Formation Of Large Igneous Provinces Research Articles

Hydrous mantle plume promoted the generation of continental flood basalts in the Tarim large igneous province

Recent research on the water content of large igneous provinces (LIPs) has revealed that water has a significant impact on the formation of LIPs. However, most studies focus on the water content of mafic–ultramafic rocks, while relatively little attention has been paid to the water content of continental flood basalts (CFB), which form the major part of LIPs and are characterized by huge volumes (> 1 × 105 km3) and short eruption times. Here, we determined water contents of clinopyroxene crystals from the Akesu diabase, which is co-genetic with flood basalts of the Tarim LIP in China. Based on these measurements, we obtained a water content of higher than 1.23 ± 0.49 wt.% for the parental magma to the Tarim CFB and a minimum water content of 1230 ± 490 ppm for the mantle source, thus indicating the presence of a hydrous mantle plume. Combined with previous studies, our results suggest that water plays a key role in the formation of the Tarim LIP. Additionally, the whole-rock compositions of the Akesu diabase indicate a contribution of pyroxenite in the mantle source. This is consistent with a model, in which water was brought into the Tarim mantle plume by a subducted oceanic plate that entered the deep mantle.

Volcanic crustal structure of the western Hikurangi Plateau (New Zealand) from marine seismic reflection imaging

Seamounts and basaltic basement can influence deformation and mass fluxes within subduction zones. We examined seamounts and volcanic units across the western Hikurangi Plateau, near the Hikurangi subduction margin, New Zealand, with seismic reflection images. Volcanism at the Hikurangi Plateau occurred in at least three phases that we attribute to (1) Early Cretaceous large igneous province formation, the top of which is marked by laterally continuous and dipping wedges of reflections that we interpret as lava flows; (2) Late Cretaceous seamounts and volcaniclastics that erupted onto the crust of the Hikurangi Plateau and make up the majority of seamount volume and basement relief; and (3) late-stage, Pliocene volcanics that erupted through and adjacent to Cretaceous seamounts and younger sediments of the north-central Hikurangi Plateau. The Pliocene volcanoes do not appear to be strongly welded to the plateau basement and may be petit spot volcanoes that are related to the displacement and accumulation of hydrous transition zone melts. Large seamounts and volcaniclastic units are evenly distributed across most of the Hikurangi Plateau near the Hikurangi margin but are absent from the Pegasus Basin. Although faults are imaged throughout the basement of the Pegasus Basin, contemporary normal faulting of the Hikurangi Plateau is uncommon, except for a zone of Quaternary normal faults near the Pliocene volcanics. These trends indicate that the Hikurangi megathrust may be more influenced by volcanic structures in the north and central Hikurangi margin, where plateau rifting and voluminous seamount eruptions have more substantially overprinted the original Early Cretaceous basement.

Cretaceous large igneous provinces: from volcanic formation to environmental catastrophes and biological crises

Abstract The Cretaceous Period was marked by the formation of numerous large igneous provinces (LIPs), several of which were associated with geologically rapid climate, environmental and biosphere perturbations, including the early Aptian and latest Cenomanian oceanic anoxic events (OAEs 1a and 2, respectively). In most cases, magmatic CO 2 emissions are thought to have been the major driver of climate and biosphere degradation. This work summarizes the relationships between Cretaceous LIPs and environmental perturbations, focusing on how volcanism caused climate warming during OAE 1a using osmium-isotope and mercury concentration data. The new results support magmatic CO 2 output from submarine LIP activity as the primary trigger of climate warming and biosphere stress before/during OAE 1a. This submarine volcanic trigger of OAE 1a (and OAE 2), two of the most climatically/biotically severe Cretaceous events, highlights the capacity of oceanic LIPs to impact Earth's environment as profoundly as many continental provinces. Cretaceous magmatism (and likely output of CO 2 and trace-metal micronutrients) was apparently most intense during those OAEs; further studies are needed to better constrain the eruption histories of those oceanic plateaus. Another open question is why the Cretaceous Period overall featured a higher rate of magmatic activity and LIP formation compared with before and afterwards.

