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  • 1
    Language: English
    In: International Journal of Earth Sciences, 2015, Vol.104(1), pp.89-102
    Description: Clinopyroxenes in some fresh anhydrous spinel peridotite mantle xenoliths from the northern Massif Central (France) and Lower Silesia (Poland), analysed for a range of incompatible trace elements by laser ablation inductively coupled plasma mass spectrometry, show unusually strong negative anomalies in Hf and Zr relative to adjacent elements Sm and Nd, on primitive mantle-normalised diagrams. Similar Zr–Hf anomalies have only rarely been reported from clinopyroxene in spinel peridotite mantle xenoliths worldwide, and most are not as strong as the examples reported here. Low Hf contents give rise to a wide range of Lu/Hf ratios, which over geological time would result in highly radiogenic εHf values, decoupling them from εNd ratios. The high 176 Lu/ 177 Hf could in theory produce an isochronous relationship with 176 Hf/ 177 Hf over time; an errorchron is shown by clinopyroxene from mantle xenoliths from the northern Massif Central. However, in a review of the literature, we show that most mantle spinel peridotites do not show such high Lu/Hf ratios in their constituent clinopyroxenes, because they lack the distinctive Zr–Hf anomaly, and this limits the usefulness of the application of the Lu–Hf system of dating to garnet-free mantle rocks. Nevertheless, some mantle xenoliths from Poland or the Czech Republic may be amenable to Hf-isotope dating in the future.
    Keywords: Mantle ; Spinel peridotite ; Clinopyroxene ; France ; Poland ; Lu–Hf geochronology
    ISSN: 1437-3254
    E-ISSN: 1437-3262
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  • 2
    Language: English
    In: Geochimica et Cosmochimica Acta, 2008, Vol.72(19), pp.4825-4844
    Description: Ureilites are ultramafic achondrites that exhibit heterogeneity in mg# and oxygen isotope ratios between different meteorites. Polymict ureilites represent near-surface material of the ureilite parent asteroid(s). Electron microprobe analyses of 〉500 olivine and pyroxene clasts in several polymict ureilites reveal a statistically identical range of compositions to that shown by unbrecciated ureilites, suggesting derivation from a single parent asteroid. Many ureilitic clasts have identical compositions to the anomalously high Mn/Mg olivines and pyroxenes from the Hughes 009 unbrecciated ureilite (here termed the “Hughes cluster”). Some polymict samples also contain lithic clasts derived from oxidized impactors. The presence of several common distinctive lithologies within polymict ureilites is additional evidence that ureilites were derived from a single parent asteroid. In situ oxygen three isotope analyses were made on individual ureilite minerals and lithic clasts, using a secondary ion mass spectrometer (SIMS) with precision typically better than 0.2–0.4‰ (2SD) for δ O and δ O. Oxygen isotope ratios of ureilitic clasts fall on a narrow trend along the CCAM line, covering the range for unbrecciated ureilites, and show a good anti-correlation with mineral mg#. SIMS analysis identifies one ferroan lithic clast as an R-chondrite, while a second ferroan clast is unlike any known meteorite. An exotic enstatite grain is derived from an enstatite chondrite or aubrite, and another pyroxene grain with Δ O of −0.4 ± 0.2‰ is unrelated to any known meteorite type. Ureilitic olivine clasts with mg#s 〈 85 are much more common than those with mg# 〉 85 which include the melt-inclusion-bearing “Hughes cluster” ureilites. Thus melt was present in regions of the parent ureilite asteroid with a bulk mg# 〉 85 when the asteroid was disrupted by impact, giving rise to two types of ureilites: common ferroan ones that were residual after melting and less common magnesian ones that were still partially molten when disruption occurred. One or more daughter asteroids re-accreted from the remnants of the mantle of the proto-ureilite asteroid. Polymict ureilite meteorites represent regolith that subsequently formed on the surface of a daughter asteroid, including impact-derived material from at least six different meteoritic sources.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 3
