Journal articles on the topic 'Bulk rock Sm-Nd'
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1
ALI, K., A. ANDRESEN, W. I. MANTON, R. J. STERN, S. A. OMAR, and A. E. MAURICE. "U–Pb zircon dating and Sr–Nd–Hf isotopic evidence to support a juvenile origin of the ~ 634 Ma El Shalul granitic gneiss dome, Arabian–Nubian Shield." Geological Magazine 149, no. 5 (December 16, 2011): 783–97. http://dx.doi.org/10.1017/s0016756811000975.
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AbstractThe calc-alkaline, gneissic El Shalul granite is the westernmost gneiss dome or core complex within the Arabian–Nubian Shield. Previous studies have indicated that it represents either a window into the underlying pre-Neoproterozoic Sahara metacraton or a melt derived from the metacraton. U–Pb LA-ICP-MS dating of magmatic zircons from two samples of the variably foliated El Shalul pluton gives ages of 637 ± 5 Ma and 630 ± 6 Ma, excluding it from representing exhumed cratonic rocks. The ages are, however, indistinguishable from the age of the Um Ba'anib pluton, constituting the core of the Meatiq Gneiss Dome, as well as several other plutons in the Eastern Desert, indicating an important magmatic pulse in the Arabian–Nubian Shield in Late Cryogenian time. Major and trace element data indicate a within-plate setting. Bulk rock Nd-isotope and Hf-isotope data on zircons from the El Shalul pluton indicate derivation of the primary melt from a relatively juvenile source, either the lower crust of a mid-Neoproterozoic volcanic arc or as a result of fractionation of a mantle-derived mafic melt. Sm–Nd bulk rock isotopic data indicate a model age of c. 720 Ma for the protolith from which the melt was derived. Time-corrected Hf-isotope data obtained on the magmatic zircons indicate that the bulk of the source rock was extracted from the mantle around 810 Ma.
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Stevenson, Ross K., P. J. Patchett, and R. F. Martin. "Sm–Nd isochron from a granodiorite–granite complex in the Portman Lake region, Northwest Territories." Canadian Journal of Earth Sciences 26, no. 12 (December 1, 1989): 2724–29. http://dx.doi.org/10.1139/e89-232.
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Whole-rock samples for a granodiorite–granite intrusion in the Portman Lake area of the Northwest Territories yield an Sm–Nd isochron with an age of 2577 ± 36 Ma. The addition of a garnet analysis results in an age of 2562 ± 21 Ma. An Sm–Nd isochron is rare for granitic samples because of small ranges in 147Sm/144Nd values and variability in the initial Nd isotopic ratios. A wide range in 147Sm/144Nd values among samples in this study is considered a result of garnet fractionation and (or) variable concentrations of titanite in the samples. The εNd values for the initial Nd isotopic ratios of the isochrons are essentially zero or bulk Earth values. The evolution of the intrusion at 2.56 Ga reflects either the remelting of (2.7–2.8 Ga) preexisting continental crust or the mixing of depleted mantle material and crust older than 2.8 Ga. In either case, the data argue for a crustal history of at least 200 Ma prior to the intrusion of the complex in this section of the Churchill Province.
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Demonterova, Elena I., Alexei V. Ivanov, Ekaterina M. Mikheeva, Anastasia V. Arzhannikova, Andrei O. Frolov, Sergei G. Arzannikov, Nikolai V. Bryanskiy, and Lyudmila A. Pavlova. "Early to Middle Jurassic history of the southern Siberian continent (Transbaikalia) recorded in sediments of the Siberian Craton: Sm-Nd and U-Pb provenance study." Bulletin de la Société géologique de France 188, no. 1-2 (2017): 8. http://dx.doi.org/10.1051/bsgf/2017009.
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The deposition of Jurassic continental sedimentary rocks in the southern part of the Siberian continent (Transbaikalia) reflects the intensification of tectonomagmatic processes in this region. The most likely cause of this intensification was associated with the formation and development of the Mongol-Okhotsk orogenic belt. The latter was controlled in its turn by the closure of the Mongol-Okhotsk Ocean, for which the timing of its closure, as well as the formation of a collisional orogeny and its subsequent collapse are still under debate. We address this question by studying sediments of the Irkutsk Basin, which were deposited in a short time span in the Middle Jurassic, most likely during the Aalenian. The Sm-Nd data for bulk-rock sandstones demonstrate that the youngest samples of the Irkutsk Basin are characterized by a prominent contribution from a source within the juvenile crust of the Mongol-Okhotsk orogenic belt. U-Pb detrital zircon ages concur with the Sm-Nd data and show that the amount of material derived from local cratonic sources decreased in time whereas material from the remote Transbaikalian sources increased. Our data provide evidence that mountain growth in Transbaikalia intensified rapidly close to the Early and Middle Jurassic boundary.
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Maneiro, Kathryn A., Ethan F. Baxter, Scott D. Samson, Horst R. Marschall, and Jack Hietpas. "Detrital garnet geochronology: Application in tributaries of the French Broad River, Southern Appalachian Mountains, USA." Geology 47, no. 12 (October 15, 2019): 1189–92. http://dx.doi.org/10.1130/g46840.1.
