Shrinkage Bubbles Research Articles

Fast, furious, and gassy: Etna's explosive eruption from the mantle

The 3930 BP Fall Stratified (FS) eruption at Mt. Etna is a rare example of a highly explosive eruption of primitive (picritic) magma directly from the mantle. The eruption produced ash plumes up to an estimated 20 km height, leading to a volcanic explosivity index (VEI) 4 (subplinian). Given its volatile-rich and primitive nature, the FS magma may have ascended rapidly from great depths to avoid fractionation and mixing within the extensive plumbing system beneath Etna. To determine the pressures from which the FS magma derived, we perform rehomogenization experiments on melt inclusions hosted in Fo90–91 olivines to resorb shrinkage bubbles and determine the initial H2O and CO2 in the melt. With measured CO2 concentrations of up to 9600 ppm, volatile solubility models yield magma storage pressures of 630–800 MPa. These correspond to depths of 24–30 km, which are comparable to the seismologically estimated Moho. Therefore, the magma's high CO2 concentration must come from carbon in the mantle (likely from subducted carbonates), as opposed to assimilation of shallow (<10 km) crustal carbonates.Diffusion modeling of H2O and forsterite zonation profiles in clear, euhedral, and crystallographically oriented olivines indicates rapid ascent of magma directly from its source region to the surface. Forsterite profiles exhibit a narrow rim of growth zoning but no detectable diffusional zoning, reflecting maximum ascent times of 1–5 days. Eighteen measured H2O profiles result in remarkably uniform decompression rates of 0.47 MPa/s (95% confidence interval of 0.16–1.28 MPa/s), which is among the fastest measured for basaltic-intermediate magmas. These decompression rates indicate that the final stage of magma ascent over the region in which H2O degasses (between the surface and ∼ 15 km) occurred extremely fast at ∼ 17.5 m/s. This eruption may provide a link between primary magma composition and eruption intensity: we propose that the unusually explosive nature of this picritic eruption was driven by high H2O and CO2 concentrations, which led to continuously rapid ascent without stalling, all the way from the Moho.

Compositions of Kimberlite Melts: A Review of Melt Inclusions in Kimberlite Minerals

This work is a comprehensive review of existing data on melt inclusions entrapped in minerals of kimberlite rocks emplaced in different cratonic settings at different times. The crystallized melt inclusions represent snapshots of kimberlite melts at different stages of their evolution. All inclusions are composed of daughter minerals and shrinkage bubbles, but no aqueous fluids and quenched silicate glasses have been so far found. More than 60 mineral species were identified among the daughter phases in the inclusions, however, such diverse phase assemblages are typical of all kimberlites studied to date. Daughter minerals are represented by various Na-K-Ca-, Na-Ca-, Na-Mg-, K-Ca-, Ca-Mg-, Ca-, Mg-, Na-carbonates, Na-Mg- and Na-carbonates with additional anions Сl–, \({\text{SO}}_{4}^{{2 - }},\) \({\text{PO}}_{4}^{{3 - }},\) alkali- sulfates, chlorides, phosphates, sulfides, oxides and silicates. Alkali carbonates, sulfates, and chlorides are usually absent among groundmass minerals the majority of kimberlites in the world, except the Udachnaya-East kimberlite in Siberia. On the other hand, this paragenesis in association with traditional kimberlite minerals, such as olivine, micas, monticellite, spinel group minerals, perovskite, rutile, ilmenite, calcite, and dolomite, is common in the crystallized melt inclusions in all studied kimberlites. Carbonates (~30 to 85 vol. %) always dominate over silicates (only up to 18 vol. %) within inclusions. All inclusions also contain variable (2 to 55 vol. %.) amounts of chlorides (halite and sylvite). When relatively low abundances of carbonate minerals (30–50 vol. %) are observed in the inclusions, chlorides (18–55 vol. %) appear to take over other minerals, including silicates that are traditionally considered as main components of “ultramafic” kimberlite parental melts. The published results on melt inclusions in the kimberlite minerals strongly imply that parental kimberlite melts were generated and further evolved within the Na2O-K2O-CaO-MgO-CO2-Cl system, that is, they were alkali-rich carbonatite/carbonatite-chloride liquids. According to various estimates, the SiO2 content in kimberlite melts at different stages of their evolution could have varied from the first to 19 wt. %. Obviously, during and after of а kimberlite bodies formation, interaction with external waters leads to serpentinization of kimberlite olivine and dissolution of a significant part of kimberlite igneous minerals, such as alkaline carbonates, sulfates, and chlorides. In the traditional approach to studying kimberlites, the role of components such as Na2O, CO2, Cl, and to a lesser extent K2O and S, F in the petrogenesis of kimberlite magmas and rocks have been largely underestimated, while olivine- and serpentine-forming components, such as of SiO2, MgO and H2O are still overestimated in contemporaneous literature.

