Large Degrees Of Melting Research Articles

Deep, hot,ancient melting recorded by ultralow oxygen fugacity in peridotites.

The oxygen fugacity (fO2) of convecting upper mantle recorded by ridge peridotites varies by more than four orders of magnitude1-3. Although much attention has been given to mechanisms that drive variations in mantle fO2 between tectonic settings1,3,4 and to comparisons of fO2 between modern rocksand ancient-mantle-derived rocks5-10, comparatively little has been done to understand the origins of the high variability in fO2 recorded by peridotites from modern mid-ocean ridge settings. Here we report the petrography and geochemistry of peridotites from the Gakkel Ridge and East Pacific Rise (EPR), including 16 new high-precision determinations of fO2. Refractory peridotites from the Gakkel Ridge record fO2 more than four orders of magnitude below the mantle average. With thermodynamic and mineral partitioning modelling, we show that excursions to ultralow fO2 can be produced by large degrees of melting at high potential temperature (Tp), beginning in the garnet field and continuing into the spinel field-conditions met during the generation of ancient komatiites but not modern basalts. This does not mean that ambient convecting upper mantle had a lower ferric to ferrous ratio in Archaean times than today nor that modern melting in the garnet field at hotspots produce reduced magmas. Instead, it implies that rafts of ancient, refractory, ultrareduced mantle continue to circulate in the modern mantle while contributing little to modern ridge volcanism.

Zircon U–Pb, Hf, and O isotopic constraints on the tectonic affinity of the basement of the Himalayan orogenic belt: Insights from metasedimentary rocks, orthogneisses, and leucogranites in Garhwal, NW India

Zircon U–Pb geochronology and Hf–O isotope geochemistry of metasedimentary rocks, orthogneisses, and leucogranites in Garhwal, NW India, provide new insights into the tectonic affinity of the basement of the Himalayan orogenic belt. Detrital zircon U–Pb ages from metasedimentary rocks in both the Lesser Himalayan Sequence (LHS) and the overlying Main Central Thrust (MCT) zone yielded dominant Paleoproterozoic ages with the youngest age peak of 1840 Ma. Along with that, the variable ɛHf (t) values (−14.56 to 8.44) indicate that these metasedimentary rocks could have been derived from the juvenile Paleoproterozoic orthogneiss basement in the Himalayas and/or the ancient Indian craton in the south. In contrast, the metasedimentary rocks in the Ediacaran Harsil Formation are dominated by the ca. 1200–800 Ma zircons with the youngest age peak of ca. 630 Ma and variable ɛHf (t) values (−16.57 to 13.32), which belong to the underlying High Himalayan Crystalline Sequence (HHCS), rather than the overlying Tethyan Himalayan Sequence. Orthogneisses in the MCT zone record ca. 1.87–1.85 Ga felsic magmatism associated with the Nuna supercontinent. The Bhagirathi leucogranites intruding the Harsil Formation were formed at ca. 496–490 Ma. These Paleoproterozoic orthogneisses and early Paleozoic leucogranites show only positive ɛHf (t) values (0 to +15) correlatable with a juvenile mantle input. However, the zircon Hf model ages older than the U–Pb ages and the oxygen isotope values (δ18O: 7.74–8.91 ‰), significantly higher than zircons crystallized from mantle-derived magma, indicate large degrees of melting and contamination of crust materials in both the granitoids. In combination with the pre-existing geochronological data, the current study indicates that the Garhwal HHCS is characterized by bimodal Neoproterozoic zircons at ca. 1100–900 Ma and ca. 860–800 Ma with wide εHf (t) values which are similar to those in the outer LHS and the Cathaysia Block (including the Jiangnan Orogen) of the South China Craton. These observations suggest that a close relationship existed between North India and South China during the Neoproterozoic since Tonian. The provenance of the Neoproterozoic HHCS is possibly a mixed source from both southern Gondwana terranes (e.g., east Antarctica) and northern South China.

Geochemical characteristics of the early Neoarchean komatiite from the North China Craton: Evidence for plume–craton interaction

Komatiite is characterized by extremely high MgO contents and H2O-poor nature and plays an important role in understanding the early mantle-derived magma processes, crust–mantle differentiation of the Archean earth, but it is sparse in the North China Craton. Although komatiitic rocks have ever been discovered in the Mengyin area, their formation ages and types are still in debate. Here, we firstly report a new komatiite outcrop near Nanbaita village (Jinan city) of the western Shandong Province and present new major and trace element contents, as well as Sr–Nd and Re–Os isotopic compositions. The rocks show spinifex texture with primary olivine and pyroxene skeletal needles have been transformed into secondary minerals (e.g., serpentine, tremolite). The komatiites display extremely high MgO, but low TiO2 and Na2O + K2O. Their chondritic Al2O3/TiO2 and GdN/YbN ratios suggest they are Aluminium-undepleted komatiites which are generated by larger degrees of melting (~50%) in relatively lower pressure (ca. 5–7 GPa) with garnet absence than Aluminium-depleted komatiites. The formation age of these komatiites was no later than 2.69 Ga constrained by the Re depletion age. Flat heavy rare earth element patterns and low 187Re/188Os and Th/Yb ratios show the primary komatiitic magma was not contaminated by substantial crustal material. The γOs value of sample 18LH51 is 0.09, which is adjacent with chondrite and other contemporaneous komatiites in the world, indicating that the Nanbaita komatiites were originated from a chondritic mantle. Furthermore, the widely distribution of ~2.7 Ga ultramafic–mafic rocks in the western Shandong Province might be the reflection of a contemporaneous worldwide superplume event.

