Glass Inclusions Research Articles

Volatiles in glasses from Mauna Loa Volcano, Hawai'i: implications for magma degassing and contamination, and growth of Hawaiian volcanoes

Glasses from Mauna Loa pillow basalts, recent subaerial vents, and inclusions in olivine were analyzed for S, Cl, F, and major elements by electron microprobe. Select submarine glasses were also analyzed for H2O and CO2 by infrared spectroscopy. The compositional variation of these tholeiitic glasses is dominantly controlled by crystal fractionation and they indicate quenching temperatures of 1,115–1,196 °C. Submarine rift zone glasses have higher volatile abundances (except F) than nearly all other submarine and subaerial glasses with the maximum concentrations increasing with water depth. The overwhelming dominance of degassed glasses on the submarine flanks of Mauna Loa implies that much of volcano's recent submarine growth involved subaerially erupted lava that reached great water depths (up to 3.1 km) via lava tubes. Anomalously high F and Cl in some submarine glasses and glass inclusions indicate contamination possibly by fumarolic deposits in ephemeral rift zone magma chambers. The relatively high CO2 but variable H2O/K2O and S/K2O in some submarine rift zone glasses indicates pre-eruptive mixing between degassed and undegassed magma within Mauna Loa's rift system. Volatile compositions for Mauna Loa magmas are similar to other active Hawaiian volcanoes in S and F, but are less Cl-rich than Lō'ihi glasses. However, Cl/K2O ratios are similar. Mauna Loa and Lō'ihi magmas have comparable, but lower H2O than those from Kilauea. Thus, Kilauea's source may be more H2O-rich. The dissimilar volatile distribution in glasses from active Hawaiian volcanoes is inconsistent with predictions for a simple, concentrically zoned plume model.

Crustal origin for coupled 'ultra-depleted' and 'plagioclase' signatures in MORB olivine-hosted melt inclusions: evidence from the Siqueiros Transform Fault, East Pacific Rise

Geochemical data from melt inclusions in olivine phenocrysts in a picritic basalt from the Siqueiros Transform Fault on the northern East Pacific Rise provide insights into the petrogenesis of mid-ocean ridge basalts (MORB). The fresh lava contains approximately 10% of olivine phenocrysts (Fo89.3 - 91.2) and rare, small (<1 mm) plagioclase phenocrysts with subhedral to irregular shapes with a range of compositions (An80-90, An57-63). Melt inclusions in olivine phenocrysts are glassy, generally rounded in shape and vary in size from a few to approximately 200 lm. Although most of the inclusions have compositions that are generally consistent with being representative of parental melts for the pillow-rim glasses, several inclusions are clearly different. One inclusion, which contains a euhedral grain of high-Al, low-Ti spinel, has a composition unlike any melt inclusions previously described from primitive phenocrysts in MORB. It has a very high Al2O3 (approximately 20 wt%), very low TiO2 (approximately 0.04 wt%) and Na2O (approximately 1 wt%) contents, and a very high CaO/Na2O value (approximately 14). The glass inclusion is strongly depleted in all incompatible elements (La =0.052 ppm; Yb =0.34; La/Sm(n) approximately 0.27), but it has large positive Sr and Eu anomalies (Sr/Sr* approximately 30; Eu/Eu* approximately 3) and a negative Zr anomaly. It also has low S (0.015 wt%) and relatively high Cl (180 ppm). We suggest that this unusual composition is a consequence of olivine trapping plagioclase in a hot, strongly plagioclase-undersaturated magma and subsequent reaction between plagioclase and the host olivine producing melt and residual spinel. Two other melt inclusions in a different olivine phenocryst have compositions that are generally intermediate between 'normal' inclusions and the aluminous inclusion, but have even higher CaO and Sr contents. They are also depleted in incompatible elements, but to a lesser degree than the aluminous inclusion, and have smaller Sr and Eu anomalies. Similar inclusions have also been described in high-Fo olivine phenocrysts from Iceland and northern Mid-Atlantic Ridge. We suggest that the compositions of these inclusions represent assimilation of gabbroic material into the hot primitive magma. The localised nature of this assimilation is consistent with it occurring within a crystal mush zone where the porosity is high as primitive magmas pass through earlier formed gabbroic 'cumulates'. In such an environment the contaminants are expected to have quite diverse compositions. Although the interaction of primitive melts with gabbroic material may not affect the compositions of erupted MORB melts on a large scale, this process may be important in some MORB suites and should be accounted for in petrogenetic models. Another important implication is that the observed variability in melt inclusion compositions in primitive MORB phenocrysts need not always to reflect processes occurring in the mantle. In particular, inferences on fractional melting processes based on geochemistry of ultra-depleted melt inclusions may not always be valid.

