Paul J. Wallace

Sulfur in basaltic magmas

Abstract The concentration of S in basaltic magmas at 1 atm pressure is strongly dependent on temperature, the fugacities of oxygen ( ƒ O 2 ) and sulfur ( ƒ S 2 ), and bulk composition. Microprobe analyses of total S in rapidly quenched, submarine basalt glasses, used in conjunction with wet chemical analyses of Fe 2 O 3 FeO and its relationship to ƒ o 2 , allow direct calculation of ƒ s 2 using an expression which relates dissolved sulfur content to sulfur fugacity. The relationship between S fugacity and dissolved S in a silicate liquid at 1 bar total pressure can be represented by the expression ln X s = a ln ƒ s 2 − b ln ƒ o 2 + c ln X FeO + d T + e + Σƒ i X i , where Xs is the mole fraction of dissolved S, a through ƒ i are experimentally calibrated regression coefficients, and the summation is over melt components, i. Back calculation of the input data yields a standard error of 0.026 wt% S for a magma with 0.1 wt% of S. Prediction of the immiscible Fe-S-O liquid saturation surface for basaltic liquids in T-X-ƒ o 2 -ƒ s 2 space is made through consideration of the heterogeneous equilibrium 1 2 S 2(gas) + FeO (silicate melt) = FeS (sulfide melt) + 1 2 O 2(gas) , using standard state thermodynamic data. Data from natural basalt glasses demonstrate that during the differentiation and Fe enrichment of basaltic magmas, the increases in S content which are observed require the ratio ƒ o 2 ƒ s 2 to decrease by 3 log10 units over the temperature range 1260 to 1050°C. This decrease is equivalent to the enthalpy change of the above reaction. Application to basalts and gases from Kilauea volcano demonstrates that during ascent, S is depleted in the magma as a result of shallow effervescence, and the gases which are evolved are in equilibrium with the magma during fire fountaining.

Gradients in H2O, CO2, and exsolved gas in a large‐volume silicic magma system: Interpreting the record preserved in melt inclusions from the Bishop Tuff

Infrared spectroscopic analyses of ∼140 melt inclusions in quartz phenocrysts from the zoned Bishop rhyolitic tuff demonstrate that systematic gradients in dissolved magmatic H2O and CO2 concentrations were present during preemptive crystallization of the magma body. Melt inclusions from the earliest erupted samples contain lower H2O (5.3±0.4 wt %) and CO2 (62±37 ppm) than inclusions from the middle of the eruption (5.7±0.2 wt % H2O; 120±60 ppm CO2). Melt inclusions from late erupted samples have much lower H2O (4.1±0.3 wt %) and higher and variable CO2 (150–1085 ppm). Trace element analyses of melt inclusions by ion microprobe show that inclusions within single pumice clasts from the early and middle Bishop Tuff have an inverse correlation between CO2 and incompatible elements. This pattern indicates that the magma was gas-saturated during crystallization, with CO2 partitioning into a coexisting gas phase. Quantitative modeling using H2O-CO2 solubility relations reveals a preeruptive gradient in exsolved gas, with gas contents varying from ∼1 wt % in the deeper regions of the magma body to nearly 6 wt % near the top. Dissolved Cl, B, Li, and Be in melt inclusions correlate negatively with CO2. Mass balance modeling of Cl loss to exsolving H2O-rich gas during crystallization provides strong corroborating evidence for the mass fractions of exsolved gas estimated from H2O, CO2, and trace element data. Pressures of quartz crystallization and melt inclusion entrapment calculated from inclusion H2O-CO2 data are consistent with progressive downward tapping of a zoned magma body during the eruption. Melt inclusion gas saturation pressures, magma volume estimates, and time-stratigraphic-compositional relations suggest that early erupted magma was stored at the top of a downward widening magma body. Melt inclusion data and the inferred gradients in dissolved H2O, CO2 and exsolved gas in the Bishop magma body suggest that gas saturation plays an important role in the formation and subsequent preservation of compositional gradients in silicic magma reservoirs.