The behaviour of ferric iron during partial melting of peridotite

In order to better understand the behaviour of Fe3+ during partial melting of a mid-ocean-ridge-basalt (MORB) mantle source, we performed partial melting experiments of a spinel peridotite at 1.5 GPa, between 1320 and 1450 °C, and over a range of oxygen fugacities (fO2) varying from FMQ-3.5 to FMQ+6 (log-units relative to FMQ = Fayalite-Magnetite-Quartz oxygen buffer). Changes in fO2 were achieved using different capsule materials and by varying the oxidation state of the starting material. Fe3+ contents of spinels and glasses were examined using Fe K-edge X-ray Absorption Near Edge Structure (XANES) spectroscopy in full-field mode. Our results show that this method allows Fe3+/ƩFe measurements to be made of spinels in our experimental charges but is not suitable for quantifying Fe3+/ƩFe of glasses, particularly at relatively low fO2 where melt Fe2O3 contents are small. We therefore employed several methods (Fe3+/ƩFe ratios of spinels measured by XANES and calculated by stoichiometry, and Fe-alloy sliding redox sensors) to assess the fO2 of the samples. These were then used to calculate the Fe3+/ƩFe ratios of glasses using published models, which were further constrained by Mössbauer spectroscopy measurements on an additional set of partial melting experiments.For a particular initial redox state, the Fe3+/ΣFe ratio in melts remains approximately constant with partial melting degree. This is explained by a slight decrease of the bulk partition coefficient of Fe2O3 (DFe2O3peridotite-melt) as the degree of partial melting increases. The Fe3+/ΣFe ratios of resulting melts are thus restricted to a relatively narrow range over large degrees of melting, as observed for global MORB glass analyses. This observation is also confirmed by pMELTS calculations employing different bulk Fe3+/ΣFe ratios. We conclude that the Fe2O3 content in MORB can be accounted for through partial melting of a mantle source containing 0.1–0.5 wt% Fe2O3 and a bulk DFe2O3peridotite-melt ranging from 0.3 to 0.1.

Impact-induced melting during accretion of the Earth

Because of the high energies involved, giant impacts that occur during planetary accretion cause large degrees of melting. The depth of melting in the target body after each collision determines the pressure and temperature conditions of metal-silicate equilibration and thus geochemical fractionation that results from core-mantle differentiation. The accretional collisions involved in forming the terrestrial planets of the inner Solar System have been calculated by previous studies using N-body accretion simulations. Here we use the output from such simulations to determine the volumes of melt produced and thus the pressure and temperature conditions of metal-silicate equilibration, after each impact, as Earth-like planets accrete. For these calculations a parametrised melting model is used that takes impact velocity, impact angle and the respective masses of the impacting bodies into account. The evolution of metal-silicate equilibration pressures (as defined by evolving magma ocean depths) during Earth's accretion depends strongly on the lifetime of impact-generated magma oceans compared to the time interval between large impacts. In addition, such results depend on starting parameters in the N-body simulations, such as the number and initial mass of embryos. Thus, there is the potential for combining the results, such as those presented here, with multistage core formation models to better constrain the accretional history of the Earth.

Petrogenesis of the melilititic and nephelinitic rock suites in the Lake Natron–Engaruka monogenetic volcanic field, northern Tanzania

The Lake Natron–Engaruka Monogenetic Volcanic Field (LNE-MVF) in northern Tanzania consists of more than 150 vents of Upper Pleistocene to Holocene age that are scattered over an area of 2500km2. Here we describe the petrological characteristics of these eruptions in detail and link the magma chemistry to eruptive behavior when the magmas reach the surface. Erupted magmas are predominantly of melilititic or nephelinitic compositions (70 and 25%, respectively), together with minor amounts of basanites (5%). The melilititic magmas form by small degrees (1–2%) of partial melting of a metasomatized upper-mantle source (containing 1–4% garnet together with both amphibole and phlogopite). The melilitites ascend very rapidly through the lithosphere prior to eruption minimizing the effect of fractional crystallization and/or crustal contamination. These eruptions also frequently carry relatively large amounts of mantle debris to the surface which is also reflected in their bulk-rock compositions. The nephelinitic rock suite, on the other hand, forms by larger degrees of melting (2–4%) at higher levels of the sub-continental lithosphere containing less garnet (≪1%). The scarcity of mantle debris in the nephelinitic eruption deposits, combined with the more evolved magma chemistry, indicates ponding in crustal reservoirs en-route to the surface. For many of the nephelinitic magmas this ponding resulted in fractional crystallization of predominantly olivine, which is also one of the principal phenocryst phases in these rocks. However, these periods of residence in the crust must have been short as none of the investigated rocks show any clear evidence of being affected by crustal contamination.Within the LNE-MVF a rough correlation between magma chemistry and resulting volcanic landforms is recognized. Large maar volcanoes and tuff cones/rings are predominantly of melilititic composition, whereas the nephelinites typically form scoria cones. This is attributed to the fact that melilititic magmas can hold more CO2 dissolved in the liquid compared to nephelinites, in combination with a rapid ascent from the upper mantle to the surface for the melilitites (<1–2days). We interpret the violent exsolution of CO2 (in response to rapid decompression) to be responsible for the higher explosivity of the melilititic eruptions compared to the nephelinitic magmas within the LNE-MVF.

