Academic literature on the topic 'Radiogenic isotopes (Sr-Nd-Pb)'

Nd isotopic composition has been determined for 16 igneous rocks, representing the wide geochemical, spatial and temporal range of post-collisional, late Cenozoic magmas in the Aegean area. Nd isotopes are used to further interpret previously published Pb and Sr isotope data. The overall pattern of late Cenozoic volcanism resulted from rapid extension, with thermal effects causing melting of hydrated, enriched, subcontinental lithosphere to produce widespread K-rich magmas. Slab break-off and intrusion of hot asthenosphere caused partial melting of rift-related continental margin basalts at the detachment point to generate adakitic magmas. Further outboard, mafic magma from enriched lithospheric mantle melted thickened lower crust to produce the granitoid plutons of the Cyclades. Nd isotopic variation in these varied rock types correlates with pre-Cenozoic palaeo-geography. Proterozoic subduction-related enrichment in Th and U, together with other large-ion lithophile elements, produced distinctive Pb isotope composition. This was later modified where Mesozoic subduction of terrigenous sediment was important, whereas subduction of oceanic carbonate sediments produced enrichment in radiogenic Sr and low Ce/Sr ratios. Late Cenozoic magmas sourced in eastern Pelagonian zone sub-continental lithospheric mantle have Nd model ages of about 1.0 Ga, and generally high 87Sr/86Sr and high 207Pb/204Pb (∼ 15.68) and 208Pb/204Pb (∼ 39.0) for low 206Pb/204Pb (∼ 18.6), but rocks to the west have more radiogenic Pb and higher Ce/Sr as a result of greater subduction of terrigenous sediment from the northern Pindos ocean. Magmas sourced from sub-continental lithosphere beneath the Apulian continental block were strongly influenced by subduction of oceanic crust and sediments north of the passive margin of north Africa. Subduction of Nile-derived terrigenous sediment in the east resulted in Nd model ages of 0.7 to 0.8 Ga and radiogenic Pb isotopes. Greater subduction of oceanic carbonate in the west resulted in magmas with higher 87Sr/86Sr and lower Ce/Sr. The strongly negative εNd for adakites in the central Aegean rules out a source from subducted oceanic basalt, and the adakite magma was probably derived from melting of hydrated Triassic sub-alkaline basalt of continental origin. Where trachytic rocks are succeeded by nepheline-normative basalts (e.g. Samos), Nd isotope data imply that early partial melting of the enriched subcontinental lithospheric mantle involved hydrous amphibole and phlogopite, but once these minerals were consumed, younger magmas were produced by partial melting dominated by olivine and orthopyroxene.

Recent alkaline lavas that have erupted across the disparate terranes of the northern Canadian Cordillera provide natural probes with which to interrogate the underlying lithosphere. The lavas range between two compositional end members, olivine nephelinite (NEPH) and hypersthene-normative olivine (Hy-NORM) basalt. The chemical signature of amphibole in the incompatible element enriched NEPH end member indicates that it is derived in the lithospheric mantle. The Hy-NORM end member is characterized by lower incompatible trace element contents but is still relatively enriched relative to primitive mantle. Although the Hy-NORM end member is always more radiogenic in Pb and Sr isotopes and less radiogenic in Nd isotopes than the NEPH end member, its isotopic signature varies with tectonic belt. In particular, Hy-NORM basalts in the Omineca Belt are strikingly more radiogenic in Sr and Pb isotopes and less radiogenic in Nd isotopes than otherwise equivalent Hy-NORM basalts in the adjacent Intermontane Belt, indicating the existence of a major lithospheric boundary between the two belts. Cordilleran and other continental Hy-NORM basalts have distinctly low Ca and high Na contents compared with their equivalents in oceanic hot spots or at mid-ocean ridges. A comparison with experimental melts of mantle peridotite indicates that these characteristics reflect smaller degrees of partial melting (<10%) in the stability field of garnet in the lower lithospheric mantle beneath the northern Cordillera. Contrary to the conclusion commonly drawn from experimental results, the Cordilleran NEPH lavas may be derived from similar or shallower depths than coeval Hy-NORM basalts.

