| 1. |
- Brinkmann, Inda, et al.
(author)
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Drought recorded by Ba/Ca in coastal benthic foraminifera
- 2022
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In: Biogeosciences. - : Copernicus GmbH. - 1726-4170 .- 1726-4189. ; 19:9, s. 2523-2535
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Journal article (peer-reviewed)abstract
- Increasing occurrences of extreme weather events, such as the 2018 drought over northern Europe, are a concerning issue under global climate change. High-resolution archives of natural hydroclimate proxies, such as rapidly accumulating sediments containing biogenic carbonates, offer the potential to investigate the frequency and mechanisms of such events in the past. Droughts alter the barium (Ba) concentration of near-continent seawater through the reduction in Ba input from terrestrial runoff, which in turn may be recorded as changes in the chemical composition (Ba/Ca) of foraminiferal calcium carbonates accumulating in sediments. However, so far the use of Ba/Ca as a discharge indicator has been restricted to planktonic foraminifera, despite the high relative abundance of benthic species in coastal, shallow-water sites. Moreover, benthic foraminiferal Ba/Ca has mainly been used in open-ocean records as a proxy for paleo-productivity. Here we report on a new geochemical data set measured from living (CTG-labeled) benthic foraminiferal species to investigate the capability of benthic Ba/Ca to record changes in river runoff over a gradient of contrasting hydroclimatic conditions. Individual foraminifera (Bulimina marginata, Nonionellina labradorica) were analyzed by laser-ablation ICP-MS over a seasonal and spatial gradient within Gullmar Fjord, Swedish west coast, during 2018-2019. The results are compared to an extensive meteorological and hydrological data set, as well as sediment and pore-water geochemistry. Benthic foraminiferal Ba/Ca correlates significantly to riverine runoff; however, the signals contain both spatial trends with distance to Ba source and species-specific influences such as micro-habitat preferences. We deduce that shallow-infaunal foraminifera are especially suitable as proxy for terrestrial Ba input and discuss the potential influence of water-column and pore-water Ba cycling. While distance to Ba source, water depth, pore-water geochemistry, and species-specific effects need to be considered in interpreting the data, our results demonstrate confidence in the use of Ba/Ca of benthic foraminifera from near-continent records as a proxy for past riverine discharge and to identify periods of drought.
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| 2. |
- Chaves, Alexandre de Oliveira, et al.
(author)
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The 920–900 Ma Bahia-Gangila LIP of the São Francisco and Congo cratons and link with Dashigou-Chulan LIP of North China craton : New insights from U-Pb geochronology and geochemistry
- 2019
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In: Precambrian Research. - : Elsevier BV. - 0301-9268. ; 329, s. 124-137
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Journal article (peer-reviewed)abstract
- A widespread mafic sill and dyke unit (the Pedro Lessa suite) adjacent to the southeastern São Francisco (SF) craton (eastern South America) is dated as 940 ± 42 Ma, 912 ± 17 Ma, and 986 ± 64 Ma (based on combined ID-TIMS and LA-ICPMS analyses on baddeleyite grains) on the Santa Maria de Itabira gabbroic intrusion. These ages suggest membership in the previously defined ca. 920–900 Ma Large Igneous Province (LIP) (herein referred to as the Bahia-Gangila LIP) of the reconstructed São Francisco and formerly adjacent Congo craton (Africa). Additional mafic and felsic units throughout the SF craton are also linked (based on a literature review), thus expanding the size of the reconstructed LIP to ca. 1,500,000 km2. Geochemistry reveals high Ti OIB signature for most units and low Ti E-MORB composition for the Gangila flood basalts, marking the initiation of attempted ocean opening. Geochemistry of the Bahia-Gangila LIP is also compared with the ca. 920–900 Ma Dashigou-Chulan LIP of the North China craton, and implications for three possible reconstructions of SF/Congo craton with the North China craton are considered.
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| 3. |
- Gumsley, Ashley, et al.
(author)
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Neoarchean large igneous provinces on the Kaapvaal Craton in southern Africa re-define the formation of the Ventersdorp Supergroup and its temporal equivalents
- 2020
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In: Geological Society of America Bulletin. - 0016-7606. ; 132:9-10, s. 1829-1844
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Journal article (peer-reviewed)abstract
- U-Pb geochronology on baddeleyite is a powerful technique that can be applied effectively to chronostratigraphy. In southern Africa, the Kaapvaal Craton hosts a well-preserved Mesoarchean to Paleoproterozoic geological record, including the Neoarchean Ventersdorp Supergroup. It overlies the Witwatersrand Supergroup and its world-class gold deposits. The Ventersdorp Supergroup comprises the Klipriviersberg Group, Platberg Group, and Pniel Group. However, the exact timing of formation of the Ventersdorp Supergroup is controversial. Here we present 2789 ± 4 Ma and 2787 ± 2 Ma U-Pb isotope dilution- thermal ionization mass spectrometry (ID-TIMS) baddeleyite ages and geochemistry on mafic sills intruding the Witwatersrand Supergroup, and we interpret these sills as feeders to the overlying Klipriviersberg Group flood basalts. This constrains the age of the Witwatersrand Supergroup and gold mineralization to at least ca. 2.79 Ga. We also report 2729 ± 5 Ma and 2724 ± 7 Ma U-Pb ID-TIMS baddeleyite ages and geochemistry from a mafic sill intruding the Pongola Supergroup and on an east-northeast-trending mafic dike, respectively. These new ages distinguish two of the Ventersdorp Supergroup magmatic events: the Klipriviersberg and Platberg. The Ventersdorp Supergroup can now be shown to initiate and terminate with two large igneous provinces (LIPs), the Klipriviersberg and Allanridge, which are separated by Platberg volcanism and sedimentation. The age of the Klipriviersberg LIP is 2791-2779 Ma, and Platberg volcanism occurred at 2754-2709 Ma. The Allanridge LIP occurred between 2709-2683 Ma. Klipriviersberg, Platberg, and Allanridge magmatism may be genetically related to mantle plume(s). Higher heat flow and crustal melting resulted as a mantle plume impinged below the Kaapvaal Craton lithosphere, and this was associated with rifting and the formation of LIPs.
