Dynamic magma mixing revealed by the 2010 Eyjafjallajökull eruption

Introduction Conclusions References Tables Figures


Introduction
Improved understanding of volcanic plumbing-systems is needed for better interpretations of precursors for volcanic eruptions.While deformation and seismic studies yield real-time information of physical changes beneath a volcano, geochemical investigation of the eruptive products allows identification of magma source and quantification of magmatic processes leading to an eruption.Here, we use petrological and geochemical evidences, obtained on a precisely dated sample-suite of lava and tephra from the 2010 Eyjafjallajökull eruption in south Iceland (Fig. 1), to evaluate the triggering mechanism for the 2010 summit eruption and to quantify the magma differentiation processes.We show that basaltic injection remobilized older silicic magma causing explosive eruption of inhomogeneous mixture of mingled magma.We also demonstrate how fast the composition and proportions of the mixing end-members changed, or the dynamics of magma mixing.

The Eyjafjallajökull 2010 eruption
Over the last fifteen years, episodic seismic swarms and inflation-induced deformation have been taken to indicate sill injections at mid-crustal depth beneath Eyjafjallajökull volcano (Gudmundsson et al., 2010;Sigmundsson et al., 2010).A deep-sourced inflation started December 2009 accompanied by decreasing depth of seismicity.Deformation and earthquake activity continued until late 20 March 2010 when a flank eruption broke out on a radial fissure at the Fimmvörðuháls Pass, between Eyjafjallajökull and Mýrdalsjökull ice-caps (Fig. 1).The eruption produced olivine-and plagioclase-phyric primitive and relatively homogeneous mildly-alkaline basalt until 12 April (Fig. 2).This was followed by a seismic swarm that migrated rapidly from depth of more than 5 km towards the summit of the volcano (Hjaltadottir et al., 2011) culminating in an explosive eruption in the early morning on 14 April.Magma-water interaction was intensive during the first two days but gradually declined, and the activity became purely magmatic by 21 April.During the first six days magma discharge was on the order of 10 6 kg s dropped to 10 4 -10 5 kg s −1 until early May when activity picked up and reached a discharge of 10 5 kg s −1 again on 5-6 May, followed by a irregular decline in discharge until the end of the eruption late May.The magma produced is of a benmoritic to trachytic composition with very fine to fine ash that disturbed air-traffic over Europe for extended periods in April and May.

Samples
Our sample suite is comprised of basaltic lava (FH-1) and tephra (FH-2) from the initial phase of the flank eruption at Fimmvörðuháls, a tephra (FH-3) collected directly from the fallout from the plume on 1 April, and lava (FH-4) from the last stage of the eruption.
The benmoritic sample suite includes tephra collected 15 April (EJ-1), a composite sample of tephra produced 17-19 April (EJ-2), and tephra from 22 April (EJ-3), 27 April (EJ-4), and 5 May tephra (EJ-5), in addition to two bread-crust bombs of trachyte composition (EJ-6,7) from the final days of the eruption, collected on 3 June 2010 from the surface of 45 m thick tephra pile on the eastern rim of the new crater.Tephra from the 1821-1823 penultimate eruption of Eyjafjallajökull was sampled from a soil section on the western flank of the volcano for comparison.The exact timing of our samples is fundamental for precisely deciphering the magma dynamics prior to and during the eruption.Figures

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Full calibration with counting time set at 10 s for all elements.During glass analyses, analytical conditions were adjusted to minimise sodium mobility; basaltic and intermediate tephra were analysed at 4 nA and 2 nA beam current, respectively, with a 10 µm defocused beam.Optimized mixture of minerals standards (synthetic and natural) and glasses (A-THO and VG2) was used for calibration.The counting time was 10 s for Na, Si, Ca, Ti and P; 20 s for Al and Mg; 30 s for Mn and 40 s for K and Fe.Secondary international glass standard USGS VG A-99 (Jarosewich et al., 1979;Thornber et al., 2002) was analysed during each session to monitor possible instrumental drift.Analyses during three days in a row, yield relative standard deviation of 0.6 % for SiO 2 , 1-2 % for TiO 2 , Al 2 O 3 and CaO, 2.2 % for FeO, 3.3 % for Na 2 O and K 2 O, using a single set of calibration values.