A melt inclusion approach to reconstructing sulfur contents and sulfide saturation of primitive basaltic melts

The study of melt inclusions in minerals is the only direct approach to estimate the contents of volatile components in silicate melts, which are essential to constrain the processes of magma generation and evolution. In particular, the content of sulfur in silicate melts is closely connected with the P-T-fO2 conditions of mantle melting and the formation of sulfide deposits. Inclusions in high-Mg olivine can provide access to the composition of the most primitive melts, but these are the most susceptible to post-entrapment modification such as ‘Fe-loss’. The loss of Fe consequently decreases sulfur solubility and leads to silicate-sulfide immiscibility and formation of daughter sulfide globules, significantly affecting the initial proportions of S contained within the silicate part (glass) of melt inclusions. In this study, we investigated the budget of sulfur in heated and quenched melt inclusions hosted in primitive olivine (87–90 mol% Fo) by accounting for sulfur stored in silicate glass (melt), daughter sulfide globules and shrinkage bubbles. Melt inclusions from MORB-like olivine-phyric rocks (Kamchatsky Mys, Far East Russia) heated for 30 min at 1200 °C and 1300 °C were compared with similar experiments on olivine-hosted melt inclusions over a 5 min duration (at 1200 °C and 1350 °C). Sulfur contents were calculated in the following types of melt inclusions, containing: (i) both sulfide globules and shrinkage bubbles, (ii) only sulfide globules, (iii) only shrinkage bubbles, and (iv) completely homogenized. Our findings indicate that S content in shrinkage bubbles and its precipitates is negligible in reheated water-poor melt inclusions, whereas daughter sulfide globules are significant repositories of initial S. Therefore, when estimating initial S contents in parental melts, the amounts stored in both daughter sulfide and silicate glass should be considered. Our experiments with MORB-type melt inclusions in forsteritic olivine phenocrysts demonstrate that effective homogenization is best achieved by briefly overheating melt inclusions prior to rapid quenching. Although this technique does not allow for the MgFe equilibrium between melt inclusions and their host olivine to be fully achieved, it promotes the dissolution of daughter sulfide globules and shrinkage bubbles, and thus the release of initial sulfur back to the melt/glass.

High-pressure homogenization of olivine-hosted CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;-rich melt inclusions in a piston cylinder: insight into the volatile content of primary mantle melts

Abstract. Experimental homogenization of olivine-hosted melt inclusions representative of near-primary basic and ultrabasic magmas is a powerful approach to investigate the nature of their source regions and the melting conditions in Earth's mantle. There is growing evidence that the total CO2 contents of olivine-hosted melt inclusions may reach values of the order of a single to several weight percent, especially in intraplate continental basalts. To be able to homogenize melt inclusions with such high CO2 contents, we developed a technique allowing for heat treating of the melt inclusions under hydrostatic pressures up to 3–4 GPa in a piston cylinder, using thick-walled Au80–Pd20 containers and molten NaCl as the surrounding medium for the inclusion-bearing olivines. We applied this technique to olivine phenocrysts from Thueyts basanite, Bas-Vivarais volcanic province, French Massif Central. Thueyts melt inclusions were chosen because of their high CO2 contents, as indicated by up to 1.19 wt % dissolved CO2 in the glasses and by the presence of shrinkage bubbles containing abundant carbonate microcrystals in addition to a CO2 fluid phase. The homogenization experiments were conducted at pressures of 1.5 to 2.5 GPa, temperatures of 1275 and 1300 ∘C, and run durations of 30 min. In all the melt inclusions treated at 2.5 GPa–1300 ∘C and half of those treated at 2 GPa–1300 ∘C, we were able to completely homogenize the inclusions, as indicated by the disappearance of the starting bubbles, and we obtained total CO2 contents ranging from 3.2 wt % to 4.3 wt % (3.7 wt % on average). In all the other melt inclusions (equilibrated at 1.5 or 2 GPa and 1300 ∘C or at 2.5 GPa–1275 ∘C), we obtained lower and more variable total CO2 contents (1.4 wt % to 2.9 wt %). In the inclusions with the highest total CO2 contents, the size of the shrinkage bubble was in most cases small (&lt;5 vol %) to medium (&lt;10 vol %): this is a strong argument in favor of an origin of these melt inclusions by homogeneous entrapment of very CO2-rich basanitic liquids (∼ 4 wt %) at pressures of 2 to 2.5 GPa. The lower total CO2 contents measured in some inclusions could reflect a natural variability in the initial CO2 contents, due for instance to melt entrapment at different pressures, or CO2 loss by decrepitation. An alternative scenario is heterogeneous entrapment of basanitic liquid plus dense CO2 fluid at lower pressures but still at least on the order of 1 GPa as indicated by dissolved CO2 contents up to 1.19 wt % in the glasses of unheated melt inclusions. Whatever the scenario, the basanites from the Bas-Vivarais volcanic province were generated in a mantle environment extremely rich in carbon dioxide.

