The mass of volatiles degassed from volcanic eruptions is often estimated by comparing the volatile concentrations in undegassed glassy melt inclusions with the volatile concentrations in the ...degassed matrix glass. However, melt inclusions are prone to post-entrapment modification, including diffusive H+ loss through the host olivine crystal lattice which lowers the H2O content of the inclusion, and the degassing of CO2 into a bubble in response to cooling and crystallisation on the inclusion walls. Such bubbles are very common in olivine-hosted melt inclusions from the AD 1783–1784 Laki eruption, south–east Iceland. We have determined the CO2 content of these bubbles using micro-Raman spectroscopy, and the CO2 concentration in the glass by SIMS. Our results show that >90% of the total inclusion CO2 may be sequestered into the bubble, which demonstrates the importance of measuring the compositions of both vapour bubbles and the glass phase in melt inclusions. We reconstruct the deep degassing path of the Laki magma by using Nb as proxy for the undegassed CO2 content of the melt inclusions. The substantial CO2/Nb variation in the Laki melt inclusions (3.8–364) can be explained by concurrent crystallisation and CO2 degassing in the Laki magmatic system. We calculate the amount of CO2 lost from individual melt inclusions, assuming CO2/Nb ≈ 435 for enriched Icelandic mantle and CO2/Nb ≈ 171 for depleted mantle. Melt inclusions with the greatest saturation pressures have lost the least CO2 prior to inclusion trapping. At any given saturation pressure, the most enriched melt inclusions have lost the most CO2, while the most depleted inclusions have lost very little CO2. Enriched primary melts with high initial CO2 concentrations are therefore useful for investigating deep degassing behaviour in magmatic systems because a range of melt inclusion saturation pressures are recorded during crystallisation and degassing. Depleted melt inclusions with low initial CO2 concentrations remain vapour-undersaturated to shallow levels and cannot be used to constrain deep degassing behaviour. The cumulative CO2 mass release from the Laki magma is determined as a function of pressure and extent of crystallisation. Using an updated petrologic method that takes into account the diversity of primary melts and CO2 sequestration into vapour bubbles, we calculate the total mass of CO2 exsolved from the Laki magma to be 304 Mt.
•CO2 in shrinkage bubbles in melt inclusions can be measured by Raman spectroscopy.•>90% of the total melt inclusion CO2 may be contained in the bubble.•The deep degassing path of the Laki magma is reconstructed using CO2/Nb systematics.•Laki melt inclusions preserve a record of concurrent crystallisation and degassing.•The total mass of CO2 exsolved from the Laki magma is ∼304 Mt.
The Askja volcanic system, North Iceland, experienced a volcano‐tectonic episode between 1874 and 1876, the climax of which was a rhyolitic, phreatoplinian to Plinian eruption at Askja central ...volcano on 28–29 March 1875. Fissure eruptions also occurred in 1875, producing the Nýjahraun lava, 45–65 km north of Askja. The Nýjahraun basalt is indistinguishable, in terms of whole‐rock major elements, from the small‐volume basaltic eruptions that took place at Askja in the early 20th century. It has been suggested that all of these basalts originated from a shallow magma chamber beneath Askja, with the Nýjahraun eruptions being fed by northward‐propagating lateral dykes. It has also been conjectured that the Holuhraun lava, located at the southern tip of the Askja volcanic system 15–25 km south of Askja, was connected with the 1874–1876 Askja volcano‐tectonic episode. We re‐examine these interpretations in light of new whole‐rock, glass and melt inclusion analyses from samples collected along the length of the Askja volcanic system. Glasses from Nýjahraun and the Askja 20th century eruptions are geochemically distinct. We suggest that the Askja 20th century basalts mixed with evolved melts in the crust, while the Nýjahraun magma evolved without such interactions. The Holuhraun basalt is more similar to lavas erupted on the Bárðarbunga‐Veiðivötn volcanic system than to postglacial basalts from Askja, indicating that particular geochemical signatures are not necessarily confined to the tectonic or structural surface expression of single volcanic systems. This has important implications for the identification and delineation of individual volcanic systems beneath the northwest sector of Vatnajökull.
