Phase relations of carbonate-bearing eclogite assemblages from 2.5 to 5.5 GPa: implications for petrogenesis of carbonatites

We have experimentally investigated the phase and melting relations of garnet + clinopyroxene + carbonate assemblages at 2.5–5.5 GPa, to assess the feasibility of carbonated eclogite as a source for some crustally emplaced carbonatites. The solidus of our composition was at 1,125 °C at 2.5 GPa, 1,225 °C at 3.5 GPa and 1,310 °C at 5.0 GPa. Melts were sodic calcio-dolomitic carbonatites, and were markedly more calcic than the dolomitic melts produced by partial melting of carbonated peridotite. Na contents of the experimental carbonatites decreased with increasing pressure when compared at similar degrees of melting, and SiO₂ contents increased with degree of melting. Experiments on a second composition with enhanced Na₂O demonstrated its strong effect in lowering melting temperatures in carbonate eclogite. Natural carbonated eclogite bodies in the peridotitic upper mantle will have a range of solidus temperatures. In many cases, carbonate will be molten in the upper 250 km. Carbonate melt would segregate from its source eclogite at very low melt fractions and infiltrate surrounding peridotitic wall rock. This would result in metasomatic enrichment of the peridotitic wall rock, but its exact nature will depend on the relative P–T positions of the eclogite + CO₂ and peridotite + CO₂ solidii. As a result of these inevitable metasomatic interactions, it is considered unlikely that carbonatite melts derived from carbonated eclogite in the upper mantle could be emplaced into the crust unmodified. However, they may have a role in metasomatically enriching and carbonating parts of the upper mantle, producing sources suitable for subsequent production of silica undersaturated silicate liquids and carbonatites ultimately emplaced in the crust.Editorial responsibility: J. Hoefs     

On the Age of Rhodesian Greenstone Belts

Deformation experiments have been carried out on a microgranodiorite under undrained conditions at a pressure of 0.448 GPa and temperatures up to 720 °C. The granodiorite contained a small amount of chlorite and amphibole minerals which decomposed at elevated temperatures, giving rise to a pore pressure. Evidence about the decomposition reaction was obtained from differential thermal analysis and optical microscopy. In some experiments small amounts of free water were enclosed with the sealed rock sample. The strength (at fracture or 2 per cent strain) fell to very low values (0.02 GPa) at 700 °C, and at temperatures of 670 °C and above there was evidence of partial melting which led to ductile behaviour. At somewhat lower temperatures the rock was relatively weak but brittle.     

Feldspathic hornblende and garnet granulites and associated anorthosite pegmatites from Doubtful Sound,Fiordland, New Zealand

Feldspathic hornblende granulites from Doubtful Sound, New Zealand with the assemblage plagioclase+hornblende+clinopyroxene+orthopy-roxene +oxide+apatite are criss-crossed by a network of garnetiferous anorthosite veins and pegmatites. The feldspathic gneiss in contact with anorthosite has a reaction zone containing the assemblage plagioclase +garnet+clinopyroxene+quartz+rutile+apatite. The garnet forms distinctive coronas around clinopyroxene. The origin of these rocks is discussed in the light of mineral and whole rock chemical analyses and published experimental work.It is thought that under conditions leading up to 750 °C, 8 kb load pressure and 5 kb H₂O pressure, partial melting occured in feldspathic hornblende granulites. The melt migrated into extensional fractures and eventually crystallised as anorthosite pegmatites and veins. The gneisses adjacent to the pegmatites from which the melt was extracted changed composition slightly, by the loss of H₂O and Na₂O, so that plagioclase reacted simultaneously with hornblende, orthopyroxene, and oxide to form garnet, clinopyroxene, quartz and rutile.     

Experimental constraints on depths of fractionation of mildly alkalic basalts and associated felsic rocks: Pantelleria,Strait of Sicily