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| 4. |
- Heimbrand, Yvette, et al.
(author)
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Cod otoliths document accelerating climate impacts in the Baltic Sea
- 2024
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In: Scientific Reports. - 2045-2322. ; 14:1
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Journal article (peer-reviewed)abstract
- Anthropogenic deoxygenation of the Baltic Sea caused major declines in demersal and benthic habitat quality with consequent impacts on biodiversity and ecosystem services. Using Baltic cod otolith chemical proxies of hypoxia, salinity, and fish metabolic status and growth, we tracked changes from baseline conditions in the late Neolithic (4500 BP) and early twentieth century to the present, in order to understand how recent, accelerating climate change has affected this key species. Otolith hypoxia proxies (Mn:Mg) increased with expanding anoxic water volumes, but decreased with increasing salinity indexed by otolith Sr:Ca. Metabolic status proxied by otolith Mg:Ca and reconstructed growth were positively related to dissolved oxygen percent saturation, with particularly severe declines since 2010. This long-term record of otolith indicators provides further evidence of a profound state change in oxygen for the worse, in one of the world’s largest inland seas. Spreading hypoxia due to climate warming will likely impair fish populations globally and evidence can be tracked with otolith chemical biomarkers.
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| 5. |
- Herrmann, Maria, et al.
(author)
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The effect of low-temperature annealing on discordance of U–Pb zircon ages
- 2021
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In: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 11:1
-
Journal article (peer-reviewed)abstract
- Discordant U–Pb data of zircon are commonly attributed to Pb loss from domains with variable degree of radiation damage that resulted from α-decay of U and Th, which often complicates the correct age interpretation of the sample. Here we present U–Pb zircon data from 23 samples of ca. 1.7–1.9 Ga granitoid rocks in and around the Siljan impact structure in central Sweden. Our results show that zircon from rocks within the structure that form an uplifted central plateau lost significantly less radiogenic Pb compared to zircon grains in rocks outside the plateau. We hypothesize that zircon in rocks within the central plateau remained crystalline through continuous annealing of crystal structure damages induced from decay of U and Th until uplifted to the surface by the impact event ca. 380 Ma ago. In contrast, zircon grains distal to the impact have accumulated radiation damage at shallow and cool conditions since at least 1.26 Ga, making them vulnerable to fluid-induced Pb-loss. Our data are consistent with studies on alpha recoil and fission tracks, showing that annealing in zircon occurs at temperatures as low as 200–250 °C. Zircon grains from these samples are texturally simple, i.e., neither xenocrysts nor metamorphic overgrowths have been observed. Therefore, the lower intercepts obtained from regression of variably discordant zircon data are more likely recording the age of fluid-assisted Pb-loss from radiation-damaged zircon at shallow levels rather than linked to regional magmatic or tectonic events.
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| 6. |
- Hoffmann, J. Elis, et al.
(author)
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Constraining the process of Eoarchean TTG formation in the Itsaq Gneiss Complex, southern West Greenland
- 2014
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In: Earth and Planetary Science Letters. - : Elsevier BV. - 1385-013X .- 0012-821X. ; 388, s. 374-386
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Journal article (peer-reviewed)abstract
- We present new major and trace element, high-precision high-field-strength-element, hafnium and neodymium isotope data for well preserved Eoarchean TTGs within the Itsaq Gneiss Complex (IGC) of southern West Greenland. These data are combined with thermodynamic model predictions of partial melting and fractional crystallization to gain new insights into continental crust formation in the Archean. Our results show that the observed compositional range of Eoarchean TTGs can be explained by a combination two processes: (1) 5-25% partial melting of amphibolite within thickened mafic crust and (2) subsequent fractional crystallization processes. The Eoarchean TTG suite of SW Greenland probably formed through mixing of melt batches that originally formed at different source depths between 10 and 14 kbar and ponded as plutons at mid-crustal levels. The trace element compositions of some TTGs point to subsequent fractional crystallization processes involving plagioclase, clinopyroxene, amphibole and garnet. Our model is consistent with recent studies proposing that the Eoarchean Itsaq Gneiss Complex TTGs from the IGC formed by re-working of mafic protocrust that stabilized as accreted juvenile crustal terranes in the Eoarchean. The model is also in good agreement with field observations from the area. (C) 2013 Elsevier B.V. All rights reserved.
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| 7. |
- Hoffmann, J. Elis, et al.
