1,721,133 research outputs found
In situ Pb geochronology of zircon with laser ablation-inductively coupled plasma-sector field mass spectrometry
No full textThe in situ Pb geochronological capabilities of a laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) coupling a magnetic sector ICP-MS with a Nd:YAG laser probe working at 213 nm have been tested on three zircon populations with different age (150-294-577 Ma) and radiogenic Pb contents (0.7-10-40 ppm). The influence of scan mode and spatial resolution on age precision and accuracy has also been evaluated. All the signals necessary to independently determine the 206Pb/238U, 207Pb/235U, 207Pb/206Pb and 208Pb/232Th ratios have been acquired. The external standardization approach has been used to correct for laser induced U/Pb elemental fractionation, instrumental mass bias and sequential acquisition of transient signals. The efficiency of the external standardization correction has been carefully evaluated in each analytical session and residual error with a spot size of 40 μm has been estimated to be in the 0.1-1.5% range. Geologically meaningful ages can be achieved in zircons with less than 1 ppm of radiogenic Pb and with a spatial resolution down to 20 μm. With a spatial resolution of 40 μm, the E-scan mode is more efficient and offers, for zircon with about 40 ppm of radiogenic Pb, an internal precision (2σ) on the apparent age to better than 1.1% for all isotope ratios. At lower radiogenic Pb contents, internal precision decreases and, for zircons with radiogenic Pb contents lower than 1 ppm, is better than 7% and 2% on the 207Pb/235U and 206Pb/238U ages, respectively. Accuracy is strictly related to Pb* in zircon and ranges between 1% and 5%. At a 20 μm of spot size, internal precision and accuracy are approximately 1.5-2 times lower than at 40 μm
Age determination of Ordovician magmatism in NE Sardinia and its bearing on Variscan basement evolution
No full textA newly discovered pre-Variscan cover-basement relationship in NE Sardinia is defined by differences in structural and metamorphic histories. The oldest metamorphic fabrics in the basement are absent from a series of metaplutons that are inferred to be comagmatic with metavolcanic rocks in the cover. Cover and basement are further distinguished by contrasting metapelite compositions. Superimposed metamorphism is represented by pre-Variscan garnet and Variscan plagioclase porphyroblasts that appear in the field as a 'mixed' garnet + albite zone. In situ U-Pb zircon dating of meta-igneous rocks from cover and basement by laser-ablation inductively coupled plasma-mass spectrometry gives ages of 458 ± 7 Ma for the Tanaunella orthogneiss, 456 ± 14 Ma for the Lodè orthogneiss, and 474 ± 13 Ma for the Lula porphyroid. The internal Sardinian Variscides thus incorporate a mid-Ordovician magmatic belt developed upon an older basement; correlation is proposed with a tectonostratigraphic domain that is at present exposed in the Alps. Within the mid-Ordovician peri-Gondwanan realm, the internal Sardinian Variscides appear to have developed along an active margin, whereas the foreland correlates with the Ibero-Aquitaine domain and is related to the evolution of a passive margin during rifting and detachment of Avalonia to form the Rheic Ocean
The contribution of amphibole from deep arc crust to the silicate Earth's Nb budget
No full textThe continental crust (CC) and the depleted mantle (DM) are generally assumed to be complementary reservoirs within the Earth. However, the mixture between CC and upper mantle does not generate the Nb/Ta and Nb/La ratios of chondrites. A reservoir with superchondritic ratios for Nb/Ta and Nb/La is thus required in the Earth's system. The occurrence of a hidden amphibole reservoir in the lower arc crust has been recently proposed. This, coupled with the capability of calcic amphibole to give rise to a superchondritic Nb/Ta and Nb/La reservoir, led us to determine to what extent amphibole-rich ultramafic rocks can account for the Nb (and Nb/Ta, Nb/La as well) imbalance on Earth. We have considered lower crust mafic and ultramafic amphibole-rich intrusive rocks fromcollisional settingsworldwide. Because CC is considered to have primarily formed in collisional setting these rocks are important for its geneticmodel. We modeled Nb, Ta and La contents of the hidden Nb reservoir by mass balance calculations between continental crust, depleted mantle and primitive mantle. Modeling shows that amphibole-rich mafic lower crust can solve the so-called Nb paradox if large volumes of materials are supposed to be returned into the mantle during the Earth's history. A possible mechanism is recycling, particularly in Precambrian times, of eclogites that underwent pre-eclogitic melting in the amphibolite facies field and then recrystallized under eclogite-facies conditions