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Abstract Nineteen single-grain detrital garnet ages from a tributary to the French Broad River (North Carolina, USA) establish a novel approach to Sm-Nd detrital garnet geochronology wherein the equilibrium bulk-rock composition lost during weathering and transport is replaced with the composition of inclusions leached from within each garnet grain. Detrital garnet ages were compared to published detrital zircon and monazite ages from the same river tributary system. Results show that 87% of the zircons have inherited Proterozoic ages; only zircon rims give Paleozoic ages. Monazites are exclusively Ordovician (weighted average: 460.9 ± 3.0 Ma). Our new detrital garnet ages (and the detrital zircon rims) record younger ages spanning the Late Ordovician to Early Devonian, likely reflecting prolonged metamorphic heating. The weighted average age of the detrital garnet population is Silurian (430.1 ± 7.2 Ma). Statistical tests confirm that the garnet population is younger than the monazite. The new detrital garnet ages illuminate a previously uninterpreted Silurian tectonometamorphic signal in this region.
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Mikhalsky, E. V., A. A. Laiba, B. V. Beliatsky, and K. Stüwe. "Geology, age and origin of the Mount Willing area (Prince Charles Mountains, East Antarctica)." Antarctic Science 11, no. 3 (September 1999): 338–52. http://dx.doi.org/10.1017/s0954102099000437.
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Mount Willing in the Prince Charles Mountains (East Antarctica) is part of the Fisher Volcano–plutonic complex which formed as part of the global-scale Grenvillian mobile belt system. Mount Willing is composed of four rock complexes: 1) a metamorphic sequence, 2) gabbro intrusions, 3) deformed felsic intrusives, and 4) abundant post-metamorphic dykes and veins. Three rock types constitute the metamorphic sequence: amphibole–biotite felsic plagiogneiss, mafic to intermediate biotite–amphibole schist, and biotite paragneiss. The bulk composition of the mafic schists classifies them as tholeiitic basalts, and rarely as basaltic andesites or andesites. Index mg ranges widely from 47 to 71. Concentrations of TiO2, P2O5, and high-field strength elements are high in some rocks. These rocks are thought to have been derived from enriched (subcontinental) mantle sources. Sm–Nd and U–Pb isotopic data indicate a series of Mesoproterozoic thermal events between 1100 and 1300 Ma. In particular, these events occurred at 1289 ± 10 Ma (volcanic activity), at 1177 ± 16 Ma (tonalite intrusion), at 1112.7 ± 2.4 and at 1009 ± 54 Ma (amphibolite facies metamorphic events). Rb–Sr systematics also indicates a thermal overprint at 636 ± 13 Ma. Mafic schists show low initial 877Sr/86Sr ratios between 0.7024 and 0.7030. Felsic rocks show higher Sri values between 0.7037 and 0.7061. Basaltic andesite metavolcanic and plutonic rocks form a calc-alkaline evolutionary trend, and probably originated from subduction-modified mantle sources in a convergent plate margin environment. An oceanic basin may have existed in central Prince Charles Mountains about 1300 Ma ago and was closed as a result of continental collision around 1000 to 800 Ma.
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Windom, Kenneth E., W. Randall Van Schmus, Karl E. Seifert, E. Timothy Wallin, and Raymond R. Anderson. "Archean and Proterozoic tectono-magmatic activity along the southern margin of the Superior Province in northwestern Iowa, United States." Canadian Journal of Earth Sciences 30, no. 6 (June 1, 1993): 1275–85. http://dx.doi.org/10.1139/e93-109.
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A Precambrian igneous body of ultramafic and mafic rocks, named the Otter Creek layered igneous complex, occurs within the basement of northwestern Iowa, United States. It is marked by a circular magnetic anomaly, one of several that lie north and west of an inferred suture between the Archean Superior Province and Early Proterozoic juvenile crust. Sm–Nd whole-rock analyses for several rock types from the Otter Creek complex yield an isochron age of 2890 ± 90 Ma, with an εNd(t) of −0.9 ± 2.4. A block of older banded iron formation, itself intruded by lamprophyre dikes, is contained within the layered sequence. The iron formation – lamprophyre block has undergone high-temperature metamorphism followed by a retrograde event. A quartz monzodiorite gneiss, with a U–Pb age of 2523 ± 5 Ma, occurs near the layered complex, but the contact relations are not known. The layered series is overlain by Proterozoic keratophyre with a U–Pb age of 1782 ± 10 Ma. These felsic pyroclastic rocks are extremely depleted in K, Rb, Ba, and Cs. Our data are consistent with Archean greenstone-belt formation, including chemical sedimentation followed by mafic–ultramafic magmatism at approximately 2.9 Ga, followed by two later episodes of magmatism, one at approximately 2.5 Ga and the other at approximately 1.78 Ga. The Otter Creek complex is the first Archean greenstone reported south of the Great Lakes Tectonic Zone (GLTZ); its 2.9 Ga age is older than those reported for the granite–greenstone rocks north of the GLTZ. The southern portion of the Superior Province thus appears to have formed later, and in a different tectonic environment, than the high-grade gneisses of the Minnesota River Valley, but before the bulk of the granite–greenstone rocks exposed in northern Minnesota, Ontario, and eastern Manitoba.
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Mitchell, Roger H. "Igneous Rock Associations 26. Lamproites, Exotic Potassic Alkaline Rocks: A Review of their Nomenclature, Characterization and Origins." Geoscience Canada 47, no. 3 (September 28, 2020): 119–42. http://dx.doi.org/10.12789/geocanj.2020.47.162.
