Terrence M. Gerlach

Metal emissions from Kilauea, and a suggested revision of the estimated worldwide metal output by quiescent degassing of volcanoes

Abstract Measurements of a large suite of metals (Pb, Cd, Cu, Zn and several others) and sulfur at Kilauea volcano over an extended period of time has yielded a detailed record of the atmospheric injection of ordinarily-rare metals from this quiescently degassing volcano, representative of an important type. We have combined the Kilauea data with data of recent studies by others (emissions from volcanoes in the Indonesian arc; the large Laki eruption of two centuries ago; Etna; estimates of total volcanic emissions of sulfur) to form the basis for a new working estimate of the rate of worldwide injection of metals to the atmosphere by volcanoes. The new estimate is that volcanoes inject a substantially smaller mass of ordinarily-rare metals into the atmosphere than was stated in a widely cited previous estimate [J.O. Nriagu, A global assessment of natural sources of atmospheric trace metals, Nature 338 (1989) 47–49]. Our estimate, which is an upper limit, is an annual injection mass of about 10,000 tons of the metals considered, versus the earlier estimate of about 23,000 tons. Also, the proportions of the metals are substantially different in our new estimate.

Magmatic Vapor Source for Sulfur Dioxide Released During Volcanic Eruptions: Evidence from Mount Pinatubo

Sulfur dioxide (SO2) released by the explosive eruption of Mount Pinatubo on 15 June 1991 had an impact on climate and stratospheric ozone. The total mass of SO2 released was much greater than the amount dissolved in the magma before the eruption, and thus an additional source for the excess SO2 is required. Infrared spectroscopic analyses of dissolved water and carbon dioxide in glass inclusions from quartz phenocrysts demonstrate that before eruption the magma contained a separate, SO2-bearing vapor phase. Data for gas emissions from other volcanoes in subduction-related arcs suggest that preeruptive magmatic vapor is a major source of the SO2 that is released during many volcanic eruptions.

Evaluation of gases, condensates, and SO2 emissions from Augustine volcano, Alaska: the degassing of a Cl-rich volcanic system

After the March–April 1986 explosive eruption a comprehensive gas study at Augustine was undertaken in the summers of 1986 and 1987. Airborne COSPEC measurements indicate that passive SO2 emission rates declined exponentially during this period from 380±45 metric tons/day (T/D) on 7/24/86 to 27±6 T/D on 8/24/87. These data are consistent with the hypothesis that the Augustine magma reservoir has become more degassed as volcanic activity decreased after the spring 1986 eruption. Gas samples collected in 1987 from an 870°C fumarole on the andesitic lava dome show various degrees of disequilibrium due to oxidation of reduced gas species and condensation (and loss) of H2O in the intake tube of the sampling apparatus. Thermochemical restoration of the data permits removal of these effects to infer an equilibrium composition of the gases. Although not conclusive, this restoration is consistent with the idea that the gases were in equilibrium at 870°C with an oxygen fugacity near the Ni−NiO buffer. These restored gas compositions show that, relative to other convergent plate volcanoes, the Augustine gases are very HCl rich (5.3–6.0 mol% HCl), S rich (7.1 mol% total S), and H2O poor (83.9–84.8 mol% H2O). Values of δD and δ18O suggest that the H2O in the dome gases is a mixture of primary magmatic water (PMW) and local seawater. Part of the Cl in the Augustine volcanic gases probably comes from this shallow seawater source. Additional Cl may come from subducted oceanic crust because data by Johnston (1978) show that Cl-rich glass inclusions in olivine crystals contain hornblende, which is evidence for a deep source (>25km) for part of the Cl. Gas samples collected in 1986 from 390°–642°C fumaroles on a ramp surrounding the inner summit crater have been oxidized so severely that restoration to an equilibrium composition is not possible. H and O isotope data suggest that these gases are variable mixtures of seawater, FMW, and meteoric steam. These samples are much more H2O-rich (92%–97% H2O) than the dome gases, possibly due to a larger meteoric steam component. The 1986 samples also have higher Cl/S, S/C, and F/Cl ratios, which imply that the magmatic component in these gases is from the more degassed 1976 magma. Thus, the 1987 samples from the lava dome are better indicators than the 1986 samples of degassing within the Augustine magma reservoir, even though they were collected a year later and contain a significant seawater component. Future gas studies at Augustine should emphasize fumaroles on active lava domes. Condensates collected from the same lava-dome fumarole have enrichments ot 107–102 in Cl, Br, F, B, Cd, As, S, Bi, Pb, Sb, Mo, Zn, Cu, K, Li, Na, Si, and Ni. Lower-temperature (200°–650°C) fumaroles around the volcano are generally less enriched in highly volatile elements. However, these lower-termperature fumaroles have higher concentration of rock-forming elements, probably derived from the wall rock.

