Volatile elements affect the behaviour of magmas during their rise through the crust, and control the timing and energy of volcanic eruptions. When rapidly released into the atmosphere, volcanic gases such as carbon monoxide, carbon dioxide, methane, sulphur dioxide, hydrogen sulphide, hydrochloric acid, and chloromethane, can have a devastating impact on the global climate and biota. The best example from the geologic record is the emplacement of large igneous provinces, which are synchronous with several major Phanerozoic mass extinctions, indicating large igneous provinces as potential triggers of global-scale climatic and environmental changes via the release of volatiles. Large igneous provinces, often volumetrically dominated by continental flood basalts, are exceptional intraplate magmatic events involving huge magma volumes (up to 10 million cubic kilometres), that are emplaced episodically, leading to a pulsed release of their volatile phases. This potentially results in rapid rise of atmospheric carbon dioxide and global climate warming.
In addition to perturbing the climate, volcanic carbon dioxide plays a key role in the storage, ascent and eruption of magma, and drives the stability and evolution of magma reservoirs, regulating flood basalt magmatism and associated degassing fluxes. The importance of exsolved volatile phases (e.g., carbon dioxide-rich fluids) is highlighted by recently developed models of magmatic plumbing systems. In these new models, magma reservoirs are dominated by a crystalline mush, forming a multi-phase (i.e., solid, liquid and gas) system in which crystals, melt, and exsolved volatiles can interact and ascend independently towards the surface. Due to the low solubility of carbon dioxide in silicate melts, exsolution of carbon dioxide-rich fluids from melts occurs deep in the crust (i.e., within magmatic plumbing system) or even in the upper mantle, whereas at shallow depths the fluids typically become more water-rich. Since the exsolution of carbon dioxide changes the physical properties (e.g., density, viscosity and buoyancy) of magmas, it may therefore play a crucial role in their ascent, and could explain the pulsed eruptive style observed for large igneous provinces. However, direct evidence of carbon dioxide abundance in the deep magmas of large igneous provinces is lacking.
In a study published in the journal Nature Communications on 7 April 2020, Manfredo Capriolo and Andrea Marzoli of the Department of Geosciences at the University of Padova, László Aradi of the Lithosphere Fluid Research Lab at Eötvös Loránd University, Sara Callegaro, also of the Department of Geosciences at the University of Padova, and of the Centre for Earth Evolution and Dynamics at the University of Oslo, Jacopo Dal Corso, Robert Newton, Benjamin Mills, and Paul Wignall of the School of Earth and Environment at the University of Leeds, Omar Bartoli, again of the Department of Geosciences at the University of Padova, Don Baker of the Department of Earth and Planetary Sciences at McGill University, Nasrrddine Youbi of the Department of Geology at Cadi Ayyad University, the Instituto Dom Luiz at the University of Lisbon, and the Faculty of Geology and Geography at Tomsk State University, Laurent Remusat of the Muséum National d’Histoire Naturelle, Richard Spiess, once again of the Department of Geosciences at the University of Padova, and Csaba Szabó, also of the Lithosphere Fluid Research Lab at Eötvös Loránd University, investigate the history of volatiles in the magmas of the Central Atlantic Magmatic Province, of Earth’s largest large igneous provinces, by analysing volatiles in melt inclusions, particularly carbon dioxide.
Capriolo et al. examine the implications of these findings for magma eruption history and subsequent impact on the global climate. The emplacement of the Central Atlantic Magmatic Province (with (peak activity at 201.6–201.1 million years ago) occurred during the early stages of the Pangaea supercontinent break-up, leading to the opening of the Central Atlantic Ocean, and is synchronous with the End-Triassic Extinction, one of the five most severe biotic crises during the Phanerozoic. At least 3 million cubic kilometres of Central Atlantic Magmatic Province basaltic magmas were erupted or intruded into the continental crust over an area of 100 million square kilometres in brief pulses, from a few centuries to a few millennia each, characterised by high eruption rates. Such short and powerful eruptions may have had a severe impact on global climate by limiting the time in which negative feedback processes, such as the weathering of calcium-magnesium silicates, can abate warming and acidification. Central Atlantic Magmatic Province magmatism coincided in time with three marked negative carbon isotope excursions bracketing the main extinction period, and with an inferred strong rise of atmospheric carbon dioxide. In general, the pulsed magmatic and degassing activities of large igneous provinces can cause a rapid rise of atmospheric carbondioxide and greenhouse conditions, which are reflected by rapid carbon¹³ negative excursions recorded in both organic matter and carbonates, testifying to a global perturbation of the exogenic (i.e., superficial) carbon cycle. A rapid input of carbon¹³-depleted volatile phases into the atmosphere–hydrosphere system is possibly triggered by the emission of volcanic carbon dioxide, enhanced by the emission of carbon dioxide and methane derived from the thermal metamorphism of intruded organic matter-rich sediments.
