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The 3 km long Bellevue Core was recovered from the Upper Zone (UZ) and Main Zone (MZ) of the Northern Limb of the Bushveld Complexçthe largest known layered mafic intrusion. We present abundance data for 57 trace elements hosted in plagioclase, clinopyroxene, low-Ca pyroxene and olivine, obtained by laser ablation inductively coupled plasma mass spectrometry. These data permit the first comprehensive investigation of trace element distribution in the Bushveld Complex on a mineral-by-mineral basis, and show that the Cr content of pyroxene is the only proxy that records three cryptic magma injections, coincident with two pyroxenite horizons and the UZ-MZ boundary. On the basis of Cr abundance, we infer that an 'anomalous' troctolitic horizon at the base of the Bellevue Core was related to injection of relatively primitive magma, analogous to the Lower or Critical Zone magmas in the Eastern and Western Limbs of the Bushveld Complex; this horizon defines the greatest inconsistency of magmatic stratigraphy between the Northern Limb and the Eastern and Western Limbs of the complex, and is prospective for platinumgroup element exploration. However, the most significant finding of this study is that plagioclase, pyroxene and olivine have undergone extensive diffusive modification. The major and trace element geochemistry of these minerals records sub-solidus equilibration during the prolonged cooling of the Bushveld Complex, controlled by diffusion rates and local partitioning between adjacent minerals. By analogy, texturally mature cumulates in any layered intrusion are also likely to be chemically re-equilibrated.

We present new major and trace element data for rare examples of preserved pillow basalts from a locality of little studied >3800 Ma rocks at the southwestern edge of the Isua Supracrustal Belt (ISB), West Greenland. The 20 samples have 47.3-59.0 wt.% SiO(2), 4.9-12.2 wt.% MgO, 6.5-11.2 wt.% CaO, 60-340 ppm Ni and 631094 ppm Cr, consistent with pre-metamorphic basaltic compositions. Nb/La((n)) and Ti/Gd((n)) are lower than primitive mantle compositions (0.1-0.6 and 0.3-0.8, respectively), with these ratios interpreted to reflect the composition of their source. Correlations of Nb/La with La/Sm, Ti/Gd and Zr/Sm and between Zr/Nb and Nb/Th are comparable to those of modern subduction related magmas, whereby slab melts enriched the overlying mantle wedge and HFSE were retained by Ti-rich phases. Trends in Ba/La vs. Ba/Nb are comparable to modern day arc related basalts, which reflect a sediment influence and may indicate that, at least early in the evolution of Eoarchean subduction zones, the subducted Eoarchean oceanic crust retained a pelagic sediment cover, with this cover influencing the chemistry of the slab-derived fluid. Three of the ISB pillow lavas show extreme enrichments in LREE (La/Sm((n)) = 1.9-2.6), large depletions in Nb, Ta, Zr and Hf relative to REE that are similar to rare high-K arc basalts from the Clark volcano of the presently active Tongan-Kermadec-New Zealand arc (Gamble, J.A., Christie, R.H.K., Wright, I.C., Wysoczanski, R.J., 1997. Primitive K-rich magmas from Clark volcano, southern Kermadec arc: a paradox in the K-depth relationship. The Canadian Mineralogist, 35, 275-290). The compositions of these highly enriched basalts indicate that the mantle source region of the Isua basalts was occasionally overprinted by small-volume sediment dominated melts rather than hydrous fluids. The compositional affinities of these 3.8 Ga pillow basalts with modern island-arc basalts provide strong evidence for the role of slab fluids and melts in basalt gene

The 2018 lower East Rift Zone (LERZ) eruption and the accompanying collapse of the summit caldera marked the most destructive episode of activity at Kı̄lauea Volcano in the last 200 years. The eruption was extremely well‐monitored, with extensive real‐time lava sampling as well as continuous geodetic data capturing the caldera collapse. This multi‐parameter dataset provides an exceptional opportunity to determine the reservoir geometry and magma transport paths supplying Kı̄lauea’s LERZ. The forsterite contents of olivine crystals, together with the degree of major element disequilibrium with carrier melts, indicates that two distinct crystal populations were erupted from Fissure 8 (termed High‐ and Low‐Fo). Melt inclusion entrapment pressures reveal that Low‐Fo olivines (close to equilibrium with their carrier melts) crystallized within the Halema’uma’u reservoir ( ∼2 km depth), while many High‐Fo olivines ( > Fo81.5; far from equilibrium with their carrier melts) crystallized within the South Caldera reservoir ( ∼3–5 km depth). Melt inclusions in High‐Fo olivines experienced extensive post‐entrapment crystallization following their incorporation into cooler, more evolved melts. This favoured the growth of a CO2 ‐rich vapor bubble, containing up to 99% of the total melt inclusion CO2 budget (median=93%). If this CO2 ‐rich bubble is not accounted for, entrapment depths are significantly underestimated. Conversely, reconstructions using equation of state methods rather than direct measurements of vapor bubbles overestimate entrapment depths. Overall, we show that direct measurements of melts and vapor bubbles by SIMS and Raman Spectroscopy, combined with a suitable H2 O −CO2 solubility model, is a powerful tool to identify the magma storage reservoirs supplying volcanic eruptions.

