Jökull


Jökull - 01.12.2006, Side 73

Jökull - 01.12.2006, Side 73
Pleistocene rhyolitic volcanism at Torfajökull, Iceland number of dated samples. (It is noteworthy that Fig- ure 6 shows more scatter, for Nb and Sr, when analy- ses from undated rhyolites are included.) Only when further ages have been obtained will the robustness of this near-linear trend be tested further. The third trend, the pronounced drop in alkalinity after 83 ka, has been followed by the dominance of subalkaline rhyolite compositions during the Holocene. The ap- parent permanence of this change is emphasised by studies (McGarvie, 1984; Macdonald et al., 1990) showing that the Holocene rhyolites, with one excep- tion, the compositionally-zoned Hrafntinnusker erup- tion, have compositions that are distinctly less evolved than those of the ring-fracture rhyolites (McGarvie et al., 1990). Furthermore, within the Holocene rhyo- lites, for which there is reasonable stratigraphic con- trol, only a slight trend of less-evolved compositions being erupted with time is present (Macdonald et al., 1990; McGarvie et al., 1990), which might indicate that some process is buffering the compositions of erupted Holocene magmas. At least two viable models can explain the third trend. The first is that the c. 16 km3 ring fracture rhyolite eruption has depleted the magma system of evolved melt, leaving only a subalkaline mush. For the past c. 65 ka this mush has been periodically in- truded by basaltic magmas, resulting both in the pro- duction of small batches of rhyolitic melt, and (dur- ing eruptive episodes) by extensive interactions be- tween intruding basaltic magmas and rhyolitic melts. Occasionally, melt generation and/or storage condi- tions has favoured the differentiation of batches of rhyolitic melt, leading (for example) to the production of the Hrafntinnusker zoned eruption (McGarvie et al., 1990). This could imply that episodes of basaltic magma injection not only trigger eruptions, but may contribute thermal energy that helps to generate fu- ture batches of rhyolitic melt (see also Gunnarsson et al., 1998). The second model ignores any direct role for basaltic magma injection in rhyolite magma production, and involves two steps, the first being the production of batches of ’dominant volume’ rhy- olitic magma (Smith 1979), and the second being the differentiation of this starting composition. This is broadly similar to models for other volcanic com- plexes, such as Pantelleria (Mahood et al., 1990) and Boina (Barberi et al., 1975). At Torfajökull, the sub- alkaline compositions that typify Holocene eruptions are assumed to represent the starting composition, and although there is disagreement over how this melt is produced (see Macdonald et al., 1990; Gunnarsson et al., 1998), for present purposes this can be ignored. Once the starting composition has been produced, it can then differentiate towards more-evolved compo- sitions such as comendites and pantellerites. Only at one Holocene eruption (Hrafntinnusker, McGarvie et al., 1990) has this occurred convincingly. Two possi- bilities stem from this: firstly that the rate of produc- tion of the starting composition is now less than it was in the past (because there has not been a return to the production of highly-evolved rhyolitic compositions such as pantellerites); and secondly that rhyolite melt replenishment is occurring at rates similar to those of the past but repeated injections of basaltic magma at c. 500-1000 year intervals (Larsen, 1984) are deny- ing the accumulating magma of sufficient residence time (and sufficient accumulated volumes) to differen- tiate towards more evolved compositions. If the model for the propagation of the Eastern Rift Zone to the south-west is correct (e.g. Oskarsson et al., 1982, 1985) then this situation is unlikely to change because episodic injections of mafic magmas into Torfajökull (from various sources), will continue (Larsen, 1984; McGarvie, 1985). Indeed, if an intensification of rift zone mafic magma production accompanies this pos- tulated rift propagation, this might well increase the frequency of tholeiitic magma injections into Torfa- jökull (McGarvie et al., 1990), though whether this will increase or decrease the volume of rhyolitic melt available for eruption is unclear at present. In seeking a more viable explanation, it is note- worthy that better-constrained examples of peralka- line magma production and evolution such as those at Olkaria and Naivasha in Kenya point to shorter timescales of 103–104 rather than 105 years (Davies and Macdonald, 1987; Heumann and Davies, 2002), with even shorter timescales implicated at other sys- tems (see Hawkesworth et al., 2004). However exam- ples of such rapid production-differentiation need to be reconciled with the broad general trend of older JÖKULL No. 56 71
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

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.