The Opening Subplinian Phase of the Hekla 1991 Eruption: Properties of the Tephra Fall Deposit

Supplementary document 1 The opening subplinian phase of the Hekla 1991 eruption: properties of the tephra fall deposit

Jonas Gudnason1,3, Thor Thordarson1, Bruce F. Houghton2 and Gudrun Larsen3

1Faculty of Earth Sciences, University of Iceland 2Department of Geology and Geophysics, University of Hawaii 3Institute of Earth Sciences, University of Iceland Corresponding author: Jonas Gudnason [email protected]

Chemical composition Glass chemistry of the 1991 tephra was analysed on a WDS Cameca SX100 electron microprobe at the Laboratoire Magmas et Volcans, Clermont –Ferrand (LMV). Standard used was natural glass from Kilauea (A99), the analytical conditions used were 15 kV voltage and 4 nA current and 10 µm beam diameter. Counting time was 10 s for Na, CA, Ti, P and Si; 20 s for Mg and Al; 30 s for Mn and 40 s for Fe and K. Before the session 10 points were measured on the standard (A99), when compared to other reported results they are indistinguishable apart from systematically higher titanium at LMV (Table 1; Jarosewich et al., 1980; Oladottir et al., 2011; Thornber et al., 2002).

The 1991 products are intermediate transitional alkalic and are basaltic icelandites (Table 1; Jakobsson et al 2008). Holocene magmatic activity at Hekla features an igneous rock series extending from mildly alkaline basalt to a sub-alkaline rhyolite, including transitional basaltic icelandites and subalkaline icelandites. Isotopic studies indicate that they are genetically unrelated and that the formation of the basaltic icelandites results from fractional crystallization of the basaltic parental magma (Sigmarsson, 1992).

When comparing the major elemental composition of tephra from the opening phase and later formed lava in the basaltic icelandite eruptions at Hekla during the latter half of the 20 th century the observed differences are minimal (Table 2). For example, the difference in concentration of highly incompatible elements, such as K2O, between the tephra and lava is <0.25 wt %, and the difference in

SiO2 concentration is only 0.65 wt % (i.e. ≤1%; Table 2). The magma produced by other post 1947 Hekla eruptions are very similar to that produced by the 1991 event (Error: Reference source not found; Gronvold et al., 1983; Gudmundsson et al., 1992; Höskuldsson et al., 2007; Thorarinsson & Sigvaldason, 1970; Thorarinsson, 1968). Of the 20th century events, only the 1947 eruption exhibits significant temporal variation in terms of major element composition, where the tephra from the initial phase is high silica icelandite (SiO2, 62.11 wt %) while the later formed lava is basaltic icelandite

(SiO2, 54.25 wt%; Table 2).

Table 1: Chemical composition of the standard VG-A99 as acquired on the electron microprobe at Laboratorie Magmas et Volcans LMV (this study) compared to published values: USGS (Thornber et al 2000), USNM (Jarosewich et al 1979), LMV (A) (Oladottir et al 2011). Numbers in brackets are one sigma standard deviation

VG-A99 LMV (this USGS USNM LMV (A) study)

SiO2 51.06 (0.46) 50.94 51.00 (0.54) 50.85 (0.35)

TiO2 3.95 (0.09) 4.06 4.17 (0.06) 4.16 (0.06)

Al2O3 12.44 (0.13) 12.49 12.65 (0.15) 12.65 (0.10) FeO 13.15 (0.16) 13.3 13.20 (0.29) 13.31 (0.18) MnO 0.19 (0.02) 0.15 0.20 (0.07) 0.20 (0.007) MgO 5.04 (0.1) 5.08 5.01 (0.09) 5.02 (0.07) CaO 9.04 (0.11) 9.3 9.14 (0.09) 9.16 (0.04)

Na2O 2.72 (0.16) 2.66 2.69 (0.11) 2.77 (0.11)

K2O 0.82 (0.03) 0.82 0.86 (0.02) 0.85 (0.02)

P2O5 0.43 (0.03) 0.38 0.46 (0.02) 0.44 (0.05) Total 98.84 99.18 99.35 99.42

Table 2: Comparison of the 20th century Hekla eruptions; Hekla 1947 tephra and lava 1(Thorarinsson and Sigvaldason 1970); Hekla 1970 tephra and lava 1(Thorarinsson and Sigvaldason 1970); Hekla 1980 tephra and lava 2(Gronvold et al. 1980); Hekla 1991 tephra average of 10 glass shards (this study) 1991 lava (Gudmundsson et al 1992) and H2000 average of 20 tephra and lava samples, according to authors undistinguishable between the two products 4(Hoskuldsson et al. 2007).

Hekla 1947 1970 1980 1991 2000 tephra - lava - tephra - lava - tephra - lava - tephra - lava - Average of lava opening effusive opening effusive opening effusive opening effusive and tephra phase phase phase phase phase phase phase phase

SiO2 62.11 54.25 55.21 54.52 54.50 54.60 55.45 54.90 55.52

TiO2 1.02 1.54 1.83 1.91 1.91 1.91 2.32 2.09 2.07

Al2O3 16.17 16.34 15.16 14.80 14.60 14.70 13.71 14.30 14.54

FeO 7.75 11.28 11.52 11.78 11.70 11.40 12.30 11.59 11.79 MnO 0.18 0.26 0.26 0.26 0.22 0.24 0.33 0.27 0.28 MgO 1.63 3.39 2.86 3.03 2.90 2.87 2.82 2.91 2.89 CaO 5.24 5.24 6.81 6.76 7.09 7.12 6.26 7.06 6.82

Na2O 4.12 4.12 4.16 4.08 4.03 3.98 3.59 4.18 4.01

K2O 0.98 0.98 1.26 1.24 1.16 1.19 1.45 1.22 1.21

P2O5 0.34 0.34 0.68 0.71 0.85 0.86 1.25 1.34 0.87

Total 99.54 97.74 99.75 99.09 98.96 98.87 99.47 99.86 100.00 Source 1 1 1 1 2 2 This study 3 4

References

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