Flood Basalts Research Articles

Tectonomagmatic evolution of Pune – Nasik Deccan Dykes: Insights from structure and dimension scaling

The Deccan Continental Flood Basalt of the Indian Peninsula is characterized by extensive basaltic eruptions ornamented with three spectacular distinct dyke swarms: the Pune – Nasik, Narmada – Tapi, and Western Coastal dyke swarms. Our study area is the Pune – Nasik dyke swarm, which has ~465 mappable dykes. These dykes exhibit different orientations with a predominant trend of N101° and vary in length from less than 1 km to ~64 km. These dykes are massively jointed and occasionally contain vesicles filled with secondary minerals like quartz and calcite. The host rock is weathered basalt of various older Deccan flows. In this study, we have calculated magmatic overpressures and magma chamber depths using the aspect ratios (length/thickness) of the dykes. The average estimated source depth is ~13 km, based on an average Young's modulus for the host rock basalt (Eavg, 7.5 GPa). Additionally, we compared the inferred magma source depths of the Pune – Nasik, and Narmada-Tapi dyke swarms which include the Nandurbar – Dhule, and Pachmarhi dykes of the Deccan Flood Basalt Province. Our findings indicate that the magma chamber source depth is greater in the Pune – Nasik dyke swarm compared with other dyke swarms. The variation in strike distribution of the Pune-Nasik dyke swarm may be attributed to several factors, including a larger magma chamber, local stress fields generated by shallow magma chamber, or the superimposition of tectonic stress fields (N-S and E-W extension) during the emplacement of dykes. This contrasts with the commonly held belief that the dykes are solely connected to a central edifice of the Reunion hotspot.

Giant plagioclase in the Steens Basalt, southeast Oregon, USA: Tracking the balance of differentiation processes in continental flood basalt evolution

Giant plagioclase basalts (GPBs), flows that contain abundant and conspicuous plagioclase crystals ≥1 cm in length, exist at many continental flood basalt provinces, and their presence can provide a window into the crustal differentiation processes and storage conditions that produced the magmas prior to eruption. GPB flows occur in the Steens Basalt of southeast Oregon, USA, one of the earliest members of the Columbia River Flood Basalt Group, but their origin in this province has not been studied in detail. We conducted quantitative textural analysis utilizing crystal size distributions (CSDs) at the outcrop and thin section scale, as well as in situ plagioclase trace element and isotopic compositional analysis, in order to elucidate the processes responsible for generating two end-member textural types of GPB found in the Steens Basalt: daisy stone glomerocryst clusters composed of radiating plagioclase laths and single isolated plagioclase. CSD patterns suggest a changing balance of multiple differentiation processes affected the magmas that created each GPB textural type. In the daisy stone plagioclase, crystals rarely exhibit resorption textures, trace element abundances are comparatively low with minimal variability, and Sr isotopes are commonly in equilibrium with groundmass. In the single isolated plagioclase, resorption textures are common, trace element abundances are generally higher and display within crystal variation, and Sr isotopes are typically not in equilibrium with groundmass. These distinctive characteristics require separate mechanisms to generate each end-member GPB texture. Daisy stone GPBs form by growth of large crystals following mafic recharge events that completely resorb entrained cumulate plagioclase. A large amount of plagioclase may then grow with minimal cooling following recharge due to saturation in plagioclase components. In contrast, single isolated GPBs are the result of partial resorption of both entrained and resident plagioclase following less voluminous and less primitive recharge events; regrowth then occurs on the partially resorbed cores when cooling resumes. Both GPB types are formed by thermal cycling, but each texture reflects the differentiation processes that were prevalent during different eruptive stages of the Steens Basalt, as the balance between recharge, crystal fractionation, and assimilation varied in time.

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.

