Rare sulfur and triple oxygen isotope geochemistry of volcanogenic sulfate aerosols

We present analyses of stable isotopic ratios ^(17)O/^(16)O, ^(18)O/^(16)O, ^(34)S/^(32)S, and ^(33)S/^(32)S, ^(36)S/^(32)S in sulfate leached from volcanic ash of a series of well known, large and small volcanic eruptions. We consider eruptions of Mt. St. Helens (Washington, 1980, ∼1 km^3), Mt. Spurr (Alaska, 1953, <1 km3), Gjalp (Iceland, 1996, 1998, <1 km^3), Pinatubo (Phillipines, 1991, 10 km^3), Bishop tuff (Long Valley, California, 0.76 Ma, 750 km^3), Lower Bandelier tuff (Toledo Caldera, New Mexico, 1.61 Ma, 600 km^3), and Lava Creek and Huckleberry Ridge tuffs (Yellowstone, Wyoming, 0.64 Ma, 1000 km^3 and 2.04 Ma 2500 km^3, respectively). This list covers much of the diversity of sizes and the character of silicic volcanic eruptions. Particular emphasis is paid to the Lava Creek tuff for which we present wide geographic sample coverage. This global dataset spans a significant range in δ^(34)S, δ^(18)O, and Δ^(17)O of sulfate (29‰, 30‰, and 3.3‰, respectively) with oxygen isotopes recording mass-independent (Δ^(17)O > 0.2‰) and sulfur isotopes exhibiting mass-dependent behavior. Products of large eruptions account for most of‘ these isotopic ranges. Sulfate with Δ^(17)O > 0.2‰ is present as 1–10 μm gypsum crystals on distal ash particles and records the isotopic signature of stratospheric photochemical reactions. Sediments that embed ash layers do not contain sulfate or contain little sulfate with Δ^(17)O near 0‰, suggesting that the observed sulfate in ash is of volcanic origin. Mass-dependent fractionation of sulfur isotopic ratios suggests that sulfate-forming reactions did not involve photolysis of SO2, like that inferred for pre-2.3 Ga sulfates from Archean sediments or Antarctic ice-core sulfate associated with few dated eruptions. Even though the sulfate sulfur isotopic compositions reflect mass-dependent processes, the products of caldera-forming eruptions display a large δ^(34)S range and exhibit fractionation relationships that do not follow the expected equilibrium slopes of 0.515 and 1.90 for ^(33)S/^(32)S vs. ^(34)S/^(32)S and ^(36)S/^(32)S vs. ^(34)S/^(32)S, respectively. The data presented here are consistent with modification of a chemical mass-dependent fractionation of sulfur isotopes in the volcanic plume by either a kinetic gas phase reaction of volcanic SO_2 with OH and/or a Rayleigh processes involving a residual Rayleigh reactant—volcanic SO_2 gas, rather than a Rayleigh product. These results may also imply at least two removal pathways for SO_2 in volcanic plumes. Above-zero Δ^(17)O values and their positive correlation with δ^(18)O in sulfate can be explained by oxidation by high-δ^(18)O and high-Δ^(17)O compounds such as ozone and radicals such as OH that result from ozone break down. Large caldera-forming eruptions have the highest Δ^(17)O values, and the largest range of δ^(18)O, which can be explained by stratospheric reaction with ozone-derived OH radicals. These results suggest that massive eruptions are capable of causing a temporary depletion of the ozone layer. Such depletion may be many times that of the measured 3–8% depletion following 1991 Pinatubo eruption, if the amount of sulfur dioxide released scales with the amount of ozone depletion

In situ dating and investigation of remarkably depleted –27.3‰ SMOW “Slushball” Earth zircons

Paleoproterozoic amphibolites and gneisses - that are remarkably depleted in ^(18)O are found in the Belomorian Belt in Karelia, Russia [1,2]. We mapped their extent to exceed 200x20km and affect metamorphosed mafic intrusions (est. ~2.4 Ga intrusion age) and host 2.6Ga gneisses found in this 1.9 Ga collisional belt. δ^(18)O values of –7 to –27.3‰ characterize minerals and rocks from several of these localities; some of these rocks are also remarkably depleted with respect to δD (-212 to –235‰ amphiboles). All have typical terrestrial Δ^(17)O values of 0‰. Based on previous paleogeographic reconstructions, we attribute the origin of these exotic O and H isotope compositions to the hydrothermal alteration associated with subglacial rifting during the Paleoproterozoic panglobal ice ages, but discuss additional possibilities: extremely low-δ^(18)O Paleo- proterozoic sea water, and excursion of Karelia to polar latitudes. Given that at high-T hydrothermal exchange equilibrium Δ^(18)O(rockwater) is close to zero, but water-rock interaction is rarely 100% efficient, the lowest measured δ^(18)O value in silicates likely gives the upper δ^(18)O bound for the altering meteoric fluid; we thus continues our quest to find the lowest δ^(18)O material such as a mineral assemblage or a tiny zircon fragment that would provide record of δ^(18)Owater

