INTRODUCTION

Erebus (Fig. 1) is an active intraplate stratovolcano (standing on the volcanic Ross Island, 77°32′ S, 167°17′ E) on a thin (17–25 km) continental crust. It is confined to the edge of the West Antarctic Rift System (Kyle et al., 1992; Encyclopedia …, 1999). The volcano is 1.3 Ma old, with its recorded activity being known for the past 172 000 years (Late Pleistocene to the present). That period saw numerous, mostly effusive, eruptions, with the activity peaking in the time spans 95 ± 9… 76 ± 4 and 27 ± 3 … 21 ± 4 ka (Harpel et al., 2004; Esser et al., 2004; Kelly et al., 2008]. The associated lavas have a composition to make a nearly continuous sequence of magmatic differentiation (the so-called Erebus lineage) in the following sequence: basanites (olivine tephrite) → phonotephrite → tephrophonolite → anorthoclase phonolite (Iacovino et al., 2013; Iverson et al., 2014). The phonolite fraction in the sequence is 20–30%. The rock forming minerals of the lavas were found to be olivine ranging between Fa55–88 in basanite and Fa43–51 in the phonotephrite; aegirine-augite; Mn–Mg–Al–Cr-bearing ulvite-magnetite; hexagonal pyrrhotine of the Fe0.96–1S composition concentrating in interstitial magnetite; feldspar ranging from orthoclase-albite-anorthite in basanites to anorthite-orthoclase-albite in phonolites; nepheline, whose concentration increases in the direction from basanite to phonotephrite. The magnetite–ilmenite thermometer gave a temperature of 1081 ± 12°С for crystallizing basanite lava (Kyle et al., 1992).

Fig. 1.
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Antarctica (a); volcanic Ross Island (b) with volcanoes: Erebus ((1) height 3794 m, crater 805 m across and 274 m deep), Terror ((2) 3230 m), Bird ((3) 1765 m), Terra Nova ((4) 2130 m); Mount Erebus (c) and Erebus crater (d) at dawn.

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Mount Erebus has demonstrated a stable phonolite composition of its eruptions during the past 40 000 years, i.e., for nearly all Quaternary time. The volcano has been in an active phase since the 1970s; it has a persistent lava lake in the crater, and periodically discharges pyroclastic material (Taziev, 1987).

In addition, lava trachytes were detected on Erebus; these are considered to be a special incidental magmatic derivate resulting from a combined fractional assimilation crystallization.

Examination of petrologic and geochemical data suggested that the primary basanite melt occurs at Erebus by 2% melting of peridotite in a pulsating high (plume) of a depleted asthenosphere mantle directly beneath the volcano. The plume diameter is approximately estimated as 40 km(Kyle et al., 1992). The isotope ratios 87Sr/86Sr = 0.702984 ± 0.000028 and 143Nd/144Nd = 0.5102 ± 0.0019 obtained for the Erebus effusive rocks are characteristic of the West Antarctica Cenozoic rift volcanoes (Panter et al., 2006) for OIB basalts in general. Each generation of basanites at Erebus is supposed to come from a new portion of mantle melt, with the subsequent fractional differentiation of the basanite melt occurring at high temperatures and under relatively dry conditions (Iverson et al., 2014).

There is a higher 87Sr/86Sr ratio (0.70425), for lava trachytes, which supports the hypothesis that they were formed by combined fractional assimilation crystallization.

Explosive events in the history of Erebus were recorded much rarer, with the most reliable dates being in the time spans 77–56, 46–32, and 18–10 ka. Traces of eruptions were detected in ice cores, providing evidence of as many as 14 dated and 20 undated (so far) tephra layers. All of this shows that explosive activity has been far less frequent than effusive activity at Erebus, and occurred as discrete events, but still is present throughout the entire eruption record (Harpel et al., 2004). The most recent period of effusive and explosive activity at Erebus began in 1972, inaugurating the modern epoch of systematic studies in the ejecta of Cenozoic volcanism in Antarctica (Kyle and Jazek, 1978; Stothers and Rampino, 1983).

