Jökull


Jökull - 01.12.1987, Side 4

Jökull - 01.12.1987, Side 4
field are almost entirely located within the ring structure and cover an area of about 140 km2 (Fig. 1). They consist mostly of steaming ground which is, as a rule, intensely altered by acid surface leaching. Steam heated waters, both of the acid sulphate and the bicarbonate types, are relatively common. In the northeastern part of the field, around Landmannalaugar, sodium-chloride type water springs are common representing boiled and variably mixed reservoir water. Chemical and mineralogical alteration associated with acid surface leaching was studied in detail in one locality by Sigvaldason (1959). Arnórsson (1969) carried out a reconnaissance survey of the major and trace element chemistry of hot spring discharges. The chem- istry of the sodium-chloride waters and fumarole gases in the Landmannalaugar area were investigated by Ar- nórsson (1985) and Arnórsson and Gunnlaugsson (1985). A special study of germanium and molybdenum in Icelandic geothermal waters (Arnórsson 1984, Ar- nórsson and ívarsson 1985) included data from the Landmannalaugar area. Using chemical geothermom- etry and mixing models Arnórsson (1985) concluded that subsurface temperatures around Landmannalaugar were some 265°C. The deuterium and ðlsO content of the sodium-chlo- ride waters indicate that they are local precipitation (Árnason 1971, Arnórsson 1985). Oxygen shift of as much as 2.5 per mil ðlsO units are observed for the waters highest in chloride. In the present contribution we summarize geochem- ical studies which have been carried out on the hot springs and fumaroles in the Torfajökull geothermal field including the associated surface hydrothermal al- teration. THE HEAT SOURCE Bödvarsson (1961) estimated the natural heat output of the Torfajökull geothermal field to be 125-750 • 106 cal/s (525-3150 • 106 J/s). This corresponds to 190-930 kg/s of steam at atmospheric pressure. Pálmason (1980) has evaluated the stored heat in the uppermost 3 km of the geothermal reservoir to be 281 • 1018 J and the theo- retical energy potential to be 964 MW electric for a 50 year production period. According to these authors Tor- fajökull is the largest geothermal field in Iceland. The acid volcanics in Iceland are generally considered to have originated by partial melting of hy drated basaltic crust due to intrusion of mantle derived olivine tholeiite magma (Óskarsson et al. 1982, 1985). Accordingly the abundant acid rocks at Torfajökull indicate emplace- ment of an unusually large body of basalt magma into the lowest part of the crust. The Torfajökull volcanic complex overlies a mantle plume (Óskarsson et al. 1985, Kurz et al. 1985). Rifting has not been active in the volcanic zone where this complex is located until during the latter part of the last glaciation (ívarsson et al. 1987). The combined effects of the mantle plume and the lack of rifting may be the cause of the presumed voluminous basaltic intrusion. A negative gravity anomaly coincides approximately with the distribution of the surface outcrops of acid volcanics (Sœmundsson 1972). A prominent gravity high occurs within the negative anomaly. Walker (1974) con- siders that this high reflects basaltic sheet intrusions and, due to the relatively low density of the acid volcanics, that the basaltic magma tended to form intrusives at the base of these rocks rather than rising through them. Walker (1974) further considers that the presumed bas- altic sheet intrusions at the base of the acid rocks consti- tute the heat source to the geothermal field. ACID SURFACE LEACHING Sigvaldason (1959) showed by his study of surface alteration of rhyolite at Hrafntinnusker that all the ma- jor elements are leached as the rock undergoes complex mineralogical changes, yet to a different extent. The ultimate alteration product becomes enriched in those elements which are leached to the least extent. They include silica and, in particular, titanium. Montmorillo- nite appears during the earliest alteration stages. When all the primary minerals have been decomposed, hema- tite, anatase and, to a lesser extent, kaolinite are found with the montmorillonite. At later alteration stages montmorillonite decomposes conjuncture with progres- sive leaching of Na, K, Ca and Mg from the rock and kaolinite grows in abundance, becoming as much as 50% of the rock by volume. Pyrite and amorphous silica are found in association with the kaolinite and montmo- rillonite at this alteration stage. In the ultimate alter- ation product montmorillonite is absent and the rock consists mostly of amorphous silica, anatase and kaoli- nite with variable amounts of native sulphur and pyrite. INTERPRETATION OF WATER CHEMISTRY Analyses exemplifying the bicarbonate, acid sulphate and sodium-chloride waters are given in Table 1. The bicarbonate waters are characterized by low chloride concentrations, near neutral pH and, in general, they 2
Side 1
Side 2
Side 3
Side 4
Side 5
Side 6
Side 7
Side 8
Side 9
Side 10
Side 11
Side 12
Side 13
Side 14
Side 15
Side 16
Side 17
Side 18
Side 19
Side 20
Side 21
Side 22
Side 23
Side 24
Side 25
Side 26
Side 27
Side 28
Side 29
Side 30
Side 31
Side 32
Side 33
Side 34
Side 35
Side 36
Side 37
Side 38
Side 39
Side 40
Side 41
Side 42
Side 43
Side 44
Side 45
Side 46
Side 47
Side 48
Side 49
Side 50
Side 51
Side 52
Side 53
Side 54
Side 55
Side 56
Side 57
Side 58
Side 59
Side 60
Side 61
Side 62
Side 63
Side 64
Side 65
Side 66
Side 67
Side 68
Side 69
Side 70
Side 71
Side 72
Side 73
Side 74
Side 75
Side 76
Side 77
Side 78
Side 79
Side 80
Side 81
Side 82
Side 83
Side 84
Side 85
Side 86
Side 87
Side 88
Side 89
Side 90
Side 91
Side 92
Side 93
Side 94
Side 95
Side 96
Side 97
Side 98
Side 99
Side 100
Side 101
Side 102
Side 103
Side 104
Side 105
Side 106
Side 107
Side 108
Side 109
Side 110
Side 111
Side 112
Side 113
Side 114
Side 115
Side 116

x

Jökull

Direkte link

Hvis du vil linke til denne avis/magasin, skal du bruge disse links:

Link til denne avis/magasin: Jökull
https://timarit.is/publication/1155

Link til dette eksemplar:

Link til denne side:

Link til denne artikel:

Venligst ikke link direkte til billeder eller PDfs på Timarit.is, da sådanne webadresser kan ændres uden advarsel. Brug venligst de angivne webadresser for at linke til sitet.