Capriolo et al. screened a suite of over 200 intrusive and effusive samples from Central Atlantic Magmatic Province basaltic lava flows and sills in North America (USA and Canada), Africa (Morocco) and Europe (Portugal), and combined several in situ analytical techniques to investigate the presence of carbon dioxide within melt inclusion bubbles and constrain their formation depth. Capriolo et al.'s multidisciplinary analytical approach reveals that gas exsolution bubbles trapped in melt inclusions are a previously unappreciated direct proxy of volatile species degassed during large igneous province magmatic activity. In the case of the Central Atlantic Magmatic Province, Capriolo et al.'s analysis confirms the abundance of carbon dioxide (up to 10 000 gigatonnes of volcanic carbon dioxide degassed during Central Atlantic Magmatic Province emplacement) and indicates that at least part of this carbon has a middle- to lower-crust or mantle origin, suggesting that Central Atlantic Magmatic Province eruptions were rapid and potentially catastrophic for both climate and biosphere.
Map of Central Atlantic Magmatic Province in central Pangea at about 200 million years ago. The black symbols indicate the provenance of the studied samples: triangle for Portugal, circle for Morocco, square for New Jersey, USA, and diamond for Nova Scotia, Canada. Capriolo et al. (2020).
About 10% of the over 200 investigated intrusive and effusive Central Atlantic Magmatic Province basaltic rocks show gas exsolution bubble-bearing melt inclusions, hosted mainly in clinopyroxene and occasionally in plagioclase, orthopyroxene and olivine. The studied Central Atlantic Magmatic Province basaltic rocks are mainly porphyritic and microcrystalline, and the principal mineral phases are labradoritic-bytownitic plagioclase, augitic (abundant) and pigeonitic (scarce) clinopyroxene, rare magnesium-rich orthopyroxene, and rare and mostly altered magnesium-rich olivine. As accessory mineral phases, magnetite is common, while ilmenite is rare. In effusive rock samples (from USA, Canada, Morocco and Portugal), glomerocrystic aggregates of augitic clinopyroxene and plagioclase are commonly present, and are interpreted as clots of partially crystallised mineral mush from the transcrustal magmatic plumbing system. In the only studied intrusive sample (from Palisades sill, USA), olivine is abundant and usually well preserved.
Representative analysed samples at transmitted light optical microscopy. (a) Porphyritic and microcrystalline texture with phenocrysts of clinopyroxene and plagioclase (Cpx and Pl; sample NEW31, New Jersey, USA). (b) Glomerocrystic aggregate of clinopyroxene (Cpx; sample NEW31, New Jersey, USA). (c) Large phenocryst of plagioclase (Pl; sample AN156A, Morocco). (d) Crystals of plagioclase, clinopyroxene and well-preserved olivine (Pl, Cpx and Ol; sample NEW136B, New Jersey, USA). Capriolo et al. (2020).