THE OBJECT OF STUDY AND THE METHODS USED

The object was a small amount of comparatively fresh, light-colored ash that was thought to come from the 2000 eruption that Ph.R. Kyle sampled at the rim of the Erebus crater. The particles in the sample mostly have elongate shapes, varying between 200 and 1000 µm across (Fig. 2). The sample was studied using the following techniques: optical microscopy (an OLYMPUS BX51 computerized unit); X-ray diffractometry (Shimadzu XRD-6000); analytic scanning electron microscopy (JSM-6400 with EDX and wave spectrometers); inductively coupled plasma mass spectrometry (Perkin Elmer ELAN 9000); X-ray fluorescence spectrometry (Shimadzu XRF-1800); isotope spectrometry (Delta V+ (Finnigan) with a Flash EA-HT 1112 elemental analyzer and a ConFlo IV gas commutator); gas chromatography (Tsvet-800 with a pyrolysis device). Analysis for trace elements was performed at the Geoanalitik TsKP, Institute of Geology and Geochemistry, Ural Branch (UrB), Russian Academy of Sciences (RAS) (Yekaterinburg), the other determinations were carried out at the Geonauka TsKP, Institute of Geology, Federal Research Institute, Komi Science Center, UrB RAS (Syktyvkar).

Fig. 2.
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The morphology and sizes of tephra particles in the sample taken from Mount Erebus.

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PARTICLE HABITUS AND SIZE

With respect to morphology and size, the particles in the sample subdivide into four types (Table 1). The first type includes the largest, elongate, nearly prismatic forms (with an elongation index of 2.5–3) that consist of intergrowths of numerous fiber-like subindividuals 10–30 µm thick (Figs. 3a–3f). The end parts of these particles contain rather deep cavities that have their axes nearly parallel to the length of the subindividuals and are very similar to gas bubbles emerging during degassing of a cooling melt (see Figs. 3e, 3f). The cavities are oriented along the direction of fiber subindividuals in the particles, which is quite consistent with the idea of their origin. The second type includes somewhat smaller and less elongate (the elongation index 1.3–1.7) angular lumpy particles having smooth-walled, isometric and oval cavities; these are even more similar to bubbles that have lost their gases as time went on.

Table 1.   The morphologic types and sizes of particles in the tephra sample studied
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Fig. 3.
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Particles, subprismatic (a–f) and angular lumpy (g–l) with signs of microvesicularity. SEM images in secondary (a, c, e, g, i, k) and elastically reflected (b, d, f, h, j, l) electrons.

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The third type includes numerous horned particles (Figs. 4a–4f) that have sizes and elongations similar to the angular lumpy particles. They too typically contain bubbles, but their sizes vary in a larger range, from relatively large ones of 20–30 µm to smaller, 1–5 µm, which are also oval or round. Lastly, we classified the fourth type as including almost the most frequent tabular forms with small-step lateral planes (see Figs. 4g–4l). Bubbles are extremely rare on these particles.

Fig. 4.
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Particles, horned (a–f) and tabular (g–l). SEM images in secondary (a, c, e, g, i, l) and in elastically reflected (b, d, f, h, j, k) electrons.

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It should be emphasized that the above data on our tephra sample are well consistent with (Kyle and Jazek, 1978; Iverson et al., 2014) for ash from cores recovered at the Byrd station, Mary Byrd Land in the western Antarctic ice cover obtained at a height of 2154 m.

CHEMISTRY, TRACE ELEMENTS, AND LITHOGENIC GASES

The bulk chemical composition classifies the tephra under study as anorthoclase phonolite with a high positive correlation (r = 0.61) between the concentrations of Na2O and K2O (Table 2). The difference from the lava phonolite consists in a slightly higher alkalinity. Nevertheless, the tephra fairly well fits the general lineage of crystallization differentiation for Erebus lavas (Fig. 5). An analysis of the glass phase in the tephra showed that it is much less acidic compared with the bulk composition, varying in the following sequence: acid phonolites → alkali trachytes → trachytes → trachyrhyodacites → trachydacites → trachyrhyolites. In addition, there are occasional cases of a trachyandesitic glass phase. We thus see that the tephra differentiates into a glass phase that is more acidic than phonolite as a whole and a microlite feldspar fraction.

