Crystal storage and transfer in basaltic systems: the Skuggafjöll eruption, Iceland

Basaltic lavas rich in large, high-anorthite plagioclase crystals are commonly erupted along slow spreading ridges and at ocean islands. Such plagioclase is often too primitive to be in equilibrium with the melts in which it is carried (Cullen et al., 1989). While some authors have preferred flotation as a mechanism for accumualting large amounts of primitve plagioclase in basatlic magmas (e.g., Flower, 1980), Lange et al. (2013) proposed that entraiment of earlier-formed cumulates represents a more feasible model. Understanding such mush disaggregation in basaltic magma reservoirs is crucial for a number of reasons: (1) timescales between disaggregation and eruption are often thought to be short (e.g., Costa et al., 2010); (2) mush crystals record information about conditions of magma storage at depth; and (3) disaggregated crystals provide a link between volcanic and plutonic realms.

We thus carried out a detailed petrological and geochemical study on the highly plagioclase-phyric Skuggafjöll eruption within the Eastern Volcanic Zone of Iceland in order to investigate crystal storage and transport processes. By using a range of petrographic and geochemical tools, including novel QEMSCAN technology, we evaluated the origin of crystals on a case-by-case basis and thus distinguished crystals grown from the carrier melt from crystals entrained from mushes.

QEMSCAN image of a glassy basalt sample from Skuggafjöll. Large pale blue crystals plagioclase crystals, khaki olivine crystals and dark green clinopyroxene crystals can be observed against a glassy and vesiculated orange groundmass. The field of view is ~20 mm across.
QEMSCAN image of a glassy basalt sample from Skuggafjöll. Large pale blue crystals plagioclase crystals, khaki olivine crystals and dark green clinopyroxene crystals can be observed against a glassy and vesiculated orange groundmass. The field of view is ~20 mm across.

Variability in whole-rock, macrocryst and melt inclusion compositions suggested that the Skuggafjöll magma experienced two stages of crystallisation. Primitive crystals from an earlier stage of crystallisation were stored in crystal mushes prior to disaggregating into to an evolved and geochemcially distinct magma, which then underwent further crystallisation before eruption. The timescale between crystal entrainment and eruption, during which crystal accumulation occurred, was short – of the order of days – and is being investigated further by PhD student I am co-supervising. Striking petrological similarities between Skuggafjöll and other highly phyric eruptions in Iceland (e.g., Halldorsson et al., 2008), as well as along mid-ocean ridges, indicate that crystal accumulation by mush disaggregation is an important mechanism for generating highly phyric magmas.

Publication

Neave, D.A., Maclennan, J., Hartley, M.E., Edmonds, M. & Thordarson, T. 2014. Crystal storage and transfer in basaltic systems: the Skuggafjöll eruption, Iceland. Journal of Petrology 55, 2311–2346. <Open Access>

Melt mixing causes negative correlation of trace element enrichment and CO2 content prior to an Icelandic eruption

Dissolved volatile elements play important roles in driving volcanic eruptions and controlling the physical properties of magmas. Degassing of magmatic volatiles also links deep geochemcial reservoirs with the Earth’s surface, closing global element cycles (e.g., Marty & Tolstikhin, 1998). However, determing the original CO2 content of mantle melts is difficult because most melts reach volatile saturation long before eruption. Measuring melt inclusions isolated hosted in primitive crystals that remained isolated from their carrier melt provides one way of investigating the CO2 content of basaltic magmas (e.g., Moore, 2008).

In this paper, we presented major, trace and volatile element analyses from >100 primitive olivine-hosted melt inclusions from a sub-glacial eruption in the Eastern Volcanic Zone of Iceland – the Skuggafjöll eruption. While our melt inclusion compositions preserved a record of primitive melt heterogeneity similar to that observed in other Icelandic systems including Laki (Neave et al., 2013), the most striking feature of our dataset was an enigmatic negative correlation between CO2 and incompatible trace element enrichment:

 

Negative correlation between melt inclusion CO2 and Ce/Y contents. The solid black line shows a mixing line between a depleted end-member shown with a black diamond and an enriched end-member shown with a white diamond that explains much of the correlated varibility in tha sample suite. Many inclusions have experineced further exsolution as illustrated by the discrepancy between predicted and measured CO2 contents. Modifued from Neave et al. (2014).
Negative correlation between melt inclusion CO2 and Ce/Y contents. The solid black line shows a mixing line between a depleted end-member shown with a black diamond and an enriched end-member shown with a white diamond that explains much of the correlated varibility in tha sample suite. Many inclusions have experineced further exsolution as illustrated by the discrepancy between predicted and measured CO2 contents. Modified from Neave et al. (2014).

We suggested that a negative correlation between CO2 and incompatibe trace element enrichment may result from the concurrent mixing, crystallisation and exsolution of CO2 from melts that have experienced varying degrees of previous CO2 loss: mixing may have been triggered by the injection of a depleted and possibily CO2-supersaturated melt (CO2/Nb > 350) into a relatively shallow magma reservoir containing an enriched melt that has already lost much of its CO2.

Another inportant finding  concerned the CO2 content of shrinkage bubbles in melt inclusions. Many recent studies have demonstrated that CO2 can be sequestered into bubbles during the cooling of melt inclusions (e.g., Hartley et al., 2014; Mironov et al., 2015; Wallace et al., 2015; Moore et al., 2015). However, despite investigating a large number number of shrinkage bubbles by Raman spectroscopy and microthermometry, we found no CO2-bearing bubbles. We therefore suggested that our subglacially quenched samples cooled sufficiently quickly to for CO2 sequestration to have been kinetically inhibited, an observation that has implications for interpreting the CO2 content of inclusions from other settings that experince rapidly quenched, such as those from mid-ocean ridges.

Comparison of the CO2 content of melt inclusions with and without inclusion-hosted bubbles: bubbles have no systematic effect on inclusion CO2 content.
Comparison of the CO2 content of melt inclusions with and without inclusion-hosted bubbles: bubbles have no systematic effect on inclusion CO2 content.

Publication

Neave, D.A., Maclennan, J., Edmonds, M. & Thordarson, T. 2014. Melt mixing causes negative correlation of trace element enrichment and CO2 content prior to an Icelandic eruption. Earth and Planetary Science Letters 400, 272–283. <Open Access>