    Language: English
    In: LITHOS, July 2011, Vol.125(1-2), pp.423-433
    Description: Scientific drilling of volcanic successions has been suggested as a way of establishing stratigraphic sequences of unexposed lava flows on large complex volcanoes, with the aim of in-depth study of magmatic processes and source geochemistry of otherwise inaccessible lava sequences. To simulate the core drilling of such sequences, lava flows from the prehistoric Vavalaci Centre exposed in the south wall of the Valle del Bove, Mount Etna, Sicily, were sampled in four stratigraphic sections. The fresh, generally strongly porphyritic Na-alkaline trachybasalts and trachyandesites show diverging sub-parallel trends of high- and low-alkali concentrations in total alkali versus silica (TAS) diagrams, whilst variations of other major and trace elements reveal two further distinct chemical groups enriched in K, REE and Ti which follow separate fractionation paths. A set of control samples was used to establish geochemical variations within a single lava flow. Primitive mantle normalised incompatible element patterns demonstrate that the lavas have highly enriched OIB signatures with a clear division in LREE, Ba, Th, Nb and Zr concentrations between the four different chemical groups. Comparison of data for the Vavalaci lavas with the compositions of other prehistoric, historic and recent eruptions of Mt. Etna indicates a temporal trend towards more basic magma compositions. The chemostratigraphy of the lavas was statistically analysed to give correlations between flows from different sections. Whilst a good number of geologically meaningful correlations were revealed, we can demonstrate that only one set of lavas was actually sampled in all four sections, whilst a number of unique lavas remain uncorrelated. Thus no individual section, or simulated borehole core, provided samples of the complete lava flow sequence without significant gaps in the stratigraphy. The trends in lava compositions are also defined in the stratigraphy, showing their evolution from low- to high-alkali lavas through the series, which may be related to temporal decrease in degree of partial melting of the mantle rather than through simple fractionation processes or mixing of magmas. ► Vavalaci Centre—mildly Na-alkaline lavas with diverging alkali enrichment. ► Separate fractionation/evolution paths for the high- and low-alkali groups. ► Geostratigraphical correlation of the lavas using a statistical approach with R. ► Schematic chronostratigraphy of the Vavalaci lavas. ► Simulation of core sampling using boreholes.
    Keywords: Mount Etna ; Vavalaci ; Alkaline Magmatism ; Stratigraphy ; Borehole Sampling ; Engineering ; Geology
    ISSN: 0024-4937
    E-ISSN: 1872-6143
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  • 4
    Language: English
    In: Gondwana Research, 2011, Vol.20(4), pp.655-672
    Description: This review considers the magmatic processes in the Carpathian–Pannonian Region (CPR) from Early Miocene to Recent times, as well as the contemporaneous magmatism at its southern boundary in the Dinaride and Balkans regions. This geodynamic system was controlled by the Cretaceous to Neogene subduction and collision of Africa with Eurasia, especially by Adria that generated the Alps to the north, the Dinaride–Hellenide belt to the east and caused extrusion, collision and inversion tectonics in the CPR. This long-lived subduction system supplied the mantle lithosphere with various subduction components. The CPR contains magmatic rocks of highly diverse compositions (calc-alkaline, K-alkalic, ultrapotassic and Na-alkalic), all generated in response to complex post-collisional tectonic processes. These processes formed extensional basins in response to an interplay of compression and extension within two microplates: ALCAPA and Tisza–Dacia. Competition between the different tectonic processes at both local and regional scales caused variations in the associated magmatism, mainly as a result