Basic phase relations of polybaric batch, percolative and critical melting

Summary ?Partial melting of the mantle is polybaric which implies that the phase relations change during partial melting. In addition to the pressure the composition of the melt depends on the melting mode. Various melting models have been suggested. Here the basic phase relations of polybaric batch, percolative, and critical melting are considered, using a simple ternary system. The percolative melts are in equilibrium with their residua, but differ somewhat in composition from those of batch melting. Critical melting is a fractional type of melting where the residuum contain interstitial melt. The critical melts differ in composition from batch melts. The linear trends of peridotites from ophiolites show that the extracted melts had nearly constant compositions, and therefore were extracted within a small pressure interval. A comparison between the trends of mantle peridotite and experimental batch melts suggests strongly that the melt extracted from the peridotites are in equilibrium with their residua. This could suggest that either batch or percolative melting are relevant melting modes for the mantle. However, isotopic disequilibria favor instead a critical mode of melting. This inconsistency can be avoided if the ascending melts are accumulated within a source region and equilibrate with the residuum before the melt is extracted from the source region. The evidence for equilibrium suggests that multisaturation of tholeiitic compositions in PT-diagrams is relevant for estimating pressure and temperature of generation of primary tholeiitic magmas. Received September 2, 2001; revised version accepted March 20, 2002     

Lead isotope systematics of vein-type antimony mineralization,Rheinisches Schiefergebirge,Germany: a case history of complex reaction and remobilization processes

Mesothermal vein-type Sb mineralization in the Rheinisches Schiefergebirge, Germany, is characterized by two different mineralization styles, which are (1) extensional quartz-stibnite vein systems, and (2) (Cu)-Pb-Sb sulphosalt assemblages in overprinted pre-existing Pb-Zn veins. A detailed Pb isotope study of 52 representative samples from both mineralization types indicates distinct compositional patterns. (Cu)-Pb-Sb sulphosalts (meneghinite, boulangerite, bournonite) formed by reaction/remobilization are characterized by Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb=18.179-18.223), which are identical to the precursor galena (²⁰⁶Pb/²⁰⁴Pb=18.168-18.223). The Pb isotope composition of sulphosalt minerals in these vein systems was controlled by lead inherited from pre-existing galena. Stibnite and Pb-sulphosalts (zinkenite, semseyite, plagionite) formed in quartz-stibnite vein systems display Pb isotope ratios (²⁰⁶Pb/²⁰⁴Pb=18.250-18.354), which are more radiogenic than galenas from Variscan Pb-Zn ore veins (²⁰⁶Pb/²⁰⁴Pb=18.162-18.303). Detailed small-scale investigation of thrust zones hosting Pb-Zn ores and crosscutting quartz-ankerite fissure veins (Ramsbeck deposit) indicates that the Pb isotope compositions of recrystallized (galena) and remobilized phases (boulangerite, semseyite, bournonite) are arranged along a linear trend. This is interpreted as mixing between primary galena with ²⁰⁶Pb/²⁰⁴Pb᜞.206 and overprinting hydrothermal fluids with a more radiogenic composition (²⁰⁶Pb/²⁰⁴Pb⁾.354), expressed by intermediate compositions (²⁰⁶Pb/²⁰⁴Pb=18.256-18.334) of newly-formed sulphosalts. The Pb isotope systematics of the vein-type Sb mineralization is in accordance with a model of Pb extraction from similar crustal sources (Palaeozoic sedimentary sequences) at different times.     

Magmatism in the Gregory Rift, East Africa: Evidence for Melt Generation by a Plume

The volume and composition of volcanic rocks associated withthe Gregory rift are interpreted in the light of inversionsperformed on the REE concentrations of the most magnesian basalts.When the estimated volume of salic rock ({small tilde}88 000km³) is converted into basalt ({small tilde}792 000 km³) thetotal volume of basaltic melt generated over the last 30 Myis at least 924 000 km³, corresponding to an average rate ofmelt production of {small tilde}0•03 km³/yr and an averagemelt thickness of between 7 and 26 km everywhere beneath therift. The mean compositions of the basaltic magmas erupted withinthe rift and on the rift flanks during the Upper Oligocene andMiocene, the Pliocene, and the Quaternary are taken to be representativeof the average compositions of melts produced by fractionalmelting in the asthenospheric mantle. When the REE concentrationsof the observed average compositions are inverted they suggestthat much of the melt was produced in the depth and temperaturerange of the transition from garnet to spinel peridotite. Fora mantle potential temperature of {small tilde}1500C the topof the melting region predicted from the inversions is at {smalltilde}70 km beneath the rift axis and {small tilde}80 km beneaththe rift flanks. Within the rift zone the predicted thicknessof melt increases from the Upper Oligocene and Miocene to thePliocene and is always greater than that predicted for the riftflanks, and the timeaveraged thickness of melt predicted is0/5 km. To generate the observed volume of melt the asthenosphericmantle must continually upwell through the melting region (extendingfrom 70 to 150 km) with a vertical velocity of between 40 and140 mm/yr. The results suggest that, volumetrically and compositionally,magmatic activity associated with the Gregory rift is quantitativelyconsistent with a model of a mantle plume upwelling beneaththinned continental lithosphere. Predictions made by such amodel are in broad agreement with geophysical observations. ^* Present address/reprint requests to: B.P. Exploration, 4/5 Long Walk, Stockley Park, Uxbridge UB11 1BP, UK     

