Browsing by Subject "CONTINENTAL FLOOD BASALTS"

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  • Luttinen, Arto V. (2018)
    In the Karoo large igneous province, the geochemical assessment of mantle source variability and structure is hampered by probable crustal contamination overprinting of compositionally diverse flood basalts. Mantle source characteristics have been defined only for exceptional, primitive rock types. Here I use a compiled dataset for over 800 samples to demonstrate that the abundance of Nb relative to Zr, Ti, and Y provides a useful geochemical tracer of mantle sources for variably contaminated rock types of the Karoo province. Variations in the relative abundance of Nb reveal emplacement of distinctive, Nb-undepleted and Nb-depleted magmas in the North Karoo and South Karoo sub-provinces, respectively, and clarify correlation between flood basalts and previously proposed mantle source components. Judging from plate tectonic reconstructions and the compositions of plausible mantle source components, the geochemical bilateral asymmetry in Karoo may reflect tapping of contrasting plume and upper mantle reservoirs in the two sub-provinces.
  • Heinonen, Jussi S.; Jennings, Eleanor S.; Riley, Teal R. (2015)
    Calculating reliable temperatures of Mg-rich magmas is problematic because melt composition and KD(Fe-Mg)ol-liq, the key parameters of many traditional thermometers, are difficult to constrain precisely. The recently developed Al-in-olivine thermometer [Coogan, L.A., Saunders, A.D., Wilson, R.N., 2014. Aluminum-in-olivine thermometry of primitive basalts: Evidence of an anomalously hot mantle source for large igneous provinces. Chemical Geology 368, 1–10] circumvents these problems by relying on the temperature-dependent exchange of Al between olivine and spinel crystallising in equilibrium with each other. This thermometer is used to re-evaluate the crystallisation temperatures of the most Mg-rich magma type identified from the Karoo large igneous province (LIP), known as the Vestfjella depleted ferropicrite suite. Previous temperature estimates for the suite were based on olivine-melt equilibria and indicated anomalously high crystallisation temperatures in excess of 1600 °C. We also present crystallisation temperatures for another Antarctic Karoo magma type, Group 3 dykes from Ahlmannryggen, which are derived from a pyroxene-rich mantle source. Our high-precision analysis of Al in olivine-spinel pairs indicate crystallisation temperatures from 1391±42 °C to 1481±35 °C for the Vestfjella depleted ferropicrite suite (Fo88–92) and from 1253±64 °C to 1303±40 °C for the Group 3 dykes (Fo79–82). Although the maximum temperature estimates for the former are over 100 °C lower than the previously presented estimates, they are still ~200 °C higher than those calculated for mid-ocean ridge basalts using the same method. Although exact mantle potential temperatures are difficult to estimate, the presented results support elevated sub-Gondwanan upper mantle temperatures (generated by a mantle plume or internal mantle heating) during the generation of the Karoo LIP.
  • Heinonen, Jussi S.; Luttinen, Arto V.; Spera, Frank J.; Bohrson, Wendy A. (2019)
    Karoo continental flood basalt (CFB) province is known for its highly variable trace element and isotopic composition, often attributed to the involvement of continental lithospheric sources. Here, we report oxygen isotopic compositions measured with secondary ion mass spectrometry for hand-picked olivine phenocrysts from similar to 190 to 180 Ma CFBs and intrusive rocks from Vestfjella, western Dronning Maud Land, that form an Antarctic extension of the Karoo province. The Vestfjella lavas exhibit heterogeneous trace element and radiogenic isotope compositions (e.g., epsilon(Nd) from -16 to +2 at 180 Ma) and the involvement of continental lithospheric mantle and/or crust in their petrogenesis has previously been suggested. Importantly, our sample set also includes rare primitive dikes that have been derived from depleted asthenospheric mantle sources (epsilon(Nd) up to + 8 at 180 Ma). The majority of the oxygen isotopic compositions of the olivines from these dike rocks (delta O-18 = 4.4-5.2%; Fo = 78-92 mol%) are also compatible with such sources. The olivine phenocrysts in the lavas, however, are characterized by notably higher delta O-18 (6.2-7.5%; Fo = 70-88 mol%); and one of the dike samples gives intermediate compositions (5.2-6.1%, Fo = 83-87 mol%) between the other dikes and the CFBs. The oxygen isotopic compositions do not correlate with radiogenic isotope compositions susceptible to crustal assimilation (Sr, Nd, and Pb) or with geochemical indicators of pyroxene-rich mantle sources. Instead, delta O-18 correlates positively with enrichments in large-ion lithophile elements (especially K) and Os-187. We suggest that the oxygen isotopic compositions of the Vestfjella CFB olivines primarily record large-scale subduction-related metasomatism of the sub-Gondwanan mantle (base of the lithosphere or deeper) prior to Karoo magmatism. The overall influence of such sources to Karoo magmatism is not known, but, in addition to continental lithosphere, they may be responsible for some of the geochemical heterogeneity observed in the CFBs.
