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Publisher's version (útgefin grein), The Holuhraun–Bárdarbunga (Iceland) eruption lasted approximately 6 months. Magma propagated laterally through a 40 km long dyke, while Bárdarbunga caldera was collapsing. This event was intensely monitored, providing an opportunity to investigate the relationships between eruption dynamics and erupted products. Whole rock and melt inclusion data do not show chemical variations of magma during the eruption. Nevertheless, zoning patterns in clinopyroxene suggest temporal variations of intensive parameters during crystallization. We investigated the chemical zoning of clinopyroxene using a data driven approach, on major and trace elements analyses from lava flow lobes emplaced during the eruption. We applied hierarchical clustering (HC) to identify compositional groups based on major and trace element chemistry. This analysis identifies five compositional groups, which can be associated with specific petrographic features. One cluster represents the chemistry of hourglass sectors, two constitute the oscillatory zoned mantle of the crystals, one cluster corresponds to a seldom present bright rim (in back scattered electron images) in the outer portions of the crystals, and a last one represents most of the outer rims. HC applied to trace elements also identifies five compositional clusters, which highlight progressively more evolved clinopyroxene compositions from the core to the rim of the crystals. Two of the clusters identified with trace elements corresponds to major element clusters. All together the data suggest that the chemical zoning in the inner portions of the clinopyroxene crystals was generated by crystallization in the magma reservoir and interaction between hot magma propagating through the dyke and unerupted magma cooling within the dyke. The fraction of zones produced by interaction with colder portion of the magma residing within the dyke dropped during the eruption, potentially signaling the thermal maturation of the dyke. Some of the analyses reveal that relatively close to the eruption time (i.e., the outer portions of the crystals) the dyke intercepted a lens of low temperature magma with a chemical composition that is distinguishable from the 2014 to 2015 Bárdarbunga eruption. Our approach can provide insights on the evolution of deep processes occurring during long-lasting eruptions by combining the analysis mineral chemistry of erupted products with multiparametric monitoring signals., LC, LP, and GS appreciated the support for fieldwork of the Faculty of Science of the University of Geneva and the access provided by the Vatnajökull National Park. LC and TS received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation Program (grant agreement no. 677493 – FEVER). LC and TS acknowledge the support of the Swiss National Science Foundation (grant no. 200021_184632). MP acknowledge the Università degli Studi di Perugia FRB2019 grant titled “ENGAGE – machinE learNinG Applications for Geological problEms.”

Publisher's version (útgefin grein), Large volume effusive eruptions with relatively minor observed precursory signals are at odds with widely used models to interpret volcano deformation. Here we propose a new modelling framework that resolves this discrepancy by accounting for magma buoyancy, viscoelastic crustal properties, and sustained magma channels. At low magma accumulation rates, the stability of deep magma bodies is governed by the magma-host rock density contrast and the magma body thickness. During eruptions, inelastic processes including magma mush erosion and thermal effects, can form a sustained channel that supports magma flow, driven by the pressure difference between the magma body and surface vents. At failure onset, it may be difficult to forecast the final eruption volume; pressure in a magma body may drop well below the lithostatic load, create under-pressure and initiate a caldera collapse, despite only modest precursors., The research presented here has benefitted from extended visits of FS during a sabbatical term to, and discussion with scientists at, the University of Leeds, ISTerre University of Savoie Mont-Blanc, USGS Cascades Volcano Observatory, and Geological Survey of Japan. We acknowledge reviews by Philip Benson and Luca Caricchi that helped to significantly improve the paper, as well as reviews of an early version of the paper by two anonymous reviewers. Financial support from the H2020 project EUROVOLC funded by the European Commission is acknowledged (grant number 731070). F.S. acknowledges support from the University of Iceland Research Fund, and R.G. acknowledges partial support through NSF grant EAR-1464546. Fissure swarms, central volcanoes and caldera outlines shown in Fig. 1 are reproduced from publications referred to (refs. 42,76) with permissions from Elsevier, and we acknowledge the use of ArticDEM (ref. 77) to plot surface and ice topography shown in Fig. 1. COMET is the NERC Centre for the Observation and Modelling of Earthquakes, Volcanoes and Tectonics, a partnership between UK Universities and the British Geological Survey.