Mantle-melt Evolution (Dynamic Source) in the Origin of a Single MORB Suite: a Perspective from Magnesian Glasses of Macquarie Island

The effects of source composition and source evolution duringprogressive partial melting on the chemistry of mantle-derivedmid-ocean ridge basalt (MORB) melts were tested using a comprehensivegeochemical and Sr–Nd–Pb isotopic dataset for fresh,magnesian basaltic glasses from the Miocene Macquarie Islandophiolite, SW Pacific. These glasses: (1) exhibit clear parent–daughterrelationships; (2) allow simple reconstruction of primary meltcompositions; (3) show exceptional compositional diversity (e.g.K₂O/TiO₂ 0·09–0·9; La/Yb 1·5–22;²⁰⁶Pb/²⁰⁴Pb 18·70–19·52); (4) preserve changesin major element and isotope compositions, which are correlatedwith the degree of trace element enrichment (e.g. La/Sm). Conventionalmodels for MORB genesis invoke melting of mantle that is heterogeneouson a small scale, followed by binary mixing of variably lithophileelement-enriched melt batches. This type of model fails to explainthe compositions of the Macquarie Island glasses, principallybecause incompatible element ratios (e.g. Nb/U, Sr/Nd) and Pbisotope ratios vary non-systematically with the degree of enrichment.We propose that individual melt batches are produced from instantaneous‘parental’ mantle parageneses, which change continuouslyas melting and melt extraction proceeds. This concept of a ‘dynamicsource’ combines the models of small-scale mantle heterogeneitiesand fractional melting. A dynamic source is an assemblage oflocally equilibrated mantle solids and a related melt fraction.Common MORB magmas that integrate the characteristics of numerousmelt batches therefore tend to conceal the chemical and isotopicidentity of a dynamic source. This study shows that isotoperatios of poorly mixed MORB melts are a complex function ofthe dynamic source evolution, and that the range in isotoperatios within a single MORB suite does not necessarily requiremixing of diverse components. KEY WORDS: mid-ocean ridge basalt; Macquarie Island; radiogenic isotopes; mantle; geochemistry     

Physical behavior of the plume source during intermittent eruptions of Hawaiian basalts

The plume surrounding a source region for magma may either compact by elastic or viscous deformation when magma leaves the source region. It is shown that the compaction is elastic. The elastic deformation implies that only a small volume fraction of the melt can leave the source region. On the other hand, the isotopic secular disequilibria show that the fraction melt must be much larger. This discrepancy is solved if the source region is veined and consists of intercalated veins and residuum, with the veins forming about 1% volume of the source region. The approximate width of the source region is estimated to be 10-20 km and its height is 1-2 km. An eruption starts when a dike is formed above the source region because of overpressure of the melt within the source region. During the repose time, the overpressure of the melt in the source region increases because of the replenishment of melt from the plume situated below.     

Melting and melt segregation in the aureole of the Glenmore Plug, Ardnamurchan

Contact metamorphism caused by the Glenmore plug in Ardnamurchan, a magma conduit active for 1 month, resulted in partial melting, with melt now preserved as glass. The pristine nature of much of the aureole provides a natural laboratory in which to investigate the distribution of melt. A simple thermal model, based on the first appearance of melt on quartz–feldspar grain boundaries, the first appearance of quartz paramorphs after tridymite and a plausible magma intrusion temperature, provides a time‐scale for melting. The onset of melting on quartz–feldspar grain boundaries was initially rapid, with an almost constant further increase in melt rim thickness at an average rate of 0.5–1.0 × 10^?9 cm s^?1. This rate was most probably controlled by the distribution of limited amounts of H₂O on the grain boundaries and in the melt rims. The melt in the inner parts of the aureole formed an interconnected grain‐boundary scale network, and there is evidence for only limited melt movement and segregation. Layer‐parallel segregations and cross‐cutting veins occur within 0.6 m of the contact, where the melt volume exceeded 40%. The coincidence of the first appearance of these signs of the segregation of melt in parts of the aureole that attained the temperature at which melting in the Qtz–Ab–Or system could occur, suggests that internally generated overpressure consequent to fluid‐absent melting was instrumental in the onset of melt movement.     

Crustal reworking in a shear zone: transformation of metagranite to migmatite

This study uses field, microstructural and geochemical data to investigate the processes contributing to the petrological diversity that arises when granitic continental crust is reworked. The Kinawa migmatite formed when Archean TTG crust in the São Francisco Craton, Brazil was reworked by partial melting at ~730 °C and 5–6 kbar in a regional‐scale shear zone. As a result, a relatively uniform leucogranodiorite protolith produced compositionally and microstructurally diverse diatexites and leucosomes. All outcrops of migmatite display either a magmatic foliation, flow banding or transposed leucosomes and indicate strong, melt‐present shearing. There are three types of diatexite. Grey diatexites are interpreted to be residuum, although melt segregation was incomplete in some samples. Biotite stable, H₂O‐fluxed melting is inferred via the reaction Pl + Kfs + Qz + H₂O = melt and geochemical modelling indicates 0.35–0.40 partial melting. Schlieren diatexites are extremely heterogeneous; residuum‐rich domains alternate with leucocratic quartzofeldspathic domains. Homogeneous diatexites have the highest SiO₂ and K₂O contents and are coarse‐grained, leucocratic rocks. Homogeneous diatexites, quartzofeldspathic domains from the schlieren diatexites and the leucosomes contain both plagioclase‐dominated and K‐feldspar‐dominated feldspar framework microstructures and hence were melt‐derived rocks. Both types of feldspar frameworks show evidence of tectonic compaction. Modelling the crystallization of an initial anatectic melt shows plagioclase appears first; K‐feldspar appears after ~40% crystallization. In the active shear zone setting, shear‐enhanced compaction provided an essentially continuous driving force for segregation. Thus, Kinawa migmatites with plagioclase frameworks are interpreted to have formed by shear‐enhanced compaction early in the crystallization of anatectic melt, whereas those with K‐feldspar frameworks formed later from the expelled fractionated melt. Trace element abundances in some biotite and plagioclase from the fractionated melt‐derived rocks indicate that these entrained minerals were derived from the wall rocks. Results from the Kinawa migmatites indicate that the key factor in generating petrological diversity during crustal reworking is that shear‐enhanced compaction drove melt segregation throughout the period that melt was present in the rocks. Segregation of melt during melting produced residuum and anatectic melt and their mixtures, whereas segregation during crystallization resulted in crystal fractionation and generated diverse plagioclase‐rich rocks and fractionated melts.     