Role of H2O in subduction-zone magmatism: New insights from melt inclusions in high-Mg basalts from central Mexico

Although there is a growing body of data on H 2 O in arc magmas, there is still considerable uncertainty about the relationship between H 2 O and various incompatible elements during enrichment of the mantle wedge by subduction processes. We report data for H 2 O, other volatiles (CO 2 , S, Cl), and trace elements in olivine-hosted melt inclusions from high-Mg basalts in central Mexico that exhibit varying degrees of subduction-related enrichment. Most melt inclusions were trapped at low pressure, but rare inclusions (Mg# 65-78, olivine hosts Fo 8 5 - 9 0 ) trapped at upper to middle crustal pressures (1-6 kbar) contain high CO 2 (250-2120 ppm). The high-pressure inclusions indicate magmatic H 2 O contents from 1.3 to 5.2 wt%. Enrichment of H 2 O relative to Nb correlates positively with K/Nb, Ba/Nb, and La/Nb, indicating a clear link between H 2 O) and trace element enrichment of the mantle wedge. Our results show that fluxing of the wedge with an H 2 O-rich component from the subducted slab is important in formation of magmas that are enriched in large ion lithophile (LILE) and light rare earth (LREE) elements relative to high field strength elements (HFSE). In contrast, magmas with low LILEs and LREEs relative to HFSEs have relatively low H 2 O, and must have formed largely by decompression melting of unmodified mantle. Our data for volcanoes <50 km apart show evidence of significant variability in the composition of H 2 O-rich subduction components that are added to the mantle wedge beneath central Mexico.

Magma degassing buffered by vapor flow through brecciated conduit margins

Obsidian pyroclasts, a common component of rhyolitic tephra, preserve a range of volatile contents, which has been used to infer syneruptive conditions of magmatic degassing. Here we show that the textures of obsidian pyroclasts provide information on physical mechanisms of magma flow and degassing along conduit margins. Obsidian clasts often contain xenoliths, sheared bands of lithic powder, and textures consistent with magma autobrecciation. These features suggest that pyroclastic obsidian primarily forms near conduit walls where magma fragments and reanneals during ascent. We use these observations to develop a degassing model for pyroclastic obsidian from the A.D. 1340 Mono Craters, California, eruptions. We suggest that degassing was buffered by continual flux of vapor through highly permeable, brecciated magma along conduit walls. Continuous reequilibration of magma with vapor of relatively constant composition not only explains the CO 2 -H 2 O and δD-H 2 O data from Mono Craters pyroclastic obsidian, but also requires much lower magmatic CO 2 values than the commonly accepted model of closed-system degassing. Taken together, the chemical and physical evidence suggests that magma brecciation along conduit walls aids the degassing of ascending rhyolite.

Volatiles in Magmas

Abstract Magmas contain dissolved gases-volatiles-when they are at depth in the Earth. When magma ascends to the Earths surface, the volatiles can no longer remain dissolved because of the decrease of pressure, and this causes the formation and expansion of gas bubbles, creating a magmatic froth that can erupt explosively, depending on the gas content. Thus volatiles play an important role in governing the eruptive behavior of volcanoes. This chapter discusses the solubility, concentrations, and degassing behavior of the major volatiles (H2O, CO2, S, Cl, F) in magmas from various tectonic environments and the methods that have been used to gain this information. The origins of volatiles in the Earth, large-scale Earth degassing processes resulting from magmatism, and volatile recycling in subduction zones are also reviewed.

