Phanerozoic Large Igneous Provinces Research Articles

Cryptic degassing and protracted greenhouse climates after flood basalt events

Large igneous provinces erupt highly reactive, predominantly basaltic lavas onto Earth’s surface, which should boost the weathering flux leading to long-term CO2 drawdown and cooling following cessation of volcanism. However, throughout Earth’s geological history, the aftermaths of multiple Phanerozoic large igneous provinces are marked by unexpectedly protracted climatic warming and delayed biotic recovery lasting millions of years beyond the most voluminous phases of extrusive volcanism. Here we conduct geodynamic modelling of mantle melting and thermomechanical modelling of magma transport to show that rheologic feedbacks in the crust can throttle eruption rates despite continued melt generation and CO2 supply. Our results demonstrate how the mantle-derived flux of CO2 to the atmosphere during large igneous provinces can decouple from rates of surface volcanism, representing an important flux driving long-term climate. Climate–biogeochemical modelling spanning intervals with temporally calibrated palaeoclimate data further shows how accounting for this non-eruptive cryptic CO2 can help reconcile the life cycle of large igneous provinces with climate disruption and recovery during the Permian–Triassic, Mid-Miocene and other critical moments in Earth’s climate history. These findings underscore the key role that outgassing from intrusive magmas plays in modulating our planet’s surface environment.

Volume and rate of volcanic CO2 emissions governed the severity of past environmental crises

The emplacement of large igneous provinces (LIPs) has been linked to catastrophic mass extinctions in Earth's history, but some LIPs are only associated with less severe oceanic anoxic events, and others have negligible environmental effects. Although it is widely accepted that massive magma outpouring can affect the environment through volatile degassing, it remains debated what controls the severity of environmental crises. Here, we demonstrate that the second-most-voluminous Phanerozoic LIP, the Kerguelen LIP, may have contributed to the early Aptian oceanic anoxic event 1a, a global event previously believed to have been caused by the Ontong Java LIP. Geochronological data show that the earliest eruptions of the Kerguelen LIP preceded the onset of oceanic anoxic event 1a by at least ∼5 million years. Analyses of CO2 abundances in melt inclusions combined with Monte Carlo simulations reveal that the volume and degassing rate of CO2 emissions from the Kerguelen LIP are an order of magnitude lower compared to LIPs that caused severe mass extinctions. We propose that the severity of volcanism-related environmental and biotic perturbations is positively correlated with the volume and rate of CO2 emissions. Our results highlight the significant importance of reducing and slowing down CO2 emission in preventing future disastrous environmental consequences.

Hot primary melts and mantle source for the Paraná-Etendeka flood basalt province: New constraints from Al-in-olivine thermometry

Continental flood basalts (CFB) are amongst the most voluminous volcanic eruptions in Earth's history. They are rapidly emplaced, and in rare cases the thick lava piles are associated with primitive magmas that have high MgO contents. The compositions of these primitive melts are consistent with a deep-sourced, high-temperature mantle plume origin. Whilst the association of CFBs with impacting mantle plumes is widely accepted, the magnitude of the thermal anomaly is not yet resolved. The development of Al-in-olivine thermometry, however, allows the crystallisation temperature of (near-)liquidus olivine to be determined without knowing the composition of the co-existing melt. This provides both a robust minimum estimate of mantle temperature and a value from which potential temperature (TP) can be back-calculated. This technique has previously confirmed that crystallisation temperatures in CFB settings can be a few hundred degrees greater than those estimated for MORB, and the results hint at a diversity in crystallisation temperatures between different CFB settings.In this study, we re-assess the TP of the mantle source of the Paraná-Etendeka CFB province by applying the Al-in-olivine thermometer to olivine–spinel pairs from picrites and ferropicrites. We show that the mean crystallisation temperature of olivine with Fo>90 in the picrites is 1458 °C, with a maximum temperature of 1511 °C. Using the mean value, we calculate a preferred TP of 1623 °C, for an assumed lithospheric thickness of 50 km and magma emplacement pressure of 0.5 GPa. This represents a thermal anomaly of around +300 °C relative to ambient mantle, and confirms that the mantle source of the Paraná-Etendeka CFB is the second hottest known from Phanerozoic Large Igneous Provinces, after the Caribbean Large Igneous Province. The ferropicrites record a cooler mean olivine crystallisation temperature of 1296 °C. Given that these low-volume melts derive from deeper and earlier melting of mantle pyroxenite, their temperature is not directly comparable to that of the picrites but they appear to require a somewhat cooler mantle source – perhaps found at the front or edges of a rising plume head.