Laser ablation inductively coupled plasma mass spectrometry
Trace element analyses in glasses were performed at the Laboratoire Magmas et Volcans (Clermont-Ferrand) using a Resonetics M50 EXCIMER laser (193 nm) coupled to an Agilent 7500 cs ICP-MS.The laser was operated at 6 mJ energy, 2 Hz repetition rate and a 11 µm spot size diameter.Ablation gas was pure helium; nitrogen (7 ml min −1 ) and argon was mixed with the carrier gas via Y-connectors between the ablation cell and ICP-MS.Analysis duration was split up in two distinct parts: 40 s background acquisition followed by 50 s data acquisition from the sample.Stability of signal intensity during ablation proved a good indicator of the analytical spot homogeneity.
The raw analyses were reduced with the Glitter software (van Achterberg et al., 2001) using CaO concentrations (measured earlier by electron microprobe) as internal standard.NIST 612 glass was used as the primary standard; NIST 610, BCR2-G and A-THO, periodically analyzed during the laser sessions, were used as reference materials for run quality control.The two latter reference glasses have similar composition as the analyzed sample and are therefore well suited to estimate precision and accuracy.Despite the small spot size, precision and accuracy were always better than 10 % for all the elements at 95 % confidence level.

Major-and trace element concentrations
About 100 mg of powder sample were fluxed with lithium metaborate (proportions 1:3) in a carbon crucible using induction furnace.The melt-pearl was immediately dissolved in diluted nitric acid and diluted 2000 times before ICP-AES analysis.Another 100 mg powder aliquot was dissolved in concentrated HF-HNO 3 , evaporated to near dryness and re-dissolved in 7M HNO 3 .The aliquot was evaporated to near dryness and subsequently diluted in HNO 3 0.4 M to reach a total dilution factor of 5000 for determination of trace element abundances by quadrupole ICP-MS (Agilent 7500, Laboratoire Magmas et Volcans).The reaction cell (He mode) was used to reduce interferences on masses ranging from 45 (Sc) to 75 (As).The signal was calibrated externally with a reference basaltic standard (BHVO-2, batch 759) dissolved as samples, and employing the GeoReM preferred values (http://georem.mpch-mainz.gwdg.de/).Both standards and pure HNO 3 0.4 M were measured every 4 samples.The external reproducibility of the method, as estimated by running repeatedly different standards (BCR-2, BIR, BEN) is <5 % (2σ) for most lithophile elements and <15 % for chalcophile elements.

Oxygen isotopes
Laser fluorination oxygen isotope analyses were performed at the University of Oregon stable isotope laboratory using a 35 W CO 2 -laser.Individual grains, bulk monomineralic fractions, and glasses ranging in weight from 1.1 to 2 mg were reacted with purified BrF 5 reagent to liberate oxygen.The gases generated in the laser chamber were purified through a series of cryogenic traps held at liquid nitrogen temperature and a mercury diffusion pump to eliminate traces of fluorine gas.Oxygen was converted to CO 2 gas using a small platinum-graphite converter, and then the CO 2 gas was analyzed on a MAT 253 mass spectrometer integrated to the laser line.was used in the standard set.Day-to-day δ 18 O variability on the standards ranged from -0.1 to +0.25 % , and these values were added to the unknown samples to correct for day-to-day variability and absolute values on SMOW scale.The obtained precision on the standards is better than 0.13 % and 0.01 % in two sessions at 1 standard deviation.

Isotope ratios of Sr and Nd
About 100-150 mg of rock powder (chips for FH-3) were weighed into Teflon beakers for the samples and rock standards and leached for an hour in warm 6 M HCl.After leaching the samples were washed in Milli-Q water and dissolved in a 2:1 mixture of concentrated HNO 3 and HF on a hotplate for 3 days.After drying down the sample residue were redissolved in 6 M HCl, dried down and redissolved again in 6 M HCl, to obtain clear sample solutions.The samples for Sr and Nd analysis were dried down and redissolved in 1 M HNO 3 and passed through TRU.Spec column chemistry, the Sr and Nd fractions was further purified through Sr. spec and LN.spec column chemistries, respectively (Pin et al., 1994(Pin et al., , 1997)).
The Sr samples were loaded onto single W-filaments and analysed at the Imperial College London MAGIC laboratories and Laboratoire Magmas et Volcans in Clermont-Ferrand on Triton TIMS in static mode.Rubidium interferences were monitored and corrected for, but where always lower than 40 ppm.Data were corrected for instrumental mass fractionation using the exponential law and interferences were monitored and corrected for in run and lower than 100 ppm.The Nd procedural blank is 40 pg, no blank correction was made.