The 12 900 years BP Laacher See eruption: estimation of volatile yields and simulation of their fate in the plume

Abstract We estimated the volatile emissions of the 12 900 years BP eruption of Laacher See volcano (Germany), using a modified petrological method. Glass inclusions in phenocrysts and matrix glasses sampled over the Laacher See tephra profile were analysed by synchrotron X-ray fluorescence microprobe and electron microprobe to obtain the emitted masses of halogens, sulphur, and water. These data were used to initialize the numerical plume model ATHAM in order to investigate the fate of volcanic gases in the plume, and to estimate volatile masses injected into the stratosphere. The scavenging efficiency of each volatile component depends on its interactions with both liquid water and ice. We found a scavenging efficiency of c. 5% for the sulphur species, and of only c. 30% for hydrogen halides, despite their high water solubility. Our simulations showed that the greatest fraction of hydrometeors freeze to ice, due to the fast plume rise and great height of the eruption column. For the dry atmospheric conditions of the Laacher See eruption, the amount of liquid water was not sufficient to completely scavenge HCl and HBr, so that a large proportion could reach the stratosphere.

Mantle genesis and crustal evolution of primitive calc-alkaline basaltic magmas from the Lesser Antilles arc

Abstract Most eruptive rocks in the Lesser Antilles arc are compositionally evolved. However, lavas with primitive characteristics do occur including, in the central part of the arc, a suite of rocks from Soufriere, St Vincent, and the Ilet à Ramiers basalt from Martinique. High-pressure experiments performed on a Soufriere basalt point to a spinel lherzolite source. Glass inclusion data and phase equilibria analysis suggest extraction of the Soufriere melt under relatively dry conditions ( c. 2 wt% H 2 O in melt). Using estimates of the H 2 O content of mantle sources fluxed by an hydrous slab-derived component, H 2 O concentrations as high as 5 wt% are considered possible for primary mantle melts in the Lesser Antilles arc. Experiments at low pressures (4 kbar) simulate the evolution of primitive melts within the arc crust. For elevated melt H 2 O concentrations (6–8.5 wt%), derivative liquids ranging from low-MgO basalt to basaltic andesite are generated at 1050–1100°C. Their crystallization at 950–1000°C yield andesitic liquids similar to those erupting at active volcanic centres such as Mt Pelée, Martinique, and Soufriere Hills, Montserrat. Therefore, experimental data support the derivation of Lesser Antilles arc eruptives by different degrees of fractionation from primary mantle melts.

Geology and petrology of ejecta from the 1999 eruption of Shishaldin Volcano, Alaska

Shishaldin Volcano erupted repeatedly during April and May 1999, with major eruptive events on 19 April and 23 April. Tephra deposits >20 cm thick were distributed across the southern flanks and hyperconcentrated debris flows ran down the northern slope. All analyzed clasts were basaltic in composition, with abundant (~29 vol%) phenocrysts of plagioclase, olivine, and magnetite. Glass-inclusion analyses indicate low volatile contents ( 900 to 58 ppm, respectively). Comparisons of sulfur contents in glass inclusions and groundmass glasses suggest significant degassing occurred even prior to eruption. Trace-element studies suggest that the magma reservoir below Shishaldin may not be a closed, long-lived chamber, and U–Th–Ra isotopic analyses indicate short crustal residence times (on the order of at most a few thousand years) for the 1999 products. Comparison of the deposits from this eruption with other similarly sized eruptions suggests the 19 April explosive event should be classified as a sub-Plinian basaltic eruption. Such eruptions are rare. The 19 April explosive event was the largest at Shishaldin during the last 175 years. However, the petrology, chemistry and deposit thickness and distribution resemble those of at least 11 scoria deposits in the last 9,000 years, suggesting that this type of eruption is common for Shishaldin.