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Lamproite is a rare ultrapotassic alkaline rock of petrological importance as it is considered to be derived from metasomatized lithospheric mantle, and of economic significance, being the host of major diamond deposits. A review of the nomenclature of lamproite results in the recommendation that members of the lamproite petrological clan be named using mineralogical-genetic classifications to distinguish them from other genetically unrelated potassic alkaline rocks, kimberlite, and diverse lamprophyres. The names “Group 2 kimberlite” and “orangeite” must be abandoned as these rock types are varieties of bona fide lamproite restricted to the Kaapvaal Craton. Lamproites exhibit extreme diversity in their mineralogy which ranges from olivine phlogopite lamproite, through phlogopite leucite lamproite and potassic titanian richterite-diopside lamproite, to leucite sanidine lamproite. Diamondiferous olivine lamproites are hybrid rocks extensively contaminated by mantle-derived xenocrystic olivine. Currently, lamproites are divided into cratonic (e.g. Leucite Hills, USA; Baifen, China) and orogenic (Mediterranean) varieties (e.g. Murcia-Almeria, Spain; Afyon, Turkey; Xungba, Tibet). Each cratonic and orogenic lamproite province differs significantly in tectonic setting and Sr–Nd–Pb–Hf isotopic compositions. Isotopic compositions indicate derivation from enriched mantle sources, having long-term low Sm/Nd and high Rb/Sr ratios, relative to bulk earth and depleted asthenospheric mantle. All lamproites are considered, on the basis of their geochemistry, to be derived from ancient mineralogically complex K–Ti–Ba–REE-rich veins, or metasomes, in the lithospheric mantle with, or without, subsequent contributions from recent asthenospheric or subducted components at the time of genesis. Lamproite primary magmas are considered to be relatively silica-rich (~50–60 wt.% SiO2), MgO-poor (3–12 wt.%), and ultrapotassic (~8–12 wt.% K2O) as exemplified by hyalo-phlogopite lamproites from the Leucite Hills (Wyoming) or Smoky Butte (Montana). Brief descriptions are given of the most important phreatomagmatic diamondiferous lamproite vents. The tectonic processes which lead to partial melting of metasomes, and/or initiation of magmatism, are described for examples of cratonic and orogenic lamproites. As each lamproite province differs with respect to its mineralogy, geochemical evolution, and tectonic setting there is no simple or common petrogenetic model for their genesis. Each province must be considered as the unique expression of the times and vagaries of ancient mantle metasomatism, coupled with diverse and complex partial melting processes, together with mixing of younger asthenospheric and lithospheric material, and, in the case of many orogenic lamproites, with Paleogene to Recent subducted material.
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Sushchevskaya, N. M., T. A. Shishkina, M. V. Portnyagin, V. G. Batanova, and B. V. Belyatsky. "Long-lasting influence of the Discovery plume on tholeiitic magmatism in the South Atlantic: data on basalts recovered by hole 513a, dsdp leg 71." Геохимия 64, no. 2 (March 15, 2019): 107–27. http://dx.doi.org/10.31857/s0016-7525642107-127.
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The paper presents the very first data on concentrations of major and trace elements; Sr, Nd, and Pb isotopic ratios of rocks; and the composition of olivine phenocrysts of 38-Ma basalts recovered by Hole 513a (DSDP Leg 71) in the South Atlantic. The bulk-rock samples and the chilled glasses are mildly magnesian (7–8 wt % MgO) and bear elevated FeO and low Na2O concentrations, as is typical of MORB of the TOR-1 type. Olivine phenocrysts (Fo84.5–88) in these rocks contain concentrations of trace elements (Ni, Mn, Cr, and Zn) that are typical of classic MORB, which are produced by partial melting mantle peridotite. The rocks are strongly depleted in incompatible elements [(La/Sm)n ~ 0.6] but have elevated Ba/Nb, K/Nb, and Pb/Ce ratios and Cu, Ag, and Au concentrations that are 1.5–4 times higher than in typical depleted MORB (N-MORB) and in most rift basalts in the South Atlantic. Isotope compositions of the basalts (average ratios 206Pb/204Pb ~ 18.0; 207Pb/204Pb ~ 15.6, 208Pb/204Pb ~ 38.0, 143Nd/144 Nd ~ 0.5130, and 87Sr/86Sr ~ 0.7040) are close to those in modern tholeiites from the southern MAR segment (SMAR) north of the Agulhas Fracture Zone. The data indicate that the magmas were derived from a strongly depleted mantle source that contained a minor (~3%) admixture of an enriched component, which is discernible in the magmas of the Discovery hotspot. The composition of the source, which is more depleted than DM, and the high degrees of melting of this source explain why the basalts from DSDP Hole 513a are enriched in chalcophile elements. It is believed that spreading magmatism at 45°–48° S in SMAR as far back as 40 Ma was already affected by the Discovery hotspot. This hotspot might be related to the Tristan plume system, and its origin and long-lasting influence on spreading magmatism in the South Atlantic are regarded as evidence of the extensive effect of the Tristan plume.
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Saleeby, Jason B. "Age and tectonic setting of the Duke Island ultramafic intrusion, southeast Alaska." Canadian Journal of Earth Sciences 29, no. 3 (March 1, 1992): 506–22. http://dx.doi.org/10.1139/e92-044.
