Native sulfur deposits on fumarolic fields at Ebeko volcano (Northern Kuriles, Russia) are enriched in chalcophile elements (As-Sb-Se-Te-Hg-Cu) and contain rare heavy metal sulfides (Ag2S, HgS, and CuS), native metal alloys (Au2Pd), and some other low-solubility minerals (CaWO4, BaSO4). Sulfur incrustations are impregnated with numerous particles of fresh and altered andesite groundmass and phenocrysts (pyroxene, magnetite) as well as secondary minerals, such as opal, alunite, and abundant octahedral pyrite crystals. The comparison of elemental abundances in sulfur and unaltered rocks (andesite) demonstrated that rock-forming elements (Ca, K, Fe, Mn, and Ti) and other lithophile and chalcophile elements are mainly transported by fumarolic gas as aerosol particles, whereas semimetals (As, Sb, Se, and Te), halogens (Br and I), and Hg are likely transported as volatile species, even at temperatures slightly above 100°C. The presence of rare sulfides (Ag2S, CuS, and HgS) together with abundant FeS2 in low-temperature fumarolic environments can be explained by the hydrochloric leaching of rock particles followed by the precipitation of low-solubility sulfides induced by the reaction of acid solutions with H2S at ambient temperatures. The elemental composition of native sulfur can be used to qualitatively estimate elemental abundances in low-temperature fumarolic gases.
Volcanic fumaroles are surficial manifestations of magmatic degassing. Fumarolic gases mainly comprise the volatile components of H2O, CO2, SO2, HCl, and HF, but high-temperature fumarolic gases (>400°C) also commonly transport many metallic and nonmetallic compounds, which have sufficiently high vapor pressure at elevated temperatures. These include native elements, oxides, halides, and more complex compounds (e.g., [
Rapid decrease in the temperature of gas at or near the surface, together with abrupt change from reducing to oxidizing conditions, results in the oversaturation of the transported volatile compounds. The latter precipitate inside or around fumaroles, forming black or colorful deposits, which are called sublimates. It has long been noticed that the zonation and compositions of high-temperature fumarolic sublimates are similar to those of ore bodies that form in magmatic environment [
Low-temperature fumaroles (i.e., 200°C and lower) are not well-characterized in terms of their gaseous transport due to their generally low concentrations of metallic elements; the saturation vapor pressures of these elements decrease by 1-2 orders of magnitude for every hundred degrees. Very low metal contents in low-temperature condensates are often obscured by an overwhelming amount of colloidal sulfur [
At the same time, metals and aerosol particles are transported by low-temperature gas and precipitate together with native sulfur around fumarole vents, forming well-known yellow incrustations. These incrustations and even well-shaped sulfur crystals often contain mineral and rock inclusions, which were also carried by and deposited from fumarolic gas. Such sulfur can help to study the gases themselves. All components carried by the gas are inevitably found in fumarolic sulfur; therefore, the composition of the gas can be estimated at a qualitative level. Unfortunately, the quantitative analysis of this gas composition using fumarolic sulfur is impossible due to the strong fractionation of elements, including sulfur itself that occurs during the discharge of gas. Most of the sulfur and other metallic and nonmetallic elements escape fumarolic vents with the gas and are dissipated in the environment; thus, only a small (and unknown) fraction of these elements is deposited in incrustations.
The existing data on volcanic sulfur composition data have been obtained using mass-spectrometry (ICP-MS), atomic emission spectroscopy (ICP-AES), scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), reflectance spectrometry [
Zeng et al. [
Kim et al. examined sulfide and sulfosalt inclusions in molten sulfur from a submarine volcano (Lau Basin, Southwestern Pacific). This sulfur contained abundant inclusions of pyrite (FeS2), covellite (CuS), and Cu-As sulfosalts, as well as measurable amounts of Au, Ag, Ga, Ge, Tl, and other chalcophile elements. The high enrichment factors of these metals suggested that they originated from magma via volcanic degassing.
In the present work, we report new data on the chemical composition of native sulfur, leachates from native sulfur, and rare minerals as inclusions in native sulfur from Ebeko volcano (North Kuriles, Russian Far East) in order to understand the transport of elements in low-temperature fumarolic gases. Native sulfur from the volcano was studied using a variety of modern methods (including Synchrotron Radiation induced X-ray Fluorescence Spectroscopy (SR-XRF), leaching by water followed by ion chromatography and ICP-AES, and dissolution in carbon disulfide followed by SEM-EDS). Ebeko was chosen because of its accessibility as well as the fact that it features various and persistent fumarolic activity occurring at several fumarolic fields. Our data show that Ebeko sulfur is enriched in volatile chalcophile elements. The presence of rare sulfides (i.e., argentite, covellite, and cinnabar) and abundant pyrite may be fully explained by the acid leaching of rock particles followed by sulfide precipitation from leachates; that is, rock aerosols may serve as a source for chalcophile metals and iron.