Pantelleria, Italy, is a continental rift volcano consisting of alkalic basalt, trachyte, and pantellerite. At 1 atm along the FMQ buffer, the least-evolved basalt (Mg #= 58.5% norm ne) yields olivine on the liquidus at 1,180° C, followed by plagioclase, then by clinopyroxene, and by titanomagnetite and ilmenite at 1,075°. After 70% crystallization, the residual liquid at 1,025° is still basaltic and also contains apatite and possibly kaersutite. A less alkalic basalt shows the same order of phase appearance. Glass compositions define an Fe-enrichment trend and a density maximum for anhydrous liquids that coincides with a minimum in Mg#.During the initial stages of crystallization at 1 atm, liquids remain near the critical plane of silica-undersaturation until, at lower temperatures, Fe-Ti oxide precipitation drives the composition toward silica saturation. Thus the qtz-normative trachytes and pantellerites typically associated with mildly ne-normative basalts in continental rifts could be produced by low-pressure fractional crystallization or by shallow-level partial melting of alkali gabbro. At 8 kbar, clinopyroxene is the liquidus phase at 1,170° C, followed by both olivine and plagioclase at 1,135°. Because clinopyroxene dominates the crystallizing assemblage and plagioclase is more albitic than at 1 atm, liquids at 8 kbar are driven toward increasingly ne-normative compositions, suggesting that higher-pressure fractionation favors production of phonolitic derivatives.Natural basaltic samples at Pantelleria are aphyric or contain 1–10% phenocrysts of plag olcpx or ol>cpx, with groundmass Fe-Ti oxides and apatite. The lack of phenocrystic plagioclase in two of the lavas suggests that crystallization at slightly higher PH₂O may have destabilized plagioclase relative to the 1-atm results, but there is no preserved evidence for significant fractionation at mantle depths as clinopyroxene is the least abundant phenocryst phase in all samples and contains only small amounts of octahedral Al. The liquid line and phenocryst compositions match more closely the 1-atm experimental results than those at 8 kbar.Although major-element trends in natural liquids and crytals reflect low-pressure fractionation, minor- and trace-element concentrations preserve evidence of multiple parental liquids. Scatter in variation diagrams exceeds that attributable to crystal accumulation in these phenocryst-poor rocks, and the large range in concentrations of P and Ti at high MgO contents cannot be produced by polybaric fractionation nor by mixing with coexisting felsic magmas. Sr and O isotope ratios rule out significant interaction with crystalline upper crust, Mesozoic shelf sediments, or Tertiary evaporites. Positive correlations of compatible and incompatible elements suggest that the basalts are not simply related to one another by closed-system fractional crystallization of a single parental magma. Increasing Ce/Yb with Ce suggests that these relations are not a product of mixing within a replenished magma chamber, nor of mixing with more felsic members of the suite, which have smaller Ce/Yb ratios. Low-pressure fractional crystallization of ol+cpx+ plag±oxides from slightly different parental magmas produced by varying degrees of melting of garnet-bearing peridotite is a possible scenario.Small and infrequently replenished magma reservoirs in this continental rift environment may account for the strongly differentiated nature of the Pantellerian basalts. There is no correlation between Mg# and eruptive frequency, in part because concentration of volatiles in residual liquids offsets the effect of Fe-enrichment on melt density, such that strong Fe-enrichment is no hindrance to eruption.     

Tholeiitic basalt magmatism of Mount Etna and its relations with the alkaline series

Mount Etna is composed for the most part of intermediate alkaline products, most of them porphyritic-the etnaïtes-, that may be defined as sodic trachybasalts or trachyandesites. The strato-volcanio itself overlies tholeiitic basalts (usually aphyric, except for olivine) belonging to three major types: olivine tholeiites (normative Ol+Hy; modal olivine and augite, titanomagnetite and ilmenite), pigeonite tholeiites (normative Hy+minor Ol or Qz; modal pigeonite and augite with minor olivine, ilmenite and titanomagnetite), transitional tholeiites, i.e. transitional between pigeonite tholeiites (aphyric) and alkali basaltic etnaïtes (porphyritic, with normative Ol+Ne or minor Hy; modal augite and olivine, titanomagnetite alone). An analcite basalt, chemically close to alkali basaltic etnaïtes, forms the small Cyclopean Islands, SE of Etna, and an alkali olivine basalt composes a neck at Paterno, SW foot of Etna.Both pigeonite tholeiites and alkali basaltic etnaïtes may be derived from a primitive olivine tholeiite magma by subtraction or addition of phases crystallized at moderate and low pressure (kaersutite±olivine, calcic plagioclase and clinopyroxene). The differentiation process implies crystal fractionation of the primitive olivine tholeiite magma at varying levels of the crust. The speed of ascent of the magma is thought to be the factor controlling the level at which differentiation may take place: in low velocity regimes, fractionation takes place at deeper levels of the crust. Slow ascent speeds would be the consequence of a developing crustal extension episode, induced by mantle diapirism that generated the olivine tholeiite magma below the Mount Etna area.     

