Large Igneous Provinces are defined as the geologically rapid emplacement (over 100 000-1 000 000 years) of hundreds of thousands to millions of cubic kilometres of lava at the surface and the associated intrusive bodies. They are dominated by thick successions of lavas known as flood basalts. These vast igneous provinces have formed several times throughout Earth’s history, on almost all of the major continents and also in the oceans. Large igneous provinces are often found far from plate boundaries. Detailed studies of individual Large Igneous Provinces have shown that they are formed of igneous rocks with diverse compositions, ranging from tholeiitic basalts, to occasional rhyolites, to strongly alkaline magmas such as lamproites and carbonatites. The generation and emplacement of Large Igneous Province magmas is linked to rapid, large-scale outgassing of volatile molecules and elements, including sulphur, water, halogens, and carbon dioxide. This surface outgassing is facilitated by extensive subterranean magmatic plumbing systems that form important pathways for the transfer of mantle and crustal carbon to the atmosphere. Among magmatic gases, carbon dioxide (CO₂) is particularly vital to the life cycle of Large Igneous Province magmatism and its climatic consequences. The centrality of CO₂ in the environmental perturbations that coincide with some Large Igneous Provinces, such as the Deccan Traps (India), Siberian Traps (Russia), Karoo–Ferrar (southern Africa and Antarctica, respectively), Ontong Java Plateau (Pacific Ocean), Columbia River Basalt Group (northwestern USA), and the Central Atlantic Magmatic Province (northwest Africa, southwest Europe, northeast and southeast North America), renders Large Igneous Province-driven climate stress an important palaeoclimate analog for the present-day climate. However, the origins, budget, isotopic composition, and fate of Large Igneous Province carbon remain pressing and challenging questions due to the evanescence of CO₂ in carbon-saturated mafic magmatic systems.
The hardened lava flows of the Deccan Traps, in western India, may have played a role in the demise of the Dinosaurs.
Gerta Keller/Science.
In a paper published in the journal Elements on 2 October 2019, Benjamin Black of the Department of Earth and Atmospheric Sciences at the City College of New York, and Sally Gibson of the Department of Earth Sciences at the University of Cambridge, discuss the relationship between the outgassing of carbon dioxide and the emplacement of Large Igneous Provinces.
Water, CO₂, sulphur, and halogens are among the most abundant constituents in volcanic gases. While some fraction of these gases is released during volcanic eruptions, the remainder may be discharged diffusely through crust, unaccompanied by volcanic eruptions. This cryptic degassing can form an important part of the overall gas release budget, especially for CO₂. Furthermore, if the establishment of lithospheric plumbing systems during the initial development of a Large Igneous Province precedes the onset of flood basalt volcanism at the surface, or if intrusive magmas solidify after the last eruptions, cryptic degassing could either precede or postdate active volcanism.
Because no Large Igneous Provinces are forming at the present day, studies of recent, analogous, volcanic activity provide an important source of information about the release of Large Igneous Province carbon. Eruptions of tholeiitic flood basalt lavas (subalkaline basalts, basalts that contain less sodium than some other basalts) may bear similarities to present-day fissure eruptions in Hawai‘i (Kï'lauea Volcano) and Iceland (Laki and Holuhraun Volcanoes). The smaller volume alkaline magmas associated with some Large Igneous Province may have had similar origins to those in the East African Rift.
Lava from a fissure on the Kï'lauea Volcano, Hawai'i, flows through a well-established channel to the ocean south of Kapoho. USGS.
At sites of present-day volcanism, both eruptive and cryptic CO₂ release can be constrained through ground-based and airborne measurements. More direct constraints on carbon in Large Igneous Province magmas can potentially be provided by petrologic and geochemical methods, but such data are scarce and require cautious interpretation. Blebs of melt trapped inside growing crystals (commonly known as 'melt inclusions') can record magmatic water, CO₂, sulphur, and halogen concentrations at the time of entrapment and prior to eruption. However, any CO₂ that has already exsolved (come out of solution and escaped) at the time of melt entrapment cannot be reliably reconstructed. Alternatively, trace element ratios such as CO₂/Niobium and CO₂/Barium from un-degassed samples can be used to infer original CO₂ content for partly degassed, primitive magmas. This is because Niobium and Barium exhibit a similar incompatible behaviour to CO₂ during mantle melting but do not degas.
