Interrelation of the stagnant slab, Ontong Java Plateau, and intraplate volcanism as inferred from seismic tomography

We investigated the seismological structure beneath the equatorial Melanesian region, where is tectonically unique because an immense oceanic plateau, a volcanic chain and subduction zones meet. We conducted a multi-frequency P-wave tomography using data collected from an approximately 2-year-long seismic experiment around the Ontong Java Plateau (OJP). High-velocity anomalies were revealed beneath the center of the OJP at a depth of ~ 150 km, the middle-eastern edge of the OJP at depths of 200–300 km, and in the mantle transition zone beneath and around the OJP; low-velocity anomalies were observed along the Caroline volcanic island chain above 450 km depth. These anomalies are considered to be associated with the thick lithosphere of the OJP, remnant dipping Pacific slab, stagnant Pacific slab, and a sheet-like upwelling. The broad stagnant slab was formed due to rapid trench retreat from 48 to 25 Ma until when the OJP with thick lithosphere collided with a subduction boundary of the Pacific and Australian plates. This collision triggered slab breakoff beneath the arc where the dipping slab remained. The stagnant Pacific slab in the mantle transition zone restricted the plume upwelling from the lower mantle causing sheet-like deformed upwelling in the upper mantle.

Tracing the subducting Pacific slab to the mantle transition zone with hydrogen isotopes

Hydrogen isotopes have been widely used as powerful tracers to understand the origin of terrestrial water and the water circulation between the surface and the deep interior of the Earth. However, further quantitative understanding is hindered due to a lack of observations about the changes in D/H ratios of a slab during subduction. Here, we report hydrogen isotope data of olivine-hosted melt inclusions from active volcanoes with variable depths (90‒550 km) to the subducting Pacific slab. The results show that the D/H ratio of the slab fluid at the volcanic front is lower than that of the slab fluid just behind the volcanic front. This demonstrates that fluids with different D/H ratios were released from the crust and the underlying peridotite portions of the slab around the volcanic front. The results also show that the D/H ratios of slab fluids do not change significantly with slab depths from 300 to 550 km, which demonstrates that slab dehydration did not occur significantly beyond the arc. Our estimated δD‰ value for the slab materials that accumulated in the mantle transition zone is > − 90‰, a value which is significantly higher than previous estimates.

Translithospheric magma plumbing system of intraplate volcanoes as revealed by electrical resistivity imaging

The magma plumbing systems of volcanoes in subduction and divergent tectonic settings are relatively well known, whereas those of intraplate volcanoes remain elusive; robust geophysical information on the magma pathways and storage zones is lacking. We inverted magnetotelluric data to image the magma plumbing system of an intraplate monogenetic volcanic field located above the stagnant Pacific slab in northeast China. We identified a complex, vertically aligned, low-resistivity anomaly system extending from the asthenosphere to the surface consisting of reservoirs with finger- to lens-like geometries. We show that magma forms as CO2-rich melts in a 150-km-deep asthenospheric plume crossing the whole lithosphere as hydrated melt, inducing underplating at 50 km depth, evolving in crustal reservoirs, and erupting along dikes. Intraplate volcanoes are characterized by low degrees of melting and low magma supply rates. Their plumbing systems have a geometry not so different from that of volcanoes in subduction settings.

Olivine chemistry of the Quaternary Datong basalts of the Trans-North China Orogen: Insights into mantle source lithology and redox–hydration state

Cenozoic intraplate basalts are widespread above the Big Mantle Wedge (BMW) and its front in East Asia. While the mantle source lithology and redox-hydration state have been demonstrated to be crucial in generation of the basalts above the BMW, their nature and role on the basalts above the front of the BMW is poorly constrained. To address this, we report olivine compositions of the Quaternary Datong basalts. The Datong basalts exhibit OIB-like trace-element compositions and depleted Sr-Nd isotopes with slightly enriched signatures (EMI) for tholeiitic basalts. Olivines of the Datong basalts show high Ni and Fe/Mn, and low Ca, Mn, and Mn/Zn values, pointing to a pyroxenite source. Applying V and Ca partition coefficients between olivine and whole-rock, respectively, the Datong basalts lie −0.44 to 0.64 log units above the fayalite-magnetite-quartz buffer for fO2, and contain 2.1-3.4 wt.% H2O but highly variable H2O/Ce values (265-1498). Both fO2 and H2O/Ce in the basalts vary with whole-rock and olivine compositions, indicating the source was the main control, thus, a heterogeneous redox-hydration state in the source: the EMI component being relatively reduced but extremely wet, and recycled oceanic crust being relatively oxidized but dry. The extremely wet EMI component was likely derived from the mantle transition zone. In the light of our findings, we propose a model in which mantle upwelling carried the recycled oceanic crust and EMI component from the MTZ to shallow mantle, due to the Pacific slab stagnating in the MTZ, to form pyroxenite, which subsequently melted to generate the Datong basalts.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5227668

Magma plumbing system and origin of the intraplate volcanoes in Mainland China: An overview of constraints from geophysical imaging

Volcanic eruptions, despite causing large-scale disasters, also provide important natural resources and are an effective way to understand the Earth's internal structure and its evolution. Herein, a comprehensive review is presented on recent progress in geophysical imaging of the structure and origin of intraplate volcanoes in Mainland China. We primarily focus on the Changbaishan, Wudalianchi, Tengchong, Hainan, and Ashikule volcanoes as they are currently active and hence, likely potential hazards during future eruptions, particularly the Changbianshan volcano. The Changbaishan and Wudalianchi volcanoes are widely believed to be caused by the dehydration of the stagnant Pacific slab in the mantle transition zone (MTZ) along with wet upwelling in the big mantle wedge (BMW). There are a number of different views regarding the formation mechanism of the Tengchong volcano. Some studies suggest that a BMW structure is also present under eastern Tibet, and the Tengchong volcano has a deep origin, similar to volcanism in northeast China. Others suggest that the Tengchong volcano is caused by a local and shallow process. Most tomographic studies suggest that the Hainan volcano is a hotspot, and its track has been located in Southeast China by combining seismological, geochemical, and numerical modeling data. A gap exists between the subducted Indian Plate and the Tarim lithosphere beneath the Ashikule volcano, which provides a channel for asthenospheric upwelling to give rise to intralplate volcanism in the Ashikule basin. The interactions of lithospheres may produce shear heating of the subcontinental lithospheric mantle (SCLM), which can generate localized melting. This process has been proposed as an explanation for the intraplate volcanism in Ashikule.