  • Heinonen, Jussi S.; Luttinen, Arto V.; Bohrson, Wendy A. (2016)
    Continental flood basalts (CFBs) represent large-scale melting events in the Earth’s upper mantle and show considerable geochemical heterogeneity that is typically linked to substantial contribution from underlying continental lithosphere. Large-scale partial melting of the cold subcontinental lithospheric mantle and the large amounts of crustal contamination suggested by traditional binary mixing or assimilation-fractional crystallization models are difficult to reconcile with the thermal and compositional characteristics of continental lithosphere,however. The well-exposed CFBs of Vestfjella, western Dronning Maud Land,Antarctica, belong to the Jurassic Karoo large igneous province and provide a prime locality to quantify mass contributions of lithospheric and sublithospheric sources for two reasons: 1) recently discovered CFB dikes show isotopic characteristics akin to mid-ocean ridge basalts, and thus help to constrain asthenospheric parental melt compositions, and 2) the well-exposed basaltic lavas have been divided into four different geochemical magma types that exhibit considerable trace element and radiogenic isotope heterogeneity (e.g., initial εNd from-16 to +2 at 180 Ma). We simulate the geochemical evolution of Vestfjella CFBs using 1) energy-constrained assimilation-fractional crystallization equations that account for heating and partial melting of crustal wallrock and 2)assimilation-fractional crystallization equations for lithospheric mantle contamination by using highly alkaline continental volcanic rocks (i.e. partial melts of mantle lithosphere) as contaminants. Calculations indicate that the different magma types can be produced by just minor (1–15 wt. %) contamination of asthenospheric parental magmas by melts from variable lithospheric reservoirs. Our models imply that the role of continental lithosphere as a CFB source component or contaminant may have been overestimated in many cases. Thus, CFBs may represent major juvenile crustal growth events rather than just recycling of old lithospheric materials.
  • Heinonen, Jussi S.; Luttinen, Arto V.; Whitehouse, M.J. (2018)
    Karoo continental flood basalt (CFB) province is known for its highly variable trace element and isotopic composition, often attributed to the involvement of continental lithospheric sources. Here, we report oxygen isotopic compositions measured with secondary ion mass spectrometry for hand-picked olivine phenocrysts from similar to 190 to 180 Ma CFBs and intrusive rocks from Vestfjella, western Dronning Maud Land, that form an Antarctic extension of the Karoo province. The Vestfjella lavas exhibit heterogeneous trace element and radiogenic isotope compositions (e.g., epsilon(Nd) from -16 to +2 at 180 Ma) and the involvement of continental lithospheric mantle and/or crust in their petrogenesis has previously been suggested. Importantly, our sample set also includes rare primitive dikes that have been derived from depleted asthenospheric mantle sources (epsilon(Nd) up to + 8 at 180 Ma). The majority of the oxygen isotopic compositions of the olivines from these dike rocks (delta O-18 = 4.4-5.2%; Fo = 78-92 mol%) are also compatible with such sources. The olivine phenocrysts in the lavas, however, are characterized by notably higher delta O-18 (6.2-7.5%; Fo = 70-88 mol%); and one of the dike samples gives intermediate compositions (5.2-6.1%, Fo = 83-87 mol%) between the other dikes and the CFBs. The oxygen isotopic compositions do not correlate with radiogenic isotope compositions susceptible to crustal assimilation (Sr, Nd, and Pb) or with geochemical indicators of pyroxene-rich mantle sources. Instead, delta O-18 correlates positively with enrichments in large-ion lithophile elements (especially K) and Os-187. We suggest that the oxygen isotopic compositions of the Vestfjella CFB olivines primarily record large-scale subduction-related metasomatism of the sub-Gondwanan mantle (base of the lithosphere or deeper) prior to Karoo magmatism. The overall influence of such sources to Karoo magmatism is not known, but, in addition to continental lithosphere, they may be responsible for some of the geochemical heterogeneity observed in the CFBs.