Although all Large Igneous Province consist of a wide variety of magma types, estimates of volatile contents have mainly been derived for tholeiitic flood basalts. Most previous estimates of the CO₂ content of flood basalts have relied on analogous, well-studied basaltic systems, such as Kï'lauea and Laki, to arrive at concentrations of 0.5–0.9 % CO₂ (by weight) in flood basalt magmas. Given that in continental settings, flood basalt magmas may require several percent weight of dissolved water and/or exsolved CO₂ to be sufficiently buoyant to erupt through low-density felsic continental crust (crust enhanced in silicon, oxygen, aluminium, sodium, and potassium), these CO₂ concentrations may be underestimates.
Alternative estimates of the CO₂ concentrations in flood basalt magmas have been derived from olivine-hosted melt inclusions. These are rare and typically found in primitive flood basalts, so they may not necessarily be representative of the main phase of more fractionated tholeiitic magmatism. Moreover, analyses of CO₂ in olivine-hosted melt inclusions in tholeiitic magmas reflect only the dissolved amount, which forms an indeterminate fraction of the total CO2 released per km3 of magma. Thus far, measurements of CO₂ in olivine-hosted melt inclusions from flood basalts have overlooked the CO₂ in shrinkage vapour bubbles (which may dominate the total CO₂ content) and so are minimum estimates. If Large Igneous Province magmas reach CO₂ saturation at high pressures, CO₂ concentrations in melt inclusions represent lower limits on initial concentrations. Consequently, it is not surprising that CO₂ measurements for melt inclusions in the Siberian Traps are lower than the estimates for flood basalts based on measurements from Hawai‘i and Iceland.
The use of volatile/nonvolatile trace element ratios (such as CO₂/niobium and CO₂/barium) to estimate original CO₂ concentrations must be applied with care to flood basalt magmas. This is because processes such as recharge, assimilation, and fractional crystallisation in crustal magma chambers can significantly modify concentrations of strongly incompatible trace elements. Primitive high-Magnesium oxide lavas, known as picrites, may sidestep this issue and thereby provide a window into initial CO₂ concentrations. Using this approach, barium and niobium concentrations in picrites from the Siberian Traps and the North Atlantic Igneous Province suggest original melt CO₂ concentrations of between 0.1 and 2 CO₂ by weight. This large range for flood basalts, along with the current lack of data for more alkaline magmas, emphasises the need for further direct constraints on carbon in Large Igneous Provinces.
Large igneous provinces occupy broad areal extents, up to 1 000 000 km² and are widely believed to have resulted from the impingement and lateral spreading of upwelling high-temperature mantle plume heads with diameters of up to 2000 km at the base of the lithosphere. According to their site of emplacement, large igneous provinces may be categorised as oceanic or continental. Large Igneous Provinces emplaced on the continents may draw carbon from three main reservoirs: the convecting mantle, the subcontinental lithospheric mantle, and sedimentary rocks and fluids in the crust. The main source of carbon for oceanic large igneous provinces is the convecting mantle.
Carbon dioxide behaves highly incompatibly during mantle melting, meaning that it partitions almost entirely into the melt phase. Consequently, the initial CO² concentrations of magmas are determined by the carbon concentration of their mantle source and by the degree of partial melting. The high helium³/helium⁴ ratios in some large igneous province magmas suggest their parental melts are formed from deep-sourced, primordial material brought up in mantle plumes. In addition to primordial carbon, mantle plumes are also likely to contain carbon that has been recycled, due to subduction and subsequent entrainment of oceanic crust by the plume. However, the fate of carbon during plate tectonic recycling is not well known. Furthermore, while some Large Igneous Province melts have incompatible trace element and strontium, neodymium, lead and hafnium isotopic ratios similar to oceanic basalts, and they appear to be derived solely from a mantle plume source, the geochemistry of many large igneous province melts testifies to additional contributions from the overlying lithosphere.
Variations in the depth and degree of melting in upwelling mantle plumes associated with flood basalts are well-established, for example, from incompatible trace element ratios that reflect the presence or absence of garnet in the residue during melting. Numerical models indicate that the extent of partial melting that occurs during upwelling of mantle plumes is primarily controlled by the temperature of the convecting mantle and the thickness of the overlying lithosphere. If the lithosphere becomes thinner through the course of Large Igneous Province magmatism, due to synemplacement (fracturing) extension or erosion, the amount of melting will be lowest at the earliest stages of plume impact and the carbon concentration in these melts will be high. Because the lithosphere is of nonuniform thickness, the amount of melting in the plume will also vary spatially at any given time, and regions of pre-existing thinning or weakness will focus plume upwelling and melting.