of extension and differences in the rheological properties and composition of the lithosphere. Extension led to disintegration of the microplates that finally developed into two basin systems: the Pannonian and Transylvanian basins. The southern border of the CPR is edged by the Adria microplate via Sava and Vardar zones that acted as regional transcurrent tectonic areas during Miocene–Recent times. Major, trace element and isotopic data of post-Early Miocene magmatic rocks from the CPR suggest that subduction components were preserved in the lithospheric mantle after the Cretaceous–Miocene subduction and were reactivated especially by extensional tectonic processes that allowed uprise of the asthenosphere. Changes in the composition of the mantle through time support geodynamic scenarios of post-collision and extension processes linked to the evolution of the main blocks and their boundary relations. Weak lithospheric blocks (i.e. ALCAPA and western Tisza) generated the Pannonian basin and the adjacent Styrian, Transdanubian and Zărand basins which show high rates of vertical movement accompanied by a range of magmatic compositions. Strong lithospheric blocks (i.e. Dacia) were only marginally deformed, where strike–slip faulting was associated with magmatism and extension. At the boundary of Adria and Tisza–Dacia strike–slip tectonics and core complex extension were associated with small volume Miocene magmatism in narrow extensional sedimentary basins or granitoids in core-complex detachment systems along older suture zones (Sava and Vardar) accommodating the extension in the Pannonian basin and afterward Pliocene–Quaternary inversion. Magmas of various compositions appear to have acted as lubricants in a range of tectonic processes. ► In the CPR the magmatism is post-collisional and magmas were generated during extension and transtensional faulting. ► Calc-alkaline, Na-, K-alkalic and ultra-K magmas mirrors the composition of: crust, mantle lithosphere and asthenosphere. ► Magmatism enhances lithospheric weakness, acting as a lubricant for tectonic processes. ► Over time, the large amount of melting in the lithosphere diminished and was followed by asthenosphere uprise melting.
    Keywords: Magmatism ; Lithosphere ; Asthenosphere ; Extension ; Core-Complex-Related Magmatism ; Transtension-Related Magmatism ; Geology
    ISSN: 1342-937X
    E-ISSN: 1878-0571
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  • 5
    Language: English
    In: Geology, 02/01/2018, Vol.46(2), pp.171-174
    Description: Evidence for meteorite impacts in the geological record may include the presence of shocked minerals, spherule layers, and geochemical anomalies. However, it is highly unusual to find unmelted crystals from the actual impactor within an ejecta layer. Here we detail the first recorded occurrence of vanadium-rich osbornite (TiVN) on Earth, from two sites on Skye, northwest Scotland, which are interpreted as part of a meteoritic ejecta layer. TiVN has only previously been reported as dust from comet Wild 2, but on Skye it has been identified as an unmelted phase. Both ejecta layer sites also contain niobium-rich osbornite (TiNbN), which has not previously been reported. An extraterrestrial origin for these deposits is strongly supported by the presence of reidite (a high-pressure zircon polymorph), which is only found naturally at sites of meteorite impact. Barringerite [(Fe,Ni) (sub 2) P], baddeleyite (ZrO (sub 2) ), alabandite (MnS), and carbon-bearing native iron spherules, together with planar deformation features and diaplectic glass in quartz, further support this thesis. We demonstrate through field relationships and Ar-Ar dating that the meteorite strike occurred during the mid-Paleocene. This is the first recorded mid-Paleocene impact event in the region and is coincident with the onset of magmatism in the British Palaeogene Igneous Province (BPIP). The Skye ejecta layer deposits provoke important questions regarding their lateral extent at the base of the BPIP and the possibility of their presence elsewhere beneath the much larger North Atlantic Igneous Province.
