Early and Late Alkali Igneous Pulses and a High- 3 He Plume Origin for the Deccan Flood Basalts

Major, Trace, and Rare-Earth Element Geochemistry of Nb-V Rich Andradite-Schorlomite-Morimotoite Garnet from Ambadungar-Saidivasan Alkaline Carbonatite Complex, India: Implication for the Role of Hydrothermal Fluid-Induced Metasomatism

In situ major, trace and rare-earth element composition of Ti-rich garnets from Ambadungar-Saidivasan alkaline carbonatite complex (ASACC) are presented to constrain its likely genesis. The garnets are characterized by high andradite (42.7–57.3), schorolomite (22.0–31.0), and morimotoite (15.6–26.5) end members. No distinct chemical zonation is noticed except for minor variations in Ti content. The garnets are enriched in LREE (average 731 ppm) and relatively depleted in HREE (average 186 ppm) and show an M-type first tetrad that leads to a convex upward pattern between Ce and Gd. Mildly positive to no Eu anomalies are observed (Eu/Eu* = 1.06–1.17). The REE patterns (LaN/YbN = 1.11–2.11) are similar to those of garnets from skarn deposits. The presence of tetrad effect in the LREE pattern suggests an active role of metasomatic processes involving hydrothermal fluids during the growth of the garnets. These garnets also contain high Nb (282–2283 ppm) and V (1083–2155 ppm) concentrations, which stand out against the composition of the host rock. Therefore, late-stage metasomatic reactions of earlier formed minerals with hydrothermal fluid enriched in Fe, Si, LREE, Nb, V, and Ti led to the formation of garnet. The primary source for these elements could be magnetite, ilmenite, and pyrochlore present in different varieties of carbonatites in the ASACC, with the required elements being released during their interaction with the hydrothermal fluid. The hydrothermal fluid was likely to be moderately acidic, and having fluoride and sulfate as the primary ligands.

Causes and consequences of asymmetric lateral plume flow during South Atlantic rifting

This article tackles the longstanding question of why some continental rifting was associated with major pulses of excess volcanism that formed “volcanic rifted margins,” yet other rifting, even nearby, was not. After reviewing the South Atlantic rifting exemplar, we present results from an improved type of three-dimensional calculation of mantle flow that avoids many of the artificial boundary condition-related pitfalls in prior numerical studies that did not predict asymmetric rift-related volcanism. Our experiments match the observed development of asymmetric volcanism during South Atlantic rifting, with important implications for plume-influenced rifting of continents. In particular, the scenario does not require a conventionally assumed “starting plume head” to provide the hotter-than-average mantle that induces excess rift magmatism.

Volcanic rifted margins are typically associated with a thick magmatic layer of seaward dipping reflectors and anomalous regional uplift. This is conventionally interpreted as due to melting of an arriving mantle plume head at the onset of rifting. However, seaward dipping reflectors and uplift are sometimes asymmetrically distributed with respect to the subsequent plume track. Here we investigate if these asymmetries are induced by preexisting lateral variations in the thickness of continental lithosphere and/or lithospheric stretching rates, variations that promote lateral sublithospheric flow of plume material below only one arm of the extending rift. Using three-dimensional numerical experiments, we find that South Atlantic rifting is predicted to develop a strong southward asymmetry in its distribution of seaward dipping reflectors and associated anomalous relief with respect to the Tristan Plume that “drove” this volcanic rifted margin, and that the region where plume material drains into the rift should experience long-lived uplift during rifting—both as observed. We conclude that a mantle plume is still needed to source the anomalously hot sublithospheric material that generates a volcanic rifted margin, but lateral along-rift flow from this plume, not a broad starting plume head, is what controls when and where a volcanic rifted margin will form.