Thinning or removal of the subcontinental lithospheric mantle may also potentially mobilize carbon. While this large and ancient Earth reservoir has been proposed as a major repository for volatiles, in part due to the infiltration of small-fraction, volatile rich, convecting, mantle-sourced melts over long periods of geological time, its carbon budget and isotopic composition are poorly known. The most concentrated accumulation of carbon is likely to be either at a depth of approximately 75 km, where experimental studies have shown that there is a depression in the CO₂-bearing peridotite solidus, or in regions where redox freezing traps carbon as graphite or diamonds, which can later be oxidized by percolating carbonatitic melts.
A final, but potentially important, source of carbon to Large Igneous Province magmas comes from crustal fluids and sedimentary rocks (such as evaporites, coals, carbonates, and hydrocarbons). The flux of carbon from these crustal sources depends on the country rock lithologies, the input and distribution of heat, and the fraction of gases that reach the atmosphere.
Large igneous provinces are commonly emplaced over several million years, but the main pulse of flood basalt volcanism occurs on timescales of a million years or less. The flux of CO₂ is, therefore, likely to vary during Large Igneous Province emplacement. and through the course of individual Large Igneous Province eruptions. Both the evolving CO₂ flux and the ratio of carbon isotopes depend on magma emplacement rates, melting conditions, carbon sources, and flushing of CO₂ through the magmatic system as a fluid phase that is not bound to magma transfer. All of these factors are likely to shift through the life cycle of a Large Igneous Province.
The tempo of magma emplacement can be constrained through studies of geochronology, physical vulcanology, palaeomagnetism, radiogenic isotope systems, or proxies such as mercury deposition. These lines of evidence retain significant uncertainties, but they do generally support the existence of short-timescale variations in volcanic activity superposed on gradually shifting long-term mean volcanic fluxes.
Geochronologic studies show that low-degree, incompatible trace element–rich, lithospheric melts often pre- and postdate the main phase outpourings of flood basalts. If lithospheric removal occurs during Large Igneous Province emplacement, this could also trigger a pulse of devolatilisation from both the foundering lithospheric material and the residual subcontinental lithosphere under a steeper geothermalmal gradient.
Thermomechanical transitions may modulate the depths of magma storage and, therefore, the country rock lithologies that are to be subjected to heating and devolatilisation; crustal metamorphism also requires heating of large volumes of rock, and outgassing from the cold upper crust may, therefore, lag behind the onset of volcanism.
On the timescales of individual eruptions, carbon outgassing can be decoupled from volcanic flux, for example when CO₂ partitions into a fluid phase. Ground-based measurements of the Holuhraun (Iceland) fissure eruption of 2014–2015 revealed that CO₂/SO₂ ratios in the volcanic plume were higher by a factor of 10 during the earliest days of the eruption.
Flipping the causal relationship, evolving CO₂ concentrations during fractionation of magmas in the deep crust have also been hypothesised to exert control over the eruptibility of flood basalt magmas. Carbon dioxide may, therefore, play a role in shaping the tempo of volcanic activity and outgassing.
The depths at which Large Igneous Province magmas become saturated in an exsolved CO₂-rich phase, and the mobility and fate of the exsolved fluid, are critical to understanding their overall carbon outgassing history. The proportion of a magmatic volatile substance, one that is initially dissolved in the melt, that reaches the atmosphere can be thought of as the outgassing efficiency. For CO₂, it is commonly assumed that the outgassing efficiency is close to 100% for extrusive flood basalt magmas, due to the very low solubility of CO₂ in basaltic melt at one atmosphere pressure. In conjunction with an assumed CO₂ content of 0.5% weight in a primitive basaltic melt, this efficiency implies an approximate CO₂ yield of 14 megatonnes per cubic kilometre of erupted magma. Importantly, if the CO₂ outgassing efficiency deviates significantly from 100%, carbon isotope fractionation due to partial degassing may shift the net isotope ratio of the carbon that is released.