The protolith age of high-grade metamorphic rocks exposed in structurally deep parts of the Omineca Crystalline Belt has been the subject of investigation and controversy for decades. We have applied multiple isotopic dating techniques to rocks of three structural culminations: the Monashee complex (which includes the Frenchman Cap and Thor–Odin gneiss domes), the Grand Forks horst, and the Vaseaux Formation, which lies in the footwall of the Okanagan Valley fault.Frenchman Cap core gneisses contain highly radiogenic Sr that scatters about a 2206 ± 117 Ma (1σ) Rb–Sr isochron with 87Sr/86Sr initial ratio of 0.700 ± 0.002. Monazite and zircon dates for the same rocks are 1851 ± 7 to 2103 ± 16 Ma (only U–Pb dates are given with 2σ errors), with lower intercepts from about 100 to 300 Ma. Sm–Nd whole-rock and crustal-residence (TDM) dates are 2.3 ± 0.2 Ga. Mafic–felsic layering in the core gneiss is also of Early Proterozoic age. There is no geochronometric evidence for Late Proterozoic or Mesozoic migmatization.Frenchman Cap mantling gneisses, including samples from above the Monashee décollement, have radiogenic Sr and unradiogenic Nd compositions that are not consistent with current inferences of a Late Proterozoic to Paleozoic depositional age. Two intrusive granitic rocks, which cut mantling gneiss, are either Early Proterozoic or Mesozoic–Cenozoic with a Proterozoic Sr isotopic signature acquired by assimilation of core gneiss. One other intrusive studied is probably Paleocene Ladybird granite. The age of the mantling gneiss is not yet consistently resolved.Grand Forks Gneiss Unit I paragneiss gives radiogenic whole-rock Sr, zircon U–Pb upper intercept, and Sm–Nd whole-rock crustal-residence dates of 1.7 ± 0.4 Ga, 1681 ± 3 Ma (2σ, but the apparent high precision is very dependent on the assumption made about the time of Pb loss), and 1.9 ± 0.3 Ga, respectively. Unit II and younger Grand Forks Gneiss units are Late Proterozoic or Phanerozoic. All isotope systems have been considerably reset on a centimetre to metre scale by Mesozoic–Cenozoic regional metamorphism. Grand Forks Sr, Pb, and Nd isotope data are much like those for Spokane area pre-Purcell basement.Vaseaux Formation micaceous schist and gneiss give radiogenic whole-rock Sr, zircon U–Pb upper intercept, and Sm–Nd crustal-residence dates of 2.1 ± 0.6 Ga, 1899 ± 49 Ma (2σ), and 2.2 ± 0.1 Ga, respectively. Hornblende-bearing schist and gneiss contain much less radiogenic Sr and more radiogenic Nd. The latter are either tectonic intercalations of Late Proterozoic to Paleozoic eugeosynclinal rocks or Mesozoic–Cenozoic mixtures of mantle-derived magma and older crustal rock. The Vaseaux Formation paragneiss is similar isotopically to paragneiss in the Frenchman Cap core gneiss. This may indicate a similar age, or that Vaseaux sedimentary rocks could be much younger and isochemically derived from a basement of Frenchman Cap character. The first alternative is favored because the three isotope systems are usually not preserved in unison through sedimentary processes. Sr isotopes, in particular, do not usually preserve a provenance age.In all three areas, late Mesozoic to early Cenozoic metamorphic monazite, hornblende, muscovite, and biotite dates provide a record of cooling from a Cretaceous to Paleocene culmination of regional metamorphism, with particularly rapid cooling during Paleocene to Eocene crustal extension and tectonic unroofing.The localities studied are tectonic windows on structural culminations that expose basement that we infer to be part of North America. Their ages fit the pattern of basement ages established for the stable craton. Their extent is consistent with the reconstruction of compressed miogeoclinal rocks. The eastern half of the Cordilleran region on both sides of the United States – Canada border is underlain by Early Proterozoic basement that was attenuated in Late Proterozoic time, compressed during Mesozoic–Cenozoic orogeny, and finally extended in early Cenozoic collapse of the thickened crust. During Mesozoic–Cenozoic orogeny the sedimentary cover of that basement was pushed approximately 200 km eastward and replaced by allochthonous terranes. The tectonic displacements documented in the southern Canadian Cordillera are truly exceptional.