Key Points
We present new geochemical data for eruptions on the Askja volcanic system
Lateral flow is not required to explain fissure eruptions north of Askja in 1875
The Holuhraun lava is compositionally similar to Veidivotn basalts
The cycling of material from Earth's surface environment into its interior can couple mantle oxidation state to the evolution of the oceans and atmosphere. A major uncertainty in this exchange is ...whether altered oceanic crust entering subduction zones can carry the oxidised signal it inherits during alteration at the ridge into the deep mantle for long-term storage. Recycled oceanic crust may be entrained into mantle upwellings and melt under ocean islands, creating the potential for basalt chemistry to constrain solid Earth–hydrosphere redox coupling.
Numerous independent observations suggest that Iceland contains a significant recycled oceanic crustal component, making it an ideal locality to investigate links between redox proxies and geochemical indices of enrichment. We have interrogated the elemental, isotope and redox geochemistry of basalts from the Reykjanes Ridge, which forms a 700 km transect of the Iceland plume. Over this distance, geophysical and geochemical tracers of plume influence vary dramatically, with the basalts recording both long- and short-wavelength heterogeneity in the Iceland plume. We present new high-precision Fe-XANES measurements of Fe3+/∑Fe on a suite of 64 basalt glasses from the Reykjanes Ridge. These basalts exhibit positive correlations between Fe3+/∑Fe and trace element and isotopic signals of enrichment, and become progressively oxidised towards Iceland: fractionation-corrected Fe3+/∑Fe increases by ∼0.015 and ΔQFM by ∼0.2 log units. We rule out a role for sulfur degassing in creating this trend, and by considering various redox melting processes and metasomatic source enrichment mechanisms, conclude that an intrinsically oxidised component within the Icelandic mantle is required. Given the previous evidence for entrained oceanic crustal material within the Iceland plume, we consider this the most plausible carrier of the oxidised signal.
To determine the ferric iron content of the recycled component (Fe2O3source) we project observed liquid compositions to an estimate of Fe2O3 in the pure enriched endmember melt, and then apply simple fractional melting models, considering lherzolitic and pyroxenitic source mineralogies, to estimate Fe2O3(source) content. Propagating uncertainty through these steps, we obtain a range of Fe2O3(source) for the enriched melts (0.9–1.4 wt%) that is significantly greater than the ferric iron content of typical upper mantle lherzolites. This range of ferric iron contents is consistent with a hybridised lherzolite–basalt (pyroxenite) mantle component. The oxidised signal in enriched Icelandic basalts is therefore potential evidence for seafloor–hydrosphere interaction having oxidised ancient mid-ocean ridge crust, generating a return flux of oxygen into the deep mantle.
•Oxidation correlates with enrichment in Reykjanes Ridge basalts.•Oxidation and enrichment correlate on 1000 km and 10 km length scales.•The geochemistry of the enriched source is consistent with having been recycled.•Recycling of oxidised oceanic crust may contribute to the solid Earth oxygen cycle.
The valence state of Fe plays a vital role in setting and recording the oxidation state of magmas, commonly expressed in terms of oxygen fugacity (
f
O
2
). However, our knowledge of how and why
f
O
...2
varies within and between magmatic systems remains patchy because of diverse challenges associated with estimating the valence state of Fe in glasses and minerals routinely. Here we investigate Fe valence systematics in magmatic clinopyroxene crystals from ocean island basalts (OIBs) erupted in Iceland and the Azores to explore whether they record information about magma Fe
3
+
contents and magmatic
f
O
2
conditions. Although many studies assume that all Fe in augitic clinopyroxene crystals from OIBs occurs as Fe
2
+
, we find that up to half of the total Fe present can occur as Fe
3
+
, with crystals from alkali systems typically containing more Fe
3
+
than those from tholeiitic systems. Thus, Fe
3
+
is a major if under-appreciated constituent of augitic clinopyroxene crystals erupted from ocean island volcanoes. Most Fe