Estimates of CO₂ outgassing that are based solely on emplacement rates of flood basalts do not account for the potential flux of CO₂ from associated intrusive magmas. Outgassing from CO₂ saturated magma bodies in the permeable upper crust may take place through gradual, passive degassing in conjunction with emissions during eruptions. In the less permeable lower crust and lithospheric mantle, dike formation and magma ascent may provide one of the only avenues for CO₂-rich exsolved fluids to reach the surface. In this case, CO₂ initially exsolved at depth could 'flush' shallower magmas, increasing CO₂ release beyond what would be expected from the volume of erupted flood basalts. The hypothesis of large-scale CO₂ flushing in the complex magmatic plumbing systems associated with flood basalts receives some support from studies of Icelandic fissure eruptions. Comparison between trace element concentrations and melt inclusion CO₂ content from the 1783–1784 Laki (Iceland) fissure eruption suggests that about 60% of the initial CO₂ cargo was degassed in the lower-to-middle crust.
The importance of deep intrusive degassing depends on the relative volumes of intrusive and extrusive magmas. Based on petrology and seismic imaging of high-velocity layers near the Moho (the Mohorovičić Discontinuity, the boundary between the Earth's crust and the mantle), previous researchers have inferred that the ultramafic cumulates that may underlie Large Igneous Provinces are comparable in volume to the erupted lavas. A range in intrusive/extrusive ratio of 0.5 to 4 implies that 30%–80% of Large Igneous Province magmas do not erupt. The efficiency with which these deep intrusive magmas degas and transfer CO₂ to the atmosphere is uncertain. It has been estimated than a 40–60% degassing rate of intrusive magmas occurred in the Laki plumbing system, but this rate could be higher if crystallisation in deep magma bodies drove further CO₂ exsolution. Assuming homogeneous CO₂ content, 50% intrusive degassing, and efficient transfer of CO₂ to the atmosphere via flushing through the magmatic system and eruption, the magmas that do erupt could carry 1.25 to 3 times their native CO₂ content assuming an intrusive/extrusive ration of 0.5 to 4. This 'excess carbon' load is, in some ways, analogous to the 'excess sulfur' released from some arc volcanic eruptions that tap a sulphur-rich exsolved phase.
The fluxes of CO₂ during Large Igneous Province magmatism, and, consequently, the viability of CO₂ as a significant driver of climate change, have been the subject of debate. Perhaps the best evidence for major perturbation of the global carbon cycle by Large Igneous Provinces comes from carbon isotope records and independent palaeoclimate and atmospheric CO₂ content proxies. In brief, these lines of evidence indicate four major aspects of carbon cycle disruption: (1) strong warming coinciding with some Large Igneous Provinces (e.g., the Siberian Traps and the North Atlantic Igneous Province), with more ambiguous evidence in other cases; (2) spikes in atmospheric CO₂ content; (3) negative carbon isotope excursions (drop in the proportion of carbon¹² in sediments); (4) ocean acidification. Taken together with geochronology aligning these environmental changes to Large Igneous Province magmatism, the most straightforward interpretation is that they were primarily caused by carbon release related to Large Igneous Province emplacement, possibly supplemented with light carbon from clathrates (chemical substances consisting of a lattice of water molecules that traps or contains other molecules) or from the metamorphism of sedimentary organic material. Accurately determining the CO₂ budget of diverse Large Igneous Province magmas is critical to testing hypotheses regarding the causal links between magmatism, warming, and carbon cycle perturbation.
On the timescales of silicate weathering and water–rock reactions, Large Igneous Provinces may play an important role as carbon sinks, because CO₂ reacts with calcium and magnesium in basalts to form carbonate minerals. By analogy to mid-ocean ridge settings, where the balance between net outgassing and net sequestration of carbon is uncertain, the capacity for Large Igneous Provincecarbon sequestration on longer timescales may rival the magnitude of potential outgassing. This balance, and the timescales of atmospheric CO₂ draw down, may depend on the extent of subaerial versus submarine volcanism in a given Large Igneous Province and the potential for water–rock reactions in hydrothermal systems.
Large igneous provinces are dominated volumetrically by vast outpourings of flood basalt lavas and their intrusive equivalents. The frequent association of alkaline and carbonatite magmatism with flood basalts in Large Igneous Provinces attests to the petrologic importance of carbon during the generation of Large Igneous Province magmas, though when and how plumbing systems associated with diverse magma types interact with each other is an unresolved question. Previous estimates of the CO₂ content of flood basalts range from 0.5% to 0.9% by weight. Revised estimates of the CO₂ content of Hawaiian (Kï'lauea) and Icelandic (Laki) basalts, along with incompatible trace elements from flood basalt picrites, suggest that primitive flood basalt magmas may commonly comprise about 1% CO₂ by weight, or possibly more. However, such primitive magmas comprise a small fraction of the erupted lavas. More detailed numerical models and geochemical measurements are required to understand the carbon concentrations in flood basalts and other magma types in Large Igneous Provincess. For example, the carbon delivery potential of voluminous tholeiite lavas in flood basalt successions is important but poorly constrained. While their parental magmas result from high degrees of partial melting and are, therefore, less likely to be intrinsically CO₂-rich they may receive a boost from CO₂-rich fluids released from deep intrusive magmas.
