Large Igneous Province Eruptions Research Articles

Dating the onset and nature of the Middle Permian Emeishan large igneous province eruptions in SW China using conodont biostratigraphy and its bearing on mantle plume uplift models

The Middle Permian Emeishan large igneous province of SW China has provided the quintessential example of the phenomenon of kilometre-scale pre-eruption domal uplift associated with mantle plume impingement on the base of the lithosphere. One key line of evidence is an interpreted zone of truncation of the platform carbonates belonging to the Maokou Formation that underlies the volcanic pile. Here we test this interpretation by conodont age dating the uppermost beds of the Maokou Formation in sections from Yunnan, Sichuan, Guizhou and Guangxi provinces, which span locations from the inner part of the igneous province to several hundred kilometres beyond its margins. The results show that eruptions began in the Jinogondolella altudaensis Zone (∼ 263 Ma) of the Middle Capitanian Stage and greatly increased in extent and volume in the J. xuanhanensis Zone (∼ 262 Ma). Pre-eruption uplift was muted, and most locations within the terrain and at many locations beyond its margins witnessed platform collapse (not uplift) with deep-water facies (radiolarian cherts and submarine fans) developing in the J. altudaensis Zone. The clearest evidence for an emergence surface occurs around the margins of the province in the J. xuanhanensis Zone. This is after the initial onset of eruptions and marks either a eustatic sequence boundary or a brief pulse of tectonic uplift contemporaneous with volcanism. As with recent studies on the basal volcanic successions of the Emeishan LIP, kilometre-scale plume-related domal uplift prior to Emeishan eruptions is not supported by these data; rather a more complex interaction between plume and lithosphere with minor localized uplift and subsidence is inferred.

Climate changes caused by degassing of sediments during the emplacement of large igneous provinces

Most mass extinctions during the last 500 m.y. coincide with eruptions of large igneous provinces (LIPs). The Cretaceous-Tertiary extinction was synchronous with the Deccan flood volcanism, the Permian-Triassic extinction with the eruption of the enormous Siberian traps, and the end-Guadalupian extinction with the Emeishan volcanic province. The causal link remains disputed, however, and many LIPs apparently had no significant impact on the biosphere. Here we show that a key control on the destructive consequences of LIP emplacement is the type of sedimentary rock in basins beneath the flood basalts. Contact metamorphism around intrusions in dolomite, evaporite, coal, or organic-rich shale generates large quantities of greenhouse and toxic gases (CO 2 , CH 4 , SO 2 ), which subsequently vent to the atmosphere and cause global warming and mass extinctions. The release of sediment-derived gases had a far greater impact on the environment than the emission of magmatic gases.

Plume Generation Zones at the margins of Large Low Shear Velocity Provinces on the core–mantle boundary

Large Igneous Province (LIP) eruption sites of the past 300 My lie vertically above 1% slow shear wave velocity ( V s) contours bounding the African and Pacific Large Low Shear Velocity Provinces (LLSVPs) at the core–mantle boundary (CMB), or in the cases of the Siberian and Columbia River LIPs, bounding one or other of two smaller, Low Shear Velocity Provinces (LSVPs). Steep gradients in V s at the CMB coincide with those 1% slow contours. The sites of 24 active hotspot volcanoes project down to the same narrowly defined borders of the LLSVPs at the CMB. Plumes that have generated LIPs and major hotspot volcanoes have risen only from the immediate neighbourhoods of the 1% slow V s contours at the CMB which thus define Plume Generation Zones (PGZs). PGZs projected vertically upward approximately match the + 10 m elevation contour of the geoid showing that the LLSVPs are a dominant control on the positively elevated geoid. Minima in the frequency distribution of shear wave velocities in the lowermost mantle near V s = − 1% indicate that regions with more negative velocities, forming ∼ 2% of total mantle mass, are likely to be of material compositionally different from the rest of the mantle. Because all LIP eruption sites with ages younger than 300 Ma lie above the borders of LLSVPs or LSVPs at the CMB, PGZ footprints are inferred to have remained in the same places for the past 300 My. Because no plumes have risen from the interior of the LLSVPs and because no lithospheric slabs have penetrated those bodies the volumes of the LLSVPs are inferred to have also remained unchanged for the past 300 My. Because the LLSVPs are the dominant control on the positively elevated areas of the geoid those too must have remained as they now are since 300 Ma. The LLSVPs are not rising buoyant objects but stable features of the deep mantle. LIPs have been erupted throughout the past 2.5 Gy indicating that PGZs comparable to those of the past 0.3 Gy and LLSVPs (of which PGZs mark the margins at the CMB) have also existed for at least that long. LLSVPs could thus form the isolated reservoir invoked by some to explain the distinctive isotopic compositions of terrestrial rocks. PGZs lie at places where the boundaries of: (i) The outer core, (ii) one of the LLSVPs or LSVPs, and (iii) the seismically faster part of the deep mantle meet. Horizontal temperature gradients across the steeply inclined margins to the LLSVPs, the interiors of which are hotter than the surrounding mantle, at the CMB are key controls for the generation of plumes. Near the CMB the association of the high temperature of the outer core with an inclined thermal boundary layer at the margins of LLSVPs facilitates the generation of mantle plumes in the PGZs.