Asthenosphere–lithosphere interaction triggered by a slab window during ridge subduction: Trace element and Sr–Nd–Hf–Os isotopic evidence from Late Carboniferous tholeiites in the western Junggar area (NW China)

Tholeiites occur in a variety of geological settings, e.g., mid-ocean ridge, back-arc basin, ocean island, island arc and intra-continent, and their geochemical and isotopic characteristics vary according to the corresponding geodynamic environments. Here we investigated the Hatu tholeiitic basalts and basaltic andesites of the western Junggar region, Central Asian Orogenic Belt (CAOB). LA-ICPMS zircon U–Pb analyses indicate that the Hatu tholeiites were generated in the Late Carboniferous (~315Ma). All the studied rock samples are characterized by flat rare earth elements pattern on chondrite-normalized plot, and negligible Nb, Ta and Ti anomalies on mid-ocean-ridge basalt normalized plots. They are also characterized by moderate positive εNd(t) (+5.25 to +5.94), εHf(t) (+13.24 to +14.89), highly radiogenic Os isotope compositions (187Os/188Os315Ma=0.1338–0.3547), and relatively low (87Sr/86Sr)i ratios (0.7044 to 0.7048). Taking into account their geological characteristics, the occurrence of nearby ophiolites and the types of contemporaneous magmatic rocks found in the western Junggar region, we propose that the Hatu basalts were generated by slab window-related processes following a spreading ridge subduction beneath the Keramay intra-oceanic island arc. During this process, deep and enriched asthenospheric mantle rose to the edge of the subducted oceanic lithosphere, its melts infiltrating the subducted oceanic lithosphere and reacting with peridotites. Therefore, the Hatu tholeiites are interpreted as a result of melting of a mixed mantle source consisting of subducted depleted oceanic lithosphere and a deep, enriched upwelling asthenospheric mantle. Incongruent dynamic melting modeling of trace element compositions indicates that the Hatu basalts could have been derived from large degrees of melting (~10%) of such a mixed mantle source. This newly recognized mechanism is a natural consequence of the diversity of contemporaneous potential mantle sources available in slab window settings.

Mesozoic rift magmatism in the North Sea region: 40Ar/39Ar geochronology of Scanian basalts and geochemical constraints

More than 100 volcanic necks composed of basanites and melanephelinites occur in Scania, southern Sweden, at the junction of two major tectonic lineaments, the Phanerozoic Sorgenfrei-Tornquist Zone (STZ) and the Proterozoic Protogine Zone. New 40Ar/39Ar isotope analyses of whole rock fragments of nine selected basalt necks suggest that the Mesozoic alkaline volcanism in the Scanian province commenced earlier than previously reported and comprised three separate volcanic episodes that span a total period of ca. 80 Myr: a first Jurassic (191–178 Ma), a second at the Jurassic/Cretaceous boundary (ca. 145 Ma), and a final middle Cretaceous episode (ca. 110 Ma). The new results allow for precise time correlations between eruption events in the Scanian and those in the North Sea volcanic provinces. The older, early Jurassic event in Scania is largely synchronous with that in the Egersund Basin and the Forties field whereas the event at ca. 145 Ma is correlated with activity in the Central Graben. These volcanic episodes also correlate in age with Kimmerian tectonic activity. Volcanic activity in the middle Cretaceous period has also been dated in the triple junction in the North Sea and offshore in the Netherland Sector. The correlation of basalt volcanism in Scania with the Egersund nephelinites strongly suggest that volcanism was triggered by repeated tectonic activity along the STZ. Geochemical data of alkaline mafic rocks in the Scanian and the North Sea volcanic provinces imply that different provinces have largely unique geochemical signatures in favour of a heterogeneous mantle in the North Sea volcanic region. However, basalts of different generations in one and the same province cannot be readily separated on the basis of geochemistry, suggesting that the same lithospheric mantle was the source of repeated volcanism over time in each province. The data suggest a low degree of melting of a volatile-bearing mantle lherzolite enriched in incompatible elements with the exception of the Forties basalts in the rift centre, produced by larger degree of melting and evolved by fractional crystallization.

Ultra-refractory mantle xenoliths from ocean islands: How do they compare to peridotites retrieved from oceanic sub-arc mantle?