Chronostratigraphy and compositional characteristics of miocene volcanism in the Sierra de Pire Mahuida, extra-Andean Patagonia, Argentina

The Pire Mahuida Volcanic Complex (PMVC), located in the extra-Andean region of southern Argentina (68° to 68° 40′W and 41° 51′ to 42° 28′ S), is a bimodal volcanic field that developed during the Miocene (17Ma-14Ma, Langhian-Burdigalian Stages). The complex partially surrounds the southern boundary of the Somún Curá basaltic plateau. The PMVC is mainly composed of acidic pyroclastic and lava facies (rhyolite/dacite flows, lava domes and coulees) and stands out as the one with the largest volume of acid rocks in the Somún Curà province, involving two evolutionary trends (alkaline and subalkaline). Subordinate in volume, basaltic flows overlie this extensive sequence of acidic rocks. Relationships between some units are difficult to establish because they are the result of different eruptive centres. However, stratigraphy, morphology and petrography allow two acid phases to be distinguished, one a mainly lavic phase and the other a mainly pyroclastic phase. U-Pb ages allow precise dating and temporal placement of the acid sequence: 1- lava flows and 2- lava domes and related pyroclastic phases. Two calderas and fissures are responsible for the emission of the felsic rocks. The basaltic facies shows a wide range of characteristics, which also allow two different groups to be distinguished. 1. The basalts of the main plateau are associated with a main fault of NW-SE pattern and 2.The younger basalts are associated with small volcanic edifices. The effusion of the complex was developed in a relatively short time with the basic episode being more prolonged than the acid episode.

40 Ar/ 39 Ar chronostratigraphy and geochemistry of Harrats Ishara and Kura in the Hamd–Jizl Basin, Madinah, Saudi Arabia

An extensive system of mafic dykes, volcanic centers, and provincial flood basalts (harrat) have erupted within the Arabian flank of the Red Sea rift since the early Oligocene. The age, spatial distribution, and composition of these features correlates with distinct changes in rift flank structural architecture as the margin progressed through successive deformational phases. A widespread pre-rift dyke swarm in the proto-Red Sea region c. 25 Ma created thermomechanical crustal conditions favourable for diffuse continental rifting and inboard extensional strain markers like the Hamd–Jizl Basin (HJB) originated amagmatically c. 23 Ma. Six million years later, basalt flows from Harrats Ishara and Kura entered the HJB at several points along strike during a 4.5 million year-long eruptive phase. Two distinct volcanic units developed in Harrat Ishara: a predominantly alkali-olivine basalt with an age range of 18.0–14.3 Ma (Ishara A) and a mostly olivine transitional basalt with an age range of 16.6–12.1 Ma (Ishara B). The contemporaneous eruption of these units indicates that multiple magma sources operated simultaneously during the middle Miocene. Both units generally decrease in alkalinity and rare earth element abundances up section, signaling either ongoing melt fractionation processes or output from different magma chambers with time.

Cenozoic Carbon Dioxide: The 66 Ma Solution

The trend in partial pressure of atmospheric CO2, P(CO2), across the 66 MYr of the Cenozoic requires elucidation and explanation. The Null Hypothesis sets sea surface temperature (SST) as the baseline driver for Cenozoic P(CO2). The crystallization and cooling of flood basalt magmas is proposed to have heated the ocean, producing the Paleocene–Eocene Thermal Maximum (PETM). Heat of fusion and heat capacity were used to calculate flood basalt magmatic Joule heating of the ocean. Each 1 million km3 of oceanic flood basaltic magma liberates ~5.4 × 1024 J, able to heat the global ocean by ~0.97 °C. Henry’s Law for CO2 plus seawater (HS) was calculated using δ18O proxy-estimated Cenozoic SSTs. HS closely parallels Cenozoic SST and predicts the gas solute partition across the sea surface. The fractional change of Henry’s Law constants, Hn−HiHn−H0 is proportional to ΔP(CO2)i, and Hn−HiHn−H0×∆P(CO2)+P(CO2)min, where ΔP(CO2) = P(CO2)max − P(CO2)min, closely reconstructs the proxy estimate of Cenozoic P(CO2) and is most consistent with a 35 °C PETM ocean. Disparities are assigned to carbonate drawdown and organic carbon sedimentation. The Null Hypothesis recovers the glacial/interglacial P(CO2) over the VOSTOK 420 ka ice core record, including the rise to the Holocene. The success of the Null Hypothesis implies that P(CO2) has been a molecular spectator of the Cenozoic climate. A generalizing conclusion is that the notion of atmospheric CO2 as the predominant driver of Cenozoic global surface temperature should be set aside.