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Concordant U–Pb zircon ages on hornblende–plagioclase pegmatites and a related granophyre from marginal and roof zones of the Duke Island ultramafic intrusion, and from leucoquartz diorite internal to hornblende–clinopyroxenite of nearby Mary Island cluster between 108 and 111 Ma. Sm–Nd mineral and bulk-rock data from pegmatite and granophyre zircon samples and from wehrlites and clinopyroxenites of the main Duke Island cumulate sequence are consistent with the consanguinity of the ultramafic cumulates and the zircon-bearing feldspathic rocks, and with an Early Cretaceous igneous age for the intrusion. Wall rocks for the intrusion consist of Ordovician–Silurian amphibolite- and greenschist-grade metamorphosed plutonic, volcanic, and minor sedimentary rocks and crosscutting Late Triassic gabbro–diorite of the Alexander terrane.The Duke Island ultramafic intrusion formed as a northwest-trending elongate funnel-shaped stratiform body commensurate with extensional brittle–ductile faulting in its roof and adjacent wall rocks. Intrusion occurred in a basinal setting within the Gravina volcanic arc along the Cordilleran continental margin. During the latter phases of intrusive activity the Gravina basin began to close by regional thrust faulting, which in the Duke Island region was west-northwest directed. The intrusion may have still had local intercumulate liquid upon initiation of thrust faulting. It responded first by open folding with incipient crystal plasticity. Once completely solidified, the intrusion behaved like a large "augen" with ductile deformation concentrated along its margins and within quartzo-feldspathic and micaceous members of its wall-rock complex. Much of the intrusion's northern margin behaved as a ductile shear zone serving as a tear fault between thrust plates to the northeast and an oblique thrust complex that roots beneath the southwest margin of the intrusion. The intrusion may thus be tectonically transported relative to its original underpinnings. North-trending high-angle faults and parallel fracture cleavage also cut the intrusion in response to axial loading as a result of its initial elongate shape and orientation relative to the thrust kinematics.
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Shaw, D. M., A. P. Dickin, H. Li, R. H. McNutt, H. P. Schwarcz, and M. G. Truscott. "Crustal geochemistry in the Wawa–Foleyet region, Ontario." Canadian Journal of Earth Sciences 31, no. 7 (July 1, 1994): 1104–21. http://dx.doi.org/10.1139/e94-099.
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Fifty-three rock samples from the Kapuskasing structural zone (KSZ) and 56 from the Wawa domal gneiss terrane (WGT), Ontario, have been analysed for major elements, 32 trace elements, and δ18O; δD was measured in a few samples.Average chemical compositions for the KSZ and WGT regions have been calculated from map unit averages weighted by regional abundance. Compared with estimates of the composition of the upper continental crust (UCC), the KSZ is enriched in Al, Fe, Mg, Ca, P, transition elements, Sc, and Sr; depleted in Si, B, most rare earth elements, Zr, Hf, Nb, Ta, Li, Na, K, U, Th, Ba, and Rb; but similar in composition to estimates of the lower continental crust. The WGT is closer in composition to the UCC. These data support the interpretation, on geophysical and petrological grounds, that the crust here is layered and has been uplifted, the WGT and KSZ regions representing progressively deeper levels.Igneous rock and orthogneiss δ18O values in the KSZ and WGT show good correlation with the weight percentages of SiO2. Paragneisses and clastic sediments and metavolcanics in the Michipicoten greenstone belt have higher ratios, as in other greenstone belts. Low δ18O and high δD values in most of the higher grade mafic gneisses show that they have never undergone low-temperature marine alteration. However, a few mafic gneisses with relict pillows show δ18O and δD values indicating low-temperature aqueous alteration. The δ18O and δD evidence throughout the two regions supports the view that no regional fluid homogenization took place.Errorchron ages of 2725 ± 130 Ma (Rb/Sr) and 2755 ± 110 Ma (Sm/Nd) were obtained for tonalite–granodiorite gneiss and agree within error with results from other workers. The intrusive Floranna Lake complex is 2580 ± 120 Ma (Rb/Sr), not significantly different from the age of the tonalite. Scatter in the data is to be attributed to localized late-stage alteration under low water–rock ratios by Sr-rich–Rb-poor brines. Model Nd ages suggest that there is no old crust [Formula: see text] in the region and that the 2700 Ma mantle was depleted relative to the bulk earth.
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Nedosekova, I. L., V. A. Koroteev, T. B. Bayanova, P. A. Serov, V. I. Popova, and M. V. Chervyakovskaya. "On the age of pyrochlore carbonatites from the Ilmeno-Vishnevogorsky Alkaline Complex, the Southern Urals (insights from Rb-Sr and Sm-Nd isotopic data)." LITHOSPHERE (Russia) 20, no. 4 (August 31, 2020): 486–98. http://dx.doi.org/10.24930/1681-9004-2020-20-4-486-498.
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Research subject. In this research, we carried out Sm-Nd- и Rb-Sr-dating of pyrochlore carbonatite from the Vishnevogorsky niobium deposit, Ilmeno-Vishnevogorsky Alkaline Complex, Southern Urals. IVC is located in the Ural fold region and is a carbonatite complex of the linear type. Rare metal (Nb-Zr-TR) deposits and occurrences are related to IVC. The age and the duration of IVC deposits formation remains a matter of debate. To determine the age of IVC carbonatites and related niobium ore, we measured Sm-Nd and Rb-Sr isotopic compositions and concentrations of the elements in the minerals (pyrochlore, calcite, apatite, biotite) and bulk sample of pyrochlore carbonatite. Materials and methods. The Sm and Nd isotopic compositions and concentrations were determined on a Finnigan MAT-262L (RPQ) seven-collector mass spectrometer in the static regime at the Geological Institute of the Kola Scientific Center, Apatity, Russia. The Sr and Rb isotopic compositions and concentrations were determined on thermos-ionization mass spectrometer Triton Plus (“Geoanalitik”, IGG UD RAN, Ekaterinburg, Russia). Results. Age of pyrochlore carbonatites from ore zone 140 (Vishnevogorsky deposit, IVC) defined by Sm-Nd and Rb-Sr isotopic methods. Mineral Sm-Nd-isochron (5 points) indicated age 229 ± 16 Ma, mineral Rb-Sr-isochron (5 points) showed similar age 250.5 ± 1.2 Ma. Conclusions. Results Sm-Nd и Rb-Sr dating indicate that the pyrochlore сarbonatites of ore zone 140 crystallized ≈ 250 Ma ago, at the stage of the postcollisional extension, possibly, in connection with exhumation complex, which was accompanied by decompression, partial melting of rocks, involving fluids, dissolution and precipitation of Ordovician-Silurian alkaline-carbonatitе complex. Thus, the formation of the IVC carbonatites and related Nb-ore, which began in Silurian (S), continued in Permian (P) and Triassic (T1-2) and was associated with the post-collision stage of tectonic activity in the Ural Fold Belt.