Gas Emissions and the Eruptions of Mount St. Helens Through 1982

The monitoring of gas emissions from Mount St. Helens includes daily airborne measurements of sulfur dioxide in the volcanic plume and monthly sampling of gases from crater fumaroles. The composition of the fumarolic gases has changed slightly since 1980: the water content increased from 90 to 98 percent, and the carbon dioxide concentrations decreased from about 10 to 1 percent. The emission rates of sulfur dioxide and carbon dioxide were at their peak during July and August 1980, decreased rapidly in late 1980, and have remained low and decreased slightly through 1981 and 1982. These patterns suggest steady outgassing of a single batch of magma (with a volume of not less than 0.3 cubic kilometer) to which no significant new magma has been added since mid-1980. The gas data were useful in predicting eruptions in August 1980 and June 1981.

Soil efflux and total emission rates of magmatic CO2 at the Horseshoe Lake tree kill, Mammoth Mountain, California, 1995–1999

Abstract We report the results of eight soil CO 2 efflux surveys by the closed circulation chamber method at the Horseshoe Lake tree kill (HLTK) — the largest tree kill on Mammoth Mountain. The surveys were undertaken from 1995 to 1999 to constrain total HLTK CO 2 emissions and to evaluate occasional efflux surveys as a surveillance tool for the tree kills. HLTK effluxes range from 1 to >10,000 g m −2 day −1 (grams CO 2 per square meter per day); they are not normally distributed. Station efflux rates can vary by 7–35% during the course of the 8- to 16-h surveys. Disturbance of the upper 2 cm of ground surface causes effluxes to almost double. Semivariograms of efflux spatial covariance fit exponential or spherical models; they lack nugget effects. Efflux contour maps and total CO 2 emission rates based on exponential, spherical, and linear kriging models of survey data are nearly identical; similar results are also obtained with triangulation models, suggesting that the kriging models are not seriously distorted by the lack of normal efflux distributions. In addition, model estimates of total CO 2 emission rates are relatively insensitive to the measurement precision of the efflux rates and to the efflux value used to separate magmatic from forest soil sources of CO 2 . Surveys since 1997 indicate that, contrary to earlier speculations, a termination of elevated CO 2 emissions at the HLTK is unlikely anytime soon. The HLTK CO 2 efflux anomaly fluctuated greatly in size and intensity throughout the 1995–1999 surveys but maintained a N–S elongation, presumably reflecting fault control of CO 2 transport from depth. Total CO 2 emission rates also fluctuated greatly, ranging from 46 to 136 t day −1 (metric tons CO 2 per day) and averaging 93 t day −1 . The large inter-survey variations are caused primarily by external (meteorological) processes operating on time scales of hours to days. The externally caused variations can mask significant changes occurring at depth; a striking example is the masking of a degassing event generated at depth and detected by a soil gas sensor network in September 1997 while an efflux survey was in progress. Thus, occasional efflux surveys are not an altogether effective surveillance tool for the HLTK, and making them effective by greatly increasing their frequency may not be practical.

Evaluation of volcanic gas analyses from Kilauea volcano

Abstract Much of the chemical variation and disequilibrium in Kilauean volcanic gas analyses is the result of contamination by meteoric water and, to a lesser extent, organic matter. Contamination by meteoric water is extensive in some samples, causing variations of two to three orders of magnitude in atomic H/C. severely contaminated with meteoric water that did not equilibrate with the “Magmatic” gases and consequently gives the analyses a disequilibrium appearance. When the contaminating H 2 O is removed, the analyses converge to equilibrium mixtures in the temperature range 1085°C to 1185°C with O 2 fugacities slightly above those for the quartz-magnetite-fayalite buffer. The restored analyses are richer in CO 2 (18–50%) and SO 2 (10–21%) and poorer in H 2 O (37–70%) than are the original analyses. There is no single gas phase composition for erupting Kilauea basalts. The restored J-series gases and the higher quality gases collected from Kilauea since 1920 form a spectrum of gas compositions related by CO 2 content. A trend of decreasing CO 2 with progressive outgassing is suggested. The extremes in the spectrum of compositions, based on presently available data, range from an “early” magmatic gas containing 50% CO 2 , 35% H 2 O, and 15% SO 2 to a more evolved gas with 70% H 2 O, 10% CO 2 and 20% SO 2 . The common presence of hydrocarbons and anomalously low atomic S/C in several Kilauea gas analyses is due to contamination of erupting lavas with organic materials. A similar origin also applies to above background levels of environmentally hazardous organohalogens in volcanic gases. Thermodynamic calculations indicate these compounds would be virtually absent in gases erupted from lavas at temperatures above 500–600°C.

Present‐day CO2 emissions from volcanos

In an effort to better understand processes that control sources of CO2 in the carbon cycle, the U.S. Global Change Research Program [CEES, 1990] identifies imbonate deposition, and burial of organic matter would deplete the CO2 content of the atmosphere in 10,000 years and the atmosphere-ocean system in 500,000 years [Holland, 1978; Berner et al., 1983]. The CO2 content of the atmosphere-ocean system has varied in the past, but not at the rate expected if CO2 were removed and not replenished. It is assumed, therefore, that CO2 de gassing from the Earths interior restores the deficit from surficial processes and balances the atmospheric CO2 budget on a time scale of 104–106yr. Earlier atmospheric balancing calculations imply present-day (pre-industrial) CO2 degassing rates of 6–7×1012 mol yr−1 [Holland, 1978; Berner et al., 1983]; recent calculations suggest degassing rates may be as high as 11×1012 mol yr−1 [Berner, 1990].