Noble gas variation during partial crustal melting and magma ascent processes

Noble gas isotopes, although present in trace amounts, are generally more reliable and less ambiguous recorders of their source than the major volatile species. In volcanic settings in particular, this advantage derives from their chemical inertness, as noble gas isotopic and elemental fractionations are strongly coupled to their source and modified only by physical processes during magma ascent and eruption. The Neogene volcano El Hoyazo (Betic Cordillera, SE Spain) is a highly favourable natural laboratory to study the links between partial crustal melting processes occurring at depth and the eruptive products at the surface, because partially melted crustal xenoliths are preserved in silicic lavas. Comparing the noble gas isotopic compositions of xenoliths and lavas has the potential to yield new insights into volatile behaviour during melting processes at inaccessible depths in the crust.At El Hoyazo, noble gases trapped in lava glasses, and the fluid/melt inclusions within xeno- and phenocrysts, provide novel information on: (i) their response to the crustal melting process including mechanisms such as magma mixing (and crustal assimilation) of two endmembers: i.e. the extracted felsic melt from the country metapelitic crust, and the basic-intermediate magma from the underplating in the region. The results reveal significant modification of magmatic noble gases by the interaction with the partially melted crust; (ii) noble gas variations during degassing and magma ascent, showing higher atmospheric influence in the lava samples from shallower depths than in the deeper lavas and minerals; and (iii) higher magmatic influence in crystals of garnet from deeper lava than in both shallower crystals of amphibole, and garnet crystals within the crustal xenoliths. In addition, we find that noble gases in melt inclusions are also likely accumulating in their shrinkage bubbles, and not only remaining dissolved in the melt.

MushPEC: Correcting Post-entrapment Processes Affecting Melt Inclusions Hosted in Olivine Antecrysts

Olivine-hosted melt inclusions (MIs) are widely used as a tool to study the early stages of magmatic evolution. There are a series of processes that affect MI compositions after trapping, including post-entrapment crystallization (PEC) of the host mineral at the MI boundaries, exsolution of volatile phases into a “shrinkage bubble” and diffusive exchange between a MI and its host. Classical correction schemes applied to olivine-hosted MIs include PEC correction through addition of olivine back to the melt until it reaches equilibrium with the host composition and “Fe-loss” correction due to Fe-Mg diffusive exchange. These corrections rely on the assumption that the original host composition is preserved. However, for many volcanic samples the crystal cargo is thought to be antecrystic, and the olivine composition may thus have been completely re-equilibrated during long crystal storage times. Here, we develop a novel MI correction scheme that is applicable when the original host crystal composition has not been preserved and the initial MI composition variability can be represented by simple fractional crystallization (FC). The new scheme allows correction of MI compositions in antecrystic hosts with long and varied temperature histories. The correction fits a set of MI compositions to modelled liquid lines of descent generated by FC. A MATLAB®script (called MushPEC) iterates FC simulations using the rhyolite-MELTS algorithm. In addition to obtaining the corrected MI compositions, the application of this methodology provides estimations of magmatic conditions during MI entrapment. A set of MIs hosted in olivine crystals of homogeneous composition (Fo77–78) from a basaltic tephra of Akita-Komagatake volcano was used to test the methodology. The tephra sample shows evidence of re-equilibration of the MIs to a narrow Mg# range equivalent to the carrier melt composition. The correction shows that olivine hosts were stored in the upper crust (c.125 – 150 MPa) at undersaturated H2O contents ofc. 1 – 2 wt% H2O).

Quantifying magmatic volatiles by Raman microtomography of glass inclusion-hosted bubbles

We present a novel application of Raman microtomography for quantitative characterisation of glass inclusion-hosted bubbles, which allows for the simultaneous identification and volumetric quantification of mineral and fluid phases filling the bubble. The combination of Raman microtomography with synchrotron XRF mapping and scanning electron microscopy provides a complete compositional and textural characterisation of the bubble. In the studied samples, minerals are systematically present on the walls of the bubbles: dominantly carbonates in samples from continental intraplate and hotspot volcanic provinces, and sulfates in the sample from subductionrelated settings. Along with fluid CO 2 , carbonates sequester 65 to 84 % of the CO 2 originally dissolved in the melt, while 18 to 60 % of the sulfur contained in the inclusion is stored in sulfides and/or sulfates. Thus, the total melt inclusion CO 2 and S contents can be underestimated (by up to ∼ 40 % and 60 %, respectively) if minerals in the bubbles are neglected. This study highlights the importance of 3D mapping of shrinkage bubbles hosted in glass inclusions for a better assessment of the bulk pre-eruptive contents of volatiles in magmas.

