.The treatment at this stage assumes constant individual crystal–liquiddistribution coefficients. Equations are developed, which areapplied in a companion paper to illustrate the behaviour thatcan be anticipated when carrier-phases play a significant rolein trace element location during melting and crystallization. KEY WORDS: uranium; thorium; platinum group elements; carrier-phase; trace element     

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     

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.     

The Highly Compatible Trace Element Paradox--Fractional Crystallization Revisited

Field relations in dissected volcanic terrains and the internalevidence of persistent low-pressure cotectic character in eruptedbasalts point to the frequent and substantial modification ofliquid compositions by some form of partial crystallizationwithin the crust In contrast, the highly compatible trace elementsdo not display the marked variations and extreme depletionswhich are predicted to result from perfect fractional crystallization(PFC). Imperfect fractional crystallization, refilling of magmachambers during fractionation and in situ crystallization areimportant factors which can help to explain this apparent paradox.This paper explores another effect, the integration of residualliquids from differing extents of partial crystallization, whichcan help to resolve this paradox, even while still permittingperfect fractional crystallization at all points in the magmachamber. Integration of such residual liquids through the thicknessof the crystallization zone is explicit, although not implemented,in the model of in situ crystallization proposed by Langmuir('Nature 340, 199–205, 1989). It may be separated as aprocess for purposes of mathematical modelling from the basicconcept of partial crystallization of small packets of magmawith remixing of the residual liquids into the main body ofmagma. Integration of melts from differing extents of partialcrystallization might in principle also be applied to the caseof lateral variations in the mass fraction crystallized withposition in the magma chamber. Integrated PFC itself can developresidual liquids which differ little from products of equilibrium(batch) crystallization (FTC) at the same average mass fractionof liquid remaining in both incompatible and compatible traceelement concentrations. For one specific combination of parametersthese integrated liquids are identical in composition at allvalues of the distribution coefficient to the EPC liquid. Atother values of the parameters the integrated liquids may even—anew paradox— have higher relative concentrations of highlycompatible elements than the EPC products. Any integration ofresidual liquids from different mass fractions of PFC rapidlyeliminates what have in the past been taken to be the diagnosticdifferences between PFC and EPC Integration of EPC liquids (towardswhich the products of imperfect fractional crystallization processeswill tend) produces even more pronounced effects, with highlycompatible elements less depleted even than in EPC and far lessdepleted than would be predicted by simple models. When interpretedaccording to oversimplified models, sequences of residual liquidsproduced in such processes might appear to be inconsistent withproducts of a partial crystallization process and to requirea process of progressively smaller mass fractions of meltingof inhomogeneous and progressively more refractory (higher mg-number)source regions. KEY WORDS: highly compatible elements; in situ crystallization; boundary layer; integrated crystallization ^*e-mail: oharamj{at}cardiff.ac.uk     

Using In Situ Trace-Element Determinations to Monitor Partial-Melting Processes in Metabasites

Peak metamorphism (800–850°C, 8–10 kbar) inthe Harts Range Meta-Igneous Complex (Harts Range, central Australia)was associated with localized partial melting by the reactionhornblende + plagioclase + quartz + H₂O = garnet + clinopyroxene+ titanite + melt. In situ trace-element determinations of prograde,peak and retrograde minerals in migmatitic metabasites and associatedtonalitic melts using laser-ablation ICP–MS has allowedmonitoring of a range of partial-melting processes (melting,melt segregation and back-reaction between crystallizing meltand restitic minerals). Mass balance calculations indicate thattitanite is a major carrier of trace elements such as Ti, Nb,Ta, Sm, U and Th, and therefore may be an important accessoryphase to control the redistribution of these elements duringthe partial melting of amphibolites. Titanite preferentiallyincorporates Ta over Nb and, hence, residual titanite mightassist in the formation of melts with high Nb/Ta. The fact thatsingle minerals record different rare earth element (REE) patterns,from prograde to peak to retrograde conditions, demonstratesthat REE diffusion is not significant up to 800°C. Therefore,trace-element analysis in minerals can be a powerful tool toinvestigate high-grade metamorphic processes beyond the limitsgiven by major elements. KEY WORDS: Harts Range; laser-ablation ICP–MS; metabasites; partial melting; trace elements     

