Libmonster ID: KZ-1579
Author(s) of the publication: Gennady KARPOV

by Gennady KARPOV, Dr. Sc. (Geol. & Mineral.), Deputy Director of the RAS SB Institute of Volcanology and Seismology (Petropavlovsk-Kamchatsky)

Vulcanism is often an unexpected, catastrophic and in many respects unexplored natural phenomenon driven by powerful physico-chemical and dynamic processes in terrestrial depths. Volcanic eruptions bring an in-depth gaseous, liquid, and solid element to the surface. It is important to study it to accumulate knowledge about the inner spheres of the planet and extend practical applications-for ore geology, volcanology, and many related disciplines such as seismology, climatology, and ecology. Despite of significant volumes of lava products that change the relief and build up the earth crust, it is well known that minute particles brought out into the atmosphere-ashes and aerosols (by estimate, about 100 mln tons of volcanic ashes get to the atmosphere) have major impact on the atmosphere, while powerful catastrophic eruptions can even change the climate of the Earth in general. Spread by the wind for hundreds of kilometers, ashes pose a great threat to air traffic. They also stimulate accumulation of loose material in soils and water bodies. In this context, it is very important to study material and mineral composition of ashes, which is a subject of the present article.

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Regular eruptions are accompanied by ejection of 5,000 t of crushed and finely crushed volcanic material per one second. Major and catastrophic eruptions bring out even more. For example, in the culmination point of eruption of Bezymianny volcano in the Kamchatka Peninsula on March 30, 1956, about 150 mln t of ashes were ejected. According to the data obtained by Nikolai Zharinov, Cand. Sc. (Tech.), and Yuri Demyanchuk, employees of the RAS SB Institute of Volcanology and Seismology, for the period from 1955 to 2009, only this very volcano ejected about 918.5 mln t of ashes. During the intense explosion of Karym-sky volcano on May 11, 1963, the eruption (volcanic) cloud reached the height of 10 km. Up to 150 kg/m2 of the mix of slag, sand and ash precipitated in the surrounding area, and 2 mm ash layer was registered in the vicinity of Petropavlovsk-Kamchatsky, 130 km away from the epicenter. Powerful ash ejections are typical of currently active volcanoes: Shevcluch, Klyuchevskoy, Bezymianny, and Kizimen in Kamchatka, Sakuradzima in Japan, St. Helen's in the USA, Gekla and Grimsvetn in Iceland, Stromboli in Italy, Popokatepetl in Mexico, Rabaul, Merapi, Krakatau in Indonesia, etc.

With distance from the center of eruption, many characteristics of the precipitating material change: composition and size of particles, structure, texture, and mineralogy. In the first place, the material is sorted by size and specific gravity of grains. Near the volcano, coarse grains (0.5-2 mm and more) of heavy minerals mainly composed of pyroxene, olivine, magnetite, etc. precipitate. Fine ashes (0.01-0.1 mm) characteristic of such sedimentary rocks as pelites and aleurites, are usually taken far away. These ashes are typically composed of angular or sometimes sharply angular particles of volcanic glass, and their share usually makes up about 10-15 percent (sometimes 30 percent) of the total mass of eruption. Pelite fractions usually include accessory minerals (i.e. minerals in small quantities) such as apatite, zircon, rutile, titanic iron ore, hematite, and pyrite. Peculiarities of the mineral composition and grain size forming a soil-pyroclastic cover (a kind of "puff-pastry" made of alternating horizons of buried soils and ashes) at different distances from the volcanoes enable geologists to reconstruct sections of deposits, decode eruption age, etc. In this article we focus only on one aspect: poorly studied native ash minerals ejected by different volcanoes all over the world.

Let's begin with the ferrum-platinum group (Fe-Pt). These minerals are seldom found due to a small size of grains; sometimes they are detected only by means of an electron-microscopic and microprobe analysis. Numerous particles of native iron were first found in newly ejected ashes of Karymsky volcano (initial phase of eruption dates from 1996). They were of elongated, sometimes of acicular shape, size 0.02-0.06 mm, of steel grey color, ductile and magnetic. The analysis showed that they are composed of native low temperature a-iron. However, the concentration of this element was less than 100 percent, which is explained by

стр. 20

Elongated particle of native iron from the ashes of the Tolbachik fissure eruption of 2012-2013 and its X-ray spectrum.

presence of some X-ray amorphous Fe oxides. Nickel, copper, and cobalt were also detected.

Later on, we identified isometric particles of native iron of 30x50 ìm in the ashes of the same volcano after a powerful eruption of July 24, 2002. These particles had ragged edges, numerous subparallel cracks and no admixtures. Generally speaking, almost all ashes ejected in the course of powerful explosions by Karymsky volcano contain Fe particles. We registered pure Fe grains and Fe grains with admixtures of manganese, nickel, cobalt, and copper. Single Fe particles were identified in the ashes of Bezymianny volcano: they were of isometric shape, 50x110 urn, admixtures of cobalt, nickel, manganese, sometimes without coat of admixture of glass. Fe grains were also identified in the ashes ejected by Sheveluch volcano. Especially high content of Fe grains were registered in the ashes ejected at the initial phase of the Tolbachik fissure eruption of 2012-2013. Most grains had an isometric shape, light grey color,

Lanceolate particle of native iron from the ashes ejected by Eyjafjallajökull volcano and its X-ray spectrum.