(author)
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Mechanisms of Archean crust formation inferred from high-precision HFSE systematics in TTGs
- 2011
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In: Geochimica et Cosmochimica Acta. - : Elsevier BV. - 0016-7037 .- 1872-9533. ; 75:15, s. 4157-4178
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Journal article (peer-reviewed)abstract
- It has been proposed that Archean tonalitic-trondhjemitic-granodioritic magmas (TTGs) formed by melting of mafic crust at high pressures. The residual mineralogy of the TTGs (either (garnet)-amphibolite or rutile-bearing eclogite) is believed to control the trace element budget of TTGs. In particular, ratios of high-field-strength elements (HFSE) can help to discriminate between the different residual lithologies. In order to place constraints on the source mineralogy of TTGs, we performed high-precision HFSE measurements by isotope dilution (Nb, Ta, Zr, Hf) together with Lu-Hf and Sm-Nd measurements on representative, ca. 3.85-2.8 Ga TTGs and related rock types from southern West Greenland, W-India and from the Superior Province. These measurements are complemented by major and trace element data for the TTGs. Texturally homogeneous early Archean (3.85-3.60 Ga old) and Mesoarchean (ca. 3.1-2.8 Ga old) TTGs have both low Ni (<11 ppm) and Cr contents (<20 ppm), indicating that there was little or no interaction with mantle peridotite during ascent. Ratios of Nb/Ta in juvenile Eoarchean TTGs range from ca. 7 to ca. 24, and in juvenile Mesoarchean TTGs from ca. 14 to ca. 27. Even higher Nb/Ta (14-42) were obtained for mig-matitic TTGs and intra-crustal differentiates, most likely mirroring further fractionation of Nb from Ta as a consequence of partial melting, fluid infiltration and migmatisation. In the juvenile TTGs, positive correlations between Nb/Ta and Gd/Yb, La/Yb, Sr/Y, Zr/Sm and Zr/Nb are observed. These compositional arrays are best explained by melting of typical Isua tholeiites in both, the rutile-bearing eclogite stability field (>15 kbar, high Nb/Ta) and the garnet-amphibolite stability field (10-15 kbar, low Nb/Ta). With respect to the low end of Nb/Ta found for TTGs, there is currently some uncertainty between the available experimental datasets for amphibole. Independent of these uncertainties, the TTG compositions found here still require the presence of both endmember residues. A successful geological model for the TTGs therefore has to account for the co-occurrence of both low-and high-Nb/Ta TTGs within the same geologic terrane. An additional feature observed in the Eoarchean samples from Greenland is a systematic co-variation between Nb/Ta and initial epsilon Hf(t), which is best explained by a model where TTG-melting occured at progressively increasing pressures in a pile of tectonically thickened mafic crust. The elevated Nb/Ta in mig-matitic TTGs and intra-crustal differentiates can shed further light on the role of intra-crustal differentiation processes in the global Nb/Ta cycle. Lower crustal melting processes at granulite facies conditions may generate high-Nb/Ta domains in the middle crust, whereas mid-crustal melting at amphibolite facies conditions may account for the low Nb/Ta generally observed in upper crustal rocks.
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| 8. |
- Hüssy, Karin, et al.
(author)
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It’s elemental, my dear watson : Validating seasonal patterns in otolith chemical chronologies
- 2021
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In: Canadian Journal of Fisheries and Aquatic Sciences. - : Canadian Science Publishing. - 0706-652X .- 1205-7533. ; 78:5, s. 551-566
-
Journal article (peer-reviewed)abstract
- Accurate age data are essential for reliable fish stock assessment. Yet many stocks suffer from inconsistencies in age interpretation. A new approach to obtain age makes use of the chemical composition of otoliths. This study validates the periodicity of recurrent patterns in25 Mg,31 P,34 K,55 Mn,63 Cu,64 Zn,66 Zn,85 Rb,88 Sr,138 Ba, and208 Pb in Baltic cod (Gadus morhua) otoliths from tag–recapture and known-age samples. Otolith P concentrations showed the highest consistency in seasonality over the years, with minima co-occurring with otolith winter zones in the known-age otoliths and in late winter – early spring when water temperatures are coldest in tagged cod . The timing of minima differs between stocks, occurring around February in western Baltic cod and 1 month later in eastern Baltic cod; seasonal maxima are also stock-specific, occurring in August and October, respectively. The amplitude in P is larger in faster-growing western compared with eastern Baltic cod. Seasonal patterns with minima in winter – late spring were also evident in Mg and Mn, but less consistent over time and fish size than P. Chronological patterns in P, and to a lesser extent Mg and Mn, may have the potential to supplement traditional age estimation or to guide the visual identification of translucent and opaque otolith patterns used in traditional age estimation.
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| 9. |
- Keulen, Nynke, et al.
