Showing papers on "Peridotite published in 2000"

Mantle solidus: Experimental constraints and the effects of peridotite composition

Abstract: [1] A review of experiments on natural peridotites allows improved constraints on the location of the mantle solidus. Available constraints on the location of the nominally dry peridotite solidus show considerable scatter, owing to interlaboratory uncertainties and the effects of bulk composition. When experiments on enriched peridotite are filtered from the database, the best fit to the solidus between 0 and 10 GPa is given by T(°C) = aP2 + bP + c where a = −5.104, b = 132.899, and c = 1120.661 and P is in gigapascals. Compared to previous models, the solidus in this parameterization is at lower temperature between 2 and 6 GPa, with the largest differences near 4 GPa, where it is 30°–60°C cooler. Consideration of experimental constraints on the peridotite solidus and of a theoretical model of melting in a simple analogue system suggests that a key variable affecting peridotite solidus temperature is the near-solidus liquid alkali concentration. The effect of alkalis on the solidus increases with bulk concentration in the peridotite but decreases with bulk partition coefficient. Thus small bulk concentrations of K can have a significant influence on the peridotite solidus, and the effect of Na diminishes with increasing pressure, as it becomes more compatible in the solidus residua. Mg # [=100 × MgO/(MgO + FeO)] variations are subordinate to alkali variations in controlling solidus temperature at lower pressures but may increase in relative importance as alkalis become more compatible in peridotite residua with increasing pressure. Increased clinopyroxene mode has the effect of making Na more compatible in residual solids and so diminishes the solidus-lowering tendencies of alkalis. As a consequence, experiments performed on a range of peridotite compositions may not reflect the likely effect of variable mantle composition on solidus temperature if they do not match the appropriate correlation between alkali content and clinopyroxene mode.

Geochemistry and tectonic significance of peridotites from the South Sandwich arc-basin system, South Atlantic

Abstract: Petrographic and geochemical studies of peridotites from the South Sandwich forearc region provide new evidence for the evolution of the South Sandwich arc–basin system and for the nature of interactions between arc magma and oceanic lithosphere. Peridotites from the inner trench wall in the north-east corner of the forearc vary from clinopyroxene-bearing harzburgites, through samples transitional between harzburgites and dunites or wehrlites, to dunites. The harzburgites are LREE depleted with low incompatible element abundances and have chromites with intermediate Cr# (ca. 0.40). Modelling shows that they represent the residues from 15–20% melting at oxygen fugacities close to the QFM buffer. The dunites have U-shaped REE patterns, low incompatible element abundances and high Cr# (0.66–0.77). Petrography and geochemistry indicate that the latter are the product of intense interaction between peridotite and melt saturated with olivine under conditions of high oxygen fugacity (QFM + 2). The transitional samples are the product of lesser interaction between peridotite and melt saturated with olivine ± clinopyroxene. The data demonstrate that the harzburgites originated as the residue from melting at a ridge (probably the early East Scotia Sea spreading centre), and were subsequently modified to transitional peridotites and dunites by interaction with South Sandwich arc magmas. The second dredge locality, near the South Sandwich Trench–Fracture Zone intersection, yielded rocks ranging from lherzolite to harzburgite that could similarly have resulted from a two-stage melting and enrichment process, but involving a more fertile mantle residue and a reacting melt that is transitional between MORB and island arc tholeiite. The South Sandwich peridotites have a similar petrogenetic history to those from Conical Seamount in the Mariana forearc in the sense that both involved interaction between arc magma and pre-existing mantle lithosphere of different provenance. However, the precise compositions of the magma and mantle components vary from location to location according to the precise tectonic setting and tectonic history. Overall, therefore, data from the South Sandwich and Izu–Bonin–Mariana systems emphasise the potential significance of peridotite geochemistry in unravelling the complex tectonic histories of forearcs past and present.