The low-Ti high-temperature dacitic volcanism of the southern Paraná-Etendeka LIP: Geochemistry, implications for trans-Atlantic correlations and comparison with other Phanerozoic LIPs

The low-Ti silicic volcanic units of the Paraná-Etendeka Large Igneous Province (LIP) have a high volume (~20,000 km3) and are widespread in South America and Africa. In this paper we present a geochemical investigation of conduit- and lava flow-related metaluminous dacitic volcanic rocks from three areas outcropping in southern Brazil. Trace-element abundances are close to those observed from the low-Ti basaltic parental melts and interbedded basaltic andesites. Assimilation and fractional crystallization models suggest significant fractionation (>60%) from basaltic melts with assimilation involving sources with variable large ion lithophiles (LIL) and high field strength (HFS) elements. Variation in Zr/Nb ratios of the silicic volcanic rocks is interpreted to be related to the source characteristics. An effective magma drainage system through structurally controlled conduits was sustained at high magmatic temperatures (~1000–1100 °C), low viscosities of ~106–104 Pa s and water contents from 0.5–1.3 wt%. Melt structures with RAI < 1 are in agreement with field descriptions of an effusive emplacement, erupting domes, coulees, lobes, and extensive SR-type lavas. Trans-Atlantic correlations indicate some compositional overlaps with the Grootberg and Wereldsend quartz latites of Namibia, but the much higher Pb contents of the Namibian quartz latites, five to ten times greater than for the Brazilian dacites, and higher Pb/Cu ratios, suggest a greater degree of assimilation/fractionation of Pb-rich material, such as shale, slate or schist, for the Namibian silicic rocks. After comparing Phanerozoic silicic units related to LIPs, we propose they can be separated into three main types: (1) large volume, high-temperature, water-poor metaluminous rocks, (2) small volume, lower temperature, more hydrous peraluminous to peralkaline rocks; and (3) high-temperature peraluminous rocks with cordierite and garnet, derived from partial melting of metapelitic basement.

Low water content in the mantle source of the Hainan plume as a factor inhibiting the formation of a large igneous province

Large igneous provinces (LIPs) are generally considered to be related to mantle plumes and to have a great importance to the supercontinent break-up events, climate change and biological evolution over Earth's history. The high melt production rates of LIPs can be achieved under rigorous melting conditions: an abnormally high temperature, substantial decompression, addition of fusible components, or remarkable enrichment in water. Although it has been repeatedly noticed that water enrichment has occurred in many Phanerozoic LIPs, the significance of water enrichment in the mantle source for the generation of LIPs has not been explicitly highlighted. The southeastern Asian basalt province (SABP), which is thought to have formed by the Hainan mantle plume from a pyroxenite-bearing source, was emplaced over a small area of about 0.037 Mkm2 over a long period of time from 28.5 Ma to Holocene, thus differing from a typical LIP (>0.1 Mkm3 in volume emplaced in one or multiple pulses of less than 5 Ma). In this work, we measured H2O contents of high-Mg# clinopyroxene phenocrysts (12–179 ppm H2O) from Cenozoic basalts in the northern Hainan Island, which is part of the SABP. These data were utilized to estimate the water content of the mantle source of these basalts, yielding values in the range of 84–360 ppm H2O, which are significantly lower than those obtained for many Phanerozoic LIPs (thousands of ppm). After calculating the mantle potential temperature for the Hainan basalts about 170°C higher than that for the MORB source mantle, we consider that the paucity of source water likely depressed the melt productivity and the velocity of plume upwelling, and induced the Hainan plume to stall at depths of 350–500 km, inhibiting the formation of a LIP by the Hainan plume, despite the occurrence of other favorable conditions. Hence, our results from the SABP provide evidence for the significance of water enrichment in the mantle source in the formation of LIPs. The low water content in the Hainan plume can be attributed to the incorporation of subducted slabs that had experienced significant dehydration.

