Lithospheric controls on magma composition along Earth’s longest continental hotspot track

East European sedimentary basins long heated by a fading mantle upwelling

A strong negative anomaly of seismic wave velocities at the core-mantle boundary (the Perm Anomaly) beneath the East European platform is attributed to the remnant of a deep mantle upwelling. The interaction between the upwelling and the East European lithosphere in the geological past and its resulting surface manifestations are still poorly understood. Using mantle plume modelling and global plate motion reconstructions, we show here that the East European lithosphere is likely to have been situated over the weakening Perm Anomaly upwelling for about 150-200 million years. As the East European platform moved above the Perm Anomaly in post-Jurassic times, the vertical tectonic movements recorded in sedimentary hydrocarbon-rich basins show either hiatus/uplift or insignificant subsidence. Analytical modelling of heat conduction through the lithosphere demonstrates that the basins have been slowly heated for a long time by the Perm Anomaly upwelling, creating suitable conditions for hydrocarbon maturation. This suggests a profound relationship between mantle plume dynamics, basin evolution, and hydrocarbon generation.

Destruction and regrowth of lithospheric mantle beneath large igneous provinces

Large igneous provinces (LIPs) are formed by enormous (i.e., frequently >106 km3) but short-lived magmatic events that have profound effects upon global geodynamic, tectonic, and environmental processes. Lithospheric structure is known to modulate mantle melting, yet its evolution during and after such dramatic periods of magmatism is poorly constrained. Using geochemical and seismological observations, we find that magmatism is associated with thin (i.e., ≲80 km) lithosphere and we reveal a striking positive correlation between the thickness of modern-day lithosphere beneath LIPs and time since eruption. Oceanic lithosphere rethickens to 125 km, while continental regions reach >190 km. Our results point to systematic destruction and subsequent regrowth of lithospheric mantle during and after LIP emplacement and recratonization of the continents following eruption. These insights have implications for the stability, age, and composition of ancient, thick, and chemically distinct lithospheric roots, the distribution of economic resources, and emissions of chemical species that force catastrophic environmental change.

Magnetotelluric support for edge-driven convection and shear-driven upwelling in the Newer Volcanics Province

Intraplate volcanic provinces present significant natural hazards to many populated regions globally but their origins are poorly understood. Though hypotheses involving mantle plumes are predominant, the Newer Volcanics Province of southeast Australia-a relatively young (< 4.5 Ma), EW trending collection of over 400 volcanic centres-is increasingly attributed to some combination of edge-driven convection (EDC) and shear-driven upwelling (SDU). In this paper, we provide magnetotelluric (MT) data in support of these geodynamic processes. Three-dimensional inversion of 49 new broadband MT sites, in combination with 143 previously collected broadband, long-period, and geomagnetic depth soundings, reveals an elongate zone of moderately low resistivity (∼ 10-300 Ω m) spanning the Mt Gambier subprovince at a depth of between 20 and 40 km. The newly defined Gambier Conductor is contiguous to, and orientationally aligned with, significant step in the seismically-defined lithosphere-asthenosphere boundary (LAB) presented by earlier studies. Moderately low resistivity is interpreted as fluid-catalysed alteration of iron-bearing crust resulting from percolating magmatic volatiles. We argue that localised low resistivity (< 10 Ω m) at ~ 25 km depth in the mid-lower crust is associated with 1.2-3.6% partial melt. Supporting evidence indicates possible crustal thickening from 5.8 Ma at a rate comparable to estimates of SDU-induced surface eruptions and previous NVP production rate estimates.

Intraplate volcanism triggered by bursts in slab flux

Long-lived, widespread intraplate volcanism without age progression is one of the most controversial features of plate tectonics. Previously proposed edge-driven convection, asthenospheric shear, and lithospheric detachment fail to explain the ~5000-km-wide intraplate volcanic province from eastern Australia to Zealandia. We model the subducted slab volume over 100 million years and find that slab flux drives volcanic eruption frequency, indicating stimulation of an enriched mantle transition zone reservoir. Volcanic isotope geochemistry allows us to distinguish a high-μ (HIMU) reservoir [>1 billion years (Ga) old] in the slab-poor south, from a northern EM1/EM2 reservoir, reflecting a more recent voluminous influx of oceanic lithosphere into the mantle transition zone. We provide a unified theory linking plate boundary and slab volume reconstructions to upper mantle reservoirs and intraplate volcano geochemistry.