Ultra-refractory harzburgitic mantle is abundantly sampled as xenoliths in ocean island magmas. Its occurrence has potentially important implications for both geochemistry and geodynamics, and we therefore need to understand how these unusually refractory compositions form. In this contribution, we compare ultra-refractory spinel harzburgite xenoliths from ocean islands with peridotites collected along active arcs, based on available major element data on rocks and their minerals. The compositions of arc peridotites vary from ultra-refractory to fertile and estimated equilibration temperatures range from about 800 to > 1300 °C. These variations are not directly related to setting (ocean–ocean or ocean–continent collision; forearc, arc or back-arc). In general, however, fertile compositions seem to be more common in continental arcs and to be accompanied by higher equilibration temperatures. Ultra-refractory peridotites appear to be most common along oceanic forearcs and give generally lower temperatures in the range 950–1050 °C. Ultra-refractory harzburgites sampled in subduction settings are chemically very similar to the ultra-refractory peridotites found as xenoliths in ocean islands. Both rock types compare quite well with the residues from experiments on the formation of primitive arc magmas. The formation of such magmas and their ultra-refractory harzburgite residues requires melting beyond the stability of clinopyroxene. Such high degrees of partial melting are probably achieved in two stages. The first stage is likely to take place as decompression melting along mid-ocean ridges or in back-arc spreading centers, whereas the second stage seems to require fluid-fluxed melting in the mantle wedge. Large degrees of melting may also be achieved in other types of settings if temperatures are sufficiently high. However, the common occurrence of ultra-refractory harzburgites together with melts generated from a strongly depleted precursor in active arcs is an argument in support of a subduction related origin of ultra-refractory harzburgites both in arc settings and in ocean islands. This implies that ultra-refractory arc material is common and may be recirculated by mantle convection. Because of their low density relative to “normal”, more fertile asthenospheric material, these ultra-refractory residues may rise as diapirs and intrude shallower parts of the asthenosphere, where they can accrete to younger oceanic plate.

Geochemistry of basalt from the North Gorda segment of the Gorda Ridge: Evolution toward ultraslow spreading ridge lavas due to decreasing magma supply

High‐density, precisely located, dive and rock‐corer basalt samples from the 65‐km‐long North Gorda ridge segment reveal compositional diversity as great as for the entire Gorda Ridge. Lava compositions along the ridge axis show considerable major and minor element diversity (MgO 9.2–4.4%, K2O 0.04–0.36%) for lavas erupted in close proximity. Although they form a near‐continuum in the higher MgO range, the samples can be separated into two groups; one is typical N‐type mid‐ocean ridge basalt (MORB) (K2O/TiO2 &lt; 0.09), and the other is a more enriched T‐MORB (K2O/TiO2 &gt; 0.09). Incompatible elements also reflect this grouping with (Ce/Yb)N &lt; 1 and Zr/Nb &gt; 20 for N‐MORB and (Ce/Yb)N &gt; 1 and Zr/Nb &lt; 20 for T‐MORB. Samples collected from off‐axis, over a distance of 4 km up the eastern rift valley wall, are all light rare earth element (LREE)‐depleted N‐MORB with a narrower compositional range (MgO of glasses 7.7 ± 0.3%, Zr/Nb = 38–50; (Ce/Yb)N &lt; 1), although isotopic ratios are comparable to those on‐axis. Lavas erupted in the past, before the present‐day deep axial valley formed on this part of the ridge, were more uniform N‐MORB, generated by larger degrees of melting when magma supply was greater. Basalts from the adjoining southern Juan de Fuca Ridge segment, with comparable spreading rate but distinctly different ridge morphology, are also all LREE‐depleted N‐MORB, but the narrow range of evolved compositions of the sheet flows covering the broad, U‐shaped valley suggests shallower, more steady state magma reservoirs underlying this ridge segment. Basalts from Escanaba, the slowest spreading segment of Gorda Ridge, include N‐, T‐, and E‐MORB that were erupted from isolated volcanic centers. The pattern of incompatible element enrichment, especially in LREE, K, Ba, and 87Sr/86Sr, with decreasing spreading rate and magma supply is even more pronounced at the ultraslow spreading Arctic ridges where most lavas are E‐MORB (Zr/Nb &lt; 10, (Ce/Yb)N &gt;1.0–3.0). Arctic E‐MORB compositions lie along a common mixing trend with those from North Gorda. As the magma budget and/or partial melting decreases, a similar enriched component, especially in K, Ba, and LREE, widely present in the oceanic mantle is apparently incorporated to a greater degree. At North Gorda, morphology and chemical characteristics appear to evolve with time toward that of ultraslow spreading ridges.

Os–Pb–Nd isotope and highly siderophile and lithophile trace element systematics of komatiitic rocks from the Volotsk suite, SE Baltic Shield

