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Aerosols have a potentially large effect on climate, particularly through their interactions with clouds, but the magnitude of this effect is highly uncertain. Large volcanic eruptions produce sulfur dioxide, which in turn produces aerosols; these eruptions thus represent a natural experiment through which to quantify aerosol–cloud interactions. Here we show that the massive 2014–2015 fissure eruption in Holuhraun, Iceland, reduced the size of liquid cloud droplets—consistent with expectations—but had no discernible effect on other cloud properties. The reduction in droplet size led to cloud brightening and global-mean radiative forcing of around −0.2 watts per square metre for September to October 2014. Changes in cloud amount or cloud liquid water path, however, were undetectable, indicating that these indirect effects, and cloud systems in general, are well buffered against aerosol changes. This result will reduce uncertainties in future climate projections, because we are now able to reject results from climate models with an excessive liquid-water-path response.

Melt inclusions formed during the early stages of magmatic evolution trap primitive melt compositions and enable the volatile contents of primary melts and the mantle to be estimated. However, the syn-and post-entrapment behaviour of volatiles in primitive high-anorthite plagioclase-hosted melt inclusions from oceanic basalts remains poorly constrained. To address this deficit, we present volatile and light lithophile element analyses from a well-characterised suite of nine matrix glasses and 102 melt inclusions from the 10 ka Gri'msvotn tephra series (i.e., Saksunarvatn ash) of Iceland's Eastern Volcanic Zone (EVZ). High matrix glass H2O and S contents indicate that eruption-related exsolution was arrested by quenching in a phreatomagmatic setting; Li, B, F and Cl did not exsolve during eruption. The almost uniformly low CO2 content of plagioclase-hosted melt inclusions cannot be explained by either shallow entrapment or the sequestration of CO2 into shrinkage bubbles, suggesting that inclusion CO2 contents were controlled by decrepitation instead. High H2O/Ce values in primitive plagioclase-hosted inclusions (182-823) generally exceed values expected for EVZ primary melts (similar to 180), and can be accounted for by diffusive H2O gain following the entrainment of primitive macrocrysts into evolved and H2O-rich melts a few days before eruption. A strong positive correlation between H2O and Li in plagioclase-hosted inclusions suggests that diffusive Li gain may also have occurred. Extreme F enrichments in primitive plagioclase-hosted inclusions (F/Nd= 51216 versus similar to 15 in matrix glasses) possibly reflect the entrapment of inclusions from high-Al/(Al + Si) melt pools formed by dissolution-crystallisation processes (as indicated by HFSE depletions in some inclusions), and into which F was concentrated by uphill diffusion since F is highly soluble in Al-rich melts. The high S/Dy of primitive inclusions (similar to 300) indicates that primary melts were S-rich in comparison with most oceanic basalts. Cl and B are unfractionated from similarly compatible trace elements, and preserve records of primary melt heterogeneity. Although primitive plagioclase-hosted melt inclusions from the 10 ka Gri'msvotn tephra series record few primary signals in their volatile contents, they nevertheless record information about crustal magma processing that is not captured in olivine-hosted melt inclusions suites., D.A.N. acknowledges support from the Natural Environment Research Council (NE/1528277/1) and the Alexander von Humboldt Foundation. SIMS analyses were supported by a Natural Environment Research Council Ion Microprobe Facility award (IMF508/1013). We thank Robin Clarke and Iris Buisman for their help with sample preparation and EPMA at the University of Cambridge. We thank Richard Hinton and all the staff at the Ion Microprobe Facility at University of Edinburgh for their assistance with SIMS analyses. We also thank Adam Kent and one anonymous reviewer for their comments that have significantly improved and streamlined this contribution.

Modelling of volcanic processes is strongly limited by a poor knowledge of the timescales of storage, mixing and final ascent of magmas into the shallowest portions of volcanic 'plumbing' systems immediately prior to eruption. It is impossible to measure these timescales directly; however, micro-analytical techniques provide indirect estimates based on the extent of diffusion of species through melts and crystals. Here, diffusion in olivine phenocrysts from the 1959 Kīlauea Iki eruption is used to constrain the timing of mixing events in the crustal plumbing system on timescales of months to years before eruption. The timescales derived from zonation of Fe-Mg in olivines, combined with contemporaneous geophysical data suggests mixing occurred on 3 timescales: (1) up to 2 years prior to eruption in the deep storage system, (2), in a shallow reservoir, between incoming hot melts and cooler, resident melt for several weeks to months prior to eruption, and (3), in the conduit and summit reservoir, between the resident magma and cooled surface lava, draining back into the vent on timescales of hours to several days during pauses between episodes. Synchronous inflation of the shallow reservoir with deep earthquake swarms and mixing suggests a fitfully open transcrustal magmatic system prior to and during eruption.