Multistage dolomitization influenced by the Emeishan large igneous province: Petrographical and geochemical evidence

The formation of large igneous provinces (LIPs) seems to require a series of conditions conducive to massive dolomitization; however, the specific mechanisms remain unclear. Based on petrographic, geochemical and fluid-inclusion microthermometry analyses, this study identifies the types of dolomites in the Maokou Formation (Middle Permian) in the eastern Sichuan Basin. It analyses the related dolomitization fluids and discusses the influencing mechanisms of the Emeishan large igneous provinces (ELIP) on dolomitization at different stages. Three types of dolomite were identified: (1) fabric-retentive, very finely crystalline, planar-s to nonplanar-a dolomite (MD1); (2) fabric-retentive, fine to medium crystalline, planar-s to nonplanar-a dolomite (MD2); and (3) medium to coarsely crystalline, nonplanar (saddle) dolomite (SD). MD1 exhibited dark-red luminescence under cathodoluminescence and rare earth element patterns that are similar to those of lime mudstone, indicating that its dolomitization fluid was medium-salinity Permian seawater. MD2 displayed orange-red luminescence under cathodoluminescence, with a high Ba content (mean: 397.5 ppm) and positive Eu anomalies (mean: 2.34), indicating that its dolomitization fluid was a mixture of shallowly buried seawater and a small amount of external hydrothermal fluid. SD showed red luminescence under cathodoluminescence, with the highest Ba content (mean: 9820.4 ppm) and significant positive Eu anomalies (mean: 3.165), indicating that its dolomitization fluid was mainly ELIP-related hydrothermal fluid with a small amount of residual seawater. Before the ELIP activity, no dolomite occurred in the study area. In the initial stage of ELIP activity, upwelling of the Emeishan mantle plume led to a shift in sedimentary patterns, promoting the occurrence of reflux dolomitization and the formation of MD1. In the intermediate stage of ELIP activity, the activation of basement faults promoted the occurrence of thermal convection dolomitization, resulting in MD2 formation. In the peak stage of ELIP activity, large-scale eruption of Emeishan basalt led to the development of hydrothermal dolomitization, forming SD. This study provides an approach to the investigation of the influences of LIPs on dolomitization processes.

Mantle Plume‐Lithosphere Interactions Beneath the Emeishan Large Igneous Province

AbstractThe formation of large igneous provinces (LIPs) has been widely believed to be linked to mantle plume activity. However, how the plume modifies the overlying lithosphere, particularly its compositional structure, remains uncertain. Here, we characterize the deep thermochemical structure beneath the Emeishan LIP (ELIP), which is a well‐known Permian plume‐related LIP in China, by taking a multi‐observable probabilistic inversion. Our results find a clear correlation between the lithospheric composition with the ELIP's concentric zones. We infer that the fertile feature of the lithospheric mantle in the ELIP's inner zone was caused by the plume‐derived fertile magmas which infiltrated into and chemically refertilized the ambient depleted lithosphere. This plume‐modified lithospheric compositional structure is likely to be preserved after the plume event, while the present lithospheric thermal structure has been mainly influenced by the subsequent thermal‐tectonic activity. Our results improve our understanding of the physicochemical interactions between the lithosphere and ancient plume.

Water Content and Hydrogen Isotopes of Olivine‐Hosted Melt Inclusions in the Dali Picrites, SW China: Reassessing the Role of Water in the Formation of the Emeishan Large Igneous Province

AbstractThe contribution of water to the formation of large igneous provinces (LIPs) remains controversial. We report the first measurement of water content and hydrogen isotope ratios in olivine‐hosted melt inclusions from the Dali picrites, which belong to the Emeishan LIP (ELIP). The inclusions have a wide range of H2O content (0.01–1.73 wt %) and H isotope ratios (δD = 87 ± 11‰ to −267 ± 14‰). The lowest H isotope ratio is below that of the average mantle but comparable to that of the dehydrated slab. These results suggest that most of the Dali melt inclusions have undergone H+ diffusion and H2O degassing. However, some melt inclusions have higher H2O content and lower δD value, which may have undergone rapid cooling and thereby preserved their magmatic H isotope and water content signatures. The reconstructed H2O content of the primary magma for the Dali picrites is ∼1.35 wt %, much lower than the previous estimate for the ELIP primary magma (3.44 ± 0.89 wt % or 4.0–5.6 wt %). Our results support a moderately hydrous (OIB‐type) magma for the ELIP, instead of a highly hydrous one (arc‐type). The mantle source of the picrites is calculated to contain 0.28–0.52 wt % H2O, and recycled oceanic crust that underwent seawater alteration and subsequent slab dehydration during subduction, causing the low δD value of the melt inclusions. We suggest that the Dali picrites may have formed by partial melting of the recycled oceanic crust and metasomatized mantle peridotite.