Coexisting clinopyroxene,garnet and amphibole from an “eclogite”, Kakanui,New Zealand

Analyses of major and rare earth elements are presented for co-existing garnet, clinopyroxene, and amphibole from a Kakanui eclogite.New and previously published analyses of garnets suggest a gradual increase of Fe and decrease of Mg from xenocrysts through garnet pyroxene eclogitic rocks to amphibole-rich eclogitic rocks. Clinopyroxenes show a parallel increase in Fe/Mg ratio and an increase in Jadeite component and decrease in Tschermak's component. These data indicate crystallization of garnet and clinopyroxene from an alkali-rich undersaturated magma and are consistent with the concept of eclogite fractionation, but rare earth data allow severe constraints to be placed on this process. The eclogites are considered to be deep-seated crystallization products of nephelinite, but eclogite fractionation is small and cannot account for the association of alkali basalt, basanite and nephelinite.     

The high-pressure assemblages at Milos,Greece

Metamorphic rocks at Milos are known in small outcrops beneath the volcanic formations, as xenoliths in the Traphores volcanic breccia and as pebbles in the Paleochora Quaternary deposits. These rocks seem to belong to three different metamorphic units which probably have intricate relationships in the basement: — the eclogites unit shows garnetjadeite or garnet-omphacite primary associations, with apparently late crystallization of lawsonite phenoblasts an recrystallization features at relatively low temperatures; — the glaucophane schist unit exhibits Jadeite+quartz or glaucophane+lawsonite primary mineralogy; — the greenschist facies unit shows low-pressure main assemblages, most generally developed after high-pressure events. The two former units involve tholeiitic meta-igneous rocks, having been spilitized before metamorphism; they generally suffered more or less advanced recrystallization features in the low-pressure field of the greenschist facies. The third unit shows only meta-sedimentary rocks, with the sole exception of one single meta-doleritic pebble having calk-alkaline affinities.Petrological and mineralogical studies, based upon 15 bulk-rock compositions and 178 probe-analysed data points, lead to suspect at least 2, perhaps 3, different metamorphic events rather than one single metamorphic evolution, to account for the 3 U distinguished. From the observation of the mineralogical assemblages and their evolution, the former events (stages 1 and 2) could be related to rapid subduction of ocean-floor or back-arc basalts, whereas, during the latter event (stage 3), the rocks experienced crystallization conditions involving both decreasing pressures and increasing temperatures.     

Phase Relations of an Armalcolite-Phlogopite Lamproite from Smoky Butte, Montana: Applications to Lamproite Genesis

Suprasolidus phase relations at pressures from 8 to 30 kb andtemperatures from 950 to 1380C have been determined experimentallyfor a glassy armalcolite–phlogopite lamproite from thechilled margin of a medium–grained lamproite from SmokyButte, Montana: The armalcolite-phlogopite lamproite has microphenocrystsof olivine in a groundmass of phlogopite, sanidine, armalcolite,clinopyroxene, chromite, priderite, apatite, and abundant glass.The lamproite is SiO₂-rich and has high F/H₂O relative to lamproitesthat have been investigated in previous experimental studies.Our data show that with decreasing temperature from the liquidusat pressures above 12 kb, melt coexists successively with:olivine; orthopyroxene + clinopyroxene; orthopyroxene + clinopyroxene+ phlogopite; clinopyroxene +phlogopite; and clinopyroxene +orthopyroxene + K-richterite. Below 12 kb, the assemblage successionis: olivine; olivine + clinopyroxene; olivine + clinopyroxene+ phlogopite; and olivine +clinopyroxene + phlogopite + armalcolite.The main difference from the natural paragenesis is that therock does not contain any orthopyroxene—a feature thatis rather remarkable inasmuch as it has 16% normative hypersthene—andthe rock differs also in that it contains sanidine and priderite.In the experiments, sanidine is observed only as ghostlike domainsin some of the glass and appears to have formed during quenching. The solid phases crystallized experimentally are generally compositionallysimilar to the minerals in the rock. These similarities andthe experimental phase relations support the concept of a rapidinitial magma ascent with only a small temperature drop andcrystallization of olivine, but not of orthopyroxene. At lowerpressures, less than 12 kb, it appears that the magma ascendedmore slowly with a larger temperature drop suggested by thesimilarity of the experimentally determined sequence of assemblagesto the paragenesis of the rock. No quasi-invariant multiphase-saturation point was found suchas might be indicative of pressure and temperature conditionsfor formation of the lamproite magma by eutectic-type partialmelting of a mantle source. The occurrence of olivine, orthopyroxene,and clinopyroxene near the liquidus, and the high proportionof normative hypersthene in the melt suggest that lherzoliteor harzburgite was probable in the magma source rock. The highSiO₂ and MgO contents of the Smoky Butte lamproites may indicatethat orthopyroxene was a source mineral even though it did notcrystallize under near-surface conditions. The curve definingthe appearance of phlogopite appears at progressively lowertemperatures from the liquidus as pressure increases, so itwould appear that either phlogopite was not the mantle K-reservoir,or it was entirely consumed during the partial melting process.The composition of the near-liquidus glass in the experimentsis likely to be the composition of the bulk rock less the verysmall amounts of olivine + clinopyroxene + orthopyroxene crystallizedwithin a few degrees below the liquidus. From the inferred compositionof this glass, anhydrous phlogopite is a potential mineral.The principal variable that determines whether phlogopite crystallizesas a near-liquidus mineral is F/H₂O; low values of this ratiopromote the presence of phlogopite as a near-liquidus mineralwhereas high values deter its crystallization. The common practiceof adding H₂O but not F in experiments to compensate for degassingmay obscure the role of phlogopite in the evolution of lamproitemagmas.     