Permian flood basalt protoliths for Jurassic amphibolites and eclogites in SW Iran: Implications for subduction of a magma-rich passive margin

Magma-rich passive margins are characterized by mafic intrusions and lavas, making their density comparable to that of the adjoining oceanic crust. As a result, these margins may sink along with the oceanic crust when subduction begins. This possibility warrants further investigation in the geologic record. A fitting example can be found in the Sanandaj-Sirjan Zone (SaSZ), north of the Neotethyan Zagros suture in Iran, where amphibolite to eclogite-facies metabasites are exposed in the Zayanderud high-pressure metamorphic complex. We present UPb zircon ages, whole rock elemental analyses, and Sr-Nd-Hf isotope data to constrain the ages, chemical and isotopic composition, as well as the tectonic setting of the metabasite protoliths. Ion microprobe UPb zircon geochronology on petrographic thin sections reveals Early–Middle Permian protolith ages for small (≤10 μm in diameter) zircon crystals that have been metamorphically overgrown in the eclogite. The elemental and Sr-Nd-Hf isotopic compositions of eclogites and amphibolites indicate a continental flood basalt affinity. Trace element and isotopic signatures closely resemble those of widespread Permian flood basalts erupted during breakup of Pangea and the opening of the Neotethys Ocean, with well-studied examples from Oman, the Himalayas, and the Asian Large Igneous Provinces in Tarim, Tianshan, and Emeishan. Earlier investigations suggested that the eclogitization occurred in the Early–Middle Jurassic. Our findings support a scenario where a magma-rich passive margin was established during the Permian and subsequently sank into the mantle at the onset of Jurassic subduction along this segment of the Neotethys. This studied example advocates a poorly explored possibility of the deep sinking of magma-rich passive margins at the beginning of subduction.

Anatomy of the Emeishan Mantle Plume Head: Insights From New Geochronologic, Geochemical, and Geologic Data

AbstractThe link between mantle plumes and the formation of large igneous provinces (LIPs) is well established although the anatomy of these remains equivocal. Recent experimental studies and geophysical data suggest that the mantle plume head is more likely to be irregular and asymmetric, rather than an axisymmetric flattened disk. The Emeishan large igneous province (ELIP) provides a unique opportunity to test this hypothesis. According to robust petrographic, geochronologic, and geochemical evidence from the late Permian basalts in the Sichuan Basin, and in conjunction with a comprehensive compilation of geologic maps and published geochemical data from the ELIP, we identified several giant radial “fingering” structures. Based on the shallow mantle source from the center to margin in the ELIP and relief of the lithosphere‐asthenosphere boundary, we propose a new mantle plume model to explain the evolution of the Emeishan plume periphery, where narrow finger‐like protrusions and plumelets developed outwards from the main body of the plume to the edges of the flattened plume head. Dragged fingers might have been torn apart into some plumelets, which dispersed and were trapped beneath the thinnest lithosphere relief, and eventually erupted to form small‐scale flood basalt in the Outer Zone of the ELIP.

Translithospheric magma plumbing system fossilized in the Emeishan large igneous province

Lighting up the magma plumbing system beneath a large igneous province (LIP) is challenging because the complex magma migration paths are often covered by flood basalts and sediments. Here, we present a three-dimensional seismic image of the Permian Emeishan LIP in Southwest China, constructed by joint inversion of Rayleigh wave dispersions and receiver functions. The results outline a cylindrical, high-velocity anomaly extending to ~135 km depth below the inner zone of this continental LIP. The geometry and magnitude of the high-velocity anomaly suggest that it represents culminated crystallized materials of primary magmas, thereby mirroring a magma plumbing system fossilized in the lithosphere. Furthermore, our geodynamic models illustrate that the nearly vertical plumbing system was controlled by slow plate motion during the magma emplacement. The plume head beneath a nearly static plate has higher thermal buoyancy and thus promotes more intensive magma emplacement. This phenomenon may apply to other LIPs throughout Earth’s history

A subduction-dismembered Neoproterozoic large igneous province in the Qinling Orogenic Belt, China: Implications for 850 Ma–initiated mantle plume activity in the greater Yangtze Block