    Keywords: Petrology Of Meteorites And Tektites ; Alloys ; Baddeleyite ; Barringerite ; Cenozoic ; Ejecta ; Europe ; Great Britain ; Hebrides ; Highland Region Scotland ; Impacts ; Inner Hebrides ; Inverness-Shire Scotland ; Isle Of Skye ; Meteorites ; Native Elements ; Nesosilicates ; Nitrides ; Orthosilicates ; Osbornite ; Oxides ; Paleocene ; Paleogene ; Reidite ; Scotland ; Silicates ; Tertiary ; United Kingdom ; Western Europe ; Zircon Group;
    ISSN: 0091-7613
    E-ISSN: 19432682
    Source: CrossRef
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  • 6
    Language: English
    In: Geology, April, 2006, Vol.34(4), p.237(4)
    Description: Subcontinental lithospheric mantle xenoliths from beneath the French Massif Central contain clinopyroxene with high Lu/Hf and Hf isotope ratios ([sup.176]Lu/[sup.177]Hf = 0.1-10.8; [[epsilon].sub.Hr] = +40 to +2600). These Lu-Hf isotope systematics yield model ages of 313-377 Ma and apparently date melt extraction in a mantle wedge during Variscan subduction, with the residual mantle subsequently forming the stabilizing mantle keel of young European continental crust. The extremely depleted Lu-Hf mantle systematics contrast with other isotopic signatures (Sr, Pb, Nd) that have been overprinted by mantle metasomatism. The high Lu/Hf values developed in mantle clinopyroxene by melt extraction, coupled with the robustness of this system to some types of metasomatism, provide a new chronometer for directly dating crust-mantle differentiation in both young and ancient continental regions. The process of metasomatic decoupling of Nd and Hf isotopes, which has been previously recognized in Hawaiian oceanic mantle and in French Massif Central subcontinental mantle, may be responsible for the prominent displacement of terrestrial rocks to lower [[epsilon].sub.Nd] at a given [[epsilon].sub.Hf] as compared to Bulk Earth reference values obtained from analysis of chondritic meteorites. Keywords: Lu-Hf isotopes, subcontinental lithosphere, mantle xenoliths, geochronology, Variscan orogeny, Europe.
    Keywords: Mantle (Geology) -- Environmental Aspects ; Orogeny -- Environmental Aspects ; Xenoliths -- Properties
    ISSN: 0091-7613
    E-ISSN: 19432682
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  • 7
    Language: English
    In: Geosphere, 06/01/2018, Vol.14(3), pp.926-950
    Description: Miocene (ca. 18 Ma) subduction-related basalts and basaltic andesites from Monte Arcuentu, southern Sardinia, Italy, show a remarkable correlation between (super 87) Sr/ (super 86) Sr (from approximately 0.705 to approximately 0.711) over a small range of SiO (sub 2) ( approximately 51-58 wt%) that contrasts with most other orogenic volcanic suites worldwide. New high-precision Pb and Hf isotope data help to constrain the petrogenesis of these rocks. The most primitive Monte Arcuentu rocks (MgO 〉8.5 wt%) were sourced from a mantle wedge metasomatized by melts derived from terrigenous sediment, likely derived from Archean terranes of northern Africa. This gave rise to magmas with high (super 87) Sr/ (super 86) Sr (0.705-0.709) and (super 207) Pb/ (super 204) Pb (15.65-15.67) with moderate epsilon Hf (-1 to +8) and epsilon Nd (-6 to +1), but it does not account for the full range of compositions observed. More evolved rocks (MgO 〈8.5 wt%) have higher (super 87) Sr/ (super 86) Sr (up to 0.711) and (super 207) Pb/ (super 204) Pb (up to 15.68), with epsilon Hf and epsilon Nd as low as -8 and -9, respectively. Mixing calculations suggest that evolved rocks with low Rb/Ba and low (super 206) Pb/ (super 204) Pb interacted with lower crust similar compositionally to that exposed today in Calabria, Italy, which was formerly in crustal continuity with Sardinia. High Rb/Ba and high (super 206) Pb/ (super 204) Pb magmas interacted with lithospheric mantle similar to that sampled by Italian lamproites. Partial melting of lower crustal and upper mantle lithologies was facilitated by the rapid extension, and subsequent passive mantle upwelling, that occurred as Sardinia drifted away from the European plate during the Oligo-Miocene (ca. 32-15 Ma). Fractional crystallization under these PT conditions involved olivine + clinopyroxene with little or no plagioclase, such that differentiation proceeded without significant increase in SiO (sub 2) . The Monte Arcuentu rocks provide insights into assimilation process in the lower crust and lithospheric mantle that may be obscured by upper crustal assimilation-fractional crystallization (AFC) processes in other orogenic suites.