U-Th-Pb “multi-phase” approach to the study of crystalline basement: application to the northernmost sector of the Ivrea-Verbano Zone (Alps)
Full text linkIn situ U-Pb geochronology was carried out on amphibolites and siliciclastic metasediments of the Kinzigite Formation exposed in the northernmost sector of the Ivrea-Verbano Zone (Finero area). The aim is to shed light on the tectono-metamorphic evolution of this intermediate-lower crustal section and its bearing with the evolution of the southern and better known sectors of the Ivrea-Verbano Zone. Based on eld observation and petrography, a metamorphic gradient gently increasing from amphibolite to upper amphibolite facies (from SE to NW) characterizes the whole metamorphic sequence. Granulite facies conditions are reached only by the slivers (septa) of the Kinzigite Formation into the External Gabbro unit of the Finero Ma c Complex. Metapelites consist mainly of biotite, quartz, plagioclase, garnet, and brolitic to prismatic sillimanite; muscovite progressively disappears as K-feldspar appears and becomes abundant. Amphibolites are made of green-brown hornblende and plagioclase and may contain clinopyroxene de ning thin layers together with plagioclase and titanite. Both metapelites and amphibolites show mylonitic deformation which is more intense towards NW, i.e. towards the lower structural levels. The mylonitic deformation strongly affected the lower crustal metabasic rocks of External Gabbro unit (Finero Ma c Complex). Zircon, monazite and titanite U-Pb geochronology was carried out with laser ablation (LA)- ICP-MS on amphibolites, migmatitic paragneiss and septa. The multi-chronological approach allowed recognizing three discrete tectono-metamorphic events, at Permian, Triassic and Jurassic. Zircon and monazite yielded Permian ages suggesting (re)crystallization during an high temperature event characterized by both metamorphism and magmatism. Titanite dating provided Triassic and Jurassic ages, that were interpreted as U-Pb resetting ages. A Triassic perturbation of the U-Pb system was also recorded by zircon and monazite as rare domains.
The tectono-metamorphic reconstruction of the evolution of the northernmost Ivrea- Verbano Zone, as revealed by the new geochronological data, is only partially in agreement with the temperature-time evolutions depicted for the southern sectors of the Ivrea-VerbanoZone. Permian ages indicating magmatism and high temperature metamorphism are common throughout the Ivrea-Verbano Zone, as well as the Jurassic ages related to local thermal pulses likely related to lithospheric thinning and associated lithospheric advection. Conversely, the occurrence of Triassic ages is peculiar of the Finero area. Two possible explanations may account for this Triassic event: Triassic ages are possibly related to the thermal effect and uid circulation during the emplacement of the External Gabbro unit; or alternatively, they are the response to the ductile deformation largely recognized in the whole area. This study is a further evidence of the necessity of approaching crystalline basement with multiple geochronometers in order to unravel the complete tectono-metamorphic evolution
Origin of titanian pargasite in gabbroic rocks from the Northern Apennine ophiolites (Italy): insights into the late-magmatic evolution of a MOR-type intrusive sequence