Ebeko (51.41°N, 176.01°E, 1156 m asl) is an active andesitic stratovolcano in the Kurile island arc, where the Pacific Plate is being subducted under the Okhotsk Plate. Ebeko is located in the northern part of Paramushir Island (Figure
Location of Ebeko volcano and a sampling scheme with fumarolic fields.
Ebeko is characterized by occasional phreatomagmatic eruptions and abundant permanent fumaroles and thermal springs [
Fumarole edifice, the Southeast field, Ebeko volcano. Photo by T. Kotenko.
(a) Acicular sulfur crystals deposited from fumarolic gas; a gas channel covered with molten sulfur is visible. (b) Layered sulfur deposits of different colors around a fumarolic vent. The red color is due to arsenic and the greenish-gray color is due to dispersed pyrite crystals. South field, Ebeko volcano. Photos by M. Zelenski.
Thirty samples of native sulfur (100–150 g each) were collected from the active and extinct fumaroles in the Northeast (NE), Southeast (SE), South, and July fumarolic fields at Ebeko volcano (Figure
The VEPP-3 electron-positron storage ring and a wiggler with a magnetic field of 2 T were used; the electron beam energy of the storage ring was 2.00 GeV; and the typical electron current was 100 mA. A silicon (111) single-crystal vacuum monochromator for the energy range of 5–47 keV and an energy resolution (Δ
The measurements were made using an irradiation energy of 30 KeV for other elements; measuring time was 100 s per sample. This allowed us to increase signal/noise ratio for light elements. The peak areas of the analyzed and standard samples were normalized to the peak area of Compton scattering. Spectra processing was performed using the nonlinear least-square method using the AXIL-PC software (MS-DOS version [
The matrix correction was performed as follows. The matrices of the studied samples and the external standard sample were almost identical. As no certified standards for native sulfur exist, we prepared our own standard. This standard comprised superpure native sulfur, with the addition of a 2.5% HCl solution containing elements (e.g., As, Sb, Se, Te, Cd, Zn, and Cu) ranging in concentration from 10 to 100 ppm. After the addition of the solution aliquots, the sulfur powder was dried at 85°C to a constant weight. For the analyses, 30 mg pellets that were six mm in diameter were molded [
The results presented here include analyses of unaltered host rocks (andesites), analyses of fumarolic sulfur, analyses of sulfur leachates, and the SEM-EDS analyses of rare mineral inclusions in sulfur.
Twelve rock samples were collected at the Ebeko summit and its vicinities, including fresh volcanic bombs from the recent 2009 eruption and samples from the lava flows downstream of the Kuzminka and Yurieva Rivers. All samples are two-pyroxene andesites. The phenocrysts include orthopyroxene, clinopyroxene, plagioclase, and Ti-magnetite. The groundmass contains microlites of plagioclase, amphibole, abundant magnetite crystals, and felsic glass. Rare Cu-rich sulfide globules up to 20
The compositions of the analyzed Ebeko andesites are given in Table
Average composition of Ebeko andesites
Oxide | Wt.% | Element | ppm | Element | ppm |
---|---|---|---|---|---|
SiO2 | 58.1 | Ag | 0.40 | Ni | 26 |
TiO2 | 0.7 | As | 4.40 | Pb | 8.1 |
Al2O3 | 17.3 | Ba | 422 | Rb | 44 |
Fe2O3 | 8.0 | Br | 2.0 | Sb | 0.44 |
MnO | 0.2 | Cd | 0.32 | | 0.05 |
MgO | 3.3 | Cr | 51 | Sn | 1.7 |
CaO | 6.9 | Cs | 1.6 | Sr | 391 |
Na2O | 3.2 | Cu | 78 | | 0.001 |
K2O | 2.2 | Ga | 13.8 | Th | 4.8 |
P2O5 | 0.20 | Ge | 2.4 | U | 1.6 |
| 0.08 | V | 133 | ||
I | 0.34 | Y | 21 | ||
Mo | 1.2 | Zn | 65 | ||
Nb | 2.25 | Zr | 109 |
Samples were collected from 30 sulfur fumaroles located on four different fumarolic fields (Figure
The most informative analysis was the SR-RXF analysis, which provided data for 31 elements (Table
Concentrations of trace elements in fumarolic sulfur from Ebeko.