стр. 21

metal gloss, and high magnetic properties. Through the microscope, they look like uniform elongated particles of white color, size 75x300 urn, with clearly seen cracks located lengthwise. They usually contain minor admixtures of manganese. Big particles (up to 500 ìm) of native Fe are rare, they have isometric drop-like shape.

The angular shape is typical of the native Fe found in the ashes of the US volcano St. Helen's (eruption of 1981). With the general elongation of up to 75-80 ìm, they are formed of quasi-separate thin leaves stuck together with admixtures of manganese and nickel, sometimes zinc. The Fe rain of complex morphology was identified in the ashes of Spurr volcano in Alaska (eruption of 1992). In the ashes ejected by Eyjafjalla-jökull volcano (eruption of 2010) we found a number of lance-shaped grains of native Fe of 30x180 ìm. In terms of structure and morphology, they are similar to Fe grains identified in St. Helen's ashes, but they are absolutely free of any admixtures.

Ore minerals accompanying iron practically in all ash samples were submicron isometric particles of native aluminum and copper (with rare inclusions of zinc and stannum), minor spherules of ferric oxide (sometimes hollow) and individual cubic crystals (less often aggregates) of ferric sulphide. Ferroalloys and intermetallic compounds of iron, manganese, chrome, nickel, titanium, copper, and molybdenum were of frequent occurrence. The smallest grains of native nickel, titanium and inclusions of molibdenum were also rarely found. A grain of mercury sulphide was identified in the ashes of the Tolbachik fissure eruption of 2012-2013.

For over 15 years of sampling studies, we identified only two grains of native nickel (Ni)--in the ashes of Bezymianny and Sheveluch volcanoes. They had an isometric elongated form, size 30(70)x200 ìm, smooth surface, clear layering and contained minor admixtures of Fe.

Speaking of the group of precious metals, we identified only one grain of native platinum and two grains of silver for the whole period of studies. The acute-angled band-like grain of platinum 0.4-0.8 ìm thick and up to 200 ìm long was detected in the sample of the basan-ite ash ejected by the Iceland volcano Eyjafjallajökull taken in the course of the powerful eruption in March 2010. This very sample contained 14 grains of ore minerals, including native Fe, hematite, ilmenite, pyrite, intermetallides of titanium, manganese, and iron. They are associated with forsterite, a high-magnesial mineral of the olivine group. Two spherical grains of native silver were registered in the ashes of Karymsky volcano (eruptions of 1996 and 2002). They had about 60 ìm in diameter composed of an aggregate of finest leaves 1-10 ìm.

Native copper (Cu) enters into the same group as gold and silver--these minerals have similar crystal structures; that is why paragenesis*of copper and si 1 -

*Paragenesis--coexistence in this or that mineral rock, related by common conditions of formation.--Ed.

стр. 22

Particle of native nickel from the ashes ejected by Sheveluch volcano and its X-ray spectrum.

ver is a regular thing. It is generally accepted that native copper occurs less often than its sulfides and oxides. However, copper is quite typical of ashes. Thus, two drop-shaped grains of copper were identified in the ashes of Karymsky volcano (eruption of 2006). Fine copper particles, often associated with zinc, were detected in the ashes of Bezymianny volcano. Zinc containing copper particles are present in the ashes ejected by St. Helen's volcano. Tens of isometric particles of native copper, often containing admixture of stannum and zinc, were detected in the ashes ejected at the initial phase of the Tolbachik fissure eruption of 2012-2013. Two isometric particles of native zinc of 50-400 urn were identified in the ashes of Karymsky volcano, and about 10 zinc grains with admixtures of iron, copper, and aluminum were identified in the ashes ejected at the initial phase of the Tolbachik fissure eruption of 2012-2013.

Submicron particles of native molybdenum were detected in glassy ash particles ejected by Karymsky volcano. As microinclusions in olivine crystals, native molybdenum is often found in the ashes of the Tolbachik fissure eruption of 2012-2013. Natural inter-metallides, in particular, complex composition particles (iron-manganese-chrome-titanium-molybde-

Particle of native platinum from the ashes ejected by Eyjafjallajökull volcano and its X-ray spectrum.

стр. 23

Rounded particle of native silver from the ashes ejected by Karymsky volcano and its X-ray spectrum.

Drop-shaped particle of native copper from the ashes ejected by Karymsky volcano and its X-ray spectrum.

num) were regularly identified in the ashes of Karymsky volcano. Isometric particles of the size of up to 50 urn, composed of iron-manganese-chrome-nickel, with traces of silicon were identified in the ashes of Sheveluch volcano.