(author)
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Meso- and Neoarchaean geological history of the Bjornesund and Ravns Storo Supracrustal Belts, southern West Greenland: Settings for gold enrichment and corundum formation
- 2014
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In: Precambrian Research. - : Elsevier BV. - 0301-9268. ; 254, s. 36-58
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Journal article (peer-reviewed)abstract
- To understand the Mesa- and Neoarchaean geological history of the Bjornesund region, southern West Greenland (ca. 62-63 degrees N), we have undertaken an integrated study combining field observations with U-Pb zircon geochronology by LA-ICP-MS, metamorphic petrology, and microstructural analyses. The Bjornesund region links the southern part of the Fiskenaesset anorthosite complex to the well-preserved metavolcanic rocks of the Ravn Story Supracrustal Belt and the Bjornesund Supracrustal Belt, so the development of this region is of key interest to unravelling the Archaean geology of the entire region. In this study, we couple the observed phases of gold-enrichment and corundum formation to the geological history. The leucogabbroic rocks of the Bjornesund region, which are part of the Fiskenaesset complex, intruded at ca. 2.95 Ga into the tholeiitic and calc-alkaline metavolcanic rocks of the two studied belts. Following isoclinal folding, the rocks were intruded by diorite (2.92 Ga) and granodiorite (2.91 and 2.88 Ga), which are now texturally orthogneisses. The area was affected by three major phases of folding; F1 (undated), F2 at 2.85-2.83 Ga, and F3 at 2.71-2.70 Ga. The former deformation event is associated with the gold-enrichment in alteration zones in shear folds in metavolcanic amphibolite near the contact with the orthogneiss. Intrusion of granitic sheets during the F3 deformation triggered the formation of corundum, which is locally of gem quality, at the contact between leucogabbroic and ultramafic rocks. Peak metamorphic conditions were calculated as 580-630 degrees C and 6-4 kbar at 2.85-2.83 Ga. Later retrogression in the southern part of the area at ca. 500-450 degrees C and 4.5-3 kbar, and extensional fault reactivation overprinted some of the older structures. (C) 2014 Elsevier B.V. All rights reserved.
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| 10. |
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| 11. |
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| 12. |
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| 13. |
- Naeraa, Tomas, et al.
(author)
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A lower crustal mafic source for the ca. 2550 Ma Qorqut Granite Complex in southern West Greenland
- 2014
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In: Lithos. - : Elsevier BV. - 0024-4937 .- 1872-6143. ; 192, s. 291-304
-
Journal article (peer-reviewed)abstract
- The late Neoarchaean Qorqut Granite Complex is the youngest large igneous intrusion in the Nuuk region in southern West Greenland, where basement is primarily of Eoarchaean and Mesoarchaean age with a tonalite-trondhjemite-granodiorite (TTG) composition. The Qorqut granite is generally undeformed and it intruded during a prolonged period, starting at ca. 2730 Ma, characterised by crustal reworking, possibly related to syn- or post accretion tectonics or continental collision. We present major and trace element whole rock chemistry and combined U/Pb, Hf and O isotope data from zircon. We obtained a mean zircon U/Pb age of 2547 +/- 4 Ma (MSWD = 0.63). Initial sHf values range from - 12 to -18 requiring a long residence time and a rather homogeneous source. Sample averaged zircon delta O-18 values range from 6.1 +/- 0.2%. to 6.5 +/- 0.3/0.7%o best interpreted with a source region of mainly unweathered mantle derived igneous rocks. Compared to the regional TTG basement, the QGC is characterised by low CaO and Na2O and high K2O, LREE and Rb contents, and a stronger fractionated REE pattern with a negative Eu anomaly. We show that the homogeneous Hf isotope signature of the granite together with its low epsilon value and its pristine oxygen isotope composition are best explained with an Eoarchaean mafic source with a Lu-176/Hf-176 around 0.015-0.019. Trace element modelling confirms that a mafic source in REE and with an eclogitic residue and with plagioclase as a fractionating phase would generate appropriate melt compositions. Modelling requires residual rutile in the source which constrain the pressures to > ca. 13-18 kbar. Zirconium saturation temperatures suggest magma temperatures in the range 750-850 degrees C. The obtained P-T conditions suggest a lower crustal source region in a thickened crustal unit consistent with a post or late continental collisional setting. (C) 2014 Elsevier B.V. All rights reserved.
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| 14. |
- Naeraa, Tomas, et al.
(author)
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Hafnium isotope evidence for a transition in the dynamics of continental growth 3.2Gyr ago
- 2012
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In: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 485:7400, s. 627-627
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Journal article (peer-reviewed)abstract
- Earth's lithosphere probably experienced an evolution towards the modern plate tectonic regime, owing to secular changes in mantle temperature(1,2). Radiogenic isotope variations are interpreted as evidence for the declining rates of continental crustal growth over time(3-5), with some estimates suggesting that over 70% of the present continental crustal reservoir was extracted by the end of the Archaean eon(3,5). Patterns of crustal growth and reworking in rocks younger than three billion years (Gyr) are thought to reflect the assembly and break-up of supercontinents by Wilson cycle processes and mark an important change in lithosphere dynamics(6). In southern West Greenland numerous studies have, however, argued for subduction settings and crust growth by arc accretion back to 3.8 Gyr ago(7-9), suggesting that modern-day tectonic regimes operated during the formation of the earliest crustal rock record. Here we report in situ uranium-lead, hafnium and oxygen isotope data from zircons of basement rocks in southern West Greenland across the critical time period during which modern-like tectonic regimes could have initiated. Our data show pronounced differences in the hafnium isotope-time patterns across this interval, requiring changes in the characteristics of the magmatic protolith. The observations suggest that 3.9-3.5-Gyr-old rocks differentiated from a >3.9-Gyr-old source reservoir with a chondritic to slightly depleted hafnium isotope composition. In contrast, rocks formed after 3.2 Gyr ago register the first additions of juvenile depleted material (that is, new mantle-derived crust) since 3.9 Gyr ago, and are characterized by striking shifts in hafnium isotope ratios similar to those shown by Phanerozoic subduction-related orogens(10-12). These data suggest a transitional period 3.5-3.2 Gyr ago from an ancient (3.9-3.5 Gyr old) crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3.2 Gyr ago that involved juvenile crust generation by plate tectonic processes.