The eruptive tempo of Deccan volcanism in relation to the Cretaceous-Paleogene boundary

Late Cretaceous records of environmental change suggest that Deccan Traps (DT) volcanism contributed to the Cretaceous-Paleogene boundary (KPB) ecosystem crisis. However, testing this hypothesis requires identification of the KPB in the DT. We constrain the location of the KPB with high-precision argon-40/argon-39 data to be coincident with changes in the magmatic plumbing system. We also found that the DT did not erupt in three discrete large pulses and that >90% of DT volume erupted in <1 million years, with ~75% emplaced post-KPB. Late Cretaceous records of climate change coincide temporally with the eruption of the smallest DT phases, suggesting that either the release of climate-modifying gases is not directly related to eruptive volume or DT volcanism was not the source of Late Cretaceous climate change.

State shift in Deccan volcanism at the Cretaceous-Paleogene boundary, possibly induced by impact

Bolide impact and flood volcanism compete as leading candidates for the cause of terminal-Cretaceous mass extinctions. High-precision (40)Ar/(39)Ar data indicate that these two mechanisms may be genetically related, and neither can be considered in isolation. The existing Deccan Traps magmatic system underwent a state shift approximately coincident with the Chicxulub impact and the terminal-Cretaceous mass extinctions, after which ~70% of the Traps' total volume was extruded in more massive and more episodic eruptions. Initiation of this new regime occurred within ~50,000 years of the impact, which is consistent with transient effects of impact-induced seismic energy. Postextinction recovery of marine ecosystems was probably suppressed until after the accelerated volcanism waned.

Palaeoposition of the Seychelles microcontinent in relation to the Deccan Traps and the Plume Generation Zone in Late Cretaceous-Early Palaeogene time

The Early Palaeogene magmatic rocks of North and Silhouette Islands in the Seychelles contain clues to the Cenozoic geodynamic puzzle of the Indian Ocean, but have so far lacked precise geochronological data and palaeomagnetic constraints. New 40Ar/39Ar and U–Pb dates demonstrate that these rocks were emplaced during magnetochron C28n; however, 40Ar/39Ar and palaeomagnetic data from Silhouette indicate that this complex experienced a protracted period of cooling. The Seychelles palaeomagnetic pole (57.55°S and 114.22°E; A9512.3°, N=14) corresponds to poles of similar ages from the Deccan Traps after being corrected for a clockwise rotation of 29.4°±12.9°. This implies that Seychelles acted as an independent microplate between the Indian and African plates during and possibly after C27r time, confirming recent results based on kinematic studies. Our reconstruction confirms that the eruption of the Deccan Traps, which affected both India and the Seychelles and triggered continental break-up, can be linked to the present active Reunion hotspot, which is being sourced as a deep plume from the Plume Generation Zone.

Supplementary material:

Experimental data are available at http://www.geolsoc.org.uk/SUP18482.

Bedout: A Possible End-Permian Impact Crater Offshore of Northwestern Australia

The Bedout High, located on the northwestern continental margin of Australia, has emerged as a prime candidate for an end-Permian impact structure. Seismic imaging, gravity data, and the identification of melt rocks and impact breccias from drill cores located on top of Bedout are consistent with the presence of a buried impact crater. The impact breccias contain nearly pure silica glass (SiO2), fractured and shock-melted plagioclases, and spherulitic glass. The distribution of glass and shocked minerals over hundreds of meters of core material implies that a melt sheet is present. Available gravity and seismic data suggest that the Bedout High represents the central uplift of a crater similar in size to Chicxulub. A plagioclase separate from the Lagrange-1 exploration well has an Ar/Ar age of 250.1 +/- 4.5 million years. The location, size, and age of the Bedout crater can account for reported occurrences of impact debris in Permian-Triassic boundary sediments worldwide.