Melt inclusions are nearly ubiquitous in glomerocrystic aggregates of clinopyroxene and plagioclase. The bubble-bearing melt inclusions usually have irregular shapes, can be singleor multi-bubble melt inclusions, and contain up to 25 bubbles per inclusion, displaying a large range of glass/bubble ratios even within the same host crystal or crystal clot (i.e., there is no proportionality between the volume of glass and the volume/number of bubbles). In detail, the estimated volume fraction of bubbles within each melt inclusion ranges from less than 0.1 to more than 0.5 approximately. Moreover, melt inclusions present a great variability in size, approximately from 5 to 50 μm on the principal axis. Bubbles within them usually have spherical shape and generally range from 1 to 15 μm in diameter. Sometimes bubbles are aggregated in the melt inclusions, probably due to post-entrapment coalescence. Some melt inclusions are partially crystallised, containing μm-sized daughter minerals in addition to, or instead of, bubbles. These crystals, likely formed from the melt after the entrapment, are mainly opaque mineral phases, such as sulphides and oxides (e.g., magnetite). The melt inclusion glass has a more silicic (mainly andesitic) and more differentiated composition compared to the host basaltic rocks, and is clearly different from typical Central Atlantic Magmatic Province basalts or basaltic andesites. The melt inclusion glass is generally enriched in silicon dioxide and aluminium oxide, and depleted in iron oxide, magnesium oxide, and calcium oxide compared to the host rocks, and would correspond to a residual melt after fractionation of about 40%vaugitic clinopyroxene, 10% plagioclase and 5% magnetite from a typical Central Atlantic Magmatic Province basalt. Such differentiation can only partly be due to post-entrapment crystallisation of the few tiny crystals within the melt inclusions or of the host clinopyroxene, which displays constant augitic composition, shows only faint chemical zonation towards the glass, and is substantially out of equilibrium with it. The most evident compositional zoning of the host clinopyroxene consists of a decrease in calcium oxide content and a slight increase in both magnesium oxide and iron oxide content close to the contact with the melt inclusions. Hence, the local thin rim around the melt inclusions of slightly calcium-depleted and iron and/or magnesium-enriched clinopyroxene suggests the probable presence of augite–pigeonite exsolution lamellae close to the boundary of melt inclusions, which likely formed at subsolidus conditions from an intermediate composition clinopyroxene that crystallized from the entrapped melt (i.e. post-entrapment crystallisation). However, the chemical disequilibrium between the melt inclusion glass and the host clinopyroxene, and the lack of significant chemical zoning within the host clinopyroxene at the contact with melt inclusions are not consistent with substantial diffusive re-equilibration within the host clinopyroxene and suggest a rapid cooling after melt entrapment. This indicates that a previously differentiated bubble-bearing melt was entrained between interstices of growing crystals, and rapidly cooled down, forming melt inclusions.
Representative single- and multi-bubble melt inclusions at transmitted light optical microscopy. The black arrows indicate the bubble-bearing melt inclusions. (a) Multi-bubble melt inclusions characterised by a highly variable range in bubble size, hosted in augitic clinopyroxene (Cpx; sample NS12, Nova Scotia, Canada). (b) Coalescent bubbles within melt inclusion, hosted in augitic clinopyroxene (Cpx; sample AN18, Morocco). (c)-(f) Single- and multi-bubble melt inclusions hosted in augitic clinopyroxene (Cpx) at increasing depth in the same crystalline aggregate, displaying different ratios between the volume of glass and the volume/number of bubbles (sample NS9, Nova Scotia, Canada). Capriolo et al. (2020).
The bubbles within melt inclusions were investigated in all the samples by confocal Raman microspectroscopy looking for carbon species (carbon monoxide, carbon dioxide, methane and elemental carbon), as well as for other important volatile compounds in volcanic systems (sulphur dioxide, hydrogen sulphide and water). In almost all analysed bubbles, carbion dioxide (within 54 bubbles of 9 samples) or elemental carbon (within 41 bubbles of 2 samples) were detected in both single- and multibubble melt inclusions of rock samples collected from all over the Central Atlantic Magmatic Province. In detail, Raman spectra show that carbon dioxide in Central Atlantic Magmatic Province bubbles is characterized by low density (roughly 0.1 g/cm³), and elemental carbon in Central Atlantic Magmatic Province bubbles is characterized by low crystallinity (i.e. it is present as disordered graphite and amorphous carbon). Carbion dioxide concentrations from 0.5 to 1.0 percent by weight in whole melt inclusions (i.e. glass plus bubbles) were calculated from the density of gaseous carbon dioxide within the bubbles and from the estimated volume fraction of these bubbles within melt inclusions. Other volatiles such as carbon monoxide, methane, sulphur dioxide and hydrogen sulphide were not detected, while water was often found within the glass of melt inclusions, but never in the bubbles. The melt inclusion glass, investigated through nanoscale secondary ion mass spectrometry, contains about 0.5–0.6 percent by weight water and 30–90 parts per million carbon dioxide.