Minette lavas and associated leucitites from the western front of the Mexican Volcanic Belt: petrology, chemistry, and origin

During the late Pliocene, K-rich minette and leucitite lavas erupted in the western Mexican Volcanic Belt near the town of Los Volcanes, a region which is located much closer to the Middle America Trench than the main line of currently active andesite stratovolcanoes. During this period the tectonic regime in western Mexico was highly complex due to the simultaneous occurrence of active subduction of the young Rivera Plate, and rifting caused by crustal extension. Most of these basic lavas contain phenocrysts of phlogopite, augite and apatite, along with microphenocrysts of leucite and Fe-Ti oxide. Olivine is absent from all but two of the flows: one an olivine leucitite, and the other a felsic minette. The phlogopite phenocrysts and high whole rock Fe2O3/FeO ratios which are characteristic of this suite record evidence of high magmatic water contents and oxygen fugacities. All of these rock types are highly enriched in the large ion lithophile and light rare earth elements, with Sr≤5100 ppm, Ba≤4800 ppm and Ce≤330 ppm. They are also mildly enriched in the high field strength elements (e.g. Zr 260–700 ppm) and display the strong relative enrichment of the LILE over the HFSE that is characteristic of magmas erupted in convergent margins. Consideration of high pressure phase equilibria in the Mg2SiO4-CaMgSi2O6-KAlSiO4-SiO2 system shows that the minettes from this region are not related through fractional crystallization to the more MgO-rich, olivinebearing minettes which have erupted in other parts of the western Mexican Volcanic Belt during the Quaternary. This conclusion is consistent with both the trace element geochemistry of these lavas and with the results of fractional crystallization models. Instead, the data suggests that these high-K magmas were derived from a source region which consists predominantly of phlogopite, clinopyroxene and apatite, and which has formed through hydrous enrichment of the subarc mantle in response to subduction.

Water and partial melting in mantle plumes: Inferences from the dissolved H2O concentrations of Hawaiian basaltic magmas

Knowledge of dissolved H2O in primary Kilauean magmas provides a constraint on H2O abundance in the deep mantle region that feeds the upwelling plume beneath Hawaii. Given an H2O/K2O mass ratio of ∼1.3 for basaltic glasses and melt inclusions from Kilauea, the mantle source is estimated to contain 450±190 ppm H2O. This value is ∼3 times greater than that estimated for the mantle source for mid-ocean ridge basalts. Consideration of OH solubility in olivine suggests that water undersaturated melting of the upwelling Hawaiian plume probably begins at a depth of ∼250 km. Thus during plume ascent through most of the upper mantle, water is partitioned between nominally anhydrous silicates and a small mass fraction of entrained hydrous silicate melt. Because water strongly influences the viscosity of olivine aggregates, partial melting and melt extraction will have important effects on the rheology of the upwelling plume beneath Hawaii.

Zoned quartz phenocrysts from the rhyolitic Bishop Tuff

Abstract Cathodoluminescence (CL) reveals growth zones in quartz phenocrysts from the rhyolitic Bishop Tuff. Melt inclusions occur in various zones and record the evolving melt composition during zonal growth. The zones form an oscillatory pattern between bright and dark CL quartz. There are three recognizable patterns of CL zoning in these crystals: (1) weakly zoned cores and bright CL rims; (2) weakly zoned cores and dark CL rims; and (3) no CL intensity difference from core to rim. Dark CL quartz generally occurs at crystal edges, contains most of the melt inclusions and is interpreted as fast-growing. Zones that occur along recognizable crystal edges (edge zones) are thicker than the same zone on adjacent faces, consistent with relatively fast growth of these zones. In each successive zone, these edge zones decrease in size toward the rim, while the zones along the crystal faces increase. Some of the melt inclusions have bright CL quartz locally associated with them. This is interpreted as the postentrapment crystallization of slow-growing quartz in the melt inclusions. Many crystals display zone discordance from the weakly zoned interiors to the rims. Most of the discordant surfaces are rational and probably are primary growth features. Pumice clasts from the southern vents are largely compositionally and texturally distinct from those from the northern vents, and this distinction is also evident in the quartz CL. The crystals that have bright CL rims are all associated with the late-erupted northern part of the Bishop Tuff. Melt inclusion compositions and CL zoning patterns suggest a common origin for early and middle-erupted quartz and the interior zones of late-erupted quartz; however, the bright CL rim on the late-erupted quartz indicates an additional stage of crystallization in late-erupted magma. Melt inclusions in individual early erupted crystals have small variations in Ba whereas inclusions in late-erupted crystals markedly increase in Ba toward the rim, which is opposite to the normal zoning of sequentially trapped melts expected during closed system crystallization differentiation. The quartz zoning features are consistent with the hypothesis of crystal settling in evolving magma that erupted late from northern vents.