A temporal and causal link between ca. 1380 Ma large igneous provinces and black shales: Implications for the Mesoproterozoic time scale and paleoenvironment

Phanerozoic large igneous provinces (LIPs) have a significant influence on global climate changes and mass extinction events (MEEs). Most of the Global Boundary Stratotype Section and Points in the Phanerozoic international chronostratigraphic scale are coeval with LIPs and are marked in the sedimentary record by global-scale MEEs and/or by ocean anoxic events represented by black shales. However, due to limited knowledge on atmospheric oxygen concentrations, ocean redox conditions, and early fossils during the Meso-Neoproterozoic Eras prior to the Ediacaran period, little is known on the climate and environmental effects of LIPs during this period of a billion years, the so-called “Boring Billion” (1.8–0.8 Ga). Here we provide geochronological and geological evidence for a temporal and genetic link between the intense ca. 1380 Ma LIP activity (found on many crustal blocks) and coeval black shales in the Nuna (Columbia) supercontinent. We further propose that the ca. 1380 Ma LIPs and black shales widely distributed in the Nuna supercontinent represent a global-scale geological event and provide a robust natural marker for the Calymmian-Ectasian boundary at 1383 Ma. Further investigation of the temporal and genetic link between the LIPs and black shales at other times can contribute to understanding the variations in atmospheric oxygen concentrations and ocean redox conditions during the Boring Billion, during which virtually nothing of Earth’s climate and MEEs is known.

The absolute paleoposition of the North China Block during the Middle Ordovician

Present-day hot spots and Phanerozoic large igneous provinces (LIPs) and kimberlites mainly occur at the edges of the projections of Large Low Shear Wave Velocity Provinces (LLSVPs) on the earth’s surface. If a plate contains accurately dated LIPs or kimberlites, it is possible to obtain the absolute paleoposition of the plate from the LIP/kimberlite and paleomagnetic data. The presence of Middle Ordovician kimberlites in the North China Block provides an opportunity to determine the absolute paleoposition of the block during the Middle Ordovician. In addition to paleobiogeographical information and the results of previous work on global plate reconstruction for the Ordovician Period, we selected published paleomagnetic data for the North China Block during the Middle Ordovician and determined the most reasonable absolute paleoposition of the North China Block during the Middle Ordovician: paleolatitude of approximately 16.6°S to 19.1°S and paleolongitude of approximately 10°W. The block was located between the Siberian Plate and Gondwana, close to the Siberian Plate. During the Cambrian and Ordovician periods, the North China Block may have moved toward the Siberian Plate and away from the Australian Plate.

Reassessment of petrogenesis of Carboniferous–Early Permian rift-related volcanic rocks in the Chinese Tianshan and its neighboring areas

The Carboniferous−Early Permian rift-related volcanic successions, covering large areas in the Chinese Tianshan and its adjacent areas, make up a newly recognized important Phanerozoic large igneous province in the world, which can be further divided into two sub-provinces: Tianshan and Tarim. The regional unconformity of Lower Carboniferous upon basement or pre-Carboniferous rocks, the ages (360–351Ma) of the youngest ophiolite and the peak of subduction metamorphism of high pressure–low temperature metamorphic belt and the occurrence of Ni-Cu-bearing mafic-ultramafic intrusion with age of ∼352Ma and A-type granite with age of ∼358Ma reveal that the final closure of the Paleo-Asian Ocean might take place in the Early Mississippian. Our summation shows that at least four criteria, being normally used to identify ancient asthenosphere upwelling (or mantle plumes), are met for this large igneous province: (1) surface uplift prior to magmatism; (2) being associated with continental rifting and breakup events; (3) chemical characteristics of asthenosphere (or plume) derived basalts; (4) close links to large-scale mineralization and the uncontaminated basalts, being analogous to those of many “ore-bearing” large igneous provinces, display Sr-Nd isotopic variations between plume and EM1 geochemical signatures. These suggest that a Carboniferous asthenosphere upwelling and an Early Permian plume played the central role in the generation of the Tianshan–Tarim (central Asia) large igneous province.