Origin of Low-K Intermediate Lavas at Nekoma Volcano, NE Honshu Arc, Japan: Geochemical Constraints for Lower-Crustal Melts

Extremely low-K basaltic andesite to andesite lavas at Nekomavolcano, situated in the frontal volcanic zone of the NE Honshuarc, were produced from melts that originated in the lower crust.Multiple incompatible trace element model calculations suggestthat extremely low-K basalt found in the same arc is a naturalanalog for the source composition. However, fractional crystallization,magma mixing, and crustal contamination models of primary low-Kbasalt cannot reproduce the Nekoma chemical composition. Derivationof melts from an extremely low-K amphibolitic lower-crustalrock with the residual mineral phases hornblende, olivine, pyroxenes,plagioclase, and magnetite is plausible. Major element compositionsof Nekoma lavas are very similar to those of experimental meltsof amphibolite dehydration melting, which further support theproposal. Light rare earth elements are slightly enriched, buttotal rare earth element abundances are relatively low, suggestinga high degree of partial melting of the source. Ba/Th ratiosare low for frontal arc lavas, reflecting modification of theratio during partial melting. Zr/Hf and Nb/Ta ratios are significantlygreater than is usual for arc lavas, suggesting an anomaloussource composition. Markedly low K, Rb, Cs contents in the extremelylow-K lavas are attributed to an extremely low-K source. Underplatingof an extremely low-K basalt originating from a hydrous depletedmantle wedge could form such an amphibolite. In contrast, Ndand Sr isotope ratios fall close to Bulk Earth values, indicatingan isotopically enriched source. Hornblende-bearing rocks maypredominate in the lower crust of the NE Honshu arc, based onthe observation of crustal xenoliths. The presence of largelow-V_p regions at lower-crustal depths beneath the frontal arcis suggested by geophysical observations. These observationsfurther support lower-crustal melting beneath Nekoma as theorigin of the intermediate low-K lavas. KEY WORDS: amphibolite source; crustal melting; low-K andesite; Sr–Nd isotopes; trace element     

Chemistry of the Shiant Isles Main Sill, NW Scotland, and Wider Implications for the Petrogenesis of Mafic Sills

Major and trace element data for the Tertiary, Shiant IslesMain Sill, NW Scotland, are used to discuss its complex internaldifferentiation. Vertical sections through the sill exhibitsharp breaks in chemistry that coincide with changes in texture,grain size and mineralogy. These breaks are paired, top andbottom, and correspond to the boundaries of intrusive units,confirming a four-phase multiple-intrusion model based on fieldrelations, petrography, mineralogy and isotopes. Whole-rockchemistry is consistent with this model and necessitates onlyminor revisions to the intrusive and differentiation mechanismspreviously proposed. The rocks contain strongly zoned minerals(e.g. olivine Fo_70–5, clinopyroxene Mg# = 75–5,plagioclase An_75–5) indicating almost perfect fractionalcrystallization, but whole-rock compositions do not show suchextreme variations. Thus, while residual liquids became highlyevolved in situ, they mainly became trapped within the crystalnetwork and did not undergo wholesale inward migration. Someinward (mainly upward) concentration of residual liquids didoccur to form a ‘sandwich horizon’, but the morevolatile-rich, late-stage liquids that did not crystallize insitu appear to have migrated to higher levels in the sill toform pegmatitic horizons. Parental liquid compositions are modelledfor the intrusive units and it is concluded that the originalparent magma formed by partial melting of upper mantle thatwas more depleted in LREE than the sources of most ScottishTertiary basaltic rocks. Incompatible trace elements in thepicrodolerite–crinanite intrusive unit support isotopeevidence that its parent magma was contaminated by crustal material.Attempts to reconcile the chemical characteristics of the sillwith a recently proposed petrogenetic model based on a singleintrusion of magma differentiated by novel, but controversial,processes fail comprehensively. It is predicted that the complexpetrogenetic history of the Shiant Isles sill is not unusualand could become the model for other large (>50 m thick)sills. KEY WORDS: alkali basalt; differentiation; geochemistry; multiple intrusion; Shiant Isles; sill     

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.     