Vapor saturation and accumulation in magmas of the 1989–1990 eruption of Redoubt Volcano, Alaska

Abstract The 1989–1990 eruption of Redoubt Volcano, Alaska, provided an opportunity to compare petrologic estimates of SO 2 and Cl emissions with estimates of SO 2 emissions based on remote sensing data and estimates of Cl emissions based on plume sampling. In this study, we measure the sulfur and chlorine contents of melt inclusions and matrix glasses in the eruption products to determine petrologic estimates of SO 2 and Cl emissions. We compare the results with emission estimates based on COSPEC and TOMS data for SO 2 and data for Cl/SO 2 in plume samples. For the explosive vent clearing period (December 14–22, 1989), the petrologic estimate for SO 2 emission is 21,000 tons, or ~12% of a TOMS estimate of 175,000 tons. For the dome growth period (December 22, 1989 to mid-June 1990), the petrologic estimate for SO 2 emission is 18,000 tons, or ~3% of COSPEC-based estimates of 572,000–680,000 tons. The petrologic estimates give a total SO 2 emission of only 39,000 tons compared to an integrated TOMS/COSPEC emission estimate of ~1,000,000 tons for the whole eruption, including quiescent degassing after mid-June 1990. Petrologic estimates also appear to underestimate Cl emissions, but apparent HCl scavenging in the plume complicates Cl emission comparisons. Several potential sources of ‘excess sulfur’ often invoked to explain petrologic SO 2 deficits are concluded to be unlikely for the 1989–1990 Redoubt eruption — e.g., breakdown of sulfides, breakdown of anhydrite, release of SO 2 from a hydrothermal system, degassing of commingled infusions of basalt in the magma chamber, and syn-eruptive degassing of sulfur from melt present in non-erupted magma. Leakage and/or diffusion of sulfur from melt inclusions do not provide convincing explanations for the petrologic SO 2 deficits either. The main cause of low petrologic estimates for SO 2 is that melt inclusions do not represent the total sulfur content of the Redoubt magmas, which were vapor-saturated magmas carrying most of their sulfur in an accumulated vapor phase. Almost all the sulfur of the SO 2 emissions was present prior to emission as accumulated magmatic vapor at 6–10 km depth in the magma that supplied the eruption; whole-rock normalized concentrations of gaseous excess S in these magmas remained at ~0.2 wt.% throughout the eruption, equivalent to ~0.7 vol.% at depth. Data for CO 2 emissions during the eruption indicate that CO 2 at whole-rock concentrations of ~0.6 wt.% in the erupted magma was a key factor in creating the vapor saturation and accumulation condition making a vapor phase source of excess sulfur possible at depth. When explosive volcanism involves magma with accumulated vapor, melt inclusions do not provide a sufficient basis for predicting SO 2 emissions. Thus, petrologic estimates made for SO 2 emissions during explosive eruptions of the past may be too low and may significantly underestimate impacts on climate and the chemistry of the atmosphere.

Three‐year decline of magmatic CO2 emissions from soils of a Mammoth Mountain Tree Kill: Horseshoe Lake, CA, 1995–1997

We used the closed chamber method to measure soil CO2 efflux over a three-year period at the Horseshoe Lake tree kill (HLTK)—the largest tree kill on Mammoth Mountain in central eastern California. Efflux contour maps show a significant decline in the areas and rates of CO2 emission from 1995 to 1997. The emission rate fell from 350 t d−1 (metric tons per day) in 1995 to 130 t d−1 in 1997. The trend suggests a return to background soil CO2 efflux levels by early to mid 1999 and may reflect exhaustion of CO2 in a deep reservoir of accumulated gas and/or mechanical closure or sealing of fault conduits transmitting gas to the surface. However, emissions rose to 220 t d−1 on 23 September 1997 at the onset of a degassing event that lasted until 5 December 1997. Recent reservoir recharge and/or extension-enhanced gas flow may have caused the degassing event.

Annual cycle of magmatic CO2 in a tree-kill soil at Mammoth Mountain, California: Implications for soil acidification

Time-series sensor data reveal significant short-term and seasonal variations of magmatic CO 2 in soil over a 12 month period in 1995–1996 at the largest tree-kill site on Mammoth Mountain, central-eastern California. Short-term variations leading to ground-level soil CO 2 concentrations hazardous and lethal to humans were triggered by shallow faulting in the absence of increased seismicity or intrusion, consistent with tapping a reservoir of accumulated CO 2 , rather than direct magma degassing. Hydrologic processes closely modulated seasonal variations in CO 2 concentrations, which rose to 65%–100% in soil gas under winter snowpack and plunged more than 25% in just days as the CO 2 dissolved in spring snowmelt. The high efflux of CO 2 through the tree-kill soils acts as an open-system CO 2 buffer causing infiltration of waters with pH values commonly of H+ ṁha −1 ṁyr −1 , mobilization of toxic Al 3+ , and long-term decline of soil fertility.