Phanerozoic Large Igneous Provinces (LIPs), HEATT (Haline Euxinic Acidic Thermal Transgression) episodes, and mass extinctions

The Phanerozoic consists of three mutually exclusive climate states: Icehouse, Greenhouse, and Hothouse. The Greenhouse is the default state, representing > 70% of the Phanerozoic. It is characterized by a weak thermal mode (pole-driven deep-water formation), with little or no continental sea ice, and weakly oxic cool-water sinking near the poles. Nitrogen limitation characterizes Greenhouse oceans in which ammonium may be the dominant bioavailable form of nitrogen. The steady-state Greenhouse default must be forced to produce either of the other two states. Continent–continent collision produces the intense chemical weathering that draws down atmospheric CO 2 and results in the Icehouse state that comprises 20–25% of the Phanerozoic. Icehouses have ice caps in contact with the ocean that produce icebergs and foster sea-ice formation. Sea-ice formation fills the ocean with highly oxic phosphorus-limited icewater except for a thin layer of warm surface water in the tropics. Of the 23 most conspicuous mass extinctions of the Phanerozoic (including the late Ediacaran), 4 are associated with ice-volume-related regressions, and 3 are problematic. We suggest that the other 16 mass extinctions occurred when Greenhouse climate was forced to Hothouse climate. Large Igneous Provinces (LIPs) can trigger the HEATT (Haline Euxinic Acidic Thermal Transgression) mechanism, which can force Greenhouse climate to a Hothouse climate state. The rapid, voluminous CO 2 emissions of LIPs cause a series of events that result in very slow but pronounced haline-mode circulation (mid-latitude-driven deep-water formation) that produces warm saline bottom water at very dry, hot subtropical and middle latitudes. This water upwells at the poles, causing warm and moist equable climate. Hothouse development promotes deep-sea anoxia that is generated in concert with CO 2 buildup and warming. The spread of anoxia favors reduced nitrogen over nitrate, and limited iron dust curtails nitrogen fixation. This leads to dominance of primary productivity by primitive green algae and/or cyanobacteria. Progression to euxinia that reaches the photic zone severely curtails primary productivity via poisoning, even as availability of usable nitrogen plummets. The result is eukaryotic mass extinction. We suggest that the 16 Phanerozoic mass extinctions referred to above resulted at least in part from the HEATT process. Evidence for this relationship at 11 of the most severe extinctions is convincing. Data sets for the other 5 are strongly suggestive of HEATT-related mass extinction, but do not yet have enough documentation to reliably characterize them as HEATT episodes.

Reconstruction and interpretation of giant mafic dyke swarms: a case study of 1.78 Ga magmatism in the North China craton

Abstract Short-lived giant mafic dyke swarms are keys to the interpretation of continental evolution and tectonics, reconstruction of continental palaeogeographical regimes, and petrogenesis of volcanism. The 1.78 Ga Taihang–Lvliang dyke swarm, one of the most significant and best-preserved Precambrian swarms in the central part of the North China craton (NCC), is reviewed and discussed. It is interpreted to have a radiating geometry that is compatible with the Xiong'er triple-junction rift, in which the Xiong'er volcanic province is proposed to be the extrusive counterpart of this swarm. It resulted in significant extension, uplift and magmatic accretion of the NCC, and it is comparable with the Phanerozoic large igneous provinces (LIPs) in areal extent ( c. 0.3 Mkm 2 ) and estimated volume ( c. 0.3 Mkm 3 ), short lifespan (&lt;20 Ma), and intraplate setting. This North China LIP is unique in that it comprises large volumes of both mafic and intermediate components. It could have resulted from extensive mantle–crust interaction, probably driven by a large-scale mantle upwelling. A plume tectonic model is favoured by several lines of supporting evidence (i.e. massive volcanic flows correlated over large areas and a giant fanning dyke swarm with plume-affinitive chemistry). It could responsible for massive sulphide (Pb–Zn) and gold (Au–Ag) ore deposits in the Xiong'er volcanic province. Dismembered remnants of this magmatism in other block(s), with potential candidates in South America, Australia and India, could identify other cratonic blocks that were formerly connected to the North China craton.

