scholarly journals Earth’s Nitrogen and Carbon Cycles

2021 ◽  
Vol 217 (3) ◽  
Author(s):  
Ralf Halama ◽  
Gray Bebout

AbstractUnderstanding the Earth’s geological nitrogen (N) and carbon (C) cycles is fundamental for assessing the distribution of these volatiles between solid Earth (core, mantle and crust), oceans and atmosphere. This Special Communication about the Earth’s N and C cycles contains material that is relevant for researchers who are interested in the Topical Collection on planetary evolution “Reading Terrestrial Planet Evolution in Isotopes and Element Measurements”. Variations in the fluxes of N and C between these major reservoirs through geological time influenced the evolution and determined the unique composition of the Earth’s atmosphere. Here we review several key geological aspects of the N and C cycles of which our understanding has significantly advanced during the last decade through field-based, experimental and theoretical studies. Subduction zones are the most important pathway of both N and C from the Earth’s surface into the deep Earth. A key question in the flux quantification is how much of the volatile elements is stored in the downgoing slab and introduced into the mantle and how much is returned back to the surface and the atmosphere through arc magmatism. For N, the retention of N as $\text{NH}_{4}^{+}$ NH 4 + in minerals has a major influence on fluxes between reservoirs. The temperature-dependent stability of $\text{NH}_{4}^{+}$ NH 4 + -bearing minerals determines whether N is predominantly retained in the slab to mantle depths (in subduction zones with a low geothermal gradient) or devolatilized (in subduction zones with a high geothermal gradient). Several lines of evidence suggest that the mantle is regassing with respect to N due to a net influx of subducted N over time, but this issue is highly debated and evidence to the contrary also exists. Nevertheless, there is consensus that the majority of the planetary N budget is stored in the Earth’s mantle, with the continental crust also constituting a significant N reservoir. For C, release from the subducting slab occurs through decarbonation reactions, dissolution and formation of carbonatitic liquids, but reprecipitation of C in the slab or the forearc mantle wedge may limit the effectiveness of direct return of C into the atmosphere. Carbon release through regional metamorphism in collision zone orogens also has potentially profound effects on C release into the atmosphere and consensus has emerged that such orogens are sources rather than sinks of atmospheric CO2. On shorter timescales, contact metamorphism through interaction of mantle-derived magmas with C-bearing country rocks, and the resulting release of large quantities of CH4 and/or CO2, has been linked to global warming events.

2016 ◽  
Vol 4 (1) ◽  
pp. 111-120 ◽  
Author(s):  
Shu-Guang Li ◽  
Wei Yang ◽  
Shan Ke ◽  
Xunan Meng ◽  
Hengci Tian ◽  
...  

Abstract Although deep carbon recycling plays an important role in the atmospheric CO2 budget and climate changes through geological time, the precise mechanisms remain poorly understood. Since recycled sedimentary carbonate through plate subduction is the main light-δ26Mg reservoir within deep-Earth, Mg isotope variation in mantle-derived melts provides a novel perspective when investigating deep carbon cycling. Here, we show that the Late Cretaceous and Cenozoic continental basalts from 13 regions covering the whole of eastern China have low δ26Mg isotopic compositions, while the Early Cretaceous basalts from the same area and the island arc basalts from circum-Pacific subduction zones have mantle-like or heavy Mg isotopic characteristics. Thus, a large-scale mantle low δ26Mg anomaly in eastern China has been delineated, suggesting the contribution of sedimentary carbonates recycled into the upper mantle, but limited into the lower mantle. This large-scale spatial and temporal variation of Mg isotopes in the mantle places severe constraints on deep carbon recycling via oceanic subduction.


1976 ◽  
Vol 13 (3) ◽  
pp. 405-421 ◽  
Author(s):  
Lee C. Pigage

Pelitic metasediments immediately southwest of Yale, British Columbia contain mineral assemblages characteristic of staurolite through sillimanite zones of the Barrovian facies series. Microprobe analyses of major constituent phases in the pelites are combined with linear regression techniques to formulate probable kyanite- and sillimanite-forming reactions.A zone some 3 km wide contains the assemblage staurolite–kyanite–garnet–biotite–muscovite–quartz–plagioclase-ilmenite-rutile, which is univariant in AFM projection. Within precision limits of microprobe analysis, this assemblage is also univariant using linear regression techniques. Univariant relations are possible if [Formula: see text] with the composition of the fluid phase being buffered by the progressing reaction. This assemblage emphasizes the need for precise analyses when using the regression method, since minor components are often within permissible error limits rather than being balanced.Pelitic and calc-silicate assemblages from the metasediments restrict estimates of pressure–temperature conditions during regional metamorphism to 6–8 kbar and 550–770 °C. Pseudomorphs after andalusite indicate that contact metamorphism preceded regional upgrading of the pelites.


