Magma-Ice interaction: late Miocene ice thickness and eruption tempo in northern Victoria Land

Antarctic ice cover fluctuations are a major driver of global changes. At the margin of the East Antarctic Ice Sheet, igneous activity of the West Antarctic rift led to intrusion of magma in the crust and to the construction of volcanic complexes (Rocchi et al., 2002; Rocchi et al., 2006). Volcanic morphologies and lithofacies of the volcanic products are uniquely valuable indicators (proxies) for reconstructing variations in ice thickness, extent and basal thermal regime (Smellie et al., 2011a; Smellie et al., 2018; Smellie et al., 2014). Glacial thermal regime affects ice stability, through variations in ice velocity, erosional and depositional rates (hence landforms) (Rocchi et al., 2006) and the response time of ice to climate change and consequent effects on global sea levels. On the other hand, glacial thermal regime is possibly affected locally by increased heat flux and (hydro)thermal activity in proximity of active volcanic complexes. The distinction between wet-based (i.e. relatively warm) and cold-based ice masses is thus a critical determinant of ice stability (Smellie et al., 2014), owing to the different physical properties of wet- and cold-based ice. Thus, being able to reliably identify the thermal regimes of past ice is important for palaeoenvironmental studies as well as for validating forward models of the effects of climate change.

Ice thickness fluctuations are effective also in modifying the stress conditions in underlying magmatic systems, and even relatively small (few tens of metres) fluctuations in ice thickness can destabilise very shallow crustal chambers (MacLennan et al., 2002; Pagli and Sigmundsson, 2008). The effect on magma chambers ca be profound in delaying or precipitating eruptions and, thus, the time available for melt fractionation  (Sigmundsson et al., 2010; Slater et al., 1998). Models of glacial retreat in Iceland have already shown that additional magma can be generated in the mantle by glacial unloading (MacLennan et al., 2002; Pagli and Sigmundsson, 2008; Sigmundsson et al., 2010). Glacier retreat also modifies the failure conditions around magma chambers as well as the pressure in the stored magma(Pagli and Sigmundsson, 2008; Sigmundsson et al., 2010). However, how these effects will influence the eruption likelihood at subglacial volcanoes worldwide and therefore the length of time that magma is stored in shallow chambers is not been reliably established in any southern hemisphere volcanoes and rarely so in the northern hemisphere (Geyer and Bindeman, 2011). Moreover, whether high frequency of eruptions may in turn favour magma ascent from depth in a top-down mechanism is also a matter of debate.

The unifying aim of this project is understanding the link between magma and ice changes: how the nature and thickness of the ice affects the type of eruption, and how eruption rate (tempo) and magma chamber replenishment are modulated by cycles of ice loading and unloading.

Late Miocene glacial-interglacial evolution

Environmental variations and evolution in time - Reconstruction of the type, thickness, extent and basal thermal regime of the ice cover coeval with subglacial eruptions. The volcanological studies will focus on identifying the lithofacies present and how they fit with published models for glaciovolcanic and nonglacial (subaerial) eruptions (Smellie et al., 2011a; Smellie et al., 2011b). A particular emphasis will be to confirm, reject or modify the reported evidence for polythermal ice conditions during the late Miocene (Smellie et al., 2014). That will be done by determining the geometry of eroded vs pristine surfaces between the erupted units (i.e. surfaces affected by overlying ice or by subaerial erosion). The major intention is to extract the details of the NVL palaeoenvironment in the warm late Miocene and reconstruct its evolution for a 2­–3 Ma period. An important new dimension to these studies is the search for volcanic units erupted during interglacial periods, that were preliminary observed on Hallett Peninsula, but their distribution and abundance are unknown. The interglacial units are important not only for characterising the environmental conditions but are also critical input for the eruption tempo aspects of MAGIC (see below). Constructing the first detailed history of terrestrial environmental conditions in NVL will give MAGIC a potentially global impact, since fluctuations in Antarctic Ice Sheet volume are a major driver of global change.

Eruption tempo vs ice loading

Feedbacks between environmental fluctuations and volcanism - Reconstruction of the composition and productivity of magmas and evaluation of scenarios of the stress field acting on a magma chambers under different glacier thickness and parameters. This aspect will also incorporate theoretical modelling to investigate in greater detail the relationships between climate, ice cap loading and resulting magmatism/magmatic compositions. Isotopic dating is also critically important and will be performed by the Ar-40/Ar-39 method, because the expected precision of radioisotopic ages for pristine Miocene/Pliocene alkali feldspars is close to, and possibly minor than, the duration of glacial cycles (c. 41 ka), investigating magmatic compositions erupted during the interglacial periods will provide a crude but effective means of identifying petrological changes under minimal (i.e. ice-poor or ice-free) lithostatic loads. They thus provide, uniquely, an end-member condition with which to compare and contrast the magmatic compositions erupted under the higher lithostatic/cryostatic loads of the glacial periods. Fortunately, from our previous work in northern Victoria Land, we know that the erupted lavas are usually extremely fresh and they also include evolved lavas which, with their significantly higher abundance of potassium and potassic minerals, makes the lavas particularly amenable to precise dating. Altered lavas can easily be screened out of our new dataset.

(1) Il progetto ha prodotto dati sul Mount Melbourne Volcanic field che evidenziano come la presenza di acqua in tutte le sue forme (neve, ghiaccio e acqua di fusione) o la sua assenza eserciti un controllo fondamentale sulla dinamica eruttiva e sulla costruzione di edifici e morfologie vulcaniche. Le nuove datazioni isotopiche, in connessione con questi dati glaciovulcanici rappresentano informazioni paleoambientali che indicano il paesaggio del Pliocene superiore- Pleistocene era rappresentato principalmente da un icefield piuttosto che da un ice sheet che copriva interamente la morfologia. Infine, lo spessore del ghiaccio in genere aumentava verso il presente.

(2) Il progetto ha prodotto dati di geochimica, geochimica isotopica. geocronologia isotopica, tessitura e chimica dei minerali, e geotermobarometria sul complesso vulcanico delle Pleiadi, northern Victoria Land. L'interpretazione di questi dati indica che la variazioni geochimiche e petrologiche dei magmi eruttati negli ultimi 850 ka siano compatibili con la variabilità del carico glaciale, come dedotta dalle variabili paleoclimatiche.

Una spedizione sul terreno è stata svolta durante la spedizione 2022-2023, con campo remoto a Cape Hallett e studio degli affioramenti glaciovulcanici dell'area.