Sally A. Gibson

Mantle plumes are a fundamental component of the Earth’s convective regime, but their long-term behaviour remains poorly understood. The Iceland plume, which is bisected by the Mid-Atlantic Ridge, presents an opportunity to constrain plume evolution. Here, we reconstruct its influence upon seafloor spreading by exploiting the geochemistry of basalts drilled south of Iceland during International Ocean Discovery Program (IODP) Expedition 395. Trace element and isotopic measurements, combined with contextual geophysical observations, demonstrate that plume influence waned after continental break-up at ~ 55 Ma, collapsed rapidly at ~ 38 Ma, and then was progressively re-established to the present day. Recovered ~ 32 Ma basalt samples have rare earth element compositions equivalent to mid-Atlantic Ridge dredge samples located south of the present-day plume influence. These compositions can be modelled by passive upwelling and melting of depleted MORB mantle with a potential temperature of ~ 1300 ^∘ C. In contrast, basalts recovered from younger (i.e. 0–14 Ma) sites show unequivocal evidence for plume influence. Together, these results imply dramatic changes in the extent of plume-ridge interaction across the North Atlantic region, providing key chemical constraints for geodynamic models of plume evolution and its imprint upon the geological record.

Some hotspot tracks, such as those formed by the Hawai’i and Galápagos mantle plumes, exhibit long-lived cross-track isotopic zonation, thought to reflect the streaking out of heterogeneous material in the plume conduit during upwelling. In lavas associated with the Galápagos mantle plume, three geochemical domains, present for at least 15 Myr, have been identified: northern, southern and central. The most extreme isotopic enrichments are observed in the northern domain of the Cocos Ridge at ~15 Ma, and in the southern domain of the Galápagos Archipelago at the present day. Owing to the northward migration of the Galápagos Spreading Center above the plume at ~5-10 Ma, this relationship suggests that geochemical enrichment in the Galápagos basalts is greatest above the region of the plume furthest from the nearby mid-ocean ridge. We examine the hypothesis that these temporal and spatial variations in geochemical enrichment reflect a ''shallow mantle control'', associated with differences in the mean depth of melting. We conducted forward melting models of a mixed peridotite-pyroxenite mantle to calculate the isotopic composition of the resulting melts formed under two different mantle flow regimes. Our results demonstrate that variations in the average pressure of melt generation, due to the influence of the nearby ridge axis, may explain the range of isotopic compositions across ~15 Ma of Galápagos plume-related volcanism. The patterns of isotopic zonation observed along the hotspot track confirm the paradigm of persistent plume striping, with variations in the degree of geochemical enrichment modulated by shallow mantle processes.

The cratonic ‘roots’ of Earth's major continents extend to depths of over 160 km and have remained stable for more than 2.5 billion years due to buoyant, refractory harzburgites formed by Archean mantle melting. However, kimberlite-hosted harzburgite mantle xenoliths from some global cratons (e.g., Kaapvaal, Siberia, Slave, Rae and Tanzania) show unusual orthopyroxene and silica enrichment, alongside titanium depletion, which cannot be explained by simple melting processes (Boyd, 1989). These are abundant in the xenolith suites and were typically entrained at depths shallower than ~150 km. Many have escaped the pervasive carbonated silica undersaturated melt metasomatism that dominates the base of global cratonic lithosphere. The origins of the orthopyroxene-rich harzburgites have long been debated: hypotheses include high-pressure melting residues (Aulbach et al., 2011), komatiite melt interaction (Tomlinson & Kamber, 2021), or subduction-related silicic melts and fluids (Bell et al., 2005).To further investigate the origin of the widespread excess silica in ancient mantle we analysed volatile (H₂O, F, Cl) contents by Secondary Ion Mass Spectrometry in well-characterised peridotites from the Kaapvaal craton. The orthopyroxene-rich harzburgites, including a diamond-bearing sample, show elevated volatile contents and depletions in Ti. The results of mass balance calculations suggest that the orthopyroxene-rich harzburgites formed by reactive infiltration of supercritical C-O-H fluids -- rich in silica, potassium, fluorine and chlorine but depleted in Ti -- fluxed from subducted oceanic lithosphere (carbonated pelites, eclogites and serpentinites). These findings highlight the role of C-O-H-F-Cl bearing fluids in shaping cratonic lithosphere globally and offer a new framework for understanding craton evolution, mantle metasomatism and diamond genesis in early Earth (Gibson et al., 2025). Figure 1. Schematic illustration of the formation of excess orthopyroxene in Archean cratons. Aulbach, S., Stachel, T., Heaman, L. M., Creaser, R. A. & Shirey, S. B. (2011). Formation of cratonic subcontinental lithospheric mantle and complementary komatiite from hybrid plume sources. Contributions to Mineralogy and Petrology 161, 947–960.Bell, D. R. et al. (2005). Silica and volatile-element metasomatism of Archean mantle: a xenolith-scale example from the Kaapvaal Craton. Contributions to Mineralogy and Petrology 150, 251–267.Boyd, F. R. (1989). Compositional distinction between oceanic and cratonic lithosphere. Earth and Planetary Science Letters 96, 15–26.Gibson, S.A., Jackson, C.J., Crosby, J.C. & Day, J.A.F. (2025). The role of C-O-H-F-Cl fluids in the making of Earth’s continental roots. Nat Commun doi:10.1038/s41467-025-62888-3Tomlinson, E. L. & Kamber, B. S. (2021). Depth-dependent peridotite-melt interaction and the origin of variable silica in the cratonic mantle. Nat Commun 12, 1082.