Valanginian climate cooling and environmental change driven by Paraná-Etendeka basalt erosion

Abstract The Valanginian Weissert Event (ca. 134 Ma) has long been linked to emplacement of the Paraná-Etendeka large igneous province (LIP). Although several Mesozoic crises were triggered by volcanic CO2 emissions and global warming, causing oceanic oxygen depletion, the Weissert Event featured climate cooling and limited marine anoxia. Here, the impact of silicate weathering on environmental change during the Weissert Event is investigated by presenting the first osmium isotope (187Os/188Os) data sets for the late Berriasian–Valanginian interval. These records document a possible rise in weathering and nutrient runoff during the latest Berriasian, followed by a switch to Paraná-Etendeka basalt erosion during the Valanginian. The high weatherability of LIP basalts enhanced global silicate weathering despite limited coeval climate warming. This weathering triggered the documented cooling, with consequent micronutrient runoff potentially aiding ocean fertilization, highlighting a mechanism linking LIP formation with environmental change that was uniquely different than that of other Mesozoic crises.

Geochemistry and petrogenesis of Late Permian basalts from the Sichuan Basin, SW China: Implications for the geodynamics of the Emeishan mantle plume

Plume-lithosphere interactions are significant in the formation of Large Igneous Provinces (LIPs). The Permian Emeishan Large Igneous Province (ELIP) is considered to be the result of a mantle plume. The Emeishan flood basalts comprise a major part of the ELIP and they define three zones: the inner, intermediate and outer zones. Both high-Ti and low-Ti basalts are present in the inner zone, whereas only high-Ti basalts are found in the intermediate zone and outer zone. However, there are only sparse outcrops in the outer zone, and so geochemical data on basalts from the outer zone are rare and the role of plume-lithosphere interaction in the petrogenesis of volcanic rocks in the outer zone remains poorly understood. In the Sichuan basin, the Basalt Formation is found between the Permian Maokou Formation limestone and the Longtan Formation marl in some drill cores as well as in outcrops in the basin. This relationship demonstrates that the basaltic layer in the basin is part of the Emeishan flood basalts. These basalts have TiO2 contents of 3.7–4.2 wt% and Ti/Y ratios of 604–720, being high-Ti sub-alkaline basalts. They display chondrite-normalized rare earth elements (REE) patterns enriched in light rare earth elements (LREE) relative to heavy rare earth elements (HREE) and have elevated large ion lithophile elements (LILE) and high field strength elements (HFSE). Lead isotope ratios are high (206Pb/204Pb(t) = 18.102–18.392, 207Pb/204Pb(t) = 15.578–15.606, 208Pb/204Pb(t) = 38.410–38.850), and εNd(t) values are −0.38 ∼ 1.17. Detailed petrology and geochemistry suggest that the high-Ti basalts from the Sichuan Basin did not experience significant contamination of crustal and lithospheric mantle material during the ascent of magma. We infer that these basalts resulted from low-degree melting of the plume mantle source and underwent fractional crystallization of clinopyroxene. The distribution and petrogenesis of the Sichuan Basin basalts in the outer zone are different from those of the basalts in the inner zone and there are clearly different plume-lithosphere interactions in different parts of the ELIP. In the inner zone, the temperature of the lithosphere mantle was markedly elevated due to underplating of the mantle plume, causing a substantial quantity of lithosphere mantle melting and the initial formation of low-Ti basalts. This was followed by melting of the mantle plume and the formation of high-Ti basalts. In the outer zone, lower temperatures further from the plume centre were insufficient to generate extensive melting of the lithospheric mantle. Consequently, only the mantle plume melted in the outer zone, resulting in the formation of high-Ti basalts with minimal lithospheric input.

Formation of Tarim Large Igneous Province and Strengthened Lithosphere Revealed Through Machine Learning

AbstractEarth history is punctuated by voluminous magmatism and the formation of large igneous provinces (LIPs). Although anomalous mantle temperatures are known to be involved in many LIPs formation, the potentially critical role of fluids remains elusive. Here we apply machine learning methods (e.g., random forest, deep neural network, and support vector machines) to train models based on global datasets of basalts associated with different settings. The trained models predict that the basalts of Tarim LIP in northwestern China show a spatial decrease in their island arc affinity from northeast to southwest, which can be correlated to fluids released from earlier southward oceanic subduction. Temporally, the fluid activity declined from 290 Ma basalts to 270 Ma mafic dykes, suggesting that the fusible components in the mantle source were waning over time and ultimately a strengthened lithosphere was generated. Our study provides new insights into the crucial role of fluids in the generation of LIPs, particularly those related to ancient subducted slabs.