Chemical maps of glomerocrystic clinopyroxene aggregates. Backscattered electrons image (a) and corresponding scanning electron microscopy with energy-dispersive X-ray spectroscopy maps (b)–(f) of a thin section area including melt inclusions and the hosting glomerocrystic clinopyroxene aggregates. In the backscattered electrons image image the brighter portions of clinopyroxene have augitic (Aug) composition and the darker ones have pigeonitic(Pgt) composition. In the scanning electron microscopy with energy-dispersive X-ray spectroscopy maps the brighter regions correspond to higher concentrations of the analysed element. These maps were acquired on sample NEW31 (New Jersey, USA). The scale bar is shown in (a). (a) Backscattered electrons image, (b) aluminium map, (c) calcium map, (d) iron map, (e) magnesium map, and (f) titanium map. Capriolo et al. (2020).
The analysed bubble-bearing melt inclusions strongly suggest that the Central Atlantic Magmatic Province magmatic system was rich in carbon dioxide. Most of the analysed bubbles contains carbon dioxide or, less frequently, elemental carbon, and no detectable amounts of any other investigated volatile phase. In particular, confocal Raman microspectroscopy allowed Capriolo et al. to distinguish and characterize both carbon dioxide and elemental carbon. The Raman spectrum of carbon dioxide is characterised by two sharp bands, usually called Fermi diad or Fermi doublet, associated to two symmetrical weak bands, usually called hot bands. Instead, the first-order Raman spectrum of elemental carbon is characterised by two different bands, the composite D band, activated in disordered graphite by lattice defects and typical of non-crystalline structures. and the single G band, typical of graphite. This last band was employed by Capriolo et al. in a crossplot to characterise the different types of elemental carbon, distinguishing disordered graphite and amorphous carbon Interestingly, carbon dioxide and elemental carbon within gas exsolution bubbles are never present together in the same samples. The elemental carbon, which is likely present as a thin film coating the inner spherical surface of the bubbles, replaces carbon dioxide in some samples, probably due to a change in the oxidation state within melt inclusions, for instance related to a diffusive loss of oxygen from the bubbles to the melt, when the latter crystallized oxides during cooling. The large variability in volume and number of bubbles observed in coexisting MIs (ranging from 1 to 25 bubbles per melt inclusion, approximately occupying from less than 0.1 to over 0.5 of the melt inclusion volume, as optically estimated in thin and thick sections) reveals heterogeneous entrapment of melt inclusions. Therefore, the bubbles within melt inclusions are interpreted as gas exsolution bubbles, formed during exsolution of a carbon dioxide-rich fluid phase likely from the silicate melt prior to, or during, their entrapment. Gas exsolution within melt inclusions after melt entrapment was probably of minor importance, particularly for melt inclusions with large bubbles, because the trapping of a bubble-free melt would have produced homogeneous melt inclusions, displaying very similar glass/bubble ratios, which were not observed in this study. The volatile-saturated melt and the volatiles may have a cogenetic origin (i.e., the melt was entrapped along with volatiles immediately after, or during, gas exsolution), or may have different origins (i.e., the melt was entrapped along with volatiles exsolved from deeper magmas, or degassed and fluxed from intruded crustal rocks).
Bubble-bearing melt inclusions at transmitted light optical microscopy and confocal Raman microspectroscopy. Left column: transmitted light photomicrographs at optical microscope of the analysed areas, bordered by dotted lines. Right column: Raman hyperspectral maps of the corresponding areas. (a), (c), (e) Photomicrographs of elemental carbon-bearing single- and multi-bubble melt inclusions (a) and (c) sample NEW31, New Jersey, USA; (e) sample AN39, Morocco. (b), (d), (f) Raman hyperspectral maps of the same samples area. (g) Photomicrograph of an irregular-shaped carbon dioxide-bearing multi-bubble melt inclusion, sample NS12, Nova Scotia, Canada. (h) Raman hyperspectral map of the same sample area. The Raman signal of carbon dioxide is weak due to its low density. However, spot analyses confirmed the presence of carbon dioxide in all bubbles. Capriolo et al. (2020).