Publisher's version (útgefin grein), The South Iceland Seismic Zone (SISZ) accommodates E‐W translation due to oblique spreading between the North American and the Eurasian plates in South Iceland with ruptures on N‐S faults. Strain is released in earthquake sequences that last days to years, at average intervals of 80–100 years. Two M6.5 earthquakes struck in the SISZ in June 2000, and two M6 earthquakes in May 2008. These events released only half of the strain accumulated since the last earthquake sequence in 1896–1912. GPS station velocities are estimated from annual campaigns and continuous measurements in the SISZ during 2001–2015. The GPS station velocities are used to calculate strain rates from a new method using the vertical derivatives of horizontal stress. Our new strain rates are obtained by integrating vertical derivatives of horizontal stress rates, rather than differentiating interpolated GPS velocities, allowing finer resolution of strain. Estimating the strain rates for eight time intervals, we find high strain rates in the SISZ (0.5–1 microstrain per year) with significant temporal and spatial variation. The strain rates in the SISZ reveal contraction in the Hengill area and dilatation over a large area in eastern SISZ, indicating inflation of the magma system beneath Hekla volcano. A prominent strain anomaly is evident in the epicentral area prior to the May 2008 earthquakes. We suggest that this signal is primarily caused by plate motion and an increase in the rate of contraction in the Hengill area, located ∼10 km west of the epicentral area., This study was funded by grants from the Icelandic Research Fund (grants 0315900-03/04/05, 060243-011/012/013, 090237-021/022/023, and 130371-051/052/053), the Icelandic Equipment Fund (grant 061059), and the University of Iceland Research fund (grant HI-6489).

Primitive basalt melt inclusions from Borgarhraun, northern Iceland, display large correlated variations in CO2 and nonvolatile incompatible trace elements (ITEs) such as Nb, Th, Rb, and Ba. The average CO2/ITE ratios of the Borgarhraun melt inclusion population are precisely determined (e.g., CO2/Nb = 391 ± 16; 2σM [two standard errors of the mean], n = 161). These data, along with published data on five other populations of undegassed mid-oceanic ridge basalt (MORB) glasses and melt inclusions, demonstrate that upper mantle CO2/Ba and CO2/Rb are nearly homogeneous, while CO2/Nb and CO2/Th are broadly correlated with long-term indices of mantle heterogeneity reflected in Nd isotopes (143Nd/144Nd) in five of the six regions of the upper mantle examined thus far. Our results suggest that heterogeneous carbon contents of the upper mantle are long-lived features, and that average carbon abundances of the mantle sources of Atlantic MORB are higher by a factor of two than those of Pacific MORB. This observation is correlated with a similar distinction in water contents and trace elements characteristic of subduction fluids (Ba, Rb). We suggest that the upper mantle beneath the younger Atlantic Ocean basin contains components of hydrated and carbonated subduction-modified mantle from prior episodes of Iapetus subduction that were entrained and mixed into the upper mantle during opening of the Atlantic Ocean basin., Maclennan is supported by Natural Environment Research Council grant NE/M000427/1. This research was supported by the Carnegie Institution of Washington and is a contribution to the Deep Carbon Observatory.

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.

Growing evidence for significant magmatic vesiculation prior to magma-water interaction (MWI) has brought into question the use of ‘diagnostic’ features, such as low vesicularities and blocky morphologies, to identify hydromagmatic pyroclasts. We address this question by quantifying co-variations in particle size, shape and texture in both magmatic and hydromagmatic deposits from the Hverfjall Fires fissure eruption, Iceland. Overlapping vesicularity and bubble number density distributions measured in rapidly quenched magmatic and hydromagmatic pyroclasts indicate a shared initial history of bubble nucleation and growth, with substantial vesiculation prior to MWI. Hydromagmatic fragmentation occurred principally by brittle mechanisms, where the length scale and geometry of fracturing was controlled by the bubble population. This suggests that the elevated fragmentation efficiency of hydromagmatic deposits is driven, at least in part, by brittle disintegration of vesicular pyroclasts due to high thermal stress generated during rapid cooling. In this way, the shape and size distributions of hydromagmatic pyroclasts, both critical input parameters for ash dispersion models, are strongly influenced by the dynamics of vesiculation prior to MWI. This result underlines the need to analyse multiple grain-size fractions to characterise the balance between magmatic and hydromagmatic processes. During the Hverfjall Fires eruption, the external water supply was sufficient to maintain MWI throughout the eruption, with no evidence for progressive exhaustion of a water reservoir. We suggest that both the longevity and the spatial distribution of MWI were determined by the pre-existing regional hydrology and represent continuous interaction between a propagating dike and a strong groundwater flow system hosted within permeable basalt lavas., This work was completed with support for KVC from the AXA Research Fund and a Royal Society Wolfson Merit Award, a Royal Society URF to ACR and a University of Bristol postgraduate Scholarship to EJL. Fieldwork was supported by a New Researchers Award from the Geologists’ Association to EJL. We thank S. Kearns and B. Buse for their assistance with SEM analysis.