The compositions of mantle-derived melts developed during the Alpine continental collision

The intrusions of Adamello and Valmasino Bregaglia are related to the Alpine continental collision and contain cumulus amphibole-rich gabbroic rocks (hornblendites, amphibole-gabbros and olivine-gabbros). Literature Nd-Sr-O data suggest that these rocks were derived from uncontaminated mantle-derived liquids. Brown amphibole and its clinopyroxene inclusions were analysed for a large number of trace elements by laser ablation ICP-MS and ion microprobe. On the basis of a suitable set of ^S/LD, the compositions of parental liquids were determined. Primary liquid compositions were calculated assuming variable degrees of early fractional crystallisation on the basis of olivine composition. Computed liquids have a marked enrichment of light, large-ion lithophile elements (LILEs), U and Th over high field strength elements (HFSEs) and rare-earth elements (REEs). The light REEs are slightly enriched relative to heavy REEs, and clear Nb and Ta negative anomalies are lacking. HFSE and HREE compositions can be related to small degrees of batch melting of a spinel facies, MORB-type source. The enrichment of highly incompatible trace elements over HFSEs and HREEs reflect metasomatic processes which could be of Alpine and/or pre-Alpine age. The latter hypothesis implies a subcontinental lithospheric origin for the mantle sources. Nevertheless, the abrupt B enrichment of Valmasino Bregaglia liquids suggests that the partial melting of the mantle sources was triggered by a B-rich fluid of Alpine origin.     

The Signature of Amphibole in Mafic Alkaline Lavas, a Study in the Northern Canadian Cordillera

Despite evidence for its involvement, the importance of amphibolein controlling the compositions of mafic alkaline magmas remainsunder-appreciated. Relatively small variations in large ionlithophile elements (LILE) with respect to other incompatibleelements, such as light rare-earth elements (LREE) or Th, requirethat amphibole was an important residual phase during the productionof Late Tertiary to Recent olivine nephelinite (Ol-NEPH) magmasbeneath the northern Canadian Cordillera. The erupted maficmagma types define a continuous array from Ol-NEPH to hypersthene-normativeolivine basalt (Hy-NORM AOB). The overall compositional arrayhas a sense of curvature which is counter to binary mixing,but can be modeled by two distinct linear melting trends: onefrom Ol-NEPH to basanite (BASAN) compositions, during whichamphibole controlled the composition of the melt, and the ratiosof LILE/LREE change significantly, but the ratios of high fieldstrength elements (HFSE) remain relatively constant; the otherfrom BASAN to Hy-NORM AOB corresponding to the melting of alherzolite assemblage, following the exhaustion of amphibole,across which the ratios of LILE/LREE remain relatively constant,but the ratios of HFSE change significantly. Other intraplate alkaline suites, such as those of the HawaiianIslands, show similar evidence for the involvement of residualamphibole in the genesis of Ol-NEPH to BASAN magmas. The meltingof any amphibole-bearing mantle assemblage is likely to be atwo-step process, regardless of whether the amphibole is segregatedas veins or distributed interstitially. In a water-undersaturatedenvironment, the first stage of melting is controlled by thebreakdown of amphibole, which produces silica-saturated liquidsbelow 12 kbar and silica-undersaturated liquids at greater depths,with little contribution from other mineral phases. In the secondstage, following the exhaustion of amphibole, the major elementcompositions of subsequent melts change rapidly to equilibratewith a lherzolite mineralogy, but the incompatible trace-elementcharacteristics of the former amphibole persist. KEY WORDS: amphibole; mafic alkaline magmas; northern Canadian Cordillera; trace elements ^*Corresponding author     

The geochemistry of repetitive cyclical volcanism from basalt through rhyolite in the uchi-confederation greenstone belt,canada

