Mid-Cretaceous marine Os isotope evidence for heterogeneous cause of oceanic anoxic events

During the mid-Cretaceous, the Earth experienced several environmental perturbations, including an extremely warm climate and Oceanic Anoxic Events (OAEs). Submarine volcanic episodes associated with formation of large igneous provinces (LIPs) may have triggered these perturbations. The osmium isotopic ratio (187Os/188Os) is a suitable proxy for tracing hydrothermal activity associated with the LIPs formation, but 187Os/188Os data from the mid-Cretaceous are limited to short time intervals. Here we provide a continuous high-resolution marine 187Os/188Os record covering all mid-Cretaceous OAEs. Several OAEs (OAE1a, Wezel and Fallot events, and OAE2) correspond to unradiogenic 187Os/188Os shifts, suggesting that they were triggered by massive submarine volcanic episodes. However, minor OAEs (OAE1c and OAE1d), which do not show pronounced unradiogenic 187Os/188Os shifts, were likely caused by enhanced monsoonal activity. Because the subaerial LIPs volcanic episodes and Circum-Pacific volcanism correspond to the highest temperature and pCO2 during the mid-Cretaceous, they may have caused the hot mid-Cretaceous climate.

A red bole zircon record of cryptic silicic volcanism in the Deccan Traps, India

Silicic magmas within large igneous provinces (LIPs) are understudied relative to volumetrically dominant mafic magmas despite their prevalence and possible contribution to LIP-induced environmental degradation. In the 66 Ma Deccan LIP (India), evolved magmatism is documented, but its geographic distribution, duration, and significance remain poorly understood. Zircons deposited in weathered Deccan lava flow tops (“red boles”) offer a means of indirectly studying potentially widespread, silicic, explosive volcanism spanning the entire period of flood basalt eruptions. We explored this record through analysis of trace elements and Hf isotopes in zircon crystals previously dated by U–Pb geochronology. Our results show that zircon populations within individual red boles fingerprint distinct volcanic sources that likely developed in an intraplate setting on cratonic Indian lithosphere. However, our red bole zircon geochemical and isotopic characteristics do not match those from previously studied silicic magmatic centers, indicating that they must derive from yet undiscovered or understudied volcanic centers associated with the Deccan LIP.

Two-pronged kill mechanism at the end-Triassic mass extinction

High-resolution biomarker and compound-specific isotope distributions coupled with the degradation of calcareous fossil remnants reveal that intensive euxinia and decalcification (acidification) driven by Central Atlantic magmatic province (CAMP) activity formed a two-pronged kill mechanism at the end-Triassic mass extinction. In a newly proposed extinction interval for the basal Blue Lias Formation (Bristol Channel Basin, UK), biomarker distributions reveal an episode of persistent photic zone euxinia (PZE) that extended further upward into the surface waters. In the same interval, shelly taxa almost completely disappear. Beginning in the basal paper shales of the Blue Lias Formation, a Lilliput assemblage is preserved consisting of only rare calcitic oysters (Liostrea) and ghost fossils of decalcified aragonitic bivalves. The stressors of PZE and decalcification parsimoniously explain the extinction event and inform possible combined causes of other biotic crises linked to emplacement of large igneous provinces, notably the end-Permian mass extinction, when PZE occurred on a broad and perhaps global scale.

A 5-km-thick reservoir with >380,000 km3 of magma within the ancient Earth's crust

Several recent studies have argued that large, long-lived and molten magma chambers1–10 may not occur in the shallow Earth’s crust11–23. Here we present, however, field-based observations from the Bushveld Complex24 that provide evidence to the contrary. In the eastern part of the complex, the magmatic layering was found to continuously drape across a ~4-km-high sloping step in the chamber floor. Such deposition of magmatic layering implies that the resident melt column was thicker than the stepped relief of the chamber floor. Prolonged internal differentiation within such a thick magma column is further supported by evolutionary trends in crystallization sequence and mineral compositions through the sequence. The resident melt column in the Bushveld chamber during this period is estimated to be >5-km-high in thickness and >380,000 km3 in volume. This amount of magma is three orders of magnitude larger than any known super-eruptions in the Earth’s history25 and is only comparable to the extrusive volumes of some of Earth’s large igneous provinces26. This suggests that super-large, entirely molten and long-lived magma chambers, at least occasionally, occur in the geological history of our planet. Therefore, the classical view of magma chambers as ‘big magma tanks’1–10 remains a viable research concept for some of Earth’s magmatic provinces.