Clinopyroxene compositions and volatile element concentrations suggest that carbon dioxide entrapment occurred within the deep magmatic roots of Central Atlantic Magmatic Province. The pressure of crystallisation of host clinopyroxene crystal clots can be calculated from mineral compositions using methods developed for magmatic systems, The geothermobarometer based on the equilibrium between clinopyroxene and a magmatic liquid was applied using whole rock composition as proxy for the original magmatic liquid composition, because the melt inclusion glass is in chemical disequilibrium with the host clinopyroxene. The calculated crystallization pressure ranges from 0.1 to 0.7gigapascals (at temperatures from 1150 to 1230°C) and is consistent with previous estimates from clinopyroxene crystallisation pressures (from 0.2 to 0.8 gigapascals) in basalts from the entire Central Atlantic Magmatic Province. These results suggest that the crystallisation of clinopyroxene in the investigated Central Atlantic Magmatic Province samples occurred predominantly within the middle continental crust (on average 12 km for a pressure/depth gradient of about 0.03 gigapascals/km). The deep origin of melt inclusions is consistent with observed volatile concentrations in both their glass and bubbles. The presence of sulphides within some melt inclusions shows that the entrapped melt became sulphide-saturated with sulphur concentrations likely exceeding 1500 parts per million. Sulphur concentrations of the same order of magnitude were estimated for Central Atlantic Magmatic Province basalts. Moreover, about 0.5–0.6 percent by weight water was detected in the melt inclusion glass through nanoscale secondary ion mass spectrometry analysis, revealing hydrated conditions for these melts. Despite the presence of water and sulphur in the melt inclusion glass, these volatiles were not detected in the bubbles. Hence, considering a realistic maximum primary concentration of about 1 percent by weight water and ca. 0.1 percent by weight sulphur dioxide in tholeiitic within-plate basaltic melts, most water and sulphur dioxide are expected to exsolve at pressures lower than 0.1 gigapascals (i.e. less than 3 km depth). Even considering that hydrogen ions may move from the bubbles into the glass, and carbon dioxide from the glass into the bubbles after melt inclusion entrapment, the observed distribution of volatile species between glass and bubbles within melt inclusions suggests the dominant occurrence of gas exsolution and bubble formation at relatively high pressures from a carbon dioxide-rich melt.
The inferred depth of carbon dioxide exsolution and entrapment indicates that this volatile species has a deep origin (at least 12 km on average). It therefore reveals that the entire carbon dioxide budget involved in Central Atlantic Magmatic Province emplacement could not have originated exclusively from assimilation and degassing of shallow intruded sediments, because sediments in the circum-Atlantic basins only reach a thickness of 5 km in eastern North America and less than 1 km in Morocco and Portugal. On the contrary, at least part of the carbon dioxide most probably derived from assimilation of deep to middle-crustal metasedimentary rocks (e.g. metacarbonates or graphite-bearing amphibolites/granulites) or from the mantle source of the Central Atlantic Magmatic Province basalts, containing significant amounts of recycled sedimentary material.
Sketch of the transcrustal plumbing system of Central Atlantic Magmatic Province basaltic magmas from the mantle to the surface. The evolution of basaltic magmas occurs at variable depth by crystallisation of minerals, which then form aggregates in crystalline mushes and entrain bubble-bearing melt, forming melt inclusions. Different volatile species exsolve at variable depth. In particular, carbon dioxide-rich fluids (white bubbles) start exsolving at great depth, whilst water-rich fluids (blue bubbles) and S-rich fluids (yellow bubbles) start exsolving at shallow depth. The black dashed arrows indicate the potential sources for the carbon inCentral Atlantic Magmatic Province magma: the mantle, the deep crust and the Palaeozoic or Triassic sedimentary basins in which Central Atlantic Magmatic Province sills intruded. The carbon within the studied melt inclusions derives from the deep sources as demonstrated with clinopyroxene geobarometry data. Capriolo et al. (2020).