Large igneous provinces generated from the margins of the large low-velocity provinces in the deep mantle

SUMMARY There is a clear correlation between downward projected large igneous province (LIP) eruption sites of the past 200 Myr and the margins of the large low-velocity provinces (LLVPs) at the base of the mantle. We established this correlation by using palaeomagnetic as well as fixed and moving hotspot reference frames. Our finding indicates that the majority of the LIPs have been generated by plumes that rose from the D �� zone at the edges of the LLVPs. Most LIP eruption sites project radially downwards to the core‐mantle boundary (CMB) within ±10 ◦ of the 1 per cent slow shear wave velocity contour in the SMEAN tomographic model. Steep shear wave velocity gradients have been mapped near the CMB along much of the lengths of the LLVP margins close to that contour which marks a faster/slower boundary (FSB) within the D �� zone. The observation that eruption sites of LIPs as old as 200 Myr can be linked to this prominent present day seismic structure shows that the FSBs of the two LLVPs have occupied their current positions for at least as long and that the process that leads to the generation of deep-seated plumes has been localized on the FSBs at the margins of the African and Pacific LLVPs for the same interval. The persistence of the LLVPs over 200 Myr is consistent with independent evidence that they are compositionally distinct and are not just simply hotter than the material making up the rest of the D �� zone.

The Link between Large Igneous Province Eruptions and Mass Extinctions

In the past 300 million years, there has been a near-perfect association between extinction events and the eruption of large igneous provinces, but proving the nature of the causal links is far from resolved. The associated environmental changes often include global warming and the development of widespread oxygen-poor conditions in the oceans. This implicates a role for volcanic CO 2 emissions, but other perturbations of the global carbon cycle, such as release of methane from gas hydrate reservoirs or shut-down of photosynthesis in the oceans, are probably required to achieve severe greenhouse warming. The best links between extinction and eruption are seen in the interval from 300 to 150 Ma. With the exception of the Deccan Trap eruptions (65 Ma), the emplacement of younger volcanic provinces has been generally associated with significant environmental changes but little or no increase in extinction rates above background levels.

Periodic components in the diversity of calcareous plankton and geological events over the past 230 Myr

We performed statistical analysis on high-resolution records of the diversity of calcareous plankton (planktonic foraminifera and calcareous nannoplankton), and on records of global sea level, marine isotopes ( δ 18O, δ 13C, 87Sr/ 86Sr), large igneous province (LIP) eruptions, and dated impact craters over the last 230 Myr. Results of Continuous Wavelet Analysis (CWT), Fast Fourier Transform (FFT) spectral analysis and cross-spectral analysis indicate that the records of diversity of calcareous nannoplankton (CN) and planktonic foraminifera (PF) and all of the geologic time series tested show similar dominant 25–33 Myr cycles. Based on the statistical results, best-fit stationary-periodic models for PF and CN evolutionary records can be constructed (with t=time in Myr): CN diversity=9.5 sin(2 π( t−7.5 Myr)/29 Myr)+4.8 sin(2 π( t−4 Myr)/15.3 Myr), and PF diversity=7.6 sin(2 π( t−12 Myr)/26 Myr)+3 sin(2 πt/9.2 Myr). These periodic models describe major patterns in the diversity history, such as: (1) a sawtooth-shaped 29-Myr cycle for calcareous nannoplankton diversity, with gradual increases and abrupt decreases in diversity; (2) plateau-shaped ∼26-Myr cycles in the planktonic foraminiferal record, with abrupt diversity increases and decreases; and (3) the presence of ∼15.3- and 9.2-Myr periodic components that modify the shapes of the 26- to 29-Myr cycles. Except for the synchronous decreases in diversity of PF and CN at 65 and ∼34 Ma, the diversity extrema for the two planktonic groups have been on average ∼2 to 3 Myr out of phase. In the ∼30-Myr-cycle band, CN diversity increased with sea-level rise and increased paleotemperatures, whereas PF diversity was, in general, greater during times of lower sea levels. Diversity of PF and CN was reduced at times of major LIP eruptions and large impacts that follow ∼15 or 30 Myr periodicities. These results suggest that the diversity of calcareous plankton since their appearance in the early Mesozoic has been modulated by long-term cyclical changes in global environmental conditions and by periodic large volcanic and impact events. The pacemaker, or pacemakers, for these cycles may be astrophysical, geophysical or some combination of the two.

Late cretaceous polar wander of the pacific plate: evidence of a rapid true polar wander event

We reexamined the Late Cretaceous-early Tertiary apparent polar wander path for the Pacific plate using 27 paleomagnetic poles from seamounts dated by (40)Ar/(39)Ar geochronology. The path shows little motion from 120 to 90 million years ago (Ma), northward motion from 79 to 39 Ma, and two groups of poles separated by 16 to 21 degrees with indistinguishable mean ages of 84 +/- 2 Ma. The latter phenomenon may represent a rapid polar wander episode (3 to 10 degrees per million years) whose timing is not adequately resolved with existing data. Similar features in other polar wander paths imply that the event was a rapid shift of the spin axis relative to the mantle (true polar wander), which may have been related to global changes in plate motion, large igneous province eruptions, and a shift in magnetic field polarity state.