Table 2.   The bulk chemical composition of ash (1) and the chemical composition of the glass phase contained therein (2–33), wt %
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Fig. 5.
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A TAS diagram (Petrograficheskii kodeks …, 2008) illustrating the chemistry of ejecta discharged as the effusive and explosive facies by Mount Erebus. The fields in the diagram: (1) picrites; (2, 3) picrites, moderately alkaline and alkaline, respectively; (4) fondites; (5–8) picrobasalts: ultrabasic, moderately alkaline, alkaline, and basic, respectively; (9) basalts; (10) trachybasalts; (11) alkaline basalts; (12) basaltic andesites; (13) basaltic trachyandesites; (14) phonotephrites; (15) andesites; (16) trachyandesites; (17) tephriphonolites; (18) low-alkali dacites; (19) dacites; (20) trachydacites; (21) trachytes; (22) alkaline trachytes; (23) phonolites; (24) low-alkali rhyodacites; (25) trachydacites; (26) trachyrhyodacites; (27) alkali rhyodacites (pantellerite); (28) low-alkali rhyolites; (29) rhyolites; (30) trachyrhyolites; (31) alkali rhyolites (comendites). Objects of study: (1) lavas in the general Erebus lineage; (2) lava phonolites; (3–5) data on our tephra sample: bulk composition, glass phase in particles, and feldspar microlites, respectively.

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The tephra was found to contain 51 trace elements, including 14 lanthanoids (Table 3). The total concentration of trace elements ranges in the interval 3765–4425 g/t, with the total lanthanoids making 626 ± 106 g/t. The list of trace elements in this tephra is similar to that for the Erebus lava vulcanites, which demonstrate a smooth and continuous growth in the concentrations and varying proportions in favor of alkaline, alkaline earth, and rare earth elements in the direction from basanites to phonolites (Kelly et al., 2007). The average total trace elements for the tephra are approximately that for lava phonolites, but are greater than those for basanites, phonotephrites–tephrophonolites, and trachytes by factors of 1.34, 1.23, 1.43, respectively. The total lanthanoids for the tephra are greater than those for basanites, phonotephrites, and tephriphonolites by factors of 1.6, and 1.3 for lava phonolites. The trend of the chondrite-normalized lanthanoid concentrations in the tephra is overall similar to that in the lavas from basanites to phonolites, but is different in having a small Eu-minimum at the tephra curves. The spider-diagram for the tephra shows a deficit in basaltoid-compatible Sr and, on the contrary, a considerable excess in incompatible elements (Rb, Zr, Y, Th, and U), as well as in heavy lanthanoids (Fig. 6). The data point of tephra composition when plotted in the Th–Hf–Ta diagram (Wood, 1980) and in the Ta/Yb–Th/Yb diagram (Boynton, 1984), as well as in the Zr–Zr/Y diagram, definitely occurs in the fields of intraplate geodynamic settings.

Table 3.   Trace elements in the Erebus tephra and lavas, g/t
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Fig. 6.
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The normalized concentrations of trace elements in the tephra sample. (1) after (Kyle et al., 1992); (2) according to this study.

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Viewed from the standpoint of Yu.G. Shcherbakov’s geochemical differentiation (Shcherbakov, 1965, 1976, 1982), the Erebus lava lineage shows a strong and orderly trend in the direction from basanites to phonolites as concerns increasing concentrations of centrifugal and deficit-centrifugal elements that compensates the loss in the concentration of centripetal and low-centrifugal elements. This is a definite confirmation of the Erebus lineage being due to the same process of magmatic differentiation. Viewed upon this background, the lava trachytes show contradictory properties. The concentrations of centrifugal and low-centrifugal elements in them are like those in phonotephrites and tephriphonolites, while the concentrations of centripetal and low-centrifugal elements makes them similar to phonolites. This is not entirely unexpected, being in agreement with the earlier inferences of American volcanologists as to a specific origin of trachytes on Erebus.

A mass occurrence of degassed bubbles in the microsculpture of tephra particles provides evidence of a considerable amount of gas in the primary melt. Our analysis revealed a broad association of components in the lithogenic gas phase, including both inorganic gases and a multitude of hydrocarbons in the range of С1–С4 alkanes and alkenes (Table 4). Judging from experimental data of (Iacovino et al., 2013), the Erebus lavas do absorb considerable amounts of volcanic gases, but this ability rapidly decreases with decreasing pressure, already in the range 700–400 MPa. It is in the lava phonotephrites that the concentration of dissolved gases becomes the lowest, approximately at the level 0.2–0.3% for Н2О and 0.05–0.2% for СО2, which is nearly identical with our data for the tephra, namely, approximately 0.15% for Н2О and 0.02% for (СО + СО2). The slightly lower concentrations of gas in our tephra seem to be a signature of its explosive origin.