The importance of melt extraction for tracing mantle heterogeneity

Numerous isotope and trace element studies of mantle rocks and oceanic basalts show that the Earth’s mantle is heterogeneous. The isotopic variability in oceanic basalts indicates that most mantle sources consist of complex assemblages of two or more components with isolated long-term chemical evolution, on both global and local scales. The range in isotope and highly incompatible element ratios observed in oceanic basalts is commonly assumed to directly reflect that of their mantle sources. Accordingly, the end-points of isotope arrays are taken to represent the isotopic composition of the different components in the underlying mantle, which is then used to deduce the origin of mantle heterogeneity. Here, a melting model for heterogeneous mantle sources is presented that investigates how and to what extent isotope and trace element signatures are conveyed from source to melt. We model melting of a pyroxenite-bearing peridotite using recent experimental constrains for melting and partitioning of pyroxenite and peridotite. Identification of specific pyroxenite melting signatures allows finger-printing of pyroxenite melts and confirm the importance of lithological heterogeneity in the Earth’s mantle. The model results and the comparison of the calculated and observed trace element-isotope systematics in selected MORB and OIB suites (e.g. from the East Pacific Rise, Iceland, Tristan da Cunha, Gough and St.Helena) further show that factors such as the relative abundance of different source components, their difference in solidus temperature, and especially the extent, style and depth range of melt aggregation fundamentally influence the relationship between key trace element and isotope ratios (e.g. Ba/Th, La/Nb, Sr/Nd, La/Sm, Sm/Yb, ¹⁴³Nd/¹⁴⁴Nd). The reason for this is that any heterogeneity present in the mantle is averaged or, depending on the effectiveness of the melt mixing process, even homogenized during melting and melt extraction. Hence to what degree mantle heterogeneity is reflected in the erupted melts is not only a function of source and melting-induced variability. It also depends on the extent of mixing during melting and melt extraction and thus strongly on the relative incompatibility of the elements considered. The observed trace element variation in erupted melts can be greater or smaller than that of their mantle sources, depending on the incompatibility of the elements investigated. The isotopic variability in erupted melts, on the other hand, is generally smaller than that of their mantle source. Melt mixing during melt extraction consequently has an important influence on the relative extent of variation, and hence the degree of correlation between the isotope and trace element ratios. Overall fewer correlations between trace element and isotope ratios are expected whenever melts are extracted from a restricted depth range, as is the case for ocean island basalts, than for cases where melts are extracted over a larger depth interval (mid ocean ridges and especially ridge centered hotspots like Iceland). While the isotopic composition of the most enriched melts may correspond closely to those of the enriched source component, even the most depleted mid ocean ridge basalts are likely to underestimate the isotopic depletion of the depleted mantle component. These observations imply that using the chemical and isotopic range observed in oceanic basalts as directly representative of that in the corresponding mantle source can be misleading, since this assumption is strictly true only for homogeneous mantle sources. In addition to identifying source or partitioning-related differences in melts from different mantle sources, inferring the true composition, origin, and distribution of heterogeneous components in the Earth’s mantle therefore requires detailed knowledge about the mechanisms of melting and melt mixing during the melt extraction process. Only if these processes and their influence on the isotope-trace element relationship are understood, can the composition and origin of the different source components, and thus mantle heterogeneity, be accurately constrained.     

Tertiary Ultrapotassic Volcanism in Serbia: Constraints on Petrogenesis and Mantle Source Characteristics