3
+
in these crystals is hosted within esseneite component (CaFe
3
+
AlSiO
6
), though some may be hosted in aegirine component (NaFe
3
+
Si
2
O
6
) in crystals from alkali systems. Observations from samples containing quenched matrix glasses suggest that the incorporation of Fe
3
+
is related to the abundance of tetrahedrally coordinated Al (
IV
Al), implying some steric constraints over Fe
3
+
partitioning between clinopyroxene and liquid (i.e.,
D
Fe
2
O
3
cpx
-
liq
values), though this may not be an equilibrium relationship. For example,
IV
Al-rich
{
h
k
0
}
prism sectors of sector-zoned crystals contain more Fe
3
+
than
IV
Al-poor
{
1
¯
11
}
hourglass sectors. Moreover,
IV
Al-rich compositions formed during disequilibrium crystallisation are enriched in Fe
3
+
. Apparent clinopyroxene-liquid Fe
2
+
–Mg exchange equilibria (i.e.,
K
D
,
Fe
2
+
-
Mg
cpx
-
liq
values) are similarly affected by disequilibrium crystallisation in our samples. Nonetheless, it is possible to reconcile our observed clinopyroxene compositions with glass Fe valence systematics estimated from olivine-liquid equilibria if we assume that
K
D
,
Fe
2
+
-
Mg
cpx
-
liq
values lies closer to experimentally reported values of 0.24
-
0.26 than values of
∼
0.28 returned from a general model. In this case, olivine-liquid and clinopyroxene-liquid equilibria record equivalent narratives, with one of our glassy samples from Iceland recording evolution under
f
O
2
conditions about one log unit above fayalite-magnetite-quartz (FMQ) equilibrium (i.e.,
∼
FMQ+1) and our glassy Azorean sample recording evolution under significantly more oxidising conditions (
≥
FMQ+2.5) before experiencing syn-eruptive reduction, likely as a result of SO
2
degassing; our other glassy sample from Iceland was also affected by reductive SO
2
degassing. Overall, our findings demonstrate that the Fe valence systematics of clinopyroxene crystals can record information about the conditions under which OIBs evolve, but that further experimental work is required to properly disentangle the effects of magma composition, disequilibrium and
f
O
2
conditions on clinopyroxene-liquid equilibria involving Fe
2
+
and Fe
3
+
.
Clinopyroxene is a major rock forming mineral capable of incorporating diverse metal cations. As a consequence, clinopyroxene preserves valuable archives of magmatic processes. Understanding ...clinopyroxene is thus essential for understanding Earth’s wider chemical evolution. However, knowledge about the relative abundances of ferrous and ferric iron in magmatic clinopyroxene remains sparse because it is not currently possible to routinely measure the valence of iron in clinopyroxene crystals without either separating single crystals for bulk analysis or securing access to Mössbauer spectroscopy or a synchrotron radiation source to perform in-situ microanalysis. This is despite magmatic clinopyroxene crystals often containing appreciable quantities of ferric iron that affect its stability and behaviour in currently ill-constrained ways and limit our ability to exploit its chemistry to robustly reconstruct conditions of magma storage and evolution. Here we integrate optimised electron probe microanalysis and Mössbauer spectroscopy on endmember and single-crystal clinopyroxene samples to re-evaluate previously discredited approaches for estimating clinopyroxene ferric iron contents by stoichiometry. By ensuring that we measured all major and minor elements in clinopyroxene crystals with sufficient precision, we show that it is possible to readily obtain stoichiometric estimates of clinopyroxene ferric-to-total iron ratios with similar precisions to those derived from Mössbauer spectroscopy (1
σ
∼
3.5% absolute). Being able to robustly determine clinopyroxene ferric iron contents enables us to propose a new empirical scheme for assigning clinopyroxene components that explicitly accounts for ferric iron, which is primarily hosted within esseneite component (CaFe
3
+
AlSiO
6
) in clinopyroxenes dominated by quadrilateral components and aegirine component (NaFe
3
+
Si
2
O
6
) in alkali clinopyroxenes. Our new scheme provides a framework for documenting the full spectrum of clinopyroxene compositions in both natural and experimental systems when analyses are performed with sufficient precision.