Magnetotelluric traverses across the southern Yukon-Tanana terrane (YTT) reveal the presence of a thick conductive layer (or layers) beneath Paleozoic crystalline rocks. These rocks have been interpreted to be flysch of probable Mesozoic age, on the basis of the occurrence of Jurassic-Cretaceous flysch in the Kahiltna assemblage and Gravina-Nutzotin belt flanking the YTT to the southwest and southeast, respectively. The Pb, Nd, Sr, and O isotopes in Cretaceous and Tertiary granitic rocks that crop out throughout the YTT were measured to determine if these rocks do in fact contain a component of flysch. Previous limited analyses indicated that the Pb isotopes of the granitic rocks could be a mixture of radiogenic Pb derived from Paleozoic crystalline rocks of the YTT with an increasing component of relatively nonradiogenic Pb with decreasing age. Our Nd, Sr, and O data, along with additional Pb isotope data, eliminate flysch as a likely source and strongly suggest that the nonradiogenic end-member was derived from mafic rocks, either directly from mantle magma or by melting of mafic crust. The lack of a sedimentary component in the granitic plutons suggests either that the plutons did not incorporate significant amounts of flysch during intrusion or that the conductive layer beneath the YTT crystalline rocks is not flysch.

ABSTRACT:The Lu-Hf and Re-Os isotope systems have been applied sparsely to elucidate the origin of granites, intracrustal processes and the evolution of the continental crust. The presence or absence of garnet as a residual phase during partial melting will strongly influence Lu/Hf partitioning, making the Lu–Hf isotope system exceptionally sensitive to evaluating the role of garnet during intracrustal differentiation processes. Mid-Proterozoic (1·1–1·5Ga ) ‘anorogenic’ granites from the western U.S.A. appear to have anomalously high εHf values, relative to their εNd values, compared with Precambrian orogenic granites from several continents. The Hf-Nd isotope variations for Precambrian orogenic granites are well explained by melting processes that are ultimately tied to garnet-bearing sources in the mantle or crust. Residual, garnet-bearing lower and middle crust will evolve to anomalously high εHf values over time and may be the most likely source for later ‘anorogenic’ magmas. When crustal and mantle rocks are viewed together in terms of Hf and Nd isotope compositions, a remarkable mass balance is apparent for at least the outer silicate earth where Precambrian orogenic continental crust is the balance to the high-εHf depleted mantle, and enriched lithospheric mantle is the balance to the low-εHf depleted mantle.Although the continental crust has been envisioned to have exceptionally high Re/Os ratios and very radiogenic Os isotope compositions, new data obtained on magnetite mineral separates suggest that some parts of the Precambrian continental crust are relatively Os-rich and non-radiogenic. It remains unclear how continental crust may obtain non-radiogenic Os isotope ratios, and these results have important implications for Re-Os isotope evolution models. In contrast, Phanerozoic batholiths and volcanic arcs that are built on young mafic lower crust may have exceptionally radiogenic Os isotope ratios. These results highlight the unique ability of Os isotopes to identify young mafic crustal components in orogenic magmas that are essentially undetectable using other isotope systems such as O, Sr, Nd and Pb.