Archean volcanic rocks in the Confederation Lake area, northwestern Ontario, Canada, are in three mafic to felsic cycles collectively 8,500 to 11,240 m thick. Each cycle begins with pillowed basalt and andesite flows and is capped with andesitic to rhyolitic pyroclastic rocks and minor flows. Seventy five samples from this succession were analyzed for major and trace elements including the rare earth elements. In two cycles, tholeiitic basalts are overlain by calcalkaline andesite to rhyolite. In the third, cycle, the tholeiitic basalts are overlain by tholeiitic rhyolites. Fe enrichment in basalts is accompanied by depletion of Ca, Al, Cr, Ni, and Sr, and enrichment in Ti, P, the rare earth elements, Nb, Zr, and Y. This is interpreted as open system fractionation of olivine, plagioclase, and clinopyroxene. Si enrichment in dacites and rhyolites is attributed to fractional crystallization of plagioclase, K-feldspar, and biotite. Tholeiitic basalt liquids are believed to be mantle-derived. Intercalated andesites with fractionated rare earth patterns appear to be products of mixing of tholeiitic basalt and rhyolite liquids and, andesites with flat rare earth patterns are probably produced by melting of previously depleted mantle. Felsic magmas are partial melts of tholeiitic basalt or products of liquid immiscibility in a tholeiitic system perhaps involving extreme fractionation in a high level magma chamber, and assimilation of sialic crust. It is concluded that Archean cyclical volcanism in this area involves the interplay of several magmatic liquids in processes of fractional crystallization, magma mixing, liquid immiscibility, and the probable existence of compositionally zoned magma chambers in the late stages of each cycle. The compositionally zoned chambers existed over the time period represented by the upper felsic portion of each cycle.     

Chemical Variations in Peridotite Xenoliths from Vitim, Siberia: Inferences for REE and Hf Behaviour in the Garnet-Facies Upper Mantle

Peridotite xenoliths in a Miocene picrite tuff from the Vitimvolcanic province east of Lake Baikal, Siberia, are samplesof the off-craton lithospheric mantle that span a depth rangefrom the spinel to garnet facies in a mainly fertile domain.Their major and trace element compositions show some scatter(unrelated to sampling or analytical problems), which is notconsistent with different degrees of partial melting or metasomatism.Some spinel peridotites and, to a lesser degree, garnet-bearingperidotites are depleted in heavy rare earth elements (HREE)relative to middle REE (MREE), whereas some garnet peridotitesare enriched in HREE relative to MREE, with Lu abundances muchhigher than in primitive mantle estimates. Clinopyroxenes fromseveral spinel peridotites have HREE-depleted patterns, whichare normally seen only in clinopyroxenes coexisting with garnet.Garnets in peridotites with similar modal and major elementcompositions have a broad range of Lu and Yb abundances. Overall,HREE are decoupled from MREE and Hf and are poorly correlatedwith partial melting indices. It appears that elements withhigh affinity to garnet were partially redistributed in theVitim peridotite series following partial melting, with feweffects for other elements. The Lu–Hf decoupling may disturbHf-isotope depletion ages and their correlations with meltingindices. KEY WORDS: garnet peridotite; lithospheric mantle; Lu–Hf isotope system; Siberia; trace elements     

Interpretation of trace element and isotope features of basalts: relevance of field relations, petrology, major element data, phase equilibria, and magma chamber modeling in basalt petrogenesis