No full textGabbroic rocks from Northern Apennine ophiolites contain accessory titanian pargasite, in interstices between plagioclase and clinopyroxene, and as rims around interstitial Fe-Ti-oxide phases. The origin of titanian pargasite has been evaluated by combining major, volatile and trace element microanalyses. Titanian pargasites show variable amounts of F and low Cl (0.03-0.23 and ≤0.03 wt%, respectively), and mg value ranging from 0.78 to 0.70 and from 0.61 to 0.53 in Mg- and Fe-rich rocks, respectively. Geothermometric calculations based on amphibole-plagioclase equilibrium yield temperatures of 900 ± 50°C and 840 ± 50°C for Mg- and Fe-rich rocks, respectively. Titanian pargasites are characterized by LREE depletion, nearly flat HREE and variable negative Eu anomalies. Total REE contents in titanian pargasites are higher and lower than in associated clinopyroxenes and apatites, respectively. In the chondrite-normalized patterns of titanian pargasites, Ba, K and Sr are markedly depleted relative to LREE, whereas Nb, Zr and Ti are slightly enriched to slightly depleted relative to neighboring REE. A separate of titanian pargasite from a Fe-rich gabbroic rock was analyzed for Sr isotopic composition; its initial 87Sr/86Sr falls within the range of modern N-MORB and is consistent with those of fresh Mg-rich gabbroic rocks. Titanian pargasite most likely formed by an igneous liquid with relatively high H2O content (4.4-6.1 wt%) and a slight LREE enrichment. The origin of this liquid has been ascribed to the percolation in the gabbroic crystal mush of a H2O-rich agent of igneous origin, WHICH could be a trondhjemite liquid or an exsolved fluid. Probably, such interaction triggered a post-cumulus crystallization process that finally yielded the precipitation of titanian pargasite
Trace-element incorporation in titanite: Constraints from experimentally determined solid/liquid partition coefficients
No full textTitanite/liquid partition coefficients for most of the trace elements relevant in petrogenetic studies are provided for titanite-saturated liquids equilibrated at 1.5 GPa and 850 °C starting from lamproitic compositions. The high compatibility observed for REE, HFSE, Sr, V and Sc, and the strong incompatibility observed for actinides, large ion lithophile and light elements are discussed in terms of available crystal-chemical mechanisms for incorporation and crystal-structure control.The exchange vectors Na1 Ca-1 and Al1 Ti-1 allow local charge balance to be achieved after incorporation of REE3+ and R5+ at the Ca and Ti site, respectively. The significant amounts of H measured are also relevant in this regard [the exchange mechanism being (OH)1 O-1 at the O1 site]. The incorporation of U4+ and Th4+ at the Ca site is scarce, and is likely balanced by that of Mg2+ at the Ti site; both these substitutions are responsible for strong structural strain.Titanite can thus be considered an important repository for REE and HFSE in metamorphic and igneous rocks, and its role must be accounted for when modelling trace-element residence during metamorphic reactions and late magmatic crystallisation history. Due to the measured differences in compatibility, titanite crystallisation increases the values of Nb/Ta and LREE/HREE ratios in residual liquids. The similar compatibility of U and Pb makes titanite suitable for U-Pb geochronology of igneous rocks only after common Pb correction. Finally, this study confirms that the titanite end member is not suitable for radioactive waste disposal due to the discussed crystal-chemical constraints, and that titanite-based waste forms should contain high amounts of Na+ and Mg2+
N-MORB crust beneath Fuerteventura in the easternmost part of the Canary Islands: evidence from gabbroic xenoliths
No full textGabbro xenoliths reported in this paper were collected in northern Fuerteventura, the Canary Island located closest to the coast of Africa. The xenoliths are very fresh and consist of Ti-Al-poor clinopyroxene + plagioclase (An87-67) + olivine (Fo72-86) ± orthopyroxene. Clinopyroxene and orthopyroxene are constantly and markedly depleted in light rare earth elements (LREE) relative to heavy REE (HREE), as expected for cumulus minerals formed from highly refractory N-MORB-type melts. In contrast, whole-rock Primordial Mantle-normalized trace element patterns range from mildly S-shaped (mildly depleted in Pr-Sm relative to both the strongly incompatible elements Rb-La and the HREE) to