This thesis provides insights into the petrogenesis of Iceland basalts via three subprojects. The first uses olivine macrocrysts as a proxy for mantle melting conditions below Iceland, the second utilizes petrological thermobarometry to resolve the crustal storage conditions of the most primitive basaltic rocks (ankaramites) of the Eyjafjallajökull volcano, and the third investigates basalt fractionation processes within the Hafnarhraun pāhoehoe lava lobe. The sub-Icelandic mantle is evidently heterogeneous in composition. Yet olivine major and minor element compositions in Iceland basalts typically concur with common mantle lherzolite as the source of magmas, with the only potential exceptions being the basalts of Eyjafjallajökull and Vestmannaeyjar volcanic systems in South Iceland. These South Iceland basalts have forsteriterich olivine with relatively high Ni and low Mn contents, together with low Sc and V and high Cr, Ti, Zn, Cu and Li contents. Elevated Ni and low Mn in olivine have been attributed to olivine-free pyroxenitic mantle source; however, the South Iceland olivine compositions are best explained by the effect of comparatively high-pressure (Pfinal>1.4 GPa) and high-temperature melting of somewhat enriched olivine-bearing mantle. I conclude this because (i) elevated Ni and low Mn in olivine can also indicate deep, high-temperature, mantle melting, (ii) the abundances of Sc, V, Ti and Zn in the South Iceland olivine are compatible with low-degree partial melts of olivine-rich mantle, and (iii) melts of olivine-free pyroxenite are, according to recent models, easily consumed in reactions with subsolidus mantle peridotite and thus unlikely to migrate to the crust and crystallize olivine. The identified high-Ni/low-Mn olivine macrocrysts suggest final mantle equilibration depths greater than 45 km for South Iceland magmas, and imply effective mantle-to-surface magma transport. Two Eyjafjallajökull ankaramite outcrops (Hvammsmúli and Brattaskjól), rich in olivine (Fo81–90) and clinopyroxene (Mg#cpx 78–90) macrocrysts (~30 vol%) in near equal amounts, have a specifically prominent “deep mantle source signature” (high-Ni/low-Mn) in olivine. To investigate the crustal storage conditions of these and other Eyjafjallajökull basaltic magmas, I analyzed olivine, clinopyroxene, spinel and melt inclusion compositions from these volcanic units. These analyzes revealed that the olivine-hosted spinel inclusions have exceptionally high Cr#spl (52–80) and TiO2 (1–3 wt%) and low Al2O3 (8–22 wt%) compared to typical chromian spinels in Iceland, in line with the postulated deep and enriched mantle source of the parental magmas. According to olivine-spinel oxybarometry, these spinels crystallized under a moderate oxygen fugacity (ΔlogFMQ 0–0.5). Furthermore, jadeite-in-clinopyroxene barometry indicates clinopyroxene crystallization at a rather low pressure (1.7–4.2 kbar; external precision ±1.4 kbar), implying a magma storage depth of 10.7±5 km. Additionally, clinopyroxe-liquid, olivine-liquid and liquid only thermometry gives varying crystallization temperatures of 1120–1195 °C, 1136–1213 °C and 1155–1222 °C, respectively, for the compositionally diverse macrocrysts. The scarcity of macrocryst plagioclase and trends in clinopyroxene compositions indicate that the mid-crustal crystallizing assemblage was olivine and clinopyroxene, and plagioclase fractionated later. Diffusive re-equilibration in Brattaskjól olivine grains suggests that this crystal assemblage mobilized and erupted from its storage within a few weeks. To conclude, the Brattaskjól and Hvammsmúli crystal cargoes are agitated wehrlitic or plagioclase-wehrlitic mushes from the mid-crust that ascended to the surface relatively rapidly. Basaltic lavas are practically never primitive