Oxygen Isotope Geochemistry of Oceanic-Arc Lavas

Abstract: Variations of oxygen isotope ratios in arc-related lavas can constrain the contributions of subducted crustal igneous rocks, sediments, and fluids to the sub-arc mantle. We have measured oxygen isotope ratios in 72 arc and back-arc lavas from five ocean–ocean subduction zone systems using laser-fluorination analyses of olivine and other phenocrysts and glass. Eighty percent of our samples have {delta}18O values for any given phase (olivine, plagioclase, glass, or biotite) within 0·2{per thousand} of the average value for that phase in upper-mantle peridotites and mid-ocean ridge basalt (MORB); the range for each phase is <=1·0{per thousand}. This result contrasts with previous studies of whole-rock samples (which are significantly more variable even after exclusion of samples believed to be altered or fractionated by magmatic differentiation) and demonstrates that most arc-related lavas contain <=1–2% of 18O-enriched crustal oxygen from any source (i.e. assimilation or subducted contributions). Elevations in {delta}18O that do occur in these basic, arc-derived magmas relative to values most common for mantle-derived lavas are associated both with ‘enriched’ radiogenic isotope signatures and, even more strongly, with chemical indices consistent with high integrated extents of melting of their peridotite sources. We interpret these relationships as evidence that melting in the sources of the high-{delta}18O lavas we have studied was fluxed by addition of high-{delta}18O aqueous fluid (or perhaps a hydrous melt) from the subducted slab, such that sources that contain relatively large components of slab-derived fluid or melt are both relatively 18O enriched and also experienced relatively large amounts of melting. We have developed a quantitative model linking the amount of melting to the extents of 18O, radiogenic isotope, and trace-element enrichment in a mantle source being fluxed by addition of aqueous fluid. Comparison of this model with observed variations in the geochemistry of lavas from the Vanuatu–Fiji–New Caledonia region (the suite of related samples showing the greatest range in {delta}18O observed in this study) constrains the amounts and chemical and isotopic compositions of slab-derived phases in the sources of these arc-related lavas. Assuming a {delta}18O value of 20{per thousand} for the slab-derived fluid, 0·5–1·0 wt % is added to the sources of most mantle-derived arc magmas; the maximum amount of slab-derived flux in the sources of arc magmas according to our results is 2·5 wt %.

The Cameroon Volcanic Line Revisited: Petrogenesis of Continental Basaltic Magmas from Lithospheric and Asthenospheric Mantle Sources

Abstract: from variable amounts of mixing between melts derived from an The volcanic activity of Mts Bambouto and Oku (Western Highlands) and of the Ngaoundere Plateau, in the continental sector of anhydrous lherzolite source (asthenospheric component) and melts the Cameroon Volcanic Line, Equatorial West Africa, ranges in from an amphibole-bearing peridotite source (lithospheric HSr age from Oligocene to Recent. It is characterized by basanitic, alkali component). New Ar/Ar ages for Mts Oku and Bambouto basaltic and transitional basaltic series. Mineral chemistry, major basalts, combined with previous Ar/Ar and K/Ar ages of and trace element bulk-rock compositions, and geochemical modelling basaltic and silicic volcanics, and with volcanic stratigraphy, suggest suggest that the magmatic series evolved mainly at low pressure a NE–SW younging of the peak magmatic activity in the Western (2–4 kbar) through fractional crystallization of clinopyroxene and Highlands. This SW younging trend, extending from the Oligocene olivine±magnetite, at moderately hydrated (H2O= 0·5–1 wt %) volcanism in northern Cameroon (e.g. Mt Oku) to the still active and QFM (quartz–fayalite–magnetite) to QFM + 1 fO2 conMt Cameroon, suggests that the African plate is moving above a ditions. Basalts from Ngaoundere (Miocene to Quaternary) and deep-seated mantle thermal anomaly. However, the age and location from the early activity (31–14 Ma) of the Western Highlands have of the Ngaoundere volcanism does not conform to the NE–SW incompatible trace element and Sr–Nd isotopic compositions similar younging trend, implying that the continental sector of the Cameroon to those of oceanic Cameroon Line basalts, pointing to a similar Volcanic Line cannot be easily interpreted as the surface expression asthenospheric mantle source. By contrast, the late (15–4 Ma) of a single hotspot system. Western Highlands basanites and alkali basalts have anomalously high concentrations of Sr, Ba and P, and low concentrations of Zr, which are exclusive features of continental Cameroon basalts. The genesis of these latter magmas is consistent with derivation from an incompatible element enriched, amphibole-bearing lithospheric

Oxygen-isotope evidence for recycled crust in the sources of mid-ocean-ridge basalts

