Magmatic evolution of Krafla, N.E. Iceland
Abstract
Krafla is an active central volcano in the, NE axial rift zone of Iceland
associated with a fissure swarm which trends NNE-SSW. Lava/hyaloclastitc samples
were collected from the volcanic system, covering the last 4 interglacial and 3 glacial
periods. A stratigraphic framework for the volcanic system has been established by
use of the Hekla tephra layers and the distinctive volcanic products of glacial and
interglacial periods. Recent volcanic activity has shown that there is a shallow magma
reservoir beneath the central volcano, which supplies magma laterally to the fissure
swarms forming dykes and to the surface directly for eruption.
The Krafla rocks contain the following phenocryst assemblages ol + plag, ol
+ plag + cpx, plag + cpx ± ol, plag + cpx + opx + FeTi oxides, plag + FeTi oxides
cpx ± fayalitic ol. These distinct assemblage suggest that the Krafla suite evolves
predominantly by fractional crystanation. They are also similar to the assemblages
found in other mid-ocean ridge basalt (MORB) suites.Most samples, excluding
rhyolites,contain plagioclase xenocrysts,implying that there has been at least 2 stages
of magma mixing prior to eruption The absence of plagioclase xenocrysts in the
rhyolites suggests that they formed in isolation from the most primitive magmas with
or without crustal assimilation.
Fractional crystallisation using the observed phenocryst assemblage explains
many aspects of the 4 major-element compositions of the suite. Least-squares
modelling confirm this observation, although it also suggests that the rhyolites
contain higher concentrations of K₂O than would. be predicted by closed-system
fractional crystallisation. This discrepancy, may be, explained by open system
fractionation and/or by crustal assimilation. Numerical modelling of phase equilibria
suggests that fractional crystallisation occurs over a range of pressures from 1-3.5
kbar. As was suspected, magma mixing also'appears to be a significant process in the
Krafla f system and may explain sorne of the scatter on major-element variation
diagrams. The Krafla suite is more differentiated than the majority of MORB suites,
consistent with the development of long-lived magma reservoirs. The suite also has
higher average values of total iron oxide and lower average values of Na₂O (for a
given MgO content), which are explicable by larger degrees of melting at'higher than
average pressures than for the majority of MORBs.
i,
Trace-element compositions also confirm'the role of fractional crystallisation.
Although the trace-element data also show that the highly incompatible trace
elements (e. g. Th, U, Rb) show have high concentrations in the rhyolites like K₂O. As
mentioned above, this enrichment in the most, incompatible elements may be
explained by either open system effects and/or by crustal assimilation. However, the
temporal variation in rock'composition shows no evidence for enrichment in the more
incompatible elements when compared with the less incompatible elements,
effectively reducing the role of open system fractionation. This suggests that the
enrichment in'inc'ompatible elements occurs through crustal assimilation of possibly
basaltic wall-rock which has undergone relatively small degrees of melting. Ratios of
incompatible trace elements also suggest that the Krafla basalts are distinctive from
most MORBs in that they appear to be derived from a less incompatible element
enriched mantle source.
The major- and the trace-element compositions of the most primitive Krafla
basalts show evidence for variable degrees of melting of mantle or possibly variations
of the source. In particular, the major-element concentrations, when compared with
data from experimental studies, suggest that the basalts derived from the smallest
degrees of melting are also produced at the highest pressures. The variation in the
incompatible trace elements appears to be "coupled" to that seen in the major
elements. The least-differentiated Krafla compositions are also compared with melt
compositions predicted from the parameterisation of melting experiments (McKenzie
& Bickle 1988). After correcting for the effects of fractional crystallisation, the Krafla
melts are most consistent with model compositions produced by a value of 1480°C for
the mantle potential temperature. Discrepancies between model compositions and the
Krafla compositions may possibly be explained by the dynamic effects of a mantle
plume on the melting processes.
The Krafla lavas show anomalously low δ¹⁸O values compared with the
majority of MORBs. The δ¹⁸O values of the lavas correlate positively with the MgO
content. This correlation is consistent with crustal assimilation of low-¹⁸O
hydrothemidly-altered country-rock. The assimilation process appears to be coupled
to fractional crystaffisation, although the linearity of the δ¹⁸Ovs MgO plot appears to
confirm that magma mixing is also operating. The (²³⁰Th/²³²Th) values of postglacial
rocks also correlate positively with MgO content, consistent with their origin by the
same crustal assimilation process. The 0- and Th-isotope ratios may be modeHed by
an assimilation with fractional crystallisation (AFC) model, providing the δ¹⁸O value
of the assimilant is assumed. An assimilant produced at the top of the magma
reservoir is likely to have a 8180 value of about -100loo. If this value of 8180 is used
for the assimilant, then a single-stage AFC model requires that the assimilant has to
contain between 3-5 ppm, Th and r=0.15-0.20. A 2-stage model, however, gives a
better fit with the O-Th isotopic data, with an assimilant containing about I ppm Th in
the first stage of differentiation and 7 ppm in the second stage. Both models can
generate the high values of the most incompatible trace elements in the rhyolites.