Results and discussion
Major-element concentrations show that the basalt of the flank eruption is mildly alkaline in composition (Table 1) similar to the magma erupted during the first half of the 1963-1967 Surtsey eruption 65 km south of Eyjafjallajökull (e.g.Jakobsson, 1979;Furman et al., 1991;Sigmarsson et al., 2009).Euhedral and normally zoned phenocrysts of olivines (Fo 86−71 ) and plagioclases (An 86−61 ) are abundant in addition to rare chromian spinel (inclusions in olivines) and clinopyroxene (Mg-number (100 x molar ratio of MgO over MgO + FeO) = 76-65).The magma is highly vesiculated and consequently the groundmass is largely crystallized (Fig. 2).The interstitial glass has evolved FeTibasaltic composition similar to segregation veins at Surtsey and Holocene lavas from the Katla volcano (Sigmarsson et al., 2009;Oladottir et al., 2008).Less evolved basaltic compositions are preserved in melt inclusions of olivine and plagioclase phenocrysts (Moune et al., 2011).In contrast, bulk samples of tephra from the explosive phase of the 2010 Eyjafjallajökull eruption are of a benmoritic composition (Table 1).During the summit eruption, phenocryst compositions varied greatly with olivines ranging from Fo 80 (Fig. 2 Table 2).These compositions plot on the same binary mixing line indicating mechanical mixing or mingling of the evolved basalts with older silicic melt.In-situ glass analyses of tephra produced between 22 April and 5 May (samples EJ-3,4,5) are all of intermediate composition.These tephra show increasingly lower whole-rock CaO/MgO values with time indicating changes in composition of mixing end-members during the eruption.
In-situ trace element measurements (see Table 3) in the three glass types of sample EJ-2, three glass inclusions in phenocrysts of the Fimmvörðuháls flank basalt, and in tephra glass from the 1821-1823 eruption confirm the role of mixing in forming the 2010 Eyjafjallajökull benmorite magma.Strong linear correlations are not only observed between incompatible element concentrations such as Rb and Th (Fig. 4), but also between those of compatible and incompatible elements (e.g.Sr versus Th.).This suggests that crystal-liquid separation had probably too little time to occur.The apparent absence of fractional crystallization despite tenfold variation in Th concentrations is best explained by rapid mechanical magma-mixing with minimal melt homogenisation prior to eruption.The whole-rock tephra trace element compositions (Table 1) plot on the same mixing line illustrating that despite changing composition of the mixing end-members during the eruption, they must have had similar Rb-Th and Sr-Th ratios.Uniform O, Sr and Nd isotope ratios in the whole rock lava and tephra samples (Table 1) support this conclusion.Delta 18 O of 5.8 ± 1 % in EJ-1 is consistent with the silicic mixing end-member being formed by fractional crystallisation of mantle derived basalt similar to those erupted laterally on Fimmvörðuháls (δ 18 O = 5.4-5.8 ± 1 % ).
During this process the global partition coefficient of Sr (D Sr ) between fractionating mineral assemblage and residual melt must have been close to unity (Fig. 4b).Introduction