Geochemical Constraints on Mantle Melting and Magma Genesis at Pohnpei Island, Micronesia

The lithospheric mantle is of paramount importance in controlling the chemical composition of ocean island basalts (OIBs), influencing partial melting and magma evolution processes. To improve the understanding of these processes, the pressure–temperature conditions of mantle melting were investigated, and liquid lines of descent were modelled for OIBs on Pohnpei Island. The studied basaltic samples are alkalic, and can be classified as SiO2-undersaturated or SiO2-saturated series rocks, with the former having higher TiO2 and FeOT contents but with no distinct trace-element composition, suggesting melting of a compositionally homogenous mantle source at varying depths. Both series underwent sequential crystallization of olivine, clinopyroxene, Fe–Ti oxides, and minor plagioclase and alkali feldspar. Early magnetite crystallization resulted from initially high FeOT contents and oxygen fugacity, and late feldspar crystallization was due to initially low Al2O3 contents and alkali enrichment of the evolved magma. The Pohnpei lavas formed at estimated mantle-melting temperatures of 1486–1626 °C (average 1557 ± 43 °C, 1σ), and pressures of 2.9–5.1 GPa (average 3.8 ± 0.7 GPa), with the SiO2-undersaturated series forming at higher melting temperatures and pressures. Trace-element compositions further suggest that garnet rather than spinel was a residual phase in the mantle source during the melting process. Compared with the Hawaiian and Louisville seamount chains, Pohnpei Island underwent much lower degrees of mantle melting at greater depth, possibly due to a thicker lithosphere.

Evidence for a Carbonatite-Influenced Source Assemblage for Intraplate Basalts from the Buckland Volcanic Province, Queensland, Australia

Eastern Australia contains a widespread suite of primitive (MgO ≥ 7.5 wt.%) intraplate basaltic provinces, including those sited along the longest continental hotspot track on Earth (≈2000 km), the Cosgrove track. The Buckland volcanic province is the most southerly basaltic province on the Cosgrove track before a >1600 km stretch that contains only sparse leucitite volcanism. Buckland is also situated just northeast of the edge of thick cratonic lithosphere where it transitions to a thinner continental lithosphere (<110 km) to the east, which may influence the production of plume-derived melts. Here, analysis of minor and trace elements in olivines in alkali basalts and basanites from the Buckland Province are combined with whole-rock compositions to elucidate the mantle source assemblages, and to calibrate minor and trace element indicators in olivine for application to source mineralogy. Olivine xenocrysts show element concentration ranges typical for peridotites; Mn and Al concentrations indicate that the ambient mantle is spinel, rather than garnet, peridotite. High modal pyroxene content is indicated by high Ni, Zn/Fe, and Fe/Mn in olivines, while high Ti/Sc is consistent with amphibole in the source. Residual phlogopite in the source of the basanites is indicated by low K/Nb in whole rocks, while apatite contains high P2O5 and low Rb/Sr (≥0.015) and Sr/La (≥13). The basanite source assemblage probably contains apatite, phlogopite, olivine, clinopyroxene and orthopyroxene, whereas the alkali basalt source assemblage is probably amphibole, olivine, orthopyroxene and clinopyroxene ± phlogopite ± apatite. Both source assemblages correspond broadly to olivine websterite, with the basanite source lying deeper than that for alkali basalt, explaining the occurrence of phlogopite in the source. This mineralogy, along with whole-rock Ti/Eu, Zr/Hf and P2O5/TiO2 values approaching those of natural carbonatites, provide evidence showing that the Buckland source consists of a peridotite that has interacted with a carbonate-rich melt whose origin may be in the deep lithosphere or asthenosphere beneath the craton. Similar enrichment processes are probably common throughout eastern Australia, controlling trace element characteristics in basaltic provinces. The topography of the underside of the lithosphere may play a significant role in determining mantle source assemblages by diverting and concentrating melt flow, and thus influence the location of basaltic provinces.

A new detailed map of total phosphorus stocks in Australian soil

Accurate data are needed to effectively monitor environmental condition, and develop sound policies to plan for the future. Globally, current estimates of soil total phosphorus (P) stocks are very uncertain because they are derived from sparse data, with large gaps over many areas of the Earth. Here, we derive spatially explicit estimates, and their uncertainty, of the distribution and stock of total P in Australian soil. Data from several sources were harmonized to produce the most comprehensive inventory of total P in soil of the continent. They were used to produce fine spatial resolution continental maps of total P in six depth layers by combining the bootstrap, a decision tree with piecewise regression on environmental variables and geostatistical modelling of residuals. Values of percent total P were predicted at the nodes of a 3-arcsecond (approximately 90 m) grid and mapped together with their uncertainties. We combined these predictions with those for bulk density and mapped the total soil P stock in the 0-30 cm layer over the whole of Australia. The average amount of P in Australian topsoil is estimated to be 0.98 t ha(-1) with 90% confidence limits of 0.2 and 4.2 t ha(-1). The total stock of P in the 0-30 cm layer of soil for the continent is 0.91 Gt with 90% confidence limits of 0.19 and 3.9 Gt. The estimates are the most reliable approximation of the stock of total P in Australian soil to date. They could help improve ecological models, guide the formulation of policy around food and water security, biodiversity and conservation, inform future sampling for inventory, guide the design of monitoring networks, and provide a benchmark against which to assess the impact of changes in land cover, land use and management and climate on soil P stocks and water quality in Australia.