The Serbian province of Tertiary ultrapotassic volcanism isrelated to a post-collisional tectonic regime that followedthe closure of the Tethyan Vardar Ocean by Late Cretaceous subductionbeneath the southern European continental margin. Rocks of thisprovince form two ultrapotassic groups; one with affinitiesto lamproites, which is concentrated mostly in the central partsof the Vardar ophiolitic suture zone, and the other with affinitiesto kamafugites, which crops out in volcanoes restricted to thewestern part of Serbia. The lamproitic group is characterizedby a wide range of ⁸⁷Sr/⁸⁶Sr_i (0·70735–0·71299)and ¹⁴³Nd/¹⁴⁴Nd_i (0·51251–0·51216), whereasthe kamafugitic group is isotopically more homogeneous witha limited range of ⁸⁷Sr/⁸⁶Sr_i (0·70599–0·70674)and ¹⁴³Nd/¹⁴⁴Nd_i (0·51263–0·51256). ThePb isotope compositions of both groups are very similar (²⁰⁶Pb/²⁰⁴Pb18·58–18·83, ²⁰⁷Pb/²⁰⁴Pb 15·62–15·70and ²⁰⁸Pb/²⁰⁴Pb 38·74–38·99), falling withinthe pelagic sediment field and resembling Mesozoic flysch sedimentsfrom the Vardar suture zone. The Sr and Nd isotopic signaturesof the primitive lamproitic rocks correlate with rare earthelement fractionation and enrichment of most high field strengthelements (HFSE), and can be explained by melting of a heterogeneousmantle source consisting of metasomatic veins with phlogopite,clinopyroxene and F-apatite that are out of isotopic equilibriumwith the peridotite wall-rock. Decompression melting, with varyingcontributions from depleted peridotite and ultramafic veinsto the final melt, accounts for consistent HFSE enrichment andisotopic variations in the lamproitic group. Conversely, themost primitive kamafugitic rocks show relatively uniform Srand Nd isotopic compositions and trace element patterns, andsmall but regular variations of HFSE, indicating variable degreesof partial melting of a relatively homogeneously metasomatizedmantle source. Geochemical modelling supports a role for phlogopite,apatite and Ti-oxide in the source of the kamafugitic rocks.The presence of two contrasting ultrapotassic suites in a restrictedgeographical area is attributable to the complex geodynamicsituation involving recent collision of a number of microcontinentswith contrasting histories and metasomatic imprints in theirmantle lithosphere. The geochemistry of the Serbian ultrapotassicrocks suggests that the enrichment events that modified thesource of both lamproitic and kamafugitic groups were relatedto Mesozoic subduction events. The postcollisional environmentof the northern Balkan region with many extensional episodesis consistent at regional and local levels with the occurrenceof ultrapotassic rocks, providing a straightforward relationshipbetween geodynamics and volcanism. KEY WORDS: kamafugite; lamproite; Mediterranean; Serbia; mantle metasomatism; veined mantle; petrogenesis     

Sr-Nd-Pb Isotopic Compositions of Peridotite Xenoliths from Spitsbergen: Numerical Modelling Indicates Sr-Nd Decoupling in the Mantle by Melt Percolation Metasomatism

Several spinel peridotite xenoliths from Spitsbergen have Sr–Ndisotopic compositions that plot to the right of the ‘mantlearray’ defined by oceanic basalts and the DM end-member(depleted mantle, with low ⁸⁷Sr/⁸⁶Sr and high ¹⁴³Nd/¹⁴⁴Nd).These xenoliths also show strong fractionation of elements withsimilar compatibility (e.g. high La/Ce), which cannot be producedby simple mixing of light rare earth element-depleted peridotiteswith ocean island basalt-type or other enriched mantle melts.Numerical simulations of porous melt flow in spinel peridotitesapplied to Sr–Nd isotope compositions indicate that thesefeatures of the Spitsbergen peridotites can be explained bychemical fractionation during metasomatism in the mantle. ‘Chromatographic’effects of melt percolation create a transient zone where thehost depleted peridotites have experienced enrichment in Sr(with a radiogenic isotope composition) but not in Nd, thusproducing Sr–Nd decoupling mainly controlled by partitioncoefficients and abundances of Sr and Nd in the melt and theperidotite. Therefore, Sr–Nd isotope decoupling, earlierreported for some other mantle peridotites worldwide, may bea signature of metasomatic processes rather than a source-relatedcharacteristic, contrary to models that invoke mixing with hypotheticalSr-rich fluids derived from subducted oceanic lithosphere. Pbisotope compositions of the Spitsbergen xenoliths do not appearto be consistently affected by the metasomatism. KEY WORDS: Spitsbergen; lithospheric mantle; metasomatism; radiogenic isotopes; theoretical modelling     

Fractionation of Nb and Ta from Zr and Hf at Mantle Depths: the Role of Titanian Pargasite and Kaersutite

Selective enrichment or depletion in either Zr and Hf (HFSE⁴⁺)or Nb and Ta (HFSE⁵⁺) is a feature commonly observed in manymantle-derived melts and amphiboles occurring as either disseminatedminerals in mantle xenoliths and peridotite massifs or in veinassemblages cutting these rocks. The fractionation of Nb fromZr seen in natural mantle amphiboles suggests that their incorporationis governed by different crystal-chemical mechanisms. An extensiveset of new partitioning experiments between pargasite–kaersutiteand melt under upper-mantle conditions shows that HFSE incorporationand fractionation depends on amphibole major-element compositionand the presence or absence of dehydrogenation. Multiple regressionanalysis shows that ^Amph/LD_Nb/Zr is strongly dependent on themg-number of the amphibole as a result of a combination of amphiboleand melt structure effects, so that the following generalizationsapply: (1) high-mg-number amphiboles crystallized from unmodifiedmantle melts more easily incorporate Zr relative to Nb leadingto an increase of the Nb/Zr ratio in the residual melt; (2)low-mg-number amphiboles, such as those found in veins cuttingperidotites, may strongly deplete the residual melt in Nb andcause very low Nb/Zr in residual melts. Implications and applicationsto mantle environments are discussed. KEY WORDS: trace elements; high field strength elements; partition coefficients; amphibole; upper mantle     

Insights into the complexity of crustal differentiation: K2O‐poor leucosomes within metasedimentary migmatites from the Southern Marginal Zone of the Limpopo Belt,South Africa