  • Heinonen, Jussi S. (Unigrafia, 2011)
    Department of Geosciences and Geography
    Mantereiset laakiobasalttiprovinssit ovat suurimpia tunnettuja ilmanalaisia vulkaanisia muodostumia (alkuperäinen tilavuus jopa 2 × 106 km3). Laakiobasaltteja esiintyy kaikilla mantereilla ja niitä tiedetään muodostuneen miltei läpi maapallon historian. Laakiobasalttien purkautumisella on ollut huomattava vaikutus maapallon ilmastoon ja elämän kehitykseen, mutta niiden synnystä tiedetään edelleen varsin vähän. Tämä johtuu osaltaan siitä, että suurin osa laakiobasalttien kantasulista on reagoinut voimakkaasti mantereisen litosfäärin kanssa ja niiden alkuperäinen, mahdollisesti litosfäärin alaisesta vaipasta peritty geokemiallinen sormenjälki on vaikeasti tunnistettavissa ja tulkittavissa. Ferropikriitit ovat poikkeuksellisen rautarikkaita (FeOtot >13–14 p. %) ja primitiivisiä (MgO ≈ 12–18 p. %) laavakiviä, joita on kuvattu muutamista laakiobasalttiprovinsseista. Toisin kuin tavalliset laakiobasaltit, ferropikriitit eivät yleensä ole merkittävästi reagoineet litosfäärin kanssa ja siksi ne tarjoavat arvokasta tietoa suoraan laakiobasalttimuodostumien alkulähteiltä. Ferropikriitit on usein yhdistetty anomalisen korkeisiin vaipan lämpötiloihin ja vaippapluumeihin, mutta näiden erikoisten kivien syntyyn liittyy useita kysymyksiä: Mistä niiden korkea rautapitoisuus on peräisin? Miten ne kytkeytyvät laakiobasalttien syntyyn? Tässä väitöskirjatyössä käsitellään Vestfjellan (Kuningatar Maudin maa, Etelämanner) ferropikriittien (FeOtot = 13–17 p. %; MgO = 13–19 p. %) sekä niihin liittyvien muiden primitiviisten magmakivien – pikriittien, meimechiittien, pikrobasalttien ja basalttien – geokemiaa ja petrologiaa. Nämä suureksi osaksi aikaisemmin tuntemattomat kivet leikkaavat juonina Karoon suuren magmaprovinssin laakiobasaltteja, jotka purkautuivat jurakaudella noin 180 miljoonaa vuotta sitten Gondwana-supermantereen repeämisprosessin alkuvaiheiden aikana. Valikoiduista näytteistä (yhteensä 31 vähintään yhdestätoista juonesta) analysoitiin mineraalien koostumuksia sekä pääalkuaine-, hivenalkuaine-, ja Sr-, Nd-, Pb- ja Os-isotooppikoostumuksia. Analysoidut näytteet voidaan jakaa hivenalkuaine- ja isotooppikoostumuksensa perusteella kahteen magmatyyppiin: (1) Köyhtynyt magmatyyppi (24 näytettä vähintään yhdeksästä juonesta) on köyhtynyt sopeutumattomimmista alkuaineista ja muistuttaa isotooppikoostumukseltaan valtamerten keskiselänteiden basaltteja; (2) Rikastunut magmatyyppi (7 näytettä vähintään kahdesta juonesta) on suhteellisen rikastunut sopeutumattomimmista alkuaineista ja muistuttaa hivenalkuaine- ja isotooppikoostumukseltaan merellisten saarten basaltteja. Kumpikaan magmatyyppi ei ole merkittävästi saastunut kuorellisella aineksella. Köyhtynyt magmatyyppi on peräisin hyvin MgO-rikkaista (jopa 25 p. %) kantasulista, jotka