The evolution and storage of primitive melts in the Eastern Volcanic Zone of Iceland: the 10 ka Grímsvötn tephra series (i.e. the Saksunarvatn ash)

Abstract

evolved assemblage is close to being in equilibrium with the matrix glass, trace element disequilibrium between primitive and evolved assemblages indicates that they were derived from different distributions of mantle melt compositions. Juxtaposition of disequilibrium assemblages probably occurred during disaggregation of incompatible trace element-depleted mushes (mean La/Yb melt = 2.1) into aphyric and incompatible trace element-enriched liquids (La/Yb melt = 3.6) shortly before the growth of the evolved macrocryst assemblage. Post-entrapment modification of plagioclase-hosted melt inclusions has been minimal and high-Mg# inclusions record differentiation and mixing of compositionally variable mantle melts that are amongst the most primitive liquids known from the EVZ. Coupled highfield strength element (HFSE) depletion and incompatible trace element enrichment in a subset of primitive plagioclase-hosted melt inclusions can be accounted for by inclusion formation following plagioclase dissolution driven by interaction with plagioclase-undersaturated melts. Thermobarometric calculations indicate that final crystal-melt equilibration within the evolved assemblage occurred at ~1140 °C and 0.0-1.5 kbar. Considering the large volume of the erupted tephra and textural evidence for rapid crystallisation of the evolved assemblage, 0.0-1.5 kbar is considered unlikely to represent a pressure of long-term magma accumulation and storage. Multiple thermometers indicate that the primitive assemblage crystallised at high temperatures of 1240-1300 °C. Different barometers, however, return markedly different crystallisation depth estimates. Raw clinopyroxene-melt pressures of 5.5-7.5 kbar conflict with apparent melt inclusion entrapment pressures of 1.4 kbar. After applying a correction derived from published experimental data, clinopyroxene-melt equilibria return mid-crustal pressures of 4 ± 1.5 kbar, which are consistent with pressures estimated from the major element Abstract Major, trace and volatile elements were measured in a suite of primitive macrocrysts and melt inclusions from the thickest layer of the 10 ka Grímsvötn tephra series (i.e. Saksunarvatn ash) at Lake Hvítárvatn in central Iceland. In the absence of primitive tholeiitic eruptions (MgO > 7 wt%) within the Eastern Volcanic Zone (EVZ) of Iceland, these crystal and inclusion compositions provide an important insight into magmatic processes in this volcanically productive region. Matrix glass compositions show strong similarities with glass compositions from the AD 1783-1784 Laki eruption, confirming the affinity of the tephra series with the Grímsvötn volcanic system. content of primitive melt inclusions. Long-term storage of primitive magmas in the mid-crust implies that low CO 2 concentrations measured in primitive plagioclase-hosted inclusions (262-800 ppm) result from post-entrapment CO 2 loss during transport through the shallow crust. In order to reconstruct basaltic plumbing system geometries from petrological data with greater confidence, mineral-mel...

Magma plumbing for the 2014–2015 Holuhraun eruption, Iceland

“…Sigmarsson et al ., ]. We note here, however, that the clinopyroxene mineral‐melt thermobarometry after Putirka [] has recently been suspected of potentially overpredicting crystallization pressures for Icelandic magmas [ Neave et al ., ]. Yet as there is no specific calibration for Icelandic magmas available and as mineral‐melt thermobarometry results are in general agreement with the bimodality obtained from OPAM pressure calculations on groundmass and whole rock compositions (see section 3.2 above), we argue that the approach employed here produces the currently most viable depth estimates of crystallization for the Holuhraun system.…”

Mantle to surface degassing of carbon- and sulphur-rich alkaline magma at El Hierro, Canary Islands

“…Indeed, recent work has highlighted the possibility that incomplete closure of the melt inclusions at depth or decrepitation of the inclusions followed by healing9 at different depths (Métrich and Wallace, 2008). Neave et al (2015) came to the similar conclusion 270 that melt inclusions from r msv tn, Iceland, must have lost CO 2 upon ascent to shallow depths. If this 271 second possibility is correct, bubbles in El Hierro melt inclusions do not contain significant amounts of 272 CO 2 , but most of the originally dissolved CO 2 was lost by deep degassing of the primitive basanite 273 prior to final melt inclusion entrapment.…”

Melt Diversity and Magmatic Evolution in the Dali Picrites, Emeishan Large Igneous Province

“…Clinopyroxene is the second‐crystallizing silicate phase in the Emeishan mafic magmas, and its saturation in the magma is closely related to melt composition and crystallization pressure (e.g., Danyushevsky, ; Ghiorso & Sack, ; Grove et al, ; Langmuir et al, ). The major and trace element compositions in the melts are coupled during partial melting (e.g., Neave et al, ; Ren et al, ; Winpenny & Maclennan, ). In the case of the Emeishan mafic magmas, the high‐Ti melts have higher Ce, Nd contents, and Ce/Y, Ce/Yb ratios and are formed by lower degrees of partial melting at greater depths than low‐Ti melts (Ren et al, ; Wang et al, ; Xu et al, ).…”