    Keywords: Isotope Geochemistry ; Igneous And Metamorphic Petrology ; Solid-Earth Geophysics ; Andesites ; Basalts ; Cenozoic ; Crust ; Enrichment ; Europe ; Fractional Crystallization ; Geochemistry ; Hafnium ; Hf-177/Hf-176 ; Igneous Rocks ; Isotope Ratios ; Isotopes ; Italy ; Lead ; Magma Contamination ; Magmas ; Mantle ; Metals ; Miocene ; Models ; Monte Arcuentu ; Nd-144/Nd-143 ; Neodymium ; Neogene ; Pb-206/Pb-204 ; Pb-207/Pb-204 ; Pb-208/Pb-204 ; Plate Tectonics ; Radioactive Isotopes ; Rare Earths ; Sardinia Italy ; Southern Europe ; Stable Isotopes ; Subduction ; Tertiary ; Volcanic Rocks;
    ISSN: Geosphere
    E-ISSN: 1553-040X
    Source: CrossRef
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  • 8
    Language: English
    In: Tectonophysics, 2011, Vol.502(1), pp.146-157
    Description: The south-eastern part of the Carpathian–Pannonian region records the cessation of convergence between the European platform/Moesia and the Tisza–Dacia microplate. Plio-Quaternary magmatic activity in this area, in close proximity to the ‘Vrancea zone’, shows a shift from normal calc-alkaline to much more diverse compositions (adakite-like calc-alkaline, K-alkalic, mafic Na-alkalic and ultrapotassic), suggesting a significant change in geodynamic processes at approximately 3 Ma. We review the tectonic setting, timing, petrology and geochemistry of the post-collisional volcanism to constrain the role of orogenic building processes such as subduction or collision on melt production and migration. The calc-alkaline volcanism (5.3–3.9 Ma) marks the end of normal subduction-related magmatism along the post-collisional Călimani–Gurghiu–Harghita volcanic chain in front of the European convergent plate margin. At ca. 3 Ma in South Harghita magma compositions changed to adakite-like calc-alkaline and continued until recent times (〈 0.03 Ma) interrupted at 1.6–1.2 Ma by generation of Na and K-alkalic magmas, signifying changes in the source and melting mechanism. We attribute the changes in magma composition in front of the Moesian platform to two main geodynamic events: (1) slab-pull and steepening with opening of a tear window (adakite-like calc-alkaline magmas) and (2) renewed contraction associated with deep mantle processes such as slab steepening during post-collisional times (Na and K-alkalic magmas). Contemporaneous post-collisional volcanism at the eastern edge of the Pannonian Basin at 2.6–1.3 Ma was dominated by Na-alkalic and ultrapotassic magmas, suggesting a close relationship with thermal asthenospheric doming and strain partitioning related to the Adriatic indentation. Similar timing, magma chamber processes and volume for K-alkalic (shoshonitic) magmas in the South Apuseni Mountains (1.6 Ma) and South Harghita area at a distance of ca. 200 km imply a regional connection with the inversion tectonics.
    Keywords: Post-Collisional Magmatism ; Slab Mechanics ; Adakite Magmas ; Alkalic Magmas ; Geology
    ISSN: 0040-1951
    E-ISSN: 1879-3266
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  • 9
    In: Journal of Petrology, 2014, Vol. 55(12), pp.2457-2480
    Description: Oxidation state is a sensitive indicator of geochemical processes within the upper mantle. Here we report results of a regional study of the oxidation state of spinel peridotite xenoliths from 45 volcanic centers distributed over ∼20 000 km 2 in the Massif Central, France. The Δlog f O 2 values relative to the fayalite–magnetite–quartz oxygen buffer (FMQ) were determined from the equilibrium between the Fe-bearing components in olivine, orthopyroxene and spinel, with the Fe 3+ content of spinel measured either by Mössbauer spectroscopy or by electron microprobe using secondary spinel standards. For the entire suite of samples, Δlog f O 2 values range between FMQ – 0·47 and FMQ + 1·66. Our data confirm the presence of two distinct lithospheric mantle domains, previously reported in the literature, lying north and south of 45°30'N, respectively. The northern domain, with its refractory bulk composition, tends to record more oxidized conditions, having Δlog f O 2 values mostly at or above FMQ + 1. The Δlog f O 2 in the southern domain is more variable, including values below FMQ. Assuming that increasing equilibration temperatures among xenoliths reflect increasing depths of origin, samples from the northern domain suggest that the shallower part of the subcontinental lithospheric mantle (SCLM) is somewhat more oxidized than at deeper levels. On the other hand, such a general observation cannot be made for the southern domain. The high Δlog f O 2 values of harzburgites suggest that they are more sensitive to resetting of their oxidation state by metasomatism than lherzolites. In terms of modally metasomatized xenoliths, the ‘melt’ leading to the addition of clinopyroxene apparently had a higher oxidation state (Δlog f O 2 〉 FMQ + 1) than the agent responsible for crystallization of amphibole (Δlog f O 2 ∼ FMQ + 0·6). Furthermore, amphibole-bearing and amphibole-free peridotites exhibit the same range in f O 2 . Cryptic metasomatism can also reset oxidation state, sometimes very effectively. Metasomatic processes are probably the reason why the xenolith suite from the Massif Central records relatively high Δlog f O 2 values compared with ‘normal’ non-cratonic SCLM. This study demonstrates the utility of using oxidation state to help characterize and delineate domains in the lithospheric mantle.
