Effects of deglaciation on the petrology and eruptive history of the Western Volcanic Zone, Iceland

Abstract

New observations and geochemical analyses of volcanic features in the 170-km-long Western Volcanic Zone (WVZ) of Iceland constrain spatial and temporal variations in volcanic production and composition associated with the last major deglaciation. Subglacial eruptions represent a significant portion of the late Quaternary volcanic budget in Iceland. Individual features can have volumes up to ∼48 km 3 and appear to be monogenetic. Subaqueous to subaerial transition zones provide minimum estimates of ice sheet thickness at the time of eruption, although water-magma interactions and fluctuating lake levels during eruption can lead to complex lithological sequences. New major and trace element data for 36 glacial and postglacial eruptive units, combined with observations of lava surface quality, passage zone heights, and 3 He exposure ages of some glacial units, indicate a maximum in volcanic production in the WVZ during the last major ice retreat. Anomalously high volcanic production rates continue into the early postglacial period and coincide with significant incompatible element depletions and slightly higher CaO and SiO 2 and lower FeO content at a given MgO. Subglacial units with strong incompatible element depletions also have lava surfaces that lack evidence of subsequent glaciation. These units likely formed after the onset of deglaciation, when rapidly melting ice sheets increased decompression rates in the underlying mantle, leading to anomalously high melting rates in the depleted upper mantle. This process also can explain the eruption of extremely depleted picritic lavas during the early postglacial period. These new observations indicate that the increased volcanic activity associated with glacial unloading peaked earlier than previously thought, before Iceland was completely ice free.

Postglacial eruptive history of the Askja region, North Iceland

“…By ∼10.8-10.5 ka, Iceland is thought to have been largely ice-free except for its highest peaks (e.g. Eason et al 2015, and references therein). The Bárðardálshraun lava, erupted from vents in the Dyngjuháls area ∼35 km south-west of Askja (i.e.…”

“…This pattern is consistent with increased melt generation in the mantle following glacial unloading. Depleted geochemical signatures are also present in late glacial eruptive units (∼11.7-10.5 ka) from the WVZ, which may indicate that melt generation and volcanic productivity peaked after the onset of deglaciation but before ice removal was complete (Eason et al 2015). In contrast, Gee et al (1998) argued that the geochemical variations observed at Theistareykir and on the Reykjanes Peninsula were best explained by processes of crystallisation, assimilation and mixing in magma chambers, with incompatible element-depleted early postglacial compositions reflecting decreased magma residence times in the crust.…”

The Importance of Icelandic Ice Sheet Growth and Retreat on Mantle CO2 Flux

“…The observed time series of incompatible trace element concentrations in Icelandic magmas and ice sheet history have been suggested to be strongly associated (Eason et al, 2015;Gee et al, 1998;Jull & McKenzie, 1996;Maclennan et al, 2002;Sinton et al, 2005). Of these studies Gee et al (1998) and Eason et al (2015) report on the Nb composition of lavas erupted and give age ranges for the erupted lavas. Dating lava flow in Iceland is complex, given the lack of reliable markers from which ages can be obtained.…”

“…(a) Ice sheet models M1 and M2 taken from the sea level models ofPeltier (2004) andPico et al (2017), respectively. (c) Observed and predicted Nb concentrations (parts per million), observations are from the Reykjanes Peninsula and the Western Volcanic Zone(Eason et al, 2015;Gee et al, 1998;Sinton et al, 2005), which are binned into three major periods: glacial (>12 ka), early postglacial (between 12 and 7 ka) and recent (<7 ka). The gray region shows estimated eruption rates from geological observations(Maclennan et al, 2002) (in square kilometer of melt per million years).…”

Formation of segregation structures in Hafnarhraun pāhoehoe lobe, SW Iceland: a window into crystal–melt separation in basaltic magma

“…Sinton et al (2005) defined Hafnarhraun as a separate unit from the neighboring Heiðin há and Leitahraun lava flow fields. The exact age of the Hafnarhraun is unknown, although it is clearly postglacial (younger than~13 ka) and older than the neighboring Heiðin há lava flow field (Jónsson 1978), for which 14 C ages indicate a maximum age around 10.3 ka (Sinton et al 2005) and 3 He exposure age a minimum of~9.3 ka (Eason et al 2015).…”