Large igneous provinces (LIP) provide invaluable clues for recognizing the growth, breakup, and cycles of supercontinents and constraining the activity of ancient mantle plumes (or superplumes). The widespread intraplate magmatism is considered as a possible fingerprint of ancient plume activities and LIPs. Here, we present the results of an integrated study on protoliths of eclogites and amphibolite-facies bimodal volcanic rocks from North Qinling Ultrahigh-pressure Metamorphic (UHPM) Terrane, Qinling Orogen, and identify a dismembered Neoproterozoic LIP. These rocks share similar protolith ages of ca. 850–800 Ma and experienced high- to ultrahigh-pressure metamorphism at ca. 500–480 Ma. The mafic rocks are mainly tholeiitic and display enrichment of LILEs and HFSEs, similar to the E-MORB/OIB characters. They also have typical features of continental flood basalts (CFBs) with low CaO/Al2O3 (0.68–0.87), Lu/Yb (0.14–0.15), Zr/Nb (8–13), high Nb/La (0.87–1.24, mean 1.07), La/Nb (0.80–1.15), La/Ta (11–20), Nb/Yb (3.0–6.2) ratios, and positive εNd(t) (+4.3 to + 5.4). Their occurrence, geochemical features, age data, and high mantle potential temperature (1500–1550 °C) suggest that they are remnants of Neoproterozoic CFBs with mantle plume origin. Based on the lateral distribution length (ca. 300 km) of meta-basaltic rocks and the maximum subduction depths (ca. 100–200 km) recorded by UHP eclogites, the identified ca. 850 Ma-initiated CFBs in the North Qinling terrane should represent an LIP, covering more than 100,000 km2. This Neoproterozoic LIP, together with the coeval volcanic sequences in the South Qinling terrane and the northern Yangtze, would have occupied a maximum landmass of 300,000 km2, which lends support for the onset of a mantle plume at ca. 850 Ma within the Rodinia supercontinent. We conclude that the Qinling Block is one of the fragments of the Rodinia supercontinent with a passive margin covered by CFBs, which was subducted to mantle depths in the Early Paleozoic.

Platinum-Group Elements and Nb-Ta geochemistry of the Deccan basalts from Kumbharli and other Ghat sections and the Koyna Seismic Zone, Maharashtra (India): Crustal contamination and implication for sulfide saturation of the magma

The Deccan basalts from Kumbharli, Nadlapur, Bijaynagar, Hazarmacchi, and Surli Ghat sections (surface samples) and the Koyna borehole-7 (KBH-7, sub-surface samples) from Maharashtra (western India) demonstrate a two-stage crystallization process. Olivine and chromite were crystallized earlier in a shallow crustal magma chamber while clinopyroxene and plagioclase phenocrysts were crystallized in the sub-volcanic conduits forming the crystal mush, and carried to the surface by the evolved Fe-Ti-rich ascending basaltic magma. The high modal abundance of plagioclase at the top and clinopyroxene at the bottom part of the Kumbharli Ghat section is responsible for an increase in Sr and a decrease in Sc content towards the upper flows. The surface and sub-surface basalt samples contain Pd = 5–31 ppb, Pt = 4–15 ppb, Ir = 1–4 ppb, Ru = 1–3 ppb, and Rh = 3–5 ppb. Negative relations of Pd/Ir, Pd/Ru, and Pd/Rh with MgO from both sets of samples indicate partitioning of Ir, Ru, and Rh to early fractionated cumulus chromite (or olivine). Magnetite is a late crystallizing phase from the evolved Fe-Ti-rich basaltic magma. With an increasing modal abundance of magnetite, there is an increase of FeO total , TiO 2 , and V with Pd indicating fractionation of Pd by magnetite. Geochemical characters show that the surface basalt samples are less contaminated compared to the sub-surface KBH-7 basalts. High Nb (6.8–13.6 ppm) and low Ta (0.2–0.37 ppm) content and elevated Nb/Ta ratios of the surface samples from the ghat sections indicate the possible interaction of the basaltic magma with the carbonate metasomatized subcontinental lithospheric mantle (SCLM). On the other hand, a high degree of partial melting with limited crustal contamination can also elevate the Nb/Ta ratios of the basalts from the ghat sections. Deccan basalts from the study areas do not bear the signature of sulfide supersaturation and subsequent immiscible sulfide segregation. This is evidenced by their high chalcophile element concentrations (Ni = 79–103 ppm, Cu = 121–259 ppm) and ratios like Cu/Zr (≥ 1), Ni/MgO (13.7–21.8) with low Cu/Pd (2140–29938) compared to other Ni-Cu (PGE) sulfide mineralized continental flood basalts. The lack of crustal sulfur in the various contaminants is perhaps the main reason for this sulfide-undersaturated character of the Deccan basalts of the current study.