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| 15. |
- Næraa, Tomas, et al.
(author)
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The ca. 2785–2805 Ma high temperature ilivertalik intrusive complex of southern west Greenland
- 2018
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In: Geosciences (Switzerland). - : MDPI AG. - 2076-3263. ; 8:9
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Journal article (peer-reviewed)abstract
- Ferroan granitoid intrusions are rare in the Archaean rock record, but have played a large role in the evolution of the Proterozoic crust, particular in relation to anorthosite-mangerite-charnockite-granite suites. Here we discuss the petrogenesis of the ca. 2785–2805 Ma ferroan Ilivertalik Intrusive Complex, which has many geochemical similarities to Proterozoic iron rich granitoids. We present major and trace element whole rock chemistry and combined in-situ zircon U-Pb, Hf and O isotope data. The intrusive complex divides into: (i) minor tabular units of mainly diorite-tonalite compositions, which are typically situated along contacts to the host basement and (ii) interior larger, bodies of mainly granite-granodiorite composition. Geochemically these two unites display continuous to semi-continuous trends in Haker-diagrams. Whole rock REE enrichment display increases from Yb to La, from 10–25 to 80–100 times chondrite, respectively. The diorite-tonalite samples are generally more enriched in REE compared to the granite-granodiorite samples. The complex has hafnium isotope compositions from around +1.5 to −2.5 epsilon units and δ18O compositions in the range of 6.3 to 6.6‰. The complex is interpreted to be derived from partial melting in a crustal source region during anomalously high crustal temperatures.
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| 16. |
- Petersson, Andreas, 1980-
(author)
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Tracing Proterozoic arc mantle Hf isotope depletion of southern Fennoscandia through coupled zircon U–Pb and Lu–Hf isotopes
- 2017
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In: Lithos. - : Elsevier BV. - 0024-4937 .- 1872-6143. ; 284–285, s. 122-131
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Journal article (peer-reviewed)abstract
- Constraints on the composition of the depleted mantle Sm–Nd and Lu–Hf crust formation ages have a long history of scientific debate. When calculating mantle extraction ages, and constructing crustal growth models, a linear evolution of incompatible trace elements in a depleted mantle since > 4 Ga is routinely used. Mantle depletion however varies regionally and over time and subduction of sediments and oceanic crust renders a mantle-wedge variously enriched relative to a modelled depleted mantle.Here we show that primitive mantle-derived subduction related gabbroic intrusions from southern Fennoscandia have Hf isotope compositions that are enriched relative to a MORB-like linear depleted mantle evolution curve. Extrapolation of primitive Paleoproterozoic gabbro suites enables the construction of a regional mantle evolution curve, providing improved constraints on model ages, crustal residence times and the fraction of juvenile versus reworked continental crust. Convergent margins are assumed to be one of the main sites of continental crust growth, and using an overly depleted mantle source yield model ages that are too old, and hence cumulative crustal growth models show too much crust generation early in the Earth's history. The approach of using the Hf isotope composition of zircon from primitive subduction related gabbroic intrusions as a proxy for mantle Hf isotope composition, piloted in this study, can be applied to other convergent margins.
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| 17. |
- Rey-Solé, Mar, et al.
(author)
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Siliceous Raw Material Exploitation at Hort de la Boquera Site (Margalef de Monsant, Tarragona, España) : First Results from La-Icp-Ms Analysis
- 2017
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In: The Exploitation of Raw Materials in Prehistory : Sourcing, Processing and Distribution - Sourcing, Processing and Distribution. - 9781443895972 - 1443895970 ; , s. 234-249
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Conference paper (peer-reviewed)abstract
- Research on the sourcing of siliceous raw materials in Prehistory has taken a step forward with the addition of geochemical techniques and elemental analysis to the archaeopetrological method. Counter to the macroscopic techniques, which retain an elevated subjective percentage in their results, geochemical techniques and especially LA-ICP-MS analysis generate objective, quantifiable and usable data, helping us to complete another mineralogical analysis.95% of the siliceous raw material from the Hort de la Boquera archaeological site (Margalef de Montsant, Tarragona, Spain) is a type of flint from an evaporitic origin that petrografically is characterized by a chalcedonic matrix with gypsum lenticules as inclusions.Siliceous Raw Material Exploitation at Hort De La Boquera Site 235Multiple flint outcrops from an evaporitic origin (Eocene-Oligocene) and with many closed characteristics such as the lithic raw material from Hort de la Boquera are located near the site (Cornudella Group-Ulldemolins Complex), making them very suitable to supply the inhabitants of the Hort de la Boquera site.We have two different but complementary goals in this research: on the one hand, to characterize, describe and create a database for all the different types of flint recovered, emphasizing their geochemical marks. On the other hand, we have the aim to compare these results with those obtained from the raw materials recovered at the site, which would provide a range of complementary data to discover the potential catchment areas and help us to understand certain behaviours of the prehistoric communities from the Hort de la Boquera site.
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| 18. |
- Saikia, Ashima, et al.
