Alteration of a Komatiite Flow from Alexo, Ontario, Canada

To investigate the ability of komatiites to preserve duringalteration a record of the geochemical and isotopic compositionof the Archean mantle, we studied the petrology and geochemistryof a komatiite flow from Alexo, Ontario. Although this flowis relatively well preserved, two main types of alteration arerecognized: (1) hydration of variable intensity throughout thewhole flow; (2) local rodingitization. In samples only slightlyaffected by hydration, the alteration was essentially isochemical;where more intense, hydration was accompanied by the mobilityof TiO₂, Zr and the light rare-earth elements (LREE). The mobilityof these reputedly immobile trace elements is influenced bytexture and secondary mineral assemblages. Rodingitization wasaccompanied by still greater mobility. An Nd isochron with anage of 272698 Ma and Nd_(T) =+ 2.5 is defined by wholerock samples.Because the spread of Sm-Nd ratios is far greater than can beexplained by magmatic processes, the age is interpreted as thatof the main alteration event during or soon after emplacement.Scattered initial Sr isotopic ratios and Nd _(T) (+ 3.6 to +0.7)provide evidence for several younger events. The Nd (T) of magnaticclinopyroxene ( plus;3.8) is higher than that of mafic tofelsicrocks and pyroxenes from the Abibiti belt (Nd _(T) +2.5). Thisresult implies: (1) the komatiites exchanged Nd with surroundingrocks during waterrock interaction; (2) the isotopic compositionof komatiites and their mantle source differed from that ofthe more common basalts of the Abitibi belt. KEY WORDS: alteration; Archean; Komatiite; trace elements; radiogenic and stable islotopes ^*Present address: Ore Genesis Research Group, VIEPS Department of Earth Science, Monash University, Clayton, Vic 3168, Australia. Telephone: (61) 3 9905-3881. Fax: (61) 3 9905–4903. e-mail: ylahaye{at}artemis.earth.monash.edu.au     

Diverse Origins of Xenoliths from Seamounts at the Continental Margin, Offshore Central California

A diverse assemblage of small mafic and ultramafic xenolithsoccurs in alkalic lava from Davidson and Pioneer seamounts locatedat the continental margin of central California. Based on mineralcompositions and textures, they form three groups: (1) mantlexenoliths of lherzolite, pyroxenite, and dunite with olivineof >Fo₉₀; (2) ocean crust xenoliths of dunite with olivine<Fo₉₀, troctolite, pyroxene-gabbro, and anorthosite withlow-K₂O plagioclase; (3) cumulates of seamount magmas of alkalicgabbro with primary amphibole and biotite and anorthosites withhigh-K₂O plagioclase. The alkalic cumulates are geneticallyrelated to, but more evolved than, their host lavas and probablycrystallized at the margins of magma reservoirs. Modeling andcomparison with experimentally derived phases suggest an originat moderate pressures (0·5–0·9 GPa). Thehigh volatile contents of the alkalic host lavas may have pressurizedthe magma chambers and helped to propel the xenolith-bearinglavas directly from deep storage at the base of the lithosphereto the eruption site on the ocean floor, entraining fragmentsof the upper mantle and ocean crust cumulates from the underlyingabandoned spreading center. KEY WORDS: basaltic magmatism; continental margin seamounts; geothermobarometry; mineral chemistry; xenoliths     

Geochemical and Isotopic (O, Nd, Pb and Sr) Constraints on A-type Granite Petrogenesis Based on the Topsails Igneous Suite, Newfoundland Appalachians

The voluminous, bimodal, Silurian Topsails igneous suite consistsmainly of ‘A-type’ peralkaline to slightly peraluminous,hypersohnis to subsolvus granites with subordinate syenite,onzonite and diabase, plus consanguineous basalts and highsilicarhyolites. _Nd(T) values from the suite range from –1.5to +5.4; most granitoid components exhibit positive _Nd(T) values(+1.1 to +3.9). Granitoid initial ⁸⁷Sr/⁸⁶Sr and most ¹⁸ O valuesare in the range expected for rocks derived from mantle-likeprotoliths (0.701–0.706 and +6 to +80/). Restricted ²⁰⁷Pb/²⁰⁴Pbvariation is accompanied by significant dispersion of ²⁰⁶Pb/²⁰⁴Pband ²⁰⁸Pb/²⁰⁴Pb. Superficially, petrogenesis by either direct(via fractionation from basalt) or indirect (via melting ofjuvenile crust) derivation from mantle sources appears plausible.Remelting of the granulitic protolith of Ordovician are-typegranitoids can be ruled out, because these rocks exhibit negative_Nd(T) and a large range in ²⁰⁷Pb/²⁰⁴Pb. Geochemical and isotopicrelationships are most compatible with remelting of hybridizedlithospheric mantle generated during arc-continent collision.A genetic link is suggested among collision-related delaminationor slab break-off events and emplacement of ‘post-erogenic’granite suites. A-type granites may recycle previously subductedcontinental material, and help explain the mass balance notedfor modern arcs. However, they need not represent net, new,crustal growth. KEY WORDS: A-type granites; juvenile crust; isotopes; Newfoundland ^*Telephone: (613) 995-4972. Fax: (613) 995-7997. e-mail: jwhalen{at}gsc.emr.ca     