Abstract: Mid-ocean-ridge basalts (MORBs) are the most abundant terrestrial magmas and are believed to form by partial melting of a globally extensive reservoir of ultramafic rocks in the upper mantle. MORBs vary in their abundances of incompatible elements (that is, those that partition into silicate liquids during partial melting) and in the isotopic ratios of several radiogenic isotope systems. These variations define a spectrum between 'depleted' and 'enriched' compositions, characterized by respectively low and high abundances of incompatible elements. Compositional variations in the sources of MORBs could reflect recycling of subducted crustal materials into the source reservoir, or any of a number of processes of intramantle differentiation. Variations in ^(18)O/^(16)O (principally sensitive to the interaction of rocks with the Earth's hydrosphere) offer a test of these alternatives. Here we show that ^(18)O/^(16)O ratios of MORBs are correlated with aspects of their incompatible-element chemistry. These correlations are consistent with control of the oxygen-isotope and incompatible-element geochemistry of MORBs by a component of recycled crust that is variably distributed throughout their upper mantle sources.

Experimental study of the phase and melting relations of homogeneous basalt+peridotite mixtures and implications for the petrogenesis of flood basalts

Abstract: Flood basalt provinces may constitute some of the most catastrophic volcanic events in the Earth's history. A popular model to explain them involves adiabatic ascent of plumes of anomalously hot peridotite from a thermal boundary layer deep in the mantle, across the peridotite solidus. However, peridotitic plumes probably require unreasonably high potential temperatures to generate sufficient volumes of magma and high enough melting rates to produce flood volcanism. This lead to the suggestion that low melting eclogitic or pyroxenitic heterogeneities may be present in the source regions of the flood basalts. In order to constrain petrogenetic models for flood basalts generated in this way, an experimental investigation of the melting relations of homogeneous peridotite + oceanic basalt mixtures has been performed. Experiments were conducted at 3.5 GPa on a fertile peridotite (MPY90)–oceanic basalt (GA1) compositional join. The hybrid basalt + peridotite compositions crystallised garnet lherzolite at subsolidus temperatures plus quenched ne-normative picritic liquids at temperatures just above the solidus, over the compositional range MPY90 to GA150MPY9050. The solidus temperature decreased slightly from ∼1500 °C for MPY90 to ∼1450 °C for GA150MPY9050. Compositions similar to GA130MPY9070 have 100% melting compressed into a melting interval which is approximately 50–60% smaller than that for pure MPY90, due to a liquidus minimum. During adiabatic ascent of hybrid source material containing a few tens of percent basalt in peridotite, the lower solidus and compressed solidus–liquidus temperature interval may conspire to substantially enhance melt productivity. Mixtures of recycled oceanic crust and peridotite in mantle plumes may therefore provide a viable source for some flood volcanics. Evidence for this would include higher than normal Fe/Mg values in natural primary liquids, consistent with equilibration with more Fe-rich olivine than normal pyrolitic olivine (i.e.

A general model for the intrusion and evolution of 'mantle' garnet peridotites in high-pressure and ultra-high-pressure metamorphic terranes

Abstract: Garnet-bearing peridotite lenses are minor but significant components of most metamorphic terranes characterized by high-temperature eclogite facies assemblages. Most peridotite intrudes when slabs of continental crust are subducted deeply (60–120 km) into the mantle, usually by following oceanic lithosphere down an established subduction zone. Peridotite is transferred from the resulting mantle wedge into the crustal footwall through brittle and/or ductile mechanisms. These ‘mantle’ peridotites vary petrographically, chemically, isotopically, chronologically and thermobarometrically from orogen to orogen, within orogens and even within individual terranes. The variations reflect: (1) derivation from different mantle sources (oceanic or continental lithosphere, asthenosphere); (2) perturbations while the mantle wedges were above subducting oceanic lithosphere; and (3) changes within the host crustal slabs during intrusion, subduction and exhumation. Peridotite caught within mantle wedges above oceanic subduction zones will tend to recrystallize and be contaminated by fluids derived from the subducting oceanic crust. These ‘subduction zone peridotites’ intrude during the subsequent subduction of continental crust. Low-pressure protoliths introduced at shallow (serpentinite, plagioclase peridotite) and intermediate (spinel peridotite) mantle depths (20–50 km) may be carried to deeper levels within the host slab and undergo high-pressure metamorphism along with the enclosing rocks. If subducted deeply enough, the peridotites will develop garnet-bearing assemblages that are isofacial with, and give the same recrystallization ages as, the eclogite facies country rocks. Peridotites introduced at deeper levels (50–120 km) may already contain garnet when they intrude and will not necessarily be isofacial or isochronous with the enclosing crustal rocks. Some garnet peridotites recrystallize from spinel peridotite precursors at very high temperatures (c. 1200 °C) and may derive ultimately from the asthenosphere. Other peridotites are from old (>1 Ga), cold (c. 850 °C), subcontinental mantle (‘relict peridotites’) and seem to require the development of major intra-cratonic faults to effect their intrusion.