Glass filler charged epoxy resin—experimental validation of boundary integral equation method for computing effective permittivity of two and three phase composites

The effective characteristics of composite materials depend on their inner structure. In this paper, we carry experimental validation of the computer simulations based on boundary integral equation method used for investigating the effective permittivity of two- and three-phase heterogeneous materials with cylindrical, spherical and hollow-spherical glass inclusions embedded in epoxy resin matrix. The results are also compared with the analytical formulae predictions.

Oxygen isotope variations in primitive tholeiites of Iceland: evidence from a SIMS study of glass inclusions, olivine phenocrysts and pillow rim glasses

Oxygen-isotope compositions are reported for glass inclusions, their host olivines (Fo 87–90) and tholeiitic pillow rim glasses from the neovolcanic rift zone of Iceland. The inclusion compositions range from enriched ([La/Sm] n=1.4–4.8) to very depleted ([La/Sm] n=0.08–0.35). The whole rocks and matrix glasses display little variation ([La/Sm] n=0.41–0.51). Glass inclusions vary somewhat stronger in 18O/ 16O ratios (δ 18O=4.0–6.2±0.6‰) than the matrix glasses (δ 18O=4.6–5.6±0.6‰). The range of olivine phenocryst composition is even wider (δ 18O=2.4–6.9±0.5‰), and in some cases the variations within individual crystals reach 1.0–2.5‰. Most glass inclusions are in O-isotopic equilibrium with the surrounding host olivine but not with the central parts of the host crystals; similarly, most olivines are out of equilibrium with their host matrix glasses. This implies that the glass inclusions and olivine phenocrysts can preserve O-isotopic compositions inherited from contrasting parental mantle-derived magmas, which might have mixed in a common volcanic system. The isotopic heterogeneity of individual olivine crystals allows one to estimate their residence times in the magma reservoir using the rate of 18O diffusion in olivine. The time required to achieve isotopic homogeneity over 90–520 μm ranges from 25 to 1030 yr. The δ 18O values found in depleted glass inclusions and their host olivines are unlikely to have resulted from magma contamination by upper crustal rocks. We suggest two possibilities, implying either an O-isotope heterogeneity of the Icelandic mantle or a mixing of two end-members: a refractory component with ‘normal-mantle’ or somewhat elevated δ 18O values, high Ca/Na and strong light rare earth element depletion mixed with a component having lower δ 18O and Ca/Na but also depleted in incompatible elements. Based on δ 18O–La/Sm and δ 18O–δ 11B relationships, the variations of δ 18O between 4.6 and 6.2‰ in enriched and depleted glass inclusions are interpreted to reflect those of primary magmas and, consequently, of their source region.

Cooling rates of Plinian-fall and pyroclastic-flow deposits in the Bishop Tuff: inferences from water speciation in quartz-hosted glass inclusions

Cooling rates inferred from water speciation in rhyolitic glass inclusions in quartz phenocrysts from the Bishop Tuff, California, vary over more than 5 orders of magnitude from about 10 to <10 -4 �C s -1 . Many of the cooling rates are within the previously published, exper- imentally calibrated range of this technique but the slowest rates (<10 -4 �C s -1 ) are extrapolated. Such extrapolation suggests cooling rates as low as 10 -8 �C s - 1 (0.3 �C year -1 ) for the interiors of thick, pyroclastic- flow deposits. The temperatures at which OH and molecular H2O species would be in equilibrium range from ~400 �C for glass inclusions from Plinian-fall deposits to ~250 �C for the most slowly cooled glassy inclusions from pyroclastic-flow deposits. Notwithstand- ing appreciable uncertainties, cooling rates correlate with depositional process (fall vs. flow deposition) and distance above the cold ground or below overlying, hot pyroclastic-flow deposits. Plinian inclusions within 2.5 m of the cold, pre-Bishop ground cooled at 1-10 �C s -1 , consistent with thermal modeling of small pumice clasts falling through cold air. Very slow, inferred cooling rates for pyroclastic-flow deposits (~10 -6 to 10 -8 �C s -1 ) also agree with thermal-conduction models. Faster cooling rates (~10 -1 to 10 -4 �C s -1 ) for inclusions in the lowermost 0.5 m of pyroclastic-flow deposits reflect the effect of quenching against the cold ground. At Blind Spring Hill (east of Long Valley Caldera), intermediate cooling rates (10 -3 to 10 -6 �C s -1 ) are observed in Plinian deposits overlain by, and partially interbedded with, pyroclastic- flow deposits, indicating that reheating of rapidly cooled Plinian inclusions can reset water speciation. Melt inclusions in pyroclastic-flow deposits are quenched to glass only if the deposit is relatively thin or has a relatively low emplacement temperature. In relatively thick or hot deposits, inclusions may be quenched to glass at the base of the deposit where it is more rapidly cooled by contact with the pre-eruption ground.