The concentrations and ratios of the major elements determine the physical properties and the phase equilibria behavior of peridotites and basalts in response to the changing energy contents of the systems. The behavior of the trace elements and isotopic features are influenced in their turn by the phase equilibria, by the physical character of the partial melting and partial crystallization processes, and by the way in which a magma interacts with its wall rocks. Concentrating on the trace element and isotope contents of basalts to the exclusion of the field relations, petrology, major element data, and phase equilibria is as improvident as slaughtering the buffalo for the sake of its tongue. The crust is a cool boundary layer and a density filter, which impedes the upward transfer of hot, dense “primary” picritic and komatiitic liquids. Planetary crusts are sites of large-scale contamination and extensive partial crystallization of primitive melts striving to escape to the surface. Escape of truly unmodified primitive melts to the surface is a rare event, requiring the resolution of daunting problems in chemical and mechanical engineering. Primary status for volumetrically abundant basalts such as mid-ocean ridge basalt, ocean island basalt, and continental flood basalts is denied by their low-pressure cotectic character, first remarked upon on petrological grounds in 1928 and on experimental grounds in 1962. These basalt liquids are products of crystal-liquid separation at low pressure. Primary status for these common basalts is further denied by the phase equilibria of such compositions at elevated pressures, when the required residual mantle mineralogy (magnesian olivine and orthopyroxene) is not stable at the liquidus. It is also denied by the picritic or komatiitic nature of partial melts of candidate upper-mantle compositions at high pressures—a conclusion supported by calculation of the melt composition, which would need to be extracted in order to explain the chemical variation between fertile and residual peridotite in natural ultramafic rock suites. The subtleties of magma chamber partial crystallization processes can produce an astounding array of “pseudospidergrams,” a small selection of which have been explored here. Major modification of the trace element geochemistry and trace element ratios, even those of the highly incompatible elements, must always be entertained whenever the evidence suggests the possibility of partial crystallization. At one extreme, periodically recharged, periodically tapped magma chambers might undergo partial crystallization by ∼95% consolidation of a succession of small packets of the magma. Refluxing of the 5% residual melts from such a process into the main body of melt would lead to eventual discrimination between highly incompatible elements in that residual liquid comparable with that otherwise achieved by 0.1 to 0.3% liquid extraction in equilibrium partial melting. Great caution needs to be exercised in attempting the reconstruction of more primitive compositions by addition of troctolite, gabbro, and olivine to apparently primitive lava compositions. Special attention is focussed on the phase equilibria involving olivine, plagioclase (i.e., troctolite), and liquid because a high proportion of erupted basalts carry these two phases as phenocrysts, yet the equilibria are restricted to crustal pressures and are only encountered by wide ranges of basaltic compositions at pressures less than 0.5 GPa. The mere presence of plagioclase phenocrysts may be sufficient to disqualify candidate primitive magmas. Determination of the actual contributions of crustal processes to petrogenesis requires a return to detailed field, experimental, and forensic petrologic studies of individual erupted basalt flows; of a multitude of cumulate gabbros and their contacts; and of upper-mantle outcrops.     

Geochemistry of eucrites: genesis of basaltic eucrites, and Hf and Ta as petrogenetic indicators for altered antarctic eucrites

We performed instrumental neutron activation analysis on a large suite of antarctic and nonantarctic eucrites, including unbrecciated, brecciated, and polymict eucrites and cumulate and noncumulate eucrites. We evaluate the use of Hf and Ta, two highly incompatible elements, as sensitive indicators of partial melting or fractional crystallization processes. Comparison with rare earth element (REE) data from nonantarctic and antarctic eucrites shows that Hf and Ta are unaffected by the terrestrial alteration that has modified the REE contents and patterns of some antarctic eucrites. The major host phases for Hf and Ta—zircon, baddeleyite, ilmenite, and titanite—are much less susceptible to terrestrial alteration than the phosphate hosts of REEs. The host phases for Hf and Ta are minor or trace phases, so sample heterogeneity is a serious concern for obtaining representative compositions. The trace lithophile and siderophile element contents of noncumulate eucrites do not allow for a single, simple model for the petrogenesis of the howardite-eucrite-diogenite suite. Fractional crystallization models cannot reproduce the compositional relationship between eucrites of the main group-Nuevo Laredo trend and those of the Stannern trend. Equilibrium crystallization models cannot explain the trace element diversity observed among diogenites. Partial melting models cannot explain the W variations among eucrites, unless source regions had different metal contents. We suggest that slight variations in oxygen fugacity of eucrite source regions during partial melting can explain the W variations without requiring different metal contents. This hypothesis may fail to account for eucrite Co contents, however.     