enriched. Estimates show that the trace element compositions of the rocks and their minerals are compatible with formation as N-MORB gabbro cumulates, which have been infiltrated at various extents (≤ 1% to > 5%) by enriched alkali basaltic melts. The enriched material is mainly concentrated along grain boundaries and cracks through mineral grains, suggesting that the infiltration is relatively recent, and is thus associated with the Canary Islands magmatism. Our data contradict the hypothesis that a mantle plume was present in this area during the opening of the Atlantic Ocean. No evidence of continental material that might reflect attenuated continental crust in the area has been found. Gabbro xenoliths with REE and trace element compositions similar to those exhibited by the Fuerteventura gabbros are also found among gabbro xenoliths from the islands of La Palma (western Canary Islands) and Lanzarote. The compositions of the most depleted samples from these islands are closely similar, implying that there was no significant change in chemistry during the early stages of formation of the Atlantic oceanic crust in this area. Strongly depleted gabbros similar to those collected in Fuerteventura have also been retrieved in the MARK area along the central Mid-Atlantic Ridge. The presence of N-MORB oceanic crust beneath Fuerteventura implies that the continent-ocean transition in the Canary Islands area must be relatively sharp, in contrast to the situation both further north along the coast of Morocco, and along the Iberian peninsula
Detection, crystal-chemical mechanisms and petrological implications of Ti-[6](4+) partitioning in pargasite and kaersutite
No full textFe-free kaersutites with different titanium contents were synthesised from oxide mixtures of "Fe-free basanite" and "Fe-free alkali-olivine basalt" compositions by equilibration at upper-mantle conditions (T = 1070 degrees C, P = 1.4 GPa) and quenching. Single-crystal structure-refinement and EMP + SIMS analyses performed on the same crystals show that Ti-[6](4+) enters all the three independent octahedral sites. The resulting site-population can be related to two different crystal-chemical mechanisms, which can be described by means of the (M(1))Ti4+ (O(3))O-2(2- M(1))Mg--1(2+) (O(3))OH2-- and M-(2,M-3)Ti4+ (T(1,2))Al-2(3+) (M(2,3))Mg--1(2+) (T(1,2))Si--2(4+) exchange vectors. They both increase amphibole stability at high temperatures.
The presence of Ti4+ at M(3) can be now straightforwardly detected from a significant increase of the equivalent-isotropic atomic displacement-parameter and confirmed by inspection of the M(3)-O bond lengths; when Ti4+ occupies M(3), short [M(3)-O] distances couple with shorter M(3)-O(I) and longer M(3)-O(3) than expected in the presence of (M(3))Fe3+ balancing for dehydrogenation. Re-consideration of previous refinements shows that (M(3))Ti4+ occurs only in a few titanian pargasite and kaersutite among the similar to 200 in the CSCC database, and is always related to low amounts of Al-[6]; ironically: it was most important in a kaersutite from the type locality. The entrance of Ti4+ at the M(3) site is thus far less favoured than that of Al and Fe3+; this fact is most probably a consequence of steric constraints and electrostatic repulsion due to the simultaneous presence of a small tetravalent cation in all the three independent sites of the octahedral strip
On the recycling of amphibole-rich ultramafic intrusive rocks in the arc crust: evidence from Shikanoshima Island (Kyushu, Japan)
No full textNew insights into the role of amphibole in arc magma petrogenesis are provided by the mineral chemistry and U-Pb geochronology of Cretaceous amphibole-rich mafic rocks and associated granitoids from Shikanoshima Island (Kyushu, Japan). In the northeastern part of Shikanoshima Island a relatively large body (about 600 m in length) of amphibole-rich mafic rocks is found within granodiorite host-rocks. The core of the mafic body consists of amphibole-rich gabbrodiorite with a porphyritic texture. Towards the host granodiorite the porphyritic texture is progressively lost and a band of relatively homogeneous medium- to fine-grained mafic rock marks the boundary with the granitoid rocks. The