A01 ガラス包有物分析に基づく三宅島火山 2000 年 8 月 18 日噴火マグマの上昇プロセス

A01 ガラス包有物分析に基づく三宅島火山 2000 年 8 月 18 日噴火マグマの上昇プロセス

Petrologic constraints on the spatial distribution of crustal magma chambers, Atka Volcanic Center, central Aleutian arc

The Atka volcanic center, the largest modern, central Aleutian magmatic complex, covers 360 km2 with an estimated eruptive volume of ~200 km3. The oldest exposed rocks, the Old Harbor Series, are a suite of basaltic andesite to dacite flows, pyroclastic rocks and lahars of presumed Tertiary age. The youngest phase of activity on Atka probably began 1–2 Ma ago near the center of the present volcanic field, and consists of basaltic and basaltic andesite flows which produced a large shield volcano, Atka Volcano. Subsequent to this activity, satellite vents formed along the margin of this structure. Collapse of the ancestral Atka Volcano produced a caldera 5 km in diameter and was accompanied by the eruption of a large dacitic flow (Big Pink). After caldera formation, four major volcanic cones (Korovin, Mount Kliuchef, Sarichef and Konia) grew around the margins of Atka caldera. This study focuses on the Atka Volcano, Korovin and Mount Kliuchef volcanic centers. Atka lavas range from basalt through dacite, with all major volcanic vents erupting lavas spanning similar compositional ranges. They are porphyritic with phenocrysts of plagioclase (25–36%), olivine (0.4–3.6%) or orthopyroxene (2.8%), clinopyroxene (1–12%) and a single Fe–Ti oxide (0–2%). Hydrous phases are lacking even in the most evolved lavas. Two generations of plagioclase occur in the Korovin suite but not in lavas from Mount Kliuchef. Korovin plagioclase also displays resorption surfaces and inclusion zones which are either absent in the phenocrysts of Mount Kliuchef lavas or very rare. Similarly, clinopyroxene in Korovin has reaction rims and abundant glass inclusions which are absent in the Mount Kliuchef lavas. Lavas erupted in the Atka volcanic field vary from 47 to 66% SiO2, although the complex is dominated volumetrically by basaltic lavas. On major-element Harker diagrams, mafic and intermediate Atka Volcano lavas form linear trends but the most siliceous lavas do not fall along these trends. By contrast, Mount Kliuchef lavas are compositionally bimodal whereas Korovin samples range from 51 to 62 wt% silica with intermediate lavas clustering around 55% SiO2. Incompatible trace-element abundances of Korovin lavas do not increase with silica content but those of Atka Volcano and Mount Kliuchef do. On incompatible-incompatible element diagrams, the three suites behave very differently. Differences also exist between the REE characteristics of the three Atka suites. The petrographic and geochemical differences between the three suites suggest formation by markedly different crustal magmatic processes. The characteristics of the Atka Volcano suite suggest formation of the basalt to dacite suite by crystal fractionation of an anhydrous phenocryst assemblage. Incompatible-incompatible element trends suggest the mafic Mount Kliuchef lavas were also produced by crystal fractionation, a conclusion supported by quantitative mass balance calculations. The difference in slope between the Atka Volcano and Mount Kliuchef suites indicates, however, that there were significant differences in these fractionation schemes. Modeling as well as geochemical characteristics suggest that the Mount Kliuchef dacites are not the product of crystal fractionation. They may represent (1) crustal melts, (2) complete melts of siliceous lava produced during earlier magmatic stages, or (3) residual liquids from the Atka Volcano magmatic system. In contrast to these suites, the Korovin lavas could not have been formed by crystal fractionation. Rather the andesites are probably the result of mixing basaltic and dacitic magmas. The Atka data suggest: (1) large volcanic centers may be supplied by a series of non-communicating, crustal magma chambers; (2) local factors may be important in determining liquid-lines-of-descent; and (3) very different magmatic processes may occur in adjacent magma chambers. If the complexity of the Atka suite is a common feature of arc magmatic plumbing systems, reconstructing differentiation processes requires analytical sample sets which are well constrained spatially and temporally.