Based on the shifting contributions of deep convecting mantle, lithospheric mantle, and crustal sources, together with the variable flux from deep intrusive magmas, the CO₂ flux from Large Igneous Province magmas probably evolves through the entire magmatic cycle. As a consequence, Large Igneous Province carbon emissions are unlikely to scale directly with volumetric eruption rates, which offer, at best, a partial picture of the tempo of carbon outgassing. Nevertheless, gross estimates based on erupted volume provide a starting place for situating Large Igneous Provinces in the context of the global carbon cycle. For a total Large Igneous Province magma volume of 1 000 000–10 000 000 km³ emplaced over 100 000–1 000 000 years, and carrying about 1% weight in CO₂, the mean annual flux would be 30–3000 mega tonnes of CO₂ per/year, with the potential for orders of magnitude deviation from this mean rate over the course of the magmatic cycle. The Laki fissure eruption released an estimated 300–900 mega tonnes of CO₂ over the course of 8 months. For comparison, the present-day global subaerially released flux of CO₂ from all volcanoes has been estimated at 300–600 mega tonnes of CO₂ per/year. During geologically brief intervals of intense outgassing, Large Igneous Provinces are likely to dominate the global flux of deep carbon to the atmosphere.
See also...
Lave fields deposited during the 1783-84 Laki Eruption in Iceland. Ulrich Latzenhofer/Fotopedia.
Alternative estimates of the CO₂ concentrations in flood basalt magmas have been derived from olivine-hosted melt inclusions. These are rare and typically found in primitive flood basalts, so they may not necessarily be representative of the main phase of more fractionated tholeiitic magmatism. Moreover, analyses of CO₂ in olivine-hosted melt inclusions in tholeiitic magmas reflect only the dissolved amount, which forms an indeterminate fraction of the total CO2 released per km3 of magma. Thus far, measurements of CO₂ in olivine-hosted melt inclusions from flood basalts have overlooked the CO₂ in shrinkage vapour bubbles (which may dominate the total CO₂ content) and so are minimum estimates. If Large Igneous Province magmas reach CO₂ saturation at high pressures, CO₂ concentrations in melt inclusions represent lower limits on initial concentrations. Consequently, it is not surprising that CO₂ measurements for melt inclusions in the Siberian Traps are lower than the estimates for flood basalts based on measurements from Hawai‘i and Iceland.
The extent of the End Permian Siberian Traps Volcanism. Jo Weber/Wikimedia Commons.
The use of volatile/nonvolatile trace element ratios (such as CO₂/niobium and CO₂/barium) to estimate original CO₂ concentrations must be applied with care to flood basalt magmas. This is because processes such as recharge, assimilation, and fractional crystallisation in crustal magma chambers can significantly modify concentrations of strongly incompatible trace elements. Primitive high-Magnesium oxide lavas, known as picrites, may sidestep this issue and thereby provide a window into initial CO₂ concentrations. Using this approach, barium and niobium concentrations in picrites from the Siberian Traps and the North Atlantic Igneous Province suggest original melt CO₂ concentrations of between 0.1 and 2 CO₂ by weight. This large range for flood basalts, along with the current lack of data for more alkaline magmas, emphasises the need for further direct constraints on carbon in Large Igneous Provinces.
Large igneous provinces occupy broad areal extents, up to 1 000 000 km² and are widely believed to have resulted from the impingement and lateral spreading of upwelling high-temperature mantle plume heads with diameters of up to 2000 km at the base of the lithosphere. According to their site of emplacement, large igneous provinces may be categorised as oceanic or continental. Large Igneous Provinces emplaced on the continents may draw carbon from three main reservoirs: the convecting mantle, the subcontinental lithospheric mantle, and sedimentary rocks and fluids in the crust. The main source of carbon for oceanic large igneous provinces is the convecting mantle.