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Full The rapid magma mixing is also reflected in the highly heterogeneous and zoned mineral compositions in the benmoritic tephra.For instance, tephra that fell during the second peak in magma discharge (i.e. 5 and 6 May) contains 50 µm zoned olivine (Fig. 2c) with 10 µm tick rim having a composition of Fo 48−50 , but a core of Fo 80 indistinguishable from olivines in the Fimmvörðuháls basalts.This suggests arrival of deepderived primitive basalts that is consistent with a deep seismic swarm (originating from a depth close to the mantle-crust boundary (Hjaltadottir et al., 2011;Bjarnason, 2008)) and increased magma output as observed by higher eruption column on 5 May.The new influx of more primitive basalt magma coincides with changes in the composition of the silicic mixing end-member changing to a less evolved composition as indicated by the mixing curves and lines on Fig. 3.The composition of the final mixing end-member is present in the EJ-5 tephra and identified as low temperature melting component of a Na-rich plagioclase (Fig. 2e-f).This suggests that the stagnant residual melt from the 1821-1823 eruption was somewhat consumed by mixing with basalts by 5 May, after which the magma chamber's carapace (Fig. 1d) was partially melted by interaction with newly injected and hotter mantle-derived basalts.Taken together, the explosive Eyjafjallajökull eruption is best explained by dynamic mixing involving older silicic intrusion that was heated up and remobilized by the injection of hot basalt magma that became more primitive with time.
The high resolution sample suite from the 2010 Eyjafjallajökull eruption allow us to estimate a) the proportions of the basalt component in the mixed magma and its variations with time, and b) the time-dependent changes in the composition of the deepderived basalt magma (Fig. 4).These estimates are obtained from the calculated binary mixing curves shown in Fig. 3b, and from the intercept of the mixing lines with the fractional crystallisation vector of the basalts applying the lever rule (see legend to The petrological and geochemical results obtained so far suggest the following scenario.The real-time deformation results obtained during the first three months of 2010 (Sigmundsson et al., 2010), were caused by the ascent and degassing of relatively primitive and slightly alkaline basalt magma that produced, via fractional crystallisation, evolved FeTi-basalts similar to those of Katla volcano (Sigmarsson et al., 2009).
This evolved basalt appears to have accumulated at depth over the three months and only shortly before the explosive eruption (13 April) encountered the partially molten 1821-1823 residual silicic magma body beneath the summit of the volcano.The silicic magma chamber appears to have hindered the rise of the basalt whereas a portion of the primitive basalt emerged further east during the Fimmvörðuháls flank eruption.
Three weeks later the flank eruption stopped when the basalt, which had partially crystallised, heated and remobilised the 1821-1823 alkaline rhyolite, was injected into the silicic magma body directly beneath the summit crater provoking the explosive eruption of mingled benmoritic magma.During the first two weeks of the explosive summit eruption, evolved basalt was involved in the mixing process and thereafter the basalt became less evolved due to inflow of deeper-derived and more primitive magma.The ascent of deeper-derived basalts may have generated the seismicity at 18-24 km depth observed in early May (Hjaltadottir et al., 2011).
Decreasing mafic end-member proportions with time in the erupted mixture strongly suggests that the basaltic injection remobilized the half-solidified residual silicic magma beneath Eyjafjallajökull and that the 2010 eruption was shut off by declining basaltic intrusion rather than emptying of a silicic magma reservoir.Therefore, the next eruption at this volcano is likely to produce silicic magma with corresponding tephra production.The strong evidences for magma mixing at the origin of recent explosive eruptions elsewhere such as at Mt.St. Helens (USA, Pallister et al., 2008) and Mt.Unzen (Japan; Nakamura, 1995) and the time-related increasing proportions of mafic enclaves in volcanics from the on-going eruption at Soufrière Hills (Montserat, Lesser Antilles; Barclay et al., 2010), clearly demonstrate that not only is magma mixing important as a triggering mechanism at hazardous volcanoes but also a very dynamic process.The results Figures

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Full of the present study clearly underline how fast magma mixing components can change.

Conclusions
The explosive summit eruption of Eyjafjallajökull in 2010 was triggered by an injection of Mg-rich basaltic magma several months earlier.That basalt stagnated below a silicic magma body, presumably residues from the penultimate rhyolitic eruption in 1821-1823, degassed, partially crystallized and evolved to a FeTi-basalt.The heat and gas liberated rose up into the half-frozen silicic magma opening a pathway for the evolved basalt that triggered the explosive eruption on 14 April through magma mingling within the silicic reservoir.In the meantime, the Mg-rich magma by-passed the central magma chamber and produced a flank eruption until the passage through the central conduit opened up.Early May, the evolved basalt was consumed by the magma mingling and deeper Mg-rich basalt rose from a depth in excess of 20 km into the silicic reservoir and caused increased magma output and corresponding higher eruption column.The additional heat brought in by the fresh intrusion caused partial melting of the microgranitic carapace causing compositional changes in the mixing end-members.
Finally, the basalt injection declined and the eruption came to a halt.Introduction

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Full  Full  Full  The first two MI in olivine with Fo73 and the two A-THO analysis are duplicate analysis of the same glass patch.The standard glass NIST610 was run as an unknown during different runs and yields the overall reproducibilty.Introduction