Anthophyllit und Hornblende in einigen metamorphen Reaktionen

Reactions which occur at the lower boundary of the hornblende-hornfels facies and in the so-called pyroxene-hornfels facies were experimentally investigated for an ultrabasic rock at 500, 1000 and 2000 bars H₂O pressure.The starting material used was a mixture of natural chlorite, talc, tremolite and quartz such that its composition, except for surplus quartz, corresponded to that of an ultrabasic rock. The atomic ratio Fe²⁺+Fe²⁺/Mg+Fe³⁺+Fe³⁺ in the system was 0.16.The lower boundary of the hornblende-hornfels facies was defined by the formation of the orthorhombic amphibole anthophyllite and hornblende according to the following idealized reaction: chlorite+talc+tremolite+quartz hornblende+anthophyllite+H₂O In effect, this reaction consists of the two bivariant reactions: chlorite+tremolite+quartz hornblende+anthophyllite+H₂O talc+chlorite anthophyllite+quartz+H₂OThe equilibrium temperatures obtained for the two reactions in the given system are practically the same and are as follows: 535±10°C at 500 bars H₂O pressure 550±20°C at 1000 bars H₂O pressure 560±10°C at 2000 bars H₂O pressure 580±10°C at 4000 bars H₂O pressureAt 2000 bars and higher temperatures within the hornblende-hornfels facies, anorthite is formed in addition to hornblende and anthophyllite, probably according to the following reaction: hornblende₁+quartz hornblende₂+anthophyllite+anorthite+H₂O; because of the formation of anorthite it is to be expected that the hornblende in this case is poorer in aluminium than the hornblende at 500 and 1000 bars. Winkler (1967) suggests renaming the pyroxene-hornfels facies as K-feldspar-cordierite-hornfels facies which, in turn, is subdivided into a lower-temperature orthoamphibole subfacies without orthopyroxene and a higher-temperature orthopyroxene subfacies without orthoamphibole. The orthopyroxene subfacies itself may in its lower temperature part still carry hornblende which finally disappears in the higher temperature part.The appearance of orthopyroxene characterizes the transition from the orthoamphibole to the orthopyroxene subfacies of the K-feldspar-cordierite hornfels facies. The following reaction takes place at pressures lower than 2000 bars: hornblende₁+anthophyllite hornblende₂+enstatite+anorthite+H₂OSince at 2000 bars an Al-poor hornblende already exists in the hornblende-hornfels facies, it is very likely that here only anthophyllite breaks down to give enstatite+quartz+H₂O.The equilibrium temperatures for these reactions which give rise to enstatite are: 650±10°C at 250 bars H₂O pressure 690±10°C at 500 bars H₂O pressure 715±10°C at 1000 bars H₂O pressure 770±10°C at 2000 bars H₂O pressureOnly after an increase in temperature to about 710°C at 500 bars and about 770°C at 1000 bars does hornblende in the system investigated here break down completely according to the reaction: hornblende = enstatite+anorthite+diopside+H₂OExcept at very small H₂O-pressures (see Fig. 3), there exists, therefore, a region within the orthopyroxene subfacies where hornblende, enstatite and anorthite coexist. As a result we have, as mentioned above, a lower-temperature and a higher-temperature part of the orthopyroxene subfacies, and it is only in the latter part that the parageneses correspond to the pyroxene-hornfels facies as stated by Eskola (1939).Summing up, the starting material consisting of chlorite, talc, tremolite plus quartz remains unchanged in the albite-epidote-hornfels facies; this gives rise in the hornblende-hornfels facies to the paragenesis hornblende+anthophyllite, or — at higher pressures — to hornblende+anthophyllite+anorthite. For the particular composition of the starting material, however, no reactions take place at the transition of the hornblende-hornfels facies to the orthoamphibole subfacies of the K-feldspar-cordierite-hornfels facies as this transition is typified by the breakdown of muscovite in the presence of quartz. However, at the end of the orthoamphibole subfacies the breakdown of anthophyllite, by which orthopyroxene is formed, heralds the onset of the orthopyroxene subfacies. In this subfacies — at greater than about 300 bars — hornblende is still present and coexists with enstatite and anorthite, but with rising temperature hornblende breaks down to give way to the paragenesis enstatite+anorthite+diopside. The experimentally determined parageneses confirm known petrographic occurrences.