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Carlson, Richard W., and Maud Boyet. "Composition of the Earth's interior: the importance of early events." Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 366, no. 1883 (September 30, 2008): 4077–103. http://dx.doi.org/10.1098/rsta.2008.0166.
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The detection of excess 142 Nd caused by the decay of 103 Ma half-life 146 Sm in all terrestrial rocks compared with chondrites shows that the chondrite analogue compositional model cannot be strictly correct, at least for the accessible portion of the Earth. Both the continental crust (CC) and the mantle source of mid-ocean ridge basalts (MORB) originate from the material characterized by superchondritic 142 Nd/ 144 Nd. Thus, the mass balance of CC plus mantle depleted by crust extraction (the MORB-source mantle) does not sum back to chondritic compositions, but instead to a composition with Sm/Nd ratio sufficiently high to explain the superchondritic 142 Nd/ 144 Nd. This requires that the mass of mantle depleted by CC extraction expand to 75–100 per cent of the mantle depending on the composition assumed for average CC. If the bulk silicate Earth has chondritic relative abundances of the refractory lithophile elements, then there must exist within the Earth's interior an incompatible-element-enriched reservoir that contains roughly 40 per cent of the Earth's 40 Ar and heat-producing radioactive elements. The existence of this enriched reservoir is demonstrated by time-varying 142 Nd/ 144 Nd in Archaean crustal rocks. Calculations of the mass of the enriched reservoir along with seismically determined properties of the D″ layer at the base of the mantle allow the speculation that this enriched reservoir formed by the sinking of dense melts deep in a terrestrial magma ocean. The enriched reservoir may now be confined to the base of the mantle owing to a combination of compositionally induced high density and low viscosity, both of which allow only minimal entrainment into the overlying convecting mantle.
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Castorina, F., F. Stoppa, A. Cundari, and M. Barbieri. "An enriched mantle source for Italy's melilitite-carbonatite association as inferred by its Nd-Sr isotope signature." Mineralogical Magazine 64, no. 4 (August 2000): 625–39. http://dx.doi.org/10.1180/002646100549652.
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AbstractNew Sr-Nd isotope data were obtained from Late Pleistocene carbonatite-kamafugite associations from the Umbria-Latium Ultra-Alkaline District of Italy (ULUD) with the aim of constraining their origin and possible mantle source(s). This is relevant to the origin and evolution of ultrapotassic (K/Na ≫2) and associated rocks generally, notably the occurrences from Ugandan kamafugites,Western Australian lamproites and South African orangeites. The selected ULUD samples yielded 87Sr/86Sr and 143Nd/144Nd ranging from 0.7100 to 0.7112 and from 0.5119 to 0.5121 respectively, similar to cratonic potassic volcanic rocks with higher Rb/Sr and lower Sm/Nd ratios than Bulk Earth. Silicate and carbonate fractions separated from melilitite are in isotopic equilibrium, supporting the view that they are cogenetic. The ULUD carbonatites yielded the highest radiogenic Sr so far reported for carbonatites. In contrast, sedimentary limestones from ULUD basement formations are lower in radiogenic Sr, i.e. 87Sr/86Sr = 0.70745–0.70735. The variation trend of ULUD isotopic compositions is similar to that reported for Ugandan kamafugites and Western Australian lamproites and overlaps the values for South African orangeites in the εSr-εNd diagram. A poor correlation between Sr/Nd and 87Sr/86Sr ratios in ULUD rocks is inconsistent with a mantle source generated by subduction-driven processes, while the negligible Sr and LREE in sedimentary limestones from the ULUD region fail to account for a hypothetical limestone assimilation process. The Nd model ages of 1.5–1.9 Ga have been inferred for a possible metasomatic event, allowing further radiogenic evolution of the source, a process which may have occurred in isolation until eruption time. While the origin of this component remains speculative, the Sr-Nd isotope trend is consistent with a simple mixing process involving an OIB-type mantle and a component with low εNd and high εSr.
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Northrup, C. J., C. Isachsen, and S. A. Bowring. "Field relations, U-Pb geochronology, and Sm-Nd isotope geochemistry of the Point Lake greenstone belt and adjacent gneisses, central Slave craton, N.W.T., Canada." Canadian Journal of Earth Sciences 36, no. 7 (July 1, 1999): 1043–59. http://dx.doi.org/10.1139/e99-004.
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Data from the Point Lake area, central Slave craton, suggest an intimate tectonic and paleogeographic association between volcano-sedimentary supracrustal rocks and adjacent gneisses. Granite plutons and orthogneisses yield U-Pb zircon crystallization ages ranging from ca. 3230 to 2818 Ma. Numerous mafic dykes cut the gneisses, and two have been dated by U-Pb zircon geochronometry at 2673 ± 3 and 2690 ± 3 Ma, ages similar to those of volcanic rocks in the Point Lake greenstone belt. Although high-strain zones form the greenstone-gneiss in most places, a structural repetition of granite about 4 km east of Keskarrah Bay is cut by numerous mafic dykes and apparently overlain depositionally(?) by pillow basalt. Mafic volcanic and plutonic rocks from Point Lake have initial (2.7 Ga) εNd values ranging from about +2.2 to -6.3, significantly lower than the depleted mantle at that time. The Nd data suggest either derivation from a more isotopically evolved reservoir, or assimilation of crust similar to the granite gneiss at Point Lake. We infer from the presence of mafic dykes of appropriate age in the basement and the low initial εNd values of some pillow basalts that the volcanic sequence developed on the older granitic crust. The supracrustal rocks may have been deposited in a back-arc basin floored at least in part by attenuated continental material. Closure of the basin, bulk east-west shortening, and sinistral oblique or strike-slip faulting then obscured the original relations between the volcanic and gneissic rocks.