Multiple factors in the origin of the Cretaceous/Tertiary boundary: the role of environmental stress and Deccan Trap volcanism

A review of the scenarios for the Cretaceous/ Tertiary (K/T) boundary event is presented and a coherent hypothesis for the origin of the event is formulated. Many scientists now accept that the event was caused by a meteorite impact at Chicxulub in the Yucatan Peninsula, Mexico. Our investigations show that the oceans were already stressed by the end of the Late Cretaceous as a result of the long-term drop in atmospheric CO2, the long-term drop in sea level and the frequent development of oceanic anoxia. Extinction of some marine species was already occurring several million years prior to the K/T boundary. The biota were therefore susceptible to change. The eruption of the Deccan Traps, which began at 66.2 Ma, coincides with the K/T boundary events. It erupted huge quantities of H2SO4, HCl, CO2, dust and soot into the atmosphere and led to a significant drop in sea level and marked changes in ocean temperature. The result was a major reduction in oceanic productivity and the creation of an almost dead ocean. The volcanism lasted almost 0.7 m.y. Extinction of biological species was graded and appeared to correlate with the main eruptive events. Elements such as Ir were incorporated into the volcanic ash, possibly on soot particles. This horizon accumulated under anoxic conditions in local depressions and became the marker horizon for the K/T boundary. An oxidation front penetrated this horizon leading to the redistribution of elements. The eruption of the Deccan Traps is the largest volcanic event since the Permian-Triassic event at 245 Ma. It followed a period of 36 m.y. in which the earth's magnetic field failed to reverse. Instabilities in the mantle are thought to be responsible for this eruption and therefore for the K/T event. We therefore believe that the K/T event can be explained in terms of the effects of the Deccan volcanism on an already stressed biosphere. The meteorite impact at Chicxulub took place after the onset of Deccan volcanism. It probably played a regional, rather than global, role in the K/T extinction.

Synchrony and Causal Relations Between Permian-Triassic Boundary Crises and Siberian Flood Volcanism

The Permian-Triassic boundary records the most severe mass extinctions in Earth's history. Siberian flood volcanism, the most profuse known such subaerial event, produced 2 million to 3 million cubic kilometers of volcanic ejecta in approximately 1 million years or less. Analysis of (40)Ar/(39)Ar data from two tuffs in southern China yielded a date of 250.0 +/- 0.2 million years ago for the Permian-Triassic boundary, which is comparable to the inception of main stage Siberian flood volcanism at 250.0 +/- 0.3 million years ago. Volcanogenic sulfate aerosols and the dynamic effects of the Siberian plume likely contributed to environmental extrema that led to the mass extinctions.

High- 3 He Plume Origin and Temporal-Spatial Evolution of the Siberian Flood Basalts

An olivine nephelinite from the lower part of a thick alkalic ultrabasic and mafic sequence of volcanic rocks of the northeastern part of the Siberian flood basalt province (SFBP) yielded a (40)Ar/(39)Ar plateau age of 253.3 +/- 2.6 million years, distinctly older than the main tholeiitic pulse of the SFBP at 250.0 million years. Olivine phenocrysts of this rock showed (3)He/(4)He ratios up to 12.7 times the atmospheric ratio; these values suggest a lower mantle plume origin. The neodymium and strontium isotopes, rare earth element concentration patterns, and cerium/lead ratios of the associated rocks were also consistent with their derivation from a near-chondritic, primitive plume. Geochemical data from the 250-million-year-old volcanic rocks higher up in the sequence indicate interaction of this high-(3)He SFBP plume with a suboceanic-type upper mantle beneath Siberia.

Age of the earliest known hominids in Java, Indonesia

40Ar/39Ar laser-incremental heating of hornblende separated from pumice recovered at two hominid sites in Java, Indonesia, has yielded well-defined plateaus with weighted mean ages of 1.81 +/- 0.04 and 1.66 +/- 0.04 million years ago (Ma). The hominid fossils, a juvenile calvaria of Pithecanthropus and a partial face and cranial fragments of Meganthropus, commonly considered part of the Asian Homo erectus hypodigm, are at least 0.6 million years older than fossils referred to as Homo erectus (OH-9) from Olduvai Gorge, Tanzania, and comparable in age with the oldest Koobi Fora Homo cf. erectus (Homo ergaster) in Kenya. These ages lend further credence to the view that Homo erectus may have evolved outside of Africa. If the ancestor of Homo erectus ventured out of Africa before 1.8 Ma, the dispersal would have predated the advent of the Acheulean culture at 1.4 Ma, possibly explaining the absence of these characteristic stone cleavers and hand axes in East Asia.