Seismological evidence for the existence of serpentinized wedge mantle

Abstract: Serpentinized peridotite is detected seismologically by mapping Poisson's ratio beneath the Kanto district, central Japan, because serpentinite has a higher Poisson's ratio than other rocks in the crust and upper mantle. We determine three-dimensional P and S wave velocity models using travel time tomography and then construct a three-dimensional map of Poisson';s ratio. The models show low velocity anomalies near the upper boundary of the slab in a depth range from 20 to 70 km. In the low velocity region, there is an area with Poisson's ratio greater than 0.3 at depths of 20–45 km, which we attribute to serpentinized peridotite. The rest of the low velocity region is interpreted as descending oceanic crust with a Poisson's ratio of about 0.25. The high Poisson's ratio area is associated with low seismicity and weak interplate coupling, which is consistent with the ductility of serpentinite.

Petrochemical constraints for dual origin of garnet peridotites from the Dabie‐Sulu UHP terrane, eastern‐central China

Abstract: Garnet peridotites occur as lenses, blocks or layers within granulite–amphibolite facies gneiss in the Dabie-Sulu ultra-high-pressure (UHP) terrane and contain coesite-bearing eclogite. Two distinct types of garnet peridotite were identified based on mode of occurrence and petrochemical characteristics. Type A mantle-derived peridotites originated from either: (1) the mantle wedge above a subduction zone, (2) the footwall mantle of the subducted slab, or (3) were ancient mantle fragments emplaced at crustal depths prior to UHP metamorphism, whereas type B crustal peridotite and pyroxenite are a portion of mafic–ultramafic complexes that were intruded into the continental crust as magmas prior to subduction. Most type A peridotites were derived from a depleted mantle and exhibit petrochemical characteristics of mantle rocks; however, Sr and Nd isotope compositions of some peridotites have been modified by crustal contamination during subduction and/or exhumation. Type B peridotite and pyroxenite show cumulate structure, and some have experienced crustal metasomatism and contamination documented by high 87Sr/86Sr ratios (0.707–0.708), low eNd(t) values (−6 to −9) and low δ18O values of minerals (+2.92 to +4.52). Garnet peridotites of both types experienced multi-stage recrystallization; some of them record prograde histories. High-P–T estimates (760–970 °C and 4.0–6.5±0.2 GPa) of peak metamorphism indicate that both mantle-derived and crustal ultramafic rocks were subducted to profound depths >100 km (the deepest may be ≥180–200 km) and experienced UHP metamorphism in a subduction zone with an extremely low geothermal gradient of <5 °C km−1.

H2O Abundance in Depleted to Moderately Enriched Mid-ocean Ridge Magmas; Part I: Incompatible Behaviour, Implications for Mantle Storage, and Origin of Regional Variations

Abstract: The H2O contents and trace-element abundances are presented for two well-studied suites of mid-ocean ridge basalt (MORB) glasses from the Northern East Pacific Rise (EPR, 9-11°N) and the South East Indian Ridge (SEIR, 127-129°E), Exactly the same region of the glass samples has been analysed for these components using microbean techniques. Our data allow examination of the fine details of H2O geochemical behaviour during MORB genesis. We demonstrate that relative H2O contents [i.e. H2O/ (another incompatible element)] vary systematically with increasing (La/Sm)(N) in MORB glasses from both the EPR and SEIR. This indicates that H2O behaves like other incompatible (in peridotite mineralogies) elements during MORB petrogenesis, and is primarily controlled by solid-melt partitioning. However, the relative H2O contents of MORB glasses from the SEIR are higher than in glasses from the EPR at a given (La/Sm)(N), demonstrating global variations in the H2O contents of MORB. Despite regional differences in relative H2O contents, the incompatible behaviour of H2O is similar in both studied regions. The relative incompatibility of H2O varies systematically with increasing (La/Sm)(N): in depleted MORB, H2O is similar to La whereas in EMORB, H2O is similar to Ce. Similar patterns of varying relative incompatibility (to REE) are displayed by Zr, Hf, and P. Our data are best explained if H2O is stored in the mantle in the same phase with LREE (clinopyroxene?) at sub-solidus. Regional variations in relative H2O contents in EMORB that have more radiogenic Sr, Nd and Pb isotopes might be explained by differences in the nature of enriched components recycled via subduction processes. However, when EMORB have the same radiogenic isotope compositions as NMORB within a segment, relative H2O contents in EMORB probably reflect local processes that lead to enrichment in incompatible elements. Regional differences in relative H2O contents of NMORB may reflect either initial variations in the Earth's mantle or inhomogeneities left after formation of the continental crust.

