Eruption age of the Sverrefjellet volcano, Spitsbergen Island, Norway

Figures & data

Fig. 1  Geology of the Sverrefjellet area and sample locations. (a) Simplified geological map of the area including Bockfjorden, Sverrefjellet and Woodfjorden, with locations and names of major Quaternary igneous bodies (after Skjelkvåle et al. 1989). North is to the top. The small square on the inset shows location of the investigated area on Spitsbergen. (b) Aerial photograph of Sverrefjellet, annotated with sample locations. North is to the right and the scene is ca. 3 km across. Adolfbreen glacier is at the upper right; alluvium in Bockfjorden is at the bottom of image. The base image is aerial photograph S70-2558, from the Norwegian Polar Programs office.

Table 1 Sample data: analysed basalts.

Sample	Location	Elevation (m)	Rationale
04SV42	Stream cut, NE apron, below glacial sediments	30	Glassy rind of pillow or pillow fragment
04SV60	Knob S of summit	346	Glassy outer rind of pillow, kilometres south of other samples
04SV73	N slope, highest outcrops of pillow basalt	430	Glassy rind of pillow, highest exposure of pillow basalt
06SV01A	Cliffs exposing pillow basalts on W slope	130	Glassy rind of pillow, approximately ca. 4 m diameter. Few small vesicles
06SV01B	Cliffs exposing pillow basalts on W slope	130	Mid-section of above pillow, with large vesicles
06SV01C	Cliffs exposing pillow basalts on W slope	130	Interior of above pillow, next to lava tube at core of pillow. More crystalline, with small vesicles
06SV04	Summit of Sverrefjellet	510	Basalt dike exposed at summit. Mostly crystalline

See Figure 1 for exact sample locations.

Fig. 2  Ar–Ar thermal release spectrum and isochron graphs for selected samples. See Fig. 1 and Table 1 for locations and the Supplementary Appendix for full data. In the thermal release spectra, statistically significant age plateaus are shown by horizontal arrows; “f39” is the fraction of ³⁹Ar release included in the plateau. Ca/K and Cl/K are also shown. Mean square weighted deviation is abbreviated as MSWD. The lower graphs are inverse isochron plots; the x-intercept is inversely proportional to sample age (see Table 2); and the y-intercept is the ³⁶Ar/⁴⁰Ar ratio of a non-radiogenic component (see Table 2). Solid lines are isochrons for selected release steps (generally excluding the lowest four and highest two steps); dashed lines are regressed through the whole dataset. Figures (a) and (b) are for splits of the same rock sample; (b) has identical plateau and isochron ages and a trapped ⁴⁰Ar/³⁶Ar consistent with air. The sample of (c) has an isochron age consistent with those of (a) and (b), but a trapped ⁴⁰Ar/³⁶Ar higher than air; hence, its calculated plateau age exceeds its isochron age. The sample of (d) shows no isochron or Ar release plateau. Its Ar is dominated by releases in the middle heating steps, probably from decomposition of carbonates and/or zeolites.

Table 2 Ar–Ar ages.

Sample	Plateau age^a (My)	MSWD^b	Isochron age^c (My)	MSWD	Isochron 40Ar/36Ar(i)	Total-gas age (My)
04SV42a	0.62±0.07	1.64	1.27±0.11	0.96	285.6±1.9	1.4
04SV42b	1.10±0.07	1.49	None	–	–	2.3
04SV60a	1.02±0.04	1.79	1.17±0.08	1.4	292.3±1.7	1.3
04SV60b	1.14±0.06	1.92	0.99±0.08	1.8	299.6±2.3	1.4
04SV73a	1.05±0.07	1.45	0.86±0.09	0.94	298.9±2.3	1.4
04SV73b	1.42±0.06	1.90	1.05±0.07	1.19	300.8±1.5	1.6
06SV01Aa	None	–	None	–	–	2.3
06SV01Ab	1.38±0.06	1.57	1.61±0.10	0.75	282.8±4.8	1.8
06SV01Ba	None	–	None		–	10.4
06SV01Bb	None	–	None	–	–	3.4
06SV01Ca	1.04±0.07	0.54	0.89±0.65	0.58	297.0±4.8	1.9
06SV01Cb	None	–	None	–	–	1.5
06SV04a	None	–	None	–	–	5.3
06SV04b	None	–	None	–	–	3.3

See Fig. 2. Uncertainties are 1σ

^aAssumes a single non-radiogenic component of ⁴⁰Ar/³⁶Ar = 295.5 (see Supplementary Appendix).

^bMean square weighted deviation.

^cGenerally exclude first four, and last two, thermal release steps (see Supplementary Appendix).

Skjelkvåle B.-L. Amundsen H.E.F. O'Reilly S.Y. Griffin W.L. Gjelsvik T. A primitive alkali basaltic stratovolcano and associated eruptive centers, northwestern Spitsbergen: volcanology and tectonic significance. Journal of Volcanology and Geothermal Research. 1989; 37: 1–19.

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