Summary diagram of Large Igneous Province (LIP) carbon fluxes ( expressed as mega tonnes of carbon per year: Mt C y ̄¹), isotope ratios (rxpressed as δ13C, in units of per mil, ‰), and various types of geological reservoirs, all placed in context. The carbonatite ledge represents a major inflection in the CO²-bearing peridotite solidus. Black & Gibson (2019).
Carbon dioxide behaves highly incompatibly during mantle melting, meaning that it partitions almost entirely into the melt phase. Consequently, the initial CO² concentrations of magmas are determined by the carbon concentration of their mantle source and by the degree of partial melting. The high helium³/helium⁴ ratios in some large igneous province magmas suggest their parental melts are formed from deep-sourced, primordial material brought up in mantle plumes. In addition to primordial carbon, mantle plumes are also likely to contain carbon that has been recycled, due to subduction and subsequent entrainment of oceanic crust by the plume. However, the fate of carbon during plate tectonic recycling is not well known. Furthermore, while some Large Igneous Province melts have incompatible trace element and strontium, neodymium, lead and hafnium isotopic ratios similar to oceanic basalts, and they appear to be derived solely from a mantle plume source, the geochemistry of many large igneous province melts testifies to additional contributions from the overlying lithosphere.
Variations in the depth and degree of melting in upwelling mantle plumes associated with flood basalts are well-established, for example, from incompatible trace element ratios that reflect the presence or absence of garnet in the residue during melting. Numerical models indicate that the extent of partial melting that occurs during upwelling of mantle plumes is primarily controlled by the temperature of the convecting mantle and the thickness of the overlying lithosphere. If the lithosphere becomes thinner through the course of Large Igneous Province magmatism, due to synemplacement (fracturing) extension or erosion, the amount of melting will be lowest at the earliest stages of plume impact and the carbon concentration in these melts will be high. Because the lithosphere is of nonuniform thickness, the amount of melting in the plume will also vary spatially at any given time, and regions of pre-existing thinning or weakness will focus plume upwelling and melting.
Thinning or removal of the subcontinental lithospheric mantle may also potentially mobilize carbon. While this large and ancient Earth reservoir has been proposed as a major repository for volatiles, in part due to the infiltration of small-fraction, volatile rich, convecting, mantle-sourced melts over long periods of geological time, its carbon budget and isotopic composition are poorly known. The most concentrated accumulation of carbon is likely to be either at a depth of approximately 75 km, where experimental studies have shown that there is a depression in the CO₂-bearing peridotite solidus, or in regions where redox freezing traps carbon as graphite or diamonds, which can later be oxidized by percolating carbonatitic melts.
Mantle plume–lithosphere interactions through the life cycle of continental Large Igneous Provinces (LIPs) can influence the melting regime of the convecting mantle and the subcontinental mantle lithosphere. (1) During the initial impingement of a mantle plume (red) beneath thick continental lithosphere (yellow and brown), low-degree partial melts from metasomatised lithospheric mantle predominate. (2) Over the course of Large Igneous Province magmatism, the lithosphere undergoes significant thinning through extension and/or foundering. Formation of tholeiitic basalts through adiabatic decompression melting in the mantle plume imposes a maximum lithospheric thickness during main-phase flood basalt magmatism of about 70 km. Black & Gibson (2019).
A final, but potentially important, source of carbon to Large Igneous Province magmas comes from crustal fluids and sedimentary rocks (such as evaporites, coals, carbonates, and hydrocarbons). The flux of carbon from these crustal sources depends on the country rock lithologies, the input and distribution of heat, and the fraction of gases that reach the atmosphere.
Large igneous provinces are commonly emplaced over several million years, but the main pulse of flood basalt volcanism occurs on timescales of a million years or less. The flux of CO₂ is, therefore, likely to vary during Large Igneous Province emplacement. and through the course of individual Large Igneous Province eruptions. Both the evolving CO₂ flux and the ratio of carbon isotopes depend on magma emplacement rates, melting conditions, carbon sources, and flushing of CO₂ through the magmatic system as a fluid phase that is not bound to magma transfer. All of these factors are likely to shift through the life cycle of a Large Igneous Province.
The tempo of magma emplacement can be constrained through studies of geochronology, physical vulcanology, palaeomagnetism, radiogenic isotope systems, or proxies such as mercury deposition. These lines of evidence retain significant uncertainties, but they do generally support the existence of short-timescale variations in volcanic activity superposed on gradually shifting long-term mean volcanic fluxes.
