Jökull


Jökull - 01.12.2006, Page 74

Jökull - 01.12.2006, Page 74
David W. McGarvie et al. and more-evolved rhyolites progressing to younger and less-evolved rhyolites seen on Figures 6 and 7 over a c. 300 ka period, and supported by data in Mc- Garvie (1985) and Ívarsson (1992). In the absence of detailed information on the pro- duction of rhyolitic magmas through time (which does not yet exist for Torfajökull, except during the Holocene), the general trends shown on Figures 6 and 7 (which span four glacial maxima, see Figure 5) can be interpreted as suggesting that variations in ice thickness may have had no effect on the composi- tion of erupted melts. This contrasts with how basaltic magma systems respond to ice loading and unload- ing, as they show excellent evidence of higher produc- tion rates (and subtly-different compositions) during deglaciation (e.g. Jull and Mackenzie, 1996; Slater et al., 1998; Maclennan et al., 2002). The reason(s) for this difference require further investigation, but it is suggested that these may be linked to differences in magma generation, with basalt dominated by man- tle processes and rhyolite dominated by crustal pro- cesses. In emphasising this difference, it is notewor- thy that immediately following the last deglaciation, whilst basaltic magma systems across Iceland were erupting at rates >30 times higher than at present, no corresponding pulse in Holocene rhyolitic eruptions occurred at Torfajökull, Iceland’s largest active rhyo- lite central volcano. CONCLUSIONS • Five new Ar-Ar ages of Pleistocene rhyolites from the Torfajökull central volcano range from 384 ka to 67 ka, and span four glacial peri- ods (Fuhne, OI stage 10), Drenthe (OI stage 8), Warthe (OI stage 6), and Weichselian (OI stages 4 and 2). • For three of the ages there is good correlation between the climatic conditions indicated by the oxygen isotope record (cold, during glacial periods), and the field evidence indicating erup- tion of rhyolite into thick ice sheets. This repre- sents the first step towards developing a rhyolite tuya proxy for past ice sheet thicknesses. • Two Ar-Ar ages of 67±9 ka and 72±7 ka that were obtained on widely-separated tuyas of the postulated c. 16 km3 ring fracture eruption (McGarvie, 1984) strengthen the argument that a large eruptive event took place during the We- ichselian, with the eruption probably occurring during (cold) OI stage 4. • Whole-rock geochemical data confirm a trend of decreasing peralkalinity with time, with the most recent (Holocene) rhyolites being dom- inated by subalkaline compositions. A near- linear trend of trace element concentrations with time requires further investigation, as it may simply be an artefact of the small data set. If the near-linear trend is real, then it raises the question of how a magma system can evacuate batches of magma of successively less-evolved composition over a period span- ning c. 300 ka, in a fashion that mimics the enrichment-depletion trace element signatures of individual compositionally-zoned eruptions. • Further work should involve improvements in the Ar-Ar method to reduce uncertainties on the ages of younger (<400 ka) rhyolites, which will enable better information on ice sheet thick- nesses for constraining paleoclimate models. ACKNOWLEDGEMENTS John Watson (OU) is thanked for doing the XRF anal- ysis. Special thanks go to Mark Davies (OU) for moti- vation and good humour during fieldwork at Torfajök- ull. Thanks also go to Smári, Nina, Eydís, and Daði at Landmannalaugar for good cheer and a solid roof dur- ing poor weather. Discussions with Ray Macdonald, John Smellie, Jennie Gilbert, Harry Pinkerton, Gret- ar Ívarsson and Magnús Tumi Guðmundsson helped to shape various parts of this paper. Ben Edwards and an anonymous reviewer are thanked for thorough and constructive comments that improved the final paper. Finally, we wish to thank Bryndís Brandsdóttir for her editorial acumen. 72 JÖKULL No. 56
Page 1
Page 2
Page 3
Page 4
Page 5
Page 6
Page 7
Page 8
Page 9
Page 10
Page 11
Page 12
Page 13
Page 14
Page 15
Page 16
Page 17
Page 18
Page 19
Page 20
Page 21
Page 22
Page 23
Page 24
Page 25
Page 26
Page 27
Page 28
Page 29
Page 30
Page 31
Page 32
Page 33
Page 34
Page 35
Page 36
Page 37
Page 38
Page 39
Page 40
Page 41
Page 42
Page 43
Page 44
Page 45
Page 46
Page 47
Page 48
Page 49
Page 50
Page 51
Page 52
Page 53
Page 54
Page 55
Page 56
Page 57
Page 58
Page 59
Page 60
Page 61
Page 62
Page 63
Page 64
Page 65
Page 66
Page 67
Page 68
Page 69
Page 70
Page 71
Page 72
Page 73
Page 74
Page 75
Page 76
Page 77
Page 78
Page 79
Page 80
Page 81
Page 82
Page 83
Page 84
Page 85
Page 86
Page 87
Page 88
Page 89
Page 90
Page 91
Page 92
Page 93
Page 94
Page 95
Page 96
Page 97
Page 98
Page 99
Page 100
Page 101
Page 102
Page 103
Page 104
Page 105
Page 106
Page 107
Page 108

x

Jökull

Direct Links

If you want to link to this newspaper/magazine, please use these links:

Link to this newspaper/magazine: Jökull
https://timarit.is/publication/1155

Link to this issue:

Link to this page:

Link to this article:

Please do not link directly to images or PDFs on Timarit.is as such URLs may change without warning. Please use the URLs provided above for linking to the website.