On the role of Earth's lithospheric mantle in global volatile cycles

In whole-Earth models of volatile cycles the outflux from our planet's convecting mantle to its surface via volcanoes, including those essential for life (e.g. C, H2O, S, F, Cl), is traditionally balanced by the influx from subducted slabs of oceanic lithosphere. Here we explore the additional modulating effect of the rigid lithospheric mantle, which forms the large laterally-continuous layer that separates Earth's convecting mantle and crust. Thermal gradients in the lithospheric mantle essentially control residence times but vary over time, and also large spatial scales, and give this reservoir a unique ability to both sequester and release volatiles. Here, we use multi-mode surface wave tomography to quantify the mass of the lithospheric mantle and reveal that this is similar beneath the world's cratons and oceans (3.5×1022kg) and a factor of 2 larger than continental off-craton regions (1.4×1022kg).By combining our estimates of mass with published volatile contents of mantle phases we demonstrate that over ‘deep time’ Earth's lithospheric mantle has sequestered significant amounts of S≈H2O>C>F>Cl. The largest volatile repository resides below the cores of continents (cratonic mantle), and contains >45% of this reservoir's inventory of H2O (7±2×1018kg), S (8±7×1018kg), C (6±3×1018kg) and F (2±1×1018kg). Approximately 44% of the global cratonic mass is located beneath North America/West Greenland and Eurasia, and so we predict these regions will dominate the volatile inventory.Although global off-craton continental mantle contains <34% of the total lithospheric mantle budget of C, H2O, S, F and Cl, the geothermal gradients of this region are more susceptible to perturbation than beneath the cratons, e.g. during formation of Large Igneous Provinces and continental break-up, and residence times of sequestered volatiles will be shorter. Due to its large volume, the oceanic mantle is also an important repository for C, H2O and S (1 to 8×1018kg), and to a lesser extent Cl and F (0.1×1018kg). While the volatile element budgets of the lithospheric mantle are less than the underlying convecting mantle (1018 to 1020kg), the physical location of this reservoir – together with its ability to store and remobilise volatiles over short and long timescales – make it an important consideration in models of whole-Earth volatile cycles that attempt to balance in- and out-gassing fluxes.

Temporal, geochemical and isotopic constraints on plume-driven felsic and mafic components in a Mesoproterozoic flood rhyolite province

Formation of the Gawler felsic large igneous province (LIP) involved a mantle plume and mafic underplating underneath a metasomatised subcontinental lithospheric mantle (SCLM). Minor extrusive basalt to andesite rocks show the evolution of mantle composition during the initial stages of LIP formation (ca. 15931590 Ma). The earliest basaltic magmatism has low Nb/Yb ratios, low (La/Yb)N, and high εNd(i), representing a relatively depleted region from within the metasomatised SCLM. The more depleted composition and higher volume of this magmatism is consistent with early plume-head driven melts. Subsequent basaltic magmatism shows increasing enrichment in the source, evidenced by increasing Nb/Yb and Th/Nb ratios and decreasing εNd(i). The final stage of basaltic magmatism is marked by an isotopically juvenile SCLM source characterised by lower Th/Nb ratios, possibly representing a rejuvenation phase.Dacite to rhyolite rocks record the behaviour of crustal melting above a mantle plume. Older dacite to rhyolite units extruded 1593.611588.47 Ma represent compositionally heterogeneous rocks that correlate temporally with the basaltic to andesitic units. The Nd isotopic record suggests an increasing SCLM component from the early to late stages of felsic magmatism, occurring synchronously with increasing enrichment in basaltic magmas. Younger dacite to rhyolite units (1586.661586.39 Ma) have no associated mafic component, are geochemically and isotopically homogeneous and represent large volumes of felsic lava extruded in 300 Kyr. The final stage of felsic magmatism (1586.39 Ma) is marked by crystal-rich dacite, decreasing εNd(i) and lower Ti-in-zircon temperatures which are interpreted to represent the waning phase of the mantle plume and increasing crustal melting.This study provides a rare example of temporally constrained geochemical and isotopic compositions in mafic and felsic volcanism in a LIP. Our detailed petrogenetic work best demonstrates the generation of the Gawler LIP occurred in an intracontinental, plume setting analogous to a mafic LIP model.