Clay Minerals ◽  
2018 ◽  
Vol 53 (3) ◽  
pp. 351-375
Author(s):  
Annette Süssenberger ◽  
Solveig Pospiech ◽  
Susanne Theodora Schmidt

ABSTRACTThe [MnO|SiO2,Al2O3,FeO,MgO] balanced ratio (i.e. the isometric log-ratio of the MnO concentration relative to the concentration of SiO2, Al2O3, FeO and MgO) of chlorite and of whole-rock composition is an effective discriminant between Mesozoic stratigraphic formations in the Magallanes Basin (Chile). The MnO content in chlorite is only controlled by the host rock chemistry and is dependent on the geological environment. The MnO content in chlorite remains unchanged at low-grade metamorphic conditions. Single-grain chlorite analysis (n = 1042, electron microprobe) and whole-rock analysis (n = 40, X-ray fluorescence) were used to discriminate stratigraphic formations and to decipher differences in the depositional environment in the Magallanes Basin. The samples are from one Upper Jurassic and three Cretaceous sedimentary units that were affected either by low-grade regional metamorphism or by Miocene contact metamorphism. The highest [MnO|SiO2,Al2O3,FeO,MgO] values are recorded in the upper Zapata Formation. The Punta Barrosa, Cerro Toro and Tobífera Formations show slightly lower [MnO|SiO2,Al2O3,FeO,MgO] values. Elevated [MnO|SiO2,Al2O3,FeO,MgO] values at the transition between Zapata and Punta Barrosa Formations record an oxygenated shallow marine environment that can be linked to the closure of the Rocas Verdes Basin and the onset of fold-and-thrust belt formation. Decreasing [MnO|SiO2,Al2O3,FeO,MgO] values from the Punta Barrosa towards the Cerro Toro Formation indicate gradually increasing water depths during the Upper Cretaceous that correlate well with the global sea level.


2020 ◽  
Author(s):  
Florence Cagnard ◽  
Didier Lahondère ◽  
Benjamin Le Bayon ◽  
Aurore Hertout ◽  
Thierry Baudin ◽  
...  

<p>The event geological maps consist in innovative numerical maps that were just designed and produced for the first time, as part of the RGF (“French Geological Referential”) mapping program in the Pyrenees. Rocks acquire their mineralogical, structural and textural characteristics through a complex geological history reflecting successive stages of transformation (i.e. metamorphism, deformation, alteration…), so called “geological events”. Classical geological maps can only represent some of these events.  In the Pyrenean orogenic belt, which results from a polyphase tectono-metamorphic history over 600 Ma (from Precambrian to present), 3400 geological events were identified. Such geological events were classified by types (e.g. deposit, volcanism, intrusion, metamorphism, weathering, hydrothermal alteration…) and time periods. They were referenced into a database and associated to mapped features (120,000 polygons and lines), coming from a compilation of 60 geological maps at 1: 50,000 scale.</p><p>In the Pyrenees, Naturally Occurring Asbestos (NOA) mostly occur in specific lithologies such as ultrabasic, basic and intermediate plutonic rocks, and meta-limestones. These rocks may be affected by different metamorphic events (i.e. hydrothermal alteration, greenschist and/or HT-LP regional metamorphism, contact metamorphism). We performed a GIS treatment to produce a predictive map of potential NOA hosting lithologies. This treatment crosses lithological and selected geological event informations (e.g different metamorphic and alteration events).</p><p>Subsequent geological field investigations with associated sampling and laboratory analyses (combining optical microscopy, microprobe and SEM analyses) allowed us to identify and characterize fibrous and asbestiform mineralogical species. Results of this work particularly emphasize: (i) the importance of actinolite-asbestos in doleritic rocks, and (ii) the occurrence of fibrous actinolite/tremolite in different marbles and skarns. Finally, we present a 1: 50,000 scale map of potential NOA occurrences in the Pyrenees.</p><p>Conversely, field observations allowed us to improve both the lithostratigraphic and the event geological maps, in particular with the identification of geological domains where intense hydrothermal alteration was not previously mapped. All the data (maps of potential NOA occurrences, field observations and results of laboratory analyses) are stored in a geospatial database, partly accessible to the public. This work illustrates a possible use of geological event maps as a powerful innovative and predictivity tool. This approach will be useful in the context of the evolution of French regulations now imposing the search for asbestos before all types of works in natural environments.</p>


1993 ◽  
Vol 130 (5) ◽  
pp. 631-638 ◽  
Author(s):  
N. J. Fortey ◽  
B. Roberts ◽  
S. R. Hirons

AbstractRegional variation of white mica (illite) crystallinity in the Skiddaw Group is set against the structural interpretation of Hughes, Cooper & Stone (1993, this issue) in which early- or pre-Ludlow deformation and slaty cleavage development (S1) were succeeded by southward thrusting and an associated development of S1 and S1 crenulation cleavages, possibly during early Devonian times. Kubier index (KI) values are plotted in relation to geological structure for a major part of the Skiddaw Group, and cross-sections constructed. The pattern is interpreted in terms of three processes: (1) diagenetic to low anchizonal burial metamorphism under a relatively high geothermal gradient during the late-arc stage (early to mid-Silurian) which preceded the orogenic phase and formation of S1; (2) upper anchizonal to epizonal metamorphism due to tectonic thickening and slaty cleavage development during the early to pre-Ludlow orogenic phase following closure of Iapetus; (3) late-tectonic uplift of already metamorphosed rocks by southward-directed movement on the Loweswater, Gasgale Gill and Causey Pike Thrusts possibly during early Devonian time. An analogous interpretation is made for the Skiddaw area, though with the addition of major post-S1 contact metamorphism.