Petrological, Geochemical and Isotopic Constraints on the Origin of the Harzburg Intrusion, Germany

We present mineralogical, petrological and geochemical datato constrain the origin of the Harzburg mafic–ultramaficintrusion. The intrusion is composed mainly of mafic rocks rangingfrom gabbronorite to quartz diorite. Ultramafic rocks are veryrare in surface outcrops. Dunite is observed only in deepersections of the Flora I drill core. Microgranitic (fine-grainedquartz-feldspathic) veins found in the mafic and ultramaficrocks result from contamination of the ultramafic magmas bycrustal melts. In ultramafic and mafic compositions cumulatetextures are widespread and filter pressing phenomena are obvious.The order of crystallization is olivine pargasite, phlogopite,spinel plagioclase, orthopyroxene plagioclase, clinopyroxene.Hydrous minerals such as phlogopite and pargasite are essentialconstituents of the ultramafic cumulates. The most primitiveolivine composition is Fo_89·5 with 0·4 wt % NiO,which indicates that the olivine may have been in equilibriumwith primitive mantle melts. Coexisting melt compositions estimatedfrom this olivine have mg-number = 71. The chemical varietyof the rocks constituting the intrusion and the mg-number ofthe most primitive melt allow an estimation of the approximatecomposition of the mantle-derived primary magma. The geochemicalcharacteristics of the estimated magma are similar to thoseof an island-arc tholeiite, characterized by low TiO₂ and alkalisand high Al₂O₃. Geochemical and Pb, Sr and Nd isotope data demonstratethat even the most primitive rocks have assimilated crustalmaterial. The decoupling of Sr from Nd in some samples demonstratesthe influence of a fluid that transported radiogenic Sr. Leadof crustal origin from two isotopically distinct reservoirsdominates the Pb of all samples. The ultramafic rocks and thecumulates best reflect the initial isotopic and geochemicalsignature of the parent magma. Magma that crystallized in theupper part of the chamber was more strongly affected by assimilatedmaterial. Petrographic, geochemical and isotope evidence demonstratesthat during a late stage of crystallization, hybrid rocks formedthrough the mechanical mixing of early cumulates and melts withstrong crustal contamination from the upper levels of the magmachamber. KEY WORDS: Harzburg mafic–ultramafic intrusion; Sr–Nd–Pb isotopes; magma evolution; crustal contamination     

Basalt and Sediment Geochemistry and Magma Petrogenesis in a Transect from Oceanic Island Arc to Rifted Continental Margin Arc: the Kermadec--Hikurangi Margin, SW Pacific

Sediment mixing and recycling through a subduction zone canbe detected in lead isotopes and trace elements from basaltsand sediments from the Kermadec-Hikurangi Margin volcanic arcsystem and their coupled back-arc basins. Sr, Nd and Pb isotopesfrom the basalts delineate relatively simple, almost overlapping,arrays between back-arc basin basalts of the Havre Trough-NgatoroBasin (⁸⁷Sr/⁸⁶Sr = 0.70255; _Nd=+9.3; ²⁰⁶Pb/²⁰⁴Pb = 18.52; ²⁰⁸Pb/²⁰⁴Pb= 38.18), island arc basalts from the Kermadec Arc togetherwith basalts from Taupo Volcanic Zone (⁸⁷Sr/⁸⁶Sr 0.7042; _Nd= +5; ²⁰⁶Pb/²⁰⁴Pb= 18.81; ²⁰⁸Pb/²⁰⁴Pb = 38.61), and sedimentsderived from New Zealand's Mesozoic (Torlesse) basement (⁸⁷Sr/⁸⁶Sr 0.715; _Nd —4; ²⁰⁶Pb/²⁰⁴Pb 18.86; ²⁰⁸Pb/²⁰⁴Pb 38.8).Basalts from the arc front volcanoes have high Cs, Rb, Ba, Th,U and K, and generally high but variable Ba/La, Ba/Nb ratios,characteristic of subduction-related magmas, relative to typicaloceanic basalts. These signatures are diluted in the back-arcbasins, which are more like mid-ocean ridge basalts. Strongchemical correlations in plots of SiO₂ vs CaO and loss on ignitionfor the sediments (finegrained muds) are consistent with mixingbetween detrital and biogenic (carbonate-rich) components. Otherdata, such as Zr vs CaO, are consistent with the detrital componentcomprising a mixture of arc- and continent-derived fractions.In chondrite-normalized diagrams, most of the sediments havelight rare earth element enriched patterns, and all have negativeEu anomalies. The multielement diagrams have negative spikesat Nb, P and Ti and distinctive enrichments in the large ionlithophile elements and Pb relative to mantle. Isotopic measurementsof Pb, Sr and Nd reveal restricted fields of Pb isotopes butwide variation in Nd and Sr relative to other sediments fromthe Pacific Basin. Rare K-rich basalts from Clark Volcano towardthe southern end of the oceanic Kermadec Island Arc show unusualand primitive characteristics ( 2% K₂O at 50% SiO₂, Ba 600p.p.m., 9–10% MgO and Ni > 100 p.p.m.) but have highlyradiogenic Sr, Nd and Pb isotopes, similar to those of basaltsfrom the continental Taupo Volcanic Zone. These oceanic islandarc basalts cannot have inherited their isotope signatures throughcrustal contamination or assimilation—fractional crystallizationtype processes, and this leads us to conclude that source processesvia bulk sediment mixing, fluid and/or melt transfer or somecombination of these are responsible. Although our results showclear chemical gradients from oceanic island arc to continentalmargin arc settings (Kermadec Arc to Taupo Volcanic Zone), overlapbetween the data from the oceanic and continental sectors suggeststhat the lithospheric (crustal contamination) effect may beminimal relative to that of sediment subduction. Indeed, itis possible to account for the chemical changes by a decreasenorthward in the sediment flux into the zone of magma genesis.This model receives support from recent sediment dispersal studiesin the Southern Ocean which indicate that a strong bottom current(Deep Western Boundary Current) flows northward along the easterncontinental margin of New Zealand and sweeps continental derivedsediment into the sediment-starved oceanic trench system. Thetrace element and isotopic signatures of the continental derivedcomponent of this sediment are readily distinguished, but alsodiluted in a south to north direction along the plate boundary. KEY WORDS: subduction zone basalts; sediments; Sr-, Nd-, Pb-isotopes; trace elements ^*Present address: School of Earth Sciences, University of Melbourne, Parkville, Vic. 3052, Australia.     