U-Pb zircon and Sm-Nd geochronology of the dom Silvério group, SE Brazil: Implications for the evolution of the superposed Araçuaí and Minas-Bahia orogens

The Araçuaí orogen, the South American portion of the Brasiliano/Pan-African Araçuaí-West Congo orogenic system (AWCO), formed during the Ediacaran-Cambrian assembly of West-Gondwana. The basement of a substantial portion of the Araçuaí orogen comprises Archean and Paleoproterozoic rocks involved in the 2.1–2.0 Ga Minas-Bahia orogen. The Dom Silvério Group, which consists of mica schists containing layers and lenses of mafic, ultramafic, and Mn-rich rocks, as well as quartzites, graphite schists, marbles, and banded iron formations, defines a ca. 10 km-wide and 150 km-long belt in the southwestern portion of the Araçuaí orogen. This succession, currently interpreted as deep marine deposits or an ocean-floor assemblage of uncertain age - Paleoproterozoic or Neoproterozoic-, potentially records processes involved in critical evolutionary stages of the superposed Minas-Bahia and Araçuaí orogens. Aiming to characterize the age and tectonic significance of the Dom Silvério Group, we performed a geochronological and geochemical investigation on samples collected from selected lithotypes. U-Pb (LA-ICP-MS) determinations on detrital zircons revealed two distinct age spectra. Zircons extracted from the lower Dom Silvério schists define a spectrum characterized predominantly by Neoproterozoic age clusters with a maximum depositional age of 601 ± 10 Ma. Quartzites from the lower and upper sections of the group do not contain zircons younger than 1.9 Ga and show unimodal age distributions around 2067 Ma. The maximum depositional age estimated for these rocks is 2020 ± 6 Ma. The spectrum of the lower section schist indicates the turbidites of the Ediacaran Salinas Formation as their most probable correlative. In contrast, the quartzites and schists of lower and upper sections correlate with the Orosirian Itacolomi Group, the post-collisional assembly of the Minas-Bahia orogen. Sm-Nd isotope analyses and whole-rock geochemistry allow the discrimination of two generations of mafic rocks. The older generation, showing TDM model ages = 1.92 to 3.6 Ga and ƐNd(t) = -12.93 to +4.63, represents components of the basement. The younger generation, with TDM model ages = 1.0 Ga to 1.25 Ga and ƐNd(t) = +0.05 to +2.29, is similar to the ophiolitic rocks that occur imbricated with Ediacaran metasedimentary successions in the central portion of the Araçuaí orogen defining the Ribeirão da Folha accretionary belt. They show Th/Yb and Nb/Yb ratios of the EMORB-OIB mantle array and Nb/Y and Zr/Y ratios compatible with OIB and oceanic plateau basalts. The Dom Silvério Group is thus a complex that involves members of the Araçuaí and Minas-Bahia orogens. Their Ediacaran components materialize the southern extension of the Ribeirão da Folha accretionary wedge and, as such, provide further evidence for oceanic consumption during the development of the southern Araçuaí orogen.