Petrological constraints on the timescales of pre-eruptive crystal storage and magma degassing provide an important framework for the interpretation of seismic, geodetic and gas monitoring data in ...volcanically active regions. We have used Fe–Mg diffusion chronometry in 86 olivine macrocrysts from the AD 1783–1784 Laki eruption on Iceland's Eastern Volcanic Zone to characterise timescales of crystal storage and transport in the lead-up to this eruption. The majority of these olivines have core compositions of Fo<76, and rim compositions in the range Fo69–Fo74 that are close to equilibrium with the Laki melt. Diffusion modelling using the greyscale intensity of backscattered electron images as a proxy for olivine composition reveals that the most probable Fe–Mg diffusion timescale for Laki olivines is 7.8 days, which reflects the characteristic olivine residence time in the carrier melt prior to eruption. A small population of Fo>81 olivines record Fe–Mg diffusion timescales of ∼124 days; these crystals are likely to have formed in mid-crustal magma chambers, been transferred to storage at shallower levels and then entrained into the Laki melt prior to eruption. Typical Fe–Mg diffusion timescales of 6–10 days are shorter than the average time interval between discrete episodes of the Laki eruption, indicating variable or pulsed disaggregation of stored crystals into the carrier liquid prior to the onset of each episode. The diffusion timescales coincide with historical accounts of strong and frequent earthquakes in southeast Iceland, which we interpret as being associated with mush disaggregation related to melt withdrawal and the initiation of dyke propagation from a crustal magma reservoir at ∼6±3 km depth to the surface. We calculate pre-eruptive CO2 fluxes of 2–6 Mt d−1, assuming a pre-eruptive CO2 outgassing budget of 189.6 Mt for the Laki eruption and a constant rate of CO2 release in the 6–10 days preceding each eruptive episode. Our dataset indicates that petrological constraints on the timescales of magmatic processes occurring in the days leading up to historic eruptions may enhance our ability to forecast the onset of future large eruptions, both in Iceland and further afield.
•Fe–Mg diffusion in olivine is used to characterise timescales of crystal storage and transport.•Olivines were entrained into their carrier liquid 6–10 days prior to the Laki eruption.•Diffusion timescales are comparable to historical records of pre-eruptive seismicity.•CO2 fluxes in the 6–10 days preceding eruption were 2–6 Mt/d.•CO2 flux monitoring may enhance accuracy of eruption forecasting in Iceland.
The 2014–2015 Holuhraun eruption, on the Bárðarbunga volcanic system in central Iceland, was one of the best-monitored basaltic fissure eruptions that has ever occurred, and presents a unique ...opportunity to link petrological and geochemical data with geophysical observations during a major rifting episode. We present major and trace element analyses of melt inclusions and matrix glasses from a suite of ten samples collected over the course of the Holuhraun eruption. The diversity of trace element ratios such as La/Yb in Holuhraun melt inclusions reveals that the magma evolved via concurrent mixing and crystallization of diverse primary melts in the mid-crust. Using olivine–plagioclase–augite–melt (OPAM) barometry, we calculate that the Holuhraun carrier melt equilibrated at 2.1 ± 0.7 kbar (7.5 ± 2.5 km), which is in agreement with the depths of earthquakes (6 ± 1 km) between Bárðarbunga central volcano and the eruption site in the days preceding eruption onset. Using the same approach, melt inclusions equilibrated at pressures between 0.5 and 8.0 kbar, with the most probable pressure being 3.2 kbar. Diffusion chronometry reveals minimum residence timescales of 1–12 days for melt inclusion-bearing macrocrysts in the Holuhraun carrier melt. By combining timescales of diffusive dehydration of melt inclusions with the calculated pressure of H
2
O saturation for the Holuhraun magma, we calculate indicative magma ascent rates of 0.12–0.29 m s
−1
. Our petrological and geochemical data are consistent with lateral magma transport from Bárðarbunga volcano to the eruption site in a shallow- to mid-crustal dyke, as has been suggested on the basis of seismic and geodetic datasets. This result is a significant step forward in reconciling petrological and geophysical interpretations of magma transport during volcano-tectonic episodes, and provides a critical framework for the interpretation of premonitory seismic and geodetic data in volcanically active regions.
•H2O contents of olivine-hosted melt inclusions from Iceland are controlled by H+ diffusion.•Trace element depleted inclusions are over-hydrated by diffusive H+ gain.•Over-hydration is a predicted ...consequence of concurrent mixing and crystallisation of diverse primary melts.•Trace element data are required to determine melt H2O content at the time of inclusion trapping.