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     

Evidence of Two Different Components in a Hercynian Peraluminous Cordierite-bearing Granite: the San Basilio Intrusion (Central Sardinia, Italy)

Detailed petrographic and geochemical data and Sr and Nd isotopecompositions of enclaves and host-granite are reported for oneof the largest strongly peraluminous cordierite-bearing intrusionsof the Hercynian Sardinia-Corsica Batholith: the San BasilioGranite. Compared with other peraluminous series, the San BasilioGranite has a ‘non-minimum melt’ composition andshows variations primarily owing to fractionation of early-crystallizedplagioclase, quartz and biotite. Crystallization age is constrainedat 305 Ma, by Rb-Sr whole-rock age [30523 Ma with (⁸⁷Sr/⁸⁶Sr)_i= 0.711050.00041], and occurred during late Hercynian tectonicevents. _Nd(305Ma) values range from –7.8 to –7.5.The San Basilio Granite contains both magmatic and metamorphicenclaves. Magmatic enclaves, similar to mafic microgranularenclaves common in calc-alkaline granitoids, are tonalitic incomposition and show a variation in silica content from 60.3to 67.7 wt % correlating with a variation in (⁸⁷Sr/⁸⁶ Sr) _(305Ma)and _{Nd (305 Ma)} from 0.7092 to 0.7109 and from –6.6 to–7.4, respectively. Together with petrographic and othergeochemical data, the Sr and Nd isotopic data record differentstages in a complex homogenization process of an unrelated maficmagma with a crustal melt. A process of simple mixing may accountfor the variations of nonalkali elements and, to some extent,of Sr and Nd isotopes, whereas the distribution of alkali elementsrequires diffusioncontrolled mass transfer. Petrographic andmineralogical data on metamorphic enclaves and geochemical modellingfor trace elements in granite indicate melt generation by high-degreepartial melting involving biotite breakdown of a dominantlyquartzo-feldspathic protolith at about T>750–800Cand P>6 kbar leaving a granulite facies garnet-bearing residue,followed by emplacement at 3 kbar. _Nd(305Ma) values of thegranite fall within the range defined by the pre-existing metamorphicrocks but (⁸⁷Sr/⁸⁶Sr) _(305Ma) ratios are lower, indicating involvementof at least two distinct components: a dominant crustal componentand a minor well-mixed mafic end-member. These data point toa decoupling between the Sr-Nd isotope systematics and majorand trace element compositions, suggesting that the effect ofthe mafic component was minor on granite major and trace elementconcentrations, but significant on Sr and Nd isotopes. The studyof the magmatic enclaves and the isotopic evidence demonstratethat unrelated mafic magmas, probably derived from the mantle,had a close spatial and temporal association with the productionof ‘on-minimum melt’ strongly peraluminous granites,and support the proposal that heat from the mafic magma contributedto crustal melting. KEY WORDS: cordierite-bearing granite; enclaves; felsic-mafic interaction; Sardinia-Corsica Batholith; Sr and Nd isotopes ^*Corresponding author.     

Zircon Hf Isotopic Evidence for Mixing of Crustal and Silicic Mantle-derived Magmas in a Zoned Granite Pluton, Eastern Australia