Differentiation of the continental crust is the result of complex interactions between a large number of processes, which govern partial melting of the deep crust, magma formation and segregation, and magma ascent to significantly higher crustal levels. The anatectic metasedimentary rocks exposed in the Southern Marginal Zone of the Limpopo Belt represent an unusually well‐exposed natural laboratory where the portion of these processes that operate in the deep crust can be directly investigated in the field. The formation of these migmatites occurred via absent incongruent melting reactions involving biotite, which produced cm‐ to m‐scale, K₂O‐poor garnet‐bearing stromatic leucosomes, with high Ca/Na ratios relative to their source rocks. Field investigation combined with geochemical analyses, and phase equilibrium modelling designed to investigate some aspects of disequilibrium partial melting show that the outcrop features and compositions of the leucosomes suggest several steps in their evolution: (1) Melting of a portion of the source, with restricted plagioclase availability due to kinetic controls, to produce a magma (melt + entrained peritectic minerals in variable proportions relative to melt); (2) Segregation of the magma at near peak metamorphic conditions into melt accumulation sites (MAS), also known as future leucosome; (3a) Re‐equilibration of the magma with a portion of the bounding mafic residuum via chemical diffusion (H₂O, K₂O), which triggers the co‐precipitation of quartz and plagioclase in the MAS; (3b) Extraction of melt‐dominated magma to higher crustal levels, leaving peritectic minerals entrained from the site of the melting reaction, and the minerals precipitated in the MASs to form the leucosome in the source. The key mechanism controlling this behaviour is the kinetically induced restriction of the amount of plagioclase available to the melting reaction. This results in elevated melt H₂O and K₂O and chemical potential gradient for these components across the leucosome/mafic residuum contact. The combination of all of these processes accurately explains the composition of the K₂O‐poor leucosomes. These findings have important implications for our understanding of melt segregation in the lower crust and minimum melt residency time which, according to the chemical modelling, is <5 years. We demonstrate that in some migmatitic granulites, the leucosomes constitute a type of felsic refractory residuum, rather than evidence of failed magma extraction. This provides a new insight into the ways that source heterogeneity may control anatexis.     

Oxygen isotope study of the Long Valley magma system, California: isotope thermometry and convection in large silicic magma bodies

Products of voluminous pyroclastic eruptions with eruptive draw-down of several kilometers provide a snap-shot view of batholith-scale magma chambers, and quench pre-eruptive isotopic fractionations (i.e., temperatures) between minerals. We report analyses of oxygen isotope ratio in individual quartz phenocrysts and concentrates of magnetite, pyroxene, and zircon from individual pumice clasts of ignimbrite and fall units of caldera-forming 0.76 Ma Bishop Tuff (BT), pre-caldera Glass Mountain (2.1-0.78 Ma), and post-caldera rhyolites (0.65-0.04 Ma) to characterize the long-lived, batholith-scale magma chamber beneath Long Valley Caldera in California. Values of '¹⁸O show a subtle 1‰ decrease from the oldest Glass Mountain lavas to the youngest post-caldera rhyolites. Older Glass Mountain lavas exhibit larger (~1‰) variability of '¹⁸O(quartz). The youngest domes of Glass Mountain are similar to BT in '¹⁸O(quartz) values and reflect convective homogenization during formation of BT magma chamber surrounded by extremely heterogeneous country rocks (ranging from 2 to +29‰). Oxygen isotope thermometry of BT confirms a temperature gradient between "Late" (815 °C) and "Early" (715 °C) BT. The '¹⁸O(quartz) values of "Early" and "Late" BT are +8.33 and 8.21‰, consistent with a constant '¹⁸O(melt)=7.8ǂ.1‰ and 100 °C temperature difference. Zircon-melt saturation equilibria gives a similar temperature range. Values of '¹⁸O(quartz) for different stratigraphic units of BT, and in pumice clasts ranging in pre-eruptive depths from 6 to 11 km (based on melt inclusions), and document vertical and lateral homogeneity of '¹⁸O(melt). Worldwide, five other large-volume rhyolites, Lava Creek, Lower Bandelier, Fish Canyon, Cerro Galan, and Toba, exhibit equal '¹⁸O(melt) values of earlier and later erupted portions in each of the these climactic caldera-forming eruptions. We interpret the large-scale '¹⁸O homogeneity of BT and other large magma chambers as evidence of their longevity (>10⁵ years) and convection. However, remaining isotopic zoning in some quartz phenocrysts, trace element gradients in feldspars, and quartz and zircon crystal size distributions are more consistent with far shorter timescales (10²-10⁴ years). We propose a sidewall-crystallization model that promotes convective homogenization, roofward accumulation of more evolved and stagnant, volatile-rich liquid, and develops compositional and temperature gradients in pre-climactic magma chamber. Crystal + melt + gas bubbles mush near chamber walls of variable '¹⁸O gets periodically remobilized in response to chamber refill by new hotter magmas. One such episode of chamber refill by high-Ti, Sr, Ba, Zr, and volatile-richer magma happened 10³-10⁴ years prior to the 0.76-Ma caldera collapse that caused magma mixing at the base, mush thawing near the roof and walls, and downward settling of phenocrysts into this hybrid melt.     