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Für die Förderung dieser Arbeit danken wir der Deutschen Forschungsgemeinschaft vielmals. Der Dank von Choudhuri gilt dem Akademischen Auslandsamt der Universität Göttingen für ein Stipendium, das ihm den Abschluß seiner Studien an der Universität Göttingen ermöglichte.     

Petrogenesis of Hawaiian tholeiites: 1, phase equilibria constraints

The most magnesian olivine phenocrysts [Mg no.=100 Mg/(Mg+Fe)=90.5] in Hawaiian tholeiites provide evidence for the earliest stages of differentiation of Hawaiian magmas. Based on the correction of olivine fractionation effects, the primitive melt compositions which have crystallised these olivines are picritic with 16 wt% MgO. They are excellent primary-melt candidates. An experimental study on a new Hawaiian picritic primary-melt estimate demonstrates multiple saturation with peridotite (harzburgite) at 2.0 GPa and 1450° C. Garnet is not a liquidus phase at pressures below 3.5 GPa, and garnet peridotite is not a liquidus phase assemblage at any pressure or temperature. This result confirms previous experimental studies on Hawaiian primary-melt estimates and conflicts with trace-elementgeochemistry-based interpretations, which claim that melt generation occurs in the presence of residual garnet. If Hawaiian tholeiite primary magmas are picritic and have equilibrated with garnet-absent peridotite residues, the geochemical and isotopic characteristics of Hawaiian tholeiites (i.e. Sm/Nd chondrites and Nd>0) are consistent with their source recently having been enriched in incompatible elements. Previous modelling shows that such characteristics are consistent with source enrichment through the migration of small melt fractions generated at depth in the presence of garnet. This may be effected either at the time of Hawaiian magma genesis through dynamic melt segregation processes or, by melting of a previously enriched mantle source; possibly oceanic lithospheric mantle which has been infiltrated by melt fractions from the underlying asthenosphere prior to Hawaiian magmatism. Alternatively, if Hawaiian primary magmas are ultramafic in composition (20 wt% MgO) they may be generated in the presence of garnet peridotite at pressures 3.0 GPa.     

Melting of Amphibole-bearing Wehrlites: an Experimental Study on the Origin of Ultra-calcic Nepheline-normative Melts

Olivine + clinopyroxene ± amphibole cumulates have beenwidely documented in island arc settings and may constitutea significant portion of the lowermost arc crust. Because ofthe low melting temperature of amphibole (1100°C), suchcumulates could melt during intrusion of primary mantle magmas.We have experimentally (piston-cylinder, 0·5–1·0GPa, 1200–1350°C, Pt–graphite capsules) investigatedthe melting behaviour of a model amphibole–olivine–clinopyroxenerock, to assess the possible role of such cumulates in islandarc magma genesis. Initial melts are controlled by pargasiticamphibole breakdown, are strongly nepheline-normative and areAl₂O₃-rich. With increasing melt fraction (T > 1190°Cat 1·0 GPa), the melts become ultra-calcic while remainingstrongly nepheline-normative, and are saturated with olivineand clinopyroxene. The experimental melts have strong compositionalsimilarities to natural nepheline-normative ultra-calcic meltinclusions and lavas exclusively found in arc settings. Theexperimentally derived phase relations show that such naturalmelt compositions originate by melting according to the reactionamphibole + clinopyroxene = melt + olivine in the arc crust.Pargasitic amphibole is the key phase in this process, as itlowers melting temperatures and imposes the nepheline-normativesignature. Ultra-calcic nepheline-normative melt inclusionsare tracers of magma–rock interaction (assimilative recycling)in the arc crust. KEY WORDS: experimental melting; subduction zone; ultra-calcic melts; wehrlite     

Long-term prediction of intermediate depth earthquakes in the southern Aegean region based on a time-predictable model