During the voyage of the H.M.S. Beagle (1831–1836), Charles Darwin had the opportunity to visit and observe volcanoes and volcanic deposits at several archipelagos in the Atlantic, Pacific, and Indian Oceans and also in the Andes. In the Galápagos Archipelago, he described hydrovolcanism and what we now know as pyroclastic surge deposits. Darwin also noted the relation between scoria and lava in Strombolian deposits and the role of effective viscosity on lava surface morphology. He observed volcanic eruptions in the Andes from a distance and speculated on the role of earthquakes in triggering eruptions. Darwin's field work on Ascension, the Cape Verde Islands, Mauritius, and the Azores included detailed work on volcanic bombs and the spatial relationship between basalt and evolved lavas. Much of his focus was on making detailed observations at the outcrop scale to deduce large-scale geologic processes, such as the relation between magmatism and mountain building and the distribution of magma in the deep Earth. While many of Darwin's theories involving volcanism did not stand the test of time, in contrast to his theories on coral reefs and the origin of species, they exemplify his genius at developing novel hypotheses and making observations to test those hypotheses.

The cratonic ‘roots’ of Earth’s major continents extend to depths of over 160 km and have remained stable for more than 2.5 billion years due to buoyant, refractory harzburgites formed by Archean mantle melting. However, mantle harzburgites from some global cratons (e.g., Kaapvaal, Siberia, Slave, Rae and Tanzania) show unusual orthopyroxene and silica enrichment, alongside titanium depletion, which cannot be explained by simple melting processes. The origins of the orthopyroxene-rich harzburgites are debated and include high-pressure melting residues, with komatiite melt interaction, or subduction-related silicic melts and fluids. To further investigate this we analysed volatile (H₂O, F, Cl) contents in Kaapvaal craton peridotites. Orthopyroxene-rich harzburgites, including a diamond-bearing sample, show elevated volatile contents, suggesting infiltration by supercritical C-O-H fluids—rich in silica, fluorine and chlorine and depleted in Ti—fluxed from subducted oceanic lithosphere (carbonated pelites, eclogites and serpentinites). These findings highlight the role of C-O-H-F-Cl bearing fluids in shaping cratonic lithosphere and offer a new framework for understanding craton evolution, mantle metasomatism and diamond genesis in early Earth.

Continental mantle represents one of Earth’s most ancient and long-lived chemical reservoirs. It plays a crucial role in the global cycling of volatile elements—such as C, H, S, F and Cl —because of its unique ability to both sequester volatiles via metasomatism and release them to the atmosphere during volcanism (Gibson and McKenzie, 2023).The widespread generation of deep-sourced, volatile-rich melts is borne out by global maps of magmas rich in CO2, H2O, S and F (e.g. kimberlites, lamproites and carbonatites). Moreover, mantle xenoliths preserve evidence of repeated episodes of pervasive, reactive percolation and stalling of these volatile-rich melts. High-precision analyses of volatile elements in the abundant nominally-volatile-free mantle minerals and accessory phases, together with analyses of volatiles of intraplate magmas, allow quantification of the storage of volatile elements in the lithospheric mantle.Recent advances in global tomography, particularly multi-mode surface wave analysis, have significantly refined estimates of lithospheric thickness. These improvements enable more reliable calculations of lithospheric mantle volume across different geodynamic environments, including cratonic regions, continental off-craton areas and oceanic domains. The results indicate that the most significant global volatile reservoir resides within the mantle beneath ancient cratons. This is primarily due to their large volume and the elevated volatile concentrations preserved within their stable ‘roots’. Our new thermal models show that the outer ~ 50 km of craton margins is especially susceptible to devolatilisation during rifting and heating events (Gibson et al., 2024b). The thermal stability of craton interiors, however, ensures these regions have acted as long-term volatile sinks for at least the past 2.5 billion years. The volatile budget of off-craton lithospheric mantle is more dynamic. Volatiles stored in these regions may have significantly shorter residence times and can be rapidly remobilized through rifting and heating events. As a result, off-craton lithospheric mantle can transition from a volatile ‘sink’ to a ‘source’ over relatively short geological timescales, potentially within a few million years.The ultimate source of volatiles stored in the continental mantle is challenging to decipher but 3He/4He exhibits a systematic behaviour with melt depletion in mantle peridotites and deviations from this global trend may be correlated with subduction events (Gibson et al., 2024a). The dynamic nature of volatile storage and release within Earth's lower lithospheric ‘lid’ underscores the need for continued refinement of mantle volatile estimates to improve our understanding of deep volatile cycling. Gibson, S. A., Crosby, J. C., Day, J. A. F., Stuart, F. M., DiNicola, L. & Riley, T. R. (2024a). Systematic behaviour of 3He/4He in Earth’s continental mantle. Geochimica et Cosmochimica Acta 384, 44–64.Gibson, S. A. & McKenzie, D. (2023). On the role of the lithospheric mantle in global volatile cycles. Earth and Planetary Science Letters 602, 117946.Gibson, S., McKenzie, D. & Lebedev, S. (2024b). The distribution and generation of carbonatites. Geology 52, 667–671.