Sample | Location | K | Ca | Mn | Fe | Ti | Ga | Ge | Y | Zr | Sr | Rb | Nb | Th | U | Ni | |
ppm | |||||||||||||||||
| |||||||||||||||||
s-1 | July field | 2000 | 1900 | l.d. | 140 | 60 | 7.0 | 4.5 | 1.6 | 1.2 | 2.2 | 1.2 | l.d. | l.d. | l.d. | 16 | |
s-1/a | 1400 | 1400 | l.d. | 400 | 50 | 6.5 | 3.04 | 1.8 | 2.1 | 7.6 | 1.2 | 0.37 | l.d. | l.d. | 15 | ||
s-2 | 2000 | 1500 | l.d. | 70 | l.d. | 5.6 | 5.34 | 0.99 | 1.5 | 1.5 | 0.75 | 0.31 | l.d. | l.d. | 14 | ||
s-3 | 1800 | 1300 | l.d. | 80 | l.d. | 6.7 | 4.6 | 1.05 | 0.42 | 1.8 | 0.63 | 0.2 | l.d. | l.d. | 13 | ||
s-4 | 2100 | 1900 | l.d. | 100 | 40 | 6.9 | 5.8 | 1.02 | l.d. | 2.1 | 0.64 | 0.43 | l.d. | l.d. | 18 | ||
s-5 | 900 | 1400 | l.d. | 100 | 70 | 6.4 | 5.1 | 0.9 | 0.44 | 1.7 | 1.4 | 0.37 | l.d. | l.d. | 17 | ||
s-6 | 2300 | 1800 | l.d. | 60 | 60 | 7.3 | 4.02 | 1.07 | 0.59 | 1.7 | 0.93 | l.d. | l.d. | l.d. | 19 | ||
s-7 | 1500 | 1500 | l.d. | 70 | 60 | 6.5 | 4.3 | 1.1 | 0.35 | 1.8 | 0.55 | 0.22 | l.d. | l.d. | 15 | ||
s-8 | 2600 | 1900 | l.d. | 240 | 70 | 6.6 | 5.1 | 1.2 | 1.4 | 3.06 | 1.09 | 0.4 | l.d. | l.d. | 13 | ||
s-9 | 2000 | 2000 | l.d. | 90 | l.d. | 5.8 | 5.0 | 1.1 | 0.64 | 1.8 | 0.75 | 0.22 | l.d. | l.d. | 15 | ||
s-10 | 2200 | 1400 | l.d. | 100 | 40 | 5.9 | 4.3 | 1.05 | 0.85 | 1.6 | 0.63 | 0.31 | l.d. | l.d. | 13 | ||
E-3 | NE field | 2900 | 800 | 50 | 2100 | 2700 | 2.9 | 0.85 | 3.4 | 78 | 130 | 7.4 | 2.2 | 2.1 | 1.8 | 2.3 | |
E-6 | 2300 | 780 | 80 | 3200 | 2600 | 3.8 | 1.0 | 2.2 | 100 | 19 | 3.4 | 2.5 | 1.6 | 0.97 | 4.3 | ||
E-5 | 1800 | 350 | 10 | 440 | 580 | 2.1 | 0.054 | 0.58 | 15 | 3.0 | 1.6 | 0.60 | 1.5 | 0.89 | 1.7 | ||
E-7 | 1800 | 360 | 50 | 5500 | 1600 | 2.7 | 0.44 | 0.76 | 47 | 7.3 | 2.2 | 2.8 | 1.3 | 0.89 | 4.4 | ||
E-9/1 | 2000 | 410 | l.d. | 110 | l.d. | 3.0 | 0.92 | 0.19 | 0.65 | 1.5 | 1.3 | 0.15 | 1.3 | 1.2 | 4.3 | ||
E-9/2 | 1800 | 730 | 30 | 900 | l.d. | 2.7 | l.d. | 1.1 | 3.8 | 5.7 | 2.7 | 0.24 | 1.3 | 1.3 | 5.5 | ||
E-8/2 | South field | 2100 | 690 | 40 | 7600 | 1800 | 2.7 | 0.31 | 1.7 | 50 | 18 | 5.0 | 3.1 | 1.2 | 1.2 | 6.9 | |
E-8/3 | 1700 | 780 | 60 | 34000 | 10000 | 1.3 | 1.8 | 6.1 | 240 | 8.7 | 4.7 | 7.8 | 4.6 | 0.76 | 13 | ||
E-11 | 1300 | 470 | 20 | 3400 | 380 | 2.1 | 0.16 | 0.43 | 14 | 1.3 | 1.1 | 0.72 | 1.3 | 0.97 | 5.8 | ||
E-12 | 2600 | 1700 | 110 | 5700 | 390 | 2.2 | 1.1 | 3.8 | 36 | 21 | 8.5 | 0.80 | 1.3 | 0.88 | 6.6 | ||
E-16/1 | SE field | 1800 | 540 | l.d. | 140 | l.d. | 2.2 | 1.3 | l.d. | 0.25 | 0.83 | 1.3 | 0.10 | 1.3 | 0.92 | 2.8 | |
E-16/2 | 2200 | 400 | l.d. | 40 | l.d. | 1.8 | 0.11 | l.d. | l.d. | 0.77 | 0.9 | 0.13 | 1.2 | 0.88 | 2.4 | ||