Dynamic evolution of West Gondwana inferred from crustal anisotropy of the South American platform

SUMMARY The amalgamation and breakup of the West Gondwana shaped the South American platform. The dynamics during the processes can be reflected by crust anisotropy of the platform, but there are no specialized crustal anisotropic measurements yet. Splitting analysis of Moho-converted shear waves in P-wave receiver functions (Pms) can reveal crustal-scale anisotropy, which is important for understanding the dynamic evolution of the crust and for the interpretation of mantle anisotropy from splitting analysis of core–mantle refracted shear waves (XKS phases). This study measured crustal anisotropy for the old and stable South American platform by Pms splitting analysis. The splitting times vary mainly in the range of 0–0.5 s, with a regional mean of 0.2 s, slightly lower than that observed in tectonically active regions. The detected crustal anisotropy shows distinct characteristics and spatial zoning, providing insights into tectonic processes. (1) Fast polarization directions at stations close to the Transbrasiliano Lineament (TBL) are oriented NNE–SSW, generally consistent with the strike of the TBL but inconsistent with the maximum horizontal compressive stress, implying that they might be formed by dynamic metamorphism during the formation of the TBL. (2) Crustal anisotropy along the passive continental margin in the east and northeast is weak. Still, the fast polarization directions tend to be oriented along the margin, implying the existence of fossil extensional crustal fabrics formed during the continental rifting of West Gondwana. (3) The Paraná Basin, one of the world's largest Large Igneous Provinces (LIP) covered by continental flood basalts, shows distinctively strong anisotropy, with fast polarization directions highly aligned with mantle anisotropy, implying that synchronous crust–mantle deformation occurred in these regions as a result of magmatism during the breakup of West Gondwana.

The Crystallization of Continental Flood Basalt Lavas: Insights from Textural Studies

Abstract Continental flood basalt (CFB) provinces are products of the largest known volumetric eruptions on Earth (~104 km3), with individual flow fields commonly covering well over 10 000 km2 with a mean lava thickness of over 5 m. Studies focusing on the emplacement style of such lava flows have relied extensively on morphological observations and comparisons with modern lava flows and experimental analogs. In the present study, we compare the textures of flood basalt lavas with those from different eruption settings all over the world using data collected from pre-existing literature to gain detailed insights into the style of eruption. Comparison of crystal size distribution data indicates that the eruption style of CFBs is similar to those of modern-day fissure eruptions (e.g. Iceland). This matches inferences based on observations of morphology. We also use a 1D thermal model to estimate the depth-dependent cooling rates within a single lava lobe and test the validity of assumptions built into the formulation of these models for large scale flood basalt lavas. The results reveal that, on average, flood basalt lavas need to conductively cool much faster than we would expect (up to order of ~102 times faster) to match the textural observations. The model is also frequently unable to replicate the observed depth-wise relative variations in length with depth for CFB lavas. Furthermore, the calculated cooling rates from crystal shapes also do not match those calculated from crystal lengths, indicating the assumptions in cooling flow models need to be modified for large CFB flow fields. Given the large areas of CFB flow fields and the relatively long eruption times inferred for the emplacement of individual flow fields, we hypothesize that inflation of lobes and formation of new lobes via breakouts combined with variable eruption rates are key processes that are missing when modeling the cooling of these flow fields. Accounting for these processes is essential to derive accurate cooling rates, which is important to better understand the environmental impact CFBs have at the time of emplacement.

The Belingwe greenstone belt, Zimbabwe – some new age constraints and implications for greenstone belt evolution

ABSTRACT The Neoarchaean Belingwe greenstone belt of the Zimbabwe craton consists of a well-preserved succession of rocks of low metamorphic grade and low strain. We present new age constraints for the Ngezi Group of the belt and discuss them in the light of greenstone belt evolution. We propose crustal destabilization of the Zimbabwe proto-craton as a result of a global flare of mantle-derived magmatism at ~ 2.75 Ga. This gave rise to extension of a Mesoarchaean granitoid-greenstone terrain, rapid subsidence and, initially, deposition of a transgressive cover succession (Manjeri Formation). High degrees of stretching of proto-cratonic and mantle lithosphere provided accommodation space for the emplacement of several kilometres of submarine volcanic rocks on deeply submerged granitoid-greenstone basement, herein referred to as proto-cratonic flood basalts. Once extension and magmatism had ceased, chemical and clastic sedimentary rocks of the Cheshire Formation were deposited prior to 2.71 Ga as a result of, and subjected to, compressional deformation, and much earlier than previously assumed.