Changing Mantle Sources and the Effects of Crustal Passage on the Steens Basalt, SE Oregon: Chemical and Isotopic Constraints

Continental flood basalts are more prone to compositional modification from passage through thicker and (or) more felsic crust in comparison to their oceanic counterparts. The Steens Basalt in southeast Oregon (~17 Ma) is among the oldest and most mafic members of the Columbia River Basalt Group and provides a record of the early stages of flood basalt volcanism. We evaluate the balance of mantle sources in time during the onset of Columbia River Basalt Group magmatism and assess the effect of crustal passage using stratigraphically controlled Sr, Nd, Pb, Hf, Os, and O isotopic compositions, as well as whole rock major and trace element data. Mixing models indicate that depleted and enriched mantle sources identified by previous workers contribute in varying proportions during the life of the magmatic system, with the greatest contribution by depleted mantle when eruption rate and presumed intrusion rate increase. During waxing, enrichment of δ18O in some flows signals cryptic deep fractionation of abundant clinopyroxene followed by shallow fractionation of olivine ± clinopyroxene ± plagioclase. Os concentrations are among the highest worldwide at a given MgO (0.29–0.86 ppb at 6.0 to 10.9 wt.%). We argue that high Os results from scavenging of sulfides by recharging magmas passing through earlier crystallized magmas. Elevated 87Sr/86Sr in the latest stage supports modest assimilation of partial melts from mafic accreted terranes, facilitated by thermal priming of crust by persistent magmatism. This work provides a more detailed schematic view of the Steens Basalt magmatic system, from mantle origin through crustal staging

Alteration of volcaniclastic deposits at Minna Bluff : geochemical insights on mineralizing environment and climate during the Late Miocene in Antarctica

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 15 (2014): 3258–3280, doi:10.1002/2014GC005422.Secondary minerals in volcaniclastic deposits at Minna Bluff, a 45 km long peninsula in the Ross Sea, are used to infer processes of alteration and environmental conditions in the Late Miocene. Glassy volcaniclastic deposits are altered and contain phillipsite and chabazite, low to high-Mg carbonates, chalcedony, and clay. The δ18O of carbonates and chalcedony is variable, ranging from −0.50 to 21.53‰ and 0.68 to 10.37‰, respectively, and δD for chalcedony is light (−187.8 to −220.6‰), corresponding to Antarctic meteoric water. A mean carbonate 87Sr/86Sr ratio of 0.70327 ± 0.0009 (1σ, n = 12) is comparable to lava and suggests freshwater, as opposed to seawater, caused the alteration. Minerals were precipitated at elevated temperatures (91 and 104°C) based on quartz-calcite equilibrium, carbonate 13C-18C thermometry (Δ47 derived temperature = 5° to 43°C) and stability of zeolites in geothermal systems (>10 to ∼100°C). The alteration was a result of isolated, ephemeral events involving the exchange between heated meteoric water and glass during or soon after the formation of each deposit. Near-surface evaporative distillation can explain 18O-enriched compositions for some Mg-rich carbonates and chalcedony. The δ18Owater calculated for carbonates (−15.8 to −22.9‰) reveals a broad change, becoming heavier between ∼12 and ∼7 Ma, consistent with a warming climate. These findings are independently corroborated by the interpretation of Late Miocene sedimentary sequences recovered from nearby sediment cores. However, in contrast to a cold-based thermal regime proposed for ice flow at core sites, wet-based conditions prevailed at Minna Bluff; a likely consequence of high heat flow associated with an active magma system.This research was funded by a collaborative grant NSF OPP 05-38033. It also was supported by UNED/NSF 250550001146, NSF grants EAR-0949191, ARC-1215551, EAR-1325054, EAR-1352212, EAR-1049351, ACS grant 51182-DNI2, DOE grants DE-FG02-13ER16402, and DE-SC0010288, a Hellman Fellowship, and a Katzner grant (BGSU).2015-02-1

Composition and evolution of the melts erupted in 1996 at Karymskoe Lake, Eastern Kamchatka: Evidence from inclusions in minerals