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.

Phosphate Scavenging During Lava‐Seawater Interaction Offshore of Kīlauea Volcano, Hawaii

AbstractThe 2018, subaerial eruption of Kīlauea volcano, Hawaii, resulted in a 5‐km‐long stretch of coastline that actively drained lava into the ocean. Nutrients were added to the surrounding ocean through the dissolution of basaltic rock and thermal upwelling of deep water, thereby fueling a large phytoplankton bloom. Lava‐impacted, surface seawater had high suspended particle loads, and concentrations of chlorophyll, silicic acid, phosphate (Pi), nitrate, and iron that were elevated up to 12, 36, 5, 960, and 1,400 times, respectively, above the background oligotrophic levels. Widespread precipitation of iron oxyhydroxides (Feox) led to extensive scavenging of the dissolved Pi pool, similar to what occurs along mid‐ocean ridge hydrothermal systems. This scavenging transformed a “fertilization” event into a Pi sink near the coast of the ocean entry; however, nutrient data from outside the bloom suggest that Pi could also desorb from the Feox as it is dispersed into the open ocean. From lava quench experiments, we estimate that the hydration state of the Feox precipitate (H2O/Fe) was 5.2–5.7, and that the equilibrium partition coefficient of Pi into Feox (solid/liquid) was 106. In addition, 33Pi radiotracer incubations were used to differentiate between biotic and abiotic uptake of Pi at Kīlauea's ocean entry. These findings are important for understanding modern‐day volcanic fertilization events, modeling nutrient dynamics during major events in Earth history (such as oxygenation of the atmosphere and the formation of large igneous provinces), and predicting the marine response to greater continental weathering in a warming climate.

Recycled carbon degassed from the Emeishan plume as the potential driver for the major end-Guadalupian carbon cycle perturbations

Massive gas emissions (e.g., CO2, CH4 and SO2) during the formation of large igneous provinces (LIPs) have been suggested as the primary cause of dramatic climatic change and the consequent ecological collapses and biotic crises. Thermogenic carbon of crustal sediments induced by intrusive magmatism throughout the LIPs is considered as the primary trigger for environmental catastrophe including mass extinction, as illustrated in the case of the Emeishan LIP in Southwest China. Here we evaluate the Emeishan LIP to address the causal link between carbon degassing and environmental crises during the end-Guadalupian of Middle Permian. An assessment of the carbon flux degassed from recycled oceanic crust in the Emeishan plume shows that recycled oceanic crust contributed significantly to the carbon flux. Using evidence from carbonate carbon isotopic records at the Gualupian-Lopingian (G-L) boundary stratotype at Penglaitan of South China, our study suggests that carbon degassed from massive recycled components in the Emeishan plume served as a major end-Guadalupian (Middle Permian) carbon isotope excursion. The model based on the Emeishan LIP also offers new insights into the important role of recycled carbon released from other LIPs in climatic change and mass extinctions, as in the cases of the end-Permian Siberian and end-Cretaceous Deccan Traps. Our work highlights that carbon released from subducted slabs is returned to the atmosphere via upwelling mantle plumes, which could drive global climatic change and mass extinction.

Metal isotope signatures from lava-seawater interaction during the 2018 eruption of Kīlauea

The 2018 eruption of Kīlauea was associated with massive input of molten lava into the coastal ocean, which altered seawater chemistry and increased phytoplankton production. In seawater plumes advected away from the site of lava entry, we observed elevated concentrations of over a dozen metals relative to background seawater and unique isotopic compositions of Fe, Cu, Ni, Cd and Zn. The δ56Fe of iron released from lava was lower than basaltic, riverine and coastal iron from Hawaiʻi, but similar to observations of other high-temperature hydrothermal vent fluids. However, rapid precipitation led to only modest enrichments in dissolved iron (<10 nM), with increasing dissolved δ56Fe likely due to fractionation associated with ligand-mediated dissolution of particulate Fe. The isotopic composition of copper and nickel show evidence for two-endmember mixing between background seawater and a lava source. While the Ni isotopic endmember reflected basaltic δ60Ni, endmember δ65Cu, δ66Zn, and δ114Cd were isotopically lighter than basalt. We hypothesize that high diffusivity and volatility of chalcophile elements leads to strong kinetic fractionation in rapidly cooling lavas, similar to Cu, Zn and Cd isotopic patterns observed in tektites. The isotopic signatures of Cu and Ni observed during the 2018 eruption of Kīlauea far exceed their normal seawater range and may be useful for identifying large-scale lava input into ocean waters during the formation of large igneous provinces and other episodes of volcanism.