Rift and plume: a discussion on active and passive rifting mechanisms in the Afro-Arabian rift based on synthesis of geophysical data

Abstract. The causal relationship between the activity of mantle plumes and continental break-up is still elusive. The Afro-Arabian rift system offers an opportunity to examine these relationships, in which an ongoing continental break-up intersects a large Cenozoic plume-related flood basalt series. In the Afar region, the Gulf of Aden, the Red Sea, and the Main Ethiopian Rift form an R-R-R triple junction within plume-related flood basalt series. We provide an up-to-date synthesis of the available geophysical and geological data from this region. We map the rift architecture in the intersection region by applying the difference in Gaussians to the topography and the bathymetry and interpreting vertical gravity gradients and Bouguer anomalies. With the aid of these methods we review the spatiotemporal constraints in the evolution of the different features of the plume–rift system. Our results show rough and irregular morphologies of the Gulf of Aden and the Red Sea arms in contrast to the symmetric, continuous, and smooth Main Ethiopian Rift. The triple junction formed by the northeastward propagation of the Main Ethiopian Rift and developed simultaneously with the abandonment of the tectonic connection between the Red Sea and the Gulf of Aden through Bab al Mandab Strait. The triple junction was the last feature to develop in the plume–rift system and marked a tectonic reorganization. By this time, all rift arms were sufficiently evolved and the break-up between Africa and Arabia was already accomplished. We argue that the classical active and passive rifting mechanisms cannot simply explain the progressive development of the Afro-Arabian rift. Instead, we propose a plume-induced plate rotation, which includes an interaction between active and passive mechanisms. In this tectonic scenario, the arrival of the Afar plume provided a push force that promoted the rotation of Arabia around a nearby pole located to the northwest of the plate boundary, enabling the rifting and, ultimately, the break-up of Arabia from Africa.

A Potential Surface Warming Regime for Volcanic Super‐Eruptions Through Stratospheric Water Vapor Increases

AbstractLarge volcanic eruptions are known to influence the climate through a variety of mechanisms including aerosol‐forced cooling and warming via emitted CO2. The January 2022 Hunga shallow underwater eruption caused an increase in stratospheric water vapor, and demonstrated how the associated positive radiative forcing can be an important component of an eruption's climate forcing. We present interactive stratospheric aerosol model simulations of super‐volcanic eruptions with a range of SO2 emissions that can produce climate warming through feedback effects produced by a large igneous province (or “flood basalt”) mid‐latitude super‐eruption using Goddard Earth Observing System Chemistry Climate Model climate model simulations. The model experiments suggest total SO2 emissions ≳4,000 Tg/4 Gt generate a multi‐year period of sustained aerosol absorptive local‐heating of the upper troposphere and lower stratosphere and hence produce net climate warming after strong initial cooling. The eruptions produce stratospheric water vapor increases of factors of 8–600. The initiation of these feedbacks within the simulations suggest they could occur for individual stratovolcano eruptions of the scale of the Toba or Tambora eruptions. We note the sensitivity of our results to volcanic sulfate aerosol microphysics and model chemistry.

Petrogenesis of flood basalts and shield basanite from Mertolemariam- Abamineos area, northwestern Ethiopian plateau: Assessments for mantle source variations

Petrographic and geochemical data (major and trace elements) are presented for Oligocene flood basalts and Miocene shield lavas from the Mertolemariam-Abamineos area, northwestern Ethiopian Plateau to examine the petrogenesis of the erupted magmas and the nature of mantle source compositions. The Mertolemariam flood basalts have mainly aphyric and plagioclase phyric textures. The Abamineos shield lavas have sparsely clinopyroxene phyric to highly plagioclase-clinopyroxene phyric textures. Geochemical classification shows that the Mertolemariam Oligocene flood basalts are sub-alkaline in composition, whereas the Abamineos Miocene shield lavas are highly alkaline in composition. The Abamineos shield has a unique composition (i.e., basanites) compared with all other shield basalts (transitional to alkaline) in the northwestern Ethiopian Plateau. Major and trace element compositions display two distinct trends between Mertolemariam flood basalts and Abamineos shield basanites. Fractional crystallization, partial melting, and crustal contamination of a homogeneous mantle source cannot explain the compositional variations between the Mertolemariam flood basalts and the Abamineos shield basanites. The trace element composition suggests that the Mertolemariam Oligocene flood basalts were more likely generated from the mixing of OIB (mantle plume) and E-MORB (enriched asthenosphere) mantle components. The Abamineos Miocene shield basanites were derived from the mantle plume (OIB) component. LREE/MREE and LREE/HREE indicate that both groups possibly originated from a mantle source within the stability field of spinel and garnet. In comparison, the Mertolemariam flood basalts were formed by a higher degree of partial melting from relatively shallow depths than the Abamineos shield basanites. We propose a scenario that explains the magmatic genesis in the northwestern Ethiopian Plateau: volcanism initiated by the initial arrival of the mantle plume (OIB-like) beneath the lithosphere, which comes across a geochemically fertile and enriched MORB (E-MORB) mantle component in the upper asthenosphere. The hot mantle plume triggered melting of the fertile and enriched MORB, and then melting occurred in the plume (OIB-like) at depth in the stability field of spinel and garnet. Melts from the mantle plume (OIB-like) and E-MORB components mixed to produce sub-alkaline flood basalts during the Oligocene in the northwestern Ethiopian Plateau Subsequently, melts from the advanced upwelling mantle plume (OIB-like) produced Miocene shield lavas in the northwestern Ethiopian Plateau.

