scholarly journals From peridotite to fuchsite bearing quartzite via carbonation and weathering: with implications for the Pb budget of continental crust

2021 ◽  
Vol 176 (11) ◽  
Author(s):  
Håkon Austrheim ◽  
Fernando Corfu ◽  
Christian J. Renggli
Keyword(s):  
Continental Crust ◽  
Mobile Elements ◽  
Chemical Fractionation ◽  
Selective Transfer ◽  
Isotopic Compositions ◽  
Rock Types ◽  
Serpentinized Peridotite ◽  
Mineral Assemblages ◽  
Se Norway

AbstractExtensive carbonation of peridotite results in listvenite, a rock composed of magnesite and quartz. At Gråberget, Røros, SE-Norway, a variably serpentinized peridotite body, surrounded by the Røros schists, a former abyssal sediment displays all stages of transformation of peridotite to quartzite. In this paper we record the sequence of steps in this process by combining the observation of mineral assemblages, textural relationships and geochemistry, and variations in Pb isotopic compositions. Initial serpentinization, a stage that also involved an enrichment in fluid-mobile elements (Pb, Sb and As), was followed by carbonation through CO2 fluids that formed soapstone, and eventually listvenite. The listvenite grades by decreasing amounts of carbonates into fuchsite bearing quartzite. The carbonates dissolved during supergene alteration and formed pores coated with oxides of Fe, Mn and Ni resulting in a brown rock color. The quartzite displays porous stylolites enriched in Pb, As and Sb and fuchsite with porous chromite grains as the only relicts of the original mineralogy in the peridotite. The dissolution of the carbonate occurred at oxidizing conditions at temperatures below 150 °C, where the solubility of magnesite is higher than that of quartz. Formation of quartzite from peridotite is supported by low REE contents and lack of zircons in the two rock types. The transformation involved enrichment of Pb, coupled with the elimination of Mg and enrichment of Si. This chemical fractionation and selective transfer of elements to the continents is an important mechanism and needs to be taken into account in models of continental evolution.

scholarly journals The role of buoyancy in the fate of ultra-high-pressure eclogite

2019 ◽  
Vol 9 (1) ◽  
Author(s):  
Timothy Chapman ◽  
Geoffrey L. Clarke ◽  
Nathan R. Daczko
Keyword(s):  
High Pressure ◽  
Continental Crust ◽  
Upper Mantle ◽  
Crustal Rocks ◽  
Ultra High Pressure ◽  
Rock Types ◽  
Mineral Assemblages ◽  
Facies Metamorphism ◽  
Point Of No Return

AbstractEclogite facies metamorphism of the lithosphere forms dense mineral assemblages at high- (1.6–2.4 GPa) to ultra-high-pressure (>2.4–12 GPa: UHP) conditions that drive slab-pull forces during its subduction to lower mantle conditions. The relative densities of mantle and lithospheric components places theoretical limits for the re-exposure, and peak conditions expected, of subducted lithosphere. Exposed eclogite terranes dominated by rock denser than the upper mantle are problematic, as are interpretations of UHP conditions in buoyant rock types. Their subduction and exposure require processes that overcame predicted buoyancy forces. Phase equilibria modelling indicates that depths of 50–60 km (P = 1.4–1.8 GPa) and 85–160 km (P = 2.6–5 GPa) present thresholds for pull force in end-member oceanic and continental lithosphere, respectively. The point of no-return for subducted silicic crustal rocks is between 160 and 260 km (P = 5.5–9 GPa), limiting the likelihood of stishovite–wadeite–K-hollandite-bearing assemblages being preserved in equilibrated assemblages. The subduction of buoyant continental crust requires its anchoring to denser mafic and ultramafic lithosphere in ratios below 1:3 for the continental crust to reach depths of UHP conditions (85–160 km), and above 2:3 for it to reach extreme depths (>160 km). The buoyant escape of continental crust following its detachment from an anchored situation could carry minor proportions of other rocks that are denser than the upper mantle. However, instances of rocks returned from well-beyond these limits require exceptional exhumation dynamics, plausibly coupled with the effects of incomplete metamorphism to retain less dense low-P phases.