Petrogenesis of the early Permian A-type granites in the Halajun region, southwest Tianshan, western Xinjiang, NW China: implications for geodynamics of Tarim large igneous province

ABSTRACT Formation of a large igneous province, an explosive magmatic event in earth, is intimately related to geodynamic processes. The Tarim large igneous province, a famous representative in China, contains A-type granites. However, the petrogenesis of the A-type granites is debated. Some researchers think that the A-type granites are developed through extensive fractionation of magma derived from partial melting of newly underplated or gabbroic rocks. Others insist that the A-type granites are developed through intensive fractional crystallization of plume-derived mafic magma. Here, we present zircon U–Pb ages and whole-rock geochemical and Sr–Nd isotopic data for the Kezilekezitage, Keziertuo, Tamu, and Halajun II intrusions in northwest Tarim to elucidate their petrogenesis and geodynamic implications. Zircon U–Pb dating yielded crystallization ages of the Kezilekezitage, Keziertuo, and Halajun II intrusions of 275.1 ± 3.1 Ma (MSWD = 1.14), 274.8 ± 1.8 (MSWD = 0.46) and 275.6 ± 0.7 (MSWD = 1.04), respectively. The whole-rock geochemistry of the four intrusions is characterized by REE and HFSE enrichment, Ba, Sr, P, Ti, and Eu depletion, εNd (t) values of – 1.2 to +0.3, two-stage Nd model ages (T2DM) of 1.14–1.02 Ga, and an A1-type granite affinity. Whole-rock zircon geothermometry indicates a high formation temperature of 721–924°C. It is concluded that these A-type granites were generated by partial melting of Neoproterozoic OIB-like rocks, coupled with extensive fractional crystallization and minor crustal contamination. The production of the A-type granites at ca. 275 Ma represented the termination and greatest degree of lithospheric thinning during large igneous province formation.

Neoarchean large igneous provinces on the Kaapvaal Craton in southern Africa re-define the formation of the Ventersdorp Supergroup and its temporal equivalents

Abstract U-Pb geochronology on baddeleyite is a powerful technique that can be applied effectively to chronostratigraphy. In southern Africa, the Kaapvaal Craton hosts a well-preserved Mesoarchean to Paleoproterozoic geological record, including the Neoarchean Ventersdorp Supergroup. It overlies the Witwatersrand Supergroup and its world-class gold deposits. The Ventersdorp Supergroup comprises the Klipriviersberg Group, Platberg Group, and Pniel Group. However, the exact timing of formation of the Ventersdorp Supergroup is controversial. Here we present 2789 ± 4 Ma and 2787 ± 2 Ma U-Pb isotope dilution-thermal ionization mass spectrometry (ID-TIMS) baddeleyite ages and geochemistry on mafic sills intruding the Witwatersrand Supergroup, and we interpret these sills as feeders to the overlying Klipriviersberg Group flood basalts. This constrains the age of the Witwatersrand Supergroup and gold mineralization to at least ca. 2.79 Ga. We also report 2729 ± 5 Ma and 2724 ± 7 Ma U-Pb ID-TIMS baddeleyite ages and geochemistry from a mafic sill intruding the Pongola Supergroup and on an east-northeast–trending mafic dike, respectively. These new ages distinguish two of the Ventersdorp Supergroup magmatic events: the Klipriviersberg and Platberg. The Ventersdorp Supergroup can now be shown to initiate and terminate with two large igneous provinces (LIPs), the Klipriviersberg and Allanridge, which are separated by Platberg volcanism and sedimentation. The age of the Klipriviersberg LIP is 2791–2779 Ma, and Platberg volcanism occurred at 2754–2709 Ma. The Allanridge LIP occurred between 2709–2683 Ma. Klipriviersberg, Platberg, and Allanridge magmatism may be genetically related to mantle plume(s). Higher heat flow and crustal melting resulted as a mantle plume impinged below the Kaapvaal Craton lithosphere, and this was associated with rifting and the formation of LIPs.