The mass estimation of volatile emission during 1199–1200 AD eruption of Baitoushan volcano and its significance

On the basis of the study of volcanic products during 1199—1200AD eruption of Baitou Mountain (Baitoushan), the released volatile content was estimated by comparing Cl, F, S, H 2 O concentrations of undegassed glass inclusions with those of degassed matrix glasses. The calculations show that volatile yields, released from the melt, are 109.88×10 6 ton of HCl, 196.80×10 6 ton of HF, 1477.84×10 6 ton of H 2 O, 23.14×106 ton of SO 2 , which could have formed 35.43×10 6 ton of H 2 SO 4 aerosol in the atmosphere. They could have substantial effect on paleoclimate and paleo-environment.

Source and H 2O content of high-MgO magmas in island arc settings: an experimental study of a primitive calc-alkaline basalt from St. Vincent, lesser antilles arc

Liquidus phase relationships have been determined for a high-MgO basalt (STV301: MgO=12.5 wt%, Ni=250 ppm, Cr=728 ppm) from Black Point, St Vincent (Lesser Antilles arc). Piston-cylinder experiments were conducted between 7.5 and 20 kbar under both hydrous and oxidizing conditions. AuPd capsules were used as containers. Compositions of supraliquidus glasses and mass-balance calculations show that Fe loss is < 10% in the majority of experiments. Two series of water concentrations in melt were investigated: (i) 1.5 wt% and (ii) 4.5 wt% H 2O, as determined by SIMS analyses on quenched glasses and with the by difference technique. The Fe 3+/Fe 2+ partitioning between Cr-Al spinel and melt and olivine-spinel equilibria show that oxidizing fO 2 were imposed (NNO + 1.5 for the 1.5 wt% H 2O series, NNO + 2.3 for the 4.5 wt% H 2O series). For both series of water concentrations, the liquid is multiply-saturated with a spinel lherzolite phase assemblage on its liquidus, at 1235°C, 11.5 kbar (1.5 wt% H 2O) and 1185°C, 16 kbar (4.5 wt% H 2O). Liquidus phases are homogeneous and comparable to typical mantle compositions. Mineral-melt partition coefficients are generally identical to values under anhydrous conditions. The modal proportion cpx/opx on the liquidus decreases from the 1.5 wt% to the 4.5 wt% H 2O series. The experimental data are consistent with STV301 being a product of partial melting of lherzolitic mantle. Conditions of multiple saturation progressively evolve toward lower temperatures and higher pressures with increasing melt H 2O concentration. Phase equilibria constraints, i.e., the necessity of preserving the mantle signature seen in high-MgO and picritic arc basalts, and glass inclusion data suggest that STV301 was extracted relatively dry (∼ 2 wt% H 2O) from its mantle source. However, not all primary arc basalts are extracted under similarly dry conditions because more hydrous melts will crystallize during ascent and will not be present unmodified at the surface. From degrees of melting calculated from experiments on KLB-1, extraction of a 12.5 wt% MgO melt with ∼ 2 wt% H 2O would require a H 2O concentration of 0.3 wt% in the sub-arc mantle. For mantle sources fluxed with a slab-derived hydrous component, extracted melts may contain up to ∼ 5.5 wt% H 2O.

