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In situ oxygen analysis of garnet in eclogite and related rocks is increasingly being used to probe the composition of subduction fluids. However, in many cases, these samples contain textural signs of both fluid flow and retrograde metamorphism, some of which may take place outside the garnet stability field. In order to test the connection between polymetamorphism and fluid infiltration, rutile rimmed by titanite from high‐grade tectonic blocks of the Franciscan Formation (California, USA) was analysed for oxygen isotope ratios and trace element concentrations. Zirconium concentrations in rutile yield temperatures of ~600°C for eclogite and hornblende eclogite from three well‐studied localities (Junction School, Tiburon and Ward Creek). Rutile trace element concentrations are generally low and consistent with a mafic protolith. Titanite surrounding rutile has inherited much of its trace element content from rutile, and Zr‐in‐titanite temperatures are spuriously high. Titanite in rutile‐free samples (blueschist and eclogite from Jenner beach) have similar compositions suggesting that they were formed at the expense of rutile as well. Oxygen isotope ratios from rutile and titanite in the same sample are fortuitously similar, indicating disequilibrium between these minerals, which formed at different times and temperatures but in equilibrium with the same oxygen reservoir. Rutile in blocks with garnets zoned in oxygen isotopes are generally in equilibrium with the rims rather than the cores. Slow oxygen diffusion in rutile and the low temperatures of formation require that rutile recrystallized after fluid interaction and before blueschist facies metamorphism. External fluid interaction of Franciscan eclogites took place near the peak of metamorphism.

Magmas with matrix glass compositions ranging from basalt to dacite erupted from a series of 24 fissures in the first 2 weeks of the 2018 Lower East Rift Zone (LERZ) eruption of Kīlauea Volcano. Eruption styles ranged from low spattering and fountaining to strombolian activity. Major element trajectories in matrix glasses and melt inclusions hosted by olivine, pyroxene and plagioclase are consistent with variable amounts of fractional crystallization, with incompatible elements (e.g., Cl, F, and H₂O) becoming enriched by 4–5 times as melt MgO contents evolve from 6 to 0.5 wt%. The high viscosity and high H₂O contents (∼2 wt%) of the dacitic melts erupting at Fissure 17 account for the explosive Strombolian behavior exhibited by this fissure, in contrast to the low fountaining and spattering observed at fissures erupting basaltic to basaltic‐andesite melts. Saturation pressures calculated from melt inclusion CO₂‐H₂O contents indicate that the magma reservoir(s) supplying these fissures was located at ∼2–3 km depth, which is in agreement with the depth of a dacitic magma body intercepted during drilling in 2005 (∼2.5 km) and a seismically imaged lowVp/Vsanomaly (∼2 km depth). Nb/Y ratios in erupted products are similar to lavas erupted between 1955 and 1960, indicating that melts were stored and underwent variable amounts of crystallization in the LERZ for >60 years before being remobilized by a dike intrusion in 2018. We demonstrate that extensive fractional crystallization generates viscous and volatile‐rich magma with potential for hazardous explosive eruptions, which may be lurking undetected at many ocean island volcanoes.