Repeat times of strong intermediate depth (60 km h 180 km) earthquakes have been determined by the use of instrumental and historical data for six seismogenic sources in the Benioff zone of the southern Aegean area. For four of these sources, at least two interevent times (three mainshocks) are available for each source. By using the repeat times for these four sources, the following relation has been determined: logT _t = 0.20M _min + 0.19M _p +a, whereT _t is the repeat time (in years),M _min the surface wave magnitude of the smallest earthquake considered,M _p the magnitude of the preceding mainshock and a parameter which varies from source to source. A multilinear correlation coefficient equal to 0.91 was determined for this relation, which indicates that the time predictable model holds to a satisfactory degree for the strong mainshocks of intermediate focal depth in the southern Aegean.By assuming that the ratioT/T _t, whereT is the observed andT _t the calculated repeat time, follows a lognormal distribution, the conditional probabilities for the occurrence of strong (M _s 6.5) and very strong (M _s 7.5) earthquakes during the period 1991–2001 in these four seismogenic sources have been calculated. These probabilities are very high (P > 0.9) for the strong and high (P > 0.5) for the very strong intermediate depth earthquakes which occur in the three sources of the shallower (h < 100 km) part of the Benioff zone where coupling occurs between the front parts of the Mediterranean lithosphere (downgoing) and the Aegean lithosphere.     

Chemical constraints on the petrogenesis of mildly alkaline lavas from Vestmannaeyjar,Iceland: the Eldfell (1973) and Surtsey (1963–1967) eruptions

The Vestmannaeyjar archipelago is composed of alkalic lavas erupted at the southern end of the active, southward propagating, Eastern Volcanic Zone. Recent eruptions include the most primitive (Surtsey) and most evolved (Eldfell) compositions found in this area. We studied time-stratigraphic sample suites from both eruptions to characterize the magmatic environment of Vestmannacyjar. All samples are nearly homogeneous in radiogenic isotopic ratios (⁸⁷Sr/⁸⁶Sr 0.70304 to 0.70327;¹⁴³Nd/¹⁴⁴Nd 0.51301 to 0.50307;²⁰⁶Pb/²⁰⁴Pb 18.96 to 19.18;²⁰⁷Pb/²⁰⁴Pb 15.50 to 15.53;²⁰⁸Pb/²⁰⁴Pb 38.47 to 38.76; KH Park and A Zindler, in preparation). Compositional trends of lavas from the two eruptions are not consistent with fractionation in a near-surface environment, but indicate rather moderate pressure evolution of small magma batches. At Eldfell, mugearite lavas can be modeled by 30% closed-system fractional crystallization of olivine+plagioclase+clinopyroxene+Fe–Ti oxides from parental hawaiite. The phase proportions are consistent with an experimentally determined moderate pressure (8 kbar) cotectic in mildly alkaline systems (Mahood and Baker 1986). Compositional variations of Surtsey lavas can be modeled by crystallization of clinopyroxene+olivine+plagioclase+minor Fe–Ti oxides. The presence of sodic plagioclase megacrysts and clinopyroxene with 8 wt% Al₂O₃ in xenoliths from Surtsey lavas are consistent with a moderate pressure fractionation event. Based on major-element and REE data the most primitive Surtsey lavas formed by small degrees of melting of a lherzolite source. The alkaline nature of Vestmannaeyjar lavas is not the result of assimilation of lower crustal melts (cf. Oskarsson et al. 1985; Steinthorsson et al. 1985).     

Polymetamorphic evolution of the Lewisian complex,NW Scotland,as recorded by U-Pb isotopic compositions of zircon,titanite and rutile

U-Pb isotopic relations in zircon and titanite of granulite and amphibolite gneisses in the Lewisian complex and bordering Laxford Front reveal complex discordance patterns indicating multiple Late Archean and Early Proterozoic crystallization, overgrowth and Pb-loss events. The earliest stages in the evolution of the complex remain poorly resolved. Zircon ages of 2710 Ma date high-grade metamorphism and magmatism probably related to tectonic and magmatic accretion in a continental arc setting. A distinct event at 2490–2480 Ma, possibly initiated by metamorphism and deformation at high-grade conditions, caused amphibolitization of the granulites and emplacement of granitic pegmatites. This event can be correlated with development of Inverian shear zones and formation of granitoid layers along the Laxford Front. The emplacement of a younger generation of granitoid sheets during the Laxfordian event fromed hydrothermal titanite at 1754 Ma in gneisses south of the Laxford Bridge and partially reset older titanite at Scourie. Growth of secondary titanite and rutile also occurred during subsequent low-grade metamorphism at 1690–1670 Ma.     