What are theoretical consequences resulting from these findings? Native metals occur in the deposits of different genesis--magmatic, metamorphic, hydrothermal, even in the ferromanganese concretions of the World Ocean. My colleagues Svetlana Glavatskykh and Lidia Vergasova, Drs Sc. (Geol. & Mineral.), identified a large group of native metals (gold, silver, copper, lead, bismuth, tungsten) and in-termetallic compounds (iron, manganese, titanium, copper, chrome, cobalt, molybdenum) in the products of volcanic exhalations (ejected gases and vapor) from fusions of mantle magnesial basalts appearing on the surface at a high velocity during Great Tol-bachik fissure eruption of 1975-1976. Segregation of metal enriched fluid gas in the course of appearance of magmatic substance on the surface is described in the works by Igor Menyalov and Lyudmila Niki-tina, Cands Sc. (Geol. & Mineral.). They pointed out that the content of volatile and chalcophylic* elements (including platinum and palladium) in the exhalations of the abovementioned eruption of 1975-1976 was much higher as compared with basalts.

Frequency of occurrence of such native metals as Fe, Cu, Zn, less often Ag, Mo, Ni, etc., in the volcanic ashes may speak for, first, a close specialization of studied magmatogenic fluid systems in terms of element composition, and, secondly, a form of transfer of ore components easily transformed in the surface conditions resulting in the formation of native phases and their alloys, for example, haloid and carbonyl metal complexes as well as metal complexes in the form of oxycentered tetrahedrons recently discovered by Sergei Krivovichev and Stanislav Filatov, Drs Sc. (Geol. & Mineral.), St. Petersburg State University. Observations over the nature of volcanic eruptions, studies of ash composition and content of ore components, including native elements, showed that ore components are better represented in the ashes ejected during most powerful eruptions. We believe that in such periods of volcanic activity easily volatile components

*Chalcophylic elements--a group of 19 chemical elements (S, Sb, Bi, As, Se, Te) and a number of heavy non-ferrous metals (Cu, etc.), tending to form native sulphides, selenides, and tellurides.--Ed.

стр. 24

Particle of native copper from the ashes of St. Helen's volcano and its X-ray spectrum.

of the magmatic fluid system basically composed of deoxidizers--hydrogen, methane, carbon monoxide, carbon bisulphide, and metals in the composition of above-listed forms of transfer--run ahead of fusion. In such conditions products of powerful explosions get enriched with native elements. This is proved by the articles by Andrei Grebennikov, Cand. Sc. (Geol. & Mineral.), and his coauthors in the Volcanology and Seismology magazine (2012, No. 4), where they, on the basis of detailed analyses, dwell on the hydrogen-methane composition of the gas phase, conditioning formation of coexisting minerals, including native iron.

The association of ore metals identified in the ashes ejected by the Eyjafjallajökull volcano that incorporated platinum, native Fe, intermetallides of iron, manganese, cobalt, chrome, nickel, titanium, molybdenum and manganese-containing ilmenite, with reference

стр. 25

Curved particle of intermetallic compound composition Fe, Mn, V, Ni, Cr, Ti, Mo from the ashes of Sheveluch volcano and its X-ray spectrum.

to the geological and structural position of Iceland on the axis of the Middle Oceanic Ridge, could speak for ingress of native elements from the peridotite upper mantle (it lies under the lithosphere at the depths of from -20 (70) to 670 km).

The obtained data on morphology, size, purity, and frequency of occurrence of particles in the ashes of active volcanoes confirm that the ore substance in mag-matic fluid systems is transferred in the gas phase and is separated in ashes in the form of submicron grains. Native metals were registered by scientists in the composition of magmatic complexes. Consequently, present-day vulcanism brings the ore substance in the form of native metals and intermetallic compounds to the

стр. 26

Particle of native copper from the ashes of the Tolbachik fissure eruption of 2012-2013 and its X-ray spectrum.

surface of the planet. In the Pacific Region, they are iron, titanium, manganese, chrome, cobalt, copper, zinc, rarely molybdenum, silver and nickel. It would be fair to assume that they are indicators of metallogenic specialization of magmas, and in the process of rising of the magmatic substance to the upper layers of the earth crust, and drop of pressure and temperature at a certain depth, relevant metals undergo mineralization processes. It is characteristic that almost all identified native metals and intermetallic compounds are inherent in products of andesitic and basaltic vulcanism, which proves a hypothesis on a single initial magma. Varying composition of substances erupted by active volcanoes is indicative of different phases of differentiation of deep magma sources under such volcanoes.

In general, it is necessary to point out that irrespective of dominating silicate particles in the volcanic ashes, ore substance (predominantly iron, in a lesser degree copper, zinc, stannum, molybdenum, nickel, and other metals) in the form of supersmall individualized particles may reach some thousandth percent. It is clear that volcanic ashes are strongly diffused by wind currents, but, proceeding from the enormous volumes of ashes, the amount of the ore substance in the native condition getting into soil and water is rather significant. This should be taken into account in assessing the overall ejection of such substance from the terrestrial depths. Another thing to be considered is that the most widespread native iron may act as a catalyst in many physico-chemical and biological processes in the earth crust and soils.


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