AbstractThe geology of Puerto Rico is divided into three regions: the north, central and SW igneous provinces. Characterized by its Jurassic ophiolitic mélange basement, lithology of the SW Igneous Province (SIP) is not related to either of the other two provinces. The ophiolitic mélange is exposed in three peridotite belts: Monte del Estado, Rio Guanajibo and Sierra Bermeja. We present geochemical data to identify the tectonic setting of the SIP peridotite formation and its relation to the evolution of the Caribbean Plate. Comparisons of spinel Cr no. (13–21), Mg no. (63.3–69.6) and TiO2suggest an abyssal peridotite origin; however, only Sierra Bermeja presents high TiO2characteristics of a mid-ocean-ridge-basalt- (MORB-) like melt reaction. Temperatures determined with two-pyroxene geothermometers indicated a cold thermal regime ofc. 800–1050°C, with characteristics of large-offset transform fault abyssal peridotites. The geochemistry and Sr–Nd–Hf–Pb isotopic compositions of basalts within the mélange were also analysed. Las Palmas amphibolites exhibited normal-MORB-like rare earth element (REE) and trace-element patterns, whereas metabasalts and Lower Cajul basalts exhibited island-arc tholeiitic-like patterns. Highly radiogenic Sr isotopes (0.70339–0.70562) of the basalts suggest seawater alteration; however, Pb–Pb and Nd–Hf isotope correlations represent the primary compositions of a Pacific/Atlantic MORB source for the amphibolites, metabasalts and Lower Cajul basalts. We propose that the SIP ophiolitic mélange was formed along a large-offset transform fault, which initiated subduction and preserved both proto-Pacific and proto-Caribbean lithospheric mantle. Younger Upper Cajul basalts exhibited enriched-MORB-like geochemical and isotopic signatures, which can be attributed to a tectonized Caribbean ocean plateau.

Abstract Although continental crust is characterized by the widespread occurrence of granitoids, the causal relationship between continental crust growth and granitic magmatism still remains enigmatic. While fractional crystallization of basaltic magmas (with or without crustal contamination) and partial melting of mafic lower crust are two feasible mechanisms for the production of granitoids in continental arc regions, the problem has been encountered in discriminating between the two mechanisms by whole-rock geochemistry. This can be resolved by an integrated study of zircon U-Pb ages and Hf-O isotopes together with whole-rock major-trace elements and Sr-Nd-Pb isotopes, which is illustrated for Mesozoic granitoids from the Gangdese orogen in southern Tibet. The results provide geochemical evidence for prompt reworking of the juvenile mafic arc crust in the newly accreted continental margin. The target granitoids exhibit high contents of SiO2 (65.76–70.75 wt%) and Na2O + K2O (6.38–8.15 wt%) but low contents of MgO (0.19–0.98 wt%), Fe2O3 (0.88–3.13 wt%), CaO (2.00–3.82 wt%), Ni (&lt;5.8 ppm), and Cr (≤10 ppm). They are enriched in large ion lithophile elements, Pb, and light rare earth elements but depleted in high field strength elements. The granitoids are relatively depleted in whole-rock Sr-Nd isotope compositions with low (87Sr/86Sr)i ratios of 0.7043–0.7048 and positive εNd(t) values of 0.5–2.6, and have relatively low 207Pb/204Pb and 208Pb/204Pb ratios at given 206Pb/204Pb ratios. Laser ablation–inductively coupled plasma–mass spectrometry and secondary ion mass spectrometry U-Pb dating on synmagmatic zircons yield ages of 77 ± 2–81 ± 1 Ma in the Late Cretaceous for their emplacement. Relict zircons have two groups of U-Pb ages in the late Mesozoic and the late Paleozoic, respectively. The whole-rock Sr-Nd isotopes in the granitoids are quite similar to those of Late Cretaceous mafic rocks in the Gangdese batholith. In addition, both synmagmatic zircons and relict zircons with Late Cretaceous U-Pb ages exhibit almost the same Hf-O isotope compositions to those of the slightly earlier mafic rocks. All these observations indicate that the granitoids were mainly derived from partial melting of the juvenile mafic arc crust. Therefore, reworking of the juvenile mafic arc crust is the mechanism for the origin of isotopically depleted granitoids in southern Tibet. It is this process that leads to differentiation of the juvenile mafic arc crust toward the felsic lithology in the continental arc. In this regard, the granitoids with depleted radiogenic isotope compositions do not necessarily contribute to the crustal growth at convergent plate boundaries.