Mantle plumes, triple junctions and transforms: A reinterpretation of Pacific Cretaceous – Tertiary LIPs and the Laramide connection

The Shatsky and Hess Rises, the Mid-Pacific Mountains and the Line Islands large igneous provinces (LIPs) present different challenges to conventional plume models. Resolving the genesis of these LIPs is important not only for a more complete understanding of mantle plumes and plume-generated magmatism, but also for establishing the role of subducted LIP conjugates in the evolution of the Laramide orogeny and other circum-Pacific orogenic events, which are related to the development of large porphyry systems. Given past difficulties in developing consistent geodynamic models for these LIPs, it is useful to consider whether viable alternative geodynamic scenarios may be provided by recent concepts such as melt channel networks and channel-associated lineaments, along with the “two mode” model of melt generation, where a deeply-sourced channel network is superimposed on the plume, evolving and adapting over millions of years. A plume may also interact with transform faults in close proximity to a mid ocean ridge, with the resultant bathymetric character strongly affected by the relative age difference of lithosphere across the fault.Our results suggest that the new two-mode melt models resolve key persistent issues associated with the Shatsky Rise and other LIPs and provide evidence for the existence of a conduit system within plumes that feed deeply-sourced material to the plume head, with flow maintained over considerable distances. The conduit system eventually breaks down during plume – ridge separation and may do so prior to the plume head being freed from the triple junction or spreading ridge. There is evidence for not only plume head capture by a triple junction but also for substantial deformation of the plume stem as the distance between the stem and anchored plume head increases. The evidence suggests that young transforms can serve as pathways for plume material migration, at least in certain plume head – transform configurations. A fortuitous similarity between the path of the Shatsky and Sio plumes, with respect to young spreading ridges and transforms, helps to clarify previously problematic bathymetric features that were not readily ascribed to fixed plumes alone. The Line Island Chain, which has been the subject of a vast number of models, is related mainly to several plumes that passed beneath the same region of oceanic crust, a relatively rare event that has resulted in LIP formation rather than a regular seamount track.Our findings have important implications for the timing and mechanism for the Laramide Orogeny in North America, demonstrating that the Hess Rise conjugate may be much smaller than traditionally thought. The Mid Pacific Mountains conjugate may not exist at all, given large parts of these LIPs were formed at an ‘off-ridge’ site. This needs to be taken into account while considering the effects of conjugate collision on mineralization and orogenic events.

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.

Contributions of Scientific Ocean Drilling to Understanding the Emplacement of Submarine Large Igneous Provinces and Their Effects on the Environment

The Ontong Java Plateau (OJP), Shatsky Rise (SR), and Kerguelen Plateau/Broken Ridge (KP/BR) represent three large igneous provinces (LIPs) located in oceanic settings. The basement lavas have been investigated through scientific ocean drilling and, in the case of the OJP, fieldwork on the emergent obducted portions of the plateau in the Solomon Islands. Such studies show that these three LIPs have very different characteristics. For example, the KP/BR still has an active hotspot, whereas the OJP and the SR do not. The OJP is remarkable in its compositional monotony across the plateau (the Kwaimbaita geochemical type), with minor compositional variation found at the margins (the Kroenke, Singgalo, and Wairahito types). Shatsky Rise shows more compositional variation and, like the OJP, has a dominant lava type (termed the type) in the early stages (Tamu Massif), but subsequent eruptions at the Ori and Shirshov massifs comprise isotopically and trace element enriched lavas, likely reflecting a change in mantle source over time. The KP/BR has highly variable basement lava compositions, ranging from lavas slightly enriched above that of normal mid-ocean ridge basalt in the northern portion (close to the South East Indian Ridge) to more enriched varieties to the south and on Broken Ridge, with a continental crust signature present in lavas from the southern and central KP/BR. The OJP and the KP/BR appear to have formed through punctuated magmatic events, whereas the SR was formed by one relatively long, drawn out event. The formation of oceanic LIPs has in many (but not all) cases been synchronous with oceanic anoxic events. This paper focuses on three oceanic plateaus to emphasize the debate surrounding the environmental impact such LIPs may have had, and also highlights the contributions of scientific ocean drilling to our knowledge of oceanic LIP formation and evolution. This new knowledge allows planning for future oceanic LIP drilling.