mantle melts owing to fractional crystallization in the crust, which, at low pressure, may be aided by volatile exsolution. Deciphering magma fractionation processes from solidified crustal intrusions is hampered by their often complex emplacement history. The emplacement of pāhoehoe lavas, however, is simpler and well understood, and hence I investigated the mechanisms of basalt fractionation from a differentiated pāhoehoe lava lobe in Hafnarhraun lava flow field. Here, volatile exsolution had facilitated separation of basaltic residual melts to form three types of melt segregations: vesicle cylinders (VC) in the core of the lobe and two types of horizontal vesicle sheets (HVS1 and HVS2) in the upper part of the lobe. Interestingly, the VC do not match chemically with the modelled residual melts of the lobe, and their formation seems to have included two stages: volatile-aided melt separation from crystallizing base of the lobe and later contamination by primitive macro- and microphenocrysts in the lava core. HVS1, which resemble VC, were formed as the ascending VC diapirs accumulated to the upper solidification front of the lava lobe. HVS2, in turn, are distinctly evolved in compositions compared to other units in the lobe and were formed as highly fractionated residual melts seeped to voids in the upper crust of the lobe. Processes analogous to segregation formation at Hafnarhraun may contribute to genesis of evolved basalts and silicic rocks in shallow magmatic systems. Overall, my work highlights the exceptional nature of South Iceland among other volcanically active regions in Iceland. Furthermore, analyses of the Hafnarhraun pāhoehoe lava reveal the processes of melt segregation formation in pāhoehoe lava lobes. I hope future research will expand on these findings, further resolving the nature of mantle melting below South Iceland and the significance of volatile-aided processes in crustal magma differentiation., Í þessari ritgerð er ljósi varpað á uppruna basaltkviku á Íslandi með þremur rannsóknarþemum. Í fyrsta þemanu er snefilefnainnihald ólívíndíla notað til að skýra hvaða skilyrði ríkja við hlutbræðslu möttulsins undir Íslandi. Í öðru þemanu er stuðst við jarðefnafræðilega hita- og þrýstimæla til þess að meta eiginleika kvikuhólfa/-þróa sem innihalda frumstæða basaltkviku (þ.e. ankaramít) í skorpunni undir Eyjafjallajökli. Þriðja þemað varðar þróunarferli basaltbráðar í helluhraunssepa í Hafnarhrauni við Þorlákshöfn. Möttulinn undir Íslandi er að samsetningu misleitur. Samt samræmist aðal- og snefilefnasamsetning ólívíndíla í íslensku basalti kviku sem á uppruna sinn að rekja til hlutbræðslu á venjulegum lherzólítmöttli. Hugsanleg undantekning frá þessari reglu er basaltkvikan sem kemur upp í eldstöðvakerfunum Eyjafjallajökli og Vestmannaeyjum, sem innihalda mjög forsterítríka ólivíndíla með tiltölulega háan Ni-styrk og lágan Mn-styrk, ásamt lágum styrk Sc og V og háum styrk Cr, Ti, Zn, Cu og Li. Þrátt fyrir að hár Ni-styrkur og lágur Mn-styrkur í frumstæðum ólivíndílum sé gjarnan rakinn til kviku sem myndast við hlutbráðnun á ólivínlausum pyroxenítmöttli, þá er samsetning umræddra ólivíndíla best skýrð með háhitabráðnun á auðguðum, ólivínríkum perídótítmöttliviðháanþrýsting(Pfinal> 1,4 GPa). Ég dreg þessa ályktun vegna þess að (i) hár Nistyrkur og lágur Mn-styrkur í ólivíni samræmist einnig bráðnun við háan hita djúpt í möttlinum, (ii) tiltölulega hár styrkur Sc, V, Ti og Zn í ólivíndílunum er í samræmi við litla hlutbráðnun á ólivínríkum möttli, og (iii) samkvæmt nýjustu líkönum hvarfast pýroxenítbráð auðveldlega við möttulperidótít og því ólíklegt að slík bráð komist upp í jarðskorpuna og kristalli ólivín. Þessi hái styrkur Ni og lágur styrkur Mn í ólivíndílunum bendir til þess að kvikan hafi síðast verið í jafnvægi við möttulefnið