The powerful eruption in the Akademii Nauk caldera on January 2, 1996, marked a new activity phase of Karymsky volcano and became a noticeable event in the history of modern volcanism in Kamchatka. The paper reports data obtained by studying more than 200 glassy melt inclusions in phenocrysts of olivine (Fo82–72), plagioclase (An92–73), and clinopyroxene (Mg# 83–70) in basalts of the 1996 eruption. The data were utilized to estimate the composition of the parental melt and the physicochemical parameters of the magma evolution. According to our data, the parental melt corresponded to low magnesian, highly aluminous basalt (SiO2 = 50.2 wt %, MgO = 5.6 wt %, Al2O3 = 17 wt %) of the mildly potassic type (K2O = 0.56 wt %) and contained much dissolved volatile components (H2O = 2.8 wt %, S = 0.17 wt %, and Cl = 0.11 wt %). Melt inclusions in the minerals are similar in chemical composition, a fact testifying that the minerals crystallized simultaneously with one another. Their crystallization started at a pressure of approximately 1.5 kbar, pro ceeded within a narrow temperature range of 1040 ± 20°C, and continued until a nearsurface pressure of approximately 100 bar was reached. The degree of crystallization of the parental melt during its eruption was close to 55%. Massive crystallization was triggered by H2O degassing under a pressure of less than 1 kbar. Magma degassing in an open system resulted in the escape of 82% H2O, 93% S, and 24% Cl (of their initial contents in the parental melt) to the fluid phase. The release of volatile compounds to the atmosphere during the eruption that lasted for 18 h was estimated at 1.7 × 106 t H2O, 1.4 × 105 t S, and 1.5 × 104 t Cl. The concen trations of most incompatible trace elements in the melt inclusions are close to those in the rocks and to the expected fractional differentiation trend. Melt inclusions in the plagioclase were found to be selectively enriched in Li. The Lienriched plagioclase with melt inclusions thought to originate from cumulate layers in the feeding system beneath Karymsky volcano, in which plagioclase interacted with Lirich melts/brines and was subsequently entrapped and entrained by the magma during the 1996 eruption

A 50-year record of NOx and SO2 sources in precipitation in the Northern Rocky Mountains, USA

Ice-core samples from Upper Fremont Glacier (UFG), Wyoming, were used as proxy records for the chemical composition of atmospheric deposition. Results of analysis of the ice-core samples for stable isotopes of nitrogen (δ15N, ) and sulfur (δ34S, ), as well as and deposition rates from the late-1940s thru the early-1990s, were used to enhance and extend existing National Atmospheric Deposition Program/National Trends Network (NADP/NTN) data in western Wyoming. The most enriched δ34S value in the UFG ice-core samples coincided with snow deposited during the 1980 eruption of Mt. St. Helens, Washington. The remaining δ34S values were similar to the isotopic composition of coal from southern Wyoming. The δ15N values in ice-core samples representing a similar period of snow deposition were negative, ranging from -5.9 to -3.2 ‰ and all fall within the δ15N values expected from vehicle emissions. Ice-core nitrate and sulfate deposition data reflect the sharply increasing U.S. emissions data from 1950 to the mid-1970s

Light Stable Isotopic Compositions of Enriched Mantle Sources: Resolving the Dehydration Paradox

Volatile and stable isotope data provide tests of mantle processes that give rise to mantle heterogeneity. New data on enriched mid‐oceanic ridge basalts (MORB) show a diversity of enriched components. Pacific PREMA‐type basalts (H2O/Ce = 215 ± 30, δDSMOW = −45 ± 5 ‰) are similar to those in the northern Atlantic (H2O/Ce = 220 ± 30; δDSMOW = −30 to −40 ‰). Basalts with EM‐type signatures have regionally variable volatile compositions. Northern Atlantic EM‐type basalts are wetter (H2O/Ce = 330 ± 30) and have isotopically heavier hydrogen (δDSMOW = −57 ± 5 ‰) than northern Atlantic MORB. Southern Atlantic EM‐type basalts are damp (H2O/Ce = 120 ± 10) with intermediate δDSMOW (−68 ± 2 ‰), similar to δDSMOW for Pacific MORB. Northern Pacific EM‐type basalts are dry (H2O/Ce = 110 ± 20) and isotopically light (δDSMOW = −94 ± 3 ‰). A multistage metasomatic and melting model accounts for the origin of the enriched components by extending the subduction factory concept down through the mantle transition zone, with slab temperature a key variable. Volatiles and their stable isotopes are decoupled from lithophile elements, reflecting primary dehydration of the slab followed by secondary rehydration, infiltration, and re‐equilibration by fluids derived from dehydrating subcrustal hydrous phases (e.g., antigorite) in cooler, deeper parts of the slab. Enriched mantle sources form by addition o