Anatectic melt inclusions in ultra high temperature granulites

AbstractPartial melting up to ultra high temperature (UHT) conditions is one of the major processes for the geochemical differentiation and reworking of the mid‐ to lower continental crust, with relevant implications on its rheological behaviour. UHT granulites from the Gruf Complex (European Central Alps) display garnet and sapphirine porphyroblasts containing a variety of primary melt inclusions (MI). Typically, MI in garnet occur as glassy and polycrystalline inclusions (i.e. nanogranitoids), the latter commonly organized in mm‐sized clusters associated with primary fluid inclusions (FI). Nanogranitoids are characterized by an elliptical faceted shape, with variable sizes ranging from 2 to 115 µm, while glassy inclusions show negative crystal shapes that usually never exceed 15 µm in diameter and present CO2‐rich shrinkage bubbles. The characteristic mineral assemblage observed in nanogranitoids consists of quartz, biotite, muscovite, plagioclase, K‐feldspar, kokchetavite and rarely aluminosilicates. Glassy and re‐homogenized MI are peraluminous and rhyolitic in composition, with SiO2 = 69 − 80 wt% and Na2O + K2O = 5 − 12 wt%. Commonly, the analysed MI have very high K2O (&gt;6 wt%) and very low Na2O (&lt;2 wt%) contents, indicative for potassic to ultrapotassic melts. Measured H2O contents of the melts range from 2.9 to 8.8 wt%, whereas CO2 concentrations are between 160 and 1738 ppm. Accordingly, calculated viscosities for re‐homogenized MI vary between 104 and 105 Pa·s. Related primary FI mainly contain CO2, with rare occurrence of CO and N2, and are commonly associated with quartz, as well as different carbonates and phyllosilicates. It is assumed that the source for the carbonic fluid was external and probably related to the degassing lithospheric mantle. Consequently, it is argued that anatexis was initially triggered by incongruent dehydration melting reactions involving biotite breakdown and proceeded in the presence of an externally derived COH‐bearing fluid. The coexistence of COH‐bearing fluid and MI indicates that partial melting occurred under conditions of fluid − melt immiscibility. Potassic to ultrapotassic MI in UHT granulites suggests that lower crustal anatexis may play a significant role in the redistribution of heat‐producing elements (such as K2O), potentially influencing the thermal structure of the continental crust.

Determination of total CO2 in melt inclusions with shrinkage bubbles

The compositions of volatiles in magmas can be estimated using melt inclusions encapsulated in phenocrysts. Shrinkage bubbles are occasionally observed in natural and rehomogenized melt inclusions. Because of the low solubility of CO2 in melts, CO2 is partitioned in both the melts and shrinkage bubbles in melt inclusions. We developed a method to determine total CO2 in melt inclusions by measuring (1) CO2 concentration in glass with secondary-ion mass spectrometry, (2) CO2 density in bubbles with micro-Raman spectrometry, and (3) bubble/glass volume ratio with micro X-ray computed tomography on rehomogenized melt inclusions. We applied this method to olivine-hosted melt inclusions in high-μ (HIMU)-type ocean island basalts from Raivavae in Austral Islands in French Polynesia.The shape of melt inclusions is predominantly related to their size; larger melt inclusions tend to be more angular than smaller ones. The volume of large melt inclusions may be overestimated by previous approaches in which the dimension of melt inclusions was measured by microscopy and the volume was calculated assuming ellipsoidal shape. CO2 dissolved in glass in rehomogenized melt inclusions ranges up to ~7000 ppm. The glass with a more alkalic composition shows greater CO2 concentration than that with a less alkalic composition in accordance with the composition-dependent CO2 solubility. By contrast, the correlation between the total CO2 content and the chemical composition of the glass is unclear because CO2 in shrinkage bubbles is generally greater than that in the glass. Some melt inclusions demonstrate an extremely high total CO2 content (up to 25,000 ppm). Because these melt inclusions show a large bubble volume proportion in melt inclusions (>12 vol%), they are likely formed by the simultaneous entrapment of melts and fluids during the host olivine growth. Excluding these melt inclusions, the total CO2 content and CO2/Nb ratio in melt inclusions were up to 7000 ppm and 200, respectively. This study offers an accurate determination of CO2 concentration and CO2/Nb of the HIMU basalts from Raivavae.

A high carbon content of the Hawaiian mantle from olivine-hosted melt inclusions

Abstract The deep mantle carbon content and flux are fundamental quantities in understanding global volatile cycles and distributions. Here, we present CO2 concentrations measured in 407 olivine-hosted melt inclusions from Hualalai, Kilauea, Koolau, Loihi, and Mauna Loa to constrain the Hawaiian mantle CO2 content and flux. Quantification of melt inclusion CO2 is complicated by the ubiquitous presence of vapor or “shrinkage” bubbles. The contribution from exsolved shrinkage bubble CO2 was determined from the measured bubble size and a CO2 equation of state, and added to the dissolved CO2 to reconstruct total melt inclusion CO2 concentrations. Bubbles typically contain ∼90% of melt inclusion C, much of which may be sequestered in precipitated phases on bubble walls, and thus not amenable to measurement by Raman spectroscopy. Based on our dataset of total (dissolved + bubble) CO2 concentrations, we estimate that parental melts from the five Hawaiian volcanoes have CO2 concentrations ranging from 3900 to 10,000 ppm CO2. Among the active volcanoes, CO2 concentrations decrease to the northwest, likely reflecting mantle source heterogeneity, although differences in CO2 degassing related to the relative depths of the magma chambers may also play a role. Mantle sources of the Hawaiian volcanoes range from 380 to 480 ppm CO2 suggesting that the Hawaiian plume is at least a factor of ∼4 more C-rich than the upper mantle sampled by mid-ocean ridge basalts. This enrichment is likely due to the presence of recycled surficial C and/or C-rich primitive material in the Hawaiian mantle.

