Ross N. Mitchell’s research while affiliated with State Key Laboratory of Lithospheric Evolution, Chinese Academy of Sciences and other places

The evolution of the lunar magnetic field can reveal the Moon's interior structure, thermal history, and surface environment. The mid-to-late stage evolution of the lunar magnetic field is poorly constrained, and thus the existence of a long-lived lunar dynamo remains controversial. The Chang'e-5 mission returned the heretofore youngest mare basalts from Oceanus Procellarum uniquely positioned at mid-latitude. We recovered weak paleointensities of 2-4 uT from the Chang'e-5 basalt clasts at 2 billion years ago, attestting to the longevity of a lunar dynamo until at least the Moon's midlife. This paleomagnetic result implies the existence of thermal convection in the lunar deep interior at the lunar mid-stage which may have supplied mantle heat flux for the young volcanism.

Large low shear-wave velocity provinces (LLSVPs) in the lowermost mantle are the largest geological structures on Earth, but their origin and age remain highly enigmatic. Geological constraints suggest the stability of the LLSVPs since at least 200 million years ago. Here, we conduct numerical modeling of mantle convection with plate-like behavior that yields a Pacific-like girdle of mantle downwelling which successfully forms two antipodal basal mantle structures similar to the LLSVPs. Our parameterized results optimized to reflect LLSVP features exhibit velocities for the basal mantle structures that are ~ 4 times slower than the ambient mantle if they are thermochemical, while the velocity is similar to the ambient mantle if purely thermal. The sluggish motion of the thermochemical basal mantle structures in our models permits the notion that geological data from hundreds of millions of years ago are related to modern LLSVPs as they are essentially stationary over such time scales.

Precambrian paleomagnetic studies are critical for testing paleogeographic reconstructions in deep time but rely on the fidelity of the assumption of the geocentric axial dipole (GAD) hypothesis. With high‐reliability data from mafic dykes and volcanic rocks, the scatter of individual virtual geomagnetic poles (VGPs) can be used to test simple GAD models. In order to conduct such a test, the VGPs must be adequate in number and in spatial coverage of the sampling sites. In this study, we targeted the 2.1 Ga Indin dyke swarm of the Slave craton. Building on previous sampling of the Indin dyke swarm in the western and central parts of southern Slave craton, we report results from 9 additional sites in the central and eastern parts of the craton, sites that significantly expand the width of the dyke swarm across the entire craton. The VGPs obtained from 7 of 9 newly identified Indin dykes are broadly similar to previously reported directions, expanding the total of VGPs for individual Indin dykes to n = 28, which is sufficient for a test of the GAD‐based statistical models using VGP scatter. The high VGP scatter of the Indin swarm can be attributed to the relatively high paleolatitude of 56° ± 6° for the Slave craton at the time of dyke emplacement. The Indin data have VGP scatter that is consistent with field models associated with the GAD hypothesis for the indicated paleolatitude, thus confirming the fidelity of the GAD field at ca. 2.1 Ga.

The redox state of a planetary mantle affects its thermal evolution. The redox evolution of lunar mantle, however, remains unclear due to limited oxygen fugacity (fO2) constraints from young lunar samples. Here, we report vanadium (V) oxybarometers on olivine and spinel conducted on 27 Chang’e-5 basalt fragments from 2.0 billion years ago. These fragments yield an average fO2 of ΔIW -0.84 ± 0.65 (2σ), which closely aligns with the Apollo samples from 3.6–3.0 billion years ago. This temporal uniformity indicates the lunar mantle remained reduced. This observation reveals that the processes responsible for oxidizing mantles of Earth and Mars either did not occur or had negligible oxidizing effects on the Moon. The long-term reduced mantle may lead to a distinctive volatile degassing pathway for the Moon. It could also make the lunar mantle more difficult to melt, preventing internal heat dissipation and consequently resulting in a slow cooling rate.