Zircon Hf isotopic data from a zoned pluton of the Moonbi supersuite,New England batholith, eastern Australia, are consistent withmagma mixing between two silicic melts, each derived from isotopicallydistinct sources. Although zircons from three zones within theWalcha Road pluton give a U–Pb crystallization age of249 ± 3 Ma, zircon populations from each zone have arange in _Hf. Zircons from the mafic hornblende–biotitemonzogranite pluton margin and intermediate zones have _Hf +5to +11, whereas those from the more felsic centre of the plutonhave _Hf +7 to +16, representing a total variation of 11 _Hfunits. The Lu–Hf depleted mantle model ages range from650 to 250 Ma, with the younger zircons present only in thefelsic pluton centre. The variation in _Hf indicates the involvementof silicic melts from at least two sources, one a crustal componentwith a Neoproterozoic model age and the other a primitive mantle-derivedcomponent with model ages similar to the U–Pb crystallizationage of the pluton. The zircons reflect the isotopic compositionsof the different proportions of crustal-derived silicic melt,relative to mantle-derived silicic melt, between melt generationand final pluton construction. The Walcha Road pluton is consideredto have formed by incremental assembly of progressively morefelsic melt batches resulting from mixing, replenishment andcrystal–melt separation, with final pluton constructioninvolving mechanical concentration as zones of crystal mush.The zoned pluton and, more broadly, the Moonbi supersuite provideexamples of magma mixing by which the more silicic units havemore juvenile isotopic compositions as a result of increasingproportions of residual melt from basalt fractionation, relativeto crustal partial melt. KEY WORDS: Australia; granite magma mixing; zircon; zoned pluton; Hf isotopes     

Fenitizing Processes Induced by Ferrocarbonatite Magmatism at Swartbooisdrif, NW Namibia

The southeastern margin of the anorthositic Kunene IntrusiveComplex, NW Namibia, has been subsequently invaded by Mesoproterozoicsyenite, nepheline syenite and ferrocarbonatite dykes alongNE- and SE-trending faults. The first generation of carbonatiteintrusions frequently contains fenitized anorthositic wall-rockfragments set in a ferrocarbonatite matrix; later, subordinateveins of massive ferrocarbonatite are almost xenolith-free andcut through the main carbonatite dykes. A mantle source forboth carbonatite generations is constrained by their respectiveoxygen and carbon isotope compositions of ankerite (¹⁸O_SMOW8·91–9·73; ¹³C_PDB –6·98 to–6·76). Na-rich fluids, released from the meltparental to the ferrocarbonatites, caused the fenitization ofboth the incorporated anorthosite xenoliths and the borderinganorthosite, syenite and nepheline syenite. This process ismainly characterized by the progressive transformation of Ca-richplagioclase, K-feldspar and nepheline into albite and/or sodalite.The changing mineral modes indicate that the fenitizing fluidswere sodium-rich and strongly Si-deficient solutions, whichalso contained significant amounts of Sr, Ba, Nb and the lightrare earth elements. On the basis of mineral equilibria studies,it is possible to reconstruct the temperature conditions forcarbonatite emplacement (c. 830 ± 200°C) and recrystallization(c. 480 ± 130°C), and for the metasomatic formationof sodalite (c. 700 ± 70°C). KEY WORDS: anorthosite; fenitization; ferrocarbonatite; sodalite; stable isotopes     

Petrology of Mafic Plutons Associated with Calc-Alkaline Granitoids, Chilliwack Batholith, North Cascades, Washington

Small (<5 km²), lithologically diverse gabbro and dioritestocks make up 2% of the 34 to 2 Ma Chilliwack batholith, andoverlap in age with associated calc-alkaline granitoids. Thesemafic plutons are similar to those in other I-type bath-oliths,and represent basaltic magmas present during batholith formation.Objectives of this study are: (1) to examine the origins ofboth interpluton and intrapluton petrologic diversity, and (2)to compare chemical and Sr-Nd isotopic traits of these gabbroswith those of Cascade are basalts. Mafic rocks in the Chilliwackare divided into a medium-K series (MKS) and a low-K series(LKS). The former contain 0.7–2.4 wt% K₂0 and are similarin composition to calc-alkaline basalts and basaltic andesites.Inverse REE modeling supports derivation of the MKS by 9–27%melting of a garnet-free, LREE-enriched source (La/b_N 2). ChilliwackLKS gabbros have chemical characteristics of low-K olivine tho-eiites,including low K₂O (0.3–0.5 wt%) and La/b_N (1.7–3.4),and high CaO (8.8–11.3 wt%) and Na₂O/K₂O (6–22).These traits suggest a source with more clinopyroxene and lowerLa/b_N than the MKS source. Differences in _Nd(O) between MKSand LKS gabbros suggest that lower Nd/Sm is a long-lived LKSsource characteristic. Lithologic variation within compositeplutons of both series resulted primarily from multiple intrusionof related magmas, in some cases differentiates of a commonparent. Two contrasting examples were studied in detail. AtMt Sefrit, MKS variation (gabbronorite-quartz diorite) is modeledby low-pressure fractionation (ol + plag + cpx), accompaniedby 10% wallrock assimilation. In contrast, chemical and Sr-Ndisotopic variation among LKS gabbro-quartz diorite at CopperLake points to crystallization dominated by clinopyroxene+plagioclaseCr-spinel,indicative of differentiation at pressures 10 kbar, althoughthe assimilant in this case is poorly constrained. Chemicaland isotopic similarities between these mafic plutons and QuaternaryCascade lavas indicate that mafic magmas present during theproduction of Chilli-wack granitoids were low-and medium-K arebasalts. KEY WORDS: are magmatism; Cascades; gabbro; granitoid; trace element ^*Present address: Department of Physics, Astronomy, & Geology, Valdosta State University, Valdosta, GA 31698, USA. Telephone: (912) 249–4847 or 333–5752 Fax: (912) 333–7389. e-mail: jtepper{at}valdosta.peachnet.edu     