E-17/1 | 2000 | 570 | l.d. | 110 | l.d. | 1.9 | 0.25 | 0.16 | 1.1 | 1.1 | 1.4 | 0.13 | 1.2 | 1.1 | 5.6 | ||
E-17/3 | 2100 | 370 | l.d. | 70 | l.d. | 2.1 | 0.63 | 0.25 | 0.21 | 0.80 | 1.1 | 0.16 | 1.2 | 1.2 | 4.9 | ||
E-17/4 | 2000 | 470 | 20 | 580 | l.d. | 1.8 | 0.71 | 0.55 | 4.0 | 5.0 | 2.1 | 0.21 | 1.2 | 1.1 | 1.3 | ||
E-18 | 1600 | 280 | 10 | 120 | l.d. | 1.9 | 0.37 | 0.24 | 0.34 | 0.86 | 0.95 | 0.21 | 1.3 | 1.2 | 4.3 | ||
E-19 | 1800 | 330 | l.d. | 40 | l.d. | 2.0 | 0.72 | l.d. | l.d. | 0.82 | 1.0 | 0.16 | 1.3 | 1.2 | 1.9 | ||
E-20 | 2000 | 670 | 30 | 5000 | l.d. | 2.3 | 0.66 | 0.47 | 5.9 | 1.3 | 1.4 | 0.17 | 1.4 | 0.77 | 6.6 | ||
E-21 | 1800 | 500 | l.d. | 1000 | l.d. | 1.8 | 0.62 | 0.27 | 10 | 0.30 | 1.1 | 0.32 | 1.3 | 0.95 | 3.4 | ||
| |||||||||||||||||
Sample | Location | Cu | Zn | Ag | Cd | Pb | Cr | V | Mo | Sn | As | Se | Sb | Te | Br | I | Hg |
ppm | |||||||||||||||||
| |||||||||||||||||
s-1 | July field | 27 | 71 | 1.1 | 0.84 | 4.7 | 67 | l.d. | l.d. | 0.26 | l.d. | 15 | l.d. | 0.78 | 1.3 | 0.76 | l.d. |
s-1/a | 27 | 53 | 0.87 | 0.73 | 4.2 | 23 | l.d. | 0.23 | l.d. | l.d. | 0.93 | 0.25 | l.d. | l.d. | 0.33 | l.d. | |
s-2 | 25 | 53 | 0.6 | 0.7 | 24 | 7.2 | 12 | l.d. | 0.26 | 66 | 440 | l.d. | 140 | 6.0 | 20 | l.d. | |
s-3 | 25 | 52 | 0.73 | 0.46 | 7.0 | 12 | l.d. | l.d. | l.d. | l.d. | 150 | l.d. | 11 | 1.8 | 3.5 | l.d. | |
s-4 | 31 | 74 | 0.94 | 0.83 | 16 | 11 | l.d. | 0.21 | 0.26 | 1.8 | 304 | 0.39 | 40 | 5.4 | 4.9 | l.d. | |
s-5 | 28 | 56 | 0.7 | 0.66 | 6.4 | 16 | 18 | 0.23 | 0.31 | l.d. | 260 | l.d. | 46 | 16 | 4.4 | l.d. | |
s-6 | 22 | 52 | 0.77 | 0.6 | 4.7 | 27 | l.d. | l.d. | l.d. | l.d. | 200 | l.d. | 28 | 1.05 | 1.3 | l.d. | |
s-7 | 28 | 49 | 0.6 | 0.48 | 3.9 | 12 | 7.1 | l.d. | l.d. | l.d. | 99 | 0.27 | 14 | 1.4 | 1.2 | l.d. | |
s-8 | 28 | 52 | 0.75 | 0.59 | 7.2 | 24 | l.d. | 0.26 | l.d. | l.d. | 160 | 0.36 | 9.1 | 0.97 | 2.1 | l.d. | |
s-9 | 24 | 49 | 0.74 | 0.61 | 3.1 | 18 | l.d. | l.d. | 0.25 | l.d. | 97 | l.d. | 8.2 | 1.7 | 1.08 | l.d. | |
s-10 | 25 | 56 | 0.73 | 0.64 | 5.4 | 18 | l.d. | l.d. | 0.31 | l.d. | 160 | l.d. | 30 | 1.7 | 0.77 | l.d. | |
E-3 | NE field | 46 | 14 | 0.079 | 0.065 | 5.6 | l.d. | 28 | 1.7 | 1.6 | 3.2 | 1.4 | 2.1 | 0.84 | l.d. | 2.3 | 0.16 |
E-5 | 9.3 | 9.7 | 0.18 | 0.082 | 2.4 | l.d. | 4.0 | 0.55 | 0.28 | 0.67 | 2.1 | 1.9 | 0.8 | 3.5 | 1.9 | 0.16 | |
E-6 | 68 | 15 | l.d. | 0.15 | 4.5 | 42 | 31 | 7.6 | 1.6 | 2.0 | 2.8 | 0.71 | 0.7 | 1.0 | 2.0 | l.d. | |
E-7 | 50 | 14 | 0.19 | 0.14 | 4.2 | 9.0 | 17 | 1.1 | 1.1 | 1.4 | 18 | 0.37 | 0.88 | 0.6 | 1.7 | l.d. | |
E-9/1 | 11 | 16 | 0.35 | 0.22 | 3.2 | l.d. | l.d. | 0.12 | 0.1 | 3.0 | 13 | 0.45 | 1.2 | 0.5 | 2.5 | 0.3 | |