The pre-eruptive water content of magma is often estimated using crystal-hosted melt inclusions. However, olivine-hosted melt inclusions are prone to post-entrapment modification by H+ diffusion as they re-equilibrate with their external environment. This effect is well established for the case of H+ loss from olivine-hosted inclusions that have cooled slowly in degassed magma. Here we present evidence for the opposite effect: the addition of H+ into inclusions that are held in melts that are enriched in H2O with respect to the trapped melts. The compositional variability in a suite of 211 olivine-hosted inclusions from the Laki and Skuggafjöll eruptions in Iceland's Eastern Volcanic Zone indicates that diffusive H+ gain governs the H2O content of incompatible trace element depleted inclusions. Individual eruptive units contain olivine-hosted inclusions with widely varying incompatible element concentrations but near-constant H2O. Furthermore, over 40% of the inclusions have H2O/Ce>380, significantly higher than the H2O/Ce expected in primary Icelandic melts or mid-ocean ridge basalts (150–280). The fact that the highest H2O/Ce ratios are found in the most incompatible element depleted inclusions indicates that hydration is a consequence of the concurrent mixing and crystallisation of compositionally diverse primary melts. Hydration occurs when olivines containing depleted inclusions with low H2O contents are juxtaposed against more hydrous melts during mixing. Melt inclusions from a single eruption may preserve evidence of both diffusive H+ loss and H+ gain. Trace element data are therefore vital for determining H2O contents of melt inclusions at the time of inclusion trapping and, ultimately, the H2O content of the mantle source regions.
Magma mixing and crystal mush disaggregation are important processes in basaltic magma reservoirs. We carried out a detailed petrological and geochemical study on a highly plagioclase-phyric eruption ...within the Eastern Volcanic Zone of Iceland-the Skuggafjoll eruption-to investigate crystal storage and transport processes within a single magmatic system. Crystal content and phase proportions vary between samples: the least phyric samples have phase proportions similar to the low-pressure, three-phase gabbro eutectic (plg:cpx:ol 11:6:3), whereas highly phyric samples are strongly enriched in plagioclase (plg:cpx:ol 8:1:1). Statistically significant geochemical variability in 28 whole-rock samples collected across the eruption can be accounted for by variable accumulation of a troctolitic assemblage containing plagioclase and olivine in an approximately 9:1 ratio. Two macrocryst assemblages are defined using compositional and textural information recorded in QEMSCAN registered images: a primitive assemblage of high-anorthite plagioclase (An sub(>83)) and high-forsterite olivine (Fo sub(>84)), and an evolved assemblage of low-anorthite plagioclase (An sub(<79)), low-forsterite olivine (Fo sub(<82)) and clinopyroxene (Mg# 82). Plagioclase and olivine have strongly bimodal composition distributions whereas the composition distribution of clinopyroxene is unimodal. The mean trace element composition of melt inclusions hosted within high-forsterite olivine and high-anorthite plagioclase macrocrysts is the same (mean Ce/Y 0.47-0.48), confirming that both primitive macrocryst phases crystallized from the same distribution of melts. Clinopyroxene macrocrysts and matrix glasses are in Ce/Yb equilibrium with each other, indicating that the evolved assemblage crystallized from melts with a more incompatible trace element-enriched composition (mean Ce/Y 0.65-71) than the primitive assemblage. Variability in whole-rock, macrocryst and melt inclusion compositions suggests that the Skuggafjoll magma experienced two stages of crystallization. Primitive macrocrysts crystallized first from incompatible trace element-depleted melts within a shallow crustal magma reservoir. These primitive macrocrysts were subsequently stored in crystal mushes that ultimately disaggregated into an evolved and incompatible trace element-enriched magma from which the evolved assemblage crystallized. On average, 17% of the erupted magma at Skuggafjoll is composed of accumulated macrocrysts entrained from crystal mushes. The timescale between mush disaggregation and eruption, during which crystal accumulation occurred, was short-of the order of years-according to simple diffusion calculations. Striking petrological similarities between Skuggafjoll and other highly phyric eruptions both in Iceland and along mid-ocean ridges indicate that crystal accumulation by mush disaggregation is likely to be an important mechanism for generating highly phyric magmas in basaltic plumbing systems.
PDF