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Gordienko, I. V. "The role of island-arc oceanic, collisional and intraplate magmatism in the formation of continental crust in the Mongolia-Trasnbaikalia region: geostructural, geochronological and Sm-Nd isotope data." Geodynamics & Tectonophysics 12, no. 1 (March 21, 2021): 1–47. http://dx.doi.org/10.5800/gt-2021-12-1-0510.
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The formation of continental crust in the Mongolia-Transbaikalia region is researched to identify the mechanisms of interactions between the crust and the mantle in the development of the Neoarchean, Proterozoic and Paleozoic magmatic and sedimentary complexes in the study area. Using the results of his own studies conducted for many years and other published data on this vast region of Central Asia, the author have analysed compositions, ages and conditions for the formation of Karelian, Baikalian, Caledonian and Hercynian structure-formational complexes in a variety of geodynamic settings. Based on the geostructural, petrological, geochemical, geochronological and Sm-Nd isotope data, he determines the crustal and mantle sources of magmatism, conducts the identification and mapping of isotopic provinces, and reveals the role of island-arc oceanic, accretion-collision and intraplate magmatism in the formation of continental crust. Considering the formation of the bulk continental crust, three main stages are distinguished: (1) Neoarchean and Paleoproterozoic (Karelian) (almost 30% of the crust volume), (2) Meso-Neoproterozoic (Baikalian) (50%), and (3) Paleozoic (Caledonian and Hercynian) (over 20%). This sequence of the evolution stages shows the predominance of the ancient crustal material in igneous rocks sources at the early stage. During the subsequent stages, tectonic structures created earlier were repeatedly reworked, and mixed crustal-mantle and juvenile sources were widely involved in the formation of the bulk continental crust in the study area.
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Maslov, A. V., O. V. Artyushkova, R. Ch Tagarieva, D. V. Kiseleva, M. V. Streletskaya, M. V. Chervyakovskaya, and N. V. Cherednichenko. "REE, Y, Th, U and Mn systematics of Upper Devonian conodonts in the West Uralian Folded Zone (Southern Urals)." LITHOSPHERE, no. 2 (June 12, 2019): 250–68. http://dx.doi.org/10.24930/1681-9004-2019-19-2-250-268.
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Research subject. This article presents the results of a study undertaken to investigate the systematics of rare-earth elements (REE), Y, Th, U and Mn in the Upper Devonian conodonts of the Western Ural region of the foreland fold of the Southern Urals (Askyn and Makarovo horizons, Kukkarauk, Ryauzyak and Lemezinsky sections).Methods. The conodonts were isolated from carbonate rocks using the conventional method of their dissolution in organic acids, mainly formic acid. Sample preparation and mass spectrometric analysis were carried out in the class 1000 and 10 000 cleanroom facilities of the Zavaritsky Institute of Geology and Geochemistry. A PerkinElmer ELAN 9000 quadrupole ICP mass-spectrometer was used to determine the concentration of the trace elements under study. Sm and Nd isotope ratios were measured from a 3% nitric acid solution by a Thermo Fischer Neptune Plus multicollector inductively coupled plasma mass-spectrometer. The long-term reproducibility and accuracy of the measurement procedure were evaluated using a standard Merck Nd solution based on the NIST Nd2O3 and yielded 143Nd/144Nd = 0.511720 ± 15 (1 SD, n = 40).Results. The established features of PAAS-normalised lanthanide distributions in the conodont bulk samples (10.4–21.8 mg), Ce-anomalies, high REE values (173–1211 ppm) and a number of other parameters indicate the leading role of late diagenetic processes in the formation of conodont REE systematics. This is also evidenced by the Y/Ho values (≈26–32) specific for the studied conodont samples.Conclusions. In general, the distribution of lanthanides in the conodonts suggests that this process was mainly controlled by a lithogenic (from 90 to more than 99%) REE source. The εNd (t) (–4.0…–2.8) values characteristic of the cono donts of the Askyn and Makarovo horizons suggest that their Nd isotopic composition was either formed under the influence of the open ocean (island arc basin), characterised by a significant share of radiogenic Nd, or due to the entry of radiogenic waters of the ocean into the shelf zone at the peak of the marine transgression that took place in the region under consideration in the Famennian.
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Vervoort, Jeffrey D., and John C. Green. "Origin of evolved magmas in the Midcontinent rift system, northeast Minnesota: Nd-isotope evidence for melting of Archean crust." Canadian Journal of Earth Sciences 34, no. 4 (April 1, 1997): 521–35. http://dx.doi.org/10.1139/e17-042.