(author)
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Trace element constraints on the parental melt of gabbroic cumulates from the Naga Ophiolite Complex, North-East India
- 2022
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In: International Journal of Earth Sciences. - : Springer Science and Business Media LLC. - 1437-3254 .- 1437-3262. ; 111:3, s. 1009-1032
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Journal article (peer-reviewed)abstract
- The best exposures of mafic cumulate (gabbros and gabbronorites) rocks of Naga Ophiolite Complex (NOC) occur north-east of Moki village, east of Wazeho in the Indian State of Nagaland. Gabbro comprises predominantly of clinopyroxene (40–50 vol%) and plagioclase (40–50 vol%), while gabbronorite mainly consist of plagioclase (40–50 vol%), clinopyroxene (30–40 vol%) and orthopyroxene (~ 10 vol%) with minor iron oxides and apatite. Compositionally clinopyroxenes are augite, plagioclases are bytownite and orthopyroxenes are enstatitic. In primitive mantle and chondrite normalized diagrams, clinopyroxenes in gabbronorite show greater enrichment in trace elements and REEs than in gabbro. However, whole rock chemistry of both rock types show similar trends except for negative Ti, Sr and Eu anomalies in gabbronorite. Whole rock geochemical results suggest gabbronorite and gabbro as cogenetic but unrelated to the ultramafic rocks of NOC. P–T estimates using clinopyroxene-only thermobarometers suggest crystallization at 1204–1273 °C and 4.4–9.4 kbar for gabbros and 1193–1230 °C and 1.3–3.6 kbar for gabbronorites. Isobaric equilibrium crystallization models indicate clinopyroxene as the first crystallizing phase in gabbro with a liquidus temperature of 1289 °C, whereas it is plagioclase in gabbronorite with liquidus temperature of 1248 °C. Liquidus temperatures are comparable to the maximum crystallization temperatures obtained for gabbro and gabbronorite. Parental melt calculated using equilibrium distribution method (EDM) corresponds to the melt compositions at trapped melt fractions = 5% and 10% for gabbro and gabbronorite, respectively. Parental melt calculated using the concentration ratio method corroborates EDM calculations.
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| 19. |
- Scherstén, Anders, et al.
(author)
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Re-Os and U-Pb constraints on gold mineralisation events in the Meso- to Neoarchaean Storo greenstone belt, Storo, southern West Greenland
- 2012
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In: Precambrian Research. - : Elsevier BV. - 0301-9268. ; 200, s. 149-162
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Journal article (peer-reviewed)abstract
- The Storo greenstone belt in Godthabsfjord, southern West Greenland consists of juxtaposed rock units of different age and origin, and hosts gold mineralisation that is associated with arsenopyrite along a contact between lithological units and along the axial plane of a large fold core. The age and origin of the gold is debatable, but in this paper, we present new arsenopyrite Re-Os and zircon U-Pb data to constrain the age of the Storo gold deposit. A 2.71 +/- 0.05 Ga arsenopyrite isochron and 2.707 +/- 0.008 Ga highly radiogenic arsenopyrite from a mineralisation along a rock contact, together with a 2.64 +/- 0.02 Ga arsenopyrite isochron along the axial plane of the fold core indicate a two-stage mineralisation process. While the 2.707 +/- 0.008 Ga highly radiogenic arsenopyrite provides firm support for an early mineralisation event, a mixing origin cannot yet be excluded for the 2.71 +/- 0.05 Ga isochron. The 2.64 +/- 0.02 Ga isochron is in perfect agreement with recent U-Pb zircon data (Nutman et al., 2007. Precambrian Research 159, 19-32) and these data are best explained by orogenic mineralisation during amphibolite facies metamorphism along structural weak planes. The initial Os-187/Os-188 value of 0.56 +/- 0.16 for the 2.64 +/- 0.02 Ga isochron indicates a crustal source for the metals, whereas the initial Os-187/Os-188 = -0.1 +/- 0.6 for the 2.71 +/- 0.05 Ga isochron remains unconstrained. Re-Os data are best explained by relatively short crustal residence times of less than 0.1 Ga, wherein the Os, and associated metals, were extracted from the mantle at a time younger than 2.8 Ga, and in which the 2.64 +/- 0.02 Ga stage formed by mobilisation of an earlier mineralisation. Detrital zircon constraints imply volcanism and sediment deposition for parts of the belt at <= 2.84 Ga and that these units were tectonically juxtaposed to similar to 3.05 Ga rocks within the belt. The units that were deposited <= 2.84 Ga record metamorphic zircon U-Pb ages of similar to 2.63 Ga, but evidence for 2.72 Ga metamorphism, which is ubiquitous in adjacent terranes, is lacking. (C) 2012 Elsevier B.V. All rights reserved.
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| 20. |
- Skridlaite, Grazina, et al.