Mafic magma recharge supplies high CO2 and SO2 gas fluxes from Popocatépetl volcano, Mexico

Since late 1994, open-vent eruptive activity and degassing at Popocatepetl volcano, Mexico, have released large masses of CO2 and SO2. Tephra and lava produced by these eruptions show evidence for mixing of mafic and silicic magmas shortly before eruption. We present the first measurements of dissolved CO2 in the mafic magma end member based on analyses of olivine-hosted melt inclusions that were trapped at pressures as high as ~400 MPa (~15 km depth) beneath the volcano. We combine our data with thermodynamic models to show that degassing of mafic magma at ~150–350 MPa pressure can explain the CO2/SO2 mass ratios (1–8) of volcanic gases released from the volcano during 1995–1997. Our results demonstrate that mafic magma recharge was responsible for the high measured fluxes of CO2 and SO2 from 1995 to 1997. The total SO2 emission of 9 Mt during this period requires intrusion and degassing of a minimum of 0.8 km3 of mafic magma. Only ~0.3% of this new mafic magma has been erupted in the form of mixed (hybrid) lava and tephra. Our results suggest that the ongoing eruption of Popocatepetl is essentially an intrusive event. More generally, we suggest that intrusion and deep degassing may explain the high gas fluxes at some other open-vent volcanoes rather than convection of magma in the uppermost parts of subvolcanic conduits.

Magma degassing and basaltic eruption styles: a case study of ∼2000 year BP Xitle volcano in central Mexico

To investigate the relationship between volatile abundances and eruption style, we have analyzed major element and volatile (H2O, CO2, S) concentrations in olivine-hosted melt inclusions in tephra from the V2000 yr BP eruption of Xitle volcano in the central Trans-Mexican Volcanic Belt. The Xitle eruption was dominantly effusive, with fluid lava flows accounting for V95% of the total dense rock erupted material (1.1 km 3 ). However, in addition to the initial, Strombolian, cinder cone-building phase, there was a later explosive phase that interrupted effusive activity and deposited three widespread ash fall layers. Major element compositions of olivine-hosted melt inclusions from these ash layers range from 52 to 58 wt.% SiO2, and olivine host compositions are Fo84� 86. Water concentrations in the melt inclusions are variable (0.2^1.3 wt.% H2O), withan average of 0.45 : 0.3 (1 c) wt.% H2O. Sulfur concentrations vary from below detection (V50 ppm) to 1000 ppm but are mostly 9 200 ppm and show little correlation withH 2O. Only the two inclusions with the highest H2O have detectable CO2 (310^340 ppm), indicating inclusion entrapment at higher pressures (700^900 bars) than for the other inclusions (9 80 bars). The low and variable H2O and S contents of melt inclusions combined withth e absence of less soluble CO 2 indicates shallow-level degassing before olivine crystallization and melt inclusion formation. Olivine morphologies are consistent with the interpretation that most crystallization occurred rapidly during near-surface H2O loss. During cinder cone eruptions, the switch from initial explosive activity to effusive eruption probably occurs when the ascent velocity of magma becomes slow enough to allow near-complete degassing of magma at shallow depths within the cone as a result of buoyantly rising gas bubbles. This allows degassed lavas to flow laterally and exit near the base of the cone while gas escapes through bubbly magma in the uppermost part of the conduit just below the crater. The major element compositions of melt inclusions at Xitle show that the short-lived phase of renewed explosive activity was triggered by a magma recharge event, which could have increased overpressure in the storage reservoir beneath Xitle, leading to increased ascent velocities and decreased time available for degassing during ascent. = 2002 Elsevier Science B.V. All rights reserved.