Timescales and mechanisms of plume–lithosphere interactions: 40Ar/ 39Ar geochronology and geochemistry of alkaline igneous rocks from the Paraná–Etendeka large igneous province

We have determined high-precision 40Ar/ 39Ar ages for alkaline igneous rocks from the western margin of the Early-Cretaceous Paraná–Etendeka large igneous province (Paraguay). These show that small-fraction melt generation occurred beneath the region in two phases; at 145 Ma and 127.5 Ma, i.e. before and at the end of the 139–127.5 Ma Paraná–Etendeka flood-basalt eruptions. Previously published 40Ar/ 39Ar ages for alkaline igneous rocks on the proto-Atlantic coastal margins range from 134 to 128 Ma and indicate that small-fraction melt generation in the east of the province was either synchronous or slightly later than the main pulse of tholeiitic volcanism (between 134 and 132 Ma). Our new 40Ar/ 39Ar phlogopite ages confirm that: (i) the earliest melts associated with the initial impact of the Tristan plume were generated in the west of the Paraná–Etendeka large igneous province and (ii) igneous activity was long lived and immediately predates continental break-up. The Early-Cretaceous Paraguayan alkaline magmas are silica-undersaturated, enriched in incompatible-trace elements, have very-low initial εNd values and probably represent melts of phlogopite-bearing, carbonate-metasomatised peridotite in the subcontinental lithospheric mantle. Our simple one-dimensional, conductive-heating models suggest that the early-phase (145 Ma) alkaline magmas were emplaced on the margins of the Rio de La Plata craton at the time of sublithospheric impact of the proto-Tristan plume. The late phase (127.5 Ma) of Paraguayan alkaline magmatism is concentrated in an intra-cratonic rift zone and melt generation appears to have been triggered by lithospheric extension, perhaps facilitated by conductive heating and thermal weakening associated with the upwelling Tristan plume. The location and timing of both alkaline and tholeiitic melt generation in the Paraná–Etendeka province appear to have been significantly influenced by the non-uniform composition and thickness of the South American and south-west African lithosphere. The long duration of Paraná–Etendeka magmatism (17 Myr) relative to other Phanerozoic large igneous provinces (e.g. Siberia, Karoo, and Deccan) may be an artefact of the limited available high-precision age data for CFB-related alkaline igneous rocks.

The Kalkarindji continental flood basalt province: A new Cambrian large igneous province in Australia with possible links to faunal extinctions

Extensive basaltic volcanism (>106 km 2 ) occurred across northern and western-central Australia during Cambrian time. The basalts are geochemically distinctive, having unusually uniform elevated incompatible element signatures (high Th/U, La/Sm, Rb/Ba) that are atypical of most other continental flood basalt provinces. Individual volcanic and intrusive suites previously assigned to local stratigraphic units are shown to share a common parental magma. This vast Cambrian igneous province is here named the Kalkarindji continental flood basalt province, Australia's oldest and largest Phanerozoic large igneous province. High-precision 4 0 Ar/ 3 9 Ar analyses of plagioclase feldspar separates from basalt flows yield ages of 508 ′ 2 Ma and 505 ′ 2 Ma (2σ), indistinguishable from previous U-Pb zircon ages for related dolerites. These ages indicate that basaltic volcanism coincided with the Early-Middle Cambrian boundary and suggest a temporal link between eruption of the Kalkarindji basalts and the end-Early Cambrian (early Toyonian) faunal mass extinction event.