Highly siderophile element (HSE; Ru, Pd, Re, Os, Ir, and Pt) abundances and Re–Os isotope systematics in combination with lithophile trace element and Sm–Nd and Pb–Pb isotopic data are reported for komatiites and basalts from the Volotsk suite in the SE Baltic (Fennoscandian) Shield. The new Sm–Nd and Pb–Pb whole-rock isochron ages of 2850 ± 84 and 2861 ± 150 Ma, respectively, define the emplacement age of the volcanic sequence. This age is statistically indistinguishable from the emplacement age of the lower mafic-ultramafic sequences of the adjacent Sumozero-Kenozero greenstone belt. The Re–Os internal isochron derived from the analysis of a whole rock B 2–4 cumulate and three chromite separates yields an age of 2878 ± 81 Ma, which is consistent with the ages obtained from the lithophile element isotopic systems. The komatiites of the Volotsk suite were derived from a mantle source characterized by depletions in highly incompatible lithophile trace elements, with the magnitude of the time-integrated LREE depletion (initial ɛNd = +2.7 ± 0.2) typical of this period in the Earth's mantle history. Yet this source had an initial γ 187Os = −0.4 ± 0.4 and, thus, evolved with a time-integrated Re/Os that was identical, within uncertainty, to that of the carbonaceous chondritic reference. In addition, the calculated relative abundances of Os, Ir, Ru, and Pt in the komatiite source are similar to chondrites and may be explained by late (post-core segregation) accretion to the Earth of materials with high absolute and chondritic relative abundances of HSE or by re-entrainment of the core. If the former interpretation is correct, the late accreted materials were well homogenized within the upper mantle by 2.9 Ga. Like many other Archean komatiites, the studied rocks have radiogenic Nd isotopic composition, indicative of their long-term relative depletions in highly incompatible lithophile trace elements, yet the long-term Re/Os in their source remained near-chondritic. Modeling indicates that the komatiite source must have experienced only a small degree of partial melting (2–3%) and extraction of basaltic melts ca. 150 Ma prior to emplacement of the komatiitic sequences. Larger degrees of melting would otherwise have led to loss of a substantial fractionation of Re in the source, which would then have evolved to a subchondritic 187Os/ 188Os by the time of komatiite formation.

U–Th–Pb and Lu–Hf isotopic constraints on the evolution of sub-continental lithospheric mantle, French Massif Central

We have carried out a Pb double-spike and Lu–Hf isotope study of clinopyroxenes from spinel-facies mantle xenoliths entrained in Cenozoic intraplate continental volcanism of the French Massif Central (FMC). U–Th–Pb and Lu–Hf isotope systematics verify the existence of different lithospheric domains beneath the northern and southern FMC. Northern FMC clinopyroxenes have extreme Lu/Hf ratios and ultra-radiogenic Hf ( ε Hf = +39.6 to +2586) that reflect ∼15–25% partial melting in Variscan times (depleted mantle model ages ∼360 Ma). Zr, Hf and Th abundances in these clinopyroxenes are low and unaffected by hydrous/carbonatitic metasomatism that overprinted LILE and light REE abundances and caused decoupling of Lu/Hf–Sm/Nd ratios and Nd–Hf isotopes ( ε Nd = +2.1 to +91.2). Pb isotopes of northern FMC clinopyroxenes are radiogenic ( 206Pb/ 204Pb > 19), and typically more so than the host intraplate volcanic rocks. 238U/ 204Pb ratios range from 17 to 68, and most samples have distinctively low 232Th/ 238U (<1) and 232Th/ 204Pb (3–22). Clinopyroxenes from southern FMC lherzolites are generally marked by overall incompatible trace element enrichment including Zr, Hf and Th abundances, and have Pb isotopes that are similar to or less radiogenic than the host volcanic rocks. Hf isotope ratios are less radiogenic ( ε Hf = +5.4 to +41.5) than northern FMC mantle and have been overprinted by silicate-melt-dominated metasomatism that affected this part of FMC mantle. Major element and Lu concentrations of clinopyroxenes from southern FMC harzburgites are broadly similar to northern FMC clinopyroxenes and suggest they experienced similar degrees of melt extraction as northern FMC mantle. 238U/ 204Pb (53–111) and 232Th/ 204Pb ratios (157–355) of enriched clinopyroxenes from the southern FMC are extreme and significantly higher than the intraplate volcanic rocks. In summary, mantle peridotites from different parts of the FMC record depletion at ∼360 Ma during Variscan subduction, followed by differing styles of enrichment. Northern FMC mantle was overprinted by a fluid/carbonatitic metasomatic agent that carried elements like U, Pb, Sr and light REE. In contrast, much of the southern FMC mantle was metasomatised by a small-degree partial silicate melt resulting in enrichment of all incompatible trace elements. The extreme mantle 238U/ 204Pb (northern and southern FMC), 232Th/ 238U (northern FMC) and 232Th/ 204Pb ratios (southern FMC), coupled with unremarkable present-day Pb isotope ratios, constrain the timing of enrichment. Mantle metasomatism is a young feature related to melting of the upwelling mantle responsible for Cenozoic FMC volcanism, rather than subduction-related metasomatism intimately associated with mantle depletion during the Variscan orogeny. The varying metasomatic styles relate to pre-existing variations in the thickness of the continental lithospheric lid, which controlled the extent to which upwelling mantle could ascend and melt. In the northern FMC, a thicker and more refractory lithospheric lid (⩾80 km) only allowed incipient degrees of melting resulting in fluid/carbonatitic metasomatism of the overlying sub-continental lithospheric mantle. The thinner lithospheric lid of the southern FMC (⩽70 km) allowed larger degrees of melting and resulted in silicate-melt-dominated metasomatism, and also focused the location of the volcanic fields of the FMC above this region.