On the occurrence of the Dragon leech Hirudo troctina Johnson, 1816 (Annelida, Hirudinea) in Sardinia (Italy)

In recent decades, medicinal leeches of the genus Hirudo have experienced a sharp decline throughout their distribution range due to their overexploitation for medical use and habitat alteration. Such a phenomenon is not fully understood because of the complex taxonomy of the genus, which remained unsettled until the beginning of the XXI century, when the implementation of DNA taxonomy allowed for a better understanding of the diversity of the genus and the distribution ranges of Hirudo species. Only the Mediterranean medicinal leech Hirudo verbana Carena, 1820 is currently reported to occur in Italy and its major islands, although records of other Hirudo species have been reported in the literature. In this study, we report the occurrence of the Dragon leech Hirudo troctina Johnson, 1816 in Sardinia and provide its molecular characterization. In addition, based on a review of the existing literature and museum collections, we provide an ample synopsis of the available evidence regarding the occurrence of the species on the island. Hirudo troctina proved to be widespread and abundant in Sardinia during the XIX century but became rarer and rarer in the XX and XXI centuries possibly due to its overexploitation for medicinal use and export in mainland Europe, the abandonment of traditional grazing practices, and the decline of wetland habitats and amphibian populations. Currently, the only recently validated occurrence sites of the species are the water bodies of the “Giara di Gesturi”, a basaltic plateau in central-southern Sardinia. No corroborated evidence of the possible occurrence of other Hirudo species on the island was found. A better understanding of Hirudo troctina distribution in Sardinia will allow the amelioration of its protection and management practices, also under the provisions of the European “Habitats Directive”, where the H. medicinalis species complex is listed in Annex V.

Volatile Mass Partitioned among the Atmosphere, Mantle, and Core in Proto-Mars

We analyze the distribution of volatiles such as water, hydrogen, and carbon within the atmosphere, mantle, and core of Mars during its accretion stages. The partitioning of these volatiles is crucial as it influences the generation of intrinsic magnetic fields, mantle convection, and magmatic activity. The study suggests that the accretion process is responsible for the presence of water and light elements within the Martian mantle and core. We employ a hybrid-type proto-atmosphere model, which considers an upper layer composed of nebula components and a lower layer made up of degassed components from the impact degassing of the simplified building blocks based on the two-component model. Our numerical analysis implies that the formation of a magma ocean at this high temperature and pressure after around the half-size of the present Mars triggers the migration of water vapor, carbon, and hydrogen from the primordial atmosphere into the planetary interior. Ultimately, several 1021 kg of water could have been distributed to the mantle and, if degassed again by the formation of flood basalts on the Tharsis Plateau, could have supplied Mars’ ancient oceans. Meanwhile, several 1020 kg of carbon and 1019 kg of hydrogen (equivalent to 1020 kg H2O) were found to be distributed in the core. Although the distribution of sulfur was simplified so that all of the sulfur is in the core, a result of sulfur accounting for 14 wt% to 19 wt.% of the core mass would be consistent with the core density suggested by InSight.