Magma mixing and crystal mush disaggregation are important processes in basaltic magma reservoirs. We carried out a detailed petrological and geochemical study on a highly plagioclase-phyric eruption within the Eastern Volcanic Zone of Iceland—the Skuggafjöll eruption—to investigate crystal storage and transport processes within a single magmatic system. Crystal content and phase proportions vary between samples: the least phyric samples have phase proportions similar to the low-pressure, three-phase gabbro eutectic (plg:cpx:ol \~ 11:6:3), whereas highly phyric samples are strongly enriched in plagioclase (plg:cpx:ol \~ 8:1:1). Statistically significant geochemical variability in 28 whole-rock samples collected across the eruption can be accounted for by variable accumulation of a troctolitic assemblage containing plagioclase and olivine in an approximately 9:1 ratio. Two macrocryst assemblages are defined using compositional and textural information recorded in QEMSCAN® images: a primitive assemblage of high-anorthite plagioclase (An\textgreater83) and high-forsterite olivine (Fo\textgreater84), and an evolved assemblage of low-anorthite plagioclase (An\textless79), low-forsterite olivine (Fo\textless82) and clinopyroxene (Mg# \~ 82). Plagioclase and olivine have strongly bimodal composition distributions whereas the composition distribution of clinopyroxene is unimodal. The mean trace element composition of melt inclusions hosted within high-forsterite olivine and high-anorthite plagioclase macrocrysts is the same (mean Ce/Y \~ 0·47–0·48), confirming that both primitive macrocryst phases crystallized from the same distribution of melts. Clinopyroxene macrocrysts and matrix glasses are in Ce/Yb equilibrium with each other, indicating that the evolved assemblage crystallized from melts with a more incompatible trace element-enriched composition (mean Ce/Y \~ 0·65–71) than the primitive assemblage. Variability in whole-rock, macrocryst and melt inclusion compositions suggests that the Skuggafjöll magma experienced two stages of crystallization. Primitive macrocrysts crystallized first from incompatible trace element-depleted melts within a shallow crustal magma reservoir. These primitive macrocrysts were subsequently stored in crystal mushes that ultimately disaggregated into an evolved and incompatible trace element-enriched magma from which the evolved assemblage crystallized. On average, \~17% of the erupted magma at Skuggafjöll is composed of accumulated macrocrysts entrained from crystal mushes. The timescale between mush disaggregation and eruption, during which crystal accumulation occurred, was short—of the order of years—according to simple diffusion calculations. Striking petrological similarities between Skuggafjöll and other highly phyric eruptions both in Iceland and along mid-ocean ridges indicate that crystal accumulation by mush disaggregation is likely to be an important mechanism for generating highly phyric magmas in basaltic plumbing sy

[1] The Askja volcanic system, North Iceland, experienced a volcano-tectonic episode between 1874 and 1876, the climax of which was a rhyolitic, phreatoplinian to Plinian eruption at Askja central volcano on 28–29 March 1875. Fissure eruptions also occurred in 1875, producing the Nýjahraun lava, 45–65 km north of Askja. The Nýjahraun basalt is indistinguishable, in terms of whole-rock major elements, from the small-volume basaltic eruptions that took place at Askja in the early 20th century. It has been suggested that all of these basalts originated from a shallow magma chamber beneath Askja, with the Nýjahraun eruptions being fed by northward-propagating lateral dykes. It has also been conjectured that the Holuhraun lava, located at the southern tip of the Askja volcanic system 15–25 km south of Askja, was connected with the 1874–1876 Askja volcano-tectonic episode. We re-examine these interpretations in light of new whole-rock, glass and melt inclusion analyses from samples collected along the length of the Askja volcanic system. Glasses from Nýjahraun and the Askja 20th century eruptions are geochemically distinct. We suggest that the Askja 20th century basalts mixed with evolved melts in the crust, while the Nýjahraun magma evolved without such interactions. The Holuhraun basalt is more similar to lavas erupted on the Bárðarbunga-Veiðivötn volcanic system than to postglacial basalts from Askja, indicating that particular geochemical signatures are not necessarily confined to the tectonic or structural surface expression of single volcanic systems. This has important implications for the identification and delineation of individual volcanic systems beneath the northwest sector of Vatnajökull.