1.8 Ga magmatism in the Fennoscandian Shield; lateral variations in subcontinental mantle enrichment

The ca. 1.8 Ga post-collisional magmatism in the southern part of the 2.1–1.86 Ga Svecofennian domain of the Fennoscandian Shield have been studied with particular reference to the character and sources of the mafic rocks in a traverse from the Archaean craton margin in east, across the juvenile Svecofennian domain to its western margin. For this purpose three key areas were selected in the eastern, central and western parts of the domain: (i) in the western part of the domain, the Tjällmo-Vättern zone (southern Sweden) of the Transscandinavian Igneous Belt (TIB) consists of extensive areas of dominantly alkali-calcic granitoids associated with calc-alkaline to tholeiitic mafic rocks. Initial ɛ Nd for the mafic rocks vary from around 0 to above +3. (ii) In the central part of the domain (SW Finland), the post-collisional rocks are represented by small intrusions consisting of calc-alkaline high-Ba–Sr granites associated with shoshonitic lamprophyres and their plutonic equivalents. Initial ɛ Nd for the rocks in this series vary between 0 and +1. (iii) In the easternmost part of the Svecofennian domain (Russian Karelia and SE Finland) shoshonitic associations occur, comprising lamprophyres and their plutonic equivalents (apatite and magnetite-rich monzodiorites), which are related to syenites and high-Ba–Sr granites by fractional crystallization. All the rock types in this shoshonitic association have strongly elevated contents of P 2O 5, LREE and LILE. Initial ɛ Nd for all rocks in Karelia fall between 0 and −1. Geochemical and isotopic results indicate that the post-collisional rocks in the central and eastern part of the domain stem from lithospheric mantle sources that were enriched during the preceding Svecofennian orogeny. The HFSE depletion, combined with the strong Sr, LILE and LREE enrichment, recall signatures of increasingly carbonate-dominated metasomatism of the mantle eastwards towards the craton margin. In contrast, the mainly LILE enriched mafic rocks from TIB in the west signal sources subjected to H 2O-dominated metasomatism, that could in part be coeval with the magmatism. In all areas the rocks carry a subduction type chemistry with continental arc affinity, however, with strongly increasing enrichment levels eastwards. Rocks in the west are derived by relatively larger degrees of melting at shallower levels, from previously depleted spinel–phlogopite–amphibole lherzolites/harzburgites, while going eastwards successively smaller melt fractions were tapping deeper, more enriched mantle sections in the garnet stability field. The enrichment agents are interpreted to be LILE-bearing H 2O-dominated fluids from dewatering slabs in the west, changing to an increasing role for CO 2-dominated fluids/melts derived mainly from subducted Svecofennian metasediments eastwards. A convergent continental margin setting with transpressional shearing was active during TIB formation in the west. Whether this shearing was instrumental in the formation of the 1.8 Ga magmatism further continentwards, or if the magmatism in the central and eastern areas was the result of extensional collapse or plume activity is presently not known.

Petrogenesis of the Paleoproterozoic rapakivi A-type granites of the Archean Carajás metallogenic province, Brazil

Three Paleoproterozoic A-type rapakivi granite suites (Jamon, Serra dos Carajás, and Velho Guilherme) are found in the Carajás metallogenic province, eastern Amazonian craton. Liquidus temperatures in the 900–870 °C range characterize the Jamon suite, those for Serra dos Carajás and Velho Guilherme are somewhat lower. Pressures of emplacement decrease from Jamon (3.2±0.7 kbar) through Serra dos Carajás (2.0±1.0 kbar) to Velho Guilherme (1.0±0.5 kbar). Oxidizing conditions (NNO+0.5) characterized the crystallization of the Jamon magma, the Velho Guilherme magmas were reducing (marginally below FMQ), and the Serra dos Carajás magmas were intermediate between the two in this respect. The three granite suites have Archean T DM model ages and strongly negative ɛ Nd values (−12 to −8 at 1880 Ma), and they were derived from Archean crust. The Jamon granite suite may have been derived from a quartz dioritic source, and the Velho Guilherme granites from K-feldspar-bearing granitoid rocks with some sedimentary input. The Serra dos Carajás granites either had a somewhat more mafic source than Velho Guilherme or were derived by a larger degree of melting. Underplating of mafic magma was probably the heat source for the melting. The petrological and geochemical characteristics of the Carajás granite suites imply considerable compositional variation in the Archean of the eastern Amazonian craton. The oxidized Jamon suite granites are similar to the Mesoproterozoic magnetite-series granites of Laurentia, and they were derived from Archean igneous sources that were more oxidized than the sources of the Fennoscandian rapakivi granites. The Serra dos Carajás and Velho Guilherme granites approach the classic reduced rapakivi series of Fennoscandia and Laurentia. No counterparts of the Mesoproterozoic two-mica granites of Laurentia have been found, however. Following the model of Hoffman [Hoffman, P., 1989. Speculations on Laurentia's first gigayear (2.0 to 1.0 Ga). Geology 17, 135–138], the origin of the ∼1.88 Ga Carajás granites is related to a mantle superswell beneath the Trans-Amazonian supercontinent. This caused breakup of the continent and was associated with magmatic underplating and resultant crustal melting and generation of A-type granite magmas. The Paleoproterozoic continent that included the Archean and Trans-Amazonian domains of the Amazonian craton was assembled at ∼2.0 Ga; its disruption was initiated at ∼1.88 Ga, at least 200 Ma earlier than in Laurentia and Fennoscandia. The Carajás granites were related to the breakup of the supercontinent, not to subduction processes.