2012 ◽  
Vol 86 (1) ◽  
pp. 48-64 ◽  
Author(s):  
Mahboobeh JAMSHIDI BADR ◽  
Fariborz MASOUDI ◽  
Alan S. COLLINS ◽  
Ali SORBI

2021 ◽  
Author(s):  
Han Hu ◽  
Alberto Vitale Brovarone ◽  
Lifei Zhang ◽  
Francesca Piccoli ◽  
Weigang Peng ◽  
...  

<p>The interaction between ascending carbonic fluids and rocks at shallow depths in orogenic systems plays an important role in carbon flux regulation. In subduction zones, most works have focused on processes related to carbon release from the subducting slab or sequestration via high-pressure (HP) carbonation of mafic or ultramafic lithologies. A significant fraction of the carbonic fluids released by deep metamorphic reactions can also reach orogenic complexes and react with crustal and exhumed metamorphic rocks. However, the amount of fluid-mediated carbonation that may take place at crustal depths in orogenic complexes is still poorly constrained.</p><p>We report the occurrence of retrograde mafic eclogites and metasomatic marbles in UHP units in the Chinese Tianshan orogenic belt. The mafic eclogites recorded two successive, superimposed metamorphic–metasomatic stages: graphite precipitation along fractures and veins at eclogite facies (Stage#1) and pervasive rock carbonation (i.e., Stage#2: silicate dissolution and carbonate precipitation) at retrograde amphibolite to greenschist facies. This work focuses on Stage#2 carbonation, which consists of the transformation of Stage#1 graphite-bearing eclogites into carbonate + paragonite (± zoisite) + quartz. We present field, microstructural, petrological, and geochemical results of carbonic fluid–rock interactions affecting exhumed mafic eclogites. These results are supported by thermodynamic modeling for low-pressure carbonation of mafic eclogite obtained by means of EQ3/6 and the Deep Earth Water model. Carbon and oxygen isotopic data and thermodynamic modeling suggest an external metasedimentary source for the Stage#2 carbonation. This deep carbon sequestration event can be referred to retrograde, greenschist-facies conditions at about 10 kbar and 450 °C, and redox conditions similar or more oxidized than the quartz–fayalite–magnetite (QFM) buffer. Our findings provide new insights into the reactivity of metastable, exhumed metamafic rocks with ascending carbonic fluids in orogenic systems. We conclude that retrograde, fluid-mediated rock carbonation can significantly impact on carbon fluxes from active collisional belts. </p>


1979 ◽  
Vol 43 (326) ◽  
pp. 201-209 ◽  
Author(s):  
P. K. Verma

SummaryThe Lower Cambrian Weymouth Formation at Nahant, Massachusetts, consisting of interbedded pelitic and calcareous rocks, was intruded by the Nahant Gabbro. The predominant metapelitic mineral assemblage of the contact aureole is quartz-muscovite-chlorite-magnetite-ilmenite. The calcareous hornfelses exhibit a varied mineral assemblage, and in some cases the variation can be spatially related to the intrusive. A number of cross-cutting thin mineral veins, many containing prehnite, are characteristically associated with these calcsilicate rocks.The minerals have been analysed by electron microprobe and this work indicates the presence of a possible solvus in the Fe3+-Al epidote solid solution series. At the physicochemical conditions of the formation of the Nahant hornfelses, the ferric mole fractions of coexisting epidotes are 0.49 and 0.98.Comparison with experimental work shows that the conditions of the contact metamorphism were T ≃ 500°C, Ptotal ≃ 2 kb, and XCO2 ≃ 0.2. However, the present assemblages are the result of a later low-grade regional metamorphism, the ultimate product of which was prehnite.


1973 ◽  
Vol 110 (6) ◽  
pp. 557-564 ◽  
Author(s):  
R. F. Holm

SummaryCharacteristics of mineralogy and texture, together with systematic changes in the enstatite content of orthopyroxene and in the anorthite content of plagioclase, indicate that intrusions of pyroxenite, norite and gabbro, and diorite and leucocratic diorite in the Dahomeyan gneiss of Ghana are genetically related, and that they belong to a subalkaline comagmatic series derived from a parental mafic magma. Partial recrystallization (deuteric) of the igneous rocks during and after intrusion and protoclasis produced minerals characteristic of temperatures higher than those that existed during the medium-grade regional metamorphism of the Dahomeyan gneiss, thus indicating that emplacement of the igneous rocks occurred late during, or perhaps after, the metamorphism. Contact metamorphism appears to have resulted in higher than normal anorthite contents of the plagioclase in the surrounding Dahomeyan gneiss.


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