CO2-rich phonolitic melt and carbonatite immiscibility in early stage of rifting: Melt inclusions from Hanang volcano (Tanzania)

Hanang volcano is the southern volcano of, the southern area of the east part of the East African Rift (the North Tanzanian Divergence) and represents volcanic activity of the first stage of continental break-up. In this study, we investigate glassy melt inclusions in nepheline phenocrysts to constrain the late stage of Mg-poor nephelinite evolution and the behaviour of volatiles (CO2, H2O, S, F, Cl) during magma storage and ascent during early stage rifting. The melt inclusions have a green silicate glass, a carbonate phase and a shrinkage bubble free of gas phase indicating that carbonatite:silicate (18:82) liquid immiscibility occurred during nephelinite magmatic evolution. The silicate glasses have trachytic composition (Na + K/Al = 1.6–7.2, SiO2 = 54–65.5 wt%) with high CO2 (0.43 wt% CO2), sulfur (0.21–0.92 wt% S) and halogens (0.28–0.84 wt% Cl; 0.35–2.54 wt% F) contents and very low H2O content (<0.1 wt%). The carbonate phase is an anhydrous Ca-Na-K-S carbonate with 33 wt% CaO, 20 wt% Na2O, 3 wt% K2O, and 3 wt% S. The entrapped melt in nepheline corresponds to evolved interstitial CO2-rich phonolitic composition (Na + K/Al = 6.2–6.9) with 6 ± 1.5 wt% CO2 at pressure of 800 ± 200 MPa after crystallization of cpx (17%), nepheline (40%) garnet (6.5%) and apatite (1.7%) from Mg-rich nephelinitic magma. During ascent, immiscibility in phonolitic melt inclusions leads to Ca-Na carbonate melt with composition within the range of carbonate melt from Oldoinyo Lengai and Kerimasi, in equilibrium with trachytic silicate melt (closed-system, P < 500 MPa). The CO2-rich phonolitic melt inclusions from Hanang volcano may represent an early stage of differentiation before Na‑carbonatitic magmatism observed at Oldoinyo Lengai.

Volatile and light lithophile elements in high-anorthite plagioclase-hosted melt inclusions from Iceland

Melt inclusions formed during the early stages of magmatic evolution trap primitive melt compositions and enable the volatile contents of primary melts and the mantle to be estimated. However, the syn- and post-entrapment behaviour of volatiles in primitive high-anorthite plagioclase-hosted melt inclusions from oceanic basalts remains poorly constrained. To address this deficit, we present volatile and light lithophile element analyses from a well-characterised suite of nine matrix glasses and 102 melt inclusions from the 10ka Grímsvötn tephra series (i.e., Saksunarvatn ash) of Iceland’s Eastern Volcanic Zone (EVZ). High matrix glass H2O and S contents indicate that eruption-related exsolution was arrested by quenching in a phreatomagmatic setting; Li, B, F and Cl did not exsolve during eruption. The almost uniformly low CO2 content of plagioclase-hosted melt inclusions cannot be explained by either shallow entrapment or the sequestration of CO2 into shrinkage bubbles, suggesting that inclusion CO2 contents were controlled by decrepitation instead. High H2O/Ce values in primitive plagioclase-hosted inclusions (182–823) generally exceed values expected for EVZ primary melts (∼180), and can be accounted for by diffusive H2O gain following the entrainment of primitive macrocrysts into evolved and H2O-rich melts a few days before eruption. A strong positive correlation between H2O and Li in plagioclase-hosted inclusions suggests that diffusive Li gain may also have occurred. Extreme F enrichments in primitive plagioclase-hosted inclusions (F/Nd=51–216 versus ∼15 in matrix glasses) possibly reflect the entrapment of inclusions from high-Al/(Al+Si) melt pools formed by dissolution-crystallisation processes (as indicated by HFSE depletions in some inclusions), and into which F was concentrated by uphill diffusion since F is highly soluble in Al-rich melts. The high S/Dy of primitive inclusions (∼300) indicates that primary melts were S-rich in comparison with most oceanic basalts. Cl and B are unfractionated from similarly compatible trace elements, and preserve records of primary melt heterogeneity. Although primitive plagioclase-hosted melt inclusions from the 10ka Grímsvötn tephra series record few primary signals in their volatile contents, they nevertheless record information about crustal magma processing that is not captured in olivine-hosted melt inclusions suites.

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

Basaltic volcanoes transfer volatiles from the mantle to the surface of the Earth. The quantification of deep volatile fluxes relies heavily on estimates of the volatile content of primitive magmas, the best archive of which is provided by melt inclusions. Available data from volcanoes producing mafic alkaline lavas in a range of tectonic settings suggest high volatile fluxes, but information remains sparse, particularly for intraplate ocean islands. Here we present measurements of volatile and trace element concentrations, as well as sulphur speciation, in olivine-hosted melt inclusions and matrix glasses from quenched basanite lava balloon samples from the 2011–2012 submarine eruption at El Hierro, Canary Islands. The results reveal remarkably high concentrations of dissolved volatiles and incompatible trace elements in this magma, with ∼80 ppm Nb and up to 3420 ppm CO2, 3.0 wt.% H2O and 5080 ppm S. Reconstructed primitive CO2 contents, considering CO2/Nb systematics and possible CO2 sequestration in shrinkage bubbles, reach weight percent levels, indicating that carbon is a major constituent of Canary Island magmas at depth and that exsolution of a CO2-rich fluid begins in the mantle at pressures in excess of 1 GPa. Correlations between sulphur concentration, sulphur speciation and water content suggest strong reduction of an initially oxidised mantle magma, likely controlled by coupled H2O and S degassing. This late-stage redox change may have triggered sulphide saturation, recorded by globular sulphide inclusions in clinopyroxene and ulvöspinel. The El Hierro basanite thus had a particularly high volatile-carrying capacity and released a minimum of 1.3–2.1 Tg CO2 and 1.8–2.9 Tg S to the environment, causing substantial stress on the local submarine ecosystem. These results highlight the important contribution of alkaline ocean island volcanoes, such as the Canary Islands, to volatile fluxes from the mantle.