Re–Os isotope systematics and platinum group element fractionation during mantle melt extraction: a study of massif and xenolith peridotite suites

Re–Os isotope and platinum group elements (PGE) systematics are presented for peridotite xenoliths from N. Lesotho (on-craton), S. Namibia (circum-cratonic), the Vitim volcanic field (Baikhal Rift), plus massif peridotites from Beni Bousera, N. Morocco. Mg–Fe variations indicate that these samples have experienced between 5% (Vitim–Beni Bousera) and 50% (Lesotho) melt extraction, providing the opportunity to examine PGE fractionation over a large melting interval. The Namibian xenoliths and Beni Bousera massif peridotites show no variation of iridium group (Ir, Ru, Os; I-PGE) abundances or inter-element fractionations relative to melt depletion indices such as Mg number or Al 2O 3. Lesotho peridotites show large variations in I-PGE abundances (Os range 0.2–13 ppb) at relatively constant Al 2O 3 that are not easily rationalised by melt-extraction models. Despite these abundance variations, there is no significant inter-element fractionation of I-PGE, e.g., (Os/Ir) n , showing that these elements are not fractionated by even very large degrees of melting (up to 50% melt extraction). Lesotho peridotites are amongst the most P-PGE (Pt, Pd)-depleted mantle rocks, with highly fractionated chondrite-normalised PGE. PGE systematics for all these peridotite suites allow a relative order of PGE compatibility to be firmly established for mantle melting: D solid/melt Os ∼D solid/melt Ir ∼D solid/melt Ru >D solid/melt Pt >D solid/melt Pd . Vitim peridotite xenoliths have very low Os contents and low Os/Ir (<70% chondritic) compared to the kimberlite-borne xenoliths and massif peridotites. The Vitim low Os/Ir is comparable with other suites of alkali-basalt-borne peridotite xenoliths and may result from syn- or post-eruption sulphide breakdown and alteration. Chondritic (Ru/Ir) n and (Os/Ir) n ratios in Lesotho and other cratonic peridotites show no evidence of anomalous PGE fractionations in the Archean subcontinental mantle and support the Late Veneer hypothesis. In most of the peridotite suites, (Pd/Ir) n values show a strong correlation with Os isotopic composition that is likely the result of melt-residue interaction. The positive variation of both (Ru/Ir) n and (Pd/Ir) n with bulk rock Al 2O 3 and Os isotopic composition for Beni Bousera and global massif peridotites indicates that these PGE ratios were modified by interaction with melts. Hence, we find no support for the intra-element PGE fractionation in the continental lithospheric mantle (CLM) representing primordial mantle heterogeneity. Highly unradiogenic Os isotope compositions appear characteristic of lithospheric peridotites with the lowest (Pd/Ir) n . In these samples, bulk-rock PGE patterns suggest that Os isotope systematics should be dominated by primary, residual, P-PGE-depleted sulphides, and hence, their bulk rock Re-depletion ages should be expected to approximate the melting age of the rock.

On the thermo‐kinetic consequences of slab melting

REE abundances in melts produced by partial melting of a hornblende‐bearing eclogite were examined using a dynamic disequilibrium‐melting model and following the P‐T path of a young subducting slab. Model calculations show that melts produced by small degrees of slab melting at 700∼900°C are more likely affected by sluggish kinetics, and have lower La/Yb ratios, higher Li2O and Na2O abundances than melts produced by large degrees of melting at higher temperatures. The extent of disequilibrium depends on deformation and melting histories of the slab, and effective grain sizes and transport properties of the minerals. The LREE abundances in the large‐degree melts are broadly similar to those in adakites. If kinetics is important in slab melting, the degrees of melting required to generate adakites may be higher than values predicted by equilibrium partial melting models and REE abundances in the melts.

Impact processing of chondritic planetesimals: Siderophile and volatile element fractionation in the Chico L chondrite

Abstract— A large impact event 500 Ma ago shocked and melted portions of the L‐chondrite parent body. Chico is an impact melt breccia produced by this event. Sawn surfaces of this 105 kg meteorite reveal a dike of fine‐grained, clast‐poor impact melt cutting shocked host chondrite. Coarse (1–2 cm diameter) globules of FeNi metal + sulfide are concentrated along the axis of the dike from metal‐poor regions toward the margins. Refractory lithophile element abundance patterns in the melt rock are parallel to average L chondrites, demonstrating near‐total fusion of the L‐chondrite target by the impact and negligible crystal‐liquid fractionation during emplacement and cooling of the dike.Significant geochemical effects of the impact melting event include fractionation of siderophile and chalcophile elements with increasing metal‐silicate heterogeneity, and mobilization of moderately to highly volatile elements. Siderophile and chalcophile elements ratios such as Ni/Co, Cu/Ga, and Ir/Au vary systematically with decreasing metal content of the melt. Surprisingly small (˜102) effective metal/silicate‐melt distribution coefficients for highly siderophile elements probably reflect inefficient segregation of metal despite the large degrees of melting. Moderately volatile lithophile elements such K and Rb were mobilized and heterogeneously distributed in the L‐chondrite impact breccias whereas highly volatile elements such as Cs and Pb were profoundly depleted in the region of the parent body sampled by Chico. Volatile element variations in Chico and other L chondrites are more consistent with a mechanism related to impact heating rather than condensation from a solar nebula. Impact processing can significantly alter the primary distributions of siderophile and volatile elements in chondritic planetesimals.