Hafnium Isotope and Trace Element Constraints on the Nature of Mantle Heterogeneity beneath the Central Southwest Indian Ridge (13{degrees}E to 47{degrees}E)

Hafnium isotope and incompatible trace element data are presentedfor a suite of mid-ocean ridge basalts (MORB) from 13 to 47°Eon the Southwest Indian Ridge (SWIR), one of the slowest spreadingand most isotopically heterogeneous mid-ocean ridges. Variationsin Nd–Hf isotope compositions and Lu/Hf ratios clearlydistinguish an Atlantic–Pacific-type MORB source, presentwest of 26°E, characterized by relatively low _Hf valuesfor a given _Nd relative to the regression line through all Nd–Hfisotope data for oceanic basalts (termed the ‘Nd–Hfmantle array line’; the deviation from this line is termed_Hf) and low Lu/Hf ratios, from an Indian Ocean-type MORB signature,present east of 32°E, characterized by relatively high _Hfvalues and Lu/Hf ratios. Additionally, two localized, isotopicallyanomalous areas, at 13–15°E and 39–41°E,are characterized by distinctly low negative and high positive_Hf values, respectively. The low _Hf MORB from 13 to 15°Eappear to reflect contamination by HIMU-type mantle from thenearby Bouvet mantle plume, whereas the trace element and isotopiccompositions of MORB from 39 to 41°E are most consistentwith contamination by metasomatized Archean continental lithosphericmantle. Relatively small source-melt fractionation of Lu/Hfrelative to Sm/Nd, compared with MORB from faster-spreadingridges, argues against a significant role for garnet pyroxenitein the generation of most central SWIR MORB. Correlations between_Hf and Sr and Pb isotopic and trace element ratios clearly delineatea high-_Hf ‘Indian Ocean mantle component’ that canexplain the isotope composition of most Indian Ocean MORB asmixtures between this component and a heterogeneous Atlantic–Pacific-typeMORB source. The Hf, Nd and Sr isotope compositions of IndianOcean MORB appear to be most consistent with the hypothesisthat this component represents fragments of subduction-modifiedlithospheric mantle beneath Proterozoic orogenic belts thatfoundered into the nascent Indian Ocean upper mantle duringthe Mesozoic breakup of Gondwana. KEY WORDS: mid-ocean ridge basalt; isotopes; incompatible elements; Indian Ocean     

A Textural Record of Solidification and Cooling in the Skaergaard Intrusion, East Greenland

The clinopyroxene–plagioclase–plagioclase dihedralangle, _cpp, in gabbroic cumulates records the time-integratedthermal history in the sub-solidus and provides a measure oftextural maturity. Variations in _cpp through the Layered Seriesof the Skaergaard intrusion, East Greenland, demonstrate thatthe onset of crystallization of clinopyroxene (within LZa),Fe–Ti oxides (at the base of LZc) and apatite (at thebase of UZb) as liquidus phases in the bulk magma is recordedby a stepwise increase in textural maturity, related to an increasein the contribution of latent heat to the total heat loss tothe surroundings and a reduction in the specific cooling rateat the crystallization front of the intrusion. The onset ofboth liquidus Fe–Ti oxide and apatite crystallizationis marked by a transient increase in textural maturity, probablylinked to overstepping before nucleation. Textural maturationat pyroxene–plagioclase–plagioclase triple junctionseffectively ceases in the uppermost parts of the Layered Seriesas a result of the entire pluton cooling below the closure temperaturefor dihedral angle change, which is 1075°C. Solidificationof the Layered Series of the Skaergaard intrusion occurred viathe upwards propagation of a mush zone only a few metres thick. KEY WORDS: magma; partial melting; asthenosphere; olivine; mantle     

A New Interpretation of Centimetre-scale Variations in the Progress of Infiltration-driven Metamorphic Reactions: Case Study of Carbonated Metaperidotite, Val d'Efra, Central Alps, Switzerland

Progress () of the infiltration-driven reaction, 4olivine +5CO₂ + H₂O = talc + 5magnesite, that occurred during Barrovianregional metamorphism, varies at the cm-scale by a factor of3·5 within an 3 m³ volume of rock. Mineral and stableisotope compositions record that X_CO2, ¹⁸O_fluid, and ¹³C_fluidwere uniform within error of measurement in the same rock volume.The conventional interpretation of small-scale variations in in terms of channelized fluid flow cannot explain the uniformityin fluid composition. Small-scale variations in resulted insteadbecause (a) reactant olivine was a solid solution, (b) initiallythere were small-scale variations in the amount and compositionof olivine, and (c) fluid composition was completely homogenizedover the same scale by diffusion–dispersion during infiltrationand subsequent reaction. Assuming isochemical reaction, spatialvariations in image variations in the (Mg + Fe)/Si of the parentrock rather than the geometry of metamorphic fluid flow. Ifinfiltration-driven reactions involve minerals fixed in composition,on the other hand, spatial variations in do directly imagefluid flow paths. The geometry of fluid flow can never be determinedfrom geochemical tracers over a distance smaller than the oneover which fluid composition is completely homogenized by diffusion–dispersion. KEY WORDS: Alpine Barrovian metamorphism; diffusion; metamorphic fluid composition; metamorphic fluid flow; reaction progress     

Fluid--Rock Interaction during Low-Pressure Polymetamorphism of the Reynolds Range Group, Central Australia