scholarly journals Igneous Rock Associations 26. Lamproites, Exotic Potassic Alkaline Rocks: A Review of their Nomenclature, Characterization and Origins

2020 ◽  
Vol 47 (3) ◽  
pp. 119-142
Author(s):  
Roger H. Mitchell
Keyword(s):  
Partial Melting ◽  
Lithospheric Mantle ◽  
Tectonic Setting ◽  
Alkaline Rock ◽  
Mantle Metasomatism ◽  
Geochemical Evolution ◽  
Alkaline Rocks ◽  
Isotopic Compositions ◽  
Economic Significance ◽  
Rock Types

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.

Sur quelques aspects de la zoneographie alpine dans les Alpes cottiennes meridionales

1962 ◽  
Vol S7-IV (4) ◽  
pp. 477-491
Author(s):  
Andre Michard
Keyword(s):  
Tectonic Activity ◽  
Structural Units ◽  
Zonal Distribution ◽  
Rock Types ◽  
Metamorphic Mineral ◽  
Mineral Assemblages ◽  
Alpine Metamorphism

Abstract A Permo-Carboniferous series and a polymetamorphic series are distinguished in the rocks of the southern Cottian Alps, Italy. The metamorphic mineral assemblages and facies of the principal rock types represented in each series and their zonal distribution are discussed. Alpine metamorphism is considered to have occurred after the tectonic activity responsible for superposition of the three structural units recognized in the region between Varaita and Stura.

scholarly journals Equilibrium crystallization of massif-type anorthosite residual melts: a case study from the 1.64 Ga Ahvenisto complex, Southeastern Finland

2020 ◽  
Vol 175 (9) ◽  
Author(s):  
Riikka Fred ◽  
Aku Heinonen ◽  
Jussi S. Heinonen
Keyword(s):  
Magma Chambers ◽  
Equilibrium Crystallization ◽  
Liquid Line ◽  
Rock Types ◽  
Residual Magma ◽  
Icp Ms ◽  
Mineral Assemblages ◽  
Residual Melt ◽  
Liquid Line Of Descent

Abstract Fe–Ti–P-rich mafic to intermediate rocks (monzodiorites and oxide–apatite–gabbronorites, OAGNs) are found as small intrusions in most AMCG (anorthosite–magnerite–charnokite–granite) suites. The origin of the monzodioritic rocks is still debated, but in many studies, they are presumed to represent residual liquid compositions after fractionation of anorthositic cumulates. In the 1.64 Ga Ahvenisto complex, SE Finland, monzodioritic rocks occur as minor dike-like lenses closely associated with anorthositic rocks. We report new field, petrographic, and geochemical (XRF, ICP-MS, EMPA) data complemented with crystallization modeling (rhyolite-MELTS, MAGFRAC) for the monzodioritic rocks, apatite–oxide–gabbronorite, and olivine-bearing anorthositic rocks of the Ahvenisto complex. The presented evidence suggest that the monzodioritic rocks closely represent melt compositions while the apatite–oxide–gabbronorite and olivine-bearing anorthositic rocks are cumulates. The monzodioritic rocks seem to form a liquid line of descent (LLD) from primitive olivine monzodiorites to more evolved monzodiorites. Petrological modeling suggests that the interpreted LLD closely corresponds to a residual melt trend left after fractional crystallization (FC) and formation of the cumulate anorthositic rocks and minor apatite–oxide–gabbronorite in shallow magma chambers. Consequent equilibrium crystallization (EC) of separate monzodioritic residual magma batches can produce the observed mineral assemblages and the low Mg numbers measured from olivine (Fo25–45) and pyroxenes (En48–63, Mg#cpx 60–69). The monzodioritic rocks and apatite–oxide–gabbronorites show similar petrological and geochemical characteristics to corresponding rock types in other AMCG suites, and the model described in this study could be applicable to them as well.