Distinguishing ultramafic‐from basalt‐hosted submarine hydrothermal systems by comparing calculated vent fluid compositions

Abstract: Submarine hydrothermal vent fluid compositions may be controlled by peridotite-seawater or basalt-seawater reactions. Previous studies of slow-spreading ridges indicate that in addition to basalts, peridotites often play a prominent role in the construction of upper oceanic crust. Therefore the surface outcrop at a submarine hydrothermal vent field may not reveal the composition of the rock that hosts the reaction zone. We can, however, predict the compositional differences between ultramafic- and basalt-hosted vent fluids by using theoretical reaction path calculations. These calculations determine equilibrium fluid compositions and mineral assemblages, yielding synthetic hydrothermal vent fluid compositions that can be compared to analytical measurements. Synthetic vent fluid compositions created from basalt and seawater reactants at 350° or 400°C and 500 bars are in close agreement with analytical measurements of end-member vent fluids from mid-ocean ridges. Twenty simulations at a 1:1 water to rock ratio using basalt compositions spanning the range of geochemical variability observed amongst mid-ocean ridge basalts yield vent fluid compositions with <20% variation in major element concentrations. We also performed 15 ultramafic-seawater simulations using dunite, lherzolite, and harzburgite compositions found in oceanic crust. All produced aqueous SiO2, K, and H2 concentrations that are distinct from the basalt-seawater calculations. These differing concentrations can be used to attribute analytical measurements of vent fluid compositions to ultramafic or basaltic reaction zones.

Tethyan ophiolites, mantle convection, and tectonic historical contingency: A resolution of the ophiolite conundrum

Abstract: The "ophiolite conundrum" describes the conflict in many ophiolite complexes between (1) structural and stratigraphic evidence for seafloor spreading in a non-island-arc environment and (2) geochemical evidence for derivation of magmas from highly depleted mantle similar to that found at present over subduction zones ("supra-subduction zone" settings). Tethyan ophiolites provide many excellent examples of the conundrum. These complexes crop out in four major belts from the western Mediterranean to the Himalaya. Many bodies display a complete ophiolite sequence (e.g., Troodos, Cyprus, and Semail, Oman), whereas others display only a partial sequence, characteristically with lavas and/or sedimentary rocks overlying serpentinized peridotite (e.g., western Mediterranean complexes, parts of Kizildag, Turkey). Chemical compositions of the lavas in these ophiolites range from standard E-MORBs (enriched mid-oceanic-ridge basalts) to highly depleted magmas (e.g., low in Nb, Ta, Ti, and some other trace elements), reminiscent of modern volcanic-arc and backarc basin (supra-subduction zone) settings. Geologic or stratigraphic evidence for any extensive arc edifice is generally lacking, and most complexes are overlain by pelagic sedimentary rocks. Modern mid-ocean ridges show compositional variations reflecting incomplete mixing of mantle components. Some mid-oceanic ridge lavas display trace element signatures (i.e., incompatible element enrichment) unlike those of MORB affinity but reminiscent of the supra-subduction zone field. These data and a new model for a two-layer mantle of depleted material in the upper 1500 km and denser, enriched mantle below suggest a resolution of the ophiolite conundrum. In this model, the composition of mantle tapped by oceanic spreading centers for the past 1100 m.y. is nonuniform or "historically contingent," i.e., it depends upon the prior history of spreading and subduction. Spreading-center magma compositions may have varied just as the patterns of continental assembly and fragmentation have altered the tectonic arrangement of Earth's surface.

