advances, the analytical limitations imposed by the inherently young age of Quaternary deposits (i.e., very small amounts of radiogenic 40Ar in-growth and often small amounts of 36Ar) remain. Our current understanding of what factors may be important in determining the suitability of glassy materials for 40Ar/39Ar dating (e.g., magma chemistry, volatile contents, eruption mechanism, degassing behavior, etc.) is limited. In this we investigate 40Ar/39Ar and major volatile contents (H2O, CO2, Cl, F, and S) in a suite of effusion-dominated rhyolites from several subglacial (Torfajökull and Prestahnúkur, Iceland) and one subaerial eruption (Hrafntinnusker, Torfajökull) that are chemically and texturally distinct to better understand the application of the 40Ar/39Ar system to glassy materials. Volatile contents preserved in fresh volcanic glass can provide a minimum estimate of the pre-eruptive volatile contents of the original magma from which they are derived, as well as track and record the degassing history of a body of magma through time (Johnson et al., 1994; Balconne-Boissard et al., 2010). Here, we look at the preserved volatile record to assess variability and the occurrence of poor or incorrect 40Ar/39Ar ages to evaluate the suitability of young, subglacial and subaerially erupted glassy material for 40Ar/39Ar dating. 2. SAMPLE LOCATIONS AND ERUPTION ENVIRONMENT Eruption products from two volcanic centers in Iceland were sampled: (1) predominantly subglacial rhyolites from Torfajökull (Fig. 1a, b) and (2) a small-volume (0.6 km3; McGarvie et al., 2007) subglacial rhyolite eruption of a single magma-batch at Prestahnúkur (Fig. 1a, c). Eruptions at both locations have involved complex volcano-ice interactions (e.g., Tuffen et al., 2001; McGarvie et al., 2006, 2007). 2.1 Torfajökull
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† Values from McGarvie et al., 2007; *Values from McGarvie et al., 1990 Table 3. Volatile concentrations for Torfajökull and Prestahnúkur rhyolites as determined by Cameca NanoSIMS 50L (OU). All concentrations are reported as ppm except water which is given in weight %. Unit H2O ± CO2 ± F ± Cl ± S ± Commentt n Bláhnúkur TJ/98-40 0.69 0.01 nd 1690 40 2000 120 6 1 p 6 TJ/98-39 < 0.1 < 5 80 5 nd nd mc, het 5 Haskerðingur TJ/98-6 0.08 0.01 nd 1650 90 1300 80 18 1 p 8 Kaldaklofsjöll TJ/98-50 0.40 0.08 640 110 250 40 30 10 120 7 pem, het 5 Hábarmur TJ/97-14 0.13 0.03 7800 2200 1320 160 290 100 30 5 pem, het 8 TJ/97-15 0.09 0.01 nd 1780 40 1620 50 45 1 p 8 Hrafntinnusker HSK-12 0.09 0.01 nd 1740 10 1590 20 45 5 p 6 HSK-32 0.08 0.01 nd 1600 30 1760 80 10 3 p 4 Prestahnúkur PK-1 0.12 0.01 nd 840 10 460 10 nd p 6 PK-6 0.13 0.01 nd 880 10 430 10 nd p 6 PK-7 0.11 0.01 nd 900 10 450 20 nd p 6 p, taken as residual (degassed), unaltered; pem = post-eruption modification; het = heterogeneous; mc = microcrystalline; n = number taken in average; bd = below detection Table 4. Summary of new ages for Torfajökull and Prestahnúkur rhyolites. Errors are 2σ. Refer to section 3.2 for full analytical details. Apparent ages in parentheses indicate altered sample which may have been