Diffusive over-hydration of olivine-hosted melt inclusions

Olivine-hosted melt inclusions are ofen used to estimate the pre-eruptive H2O content of magmas (Métrich & Wallace, 2008). However, it has been noted for a number of years that H2O appears to ‘leak’ out of melt inclusions during ascent and eruption (Massare et al., 2002; Chen et al., 2013). Rare cases of H2O gain have also been noted (Kolezsar et al., 2009). Recent experiments and modelling has clarified the mechanisms of H2O loss– by diffusive re-equilibration through the host crystal – and has opened up the possibily of extracting timescales from the extent of H2O exchange (Gaetani et al., 2012; Bucholz et al., 2013).

In this study, led by Margaret Hartley at the University of Manchester, we showed that different populations of melt inclusions from the Laki and Skuggafjöll eruptions in the Eastern Volcanic Zone of Iceland experienced diffusive loss or diffusive gain of H2O. Some rapidly quenched melt inclusions from the Laki tephra and subglacially-quenched pillow glasses from Skuggafjöll had coherent H2O/Ce values of ~180 that we interpreted as the primary mantle value (e.g., Michael, 1995). However, many inclusions from the Laki lava flow had very low H2O/Ce values consistent with H2O loss during transport in an extensive lava tube network at the surface. Conversely, most inclusions from Skuggafjöll, as well as most low-Ce, primitive inclusions from Laki, had elevated H2O/Ce values of up to ~1000 that are indicative of H2O gain during storage in the crust.

A figure from Hartley et al. (2015) summarising the processes by which diffusive loss and gain of H2O has been observed in the Laki and Skuggafjöll eruptions.

Using the diffusive re-equilibration model of Bucholz et al. (2013), we placed minimum constraints on the residence times of dehydrated inclusions in the Laki lava flow and over-hydrated inclusions in evolved melts immediately prior to the eruptions. The timescales were on the order of days to tens of days in both cases. Finally, we demonstrated that diffusive gain, as well as diffusive loss, can be observed in a number of global datasets where primitive, H2O-poor inclusions are mixed into more enriched and/or evolved melts before eruption. Thus, rather than viewing the open system nature of olivine-hosted melt inclusions as weakness, it can be exploited to gain further insights into pre-eruptive magma processes.

Publication

Hartley, M.E., Neave, D.A., Maclennan, J., Edmonds, M. & Thordarson, T. 2015. Diffusive over-hydration of olivine-hosted melt inclusions. Earth and Planetary Science Letters 425, 168–178. <Open Access>

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>

Crystal-melt relationships and the record of deep mixing and crystallisation in the AD 1783 Laki eruption, Iceland

Basaltic magmas are often assembled from a diversity of mantle melts that mix and crystallise en route to the Earth’s surface (Sobolev & Shimizu, 1993; Maclennan, 2008). Thus, before any attempt can be made at determining the depths of any pre-eruptive processes, it is essential to understand how melts and and crystals relate to each other.

In this paper, we investigated how the magma that fed the large and environmentally impacting AD 1783–84 Laki eruption was assembled. Olivine-hosted melt inclusion compositions revealed that concurrent mixing and crystallisation of variable mantle melts occurred deep within Laki plumbing system. Indeed, the presence of high-anorthite plagioclase compositions more primitive than any other crystal or melt inclusion composition measured confirmed that the difference components of the Laki lava cannot all be related to the carrier liquid by single liquid line of descent. Furthermore, crystal zonation patterns indicated that multiple crystal mush formation and disaggregation events took place prior to eventual eruption. Combining clinopyroxene-melt barometry with information from crystal textures indicates that most crystallisation took place within the mid-crust, the depth of much recent seismogenic magmatism in the Eastern Volcanic Zone of Iceland (Tarasewicz et al. 2012).

 

Syhthesis of deep magmatic processes in the Laki plumbing system. Modified from Neave et al. (2013)
Deep magmatic processes in the Laki plumbing system. Modified from Neave et al. (2013).

Publication

Neave, D.A., Passmore, E., Maclennan, J., Fitton, J.G. & Thordarson, T. 2013. Crystal-Melt Relationships and the Record of Deep Mixing and Crystallization in the AD 1783 Laki Eruption, Iceland. Journal of  Petrology 54, 1661–1690. <Open Access>

Melting, differentiation and degassing at the Pantelleria volcano, Italy

Pantellerites are Fe- and volatile-rich, peralkaline rhyolites that erupt primarily in continental rift settings. As no eruptions of pantelleritic magma have been observed, interpreting the diversity of volcanic phenomena at pantelleritic volcanoes is challenging. Explosive eruptions range in scale from large ignimbrite-forming events like the ~45 ka Green Tuff eruption on Pantelleria to small cone-forming events. Effusive eruptions form structures as diverse as low-aspect-ratio lava domes and high-aspect-ratio lava shields. Although fewer in number than their calcalkaline counterparts, peralkaline rhyolite volcanoes nevertheless present a range of hazards.

The evolution of peralkaline magmas has been the subject of much recent debate, with some authors advocating for pantellerite genesis by melting alkali gabbros (e.g., Avanzinelli et al., 2004), and others favouring extensive fractional crystallisation (e.g., White et al., 2009). The volatile content of pantellerite melts had also been the subject of considerable uncertainty until a recent studied have confirmed the water-rich nature of pantelleritic melts.

In this paper, we presented major element, trace element and volatile compositions from glasses, crystals and melt inclusions from a number of post-Green Tuff eruptions from Pantelleria. The main outcomes were:

  1. A quantification of the degree of aluminous lherzolite melting required to generate the alkali basalts present around northwest coast of Pantelleria (~2%).
  2. A confirmation that pantellerites can be generated by extensive fractional crystallisation (~95%) of alkali basalts.
  3. High precision analyses of glass and melt inclusion volatile contents (H2O, CO2, Li, F, Cl, S) that confirm the H2O- and halogen-rich of pantellerite melts
  4. An evaluation that explosive peralkaline eruptions may emit much more sulphur than metauluminous eruptions of an equivalent size, up to ~100 Mt for a Green Tuff-sized eruption, becasue of the high sulphur solubilty in Fe- and alkali-rich melts.

Dammusi on Pantellieria
Dammusi on Pantellieria

Publication

Neave, D.A., Fabbro, G., Herd, R.A., Petrone, C.M. & Edmonds, M. 2012. Melting, Differentiation and Degassing at the Pantelleria Volcano, Italy. Journal of Petrology 53, 637–663.