Highly siderophile element (HSE: Re, Os, Ir, Ru, Pt, Pd) abundances are reported for eucrites, diogenites and howardites from Vesta. The four orders of magnitude variation (1 × 10–3 to 1 × 10–6 × CI chondrite) and measured 187Os/188Os (0.103 to 3.07) ranges in eucrites primarily reflect post-crystallization impactor additions and constrain meteorites previously identified as pristine (<1 μg/g Ni) as having limited (<0.1%) chondritic HSE additions. Pristine eucrites have fractionated inter-element HSE ratios with respect to chondrites, with a wide range of 187Os/188Os, making them distinct and more HSE-rich than lunar mare basalts. These eucrites define the crustal Vesta HSE composition as broadly similar to modern terrestrial basaltic oceanic crust. Models assuming either residual metal in the mantle, or its complete exhaustion, during eucrite melt extraction predict an HSE enriched bulk silicate Vesta compared to bulk silicate Earth or bulk silicate Mars, requiring between 0.4 to 1.9% post-core formation accretion (PCFA). A preferred alternative model using regression of the HSE to MgO contents of eucrites and diogenites derives a composition between bulk silicate Earth and bulk silicate Moon, requiring ∼0.12% PCFA. Such a composition is like the HSE contents measured for peridotites considered to come from Vesta. The estimates of PCFA from this large dataset of vestan meteorites are all higher than previously determined estimates, requiring a proportional increase in mass from leftover planetesimal and asteroidal sources to the inner Solar System planets.
Hydrothermal circulation along mid-ocean ridges is primarily controlled by the interplay between magmatic and tectonic processes. The Mohns Ridge, shaped by the Jan Mayen/Iceland plume and a shift from orthogonal to oblique accretion, provides a natural laboratory to test whether melt-flux, tectonic strain, and axial volcanic ridge (AVR) maturity predict hydrothermal venting, using a multiproxy approach combining bathymetry, magnetic, gravity, and seismicity data. Variations in mantle Bouguer anomaly and bathymetry along the axis show how plume distance and ridge obliquity jointly control melt distribution, dividing the ridge into three regions: high-melt-flux south of the 71.3°N, diffuse high-melt-flux between 71.3°N and 72.2°N, and a focused but melt-limited region northward. While AVR volume appears as an unreliable proxy for melt-flux and AVR maturity, along-axis correlation between magnetic, mantle Bouguer anomalies and sediment thickness distinguishes between well-developed active, well-developed inactive, and infrequent but recently active AVRs. Melt distribution style (focused vs. diffuse) and AVR magmatic state primarily dictate hydrothermal vent distribution: well-developed active AVR in focused regions promotes high-temperature venting, while a diffuse-melt distribution appears to hinder their development. The apparent tectonic strain in the axial valley shows no significant relationship with melt-flux or magnetic anomaly, reflecting the differing timescales between crustal accretion, alteration and surface activity. Similarly, seismic activity shows no relationship with AVR maturity, melt emplacement style, or hydrothermal venting, indicating that short-term events are decoupled from the long-term evolution of AVRs. Hydrothermal activity instead correlates more closely with the long-term magmatic evolution of AVRs than with these short-term events.
Redox conditions strongly influence mantle phase equilibria and arc magmatism, yet the processes driving elevated oxygen fugacity (fO2) in subarc mantle remain unresolved. We investigate six harzburgitic peridotite xenoliths from Shiveluch volcano, Kamchatka, to estimate fO2 shifts induced by metasomatic reactions. Subduction–derived silicate melts and hydrous fluids generated fibrous pyroxenes at the expense of olivine, produced amphibole, phlogopite, anhydrite, and reaction textures at spinel rims. Spinel displays systematic core–to–rim increase in Fe3+/FeT (from 0.17–0.39 to 0.48–0.57) and inverse correlation between Mg# and Cr#. Fe3+/FeT was reset at the spinel rim, followed by progressive oxidation through diffusion–driven A– and B–site substitutions involving Mg2+–Fe2+ and Fe3+–Al3+–Cr3+ exchange. fO2 (ΔFMQ) estimates, using the Ballhaus and Wood oxybarometers (hereafter BO and WO), range from 0.9–2.5 in spinel cores to 2.8–4.1 in rims by BO and from 1.0–2.4 to 2.8–3.2 by WO. Individual xenoliths record redox increases of up to 2.9 log units (BO) or 1.9 log units (WO). The two oxybarometers agree within ±0.5 log units at moderate fO2 but diverge at higher oxidation states (Fe3+/ΣR3+spinel>0.30). Spinel core fO2 values overlap those of previously studied local harzburgites, whereas rim values overlap those of clinopyroxenites from Shiveluch volcano, revealing significant redox heterogeneity (BO: 0.9–4.1; WO: 1.0–3.2) in the Kamchatka arc mantle spanning a large segment of the global arc mantle fO2 spectrum. The metasomatically modified rims in all samples record fO2 values of approximately FMQ +3 (WO) suggesting the strong oxidising character of the melt/fluid–deriving source beneath the Kamchatka arc, highlighting this region as among the most oxidised mantle domains yet documented.