á meira en 45 km dýpi ogaðkvikanhafiflusthrattfrámöttlitilyfirborðs. Tvær ankaramítmyndanir í Eyjafjöllum, Hvammsmúli og Brattaskjól, sem auðugar eru af ólivíndílum (Fo81–90) og klínópýroxendílum (Mg#cpx 78–90) (~ 30%), nokkurn veginn í sama magni,sýnamerkiumupprunadjúptímöttlinum, þ.e. hátt Ni-magn / lágt Mn-magn í ólivíndílunum. Efnasamsetningólivíns,klínópýroxens,spínilsog bráðarinnlyksna var greind í sýnum frá þessum myndunum til þess að meta dýpi kvikuþróa þar sem þessar kvikur safnast fyrir í skorpunni fyrir gos. Þessar greiningarsýna að spínilkristallarnir, sem eru til staðar sem innlyksur í ólivíndílum, hafa óvenju hátt Cr#spl (52–80) og mikið af TiO2 (1–3 þ.%) og lítið af Al2 O3 (8–22 þ.%) í samanburði við dæmigerða krómspínla í íslensku basalti. Þetta er í takt við ályktunina um djúpstæðan uppruna móðurkvikunnar frá auðguðum möttli. Samkvæmt ólivín-spínil súrefnisþrýstingsmælinum kristölluðust þessir spínlar við hóflegan súrefnisstyrk (ΔlogFMQ 0–0,5).Jafnframtbendirþrýstimælirsembyggirá magnijaðeítþáttaríklínópýroxenitilaðkristöllun klínópýroxens hafi átt sér stað við frekar lágan þrýsting (1,7–4,2±1,4 kbar),semgefurtil kynna 10,7 ± 5 km dýpi fyrir kvikuþróna.Að auki gefa hitamælarfyrir klínópýroxen-gler, ólivín-gler og gler eingöngu mismunandi kristöllunarhitastig fyrir hinar mismundi gerðir fasa, nefnilega 1120– 1195 °C, 1136–1213 °C og 1155–1222 °C. Lítið magn plagíóklasdíla ásamt samsetningu klínópýroxendílanna bendir til þess að þessar kvikur hafi kristallað ólivín og klínópýroxen í kvikuþrónni og myndun plagíóklasdíla hafist síðar. Líkanreikningar á efnasveimi í ólivíni frá Brattaskjóli bendir til að þessi dílafarmur hafi farið að stað úr geymslurýminu og borist til yfirborðs í eldgosi innan nokkurra vikna. Niðurstaðan er að dílafarmurinn í Brattaskjóli og Hvammsmúla sé að uppruna kristalríkur massi í miðskorpunni með steindafylki wehrlíts eða plagíóklas-wehrlíts sem reis tiltölulega hratt til yfirborðs. Basalthraun eru nær aldrei með sömu samsetningu og frumstæðar möttulbráðir vegna hlutkristöllunar sem á sér stað í jarðskorpunni, en hlutkristöllunin getur við lágan þrýsting orðið fyrir áhrifum af aðskilnaði gastegunda. Erfitt getur verið að greina áhrif þróunarferla í kviku með því að skoða storknuð innskot vegna flókinnar sögu þeirra oft á tíðum. Hins vegar er myndun helluhrauna vel skilin og hef ég því rannsakað kvikuþróunarferli basalts með því að skoða þróaðan helluhraunssepa í Hafnarhrauni. Í þessu tilfelli átti aðskilnaður gass þátt í aðskilnaði afgangsbráðar með samsetningu basalts. Bráðaraðskilnaðurinn var af þrennu tagi: blöðrusívalningar (VC) í kjarna hraunsepans og tvær gerðir láréttra blöðrulaga (HVS1 og HVS2) í efri hluta sepans. Áhugavert er að efnasamsetning VC fellur ekki að líkanreikningum fyrir sepann og myndun þeirra virðist hafa orðið í tveimur þrepum: bráðaraðskilnaður með hjálp gass í botni sepans þar sem kristöllun átti sér stað og síðar mengun af frumstæðum stór- og smádílum í kjarna hraunsins. HVS1 líkjast VC og mynduðust þegar VC-sívalningar risu og söfnuðust fyrir á storknunarmörkum hraunsepans. Hins vegar eru HVS2 greinilega þróaðir í samanburði við hinar gerðirnar í sepanum og mynduðust við að mjög þróaðar afgangsbráðirseytluðu í holrými í efri skorpu sepans. Ferli lík þeim sem mynduðu afgangsbráðirnar í Hafnarhrauni gætu komið við sögu við myndun þróaðs basalt og súrs bergs í grunnstæðum kvikukerfum. Heiltyfirlitiðþásýnarannsóknirmínarhversu einstakt Suðurgosbeltið er meðal virkra gosbelta á Íslandi. Þá hafa rannsóknirnar á Hafnarhrauni leitt í ljós ferli bráðaraðskilnaðar í helluhraunssepum. Ég vona að framtíðarrannsóknir muni byggja á þessum uppgötvunum og leiða til betri skilnings á eðli möttulbráðnunar undir Suðurlandi og því hversu mikilvægt gas er fyrir kvikuþróunarferli í skorpunni.