Immiscible sulfide droplets in pseudotachylyte: Evidence for high temperature (> 1200 °C) melts

In the North Cascade Mountains, Washington, rocks that underwent friction melting commonly contained sulfide minerals, mostly pyrrhotite and pyrite. During pseudotachylyte melt formation, sulfides melted to form immiscible sulfide droplets present in five distinct textural settings. The largest droplets formed through melting of lithic clasts, whereas micron-scale sulfide droplets are common in many of the pseudotachylytes veins.Microprobe analysis indicates that nearly all droplets are pyrrhotite. The disappearance of pyrite indicates that melt temperatures must have exceeded 750 °C, but other indications suggest that the melt temperature must have been much higher. The extremely common presence of pyrrhotite droplets suggests that pyrrhotite from the protolith melted, requiring a minimum melt temperature of 1200 °C. In some samples, evidence for fluid-rich bubbles, and possible silicate spherules indicates three coexisting immiscible phases within the silicate melt. The presence of sulfide droplets appears to be common, especially in relatively low oxygen-fugacity melts that formed at shallow crustal levels. This can provide a good textural marker of melting and therefore of pseudotachylyte formation, especially where other indications of melting (i.e., high temperature microlites, vesicles, etc.) are lacking, and illustrates the extreme temperatures possible along frictionally sliding surfaces during seismic events.     

Petrology of kamafugites and kimberlites from the Alto Paranaíba Alkaline Province, Minas Gerais, Brazil

Mafic rocks representative of the alkaline magmatism of the Alto Paranaíba Province in southwestern Minas Gerais, Brazil were studied by means of petrography, mineral, whole-rock and isotope geochemistry with the objective of better understanding this Cretaceous magmatism and the characteristics of the magma sources. Because of the variety and complexity of lithotypes examined in this research and the paucity of detailed studies of these Brazilian rocks in the literature, this study also attempts to establish parameters that allow for a clear distinction between kimberlite and kamafugite. Fifty-two occurrences are described and classified as kimberlite or kamafugite. Among the kamafugites, both ugandite (characterized by the presence of leucite) and mafurite (with kalsilite) end members have been characterized. Mineral compositions were found to be efficient in distinguishing between kimberlites, mafurites and ugandites in the province, primarily on the basis of phlogopite composition. The Re-Os isotope systematics permitted a better understanding of the relation of the sublithospheric mantle source to the magmatism in the region. Kimberlites, mafurites and ugandites have different ¹⁸⁷Os/¹⁸⁸Os ratios (0.117 to 0.129, 0.127 to 0.145 and 0.142 to 0.147, respectively). The Rb-Sr and Sm-Nd isotope systematics failed to indicate first-order differences between kamafugites and kimberlites, whilst ²⁰⁶Pb/²⁰⁴Pb ratios for the kimberlites are higher than those for the other rock types. Kimberlite and kamafugite isotopic compositions appear to be related to the mixture of at least two dominant mantle components: one with an isotopic signature similar to that of lithospheric peridotites, i.e., with ¹⁸⁷Os/¹⁸⁸Os ratios of the order of 0.118, similar to those observed in mantle-derived xenoliths entrained in kimberlites intruded in the Kaapvaal, Wyoming, and Siberian cratons; another with higher ¹⁸⁷Os/¹⁸⁸Os ratios of the order of 0.135, within the range of ratios reported for pyroxenite veins in alpine-type peridotites and ocean island basalts. Different melting depths of heterogeneous lithospheric sources by a mantle plume are suggested to explain the isotopic characteristics of the Alto Paranaíba Alkaline Province alkaline rocks.     

Melting Behaviour of Model Lherzolite in the System CaO-MgO-Al2O3-SiO2-FeO at 0{middle dot}7-2{middle dot}8 GPa

Fe–Mg exchange is the most important solid solution involvedin partial melting of spinel lherzolite, and the system CaO–MgO–Al₂O₃–SiO₂–FeO(CMASF) is ideally suited to explore this type of exchange duringmantle melting. Also, if primary mid-ocean ridge basalts arelargely generated in the spinel lherzolite stability field bynear-fractional fusion, then Na and other highly incompatibleelements will early on become depleted in the source, and themelting behaviour of mantle lherzolite should resemble the meltingbehaviour of simplified lherzolite in the CMASF system. We havedetermined the isobarically univariant melting relations ofthe lherzolite phase assemblage in the CMASF system in the 0·7–2·8GPa pressure range. Isobarically, for every 1 wt % increasein the FeO content of the melt in equilibrium with the lherzolitephase assemblage, the equilibrium temperature is lower by about3–5°C. Relative to the solidus of model lherzolitein the CaO–MgO–Al₂O₃–SiO₂ system, melt compositionsin the CMASF system are displaced slightly towards the alkalicside of the basalt tetrahedron. The transition on the solidusfrom spinel to plagioclase lherzolite has a positive Clapeyronslope with the spinel lherzolite assemblage on the high-temperatureside, and has an almost identical position in P–T spaceto the comparable transition in the CaO–MgO–Al₂O₃–SiO₂–Na₂O(CMASN) system. When the compositions of all phases are describedmathematically and used to model the generation of primary basalts,temperature and melt composition changes are small as percentmelting increases. More specifically, 10% melting takes placeover 1·5–2°C, melt compositions are relativelyinsensitive to the degree of melting and bulk composition, andequilibrium and near-fractional melting yield similar melt compositions.FeO and MgO are the oxides that exhibit the greatest changein the melt with degree of melting and bulk composition. Theamount of FeO decreases with increasing degree of melting, whereasthe amount of MgO increases. The coefficients for Fe–Mgexchange between the coexisting crystalline phases and melt,K_dFe–Mg^xl–liq, show a relatively simple and predictablebehaviour with pressure and temperature: the coefficients forolivine and spinel do not show significant dependence on temperature,whereas the coefficients for orthopyroxene and clinopyroxeneincrease with pressure and temperature. When melting of lherzoliteis modeled in the CMASF system, a strong linear correlationis observed between the mg-number of the lherzolite and themg-number of the near-solidus melts. Comparison with meltingin the CMASN system indicates that Na₂O has a strong effecton lherzolite melting behaviour only at small degrees of melting. KEY WORDS: CMASF; lherzolite solidus; mantle melting     