Fluid history of UHP metamorphism in Dabie Shan, China: a fluid inclusion and oxygen isotope study on the coesite-bearing eclogite from Bixiling

Abstract: The coesite-bearing eclogites and associated ultramafic rocks of Bixiling form the largest metamorphic complex in the Dabie–Sulu ultrahigh-pressure (UHP) metamorphic belt. They mainly consist of “fresh” eclogite, kyanite-rich eclogite, retrograded eclogite and garnet peridotite. Fluid inclusion and oxygen isotope studies have been carried out on the different types of eclogite and peridotite in order to identify the role of fluids during the metamorphic evolution culminating in UHP metamorphism. Five types of fluid inclusions were distinguished based on textural criteria and fluid compositions: (1) primary Ca-rich brines in quartz blebs in kyanite; (2) primary NaCl-dominated high-salinity inclusions in omphacite and kyanite; (3) primary NaCl-dominated medium- to high-salinity inclusions in matrix quartz; (4) carbonic inclusions in omphacite and matrix quartz; (5) secondary low-salinity aqueous (or pure water) inclusions in matrix quartz. The Ca-rich fluid inclusions in quartz blebs in kyanite represent the earliest recognizable fluids (prograde metamorphism) as they largely escaped late re-equilibration. Fluid inclusions in omphacite and kyanite may have been trapped during peak metamorphic conditions, whereas low-salinity aqueous inclusions in matrix quartz were trapped during the latest stage of uplift. UV-laser oxygen isotope measurements show that garnet and clinopyroxene from the “fresh” eclogite, kyanite-rich eclogite and garnet peridotite have narrow δ18O values ranging from 3.0 to 3.9‰. In contrast, garnet and omphacite in retrograded eclogite have δ18O values of −1.8 to −1.2 and of −1.1 to −0.6‰, respectively. The difference in oxygen isotope composition is interpreted to result from partial oxygen isotope exchange between the UHP complex and retrograde fluids during late exhumation. Fluids derived from the surrounding gneiss were probably responsible for the low-salinity solutions found in secondary fluid inclusions and the lowering of the δ18O values of the retrograded eclogite.

Deep subduction of mantle-derived garnet peridotites from the Su-Lu UHP metamorphic terrane in China

Abstract: Garnet peridotites from the southern Su-Lu ultra-high-pressure metamorphic (UHPM) terrane, eastern China, contain porphyroblastic garnet with aligned inclusions comprising a low-P–T mineral assemblage (chlorite, hornblende, Na-gedrite, Na-phlogopite, talc, spinel and pyrite). Orthopyroxene porphyroblasts show fine exsolution lamellae of clinopyroxene and minor chromite. A clinopyroxene inclusion in garnet shows some orthopyroxene exsolution lamellae. Both the rims of porphyroblastic pyroxene and garnet and the matrix pyroxene and garnet crystallized at the expense of olivine. This is interpreted as a result of metasomatism of the peridotites by an SiO2-rich melt at UHP conditions. A chromian garnet further overgrew on the rims of the garnet. The XMg values (Mg/(Mg+Fe)) of porphyroblastic garnet decrease from core to rim and vary in different peridotite samples, while the compositions of both the porphyroblastic and the matrix pyroxene are similar in terms of Ca–Mg–Fe. The Mg-rich cores of porphyroblastic garnet and orthopyroxene record high temperatures and pressures (c. 1000 °C, ≥5.1 GPa), whereas the matrix minerals, including the rims of porphyroblasts, record much lower P–T (c. 4.2 GPa, c. 760 °C). Sm–Nd data give apparent isochron ages of c. 380 Ma and negative eNd(0) values (c.−9). These dates are considered meaningless due to isotopic disequilibrium between garnet cores and the rest of the rocks. The isotopic disequilibrium was probably caused by metasomatism of the peridotites by melt/fluids derived from the coevally subducted crustal materials. On the other hand, the Rb–Sr isotopic systems of phlogopite and clinopyroxene appear to have reached equilibrium and record a cooling age of c. 205 Ma. It is suggested that the garnet peridotites were originally emplaced into a low-P–T environment prior to the c. 220 Ma continental collision, during which they were subducted together with crustal rocks to mantle depth and subjected to UHP metamorphism. An important corollary is that at least some of the coevally subducted crustal rocks in the Su-Lu terrane have been subjected to peak metamorphism at P–T conditions much higher than presently estimated (≥2.7 GPa, ≤800 °C).