amphibole-rich porphyritic gabbrodiorite consists of large amphibole grains (up to 60 vol. %) characterized by brown cores, occasional inclusions of clinopyroxene, and green rims. These large amphibole grains are dispersed in a fine-grained matrix consisting of green amphibole, clinopyroxene and plagioclase. Literature whole-rock data on the mafic rocks from Shikanoshima Island suggest that they are the intrusive counterparts of high-Mg andesite (HMA). Major and trace element mineral compositions reveal a marked chemical contrast between the brown amphibole (and its inclusions) and the matrix minerals, suggesting that they are not on the same liquid line of descent. The brown amphibole and its clinopyroxene inclusions were inherited from amphibole-rich ultramafic intrusive crustal rocks (e.g. hornblendites) crystallized from a melt with a chemical composition close to that of continental arc basalts. U-Pb geochronological data suggest that the xenocrystic material is about 20 Myr older than the matrix minerals. The matrix mineral crystallized from a parental liquid similar to sanukite-type HMA and with a trace element signature characterized by strong enrichment in elements with high crustal affinity and depletion in heavy rare earth elements. Green amphibole is a common mineral in all the studied lithologies; this allowed us to monitor the compositional variations in the liquid from which it crystallized moving from the core of the mafic complex to the host granodiorite. The data reveal that the interstitial melt had interacted with a melt enriched in elements with a high crustal affinity that, given the close association in the field, is inferred to be the host granitoid. These results favour an origin for sanukite-type HMA not from primary mantle melts but from mantle melts that have been affected by crustal processes and have been contaminated by crustal material. The major and trace element composition of the brown amphibole from the Shikanoshima Island mafic rocks is compared with that of brown amphibole from other amphibolite-rich intrusive rocks in orogenic settings worldwide (Alpine chain and Ross Orogen). The observed similarities suggest that the amphibole-rich mafic rocks are the expression of a magmatic process with a common geochemical affinity that is independent of the age and local geodynamic setting and thus related to a specific petrogenetic process. Amphibole-rich mafic and ultramafic intrusive rocks could be a common feature of all collisional systems and thus represent a 'hidden' amphibole reservoir in the arc crust. We show that amphibole plays a major role in the petrogenesis of sanukite-type HMA but is also expected to play a major role in the differentiation of many other arc magmas
New constraints on the origin and age of Variscan eclogitic rocks (Ligurian Alps, Italy)
No full textGabbro and eclogite boudins are preserved within the amphibolites of the composite para- and ortho-gneiss Variscan basement of the Savona Crystalline Massif (Ligurian Briançonnais, Italy). Whole rock trace element patterns, low initial εNd (+5.4 to +8.8) data and trace element analyses on relict igneous clinopyroxene revealed that the mafic rocks were derived from depleted mantle melts, which most likely underwent crustal contamination during emplacement. Gabbros have a cumulus origin controlled by clinopyroxene and plagioclase segregation, whereas the eclogites represent evolved melts. U-Pb and trace element micro-analyses on zircons separated from one amphibolitised gabbro and one eclogite help to constrain coeval ages at -468 Ma for their igneous protoliths. The occurrence of a few inherited zircons confirms the involvement of a crustal component in the petrogenesis of the mafic rocks. In the eclogite, concordant zircon ages younger than the protolith age testify to metamorphic re-crystallisation (or new growth) from about 420 to 305 Ma. Zircon textures and trace element compositions indicate that eclogite facies metamorphism occurred 392-376 Ma ago. The younger zircon portions yielding a mean Concordia age of 333 ± 7 Ma are related to equilibration or new growth during the post-eclogite, amphibolite-facies equilibration
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