Volatile contents of mafic-to-intermediate magmas at San Cristóbal volcano in Nicaragua

San Cristóbal volcano in northwest Nicaragua is one of the most active basaltic–andesitic stratovolcanoes of the Central American Volcanic Arc (CAVA). Here we provide novel constraints on the volcano's magmatic plumbing system, by presenting the first direct measurements of major volatile contents in mafic-to-intermediate glass inclusions from Holocene and historic-present volcanic activity. Olivine-hosted (forsterite [Fo] <80; Fo<80) glass inclusions from Holocene tephra layers contain moderate amounts of H2O (0.1–3.3wt%) and S and Cl up to 2500μg/g, and define the mafic (basaltic) endmember component. Historic-present scoriae and tephra layers exhibit more-evolved olivines (Fo69–72) that contain distinctly lower volatile contents (0.1–2.2wt% H2O, 760–1675μg/gS, and 1021–1970μg/g Cl), and represent a more-evolved basaltic–andesitic magma. All glass inclusions are relatively poor in CO2, with contents reaching 527μg/g (as measured by nanoscale secondary ion mass spectrometry), suggesting pre- to postentrapment CO2 loss to a magmatic vapor. We use results of Raman spectroscopy obtained in a population of small (<50μm) inclusions with CO2-bearing shrinkage bubbles (3–12μm) to correct for postentrapment CO2 loss to bubbles, and to estimate the original minimum CO2 content in San Cristóbal parental melts at ~1889μg/g, which is consistent with the less-CO2-degassed melt inclusions (MI) (>1500μg/g) found in Nicaragua at Cerro Negro, Nejapa, and Granada. Models of H2O and CO2 solubilities constrain the degassing pathway of magmas up to 425MPa (~16km depth), which includes a deep CO2 degassing step (only partially preserved in the MI record), followed by coupled degassing of H2O and S plus crystal fractionation at magma volatile saturation pressures from ∼195 to <10MPa. The variation in volatile contents from San Cristóbal MI is interpreted to reflect (1) Holocene eruptive cycles characterized by the rapid emplacement of basaltic magma batches, saturated in volatiles, at depths of 3.8–7.4km, and (2) the ascent of more-differentiated and cogenetic volatile-poor basaltic andesites during historic-present eruptions, having longer residence times in the shallowest (<3.4km) and hence coolest regions of the magmatic plumbing system. We also report the first measurements of the compositions of noble-gas isotopes (He, Ne, and Ar) in fluid inclusions in olivine and pyroxene crystals. While the measured 40Ar/36Ar ratios (300–304) and 4He/20Ne ratios (9–373) indicate some degree of air contamination, the 3He/4He ratios (7.01–7.20Ra) support a common mantle source for Holocene basalts and historic-present basaltic andesites. The magmatic source is interpreted as generated by a primitive MORB-like mantle, that is influenced to variable extents by distinct slab fluid components for basalts (Ba/La~76 and U/Th~0.8) and basaltic andesites (Ba/La~86 and U/Th~1.0) in addition to effects of magma differentiation. These values for the geochemical markers are particularly high, and their correlation with strong plume CO2/S ratios from San Cristóbal is highly consistent with volatile recycling at the CAVA subduction zone, where sediment involvement in mantle fluids influences the typical relatively C-rich signature of volcanic gases in Nicaragua.

Oxygen isotopes and volatile contents of the Gorgona komatiites, Colombia: A confirmation of the deep mantle origin of H2O

We report O isotopes in olivine grains (Fo89–93) and volatile contents (CO2, H2O, F, S, Cl) in olivine-hosted melt inclusions from one Gorgona picrite and five komatiites with the aim of constraining the origin of H2O in these magmas. These samples have previously been analysed for major and trace elements and volatile concentrations (H2O, S, Cl) and B isotopes in melt inclusions. A distinctive feature of the included melts is relatively high contents of volatile components and boron, which show positive anomalies in, otherwise depleted, primitive mantle normalised trace and rare earth element patterns and range in δ11B from −11.5 to 15.6‰. In this study, the olivines were systematically analysed for O isotopes (1) in the centre of grains, (2) near the grain boundaries and, (3) as close as possible to the studied melt inclusions. The majority of olivines (∼66%) are “mantle”-like, 4.8‰≤δ18O≤5.5‰, with a subordinate but still significant number (∼33%) above, and only 2 grains below, this range. There is no systematic difference between the central and marginal parts of the grains. Higher than “mantle” δ18OOl values are ascribed to low-T (<300 °C) serpentinisation along inner fractures and grain boundaries of olivine phenocrysts. The measured concentrations of volatile components in the melt inclusions corrected for the effects of post-entrapment crystallisation and H2O–CO2 exsolution in inclusion shrinkage bubbles are: 286–1748 μg/g CO2, 0.2–0.86 wt.% H2O, 48–82 μg/g F, 398–699 μg/g S and 132–198 μg/g Cl. They correspond to a pressure of 86±44MPa or ∼2.5-km crustal depth of olivine crystallisation. The correlations of S and, to a lesser extent, of H2O, with highly incompatible lithophile elements and the correlation of F with Cl, but no relationships of H2O with Cl, rule out shallow depth magma degassing and/or crustal contamination. Our new δ18O olivine and volatile component data combined with the existing, highly variable δ11B values for melt inclusions also support the deep mantle origin of H2O (and probably other volatiles) in the Gorgona mafic and ultramafic magmas.