    Keywords: Oxidation State, Mantle Peridotite, Metasomatism, Lithospheric Mantle, Massif Central
    ISSN: 0022-3530
    E-ISSN: 1460-2415
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  • 10
    In: Journal of Petrology, 2017, Vol. 58(8), pp.1567-1589
    Description: Hf isotope data for zircons and whole-rocks from lower crustal mafic granulite and pyroxenite xenoliths from NW Russia are presented together with the results of U–Pb zircon dating, Sm–Nd and Rb–Sr isotopic compositions of bulk-rocks and minerals, and trace element compositions of minerals. Most zircons preserve a record of only the youngest metamorphic events, but a few Grt-granulite xenoliths retain Archean magmatic zircons from their protolith. Metamorphic zircons have highly variable ɛHf( t ) values from –25 to –4. The least radiogenic zircons were formed by recrystallization of primary magmatic Archean zircons. Zircons with the most radiogenic ɛHf grew before garnet or were contemporaneous with its formation. Zircons with ɛHf( t ) from –15 to –9 formed by various mechanisms, including recrystallization of pre-existing metamorphic zircons, subsolidus growth in the presence of garnet and exsolution from rutile. They inherited their Hf isotopic composition from clinopyroxene, pargasite, rutile and earlier-formed zircon that had equilibrated with garnet. Subsolidus zircons were formed in response to a major change in mineral association (i.e. garnet- and zircon-producing reactions including partial melting). Recrystallized zircons date the onset of high-temperature conditions without a major change in mineral association. Age data for metamorphic zircons fall into five groups: 〉1·91 Ga, 1·81–1·86 Ga, 1·74–1·77 Ga, 1·64–1·67 Ga and 〈1·6 Ga. Most ages correlate with metamorphic events in the regional upper crust superimposed onto rocks of the Belomorian belt during formation of the Lapland Granulite Belt. Zircon formation and resetting at 1·64–1·67 Ga significantly postdates Lapland–Kola orogenic events and may relate to the onset of Mesoproterozoic rifting. The youngest ages (1·6–1·3 Ga) correspond to an event that affected only a few grains in some samples and can be explained by interaction with a localized fluid. The observed garnet-granulite associations were formed at 1·83 Ga in Arkhangelsk xenoliths and 1·74–1·76 Ga in most Kola xenoliths. By the end of the Lapland–Kola orogeny, the rocks were already assembled in the lower crust. However, no addition of juvenile material has been detected and preservation of pre-Lapland–Kola metamorphic zircon indicates that some xenoliths represent an older lower crust. Granulites, pyroxenites and Phl-rich rocks have a common metamorphic history since at least c. 1·75 Ga. At about 1·64 Ga metasomatic introduction of phlogopite took place; however, this was only one of several phlogopite-forming events in the lower crust.
    Keywords: Lower Crust ; Xenoliths ; Zircon ; Hf Isotopes ; Kola Peninsula
    ISSN: 0022-3530
    E-ISSN: 1460-2415
    Source: Oxford University Press
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