The Generation of Granitic Magmas by Intrusion of Basalt into Continental Crust

When basalt magmas are emplaced into continental crust, meltingand generation of silicic magma can be expected. The fluid dynamicaland heat transfer processes at the roof of a basaltic sill inwhich the wall rock melts are investigated theoretically andalso experimentally using waxes and aqueous solutions. At theroof, the low density melt forms a stable melt layer with negligiblemixing with the underlying hot liquid. A quantitative theoryfor the roof melting case has been developed. When applied tobasalt sills in hot crust, the theory predicts that basalt sillsof thicknesses from 10 to 1500 m require only 1 to 270 y tosolidify and would form voluminous overlying layers of convectingsilicic magma. For example, for a 500 m sill with a crustalmelting temperature of 850 ?C, the thickness of the silicicmagma layer generated ranges from 300 to 1000 m for countryrock temperatures from 500 to 850?C. The temperatures of thecrustal melt layers at the time that the basalt solidifies arehigh (900–950?C) so that the process can produce magmasrepresenting large degrees of partial fusion of the crust. Meltingoccurs in the solid roof and the adjacent thermal boundary layer,while at the same time there is crystallization in the convectinginterior. Thus the magmas formed can be highly porphyritic.Our calculations also indicate that such magmas can containsignificant proportions of restite crystals. Much of the refractorycomponents of the crust are dissolved and then re-precipitatedto form genuine igneous phenocrysts. Normally zoned plagioclasefeldspar phenocrysts with discrete calcic cores are commonlyobserved in many granitoids and silicic volcanic rocks. Suchpatterns would be expected in crustal melting, where simultaneouscrystallization is an inevitable consequence of the fluid dynamics. The time-scales for melting and crystallization in basalt-inducedcrustal melting (10²–10³ y) are very short compared tothe lifetimes of large silicic magma systems (>10⁶ y) orto the time-scale for thermal relaxation of the continentalcrust (> l0⁷ y). Several of the features of silicic igneoussystems can be explained without requiring large, high-level,long-lived magma chambers. Cycles of mafic to increasingly largevolumes of silicic magma with time are commonly observed inmany systems. These can be interpreted as progressive heatingof the crust until the source region is partially molten andbasalt can no longer penetrate. Every input of basalt triggersrapid formation of silicic magma in the source region. Thismagma will freeze again in time-scales of order l0²–10³y unless it ascends to higher levels. Crystallization can occurin the source region during melting, and eruption of porphyriticmagmas does not require a shallow magma chamber, although suchchambers may develop as magma is intruded into high levels inthe crust. For typical compositions of upper crustal rocks,the model predicts that dacitic volcanic rocks and granodiorite/tonaliteplutons would be the dominant rock types and that these wouldascend-from the source region and form magmas ranging from thosewith high temperature and low crystal content to those withhigh crystal content and a significant proportion of restite.     

Disequilibrium Melting and the Rate of Melt-Residuum Separation During Migmatization of Mafic Rocks from the Grenville Front, Quebec

Migmatites are developed in Archaean metabasites south of theGrenville Front. Relative to equivalent greenschist facies metabasites,those hosting the migmatites have undergone some mobilizationof CaO, Na₂O, and Sr, and, in the case of sheared metabasites,the introduction of K₂O, Ba, Cs, and Rb, before migmatization.Three types of anatectic migmatite are recognized, based ontheir leucosome-melanosome relationships: (1) non-segregatedmigmatites in which new leucocratic and magic phases are intimatelymixed in patches up to 15 cm across, (2) segregated migmatitesin which the leucosomes are located in boudin necks and shearbands, and are separated from their associated mafic selvedgesby 5–100 cm, and (3) vein-type migmatites where discordantleucosomes lack mafic selvedges. The non-segregated and segregatedmigmatites have a local and essentially isochemical origin,whereas the vein-type represent injected melt. Leucosomes fromthe segregated and vein-type migmatites have similar tonaliticmajor oxide compositions, but they differ greatly in their trace-elementcharacteristics. The vein-type leucosomes are enriched in K₂O, Ba, Cs, Rb, LREE,Th, Hf, Zr, and P2O5 relative to their metabasite hosts, andhave greater La/Yb_N ratios (27 compared with 0?6–17).These veins may have formed by between 5 and 25%equilibriumbatch partial melting of Archaean metabasalt, leaving garnet+ hornblende in the residuum. In contrast, leucosomes from the segregated migmatites are depletedin REE, Sc, V, Cr, Ni, Co, Ti, Th, Hf, Zr, Nb, and P₂O₅ relativeto their source rocks; the associated mafic selvedges are enrichedin these elements. The leucosomes and mafic selvedges both haveLa/Yb_N ratios that are similar to those of the source metabasitesirrespective of whether the source is LREE depleted or LREEenriched. The abundances of many trace elements in the leucosomesappear to be controlled by the degree of contamination withresiduum material. Zr concentrations in the leucosomes are between10 and 52% of the estimated equilibrium concentrations in felsicmelts at the temperature (750–775 ?C) of migmatization.A numerical simulation of disequilibrium melting using bothLREE-depleted and LREE-enriched sources yields model melts withtrace element abundances that match those of the natural leucosomes.Mafic selvedge compositions indicate that the segregated migmatitesrepresent a range of between 12 and 36% partial melting of theirhost metamatization. Based upon calculated dissolution times for zircon in wet melts,the melt and residuum were separated in less than 23a, otherwisemelts would have become saturated in Zr. Rapid melt extractionis thought to be driven by pressure gradients developed duringnon-coaxial deformation of the anisotropic palaeosome duringmigmatization. The common occurrence, based on published work, of disequilibriumcompositions in migmatite leucosomes implies that during mid-crustalmelting the melt-segregation rates are greater than the rateof chemical equilibration between melt and the residual solid.In contrast, at the higher temperatures of granite formation,the rate of chemical equilibration exceeds that of melt-segregationand equilibrium melt compositions are reached before segregationcan occur. On the basis of their trace element characteristics,the melt which forms segregated migmatites cannot be the sameas that which forms the vein-like migmatites, or granitoid plutons.     