Magma transport and olivine crystallization depths in Kīlauea’s east rift zone inferred from experimentally rehomogenized melt inclusions

Concentrations of H2O and CO2 in olivine-hosted melt inclusions can be used to estimate crystallization depths for the olivine host. However, the original dissolved CO2 concentration of melt inclusions at the time of trapping can be difficult to measure directly because in many cases substantial CO2 is transferred to shrinkage bubbles that form during post-entrapment cooling and crystallization. To investigate this problem, we heated olivine from the 1959 Kīlauea Iki and 1960 Kapoho (Hawai‘i) eruptions in a 1-atm furnace to temperatures above the melt inclusion trapping temperature to redissolve the CO2 in shrinkage bubbles. The measured CO2 concentrations of the experimentally rehomogenized inclusions (⩽590ppm for Kīlauea Iki [n=10]; ⩽880ppm for Kapoho, with one inclusion at 1863ppm [n=38]) overlap with values for naturally quenched inclusions from the same samples, but experimentally rehomogenized inclusions have higher within-sample median CO2 values than naturally quenched inclusions, indicating at least partial dissolution of CO2 from the vapor bubble during heating. Comparison of our data with predictions from modeling of vapor bubble formation and published Raman data on the density of CO2 in the vapor bubbles suggests that 55–85% of the dissolved CO2 in the melt inclusions at the time of trapping was lost to post-entrapment shrinkage bubbles. Our results combined with the Raman data demonstrate that olivine from the early part of the Kīlauea Iki eruption crystallized at <6kmdepth, with the majority of olivine in the 1–3kmdepthrange. These depths are consistent with the interpretation that the Kīlauea Iki magma was supplied from Kīlauea’s summit magma reservoir (∼2–5kmdepth). In contrast, olivine from Kapoho, which was the rift zone extension of the Kīlauea Iki eruption, crystallized over a much wider range of depths (∼1–16km). The wider depth range requires magma transport during the Kapoho eruption from deep beneath the summit region and/or from deep beneath Kīlauea’s east rift zone. The deeply derived olivine crystals and their host magma mixed with stored, more evolved magma in the rift zone, and the mixture was later erupted at Kapoho.

Petrology and volatile content of magmas erupted from Tolbachik Volcano, Kamchatka, 2012–13

Abstract We report petrography, and bulk rock, mineral and glass analyses of eruptive products of the 2012–13 eruption of Tolbachik volcano, Central Kamchatka Depression, Russia. Magmas are shoshonitic in composition, with phenocrysts of olivine and plagioclase; clinopyroxene phenocrysts are scarce. Samples collected as bombs from the active vent, from liquid lava at the active lava front, and as naturally solidified “toothpaste” lava allow us to quantify changes in porosity and crystallinity that took place during 5.25 km of lava flow and during solidification. Olivine-hosted melt inclusions from rapidly-cooled, mm-size tephra have near-constant H 2 O contents (1.19 ± 0.1 wt%) over a wide range of CO 2 contents ( 2 O and 50 ppm CO 2 . The presence of small saturation, rather than shrinkage, bubbles testifies to volatile saturation at the time of entrapment. Calculated saturation pressures are 0.3 to 1.7 kbar, in agreement with the depths of earthquake swarms during November 2012 (0.6 to 7.5 km below the volcano). Melt inclusions from slowly-cooled and hot-collected lavas have H 2 O contents that are lower by an order of magnitude than tephras, despite comparable CO 2 contents. We ascribe this to diffusive H 2 O loss through olivine host crystals during cooling. The absence of shrinkage bubbles in the inclusions accounts for the lack of reduction in dissolved CO 2 (and S and Cl). Melt inclusions from tephras experienced 2 O-poor and K 2 O-rich basaltic parent.

Bubbles do matter!

Silicate melt inclusions (SMIs) are a very reliable tool for reconstructing the composition of magma (silicate melt + volatiles) and tracing its evolution from mantle depths to the Earth’s surface. Although the scientific community already knew about the ability of shrinkage bubbles in SMIs to trap a significant fraction of CO2, quantitative estimates were still lacking. In the article “Melt inclusion CO2 contents, pressures of olivine crystallization, and the problem of shrinkage bubbles” by Wallace et al. (2015) (included in the special collection “Glasses, melts, and fluids, as tools for understanding volcanic processes and hazards”) the authors provide a numerical estimate of the amount of initial CO2 dissolved in the melt that is lost to shrinkage bubbles and a computational method to estimate such an amount in absence of experimental studies and direct measurements by micro-Raman spectroscopy.