ABSTRACTU-Pb zircon geochronology of Mesoproterozoic (Subjotnian) rapakivi complexes in central Sweden yields: 1526 ± 3 Ma (Mullnäset), 1524 ± 3 Ma (Mårdsjö), 1520 ± 3 Ma (Nordsjö) and 1497 ± 6 Ma (Rödön). Together with complexes further S in Sweden, they constitute the westernmost, youngest (1·53−1·47 Ga) belt of rapakivi magmatism in the Fennoscandian shield.The low initial εNd values (−8·9 to −4·8) of all studied Subjotnian basic, intermediate and silicic rocks, require an input from an old (Archaean) low-radiogenic source component, as evidence for Palaeoproterozoic protoliths in the age range 2·5−2·1 Ga is lacking in this region. Crustal, early Svecofennian + Archaean (roughly 30−40%) sources are suggested for the Subjotnian A-type granites and syenites, where the granites derive from undepleted, granodioritic, and the syenites from monzodioritic (±depleted crustal) protoliths. The basic rocks originate from a depleted mantle acquiring the enriched Nd isotopic signatures during interaction with an Archaean lower crust (20−40%), largely depleted after rapakivi melt extraction. Pb isotope data from feldspars (207Pb/204Pb to 15·018−15·542) support the presence of Archaean components in the magmas.The results indicate that an Archaean basement is underlying relatively wide areas of Svecofennian formations in central Sweden. This old basement section was most likely rifted off the Archaean craton in the NE in Palaeoproterozoic times.

The focus of this work is magma-crust interaction processes and associated crustal volatile release in subduction zone volcanoes, drawing on rock, mineral, and gas geochemistry as well as experimental petrology. Understanding the multitude of differentiation processes that modify an original magma during ascent to the surface is vital to unravel the contributions of the various sources that contribute to the final magmas erupted at volcanoes. In particular, magma-crust interaction (MCI) processes have been investigated at a variety of scales, from a local scale in the Vesuvius, Merapi, and Kelut studies, to a regional scale, in the Java to Bali segment of the Sunda Arc.  The role of crustal influences is still not well constrained in subduction systems, particulary in terms of the compositional impact of direct magma crust interplay. To address this shortcoming, we studied marble and calc-silicate (skarn) xenoliths, and used high resolution short timescale experimental petrology at Vesuvius volcano. The marbles and calc-silicates help to identify different mechanisms of magma-carbonate and magma-xenolith interaction, and the subsequent effects of volatile release on potential eruptive behaviour, while sequential short-duration experiments simulate the actual processes of carbonate assimilation employing natural materials and controlled magmatic conditions. The experiments highlight the efficiency of carbonate assimilation and associated carbonate-derived CO2 liberated over short timescales. The findings at Merapi and Kelut demonstrate a complex magmatic plumbing system underneath these volcanoes with magma residing at different depths, spanning from the mantle-crust boundary to the upper crust. The erupted products and volcanic gas emissions enable us to shed light on MCI-processes and associated volatile release in these systems. The knowledge gained from studying individual volcanoes (e.g., Merapi and Kelut) is then tested on a regional scale and applied to the entire Java and Bali arc segment. An attempt is presented to distinguish the extent of source versus crustal influences and establish a quantitative model of late stage crustal influence in this arc segment. This thesis therefore hopes to contribute to our knowledge of magma genesis and magma-crust interaction (MCI) processes that likely operate in subduction zone systems worldwide.