E-9/2 | 11 | 17 | 0.16 | 0.14 | 3.3 | 7.0 | l.d. | l.d. | 0.13 | 1.1 | 7.1 | 0.16 | 0.78 | 0.94 | 2.0 | 0.7 | |
E-8/2 | South field | 66 | 18 | 0.12 | 0.16 | 4.7 | l.d. | 12 | 0.94 | 1.0 | 1.7 | 7.5 | 0.13 | 0.84 | l.d. | 2.1 | l.d. |
E-8/3 | 600 | 12 | 0.85 | 0.099 | 11 | 3.5 | 110 | 8.9 | 7.9 | 19 | 20 | 5.1 | 33 | 9.4 | 7.7 | l.d. | |
E-11 | 81 | 13 | 0.16 | 0.072 | 4.0 | 15 | l.d. | 1.1 | 1.1 | 13 | 42 | 0.4 | 15 | 0.67 | 1.5 | 0.41 | |
E-12 | 21 | 19 | 0.11 | 0.13 | 6.3 | 23 | 3.6 | 1.3 | 0.98 | 12 | 58 | 5.0 | 0.99 | 3.7 | 5.6 | l.d. | |
E-16/1 | SE field | 8.3 | 19 | 0.16 | 0.12 | 5.8 | l.d. | l.d. | 0.41 | 0.11 | 49 | 47 | 0.4 | 5.2 | 22 | 3.6 | 0.15 |
E-16/2 | 7.5 | 11 | 0.11 | 0.13 | 3.6 | l.d. | l.d. | l.d. | 0.18 | 180 | 41 | 10 | 3.5 | 2.5 | 3.9 | 2.2 | |
E-17/1 | 9.7 | 17 | 0.24 | 0.27 | 4.6 | l.d. | l.d. | 0.19 | 0.18 | 8.7 | 54 | 0.29 | 0.58 | 14 | 13 | 0.19 | |
E-17/3 | 9.3 | 13 | 0.14 | 3.9 | 4.2 | 3.9 | 7.2 | 0.15 | 0.17 | 2.6 | 26 | 0.12 | 0.34 | 2.7 | 3.7 | 0.15 | |
E-17/4 | 10 | 17 | 0.33 | 0.18 | 3.5 | 16 | l.d. | 0.54 | 0.24 | 68 | 21 | 68 | 0.56 | 3.0 | 13 | 2.1 | |
E-18 | 11 | 13 | 0.16 | 0.064 | 5.0 | 12 | 4.7 | 0.17 | 0.11 | 22 | 64 | 0.35 | 5.7 | 1.6 | 48 | 0.48 | |
E-19 | 7.9 | 13 | 0.13 | 0.28 | 5.2 | l.d. | l.d. | l.d. | 0.08 | 16 | 70 | 0.15 | 8.5 | 1.9 | 46 | l.d. | |
E-20 | 15 | 14 | 0.19 | 0.13 | 4.6 | 11 | l.d. | 0.94 | 0.5 | 73 | 45 | 1.5 | 11 | 0.52 | 2.0 | 0.22 | |
E-21 | 14 | 13 | 0.063 | 0.18 | 5.3 | 10 | 4.5 | 1.1 | 0.56 | 5.1 | 61 | 0.15 | 28 | 0.64 | 2.2 | 0.20 |
The lithophile elements (Ca) and a number of chalcophile elements (e.g., Cu, Zn, Ag, and Cd) exhibit moderate correlations between each other (Figure
Variation diagrams for elements with different behaviors. (a) The chalcophile elements of Zn and Cd are moderately correlated between each other and have distinctly different concentrations in the July field (high) and in other fields (low). (b) The chalcophile elements of Se and Te are moderately correlated but exhibit no dependence on fumarolic fields. (c) Iron is not correlated with the chalcophile element Zn but has high concentrations in only one field. (d) The lithophile elements of K and Ca are not correlated.
Elements occur in sulfur in both water-insoluble and water-soluble forms; the presence of the latter was confirmed via the treatment of sulfur using pure water followed by analyses of the resulting solutions (leachates). This was especially useful for the estimation of anion concentrations but also provided some additional information about cations. The compositions of the sulfur leachates are provided in Table
Concentrations of anions and cations in sulfur leachates.