Marbles and metapelites from the Reynolds Range Group (centralAustralia) were regionally metamorphosed at low pressure duringM₂ at 1.6 Ga, M₂ ranged in grade from greenschist to granulitefacies along the length of the Reynolds Range, and overprinted1.78 Ga granites and their contact aureoles in the ReynoldsRange Group metasediments. At all M₂ grades the marbles andmetapelites have highly variable oxygen isotope ratios [marbles:¹⁸O(carb) 14–20%; metapelites: ¹⁸O 6–14%). Similarly, 1.78 Ga granites have highly variable oxygen isotope ratios(¹⁸O 5–13%), with the lowest values occurring at thegranite margins. In all rock types, the lowest oxygen isotopevalues are consistent with the infiltration of channelled magmaticand/or meteoric fluids. The variable lowering of oxygen isotopevalues resulted from pre-M₂ contact metamorphism and fluid—rockinteraction around the 1.78 Ga granites. In contrast, mineralassemblages in the marbles define a trend of increasing X_CO2with increasing grade from <0.05 (greenschist facies) to0.7–1.0 (granulite facies). This, together with the lackof regionally systematic resetting of oxygen isotope ratios,implies that there was little fluid—rock interaction duringprograde regional metamorphism. KEY WORDS: low pressure; polymetamorphism; fluids; stable isotopes; petrology ^*Corresponding author Fax: 61–3–94791272. e-mail: geoisb{at}lure.latrobe.edu.au     

Geochemical and Isotopic Heterogeneities along an Island Arc-Spreading Ridge Intersection: Evidence from the Lewis Hills, Bay of Islands Ophiolite, Newfoundland

This study focuses on the origin of magma heterogeneity andthe genesis of refractory, boninite-type magmas along an arc–ridgeintersection, exposed in the Lewis Hills (Bay of Islands Ophiolite).The Lewis Hills contain the fossil fracture zone contact betweena split island arc and its related marginal oceanic basin. Threetypes of intrusions, which are closely related to this narrowtectonic boundary, have been investigated. Parental melts inequilibrium with the ultramafic cumulates of the PyroxeniteSuite are inferred to have high MgO contents and low Al₂O₃,Na₂O and TiO₂ contents. The trace element signatures of thesePyroxenite Suite parental melts indicate a re-enriched, highlydepleted source with 0·1 x mid-ocean ridge basalt (MORB)abundances of the heavy rare earth elements (HREE). Initial_Nd values of the Pyroxenite Suite range from -1·5 to+0·6, which overlap those observed for the island arc.Furthermore, the Pyroxenite Suite parental melts bear strongsimilarities to boninite-type equilibrium melts from islandarc-related pyroxenitic dykes and harzburgites. Basaltic dykessplit into two groups. Group I dykes have 0·6 x MORBabundances of the HREE, and initial _Nd values ranging from +5·4to +7·5. Thus, they have a strong geochemical affinitywith basalts derived from the marginal basin spreading ridge.Group II dykes have comparatively lower trace element abundances(0·3 x MORB abundances of HREE), and slightly lower initial_Nd values (+5·4 to +5·9). The geochemical characteristicsof the Group II dykes are transitional between those of GroupI dykes and the Pyroxenite Suite parental melts. Cumulates fromthe Late Intrusion Suite are similarly transitional, with _Ndvalues ranging from +2·9 to +4·6. We suggest thatthe magma heterogeneity observed in the Lewis Hills is due tothe involvement of two compositionally distinct mantle sources,which are the sub-island lithospheric mantle and the asthenosphericmarginal basin mantle. It is likely that the refractory, boninite-typeparental melts of the Pyroxenite Suite result from remeltingof the sub-arc lithospheric mantle at an arc–ridge intersection.Furthermore, it is suggested that the thermal-dynamic conditionsof the transtensional transform fault have provided the prerequisitefor generating magma heterogeneity, as a result of mixing relationshipsbetween arc-related and marginal basin-related magmas. KEY WORDS: Bay of Islands ophiolite; transform (arc)–ridge intersection; boninites; rare earth elements, Nd isotopes     

Geochemistry of the Wrangellia Flood Basalt Province: Implications for the Role of Continental and Oceanic Lithosphere in Flood Basalt Genesis

The Wrangellia terrane of North America contains a large volumeof Middle to Late Triassic oceanic flood basalts which wereemplaced on top of a preexisting island arc. Nd-, Sr-, and Pb-isotopiccompositions reflect derivation from a plume source with _Nd(T)+6 to + 7, ⁸⁷Sr/⁸⁶Sr_i0•7034, and ²⁰⁶Pb/²⁰⁴Pb_i19•0.Major and trace element compositions suggest the Wrangelliaflood basalts (WFB) formed through relatively small degreesof partial melting at greater depths than estimated for otheroceanic plateaux such as Ontong Java. It appears that the WFBdid not form in a rifting environment, and that preexistingarc lithosphere limited the ascent and decompression meltingof the source plume. Rocks from the preexisting arc are stronglydepleted in high field strength elements (HFSEs) relative tolarge ion lithophile elements (LILEs), but the WFB are not.Assimilation of arc lithospheric mantle or crust was thereforegenerally minor. However, some contamination by arc componentsis evident, particularly in basalts erupted in the early stagesof volcanism. Minor isotopic shifts, to lower _Nd(T) and ²⁰⁶Pb/²⁰⁴Pb_iand higher ⁸⁷Sr/⁸⁶Sr_i, are accompanied by shifts in trace elementratios towards more arclike signatures, e.g. low Nb/Th and Nb/La.Arc contamination is greatest in the most evolved basalts, indicatingthat assimilation was coupled with fractional crystallization.A comparison of the WFB with other continental and oceanic floodbasalts reveals that continental flood basalts generally formthrough smaller degrees of melting than oceanic flood basaltsand that the contribution of material from the crust and litho-sphericmantle is significantly greater. KEY WORDS: oceanic flood basalts; Wrangellia terrane; petrogenesis; Sr-Nd-Pb isotopes ^*Corroponding author     