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Full  (Oladottir et al., 2008).Mixing lines between these evolved basalt compositions and the two silicic end-members, the 1821-1823 rhyolite and oligoclase melt of 5 May (dash-dot line) are also shown.The intercepts of these lines with the basaltic fractionation vector is used to estimate the degree of basaltic evolution (expressed as mixing proportions between evolved and primitive basalts in Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Five aliquots of standards were analyzed together with the unknown samples; Gore Mt Garnet (δ 18 O = 5.75 % ) Discussion Paper | Discussion Paper | Discussion Paper | 710251 ± 8 (2σ).Neodymium was analyzed in static mode and data were corrected for instrumental mass fractionation using the exponential law and 146 Nd/ 144 Nd = 0.7219, samples were analyzed in two analytical session during the first average value of the JNdi standard was 143 Nd/ 144 Nd = 0.512099 ± 20, during the second 143 Nd/ 144 Nd = 0.512059 ± 20.Sample data were normalized to 143 Nd/ 144 Nd value of JNdi of 0.512113.Discussion Paper | Discussion Paper | Discussion Paper | ) to Fo 46 , feldspars vary from An 69 to An 9 , and Mg-number of clinopyroxene range from 72 down to 19.Magnetite is abundant and traces of apatite, pyrite and orthopyroxene are also present.Both the plagioclases and the clinopyroxenes display an inverse chemical zonation (e.g.Fig.2e-f) with a core having, respectively, lower An content and Mg-number.Such compositional zonation is readly explained by magma mixing.Noteworthy are microsyenitic fragments composed of anorthoclase (An 1.2 Or 32 ), tridymite, ferrohedenbergite (Mg-number = 19; En 11 Fs 48 Wo 41 ) and fluorite emitted during the first days of the summit eruption (Fig.2d).The major-element concentration variations for the whole-rock and glass analysis are shown in Fig.3where CaO vs MgO are plotted (a) and the molar ratios of CaO over Al 2 O 3 is displayed as function of the Mg-number (b).The whole-rock CaO/MgO Introduction Discussion Paper | Discussion Paper | Discussion Paper |decreased from 2.41 in the initial phase (sample EJ-1) to 1.48 in bread-crust bombs from the final stage of the eruption.Whole-rock sample of the first tephra (15 April) plots on a binary mixing line defined by the interstitial glass of the Fimmvörðuháls basalt and the glass composition of the 1821-1823 AD rhyolitic tephra.Three glass compositions are detected in the composite tephra from 17-19 April (sample EJ-2); basalt with SiO 2 of 49-51 %, benmorite (SiO 2 = 60-61 %) and trachyte (SiO 2 = 69-70 %; Fig.3 and Discussion Paper | Discussion Paper | Discussion Paper |

Fig. 3 )
Fig.3).The results indicate that the proportions of the basalt decreased from approximately 50 % late April to less than 30 % a month later whereas the evolved FeTi-basalt composition early in the eruption was progressively replaced by more primitive basalt composition at the end.
Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Pin C., Briot D., Bassin C., and Poitrasson F.: Concomitant separation of strontium and samarium-neodymium for isotopic analysis in silicate samples, based on specific extraction chromatography, Anal.Chim.Ac., 298, 209-217, 1994.Pin, C. and Zaldegui, J. F. S.: Sequential separation of light rare-earth elements, thorium, and uranium by miniaturized extraction chromatography: Application to isotopic analyses of Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

Fig. 5 .
Fig. 5. Mixing proportions as a function of time of primitive basalt in the mafic end-member (scale on left y-axis) and that of basalt melt in the benmoritic tephra of Eyjafjallajökull.Arbitrary 5 % error is assigned to the estimated magma mixing proportions that are derived from Fig. 3.The decrease of the basalt component suggests that supply of silicic magma at depth is abundant (see text for further details).

Table 1 .
Whole-rock major-and trace-element concentrations and Sr-, Nd-and O-isotope ratios in Eyjafjallajökull 2010 products.
Major-and trace-element concentrations are given in wt %, and ppm, respectively.Abbreviations SE and sd denote standard error and standard deviation, respectively.Apastrophes behind a sample number denote a replicate analysis.Introduction Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

Table 3 .
Trace element concentrations analysed by laser ablation ICP-MS in melt inclusions (MI) in olivines from flank basalt and in benmoritic tephra erupted from the first explosive phase.