Chemistry of glass inclusions in olivines of the CR chondrites Renazzo, Acfer 182, and El Djouf 001

Glass inclusions in olivines of the Renazzo, El Djouf 001, and Acfer 182 CR-type chondrites are chemically divers and can be classified into Al-rich, Al-poor, and Na-rich types. The chemical properties of the glasses are independent of the occurrence of the olivine (isolated or part of an aggregate or chondrule) and its composition. The glasses are silica-saturated (Al-rich) or oversaturated (Al-poor, 24% normative quartz). All glasses have chondritic CaO/Al 2O 3 ratios, unfractionated CI-normalized abundances of refractory trace elements and are depleted in moderately volatile and volatile elements. Thus the glasses are likely to be of a primitive condensate origin whose chemical composition has been established before chondrule formation and accretion, rather then the product of either crystal fractionation from chondrule melts or part melting of chondrules. Rare Na-rich glasses give evidence for elemental exchange between the glass and a vapor phase. Because they have Al 2O 3 contents and trace element abundances very similar to those of the Al-rich glasses, they likely were derived from the latter by Ca exchange (for Na) with the nebula. Elemental exchange reactions also have affected practically all olivines (e.g., exchange of Mg of olivine for Fe 2+, Mn 2+, and Cr 3+). Glasses formed contemporaneously with the host olivine. As the most likely process for growing nonskeletal olivines from a vapor we consider the VLS (vapor-liquid-solid) growth process, or liquid-phase epitaxy. Glasses are the possible remnants of the liquid interface between growing crystal and the vapor. Such liquids can form stably or metastably in regions with enhanced oxygen fugacity as compared to that of a nebula of solar composition.

Experimental study of Cl solubility in hydrous alkaline melts: constraints on the theoretical maximum amount of Cl in trachytic and phonolitic melts

Cl solubility in evolved alkaline melts was investigated at 860–930 °C and pressures of 25 to 250 MPa using natural trachytes and a synthetic phonolite equilibrated with subcritical fluids in the H2O–(Na,K)Cl system (i.e. silicate melt coexisted with water-rich aqueous fluid and a saline brine). Fluid phase characteristics were identified by examination of fluid inclusions present in the run product glasses and the fluid bulk composition was calculated by mass balance. The Cl contents of trachytic glasses coexisting with subcritical fluids increase linearly with decreasing pressure from 250 to 25 MPa and range from 0.37 to 0.90 wt%; Cl in the phonolitic glass ranges from 0.35 to 0.59 wt%. These values are approximately twice those found in metaluminous rhyolitic melts under similar conditions. Variations from peralkaline to peraluminous composition has little effect on Cl solubility in trachytes, whereas it is a more important factor in phonolites. More generally, melt structure, in particular non-bringing oxygen, appears to strongly influence Cl solubility in silicate melts. The negative correlation between pressure and melt Cl content is governed by the large negative partial volume of NaCl in the vapour phase. No change in Cl solubility is observed between 200 and 250 MPa. Comparison of our experimental results with Cl abundance in glass inclusion and matrix glass from Italian volcanoes can be used to identify those eruptive products preserved in the geologic record which may have been associated with large Cl emissions.

Orthopyroxene-bearing tholeiites of the Iblean Plateau (Sicily): constraints on magma origin and evolution from glass inclusions in olivine and orthopyroxene