Re--Os and Sm--Nd Isotope Geochemistry of the Stillwater Complex, Montana: Implications for the Petrogenesis of the J-M Reef

Re—Os and Sm—Nd isotopic data have been obtainedfor mafic and ultramafic cumulates from the 2700-Ma StillwaterComplex and associated fine-grained sills and dykes, so as tobetter constrain the geochemical characteristics of Stillwaterparental magmas and to trace the source(s) of the precious metalsthat have been concentrated in the J-M Reef, the major platinum-groupelement mineral deposit in the complex. Initial Os isotopiccompositions (¹⁸⁷Os/¹⁸⁸Os) for chromitites from the Ultramaficseries range from a radiogenic isotopic composition of 0.1321(_Os = +21) for the platinum group element (PGE)-enriched B chromititeseam from the West Fork area to a near-chondritic isotopic compositionof 0.1069–0.1135 (_Os=–2 to +4.1) for the PGE-poorG and H chromitite seams, respectively, near the middle of theUltramafic series. Osmium isotopic data for the PGE-rich B chromititeseam are generally isochronous with whole-rock and mineral datafor the J-M Reef (_Os = + 12 to + 34). Re—Os isotopic datatherefore document a contrast between PGE-poor cumulates fromthe Ultramafic series and PGE-enriched cumulates from both theUltramafic series and the J-M Reef, suggesting that Os and probablythe other PGE were derived from at least two isotopically distinctsources. Moreover, these Re-Os isotopic characteristics correlatewith petrogenetic subdivisions of the Stillwater Complex basedon field mapping, petrology, REE geochemistry, and Sm—Ndisotope geochemistry. The data are best explained by mixingof two magma types, referred to as U-type and A-type magmas,with differing major element, trace element, and precious metalabundances and isotopic compositions. Although crustally contaminatedkomatiites can mimic the Os and Nd isotopic characteristicsof the U-type magma, the combination of low initial Os isotopicvalues (_Os0) with low initial Nd isotopic values (_Nd–1),high ²⁰⁷Pb/²⁰⁴Pb for a given ²⁰⁶Pb/²⁰⁴Pb (Wooden et al., 1991),and high (Ce/Yb)_n ratios in U-type cumulates and fine-grainedsills and dykes is more consistent with the involvement of aRe-poor, but trace-element-enriched portion of the subcontinentallithospheric mantle in the petrogenesis of Stillwater U-typemagmas. However, the radiogenic initial Os isotopic compositionsof the J-M Reef and other portions of the intrusion with elevatedPGE concentrations suggest that A-type parental magmas incorporatedOs from radiogenic early Archaean crust. The relatively largerange in (Ce/Yb)_n, _Os, and _Nd values suggests that mixing ofgeochemically distinct magmas may have been an important processthroughout the history of the Stillwater magma chamber. Magmamixing may then explain not only the PGE-enriched J-M Reef butalso the anomalous enrichment of the PGE in the B chromititeseam from the West Fork area and the variable values observedin other chromitite seams of the Ultramafic series. The intimateassociation of these magma types, derived from or modified inthe Archaean continental lithosphere, may then be crucial tothe formation of magmatic PGE mineral deposits.     

An Experimental Study of Fe-Al Solubility in the System Corundum-Hematite up to 40 kbar and 1300{degrees}C

The mutual solubility in the system corundum–hematite[-(Al, Fe³⁺)₂O₃] was investigated experimentally using bothsynthetic and natural materials. Mixtures of -Al₂O₃ and -Fe₂O₃(weight ratios of 8:2 and 10:1) were used as starting materialsfor synthesis experiments in air at 800–1300°C withrun times of 7–34 days. Experiments at 8–40 kbarand 490–1100°C were performed in a piston-cylinderapparatus (run times of 0·8–7·4 days) usinga natural diasporite consisting of 60–70 vol. % diasporeand 20–30 vol. % Ti-hematite. During the diasporite–corunditetransformation, the FeTiO₃ component (12–18 mol %) ofTi-hematite only slightly increased, implying that oxygen fugacitywas maintained at high values. Run products were studied byelectron microprobe and X-ray diffraction (Rietveld) techniques.An essentially linear volume of mixing exists in the solid solutionwith a slight positive deviation at the hematite side. Up to1000°C, corundum contains <4 mol % Fe₂O₃ and hematite<10 mol % Al₂O₃; at 1200°C these amounts increase to9·3 and 17·0 mol %, respectively. At 1300°Chematite was no longer stable and coexists with the orthorhombic phase . The present results agree with corundum (solvus) compositions obtained inprevious studies but indicate a larger solubility of Al in hematite.The miscibility gap in the solution can be modelled with anasymmetric Margules equation with interaction parameters (2uncertainties): ; ; ; . Application of the corundum–hematite solution as a solvus geothermometer is limited because of thescarcity of suitable rock compositions. KEY WORDS: corundum; hematite; corundum–hematite miscibility gap; experimental study; Margules model; metabauxite     

