Testing the Links Between Deglaciation and Magma Evolution in Iceland Using
Geochemical Signatures From Basaltic Table Mountains
[*Kerri C. Schorzman*] (University of New Hampshire, 56 College Rd., Durham, NH,
USA, 03824; ph: (603) 862-1718; Fax: (603) 862-2649; email: k.schorzman@unh.edu);
Joe Licciardi (University of New Hampshire, 56 College Rd., Durham, NH, USA, 03824;
ph: (603) 862-1718; Fax: (603) 862-2649; email: joe.licciardi@unh.edu); Julie Bryce
(University of New Hampshire, 56 College Rd., Durham, NH, USA, 03824; ph: (603)
862-1718; Fax: (603) 862-2649; email: julie.bryce@unh.edu)
Basaltic table mountains in the neovolcanic zones of Iceland preserve a unique history of
the interplay between glaciation and hot spot volcanism. Geochemical signatures in the
eruptive units that comprise these subglacially erupted landforms provide insight into a
variety of geologic processes associated with rift-related volcanism including trends in
melt production rates, variations in magma composition, and the influence of deglaciation
on mantle processes. To address how glacial loading and unloading may have affected
mantle processes beneath Iceland, we measured major and trace element concentrations
in samples from lithostratigraphic units of thirteen table mountains in the northern (NVZ)
and western (WVZ) volcanic zones. The eruptive ages of the sampled table mountains
were recently inferred from cosmogenic 3He surface exposure dating of their subaerially
erupted summit lavas (Licciardi et al., 2007), affording an opportunity to evaluate both
spatial and temporal trends in the geochemical data.
Several previous studies have linked glacier dynamics in Iceland with changing eruption
rates and marked differences in magma compositions (cf. Sinton et al., 2005). For
example, Slater et al. (1998) and Maclennan et al. (2002) attributed temporal variations of
incompatible trace element concentrations to increased mantle melting rates during
deglaciation. This mechanism is supported by the theoretical model of Jull and
Mackenzie (1996) which showed that rapid glacial unloading can stimulate increased
melt generation in the upper mantle. In contrast, Gee et al. (1998) argued that temporal
variations in geochemistry could arise entirely from magma chamber processes related to
crustal instability during ice removal.
Few studies have documented compositional trends in lithostratigraphic units within
Icelandic table mountains (e.g., Moore and Calk, 1991; Werner et al., 1996), and the
absence of age control has prevented previous studies from examining the geochemical
data in the context of a chronology of table mountain formation.
Major and trace element compositions have now been measured in samples collected
from the subaerial cap lavas of all thirteen table mountains exposure-dated by Licciardi et
al. (2007). Geochemical data have also been obtained from the pillow lava bases of five
of the table mountains, enabling a base-summit comparison of geochemical signatures.
All samples are tholeiitic basalt, ranging in MgO composition from 7 to 10 wt %.
Preliminary analyses indicate that for the five table mountains with paired base-summit
samples, basal pillow lavas are consistently less evolved than their subaerial cap lava
counterparts. Results also suggest geographic controls on variations in magma sources
between table mountains in the NVZ and WVZ. Ongoing analyses are focusing on
correcting for crustal-level processes so that we may properly evaluate trends in
geochemical signatures sensitive to the degree and/or pressure of melting. By combining
age constraints from exposure dating with geochemical modeling of melting processes
and source-region characteristics, we will test hypotheses that link ice unloading with
changing mantle melting conditions.
ORAL
CORRESPONDING AUTHOR: Kerri C. Schorzman, student
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