Mantle plume-related magmas typically have higher chalcophile and siderophile element (CSE) contents than mid-ocean ridge basalts (MORB). These differences are often attributed to sulfide-under-saturation of plume-related melts. However, because of eruption-related degassing of sulfur (S) and the compositional, pressure, temperature and redox effects on S- solubility, understanding the magmatic behavior of S is challenging. Using CSE data for oceanic plateau basalts (OPB), which rarely degas S, we show that many OPB are sulfide- saturated. Differences in the timing of sulfide-saturation between individual OPB suites can be explained by pressure effects on sulfur solubility associated with ascent through over- thickened crust. Importantly, where S-degassing does occur, OPB have higher CSE contents than S-undegassed melts at similar stages of differentiation. This can be explained by resorption of earlier-formed sulfides, which might play an important role in enriching degassed melts in sulfide-compatible CSE and potentially contributes to anomalous enrich- ments of CSE in the crust.

Mantle plume-related magmas typically have higher chalcophile and siderophile element (CSE) contents than mid-ocean ridge basalts (MORB). These differences are often attributed to sulfide-under-saturation of plume-related melts. However, because of eruption-related degassing of sulfur (S) and the compositional, pressure, temperature and redox effects on S- solubility, understanding the magmatic behavior of S is challenging. Using CSE data for oceanic plateau basalts (OPB), which rarely degas S, we show that many OPB are sulfide- saturated. Differences in the timing of sulfide-saturation between individual OPB suites can be explained by pressure effects on sulfur solubility associated with ascent through over- thickened crust. Importantly, where S-degassing does occur, OPB have higher CSE contents than S-undegassed melts at similar stages of differentiation. This can be explained by resorption of earlier-formed sulfides, which might play an important role in enriching degassed melts in sulfide-compatible CSE and potentially contributes to anomalous enrich- ments of CSE in the crust.

The 2018 lower East Rift Zone (LERZ) eruption and the accompanying collapse of the summit caldera marked the most destructive episode of activity at Kı̄lauea Volcano in the last 200 years. The eruption was extremely well‐monitored, with extensive real‐time lava sampling as well as continuous geodetic data capturing the caldera collapse. This multi‐parameter dataset provides an exceptional opportunity to determine the reservoir geometry and magma transport paths supplying Kı̄lauea’s LERZ. The forsterite contents of olivine crystals, together with the degree of major element disequilibrium with carrier melts, indicates that two distinct crystal populations were erupted from Fissure 8 (termed High‐ and Low‐Fo). Melt inclusion entrapment pressures reveal that Low‐Fo olivines (close to equilibrium with their carrier melts) crystallized within the Halema’uma’u reservoir ( ∼2 km depth), while many High‐Fo olivines ( > Fo81.5; far from equilibrium with their carrier melts) crystallized within the South Caldera reservoir ( ∼3–5 km depth). Melt inclusions in High‐Fo olivines experienced extensive post‐entrapment crystallization following their incorporation into cooler, more evolved melts. This favoured the growth of a CO2 ‐rich vapor bubble, containing up to 99% of the total melt inclusion CO2 budget (median=93%). If this CO2 ‐rich bubble is not accounted for, entrapment depths are significantly underestimated. Conversely, reconstructions using equation of state methods rather than direct measurements of vapor bubbles overestimate entrapment depths. Overall, we show that direct measurements of melts and vapor bubbles by SIMS and Raman Spectroscopy, combined with a suitable H2 O −CO2 solubility model, is a powerful tool to identify the magma storage reservoirs supplying volcanic eruptions.