The calculated depth of entrapment (roughly 12 km) allows an estimation of the carbon dioxide concentration originally present in Central Atlantic Magmatic Province magmas. The carbon dioxide saturation in basaltic melts is achieved at about 1000 parts per million at 0.2 gigapascals, increasing by ca. 500 parts per million for each 0.1 gigapascals. Considering the calculated crystallisation depths, the minimum estimate for the carbon dioxide concentration of Central Atlantic Magmatic Province magma, before gas exsolution, is between about 500 and 4000 parts per million. Such values are consistent with the carbon dioxide concentrations in the melt inclusions, calculated from carbon dioxide density within the bubbles, which range from 0.5 to 1.0 percent by weight. Moreover, starting from the minimum calculated values of the carbon dioxide concentration within melt inclusions (i.e. 0.5–0.6 percent by weight) as representative of Central Atlantic Magmatic Province magma, assuming an average density of 2.90 g/cm³ for basaltic rocks and considering 5–6 million km³ for the total volume of Central Atlantic Magmatic Province (in order to take into account the deep plumbing system), the total amount of degassed volcanic carbon dioxide during Central Atlantic Magmatic Province mplacement would be up to 100 000 gigatonnes. Interestingly, the values estimated for the carbon dioxide concentration of Central Atlantic Magmatic Province magma (0.5–1.0 percent by weight) and for the total amount of degassed volcanic carbon dioxide during Central Atlantic Magmatic Province emplacement (up to 100 000 gigatonnes) are consistent with those assessed in several other large igneous provinces, using different approaches
The high-volume fractions of carbon dioxide and elemental carbon-bearing bubbles within Central Atlantic Magmatic Province melt inclusions, along with the inferred depths of formation, reveal the high abundance (0.5–1.0 percent by weight) of carbon dioxide in the Central Atlantic Magmatic Province transcrustal magmatic plumbing system. The carbon dioxide-bearing bubbles identified in Central Atlantic Magmatic Province melt inclusions can be interpreted as batches of ascending volatiles entrapped in crystalline mush shortly prior to its mobilization and prior to eruption. This evidence for carbon dioxide saturation in the basaltic magmas at depth can explain the pulsed eruptive style of Central Atlantic Magmatic Province, where carbon dioxide acts as propellant for magma ascent, causing rapid and violent eruptive pulses. For instance, carbon dioxide-rich Hawaiian basalts have been shown to rapidly rise from over 5 km depth and to cause high fountaining eruptions.
The presence of large amounts of carbon dioxide-bearing bubbles, the pulsed eruption and the efficient degassing of carbon dioxide from the basaltic magmas, strengthens the role of Central Atlantic Magmatic Province in triggering end-Triassic extreme greenhouse conditions The rate of volatile release plays a fundamental role in determining the severity of the surface environmental response; more rapid release increases the maximum transient concentration of atmospheric carbon dioxide and the subsequent severity of any environmental cascade. Assuming 0.5–1.0 percent by weight carbon dioxide in Central Atlantic Magmatic Province basalts, as suggested by the average carbon dioxide density and most common glass/bubble ratio within melt inclusions, and considering its efficient rise to the atmosphere through the magmatic transcrustal roots, it is possible that just a single Central Atlantic Magmatic Province volcanic pulse may have severely affected the end-Triassic climate. In fact, a single short-lived Central Atlantic Magmatic Province magmatic pulse (roughly 100 000 km³ erupted over 0.5 thousand years may emit about the same total amount of projected anthropogenic carbon dioxide emissions over the 21st century, according to the Representative Concentration Pathway 4.5 (a Representative Concentration Pathway is a greenhouse gas
concentration trajectory adopted by the Intergovernmental Panel on Climate Change; four pathways were used
for climate modeling and research for the Intergovernmental Panel on Climate Change Fifth Assessment Report
in 2014.). This scenario for rapid carbon dioxide emissions predicts a global temperature increase of about 2 °C and an oceanic pH decrease of about 0.15 units over 100 years, and suggests that the end-Triassic climatic and environmental changes, driven by carbon dioxide emissions, may have been similar to those predicted for the near future.
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