An overview of the plumbing systems of Large Igneous Provinces and their significance

Identification of large volume, short duration mafic magmatic events of intraplate affinity in both continental and oceanic settings on the Earth and other planets provides invaluable clues for understanding several vital geological issues of current concern. Of particular importance is understanding the assembly and dispersal of supercontinents through Earth's history, dramatic climate change events including mass extinctions, and processes that have produced a wide range of LIP-related resources such as Ni-Cu-PGE, Au, U, base metals, and petroleum. This current volume presents some of the latest developments and new information on the temporal and spatial distribution of LIPs in both the Precambrian and Phanerozoic, their origin, the plumbing system of mafic dyke swarms, sill provinces, and layered inrusions, and links to mantle plumes/superplumes events, supercontinent reconstructions and associated metallogeny.

Global-scale emergence of continental crust during the Mesoarchean–early Neoarchean

The timing of the emergence of subaerial landmasses is equivocally constrained as post-Archean and continues to be a much-debated issue. In this study, we document exceptionally 18O-depleted (δ18O < 4.7‰) Mesoarchean to early Neoarchean magmatism in India that shows a similarity with the coeval low-δ18O magmas reported from Australia, South America, and northern China. Such global-scale low-δ18O magmatism would require high-temperature meteoric water–rock interaction in the uppermost crust synchronous with magma generation, necessitating the emergence of a substantial volume of the continental crust. The timing of this low-δ18O magmatism coincides with the development of extensive, subaerial large igneous provinces, a downward shift in δ18O and Δ17O values in pelitic rocks, the rise of normalized 87Sr/86Sr in seawater, and an intermittent upsurge in the proportion of atmospheric oxygen. We propose that the emergence of substantial volumes of continental crust initiated at ca. 3.2 Ga and peaked at 2.8–2.6 Ga, facilitating the generation of globally distributed low-δ18O magmas, and this event can be linked to the first appearance of atmospheric oxygen.

Textural analysis and emplacement conditions of well-preserved Orosirian felsic volcanic rocks of Northern Amazon Craton, Brazil

Located in the Amazon Craton, the Uatumã magmatism (1.89-1.87 Ga) consists in one of the oldest Silicic Large Igneous Provinces (SLIPs) on Earth. For a long time, the access to these deposits in the northern Amazon Craton (Erepecuru–Trombetas Domain) has been set back for volcanological studies due to dense vegetation cover and the absence of roads. Recent studies identify two Orosirian volcanic units in the region: the Iricoumé Group (1.89-1.87 Ga) related to the Uatumã magmatism, and the Igarapé Paboca Formation (1.99-1.94 Ga), associated to an older magmatism. Both units are widespread in the Erepecuru–Trombetas Domain and include effusive and explosive deposits. In this paper, we apply textural analyses and rheological estimations to determine the eruption and emplacement conditions of these two volcanic sequences. Textural analyses were carried out through fieldwork and petrography, including a systematic classification of lavas and volcaniclastic rocks. Rheological parameters were determined using geochemistry data to obtain melt viscosity (η) and temperature, zircon saturation (TZr), liquidus (TL), and glass transition temperatures (TG), for anhydrous and hydrous compositions. Textural analyses indicate the predominance of volcaniclastic facies with abundant eutaxitic and parataxitic textures. Rheological estimations reveal TL of 1020ºC, TZr 650-905ºC, and TG 640-753ºC for anhydrous Iricoumé Group melts. Eruptive viscosity estimations range from 8.4 to 11.7 log η (Pa.s). Igarapé Paboca melts present higher temperatures, with TL of 1050ºC, TZr 710-880ºC, and TG 670-740 ºC. Modeling using hydrous compositions indicate that minute amounts of water can strongly affect the rheology of the studied melts, reducing η, TL, TZr, and TG. The petrographic features indicative of hydrous magma reinforces the role of H2O as a controlling agent in the fragmentation of Iricoumé and Igarapé Paboca melts. The pyroclastic samples are marked by elevated ∆TZr - TG relationships indicative of high emplacement temperatures above the TG. Our results indicate that the high temperatures and the presence of network-modifier cations in the studied melts favored the development of extensive welded ignimbrites associated with low-eruption columns, likely developed in fissural and/or caldera systems.