The Ni-S System and Related Minerals

The system Ni-S has been studied systematically from 200^? to1, 030^? C by means of evacuated, sealed silica-glass tube experimentsand differential thermal analyses. Compounds in the system areNi₃S₂ (and a high temperature, non-quenchable Ni_3?S₂ phase),Ni₇S₆, Ni₁–S₄ Ni₃S₄, and NiS₂. The geologic occurrenceof the minerals heazlewoodite (Ni₂S₂), millerite (ßSNi₁-₂S),polydymite (Ni₃S₄), and vaesite (NiS₂) can now be describedin terms of the stability ranges of their synthetic equivalents. Hexagonal heazlewoodite, which is stoichiometric within thelimit of error of the experiments, inverts on heating to a tetragonalor pseudotetragonal phase at 556^? C. This high-temperature phase(Ni₃ has a wide field of stability, from 23.5 to 30.5 wt percent sulfur at 600^? C, and melts incongruently at 806^??3^? C.The ßNi₇S₆ phase inverts to Ni₇₈ at 397^? C6 when inequilibrium with Ni₃S₂, and at 400^? C when in equilibrium withNiS. Crystals of Ni₇S₆ break down to Ni₃-S₂+NiS at 573^??3^?C.The low-temperature form of Ni₁-S₁ corresponding to the mineralmillerite, is rhombohedral, and the high-temperature form hasthe hexagonal NiAs structure. Stoichiometric NiS inverts at379^??3^?C, whereas Ni₁-S with the maximum nickel deficiency invertsat 282^??5OC. The Ni₁-alphS-NiS₂ solvus was determined to 985^??3^?C,the eutectic temperature of these phases. Stoichiometric NiSis stable at 600^?C but breaks down to Ni₂-S₂ and Ni₁-S below797^?C, whereas Ni₁-S with 38.2 wt per cent sulfur melts congruentlyat 992^??3^?C. Vaesite does not vary measurably from stoichiometricNiS₂ composition, and melts congruently at 1.007?5^?C. Polydymitebreaks down to aNi-S? vaesite at 356^??3^?C. Differential thermalanalyses showed the existence of a two-liquid field in the sulfur-richportion of the system above 991^?C and over a wide compositionalrange.     

Early-Middle Jurassic Dolerite Dykes from Western Dronning Maud Land (Antarctica): Identifying Mantle Sources in the Karoo Large Igneous Province

A suite of dolerite dykes from the Ahlmannryggen region of westernDronning Maud Land (Antarctica) forms part of the much moreextensive Karoo igneous province of southern Africa. The dykecompositions include both low- and high-Ti magma types, includingpicrites and ferropicrites. New ⁴⁰Ar/³⁹Ar age determinationsfor the Ahlmannryggen intrusions indicate two ages of emplacementat 178 and 190 Ma. Four geochemical groups of dykes have beenidentified in the Ahlmannryggen region based on analyses of60 dykes. The groups are defined on the basis of whole-rockTiO₂ and Zr contents, and reinforced by rare earth element (REE),⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotope data. Group 1 were intrudedat 190 Ma and have low TiO₂ and Zr contents and a significantArchaean crustal component, but also evidence of hydrothermalalteration. Group 2 dykes were intruded at 178 Ma; they havelow to moderate TiO₂ and Zr contents and are interpreted tobe the result of mixing of melts derived from an isotopicallydepleted source with small melt fractions of an enriched lithosphericmantle source. Group 3 dyke were intruded at 190 Ma and formthe most distinct magma group; these are largely picritic withsuperficially mid-ocean ridge basalt (MORB)-like chemistry (flatREE patterns, ⁸⁷Sr/⁸⁶Sr_i 0·7035, Nd_i 9). However, theyhave very high TiO₂ (4 wt %) and Zr (500 ppm) contents, whichis not consistent with melting of MORB-source mantle. The Group3 magmas are inferred to be derived by partial melting of astrongly depleted mantle source in the garnet stability field.This group includes several high Mg–Fe dykes (ferropicrites),which are interpreted as high-temperature melts. Some Group3 dykes also show evidence of contamination by continental crust.Group 4 dykes are low-K picrites intruded at 178 Ma; they havevery high TiO₂–Zr contents and are the most enriched magmagroup of the Karoo–Antarctic province, with ocean-islandbasalt (OIB)-like chemistry. Dykes of Group 1 and Group 3 aresub-parallel (ENE–WSW) and both groups were emplaced at190 Ma in response to the same regional stress field, whichhad changed by 178 Ma, when Group 2 and Group 4 dykes were intrudedalong a dominantly NNE–SSW strike. KEY WORDS: flood basalt; depleted mantle; enriched mantle; Ahlmannryggen; Karoo dyke