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The North Shore Volcanic Group (NSVG) of northeast Minnesota is a thick (9 km) sequence of plateau volcanic rocks that constitutes an important part of the Midcontinent rift system. This volcanic sequence is unique among the Midcontinent rift lavas, because it is composed of up to 25% rhyolite flows. We have analyzed Sm- and Nd-isotope compositions of 20 of the largest rhyolite and icelandite flows from the NSVG and seven comparably sized granophyres in the subjacent Duluth and Beaver Bay complexes. The lavas vary in composition from primitive basalt and basaltic andesite to icelandite and rhyolite, with a bimodal distribution. The rhyolites have much lower initial εNd values (−2 to −15, most samples < −10) than either the icelandites (0 to −6) or granophyres (0 to −8). Most rhyolites cannot be related to either the icelandites or more mafic magmas by simple fractionation, but rather have been produced by melting and assimilation of older, evolved crust. We suggest that the bimodal magmatism in the NSVG, and probably throughout the Midcontinent rift, has been produced by two fundamentally different processes. The bulk of the magmatism is basaltic; magmas originate in the mantle and migrate through the lithosphère with minor compositional change. Assimilation and fractional crystallization occur to varying degrees in the crust and, in some cases, produce icelandites, some small-volume rhyolites, and the granophyres, with Nd compositions dominated by the mantle component. The melting that produced the large-volume rhyolites is the result of a multistage process induced by these mantle-derived magmas that pond within the crust. This process appears to occur during a period of slowed extension and causes widespread heating and eventually localized extensive melting of the crust.
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Slezak, Paul, Murray W. Hitzman, David van Acken, Eoin Dunlevy, David Chew, Foteini Drakou, and Mark Holdstock. "Petrogenesis of the Limerick Igneous Suite: insights into the causes of post-eruptive alteration and the magmatic sources underlying the Iapetus Suture in SW Ireland." Journal of the Geological Society, November 22, 2022. http://dx.doi.org/10.1144/jgs2022-039.
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The Limerick Igneous Suite (LIS) in Co. Limerick, SW Ireland, contains two distinct basaltic igneous units: the Knockroe and Knockseefin, which are expressed as hypabyssal intrusions, porphyritic dykes, diatremes, lava flows, agglomerates, and tuffs. These units make up two distinct evolutionary trends: the Knockroe igneous units which range from alkaline basalts to trachyandesites and the Knockseefin igneous units which range from alkaline basalts to basanites. Uranium‒Pb dating of apatite establishes a primary crystallisation age of c. 350 Ma for the Knockroe units. Strontium and Nd isotopes from the least altered Knockroe bulk rock samples range from 0.70301 – 0.70454 and 0.512457 – 0.512493, respectively. The Sr isotopes for the least altered Knockseefin samples are similar, ranging from 0.70325 – 0.70386, but the Nd values are slightly more radiogenic, ranging from 0.512431 – 0.512437. Altered samples are buffered against changes in Nd, but some show excursions towards radiogenic Sr, suggesting contamination from Carboniferous seawater and/or introduction of Rb. Rubidium–Sr calculations conducted on altered Knockroe samples, returned ages within uncertainty of the U‒Pb dates. The bulk rock isotope values, normalised trace element plots as well as Zr/Sm, Ce/Pb, and Nb/U values demonstrate the LIS is comparable to ocean island basalts (OIBs) and Ce/Y and Zr/Nb values suggest the units likely originated from low degrees of partial melting likely caused by extension related to the assembly of Laurentia and Gondwana. Supplementary material: https://doi.org/10.6084/m9.figshare.c.6297395
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Dora, M. L., Tushar Meshram, Srinivas R. Baswani, Vivek P. Malviya, Satya Narayana Mahapatro, Jitendra K. Dash, Rajkumar R. Meshram, et al. "The Paleo-Mesoarchaean Gondpipri Mafic-Ultramafic Intrusions, Western Bastar Archaean Craton, Central India: Insights from Bulk-Rock Geochemistry and Sm-Nd and S Isotope Studies on the Formation of Ni-Cu-PGE Mineralization." Economic Geology, June 20, 2022. http://dx.doi.org/10.5382/econgeo.4947.
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Abstract Magmatic Ni-Cu-platinum group element (PGE)-Te mineralization in the Gondpipri mafic-ultramafic layered intrusion of ca. 3323 ± 74 Ma age, western Bastar craton, central India, is one of the most prospective exploration targets for magmatic sulfides in India. The Gondpipri layered intrusion is divided into two distinct group of rocks based on their mineralization potential, which includes (1) mineralized layered gabbro and pyroxenite and (2) a barren olivine gabbro intrusion. The host rocks show Cu + Ni concentrations up to 5,000 ppm with a Cu/Ni ratio <1 and all platinum group element (PGE) values between 0.1 and 1.1 ppm. Mineralization occurs in two modes: type I mineralization occurring as blebs, specks, and dissemination and type II mineralization occurring as stringers and minor veins. The geochemical data suggest that the parental magma of the host rock was generated at depths between spinel and garnet peridotite mantle source regions and subsequently modified by assimilation fractional crystallization (AFC) of the continental crust. High large ion lithophile elements, Th/Yb ratios of the studied rocks, and Sm-Nd isotope studies are consistent with a depleted mantle source. The geochemical proxies such as Th versus Ba/Th and (Ta/La)PM versus (Hf/Sm)PM and higher Sr/Nd (2.21–82.58) ratios indicate involvement of fluid-related subduction metasomatism and enrichment processes in an island-arc tectonic setting. Mineral assemblages and textural relationship between platinum group minerals (PGMs) and base metal sulfides suggest that sulfide-silicate liquid immiscibility was brought about by the precipitation of magnetite/Cr magnetite resulting in sulfide saturation in the melt by decreasing S solubility. Sulfur isotope compositions (δ34S: 1.61–3.30‰) and Sm-Nd geochemistry suggest that the sulfur was added in the tholeiitic magma by magmatic process. Crustal contamination played a significant role in sulfide saturation and in bringing about PGE and Te, As, Bi, Sb, Se (TABS) mineralization. PGM-NiTeBi developed at relatively low temperatures, where moncheite (PtPd)Te2 and merenskyite (PdTe) were formed at 650°C. The identification of Ni-Cu-PGM-Te in the margin of the western Bastar craton boosts deeper subsurface exploration.