Geothermometry of exsolved augites from the Laramie Anorthosite Complex,Wyoming

Exsolved augite pyroxenes from the ferromonzonite border facies of the ferrosyenite in the Laramie Anorthosite Complex have been studied with the transmission electron microscope and the electron microprobe to determine their exsolution histories. The Lindsley and Andersen (1983) geothermometer gives initial crystallization temperatures of 1000° C for the bulk augite crystal (Wo₃₂ En₂₂ Fs₄₆). Exsolved lamellae are predominantly pigeonites with very low calcium contents (Wo_1–3 En_23–24 Fs_71–74) and have formation temperatures estimated to be in the range of 600 to 975° C. The uniform compositions of lamellae and hosts, despite the range in lamellar size and orientation, suggest that either 1) the ferromonzonite experienced an extended plateau in cooling or a reheating event at 600 to 650° C or 2) the pyroxenes recorded a blocking temperature. Two-feldspar geothermometry on exsolved feldspars also records 600° C and suggests that these low temperatures are not blocking temperatures.     

Experimental Constraints on the Role of Garnet Pyroxenite in the Genesis of High-Fe Mantle Plume Derived Melts

The anhydrous phase relations of an uncontaminated (primitive),ferropicrite lava from the base of the Early Cretaceous Paraná–Etendekacontinental flood basalt province have been determined between1 atm and 7 GPa. The sample has high contents of MgO (14·9wt %), FeO^* (14·9 wt %) and Ni (660 ppm). Olivine phenocrystshave maximum Fo contents of 85 and are in equilibrium with thebulk rock, assuming a of 0·32. A comparison of our results with previous experimental studiesof high-Mg rocks shows that the high FeO content of the ferropicritecauses an expansion of the liquidus crystallization field ofgarnet and clinopyroxene relative to olivine; orthopyroxenewas not observed in any of our experiments. The high FeO contentalso decreases solidus temperatures. Phase relations indicatethat the ferropicrite melt last equilibrated either at 2·2GPa with an olivine–clinopyroxene residue, or at 5 GPawith a garnet–clinopyroxene residue. The low bulk-rockAl₂O₃ content (9 wt %) and high [Gd/Yb]_n ratio (3·1)are consistent with the presence of residual garnet in the ferropicritemelt source and favour high-pressure melting of a garnet pyroxenitesource. The garnet pyroxenite may represent subducted oceaniclithosphere entrained by the upwelling Tristan starting mantleplume head. During adiabatic decompression, intersection ofthe garnet pyroxenite solidus at 5 GPa would occur at a mantlepotential temperature of 1550°C and yield a ferropicriteprimary magma. Subsequent melting of the surrounding peridotiteat 4·5 GPa may be restricted by the thickness of theoverlying sub-continental lithosphere, such that dilution ofthe garnet pyroxenite melt component would be significantlyless than in intra-oceanic plate settings (where the lithosphereis thinner). This model may explain the limited occurrence offerropicrites at the base of continental flood basalt sequencesand their apparent absence in ocean-island basalt successions. KEY WORDS: continental flood basalt; ferropicrite; mantle heterogeneity; mantle melting; phase relations; pyroxenite     

Experimental reactions of amphibolite with boron-bearing aqueous fluids at 200 MPa: implications for tourmaline stability and partial melting in mafic rocks

Reactions between hornblende-plagioclase amphibolite and acidic and alkaline B-bearing aqueous fluids have been investigated by experiments at 475°–600° C and 200 MPa. At 600° C, hornblende+calcic plagioclase react to form tourmaline+danburite+clinopyroxene+quartz in acidic fluids containing 0.5–1.0 wt% B₂O₃.Tourmaline is precipitated directly from acidic fluids, and the reaction is driven by neutralization of fluids by Na±Ca derived from the breakdown of reactant solids. The concentration of B₂O₃ in fluids needed to stabilize tourmaline increases as pH increases (above approximately 6.0), and tourmaline is unstable in alkaline fluids (pH > approximately 6.5–7.0) regardless of B concentration. In addition to acid-base relations, tourmaline stability is favored by comparatively higher activity coefficients for Al species in acidic fluids. The concentrations of Al and Si in fluid increase with alkalinity, with the eventual production of felsic borosilicate melts through partial melting of the plagioclase component of the amphibolite. In seeded experiments, tourmaline also contributes components to melt. Partial melting is evident in the range 500°–525° C at 200 MPa in experiments with 8wt% B₂O₃ in fluid as Na₂B₄O₇. The experimental results are applied primarily to metasomatic reactions between mafic rocks and borate fluids derived from granitic magmas, but tourmaline stability and partial melting in mafic regional metamorphic systems are also discussed briefly.     