Residence and redistribution of REE, Y, Zr, Th and U during granulite-facies metamorphism: behaviour of accessory and major phases in peraluminous granulites of central Spain

Accessory minerals are thought to play a key role in controlling the behaviour of certain trace elements such as REE, Y, Zr, Th and U during crustal melting processes under high-grade metamorphic conditions. Although this is probably the case at middle crustal levels, when a comparison is made with granulite-facies lower crustal levels, differences are seen in trace element behaviour between accessory minerals and some major phases. Such a comparison can be made in Central Spain where two granulite-facies terranes have equilibrated under slightly different metamorphic conditions and where lower crustal xenoliths are also found. Differences in texture and chemical composition between accessory phases found in leucosomes and leucogranites and those of melanosomes and protholiths indicate that most of the accessory minerals in melt-rich migmatites are newly crystallized. This implies that an important redistribution of trace elements occurs during the early stages of granulite-facies metamorphism. In addition, the textural position of the accessory minerals with respect to the major phases is crucial in the redistribution of trace elements when melting proceeds via biotite dehydration melting reactions. In granulitic xenoliths from lower crustal levels, the situation seems to be different, as major minerals show high concentration of certain trace elements, the distribution of which is thus controlled by reactions involving final consumption of Al-Ti-phlogopite. A marked redistribution of HREE–Y–Zr between garnet and xenotime (where present) and zircon, but also of LREE between feldspars (K-feldspar and plagioclase) and monazite, is suggested.     

Origin of calc-alkaline series lavas at Medicine Lake Volcano by fractionation,assimilation and mixing

The results of experimental studies and examination of variations in major elements, trace elements and Sr isotopes indicate that fractionation, assimilation and magma mixing combined to produce the lavas at Medicine Lake Highland. Some characteristics of the compositional differences among the members of the calc-alkalic association (basalt-andesite-dacite-rhyolite) can be produced by fractional crystallization, and a fractionation model reproduces the major element trends. Other variations are inconsistent with a fractionation origin. Elevated incompatible element abundances (K and Rb) observed in lavas intermediate between basalt and rhyolite can be produced through assimilation of a crustal component. An accompanying increase in ⁸⁷Sr/⁸⁶Sr from ∼ 0.07030 in basalt to ∼0.7040 in rhyolite is also consistent with crustal assimilation. The compatible trace element contents (Ni and Sr) of intermediate lavas can not be produced by fractional crystallization, and suggest a magma-mixing origin for some lavas. Unusual phenocryst assemblages and textural criteria in these lavas provide additional evidence for magma mixing. A phase diagram constructed from the low pressure melting experiments identifies a distributary reaction point, where olivine+augite react to pigeonite. Parental basalts reach this point at low pressures and undergo iron-enrichment at constant SiO₂ content. The resulting liquid line of descent is characteristic of the tholeiitic trend. Calc-alkalic differentiation trends circumvent the distributary reaction point by three processes: fractionation at elevated pH₂O, assimilation and magma mixing.     

Garnet-field Melting and Late-stage Refertilization in 'Residual' Abyssal Peridotites from the Central Indian Ridge

The role of residual garnet during melting beneath mid-oceanridges has been the subject of many recent investigations. Toaddress this issue from the perspective of melting residues,we obtained major and trace element mineral chemistry of residualabyssal peridotites from the Central Indian Ridge. Many clinopyroxeneshave ratios of middle to heavy rare earth elements (MREE/HREE)that are too low to be explained by melting in the stabilityfield of spinel peridotite alone. Several percent of meltingmust have occurred at higher pressures in the garnet peridotitestability field. Application of new trace element partitioningmodels, which predict that HREE are compatible in high-pressureclinopyroxene, cannot fully explain the fractionation of theMREE from the HREE. Further, many samples show textural andchemical evidence for refertilization, such as relative enrichmentsof highly incompatible trace elements with respect to moderatelyincompatible trace elements. Therefore, highly incompatibleelements, which are decoupled from major and moderately incompatibletrace elements, are useful to assess late-stage processes, suchas melt entrapment, melt–rock reaction and veining. Moderatelyincompatible trace elements are less affected by such late-stageprocesses and thus useful to infer the melting history of abyssalperidotites. KEY WORDS: abyssal peridotites; mantle melting; garnet     