# | L-1 | L-2 | L-3 | L-4 | L-5 | L-6 | L-7 | L-8 | L-9 |
---|---|---|---|---|---|---|---|---|---|
Location | SE field | July field | NE field | ||||||
pH | 0.88 | 0.54 | 0.73 | 1.1 | 1.8 | 1.9 | 1.8 | 1.8 | 1.8 |
ppm | |||||||||
F− | 0.0032 | 0.047 | 0.042 | 0.24 | l.d. | l.d. | l.d. | l.d. | l.d. |
| 120 | 30 | 290 | 22 | 38 | 42 | 8 | 19 | 5.6 |
Cl− | 6400 | 19000 | 12000 | 6000 | 2.8 | 9.5 | 130 | 1090 | 38 |
SiO2 | 0.19 | 0.69 | 0.77 | 3.7 | 1.1 | 20 | 28 | 1.7 | 17 |
Al | 1.1 | 0.804 | 0.72 | 0.97 | 0.3 | 7.7 | 5.5 | 12 | 2.2 |
As | 1.7 | 4.8 | 4.2 | 8.6 | 0.035 | 0.033 | 0.013 | 0.023 | 0.011 |
B | 21 | 36 | 54 | 84 | 0.086 | 0.19 | 0.038 | 0.33 | 0.02 |
Ba | 0.065 | 0.032 | 0.016 | 0.019 | 0.01 | 0.1 | 0.066 | 0.032 | 0.0078 |
Bi | 0.24 | 0.4 | 0.37 | 1.1 | 0.035 | 1.4 | 0.406 | 0.22 | 0.15 |
Ca | 1.5 | 1.8 | 1.4 | 2.8 | 1.4 | 8.8 | 3.5 | 9.9 | 1.9 |
Co | 0.0083 | 0.014 | 0.0012 | 0.013 | 0.002 | 0.024 | 0.019 | 0.0061 | 0.0078 |
Cr | 0.809 | 1.7 | 0.84 | 0.62 | 0.002 | 0.066 | 0.303 | 0.054 | 0.13 |
Cu | 0.17 | 0.3 | 0.302 | 0.93 | 0.049 | 1.2 | 0.172 | 0.17 | 0.12 |
Fe | 8.3 | 15 | 9.1 | 8.5 | 0.35 | 5.8 | 6.5 | 14 | 4.4 |
Ga | 0.076 | 0.0472 | 0.21 | 0.026 | 0.035 | 0.033 | 0.013 | 0.023 | 0.011 |
I | 1.9 | 1.1 | 6.2 | 13 | 0.6 | 0.67 | 0.27 | 0.47 | 0.2 |
K | 0.4 | 0.39 | 0.29 | 0.35 | 0.18 | 0.45 | 0.68 | 1.1 | 0.3 |
Mg | 0.29 | 0.27 | 0.45 | 0.51 | 0.15 | 0.84 | 0.6 | 1.5 | 0.39 |
Mn | 0.6 | 1.3 | 0.72 | 0.58 | 0.034 | 0.57 | 0.27 | 0.57 | 0.21 |
Na | 1.3 | 1.6 | 1.3 | 1.7 | 2.9 | 4.2 | 5 | 2.5 | 1.4 |
Ni | 0.35 | 0.58 | 0.49 | 0.55 | 0.022 | 1.5 | 0.29 | 0.77 | 0.44 |
Pb | 0.36 | 0.48 | 0.33 | 0.25 | 0.035 | 0.0709 | 0.027 | 0.063 | 0.042 |
Sb | 0.2 | 0.094 | 0.4 | 1.1 | 0.07 | 0.067 | 0.027 | 0.047 | 0.022 |
Sn | 0.032 | 0.047 | 0.034 | 0.14 | 0.035 | 0.033 | 0.013 | 0.023 | 0.011 |
Sr | 0.037 | 0.0089 | 0.009 | 0.019 | 0.009 | 0.087 | 0.052 | 0.12 | 0.024 |
Ti | 0.209 | 0.14 | 0.17 | 0.12 | 0.004 | 0.075 | 0.05 | 0.042 | 0.016 |
V | 0.0064 | 0.0075 | 0.0072 | 0.0054 | 0.002 | 0.021 | 0.024 | 0.024 | 0.0043 |
Zn | 0.38 | 0.61 | 0.55 | 0.91 | 0.13 | 0.82 | 0.2 | 0.25 | 0.085 |
| |||||||||
ppb | |||||||||
Cd | 36 | 2.8 | 2 | 1.9 | 2 | 2.7 | 0.8 | 1.4 | 0.7 |
Li | 1.9 | 2.8 | 2 | 1.9 | 2 | 4.7 | 2.8 | 1.9 | 1.6 |
Y | 1.9 | 1.9 | 1.4 | 1.9 | 1.4 | 2.7 | 1 | 2.9 | 0.9 |
Zr | 3.8 | 3.8 | 3 | 4.5 | 1 | 2 | 0.8 | 1.9 | 0.7 |
Average elemental concentrations in solid sulfur (July fumarolic field) compared to corresponding elemental concentrations in sulfur leachates from the same field. Elements are arranged from high to low average concentrations in sulfur.
To examine the forms of occurrence of elements in native sulfur, we studied the samples using a scanning electron microscope with an Energy-Dispersive Spectrometer. Sulfur was studied: (1) as split unpolished pieces; (2) as polished pieces; and (3) after it was dissolved in carbon disulfide (CS2) and its insoluble residue was examined. The last method provided the most interesting results because it allowed us to observe extremely rare inclusions.
The SEM-EDS analysis of trace minerals in sulfur is useful for two reasons. First, micron- and even submicron-sized mineral crystals composed of heavy elements are clearly visible under the electron microscope in back-scattered imaging mode (BSE) and can therefore be counted and studied. Second, very small and rare phases containing small amounts of elements can be reliably analyzed under an electron microscope, but such vanishingly low concentrations cannot be measured using conventional chemical or ICP-MS analyses. The list of the rare phases observed in the fumarolic sulfur of Ebeko volcano includes 21 rarely and moderately occurring minerals (Table
Phase list and occurrence frequencies of minerals in fumarolic sulfur from Ebeko.