Isotope-abundance variations of selected elements (IUPAC Technical Report)

2002 ◽  
Vol 74 (10) ◽  
pp. 1987-2017 ◽  
Author(s):  
Tyler B. Coplen ◽  
John Karl Böhlke ◽  
P. De Bièvre ◽  
T. Ding ◽  
N. E. Holden ◽  
...  
Keyword(s):  
Chemical Elements ◽  
Radioactive Decay ◽  
Atomic Weight ◽  
Chemical Fractionation ◽  
Isotope Abundance ◽  
Isotopic Compositions ◽  
Technical Report ◽  
Isotopic Abundances ◽  
Calcium Iron ◽  
Physical And Chemical

Documented variations in the isotopic compositions of some chemical elements are responsible for expanded uncertainties in the standard atomic weights published by the Commission on Atomic Weights and Isotopic Abundances of the International Union of Pure and Applied Chemistry. This report summarizes reported variations in the isotopic compositions of 20 elements that are due to physical and chemical fractionation processes (not due to radioactive decay) and their effects on the standard atomic-weight uncertainties. For 11 of those elements (hydrogen, lithium, boron, carbon, nitrogen, oxygen, silicon, sulfur, chlorine, copper, and selenium), standard atomic-weight uncertainties have been assigned values that are substantially larger than analytical uncertainties because of common isotope-abundance variations in materials of natural terrestrial origin. For 2 elements (chromium and thallium), recently reported isotope-abundance variations potentially are large enough to result in future expansion of their atomic-weight uncertainties. For 7 elements (magnesium, calcium, iron, zinc, molybdenum, palladium, and tellurium), documented isotope variations in materials of natural ter- restrial origin are too small to have a significant effect on their standard atomic-weight uncertainties. This compilation indicates the extent to which the atomic weight of an element in a given material may differ from the standard atomic weight of the element. For most elements given above, data are graphically illustrated by a diagram in which the materials are specified in the ordinate and the compositional ranges are plotted along the abscissa in scales of (1) atomic weight, (2) mole fraction of a selected isotope, and (3) delta value of a selected isotope ratio.

scholarly journals Measurement of geologic nitrogen using mass spectrometry, colourimetry, and a newly adapted fluorometry technique

2016 ◽  
Author(s):  
Benjamin W. Johnson ◽  
Natashia Drage ◽  
Jody Spence ◽  
Nova Hanson ◽  
Rana El-Sabaawi ◽  
...  
Keyword(s):  
Mass Spectrometry ◽  
Continental Crust ◽  
Large Scale ◽  
Organic N ◽  
Stable Form ◽  
Earth System ◽  
N Budget ◽  
Rock Types ◽  
The Earth ◽  
Assess Quality

