Syvä avoin varasto ja matala suljettu kuljetussysteemi laakiobasalttisekvenssille paljastui Magmakammiosimulaattorin avulla

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.Peer reviewe

Rikastuneita mantereisia laakiobasaltteja köyhtyneistä vaipan sulista: litosfäärikontaminaation mallinnus Antarktiksen Karoo-laavoille

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.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.Peer reviewe

Criterion for traffic phases in single vehicle data and empirical test of a microscopic three-phase traffic theory

A microscopic criterion for distinguishing synchronized flow and wide moving jam phases in single vehicle data measured at a single freeway location is presented. Empirical local congested traffic states in single vehicle data measured on different days are classified into synchronized flow states and states consisting of synchronized flow and wide moving jam(s). Then empirical microscopic characteristics for these different local congested traffic states are studied. Using these characteristics and empirical spatiotemporal macroscopic traffic phenomena, an empirical test of a microscopic three-phase traffic flow theory is performed. Simulations show that the microscopic criterion and macroscopic spatiotemporal objective criteria lead to the same identification of the synchronized flow and wide moving jam phases in congested traffic. It is found that microscopic three-phase traffic models can explain both microscopic and macroscopic empirical congested pattern features. It is obtained that microscopic distributions for vehicle speed difference as well as fundamental diagrams and speed correlation functions can depend on the spatial co-ordinate considerably. It turns out that microscopic optimal velocity (OV) functions and time headway distributions are not necessarily qualitatively different, even if local congested traffic states are qualitatively different. The reason for this is that important spatiotemporal features of congested traffic patterns are it lost in these as well as in many other macroscopic and microscopic traffic characteristics, which are widely used as the empirical basis for a test of traffic flow models, specifically, cellular automata traffic flow models.Comment: 27 pages, 16 figure

Primitiivisten magmojen perättäiset vuorovaikutukset felsisen ja mafisen kuoren kanssa tallennettuna Karoon suuren magmaprovinssin gabrojuoniin Etelämantereella

Two subvertical gabbroic dikes with widths of similar to 350 m (East-Muren) and >= 500 m (West-Muren) crosscut continental flood basalts in the Antarctic extension of the similar to 180 Ma Karoo large igneous province (LIP) in Vestfjella, western Dronning Maud Land. The dikes exhibit unusual geochemical profiles; most significantly, initial (at 180 Ma) epsilon(Nd) values increase from the dike interiors towards the hornfelsed wallrock basalts (from - 15.3 to - 7.8 in East-Muren and more gradually from - 9.0 to - 5.5 in West-Muren). In this study, we utilize models of partial melting and energy-constrained assimilation-fractional crystallization in deciphering the magmatic evolution of the dikes and their contact aureoles. The modeling indicates that both gabbroic dikes acquired the distinctly negative epsilon(Nd) values recorded by their central parts by varying degrees of assimilation of Archean crust at depth. This first phase of deep contamination was followed by a second event at or close to the emplacement level and is related to the interaction of the magmas with the wallrock basalts. These basalts belong to a distinct Karoo LIP magma type having initial epsilon(Nd) from - 2.1 to + 2.5, which provides a stark contrast to the epsilon(Nd) composition of the dike parental magmas (- 15.3 for East-Muren, - 9.0 for West-Muren) previously contaminated by Archean crust. For East-Muren, the distal hornfelses represent partially melted wallrock basalts and the proximal contact zones represent hybrids of such residues with differentiated melts from the intrusion; the magmas that were contaminated by the partial melts of the wallrock basalts were likely transported away from the currently exposed parts of the conduit before the magma-wallrock contact was sealed and further assimilation prevented. In contrast, for West-Muren, the assimilation of the wallrock basalt partial melts is recorded by the gradually increasing epsilon(Nd) of the presently exposed gabbroic rocks towards the roof contact with the basalts. Our study shows that primitive LIP magmas release enough sensible and latent heat to partially melt and potentially assimilate wallrocks in multiple stages. This type of multi-stage assimilation is difficult to detect in general, especially if the associated wallrocks show broad compositional similarity with the intruding magmas. Notably, trace element and isotopic heterogeneity in LIP magmas can be homogenized by such processes (basaltic cannibalism). If similar processes work at larger scales, they may affect the geochemical evolution of the crust and influence the generation of, for example, massif-type anorthosites and "ghost plagioclase" geochemical signature.Peer reviewe

The nature of the Karoo mantle source region from the perspective of olivine in the Luenha picrites: an oxygen isotopic and chemical study

The Karoo large igneous province has been divided into rift zone and basin-related groups, with picrites from the Luenha river, Mozambique, representing an end-member of the latter. New O isotope, major and trace element data for olivine have been combined with MELTS crystallisation modelling to deconvolve compositional diversity associated with magma differentiation from source-derived heterogeneity. Three olivines populations have been discerned as follows: (1) the 'main trend', which records crystallisation from a variety of magma compositions; (2) the 'low Cu trend', which is inferred to represent xenocrysts or antecrysts; and (3) the 'high CaO' olivines, which record polybaric crystallisation of a primitive, little fractionated magma. The trace element variability in olivine phenocrysts relates partially to sampling of different parts of the same overall magma transport and storage systems, and partly to heterogeneity of parental magmas and their mantle sources. When the measured delta O-18(olivine) values have been converted into delta O-18(melt) values, the mean delta O-18(melt) values for the 'main trend' and 'low Cu' groups are indistinguishable from each other (5.7 +/- 0.1 parts per thousand, 2 sigma); however, the mean delta O-18(melt) value of 6.1 +/- 0.1 parts per thousand for the 'high CaO' group is distinctly enriched. These data record source heterogeneity and suggest contributions from two mantle sources, one with elevated delta O-18, and another with more 'typical' mantle delta O-18. Combining these data with previously reported trace element and Nd and Sr isotope data support derivation from a mantle source similar to non-chondritic bulk silicate earth, but with minor contributions (1-2% for the enriched magmas) from a recycled sedimentary component. This points to the importance of a primitive mantle source for the basin-related successions in Karoo.Peer reviewe