Geochronologic studies show that low-degree, incompatible trace element–rich, lithospheric melts often pre- and postdate the main phase outpourings of flood basalts. If lithospheric removal occurs during Large Igneous Province emplacement, this could also trigger a pulse of devolatilisation from both the foundering lithospheric material and the residual subcontinental lithosphere under a steeper geothermalmal gradient.
Thermomechanical transitions may modulate the depths of magma storage and, therefore, the country rock lithologies that are to be subjected to heating and devolatilisation; crustal metamorphism also requires heating of large volumes of rock, and outgassing from the cold upper crust may, therefore, lag behind the onset of volcanism.
On the timescales of individual eruptions, carbon outgassing can be decoupled from volcanic flux, for example when CO₂ partitions into a fluid phase. Ground-based measurements of the Holuhraun (Iceland) fissure eruption of 2014–2015 revealed that CO₂/SO₂ ratios in the volcanic plume were higher by a factor of 10 during the earliest days of the eruption.
Flipping the causal relationship, evolving CO₂ concentrations during fractionation of magmas in the deep crust have also been hypothesised to exert control over the eruptibility of flood basalt magmas. Carbon dioxide may, therefore, play a role in shaping the tempo of volcanic activity and outgassing.
The depths at which Large Igneous Province magmas become saturated in an exsolved CO₂-rich phase, and the mobility and fate of the exsolved fluid, are critical to understanding their overall carbon outgassing history. The proportion of a magmatic volatile substance, one that is initially dissolved in the melt, that reaches the atmosphere can be thought of as the outgassing efficiency. For CO₂, it is commonly assumed that the outgassing efficiency is close to 100% for extrusive flood basalt magmas, due to the very low solubility of CO₂ in basaltic melt at one atmosphere pressure. In conjunction with an assumed CO₂ content of 0.5% weight in a primitive basaltic melt, this efficiency implies an approximate CO₂ yield of 14 megatonnes per cubic kilometre of erupted magma. Importantly, if the CO₂ outgassing efficiency deviates significantly from 100%, carbon isotope fractionation due to partial degassing may shift the net isotope ratio of the carbon that is released.
Estimates of CO₂ outgassing that are based solely on emplacement rates of flood basalts do not account for the potential flux of CO₂ from associated intrusive magmas. Outgassing from CO₂ saturated magma bodies in the permeable upper crust may take place through gradual, passive degassing in conjunction with emissions during eruptions. In the less permeable lower crust and lithospheric mantle, dike formation and magma ascent may provide one of the only avenues for CO₂-rich exsolved fluids to reach the surface. In this case, CO₂ initially exsolved at depth could 'flush' shallower magmas, increasing CO₂ release beyond what would be expected from the volume of erupted flood basalts. The hypothesis of large-scale CO₂ flushing in the complex magmatic plumbing systems associated with flood basalts receives some support from studies of Icelandic fissure eruptions. Comparison between trace element concentrations and melt inclusion CO₂ content from the 1783–1784 Laki (Iceland) fissure eruption suggests that about 60% of the initial CO₂ cargo was degassed in the lower-to-middle crust.
A fissure eruption in Hawai'i. Wikimedia Commons.
The importance of deep intrusive degassing depends on the relative volumes of intrusive and extrusive magmas. Based on petrology and seismic imaging of high-velocity layers near the Moho (the Mohorovičić Discontinuity, the boundary between the Earth's crust and the mantle), previous researchers have inferred that the ultramafic cumulates that may underlie Large Igneous Provinces are comparable in volume to the erupted lavas. A range in intrusive/extrusive ratio of 0.5 to 4 implies that 30%–80% of Large Igneous Province magmas do not erupt. The efficiency with which these deep intrusive magmas degas and transfer CO₂ to the atmosphere is uncertain. It has been estimated than a 40–60% degassing rate of intrusive magmas occurred in the Laki plumbing system, but this rate could be higher if crystallisation in deep magma bodies drove further CO₂ exsolution. Assuming homogeneous CO₂ content, 50% intrusive degassing, and efficient transfer of CO₂ to the atmosphere via flushing through the magmatic system and eruption, the magmas that do erupt could carry 1.25 to 3 times their native CO₂ content assuming an intrusive/extrusive ration of 0.5 to 4. This 'excess carbon' load is, in some ways, analogous to the 'excess sulfur' released from some arc volcanic eruptions that tap a sulphur-rich exsolved phase.