The permeability controlled accumulation of primary magma

The initial accumulation of primary magma occurs just after the mantle has become permeable. The accumulation is caused by the compaction of the residuum, which either may be controlled by the rate of creep, or by the rate of flow of the interstitial melt. Experimental results suggest that the rate of compaction is controlled by the permeability, and a model for the accumulation process is worked out on this basis. The compaction causes the formation of a lower compaction boundary and an upper layer of melt. The ascending mantle of plumes and convection currents will form layers of melt situated 20–100 m apart. The type of partial melting for this accumulation is critical melting.     

Combined Trace Element and Pb-Nd-Sr-O Isotope Evidence for Recycled Oceanic Crust (Upper and Lower) in the Iceland Mantle Plume

We present the results of a comprehensive major element, traceelement and Sr–Nd–Pb–O isotopic study of post-glacialvolcanic rocks from the Neovolcanic zones on Iceland. The rocksstudied range in composition from picrites and tholeiites, whichdominate in the main rift systems, to transitional and alkalicbasalts confined to the off-rift and propagating rift systems.There are good correlations of rock types with geochemical enrichmentparameters, such as La/Sm and La/Yb ratios, and with long-termradiogenic tracers, such as Sr–Nd–Pb isotope ratios,indicating a long-lived enrichment/depletion history of thesource region. ⁸⁷Sr/⁸⁶Sr vs ¹⁴³Nd/¹⁴⁴Nd defines a negative array.Pb isotopes define well-correlated positive arrays on both ²⁰⁶Pb/²⁰⁴Pbvs ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb diagrams, indicating mixing ofat least two major components: an enriched component representedby the alkali basalts and a depleted component represented bythe picrites. In combined Sr–Nd–Pb isotopic spacethe individual rift systems define coherent mixing arrays withslightly different compositions. The enriched component hasradiogenic Pb (²⁰⁶Pb/²⁰⁴Pb > 19·3) and very similargeochemistry to HIMU-type ocean island basalts (OIB). We ascribethis endmember to recycling of hydrothermally altered upperbasaltic oceanic crust. The depleted component that is sampledby the picrites has unradiogenic Pb (²⁰⁶Pb/²⁰⁴Pb < 17·8),but geochemical signatures distinct from that of normal mid-oceanridge basalt (N-MORB). Highly depleted tholeiites and picriteshave positive anomalies in mantle-normalized trace element diagramsfor Ba, Sr, and Eu (and in some cases also for K, Ti and P),negative anomalies for Hf and Zr, and low ¹⁸O_olivine values(4·6–5·0) below the normal mantle range.All of these features are internally correlated, and we, therefore,interpret them to reflect source characteristics and attributethem to recycled lower gabbroic oceanic crust. Regional compositionaldifferences exist for the depleted component. In SW Icelandit has distinctly higher Nb/U (68) and more radiogenic ²⁰⁶Pb/²⁰⁴Pbratios (18·28–18·88) compared with the NErift (Nb/U 47; ²⁰⁶Pb/²⁰⁴Pb = 18·07–18·47).These geochemical differences suggest that different packagesof recycled oceanic lithosphere exist beneath each rift. A thirdand minor component with relatively high ⁸⁷Sr/⁸⁶Sr and ²⁰⁷Pb/²⁰⁴Pbis found in a single volcano in SE Iceland (Öræfajökullvolcano), indicating the involvement of recycled sediments inthe source locally. The three plume components form an integralpart of ancient recycled oceanic lithosphere. The slope in theuranogenic Pb diagram indicates a recycling age of about 1·5Ga with time-integrated Th/U ratios of 3·01. Surprisingly,there is little evidence for the involvement of North AtlanticN-MORB source mantle, as would be expected from the interactionof the Iceland plume and the surrounding asthenosphere in formof plume–ridge interaction. The preferential samplingof the enriched and depleted components in the off-rift andmain rift systems, respectively, can be explained by differencesin the geometry of the melting regions. In the off-rift areas,melting columns are truncated deeper and thus are shorter, whichleads to preferential melting of the enriched component, asthis starts melting deeper than the depleted component. In contrast,melting proceeds to shallower depths beneath the main rifts.The longer melting columns also produce significant amountsof melt from the more refractory (lower crustal/lithospheric)component. KEY WORDS: basalts; trace element and Sr, Nd, Pb, O isotope geochemistry; Iceland plume; isotope ratios; oceanic crustal recycling; partial melting; plume–ridge interaction