Composition | Name | Occurrence |
---|---|---|
Ag2S | Argentite | Moderate |
As4S4 | Realgar | Moderate |
Au2Pd | Palladian gold | Single |
BaSO4 | Barite | Moderate |
B(OH)3 | Sassolite | Rare |
CaF2 | Fluorite | Rare |
CaSO4 | Anhydrite | Moderate |
CaWO4 | Scheelite | Moderate |
CdIn2S4 | Cadmoindite | Rare |
(Cu,Sn)S | Copper-tin sulfide | Rare |
Cu3SbS4 | Famatinite | Rare |
CuS | Covellite | Rare |
Cu2S | Chalcocite | Rare |
FeS2 | Pyrite | Major |
HgS | Cinnabar | Rare |
KAl3(SO4)2(OH)6 | Alunite | Major |
(NH4)As4O6(I) | Iodine-lucabindiite | Rare |
Ni, Fe, S | Native nickel | Single |
PbCl2 | Cotunnite | Moderate |
PbSO4 | Anglesite | Rare |
S | Native sulfur | Major |
Sb2S3 | Stibnite | Rare |
SiO2 | Opal | Major |
TiO2 | Rutile | Moderate |
ZrSiO4 | Zircon | Moderate |
| ||
Pyroxene | Moderate | |
Plagioclase | Moderate | |
Magnetite | Moderate | |
Glass | Moderate |
Major and rare phases from fumarolic incrustations of Ebeko volcano. (a) Molten and solidified sulfur with “cheese texture.” (b) Opal with contraction cracks surrounded by sulfur. (c) Octahedral pyrite crystals mixed with silicate particles. (d) Aggregate of sassolite (H3BO3) crystals. (e) Covellite (CuS) crystals on sulfur. (f) Chalcocite (Cu2S) aggregates and a particle of TiO2 (rutile?) inside a pore. (g) Barite, magnetite, rutile, and opal aggregates extracted from sulfur. (h) Euhedral crystals of scheelite. SEM BSE images.
Rare phases from fumarolic incrustations of Ebeko volcano. (a) Small aggregate of palladian gold attached to sulfur surface. (b) Aggregate and a single crystal of cadmoindite (CdIn2S4) on sulfur surface. (c) Aggregate of argentite (Ag2S) crystals. (d) Aggregate of cinnabar (HgS) crystals. (e) Crystals of (NH4)As4O6I (white) attached to sulfur (gray). (f) Two globules of famatinite (Cu3SbS4) (white) surrounded by sulfur (light gray) and alunite (gray). (g) Aggregate of Cu-Sn sulfide (white) on sulfur surface (gray). (h) Aggregate of cotunnite (PbCl2) (white crystals) mixed with silicate particles. SEM BSE images.
Some rare phases were observed, such as palladium gold (Au2Pd) (Figure
Silicate inclusions (particles) originating from wall rock (i.e., minerals, glass, and larger rock fragments) are sometimes abundant and reach 200–300
Due to the extremely low vapor pressures of the majority of metallic element compounds below 200°C (e.g., [
It has long been known that elements can be transported within fumarolic gases either as gaseous species or in the form of fine rock aerosol (e.g., [
Similar results can be achieved using the so-called titration method if we would subtract small fractions of rock from the total composition of the incrustations. When the concentration of the chosen reference element approaches zero, it can be concluded that the correct amount of titrant (rock) has been subtracted and that all remaining elements have originated from a source other than rock aerosol. This approach was implemented on the plot shown in Figure
Elemental concentrations in sulfur compared to their corresponding concentrations in “5% rock” (Ebeko andesites). The “5% rock” approximates the contribution of rock particles to the composition of fumarolic incrustations. For each element, its minimum, maximum, and average concentrations are shown. Elements are arranged from high to low concentrations in rock (Ebeko andesite). The distance in log units on the plot between a point showing the concentration of an element in sulfur and a point showing the concentration of the same element in “5% rock” corresponds to the “enrichment factor” for this element.
The plot in Figure
The ratios of the average elemental concentrations in sulfur to the elemental concentrations in “5% rock” are plotted on a separate graph (Figure
Logarithms of elemental ratios
In the case of low-temperature fumaroles, approaches such as enrichment factors or rock titration may be less accurate than when applied to high-temperature gases because (1) the majority of aerosols are particles of altered rocks with unknown compositions and (2) aerosol particles continue to change their compositions after they are deposited in incrustations because they are affected by low-temperature acid condensates of fumarolic gases containing hydrochloric and sulfuric acids. The concentrations of any reference element in low-temperature incrustations can arbitrarily change because of acid alteration (see below), which will eventually decrease the accuracy of the calculations. Additionally, the “enrichment factor conception” does not allow distinguishing between the volatile transport of elements and the transport of condensed volatile species that were gaseous at higher temperatures but condensed somewhere in a fumarolic channel beneath the surface (e.g., sulfides and chlorides of chalcophile elements).