Assessing the presence of garnet-pyroxenite in the mantle sources of basalts through combined hafnium-neodymium-thorium isotope systematics

Abstract: [1] The existence of an enriched component in the mantle with a pyroxenitic or eclogitic composition and its importance for basalt genesis has been discussed for over two decades Inferences about the depth of melting as well as the dynamics of melting based on the presence of garnet and the location of the spinel-garnet transition are different if garnet-pyroxenite is present in the peridotitic mantle Trace element partition coefficients are dependent on composition, and the differences between garnet-pyroxenite and peridotite are large enough to produce significant differences in trace element fractionation between melts derived from these different lithologies Melts derived from garnet-pyroxenite or eclogite-bearing sources will have small or no 230Th excesses, which are largely independent of melting and upwelling rate Melts derived from garnet-peridotite will have significant 230Th excesses, which are dependent on melting and upwelling rate We show that the combined hafnium-neodymium-thorium (Hf-Nd-Th) isotope and trace element data can distinguish between melts derived from peridotitic and pyroxenitic or eclogitic sources We also present new Hf isotope data for Hawaiian basalts and use the combined Hf-Nd-Th isotope and trace element systematics to argue against the existence of garnet-pyroxenite or eclogite in the source of Hawaiian basalts It is especially the large variation in degree of melting for relatively constant isotopic composition that allows us to rule out garnet-pyroxenite in the source of the Hawaiian basalts

An Archean arc basalt–Nb-enriched basalt–adakite association: the 2.7 Ga Confederation assemblage of the Birch–Uchi greenstone belt, Superior Province

Abstract: The Uchi subprovince of the Archean Superior Province is a series of greenstone belts extending 600 km east–west along the southern margin of the North Caribou Terrane protocontinent. The 2.7 Ga Confederation tectonostratigraphic assemblage of the Birch–Uchi greenstone belt, northwest Ontario, is dominated by volcanic suites of mafic, intermediate and felsic composition. Tholeiitic basalts range compositionally from Mg# 59–26 evolving continuously to greater REE contents (La=2–19 ppm; Th/Lapm˜1), with small negative Nb anomalies. Primitive tholeiites are similar to modern intraoceanic arc basalts, whereas evolved members extend to greater concentrations of Ti, Zr, V, Sc, and Y, and lower Ti/Zr, but higher Ti/Sc and Ti/V ratios characteristic of back arc basalts. Calc-alkaline basalts to dacites are characterised by more fractionated REE (La/Ybn=1–8), high Th/Nbpm ratios and deeper negative Nb anomalies; they plot with modern oceanic arc basalts and some may qualify as high magnesium andesites. The two suites are interpreted as a paired arc–back arc sequence. A third group of Nb-enriched basalts (NEB; Nb=9–18 ppm) extend to extremely high TiO2, Ta, P2O5, Sc and V contents, with strongly fractionated REE and ratios of Nb/Ta and Zr/Hf greater than primitive mantle values whereas Zr/Sm ratios are lower. The most abundant rhyolitic suite has extremely enriched but flat trace element patterns and is interpreted as strongly fractionated tholeiitic basalt liquids. A second group are compositionally similar to Cenozoic adakites and Archean high-Al, high-La/Ybn tonalites; they possess Yb ≤ 0.4 ppm, Y ≤ 6 ppm and Sc ≤ 8 ppm, with La/Ybn of 19–30 and Zr/Sm of 50–59. They are interpreted as melts of ocean lithosphere basaltic crust in a hot shallow subduction zone. Adakites are associated with NEB in Cenozoic arcs where there is shallow subduction of young and/or hot ocean lithosphere, often with oblique subduction. Slab melt adakites erupt, or metasomatise sub-arc mantle peridotite to generate an HFSE-enriched source that subsequently melts during induced mantle convection. The Archean adakite–NEB association erupted during development of the tholeiitic to calc-alkaline arc and its associated back arc. Their coexistence in the Confederation assemblage of the Birch–Uchi greenstone belt implies convergent margin processes similar to those in Cenozoic arcs.