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Giovanardi, T., V. A. V. Girardi, C. T. Correia, S. Sinigoi, C. C. G. Tassinari, and M. Mazzucchelli. "U-Pb zircon SHRIMP data from the Cana Brava layered complex: new constraints for the mafic-ultramafic intrusions of Northern Goiás, Brazil." Open Geosciences 7, no. 1 (January 13, 2015). http://dx.doi.org/10.1515/geo-2015-0015.
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AbstractThe Cana Brava Complex is the northernmost and least well known layered intrusion of a discontinuous belt of mafic-ultramafic massifs within the Brasilia Belt, which also comprises theNiquelândia and Barro Alto complexes. Available geochronological data from a range of techniques (K/Ar, Ar/Ar, Rb/Sr, Sm/Nd and U/Pb) provide a range of possible ages (time span from 3.9 Ga to 450 Ma), hence a precise and reliable age for the Cana Brava Complex is still lacking. Also, preliminary isotopic and geochemical data of the Cana Brava Complex suggest a significant crustal contamination, which could have affected bulk-rock Sr and Nd systematics resulting in meaningless age determinations. In this paper, we present new U-Pb SHRIMP zircon analyses from four samples of different units of the Cana Brava Complexwhich suggest that the intrusion occurred during the Neoproterozoic, between 800 and 780 Ma, i.e. at the same age ofNiquelândia. Discordant older
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Ganguly, Jibamitra. "Academic Reminiscences and Thermodynamics-Kinetics of Thermo-Barometry-Chronology." Geochemical Perspectives, April 2021, 1–144. http://dx.doi.org/10.7185/geochempersp.10.1.
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This article has three major components that include, in addition to the technical aspects, reminiscences of my academic upbringing, my move to the USA from India, and my professional career. I have recounted many stories that I hope convey some sense of time, especially in these two countries with vastly different cultures, my personal journey with its ups and downs and how I made the transition to an academic career path in USA even though that was not in my future plan as a young man. The development of the field of thermobarometry and its integration with diffusion and crystal kinetic modelling of compositional zoning (or lack thereof) and cation ordering in minerals have led to important quantitative constraints on the pressure-temperature-time evolution of terrestrial rocks and meteorites. I review the historical developments in these areas and a segment of my own research spanning the period of 1964-2021. The foundational works of the thermometry of metamorphic rocks and palaeothermometry were laid at the University of Chicago around 1950. Subsequently, the synergetic growth of thermodynamics and experimental studies in petrology in the 1960s and 1970s, along with the introduction of electron microprobe as a nondestructive analytical tool with micron scale resolution, gave a major boost to the field of thermobarometry. There were also significant new developments in the field of thermodynamics of solid solutions in the petrology community and demonstration from observational data, countering strong scepticism, that the principles of classical thermodynamics were applicable to “complex natural systems”. The section on thermodynamic basis of thermobarometry concludes with a discussion of the thermodynamics of trace element and single mineral thermometry. I further deal with the experimental protocols, along with selected examples, for phase equilibrium studies that provide the bedrock foundation for the field of thermobarometry based on elemental compositions of coexisting minerals in a rock. It is followed by an account of the controversies and international meetings relating to the aluminum silicate and peridotite phase diagrams that play crucial roles in the thermobarometry of metamorphic rocks and mantle xenoliths, respectively. The construction of quantitative petrogenetic grids to display stability relations of minerals in multicomponent–multiphase systems came into play in the field of metamorphic petrology in the mid-1960s and early 1970s. Augmented by experimental data, these petrogenetic grids led to important discoveries about the P-T-<em>f</em>(O<sub>2</sub>) and bulk compositional controls on the stability of certain “index” minerals that are used to define metamorphic isograds and different types of regional metamorphism; one such grid also opened up a new field that came to be known as ultra-high temperature metamorphism. The construction of petrogenetic grids has now evolved to computer based calculations of complex equilibrium P-T phase diagrams, commonly referred to as “pseudosections”, by minimisation of Gibbs free energy of a system with fixed bulk composition. I discuss these historical developments and modern advancements. Subsequently I highlight some aspects of thermobarometry and diffusion kinetic modelling of selected natural samples along with their broader implications and present a critical discussion of different protocols for thermobarometry of natural assemblages. Following up on the introductory historical perspective of development of palaeothermometry, I discuss the modern advancements using density functional theory (DFT). Examples of DFT based calculations have been shown for hydrogen isotope fractionation in mineral-water/hydrogen systems and “clumped isotope” thermometry. The hydrogen isotope fractionation data led the development of new low temperature palaeothermometers using serpentine-talc/brucite mineral pairs. These results enable simultaneous solutions of both temperature and source of fluid in the serpentinisation process of rocks. The final section is devoted to high temperature thermochronology dealing with the problems of closure temperature of decay systems in minerals and the use of bulk and spatial resetting of mineral age according to a specific decay system to determine cooling rates of the host rocks. Complications arise in the interpretation of mineral ages determined by such decay systems as <sup>176</sup>Lu-<sup>176</sup>Hf or the short- lived system <sup>53</sup>Mn-<sup>53</sup>Cr in which the parent nuclide has a much lower closure temperature than the corresponding daughter product. Numerical simulations help explain the discrepancy between the <sup>176</sup>Lu-<sup>176</sup>Hf and <sup>147</sup>Sm-<sup>143</sup>Nd ages of garnets in metamorphic rocks and enable construction of the entire T-t cycle from the discrepant ages and some additional constraints.