This work focuses on crustal interaction in magmatic systems, drawing on experimental petrology and elemental and isotope geochemistry. Various magma-chamber processes such as magma-mixing, fractional crystallisation and magma-crust interaction are explored throughout the papers comprising the thesis. Emphasis is placed on gaining insights into the extent of crustal contamination in ocean island magmas from the Canary Islands and the processes of magma-crust interaction observed both in nature and in experiments. This research underscores that the compositions of ocean island magmas, even primitive types which are classically used as probes of the mantle, are susceptible to modification by crustal contamination. The principal mechanisms of contamination identified from work on both Tenerife and Gran Canaria (Canary Islands) are assimilation and partial melting of the pre-existing island edifice and intercalated sediments by newly arriving magma (i.e. “island recycling”). The information that we can gain from studying solidified magma and entrained crustal xenoliths concerning the rates and mechanisms of crustal assimilation is, however, limited. To address this shortcoming, a series of time-variable crustal carbonate assimilation experiments were carried out at magmatic pressure and temperature using natural materials from Merapi volcano, Indonesia. A temporally constrained reaction series of carbonate assimilation in magma has hence been constructed. The experiments were analysed using in-situ techniques to observe the progressive textural, elemental, and isotopic evolution of magma-carbonate interaction. Crucially, carbonate assimilation was found to liberate voluminous crustally-derived CO2 on a timescale of only seconds to minutes in the experiments. This points to the role of rapid crustal degassing in volcanic volatile budgets, and, pertinently, in magnifying hazardous volcanic behaviour. This thesis, therefore, delivers detailed insights into the processes of magma-crust interaction from experiments and geochemistry. The outcomes confirm that crustal processes are significant factors in both, i) ocean island magma genesis, and ii) magma differentiation towards compositions with greater explosive potential which can, in turn, manifest as hazardous volcanism.
Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 707

O objetivo primário deste estudo é aprimorar o conhecimento acerca das fontes do manto e dos mecanismos envolvidos na gênese dos basaltos da Província Magmática do Paraná, que constitui uma das maiores manifestações de basaltos continentais do mundo. Para tanto, foram determinadas as concentrações de terras raras (La, Ce, Nd, Sm, Eu, Tb, Yb e Lu), outros elementos traço (Cs, Rb, Ba, U, Th, Ta, Hf, Co e Sc) e elementos altamente siderófilos (Os, Ir, Ru, Pt, Pd e Re), juntamente com razões isotópicas dos sistemas Rb-Sr, Sm-Nd, U-Th-Pb e Re-Os em basaltos com alto-Ti (Paranapanema e Pitanga) que ocorrem no norte da PMP. Além disso, foram determinadas as concentrações de elementos altamente siderófilos e as razões isotópicas de 187Os/188Os amostras representativas de basaltos com baixo-Ti (Esmeralda) do sul da PMP. Os dados geoquímicos e as razões isotópicas de Sr, Nd e Pb obtidos são consistentes com dados da literatura, porém, refinam as variações (extremos) isotópicas dos magmas-tipo Paranapanema e Pitanga. Esses dados, juntamente com as concentrações de elementos altamente siderófilos e das razões isotópicas de Os, inéditas na literatura, sugerem que as fontes dos basaltos (astenosfera ou manto litosférico subcontinental) sofreram metassomatismo significativo, com a intrusão de veios piroxeníticos, relacionado a antigas subducções e/ou processos de delaminação.
The primary goal of this study is to improve the understanding about the mantle sources and the mechanisms involved in the basalt genesis from Paraná Magmatic Province (PMP), which is one of the largest known continental flood basalts of the world. Therefore, the concentrations of rare earths (La, Ce, Nd, Sm, Eu, Tb, Yb and Lu), other trace elements (Cs, Rb, Ba, U, Th, Ta, Hf, Co and Sc) and highly siderophile elements (Os, Ir, Ru, Pt, Pd and Re) were determined, along with isotope ratios regarding Rb-Sr, Sm-Nd, U-Th-Pb e Re-Os systematics in high-Ti basalts (Paranapanema and Pitanga) from northern PMP. In addition, the highly siderophile element concentrations, as well as 187Os/188Os isotope ratios, were measured in selected samples of low-Ti basalts (Esmeralda) from southern PMP. The geochemical and Sr-Nd-Pb isotope results of the present study are consistent with literature data, but refine the isotope variations (extreme) for the Paranapanema and Pitanga magma-types. These data, along with the concentrations of highly siderophile elements and Os isotope ratios suggest that the basalt mantle sources (asthenosphere or subcontinental lithospheric mantle) were affected by significant metasomatism (piroxenitic vein hybridization), related with old subduction and/or delamination processes.