muodostuivat Karoon päävaiheen aikana korkeissa vaipan lämpötiloissa (>1600 °C) ja paineissa (n. 5–6 GPa) pääosin vesipitoisesta, köyhtyneestä ylävaipan peridotiitista. Rikastuneen magmatyypin lähteenä ovat vaipan heterogeeniset pyrokseenipitoiset komponentit, jotka muodostuivat joko subduktoituneen merellisen kuoren reagoidessa vaipan peridotiitin kanssa tai sulametasomatoosin seurauksena. Geokemiallisen mallinnuksen perusteella monet litosfäärin aineksilla saastuneista Karoon laakiobasalteista ovat saaneet alkunsa samasta vaippalähteestä kuin köyhtynyt magmatyyppi. Tästä lähteestä ovat todennäköisesti peräisin myös isotooppikoostumukseltaan hyvin samankaltaiset Intian Valtameren keskiselänteen basaltit. Nämä havainnot ja köyhtyneelle magmatyypille arvioidut korkeat lähdelämpötilat viittaavat siihen, että Karoon laakiobasaltit saivat suurimmaksi osaksi alkunsa Gondwana-supermantereen alaisen vaipan sisäisen lämpenemisen, ei niinkään ylävaippaan tunkeutuneen syvän vaippapluumin, seurauksena. Tutkimukseni tukee näkemystä siitä, että ferropikriittiset sulat voivat syntyä monin eri tavoin: poikkeuksellisen korkea rautapitoisuus saavutetaan todennäköisimmin, jos lähdemateriaali vaipassa sulaa (1) alhaisella asteella (2) ja/tai korkeassa paineessa ja/tai (3) lähdemateriaali sisältää rikastuneita komponentteja (esim. pyrokseniittia tai metasomatoitunutta peridotiittia). On kuitenkin huomionarvoista, että ferropikriittinen kokokivikoostumus voi olla myös seurausta akkumulaatiosta, sekundaarisesta muuttumisesta tai fraktioivasta kiteytymisestä, ja erityistä huomiota on siksi kiinnitettävä kantamagman koostumuksen määrittämiseen.
  • Heinonen, Jussi S.; Fusswinkel, Tobias (2017)
    Nickel contents and Mn/Fe in olivine phenocrysts have been suggested to reflect the mineral composition of the mantle source of the host magma. This hypothesis is tested here against a well-characterized suite of meimechitic (or Ti-rich komatiitic) dikes from the Antarctic extension of the Jurassic ~180 Ma Karoo large igneous province. The presented trace element data on Fo82–92 olivines show relatively high Ni (2430–3570 ppm) and low 100*Mn/Fe (1.32–1.5; Mn = 890–1570 ppm), compatible with pyroxenite-rich sources (Xpx = 37–75%). Many other mantle source indicators (parental melt MgO and whole-rock Zn/Fe, MgO/CaO, FC3MS, Zr/Y vs. Nb/Y, and radiogenic isotope compositions) suggest dominantly or solely peridotitic mantle sources, however. Therefore, the measured high Ni and low Mn/Fe are likely to reflect high temperatures and pressures of melting and possibly high water contents in such peridotite sources. We recommend considerable caution when using Ni and Mn contents of olivine as source indicators, as they may only serve for qualitative comparison of primitive volcanic rocks that originated under fairly similar mantle conditions.