(author)
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Iron-rich Ca–Mg skarns from the SW East European Craton (Lithuania) : Microstructural study, mineral reactions and direct age constraints of ore-forming events using LA-ICPMS
- 2024
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In: Lithos. - 0024-4937. ; 482-483
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Journal article (peer-reviewed)abstract
- The Varena Iron Ore deposit in the SW East European Craton is a significant ore body that occurs within metamorphosed and hydrothermally reworked Paleoproterozoic dolostones. We have performed microstructural investigations supplemented with mineral chemistry and geochronological investigations (LA-ICP-MS) to obtain age constraints on the ore-forming event(s) and improve the understanding of the conditions during mineralization process. Mineral chemistry and textures suggest a drop in pressure after the event of peak metamorphic skarn formation. Influx of oxidized, iron-rich H2O fluids resulted in (1) Mg mobility that caused secondary dolomitization of calcite, (2) dissolution of metamorphic magnetite and formation of a new, inclusion-rich (Mag-1) and inclusion-poor (Mag-2) magnetite, and (3) replacement of the peak skarn assemblages. During these fluid-related processes, accessory phases of monazite, baddeleyite, and zircon were formed. Their U–Pb dating yield individually robust ages of 1721 ± 9 Ma (monazite, 23 spots), 1703 ± 10 Ma (baddeleyite, 18 spots) and 1706 ± 54 Ma (zircon, 14 spots), respectively. The weighted mean age of 1713 ± 7 Ma (2σ internal) is considered to represent the best age estimate of the iron-ore mineralization in the Varena Iron Ore deposit, and possibly also dates influx of P, REEs etc. into the system. This mineralization event is contemporaneous with ca. 1.73–1.70 Ga metamorphic reworking of the host rocks in the region and may be linked to regional continental-margin type Transscandinavian Igneous Belt (TIB) magmatism in south-central Sweden.
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| 21. |
- Smit, Mattijs, et al.
(author)
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Formation of Archean continental crust constrained by boron isotopes
- 2019
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In: Geochemical Perspective Letters. - : European Association of Geochemistry. - 2410-3403. ; , s. 23-26
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Journal article (peer-reviewed)abstract
- The continental crust grew and matured compositionally during the Palaeo- to Neoarchean through the addition of juvenile tonalite-trondhjemite-granodiorite (TTG) crust. This change has been linked to the start of global plate tectonics, following the general interpretation that TTGs represent ancient analogues of arc magmas. To test this, we analysed B concentrations and isotope compositions in 3.8-2.8 Ga TTGs from different Archean terranes. The 11B/10B values and B concentrations of the TTGs, and their correlation with Zr/Hf, indicatedifferentiation from a common B-poor mafic source that did not undergo addition of B from seawater or seawater-altered rocks. The TTGs thus do not resemble magmatic rocks from active margins, which clearly reflect such B addition to their source. The B- and 11B-poor nature of TTGs indicates that modern style subduction may not have been a dominant process in the formation of juvenile continental crust before 2.8 Ga.
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| 22. |
- Szilas, Kristoffer, et al.
(author)
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Complex calc-alkaline volcanism recorded in Mesoarchaean supracrustal belts north of Frederikshab Isblink, southern West Greenland: Implications for subduction zone processes in the early Earth
- 2012
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In: Precambrian Research. - : Elsevier BV. - 0301-9268. ; 208, s. 90-123
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Journal article (peer-reviewed)abstract
- We present new geochemical data for three Mesoarchaean supracrustal belts (Ravns Storo, Bjornesund and Perserajoorsuaq) situated north of Frederikshab Isblink in southern West Greenland for which we propose the collective name 'The Ikkattup Nunaa Supracrustal Association'. They comprise mainly amphibolites of tholeiitic basalt composition and leucoamphibolites of calc-alkaline andesite composition. Both lithological units are cut by aplite sheets of tonalite-trondhjemite-granodiorite (TTG) composition with U-Pb zircon ages of c. 2900 Ma. Lu-Hf and Sm-Nd isochrons based on whole rock amphibolite and leucoamphibolite samples yield ages of 2990 +/- 41 Ma and 3020 +/- 78 Ma, respectively, which are within error of the age of the Fiskenaesset Complex situated 5-25 km to the north. Leucoamphibolites from the three supracrustal belts show apparent geochemical mixing trends between tholeiitic amphibolites and TTG gneisses, as the end-members. By assimilation-fractional-crystallisation (AFC) modelling we can show that one group of leucoamphibolites can indeed be explained by contamination of the parental melts by a TTG-like end-member and another group of high P2O5, La and Nb leucoamphibolites can be explained by contamination involving a hypothetical low-silica adakite end-member. However, the leucoamphibolites are juvenile with epsilon Nd-(2970 Ma) from +2.1 to +3.5 and epsilon Hf-(2970 (Ma)) of +3.5 to +4.3. Thus, the mafic source of the felsic contaminant melts must have been derived from a depleted mantle source more or less at the same time (<60 Ma) as the volcanism took place. Our preferred interpretation of the geochemical and isotope data is that the protoliths of the supracrustal rocks formed in an island arc setting, where early tholeiitic volcanism gave way to calc-alkaline volcanism in a maturing arc. The apparent AFC trends are thus explained by in situ partial melting of basaltic arc crust to form juvenile TTG- and adakite-melts that mixed with mafic magmas or contaminated their mantle source to produce the calc-alkaline leucoamphibolite protolith. This model has important implications for the general interpretation of other Archaean supracrustal belts, because AFC and geochemical mixing trends towards a TTG end-member are not uniquely diagnostic of crustal contamination, but may rather reflect processes operating at lower crustal or source levels in arcs, such as melting-assimilation-storage-homogenisation (MASH) or slab-melt (low-silica adakite) metasomatism of the mantle source, respectively. (C) 2012 Elsevier B.V. All rights reserved.
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| 23. |
- Szilas, Kristoffer, et al.