The migration and composition of magmatism over time can provide important insights into the tectonic evolution of an orogen like the Variscan Belt. To identify Large Igneous Provinces (LIPs), key criteria include large magmatic volume, intraplate-origin volcanic geochemistry, and significant plumbing systems. Based on such criteria, we present evidence of ca. 347–330 Ma LIP “fragments” in the South-Western Branch of the Variscan Belt (Morocco), exemplified by the Variscan Central Jebilet Massif. The interpretations are based on four new zircon Usingle bondPb ages obtained by sensitive high-resolution ion microprobe (SHRIMP), a geochemical database of Carboniferous mafic sills, dykes, and gabbroic intrusions together, with subordinate layered ultramafic intrusions, silicic intrusive and volcanic rocks of Central Jebilet Massif, combined with previously published and unpublished data including Srsingle bondNd isotope analyses. Geochemistry data indicate that the early Carboniferous magmatism of the Jebilet Massif is plume-related. Furthermore, primary magmas of the mafic rocks were generated in an intraplate setting and derived by partial melting of complex sources involving asthenosphere, lithospheric mantle, and subducting slab components (Rheic dead slabs), and were modified by crustal contamination during ascent. Magmatic rocks in the same stratigraphic position also occur in other Carboniferous basins including Western Meseta (Rehamna and Moroccan Central Massif). The newly obtained and compiled zircon Usingle bondPb ages from Western Meseta rocks, encompassing an area of ~400,000 km2, indicate that magmatism occurred between ca. 347–330 Ma, coeval with volcanic activity in the Eastern Meseta in northeastern Morocco. The similar emplacement ages, in combination with the tectonic reconstruction of northwestern Gondwana at ca. 330 Ma, suggest that the igneous subprovinces of the Jebilet, Rehamna, and Moroccan Central Massif in Western Meseta, along with Tazekka, Debdou, and Mekkam in Eastern Meseta, the igneous rocks of the Maritimes (Magdalen) Basin, the St. Jean du Doigt bimodal layered intrusion (Brittany, France), and other equivalents such as the Iberian Pyrite Belt and the Southern Vosges magmatism, may represent the eroded and/or deformed remnants of a Large Igneous Province (LIP), which we name here the North Gondwana–Avalonia LIP. We argue that this newly identified LIP was formed by a mantle plume that may have played a role in the breakup along the northwestern margin of the precursor megacontinent Gondwana and the assembly of Pangea. The plume was likely centered under the thick lithosphere of Avalonia. The large-scale sublithospheric plume-flow channeling from the plume head led to the development of widespread tholeiitic/alkaline magmatism in the thinned lithosphere of Western Meseta, interpreted here as a large thin-spot domain, and calc-alkaline/alkaline magmatism in the thickened lithosphere of the Eastern Meseta. The mantle plume may have been most active during the periods of ca. 390–330 Ma (Maritimes Event), ca. 370–338 Ma (Iberia Event), ca. 347–330 Ma (Meseta Event), and the multipulsed ca. 300 Ma, 290–275 Ma, and 250 Ma European North West African Magmatic Province (EUNWA or EUNWAMP), which were the periods when most of the Variscan mafic rocks were produced in these areas.

In a deforming partially molten rock, melt concentrates into a grain‐scale melt pocket aligned at a preferred orientation (melt‐preferred orientation, or MPO). However, observing this texture alone provides limited information on the 3D orientation and geometry of these melt pockets, which are critical parameters for estimating permeability. Here, we modeled the MPO of experimentally deformed peridotites by simulating melt streaks arising from melt pockets of various shapes and 3D orientations. The model aims to identify 3D distribution and characteristics of melt pockets that could account for the observed length, thickness, and the probability of melt streaks. Results show that melt pockets at preferred orientation exhibit greater length, thickness, and number density compared to those perpendicular. These results can be incorporated into the simulation of melt flow through individual melt pockets, which allows us to estimate the permeability corresponding to the observed MPO. We found that the permeability of vertically compressed peridotites increases with increasing compressive strain and a more elongated and thickened shape for melt pocket aligned at preferred orientation. The vertical permeability in the sample with 30% compressive strain is at least 40 times larger than that of an undeformed sample. For peridotites deformed under simple shear, the permeability exhibits an anisotropy of at least three. Such anisotropic permeability, coupled with the formation of melt‐rich bands and other melt channels, is believed to cause lateral melt focusing beneath mid‐ocean ridges.