  • Heinonen, Jussi S.; Kurz, Mark D. (2015)
    The massive outpourings of Karoo and Ferrar continental flood basalts (CFBs) ~180 Ma ago mark the initial Jurassic rifting stages of the Gondwana supercontinent. The origin and sources of these eruptions have been debated for decades, largely due to difficulties in defining their parental melt and mantle source characteristics. Recent findings of Fe- and Mg-rich dikes (depleted ferropicrite suite) from Vestfjella, western Dronning Maud Land, Antarctica, have shed light on the composition of the deep sub-Gondwanan mantle: these magmas have been connected to upper mantle sources presently sampled by the Southwest Indian Ocean mid-ocean ridge basalts (SWIR MORBs) or to high 3He/4He plume-entrained non-chondritic primitive mantle sources formed early in Earth’s history. In an attempt to determine their He isotopic composition and relative contributions from magmatic, cosmogenic, and radiogenic He sources, we performed in-vacuo stepwise crushing and melting analyses of olivine mineral separates, some of which were abraded to remove the outer layer. The best estimate for the mantle isotopic composition is given by a sample with the highest amount of He released (> 50%) during the first crushing step of an abraded coarse fraction. It has a 3He/4He of 7.03 ± 0.23 (2σ) times the atmospheric ratio (Ra), which is indistinguishable from those measured from SWIR MORBs (6.3‒7.3 Ra; source 3He/4He ~6.4–7.6 Ra at 180 Ma) and notably lower than in the most primitive lavas from the North Atlantic Igneous Province (up to 50 Ra), considered to represent the epitome magmas from non-chondritic primitive mantle sources. Previously published trace element and isotopic (Sr, Nd, and Pb) compositions do not suggest a direct genetic link to any modern hotspot of Indian or southern Atlantic Oceans. Although influence of a mantle plume cannot be ruled out, the high magma temperatures and SWIR MORB-like geochemistry of the suite are best explained by supercontinent insulation of a precursory Indian Ocean upper mantle source. Such a model is also supported by the majority of the recent studies on the structure, geochronology, and petrology of the Karoo CFBs.
  • Heinonen, Jussi S.; Carlson, Rick W.; Riley, Teal R.; Luttinen, Arto V.; Horan, Mary F. (2014)
    The great majority of continental flood basalts (CFBs) have a marked lithospheric geochemical signature, suggesting derivation from the continental lithosphere, or contamination by it. Here we present new Pb and Os isotopic data and review previously published major element, trace element, mineral chemical, and Sr and Nd isotopic data for geochemically unusual mafic and ultramafic dikes located in the Antarctic segment (Ahlmannryggen, western Dronning Maud Land) of the Karoo CFB province. Some of the dikes show evidence of minor contamination with continental crust, but the least contaminated dikes exhibit depleted mantle –like initial εNd (+9) and 187Os/188Os (0.1244–0.1251) at 180 Ma. In contrast, their initial Sr and Pb isotopic compositions (87Sr/86Sr = 0.7035–0.7062, 206Pb/204Pb = 18.2–18.4, 207Pb/204Pb = 15.49–15.52, 208Pb/204Pb = 37.7–37.9 at 180 Ma) are more enriched than expected for depleted mantle, and the major element and mineral chemical evidence indicate contribution from (recycled) pyroxenite sources. Our Sr, Nd, Pb, and Os isotopic and trace element modeling indicate mixed peridotite-pyroxenite sources that contain ~10–30 % of seawater-altered and subduction-modified MORB with a recycling age of less than 1.0 Ga entrained in a depleted Os-rich peridotite matrix. Such a source would explain the unusual combination of elevated initial 87Sr/86Sr and Pb isotopic ratios and relative depletion in LILE, U, Th, Pb and LREE, high initial εNd, and low initial 187Os/188Os. Although the sources of the dikes probably did not play a major part in the generation of the Karoo CFBs in general, different kind of recycled source components (e.g., sediment-influenced) would be more difficult to distinguish from lithospheric CFB geochemical signatures. In addition to underlying continental lithosphere, the involvement of recycled sources in causing the apparent lithospheric geochemical affinity of CFBs should thus be carefully assessed in every case.