The widespread Pliocene (ca. 2 Ma) orthopyroxene-bearing tholeiites represent by far the most voluminous but short-lived magmatic phase of the Cenozoic Iblean volcanic province. We have studied major, volatile (H 2O, S, and Cl), and light lithophile (Li, Be, and B) elements and 7Li/ 6Li and 11B/ 10B isotopic ratios of olivine- and orthopyroxene-hosted glass inclusions and their host pillow-rim glasses using EPMA and SIMS. Most variations of major elements, H 2O (0.2–0.5 wt.%), Cl (100–350 ppm), and especially of Li (4.6–5.8 ppm), Be (0.5–0.8 ppm) and B (0.6–1.1 ppm) are believed to reflect heterogeneity of the Iblean magma source. Crystallization of these magmas occurred at shallow crustal depths in the range of 1250–1140 °C and oxygen redox conditions corresponding to FMQ–NNO buffers. Olivine [75.8–83.8 mol% Fo], orthopyroxene [Mg#=80.3–85.6], plagioclase [54.2–69.9 mol% An] and spinel [Mg#=40.9–49.6 and Cr#=59.6–63.8] are common liquidus phases. Variable sulfur concentrations in glass inclusions (280–1330 ppm S) and host matrix glasses (160–670 ppm S) reflect degassing of rising and erupting magma. The abundances of SO 4 2− dissolved in the melt (9–38% of total S) fit well to the redox conditions deduced from Fe 2+/Fe 3+ratios of the coexisting spinel. Glass inclusions scatter significantly in 7Li/ 6Li and 11B/ 10B isotopic ratios even within single specimens ( δ 7Li=−3.4‰ to +1.2‰ and δ 11B=−17.1‰ to −3.1‰). The lowest δ 7Li (−3.4‰ to +1.2‰) and δ 11B (−13.9‰ to −16.1‰) probably reflect isotopically light mantle domains beneath the Iblean Plateau. The shift of δ 11B toward more positive values (up to −3.1‰) at nearly the same low δ 7Li (−3.4‰ to 0.9‰) is explained by assimilation of <2 wt.% altered basaltic rocks.

Micro X-ray absorption near edge structure at the sulfur and iron K-edges in natural silicate glasses

Both sulfur and iron chemical environments were studied in natural compounds; sulfides, sulfates and silicate basaltic glasses which are supposed to have variable oxidation states, using XANES (X-ray absorption near edge structure) spectroscopy. Although X-ray absorption methods are well suited for such investigations, the size of natural glass inclusions trapped in volcanic minerals, ranging from a few micrometers to several tenths of micrometers, makes micro-spectroscopy necessary. Hence, we present the μ-XANES spectra at the sulfur and iron K-edges of olivine-hosted glass inclusions containing from 1000 to 1650 ppm S, and between 5.5 and 7.8 wt.% Fe. By combining both high energy and spatial resolutions, we demonstrate the ability of the μ-XANES to produce quantitative determination of sulfur and iron valence states. We have identified various species for sulfur, specifically S II−, S VI and possibly S IV, in basaltic glass inclusions hosted in olivine grains. We propose a method to calibrate the proportion of Fe dissolved as Fe 3+ in basaltic glasses for which the Fe 3+/ΣFe ratio varies between 0.05 and 0.48 with a relative precision of less than 10%.

玄武岩、安山岩、流紋岩中のガラス包有物の含水量をラマン顕微鏡を用いて測定する

玄武岩、安山岩、流紋岩中のガラス包有物の含水量をラマン顕微鏡を用いて測定する

Glass inclusions and melt volatile contents at Parícutin Volcano, Mexico

Olivine-hosted glass inclusions were investigated from tephra samples erupted at Paricutin volcano on four different dates: May 26 and August 1, 1943; January 23, 1945; and March 31, 1948. These dates span the first two thirds of the 9 year eruption, during which time the tephra/lava mass-eruption rate fell dramatically. They also span the strong whole-rock compositional shift of 1947, attributed to the increased importance of crustal contamination. Nine of the 26 analyzed glass inclusions have lower SiO2 contents than any previously analyzed Paricutin lava sample, ranging to below 53 wt%. These silica-poor glasses are found in olivines erupted in 1943 and 1945, and provide evidence for melts that are parental to the main Paricutin lava suite. Total water contents in the glass inclusions measured by Fourier transform infrared (FTIR) spectroscopy vary considerably in all individual samples, with a total range of 1.8–4.0 wt%. Total water contents are not correlated with SiO2 of the glass, Mg# of the adjacent host olivine, or eruption date. Only two glass inclusions have carbonate contents (248 and 296 ppm CO2) above the FTIR detection limit of ~50 ppm CO2; importantly, these inclusions also have the highest total water contents and among the highest SO3t values. These two inclusions were trapped at minimum depths of 9.0–9.6 km beneath the volcano. Thus, early degassing likely stripped most carbon from Paricutin melts at mid-crustal levels. Other glass inclusions yield minimum entrapment depths of 1.3–5.1 km based on water solubility limits. Total sulfur (0.30 to 0.01 wt% SO3) declines as SiO2 contents increase from 52.7 to 60.5 wt%. This trend and the wide range of glass inclusion total water contents are interpreted to reflect degassing accompanied by fractional crystallization and assimilation at upper crustal levels.