Abstract. Long viewed as a mostly noble, atmospheric species, recent work demonstrates that nitrogen in fact cycles throughout the Earth system, including the atmosphere, biosphere, oceans, and solid Earth. Despite this new-found behaviour, more thorough investigation of N in geologic materials is limited due to its low concentration (1 to 10 s ppm) and difficulty in analysis. In addition, N can exist in multiple species (NO3−, NH4+, N2, organic-N), and determining which species is actually quantified can be difficult. In rocks and minerals, NH4+ is the most stable form of N over geologic time scales. As such, techniques designed to measure NH4+ can be particularly useful. We measured a number of geochemical rock standards using three different techniques: mass spectrometry, colourimetry, and fluorometry. The fluorometry approach is a novel adaptation of a technique commonly used in biologic science, applied herein to geologic NH4+. Briefly, NH4+ can be quantified by HF-dissolution, neutralization, addition of a fluorescing reagent, and analysis on a standard fluorometer. We reproduce published values for several rock standards (BCR-2, BHVO-2, and G-2), especially if an additional distillation step is performed. While it is difficult to assess quality of each method, due to lack of international geologic N standards, fluorometry appears better suited to analyzing mineral-bound NH4+ than mass spectrometry, and is a simpler, quicker alternative to colourimetry. To demonstrate a potential application of fluorometry, we calculated a continental crust N budget based on new measurements. We used glacial tills as a proxy for upper crust and analyzed several poorly constrained rock types (volcanics, mid-crustal xenoliths) to determine that the continental crust contains ∼ 2 × 1018 kg N. This estimate is consistent with recent budget estimates, and shows that fluorometry is appropriate for large-scale questions where high sample throughput is helpful. Lastly, we report the first δ15N values of six rock standards: BCR-2 (1.05 ± 0.4 ‰), BHVO-2 (−0.3 ± 0.2 ‰), G-2 (1.23 ± 1.32 ‰), LKSD-4 (3.59 ± 0.1 ‰), Till-4 (6.33 ± 0.1 ‰), and SY-4 (2.13 ± 0.5 ‰). The need for international geologic N standards is crucial for further investigation of the Earth system N cycle, and we suggest that existing rock standards may be suited to this need.

Combined Sm-Nd and Rb-Sr isotope systematics in the Donegal granitoids and their petrogenetic implications

1990 ◽  
Vol 127 (1) ◽  
pp. 75-80 ◽  
Author(s):  
C. S. Dempsey ◽  
A. N. Halliday ◽  
I. G. Meighan
Keyword(s):  
Continental Crust ◽  
Isotope Ratios ◽  
Amphibolite Facies ◽  
Sr Isotope ◽  
Isotopic Data ◽  
Isotopic Compositions ◽  
Isotopic Variations ◽  
Isotope Systematics

AbstractThe metaluminous to peraluminous granitoids of the Donegal batholith, northwest Ireland, were emplaced at c. 400 Ma into greenschist-amphibolite facies metasediments of the Dalradian Supergroup. Sm-Nd and Rb-Sr isotopic data are provided for eleven granitoid samples from six of the plutons and one specimen from the northeast granodiorite pluton of the Newry complex, Co. Down; the Donegal results reveal essentially similar initial Sr isotope ratios (0.7051–0.7068) but highly variable initial eNd values, −1.2 to −8.3 (and −0.5 for Newry). Certain granitoids have distinctive Nd isotopic compositions characteristic of the involvement of old, LREE-enriched continental crust in some cases or young crust and/or mantle-derived magmas in others. The Nd and Sr isotopic variations can be explained by a variety of mixing hypotheses.

Accretion of continental crust: thermal and geochemical consequences

1981 ◽  
Vol 301 (1461) ◽  
pp. 347-357 ◽  
Keyword(s):  
Continental Crust ◽  
Numerical Models ◽  
Granulite Facies ◽  
Vapour Phase ◽  
Controlled Processes ◽  
Sialic Crust ◽  
Partial Melt ◽  
The Past ◽  
Profound Influence ◽  
Mineral Assemblages

The irreversible chemical differentiation of the Earth’s mantle to produce sialic crust over the past 3900 Ma has most probably occurred during widely separated, but short-lived, accretion episodes. These episodes involved the massive addition of juvenile sialic magma to the Earth’s surface, thickening pre-existing crust. Simple numerical simulations, based on tectonic, petrological and geochemical observations on Archaean high-grade orthogneiss terranes, have been used to explore the metamorphic and geochemical consequences of massive thickening of sialic crust during short-lived accretion episodes. The location of the main sites of magmatic addition within the crust exert a profound influence on the thermal régimes. Geochemical differentiation of the continental crust by partial-melt and vapour-phase-controlled processes, and the development of granulite facies mineral assemblages can be integrated with the simple numerical models. Finally, the survival of thick Archaean continental crust imples the contemporaneous stabilization of thick lithospheric substructures to the newly formed continental masses.

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