(author)
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Highly refractory Archaean peridotite cumulates : Petrology and geochemistry of the Seqi Ultramafic Complex, SW Greenland
- 2018
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In: Geoscience Frontiers. - : Elsevier BV. - 1674-9871. ; 9:3, s. 689-714
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Journal article (peer-reviewed)abstract
- This paper investigates the petrogenesis of the Seqi Ultramafic Complex, which covers a total area of approximately 0.5 km2. The ultramafic rocks are hosted by tonalitic orthogneiss of the ca. 3000 Ma Akia terrane with crosscutting granitoid sheets providing an absolute minimum age of 2978 ± 8 Ma for the Seqi Ultramafic Complex. The Seqi rocks represent a broad range of olivine-dominated plutonic rocks with varying modal amounts of chromite, orthopyroxene and amphibole, i.e. various types of dunite (s.s.), peridotite (s.l.), as well as chromitite. The Seqi Ultramafic Complex is characterised primarily by refractory dunite, with highly forsteritic olivine with core compositions having Mg# ranging from about 91 to 93. The overall high modal contents, as well as the specific compositions, of chromite rule out that these rocks represent a fragment of Earth's mantle. The occurrence of stratiform chromitite bands in peridotite, thin chromite layers in dunite and poikilitic orthopyroxene in peridotite instead supports the interpretation that the Seqi Ultramafic Complex represents the remnant of a fragmented layered complex or a magma conduit, which was subsequently broken up and entrained during the formation of the regional continental crust. Integrating all of the characteristics of the Seqi Ultramafic Complex points to formation of these highly refractory peridotites from an extremely magnesian (Mg# ∼ 80), near-anhydrous magma, as olivine-dominated cumulates with high modal contents of chromite. It is noted that the Seqi cumulates were derived from a mantle source by extreme degrees of partial melting (>40%). This mantle source could potentially represent the precursor for the sub-continental lithospheric mantle (SCLM) in this region, which has previously been shown to be ultra-depleted. The Seqi Ultramafic Complex, as well as similar peridotite bodies in the Fiskefjord region, may thus constitute the earliest cumulates that formed during the large-scale melting event(s), which resulted in the ultra-depleted cratonic keel under the North Atlantic Craton. Hence, a better understanding of such Archaean ultramafic complexes may provide constraints on the geodynamic setting of Earth's first continents and the corresponding SCLM.
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| 24. |
- Szilas, Kristoffer, et al.
(author)
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Origin of Mesoarchaean arc-related rocks with boninite/komatiite affinities from southern West Greenland
- 2012
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In: Lithos. - : Elsevier BV. - 0024-4937. ; 144, s. 24-39
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Journal article (peer-reviewed)abstract
- We report whole-rock elemental and Sm-Nd isotope geochemical data from mafic-ultramafic supracrustal rocks from the Nunatak 1390 area in southern West Greenland. Additionally, we report the metamorphic temperature history for these rocks as derived from tourmaline thermometry on a tourmalinite inlier, as well as in situ U-Pb, Hf and O isotopic data from zircons extracted from tonalite-trondhjemite-granodiorite (TTG) gneisses that intruded the mafic-ultramafic sequence. The supracrustal rocks from the Nunatak 1390 area have a minimum age of c. 2900 Ma defined by U-Pb zircon ages of cross-cutting aplite sheets of TTG composition. The supracrustal sequence comprises mafic rocks with pillow structures and ultramafic rocks with no evidence of their protolith. They all have amphibolite-facies mineral assemblages and a peak metamorphic temperature of approximately 550 degrees C. The mafic sequence has relatively flat trace element patterns (La-N/Sm-N of 0.70-2.4) and mostly negative Nb-anomalies (Nb/Nb* of 0.30-1.0) and resembles modern island arc tholeiites. The mafic sequence can be divided into a high- and low-Ti group, where the former group has lower MgO, and significantly higher contents of incompatible elements such as TiO2, P2O5, Zr, Nb and Th. The ultramafic rocks have major and trace element compositions similar to Ti-enriched/Karasjok-type komatiites described in the literature. However, there are no textural indications that the ultramafic rocks from Nunatak 1390 are komatiites sensu stricto. The low-Ti group of the mafic sequence appears to have been derived from a N-MORB source, whereas the high-Ti group and the ultramafic rocks appear to have been derived from a mantle source that is more enriched than the N-MORB source. However, there is no difference in the initial epsilon Nd of the mafic and ultramafic rocks. Additionally, assimilation-fractional-crystallisation (AFC) modelling is consistent with this enrichment being caused by introduction of juvenile low-silica adalcite (slab-melt) into the mantle source region. Accordingly, we propose that the mafic and ultramafic rocks were derived from a similar type of mantle source, but that the ultramafic rocks were derived from a previously depleted mantle source that was refertilised by slab melts in a subduction zone setting. The high MgO contents of the ultramafic rocks could thus reflect a second stage of partial melting of a refractory mantle in a process similar to that which is suggested for the formation of modern boninites. We propose that the mafic-ultramafic sequence represents an island arc that evolved initially as a juvenile complex (c. 3000 Ma). However, inherited zircon grains in aplites and Hf isotope data recorded by the second intrusive TTG phase (c. 2850-2870 Ma), show that mixing with older pre-existing crust occurred during this event. Because the regional crust is dominated by TTGs of this younger age, our data suggests that it likely formed by accretion and melting of arcs of different ages and/or contamination of juvenile arcs by pre-existing continental crust rather than entirely by juvenile arc differentiation or melting. Our data thus supports melting of thickened mafic crust in an accretionary setting, rather than direct slab melting, as a mechanism for Archaean crust formation. (C) 2012 Elsevier B.V. All rights reserved.
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