The Skaergaard Layered Series. Part VI. Excluded Trace Elements

In contrast to the smooth trends of major elements and mineralcompositions, the excluded trace elements in the SkaergaardLayered Series have an irregular distribution that does notconform to the normal trends of Rayleigh-type fractionation.Their concentrations are about constant or even decline throughthe Lower and Middle Zones before increasing sharply to reachmaximum concentrations 100–200 m above the Sandwich Horizon.As in the case of included elements, the relative concentrationsof excluded elements in coexisting phases deviate widely fromthose predicted by experimentally determined partition coefficientsunder presumed magmatic conditions. This is seen most clearlyin the immiscible melanogranophyres and conjugate ferrogabbros.Although the major elements conform to the experimentally determinedrelations for immiscible liquids, the trace elements do not;they follow a totally independent trend. The abrupt increasein the concentrations of excluded elements in the upper partof the intrusion could plausibly be attributed to an additionof new magma or to a density inversion that resulted in upwardmigration of a late liquid or fluid, but these possibilitiesare inconsistent with the compositional and spatial relationsof the upper parts of the intrusion. Although a late residualliquid certainly migrated upward, the most likely explanationfor the observed distribution of excluded elements is that thepartition coefficients were altered by volatile components,which gradually increased during the early stages of crystallizationthen began to exsolve near the top of the Middle Zone. KEY WORDS: igneous differentiation; Skaergaard intrusion     

The Characteristics and Mechanism of Island-Arc Volcanism on the Central and Southern Fildes Peninsula, King George Island, Antarctica

The volcanic rock series on the Fildes Peninsula is the product of the later subduction of the Pacific platebeneath the Antarctic plate. It consists mainly of basalt, basaltic andesite and andesite with minor dacite. Itsisotopic ages range from 64.6±1 to 43±2 Ma, belonging to Palaeocene to Eocene. Volcanism in the area maybe divided into two phases. The contents of major oxides, rare earth elements (REE) and trace elements in vol-canic rocks formed in different phases show regular changes, which are mainly related to the rock associationsof these phases. Isotope geochemical studies indicate that the primitive magma in the area originating by par-tial melting in the upper mantle underwent fractional crystallization and ascended to the high-level (shallow)magma chamber. Before eruption the primitive basalt-andesitic magma was subjected to differentiation in thehigh-level magma chamber, forming zones of derivative magmas of different compositions. In various phasesmagma-conducting faults experienced periodic extension and cut through various derivative magma zones indifferent parts of the peninsula, leading to the eruption of magmas of different compositions on the surface andthe formation of volcanic rock associations of corresponding compositions.     

Trace element constraints on pegmatite genesis: Tin Mountain pegmatite,Black Hills,South Dakota

The Tin Mountain pegmatite is a small, zoned granitic body that is extremely enriched in Rb and Li, but has moderate concentrations of Sr and Ba. These trace elements are modelled using granitic distribution coefficients in order to test the potentials of partial melting of metasedimentary rocks and fractionation of a less-evolved granitic melt to have produced the parental liquid to the Tin Mountain pegmatite. Batch melting of any reasonable metasedimentary source rock would likely have yielded melts that were either insufficiently enriched in Rb and Li to be the parental liquid, or that had Sr and Ba concentrations that were much higher than those estimated for the parental liquid. The modelling of simple fractional crystallization and equilibrium crystallization of a granitic melt within the compositional range of the spatially associated Harney Peak Granite gives calculated melt compositions with either lower Sr and Ba concentrations or inadequate Rb and Li enrichments, to be the parent liquid of the pegmatite. At least two variants from simple crystal-liquid fractionation models can, however, successfully account for the derivation of the parent liquid: 1) generation of a Rb-, Li-, Ba- and Sr-rich granitic melt (outside of the compositional range of the sampled portions of the Harney Peak Granite complex) by low degrees of partial melting of metasedimentary rocks found in the Black Hills, followed by moderate extents of fractional or equilibrium crystallization, 2) derivation from Harney Peak granite via a complex, multi-stage crystal-liquid fractionation process, such as progressive equilibrium crystallization.