The Mohorovičić Discontinuity, or Moho (red). Geology.com
The fluxes of CO₂ during Large Igneous Province magmatism, and, consequently, the viability of CO₂ as a significant driver of climate change, have been the subject of debate. Perhaps the best evidence for major perturbation of the global carbon cycle by Large Igneous Provinces comes from carbon isotope records and independent palaeoclimate and atmospheric CO₂ content proxies. In brief, these lines of evidence indicate four major aspects of carbon cycle disruption: (1) strong warming coinciding with some Large Igneous Provinces (e.g., the Siberian Traps and the North Atlantic Igneous Province), with more ambiguous evidence in other cases; (2) spikes in atmospheric CO₂ content; (3) negative carbon isotope excursions (drop in the proportion of carbon¹² in sediments); (4) ocean acidification. Taken together with geochronology aligning these environmental changes to Large Igneous Province magmatism, the most straightforward interpretation is that they were primarily caused by carbon release related to Large Igneous Province emplacement, possibly supplemented with light carbon from clathrates (chemical substances consisting of a lattice of water molecules that traps or contains other molecules) or from the metamorphism of sedimentary organic material. Accurately determining the CO₂ budget of diverse Large Igneous Province magmas is critical to testing hypotheses regarding the causal links between magmatism, warming, and carbon cycle perturbation.
On the timescales of silicate weathering and water–rock reactions, Large Igneous Provinces may play an important role as carbon sinks, because CO₂ reacts with calcium and magnesium in basalts to form carbonate minerals. By analogy to mid-ocean ridge settings, where the balance between net outgassing and net sequestration of carbon is uncertain, the capacity for Large Igneous Provincecarbon sequestration on longer timescales may rival the magnitude of potential outgassing. This balance, and the timescales of atmospheric CO₂ draw down, may depend on the extent of subaerial versus submarine volcanism in a given Large Igneous Province and the potential for water–rock reactions in hydrothermal systems.
Large igneous provinces are dominated volumetrically by vast outpourings of flood basalt lavas and their intrusive equivalents. The frequent association of alkaline and carbonatite magmatism with flood basalts in Large Igneous Provinces attests to the petrologic importance of carbon during the generation of Large Igneous Province magmas, though when and how plumbing systems associated with diverse magma types interact with each other is an unresolved question. Previous estimates of the CO₂ content of flood basalts range from 0.5% to 0.9% by weight. Revised estimates of the CO₂ content of Hawaiian (Kï'lauea) and Icelandic (Laki) basalts, along with incompatible trace elements from flood basalt picrites, suggest that primitive flood basalt magmas may commonly comprise about 1% CO₂ by weight, or possibly more. However, such primitive magmas comprise a small fraction of the erupted lavas. More detailed numerical models and geochemical measurements are required to understand the carbon concentrations in flood basalts and other magma types in Large Igneous Provincess. For example, the carbon delivery potential of voluminous tholeiite lavas in flood basalt successions is important but poorly constrained. While their parental magmas result from high degrees of partial melting and are, therefore, less likely to be intrinsically CO₂-rich they may receive a boost from CO₂-rich fluids released from deep intrusive magmas.
Based on the shifting contributions of deep convecting mantle, lithospheric mantle, and crustal sources, together with the variable flux from deep intrusive magmas, the CO₂ flux from Large Igneous Province magmas probably evolves through the entire magmatic cycle. As a consequence, Large Igneous Province carbon emissions are unlikely to scale directly with volumetric eruption rates, which offer, at best, a partial picture of the tempo of carbon outgassing. Nevertheless, gross estimates based on erupted volume provide a starting place for situating Large Igneous Provinces in the context of the global carbon cycle. For a total Large Igneous Province magma volume of 1 000 000–10 000 000 km³ emplaced over 100 000–1 000 000 years, and carrying about 1% weight in CO₂, the mean annual flux would be 30–3000 mega tonnes of CO₂ per/year, with the potential for orders of magnitude deviation from this mean rate over the course of the magmatic cycle. The Laki fissure eruption released an estimated 300–900 mega tonnes of CO₂ over the course of 8 months. For comparison, the present-day global subaerially released flux of CO₂ from all volcanoes has been estimated at 300–600 mega tonnes of CO₂ per/year. During geologically brief intervals of intense outgassing, Large Igneous Provinces are likely to dominate the global flux of deep carbon to the atmosphere.
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