The geodynamic processes that formed Earth’s earliest continents are intensely debated. Particularly, the transformation from ancient crustal nuclei into mature Archaean cratons is unclear, primarily owing to the paucity of well-preserved Eoarchaean–Palaeoarchaean ‘protocrust’. Here, we report a newly identified Palaeoarchaean continental fragment—the Baishanhu nucleus—in northeastern North China Craton. U–Pb geochronology shows that this nucleus preserves five major magmatic events during 3.6–2.5 Ga. Geochemistry and zircon Lu–Hf isotopes reveal ancient 4.2–3.8 Ga mantle extraction ages, as well as later intraplate crustal reworking. Crustal architecture and zircon Hf–O isotopes indicate that proto-North China first formed in a stagnant/squishy lid geodynamic regime characterised by plume-related magmatic underplating. Such cratonic growth and maturation were prerequisites for the emergence of plate tectonics. Finally, these data suggest that North China was part of the Sclavia supercraton and that the Archaean onset of subduction occurred asynchronously worldwide.

Due to the scarcity of rock samples, the Hadean Era predating 4 billion years ago (Ga) poses challenges in understanding geological processes like subaerial weathering and plate tectonics that are critical for the evolution of life. The Jack Hills zircon from Western Australia, the primary Hadean samples available, offer valuable insights into magma sources and tectonic genesis through trace element signatures. However, a consensus on these signatures has not been reached. To address this, we developed a machine learning classifier capable of deciphering the geochemical fingerprints of zircon. This allowed us to identify the oldest detrital zircon originating from sedimentary-derived “S-type” granites. Our results indicate the presence of S-type granites as early as 4.24 Ga, persisting throughout the Hadean into the Archean. Examining global detrital zircon across Earth’s history reveals consistent supercontinent-like cycles from the present back to the Hadean. These findings suggest that a significant amount of Hadean continental crust was exposed, weathered into sediments, and incorporated into the magma sources of Jack Hills zircon. Only the early operation of both subaerial weathering and plate subduction can account for the prevalence of S-type granites we observe. Additionally, the periodic evolution of S-type granite proportions implies that subduction-driven tectonic cycles were active during the Hadean, at least around 4.2 Ga. The evidence thus points toward an early Earth resembling the modern Earth in terms of active tectonics and habitable surface conditions. This suggests the potential for life to originate in environments like warm ponds rather than extreme hydrothermal settings.

Geodynamic models for Pangea assembly require knowledge of Paleozoic mantle convection patterns. Application of basic geodynamic principles to Neoproterozoic–Paleozoic plate reconstructions yields Pangea in the incorrect configuration (predicting that Pangea should have formed by consumption of the exterior paleo-Pacific Ocean instead of Iapetus, Rheic, and Proto-Tethys oceans). We contend that the mantle legacy of Late Neoproterozoic–Cambrian amalgamation of Gondwana must be factored into models for Pangea amalgamation. Proxy data suggest that the mantle downwelling driving Pan-African collisions and Gondwana assembly evolved into a mantle upwelling as evidenced by the interplay between subduction-related and plume-related tectonics around the periphery of Gondwana. Orthoversion theory, whereby a supercontinent assembles ∼90° away from the centre of the previous supercontinent, suggests that Gondwana amalgamated above an intense downwelling along a meridional subduction girdle that bisected two antipodal sub-equatorial upwellings. Several processes beneath and around Gondwana reduced the intensity of the original downwelling, as evidenced by plume-related activity along its margins, initiation of subduction zone roll-back, and the export of terranes from Gondwana that collided with the margin of Laurentia–Baltica. As upwelling beneath it intensified, Gondwana migrated along the girdle until it collided with Laurentia–Baltica, resulting in the final assembly of Pangea.