Melt inclusion CO2 contents, pressures of olivine crystallization, and the problem of shrinkage bubbles

The H2O and CO2 contents of melt inclusions can potentially be used to infer pressures of crystallization and inclusion entrapment because the solubility of mixed H2O-CO2 vapor has been determined experimentally for a wide range of melt compositions. However, melt inclusions commonly develop a shrinkage bubble during post-entrapment cooling and crystallization because these processes cause a pressure drop in the inclusion. This pressure drop causes a vapor bubble to nucleate, leading to exsolution of low-solubility CO2 from the trapped melt. To investigate the loss of CO2 into such bubbles, we experimentally heated large, naturally glassy melt inclusions in olivine (Fo contents of 88.1 ± 0.2) from a Mauna Loa picrite to rehomogenize the inclusions. Rapid heating to 1420 °C using a hightemperature heating stage dissolved the shrinkage bubbles into the melt. CO2 contents measured by FTIR spectroscopy and recalculated for melt in equilibrium with the olivine host are 224-505 ppm (n = 11) for heated inclusions, much higher than the CO2 contents of naturally quenched inclusions from the same sample (38-158 ppm; n = 8). Pressures of inclusion entrapment calculated from the H2O and CO2 data for the heated inclusions range from 0.5 to 1.1 kbar, indicating that Mg-rich olivine crystallized at very shallow depths beneath the surface of Mauna Loa. Our results indicate that 40-90% (average 75%) of the original CO2 dissolved in the melt at the time of inclusion entrapment can be lost to the shrinkage bubble during post-entrapment cooling. We show that the computational method of Riker (2005), which predicts the pre-eruption shrinkage bubble size as a function of the difference between trapping temperature and pre-eruption temperature, successfully reproduces our experimental results. Our results demonstrate that the mass of CO2 contained in shrinkage bubbles must be considered to accurately infer original pressures of crystallization for melt inclusions. However, the effect is expected to be smaller for more H2O-rich melt inclusions than those studied here because the vapor bubble in such inclusions will have lower mole fractions of CO2 than the low-H2O inclusions in our study.

Silicate melt inclusions in the phenocrysts of the Szomolya Ignimbrite, Bükkalja Volcanic Field (Northern Hungary): Implications for magma chamber processes

Silicate melt inclusions provide important information on the magma evolution processes and therefore they are the subjects of rapidly growing geochemical studies. However, the original geochemical fingerprint of the trapped melts could be often masked by post-entrapment processes. In this paper, we investigate silicate melt inclusions hosted by various minerals (plagioclase, orthopyroxene, clinopyroxene, quartz) of a silicic ignimbrite and the enclosed igneous lithic clasts in order to reconstruct the magma chamber processes. The Szomolya Ignimbrite was formed at the initiation of the widespread Neogene volcanism of the Pannonian Basin, eastern–central Europe. It is composed of fresh rhyolitic pyroclastics and contains abundant andesitic lithic clasts. The melt inclusions in the phenocrysts are composed mostly of glass with or without shrinkage bubble suggesting rapid cooling. They all have rhyolitic composition, but show significant chemical variability. The melt inclusions in quartz and plagioclase of the ignimbrite have similar compositions as the glass shards. They represent a strongly silicic and potassic (SiO 2 = 76.5 to 79 wt.%; K 2O / Na 2O = > 2) melt resided at the top of the magma chamber at about 2–3 km depth. The melt inclusions in the phenocrysts of the lithic clast show larger chemical variation. It is remarkable that the plagioclase-hosted melt inclusions differ significantly from all the other glass data. This can be explained by the influence of significant post-entrapment crystallization (PEC), i.e. chemical modifications between the entrapment and quenching. Based on major and trace element mass balance modeling, we show that about 10% PEC took place in the plagioclase-hosted melt inclusions, whereas PEC was less in the pyroxene-hosted melt inclusions (about 3%). After correcting this modification, the melt inclusions from the pyroxenes still show wide chemical variation forming linear trends in the Harker diagrams overlapping partly the compositional trend of the groundmass glass. We interpret this as incorporation of heterogeneous melt batches during the mineral growth at the solidification front of the magma chamber at about 5–6 km depth. We assume that there is a genetic relationship between the andesitic lithic clasts and the host ignimbrite. The former ones could reflect crystallization at the solidification front at greater depth, whereas the latter one could represent the silicic residual melt accumulating at the top of the magma chamber. Difference in trace element compositions can be explained by late stage removal of accessory minerals such as zircon and allanite, which strongly controlled the trace element budget of the most differentiated melt. Withdrawal of the rhyolitic magma could enhance the disruption of the deeper solidification front. Its mineral assemblage (clinopyroxene, orthopyroxene and plagioclase) came in contact with a slightly more evolved melt, which quenched suddenly due to the eruption of the silicic magma.