Olivine-hosted melt inclusions in Hawaiian picrites: equilibration, melting, and plume source characteristics

Olivine-hosted melt inclusions in tholeiitic picrites from five Hawaiian volcanoes (Koolau, Mauna Loa, Kilauea, Loihi, and Hualalai) have major and trace element compositions that illustrate the magmatic characteristics of ocean island volcanoes and the nature of mantle plumes. The geochemistry of these melt inclusions reflects the well known geochemical features that distinguish Hawaiian shield volcanoes, but with considerably greater diversity than whole rock compositions, providing a higher resolution of the magmatic processes contributing to Hawaiian plume magmatism. Naturally quenched inclusions from Kilauea, Mauna Loa, and Hualalai have been modified by crystallization of olivine on the walls of the inclusion and diffusive interaction with the host crystal. In contrast, melt inclusions in two Loihi picrites have not been affected by re-equilibration with their host olivines, reflecting a relatively brief interval between crystallization of the olivines and eruption of these lavas. Corrected major element compositions of experimentally melted inclusions from two Koolau picritic tholeiites are similar to those of erupted lavas from this volcano and document the presence of Koolau melts with at least 14% MgO. Trace element characteristics of melt inclusions from Mauna Loa, Kilauea, and Loihi can be produced by melting of a moderately depleted, garnet lherzolite source. The extent of melting generally increases from Loihi<Kilauea<Mauna Loa, although rare inclusions from Mauna Loa also indicate contributions of relatively small degree (2–4%) melts to these lavas. Extent of melting and isotopically defined source components appear to be linked in the melting regime, with the Mauna Loa component being sampled preferentially at larger degrees of melting. Melt inclusions with trace element characteristics indicating a recycled basaltic eclogite source were not found at Mauna Loa or any of the other volcanoes. Compositions of the Koolau inclusions do require a unique source component, however, possibly reflecting contributions from ancient lithosphere, either within the plume or entrained in the upper mantle.

Middle/Late Cambrian intracontinental rifting in the central West Sudetes, NE Bohemian Massif (Czech Republic): geochemistry and petrogenesis of the bimodal metavolcanic rocks

AbstractThe Early Palaeozoic East Krkonoše Complex (EKC) situated in the central West Sudetes, NE Bohemian Massif, is a volcano‐sedimentary suite containing abundant mafic and felsic volcanics metamorphosed to greenschist facies.The trace element distribution patterns and Nd isotope signatures (ENd500 = + 3.1 to + 6.6) of the metabasites (metabasalts) indicate that they may be related to a rising mantle diapir associated with intracontinental rifting. At the early stage, limited melting of an upwelling asthenosphere produced alkali basalts and enriched tholeiites which compositionally resemble oceanic island basalts. A later stage of rifting with larger degrees of melting at shallower depths generated tholeiitic basalts with E‐MORB to N‐MORB characteristics.The values of (87Sr/86Sr)i = 0.706 and ENd500 = − 5 ±1 of the porphyroids (metarhyolites) as well as the lack of rocks with intermediate compositions suggest that the felsic rocks were formed by a partial melting event of continental crust triggered by mantle melts.The geochemistry of the EKC bimodal metavolcanics and their association with abundant terrigenous metasediments suggest that the felsic–mafic volcanic suite was generated during intracontinental rifting. This process, widespread in Western and Central Europe during the Early Palaeozoic, is evidence of large‐scale fragmentation of the northern margin of the Gondwana supercontinent. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Pb, Sr, and Nd isotopic characteristics of Tertiary Red Sea rift volcanics from the central Saudi Arabian Coastal Plain

Alkalic to subalkalic bimodal Tertiary igneous rocks from the Red Sea coastal plain at Al Lith, Saudi Arabia, represent early rift volcanism in the central part of the Red Sea rift. Initial 143Nd/144Nd and 87Sr/86Sr plot within or near the enriched mid‐ocean ridge basalt (MORB) field and show a variation in ε‐Nd values from +4.1 to +7.7. The data are intermediate between more depleted basalts from the Red Sea spreading axis and less depleted alkalic basalts from the southeastern Arabian shield and carbonatites from the East African rift. Pb isotopic compositions in alkalic and tholeiitic basalts show a variation in 206Pb/204Pb from 18.0 to 18.9 and plot in and slightly above the MORB envelope. Relatively low 206Pb/204Pb and variable 207Pb/204Pb and 208Pb/204Pb in tholeiitic volcanics are compatible with a component from either old lithospheric mantle or lower crust containing ancient sediment. The isotopic compositions suggest mixing of asthenospheric and lithospheric sources but show no evidence for changing proportions of these components during early rift volcanism at Al Lith. Volcanism in the central part of the Red Sea is tholeiitic, consistent with a larger degree of melting or melting at increasingly shallower depths as rifting progresses.