Magma ascent and contamination beneath one intraplate volcano: evidence from S and O isotopes in glass inclusions and their host clinopyroxenes from Miocene basaltic hyaloclastites southwest of Gran Canaria (Canary Islands)

We report ion microprobe analyses of S and O isotopes in glass inclusions and their host clinopyroxenes (Cpx) from Miocene basaltic hyaloclastites drilled during Ocean Drilling Program (ODP) Leg 157. This is the first study where high-precision in situ δ34S and δ18O measurements were applied in order to constrain the isotopic variability of intraplate magmas and to have insights into establishment of isotopic equilibrium between the melt and crystallizing phenocrysts. Glass inclusions range from tholeiitic to alkali basaltic composition and contain varying to very high proportions of sulfur dissolved as sulfate (S6+/Stotal = 0.40 to 0.87) at S contents of 800 to 2100 ppm. The δ34S values varying from -1.0 to +8.5 ± 1.5‰, exceed significantly the range reported for fresh submarine midocean ridge basalt (MORB) and oceanic island basalt (OIB) glasses, and positively correlate with Cl concentrations and S6+/Stotal ratios. The δ18O values in glass inclusions and their host Cpx crystals analyzed within 50 μm distance away from inclusions are also strongly variable (inclusions: +5.0 to +8.2 ± 0.7‰; Cpx: +5.7 to +8 ± 0.7‰) and are in agreement with O-isotope equilibrium between Cpx and basaltic melt. A broad δ18O range (+5.6 to +7.3‰) decoupled from that of the melt inclusions was also found in the Cpx crystals measured >500 μm away from inclusions.The variations of δ34S and δ18O in glass inclusions are interpreted as resulting from a combination of degassing and assimilation processes. Hydrothermally altered basaltic crust (δ34S = +4.0 to +9.6‰, δ18O = +9.0‰) mixed with oceanic sediments (δ18O ≈ 25‰) and containing up to 5 wt.% of seawater sulfate (δ34S = +21‰, δ18O = +9.5‰) represents the most probable contaminating end-member. The oxygen isotope heterogeneity found in most Cpx crystals and their glass inclusions implies that magma contamination and inclusion-trapping occurred during a few years only, i.e., too short for O isotopic homogeneity to be reestablished in the entire crystals.

Numerical modeling of major element distribution between chromian spinel and basaltic melt, with application to chromian spinel in MORBs

A set of empirical equations is developed which allows calculation of chromian spinel composition in equilibrium with a basaltic melt under a known set of conditions. These equations are calibrated with published experimental data for the temperature range from 1,093 to 1,490 °C and a pressure of up to 20 kbar. It is demonstrated that the composition of chromian spinel from the experiments, which crystallizes from basaltic, boninitic, and komatiitic melt and varies from high-Al to high-Cr, and high-Fe3+ can be successfully reproduced using the suggested model. The composition of chromian spinel has been calculated using the glass composition for a set of primitive basaltic lavas using the suggested set of empirical equations. Good agreement between the calculated composition and composition of rims of chromian spinel included in glass is achieved for compositionally diverse spinel from a Hawaiian sample and 15 MORB samples. The Fe2O3 content in the chromian spinels is, however, often variable and higher than the calculated Fe2O3 content. Additional calculations using 76 published MORB glass analyses reproduced most of the MORB chromian spinel range, except for the most Cr- and Mg-rich. The crystallization of these Cr-rich chromian spinels likely occurred from a more primitive Al-poor melt than that of the 76 MORB glasses. An example of a more primitive glass can occasionally be found as glass inclusions in these Cr- and Mg-rich chromian spinel microphenocrysts.