Although the direct deposition of sulfides from high-temperature fumarolic gas is a phenomenon that frequently occurs on volcanoes [
Fumarolic gases condense upon cooling to produce acid solutions at temperatures below or slightly above the boiling point of pure water at a given hypsometric level (~99°С for Ebeko). If this condensation occurs in the absence of air (e.g., inside a fumarolic edifice similar to that shown in Figure
It is known from chemistry that a compound will form a precipitate if the ion product Q in the solution is greater than the solubility product of the compound Ksp: Me2+(aq) + S2−(aq) = MeS↓. Because H2S is a weak acid, the pH of the solution affects the concentrations of sulfide ions S2−(aq) in the solution: S2−(aq) + H3O+(aq) = HS-(aq) + H2O; HS-(aq) + H3O+(aq) = H2S. Strong acid (HCl) removes S2−(aq) ions from the solution, and the ion product Q = [Me2+(aq)] × [S2−(aq)] decreases. Only sulfides with low Ksp (e.g., HgS, Ag2S) can precipitate from solutions with low pH, whereas elements that form sulfides with higher Ksp remain dissolved. This phenomenon has been widely used in analytical chemistry to separate ions of different metals. For example, Hillebrand et al. [
The behavior of a metal sulfide can be estimated more precisely based on its solubility product Ksp (Supplementary Table
Chemical processes in low-temperature sulfur fumaroles. Fumarolic gases (mainly H2O, CO2, SO2, H2S, and HCl) carry volatile metal species and some amount of rock aerosol. At the fumarole vent, the temperature abruptly decreases and volatile species condense, forming solid phases and acid condensate that is enriched in HCl, while H2S mainly escapes. Sulfur is precipitated due to the reaction between H2S and SO2 and the partial oxidation of H2S. Hot acid condensate can attack rock particles, extracting metals. Then, the condensate cools to ambient temperature and the chlorides of metals (e.g., CuCl2, FeCl2) from the acid solution react with H2S to form insoluble sulfides. Petrogenic elements do not form sulfides, and their chlorides are washed away from incrustations by atmospheric precipitates.
Whereas elements with insoluble sulfides are retained in sulfur incrustations, petrogenic elements (i.e., Na, K, Mg, Ca, Al, and partially Fe) can be washed away under low-temperature conditions (<100°C) by atmospheric precipitation due to the solubility of their chlorides. Ca and Al may be partially retained in the presence of H2SO4 in forms gypsum/anhydrite and alunite. Such rinsing may significantly decrease the concentrations of Ca and Mg in incrustations, thus making the calculations of the enrichment factors or rock titration less reliable (see previous section). In fact, if a nonvolatile reference element is depleted, then these calculations will show enrichment in all other elements. Thus, for low-temperature incrustations, other reference elements or other calculation methods should be implemented in further studies.
(1) Low-temperature sulfur incrustations from Ebeko volcano contain appreciable amounts of trace elements stored in rock particles, metal sulfides (e.g., argentite (Ag2S), covellite (CuS), cadmoindite (CdIn2S4), famatinite (Cu3SbS4)), chlorides (cotunnite (PbCl2)), sulfates (anhydrite (CaSO4), alunite (KAl3(SO4)2(OH)6), anglesite (PbSO4), barite (BaSO4)), and tungstates (scheelite (CaWO4)). Native sulfur, opal, alunite, and pyrite are the major minerals in these incrustations. The SR-XRF analysis of native sulfur represents a reliable analysis that does not require complex digesting procedures.
(2) Because fumarolic incrustations are derived from cooling gas, their compositions can be used to assess the composition of this gas. This approach was implemented for the low-temperature fumaroles of Ebeko volcano, whose gas contains extremely low concentrations of volatile metal species; thus, the data obtained directly from gas condensates may be inconclusive. Rock titration was used to assess the modes of transport of different elements. Only ten elements (Cd, Ag, Hg, Se, Te, As, Sb, Pb, Br, and I) are sufficiently enriched in sulfur to suggest that they experienced gaseous transport. All other elements, including the chalcophile elements of Cu and Zn, originated from rock particles.
(3) Ultra-acid and hot (pH ~ 0.5; ~100°C) gas condensates attack rock particles impregnated in fumarolic sulfur, which results in the extraction of cations from the aforementioned particles. At lower (ambient) temperatures, such cations in solution further interact with fumarolic H2S to precipitate rare, low-solubility sulfides (Ag2S, HgS, and CuS) as well as abundant pyrite (FeS2). The silicate matrix remaining after this acid leaching forms opal with rare inclusions of TiO2. The concentrations of petrogenic elements (i.e., Na, K, Mg, Ca, Al, and Fe) in sulfur incrustations can be significantly decreased by acid leaching followed by washing away with atmospheric precipitation.
The authors declare that they have no conflicts of interest.
The authors are grateful to T. A. Kotenko and L. V. Kotenko for their helpful logistical support during fieldwork. Financial support for this study was provided by the Russian Science Foundation (Grant no. 16-17-10145). Fieldwork was supported by the Russian State Assignment Project 0330-2016-0001.
The supplementary materials file contains four supplementary tables. Table