The deep cratonic ‘roots’ of Earth's major continents extend over 160 km and have remained stable for more than 2.5 billion years. This longevity is primarily due to the presence of low-density, refractory peridotites (harzburgites) that formed by extensive mantle melting in the Archean. However, mantle harzburgites from some global cratons (e.g., Kaapvaal, Siberia, Slave, Rae and Tanzania) often show unusual enrichments in orthopyroxene and silica, alongside titanium depletion, which cannot be explained by simple melting processes. The proposed origins of the orthopyroxene-rich harzburgites are highly controversial and include high-pressure melting residues, komatiite interactions, or subduction-related silicic melts and fluids. The uncertainty primarily arises because of the inherent difficulties involved in reconstructing the tectonic settings responsible for the stabilisation of early Earth. To investigate further, we analysed volatile (H₂O, F, Cl) contents in Kaapvaal craton peridotites. Our new results show that harzburgites rich in orthopyroxene, including a diamond-bearing sample, also have higher volatile concentrations. This suggests infiltration by super-critical C-O-H fluids -- rich in silica, fluorine and chlorine fluxed from subducted oceanic lithosphere (carbonated pelites and serpentinites) -- was important in driving olivine-to-orthopyroxene transformation and diamond formation under reduced conditions during the Archean. These findings highlight the role of C-O-H-F-Cl bearing fluids in shaping cratonic lithosphere and offer a new framework for understanding craton evolution, mantle metasomatism and diamond genesis in early Earth.

Most published shear-wave (VS) velocity models of cratons include a VS increase with depth below the Moho, with a maximum at 100-150 km depth. This feature is seen in regional and global 3D tomography models and in regional 1D VS profiles. Taken at face value, it implies an oscillatory geotherm, with a ubiquitous temperature decrease below the Moho, which is implausible. The VS increase with depth has thus been attributed to strong compositional layering in the lithosphere. One recent model postulated widespread hydration and metasomatism in the uppermost cratonic mantle, decreasing VS just below the Moho. An alternative model suggested a strong enrichment of the lower cratonic lithosphere in eclogite and diamond, increasing VS but implying an unusual lithospheric composition. Here, we assemble a representative dataset of phase-velocity curves of Rayleigh and Love surface waves for cratons globally, including the all-craton averages, averages over regions in southern Africa, and interstation measurements elsewhere. We perform both thermodynamic and purely seismic inversions and show that the sub-Moho VS increase is not required by the data. Models with equilibrium, conductive lithospheric geotherms and ordinary, depleted-peridotite compositions fit the surface-wave data fully. A model-space mapping quantifies the strong trade-off between seismic velocities just below the Moho and at 100-150 km depth, which is the cause of the ambiguity. The reason why most seismic models contain a VS increase with depth below the Moho is regularization that penalizes deviations from global average reference models, which are much slower than cratonic VS profiles.

Conventional methods of inorganic impurity analysis do not provide high enough depth resolution for many scientific questions in ice core science. In this study, we present a setup of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for high-resolution glacier ice impurity analysis to the sub-millimetre scale. This setup enables ice core chemical impurity analysis to a depth resolution of ∼182 µm while consuming only very small amounts of ice. The system performs simultaneous analysis of sodium, magnesium and aluminium incorporated in the ice matrix. In this case study within the framework of the WACSWAIN (WArm Climate Stability of the West Antarctic Ice Sheet in the last INterglacial) project, our method is applied to a selection of samples from the Skytrain ice core (West Antarctica) over a total length of 6.7 m consisting of about 130 single samples. The main goal of this study is to use the new LA-ICP-MS method to extract meaningful climate signals on a depth resolution level beyond the limits of continuous-flow analysis (CFA). A comparison between low-resolution CFA data and the high-resolution LA-ICP-MS data reveals generally good agreement on the decimetre scale. Stacking of parallel laser measurements together with frequency analysis is used to analyse the high-resolution LA-ICP-MS data at millimetre resolution. Spectral analysis reveals that despite effects of impurity accumulation along ice crystal grain boundaries, periodic concentration changes in the Skytrain ice core on the millimetre scale can be identified in ice from 26.8 ka (kiloyears before present, i.e. 1950 CE). These findings open new possibilities for climate data interpretation with respect to fast changes in the last glacial period and beyond, for example within the Beyond EPICA oldest-ice project.