Physical evolution of Grande Ronde Basalt magmas, Columbia River Basalt Group, north-western USA

Summary In this paper we present what is, to the best of our knowledge, the first comprehensive study of clinopyroxenes and plagioclases contained in the flows of the Grande Ronde Basalt member of the Columbia River Basalt Group (northwestern USA). The rocks have MgO(wt%)<6%, and trace amounts of Cr and Ni. About 56% of extracted solid containing normative clinopyroxene and plagioclase explains the liquid line of descent from the more mafic sample (MgO wt%=5.89) to the most evolved. The most ubiquitous phases in the basalts are plagioclase and augite. Ilmenite and magnetite are accessories in all rocks. Olivine is present in small amount only in one sample (RT 89-7). Based on principles of Ca–Na plagioclase–liquid exchange, estimates of pre-eruptive magmatic water are < 2.4wt%. From clinopyroxene–liquid equilibria, calculated pressures and temperatures of ascending magmas are between 1atm and 0.617GPa, and 1068°C and 1166°C, respectively. Compositions of magnetite–ilmenite pairs and olivine–clinopyroxene–oxide assemblages yield post-eruptive oxygen fugacities of NNO=–1.923, and one pre-eruptive value of NNO=– 2.455. A simple model of asthenospheric melting and magma ponding in the lower crust fits the physical parameters.Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1007/s00710-003-0017-1     

Mineralogy and proposed P-T paths of basaltic lavas from Rabaul caldera,Papua New Guinea

Rabaul caldera is a large volcanic depression at the north-east tip of New Britain, Papua New Guinea. The lavas range in composition from basalt to rhyolite and have a calc-alkalic affinity but also display features typical of tholeiites, including moderate absolute iron enrichment in flows cropping out around the caldera. The basalts contain phenocrysts of plagioclase and clinopyroxene with less abundant olivine and titanomagnetite. In the basaltic andesites olivine is rare, while orthopyroxene and titanomagnetite are common along with plagioclase and clinopyroxene. Orthopyroxene is also found mantling olivine in some of the basalts while in both rock types pigeonitic augite is a fairly common constituent of the groundmass. Plagioclase in both basalt and basaltic andesite often exhibits sieve texture and analysis of the glass blebs show them to be of similar composition to the bulk rock. Phenocrystic clinopyroxene is a diopsidic augite in both basalt and basaltic andesite. Al₂O₃ content of the clinopyroxene is moderately high (4%) and often shows considerable variation in any one grain. Calculations show that the microphenocrysts probably crystallised near the surface, while phenocrysts crystallised at around 7 kb (21 km). Neither the basalts nor the basaltic andesites would have been in equilibrium at any geologically reasonable P and T with quartz eclogite. Equilibration between mantle peridotite and a. typical Rabaul basaltic liquid could have occurred around 35 kb and 1270 °C. A basaltic andesite liquid yields a temperature of 1263 °C and a pressure of 28 kb for equilibration with mantle peridotite.Partial melting of sufficient volumes of mantle peridotite at these P's and T's requires about 15% H₂O, but there is no evidence that these magmas ever contained large amounts of water. It is proposed that the Rabaul magmas were initially generated by partial melting of subducted lithosphere and subsequently modified by minor partial melting as they passed through the overlying mantle peridotite.     

Anhydrous partial melting of an iron-rich mantle I: subsolidus phase assemblages and partial melting phase relations at 10 to 30 kbar

Anhydrous partial melting experiments, at 10 to 30 kbar from solidus to near liquidus temperature, have been performed on an iron-rich martian mantle composition, DW. The DW subsolidus assemblage from 5 kbar to at least 24 kbar is a spinel lherzolite. At 25 kbar garnet is stable at the solidus along with spinel. The clinopyroxene stable on the DW solidus at and above 10 kbar is a pigeonitic clinopyroxene. Pigeonitic clinopyroxene is the first phase to melt out of the spinel lherzolite assemblage at less than 20°C above the solidus. Spinel melts out of the assemblage about 50°C above the solidus followed by a 150° to 200°C temperature interval where melts are in equilibrium with orthopyroxene and olivine. The temperature interval over which pigeonitic clinopyroxene melts out of an iron-rich spinel lherzolite assemblage is smaller than the temperature interval over which augite melts out of an iron-poor spinel lherzolite assemblage. The dominant solidus assemblage in the source regions of the Tharsis plateau, and for a large percentage of the martian mantle, is a spinel lherzolite.