Table 4.   The composition of gas pyrolysates obtained by heating the Erebus tephra sample
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From the viewpoint of petrology, the composition of lithogenic gases identified in the Erebus tephra correlates with that in products of crust–mantle interaction, being much nearer to mantle formations proper. This is indicated by a considerable amount of carbon monoxide in the pyrolysate (Fig. 7a) and by the dominance of С2–С4 hydrocarbons over methane (see Fig. 7b). In addition, the Erebus tephra has the composition of hydrocarbons in the pyrolysate similar to the basaltic andesite lavas discharged during the Tolbachik eruption of 2012–2013 (see Fig. 7c), thus providing evidence that the composition of lithogenic gases in volcanic ejecta is controlled by the depth at which the material source resides rather than by the composition and facies nature of the ejecta.

Fig. 7.
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Petrologic aspects in the origin of lithogenic gases in the Erebus tephra. (a) proportions of inorganic gases in mantle–crust derivates (A stands for mantle derivates (diamonds), B for products of mantle–crust interactions (minerals in diamond-bearing parastereseis), C stands for crustal derivates (Petrovsky et al., 2008); (b) same for hydrocarbon gases; (c) proportions of hydrocarbon gases in pyrolysates obtained by heating TFE-50 lavas to a temperature of 400°С (1) and in the range 400–1000°С (Silaev et al., 2019) (2). Star shows the composition of lithogenic gases in the tephra.

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MINERAL AND PHASE COMPOSITION

An X-ray diffractogram of the Erebus tephra shows a wide radiation band (FWHM = 11°) in the angle range 2Ө 22°–37° whose maximum occurs at 26.7°, which corresponds to d/n = 3.3 Å. This must be volcanic glass. Two narrow reflexes are observed in the region of extremum for the band, 3.34 and 3.22 Å, indicating an insignificant admixture of quartz and feldspar, respectively. The X-ray data thus show, when combined with analytic SEM results, that the tephra largely consists of intermediate to acidic glass. The insignificant admixture includes numerous microminerals like oxygen salts, magnetite, rutile, pyrite, native metals, and chlorides, aside from feldspars and quartz. All of these minerals make a paragenesis that is very typical of exactly the explosive facies of Cenozoic volcanism (Karpov et al., 2017).

The feldspars in this tephra (Table 5) are occasional Са-bearing orthoclase and a definitely dominant anorthoclase having the composition (Na0.61–0.69K0.24–0.31 Ca0.06–0.08)[Al0.89–0.92Fe0.15–0.18O8]. Regarded in its endmember composition, the latter mineral is anorthite–orthoclase–albite (mol %): albite 60–70, orthoclase 25–30, and anorthite 3–8, which is in complete agreement with the region of complete miscibility for the endmembers. It is a known fact (Kelly et al., 2007) that the anorthoclase that is present in lavas has a much more variable composition: one has orthoclase–albite–anorthite in basanites and anorthite–orthoclase–albite in phonolites, but anorthite is much more abundant (up to 23 mol %) there than in the tephra phonolite.

Table 5.   The chemical (wt %) and the endmember(mol %) compositions of feldspars
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Zircon in the tephra was found in the glass in the form of occasional inclusions, frequently as pyramidal prismatic crystals (typical of zircon) (6.9 ± 7.4) × (3.7 ± 2.7) µm in size (Figs. 8a, 8b). The composition of this zircon (Table 6) was found using the formula (Zr0.97–0.99 Hf0–0.01)[SiO4].

Fig. 8.
figure 8

Exhalations of accessory silicates and phosphates in the tephra. Minerals: ZRC zircon, AP apatite, MON monazite. SEM images in secondary (a, c, e) and in elastically reflected (b, d, f) electrons

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Table 6.   The composition of zircon (1–3), apatite (4–10), and barite (11, 12)
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The phosphate group includes apatite and monazite. The former mineral is chlorohydroxylapatite having the composition (Ca9.75–10.06Sr0–0.35Mn0–0.21) 9.89–10.06[P5.64S0–0.36O24](OH)1.66–2.34Cl0–0.59 and releasing minute, mostly oval, exhalations (0.9 ± 0.6) × (0.14 ± 0.21) µm in size (see Figs. 8c, 8d). The other phosphate in the tephra is La-Ce-Nd monazite; it is encountered in growths with rutile and orthoclase as isometric grains (5–7) × (3–4) µm in size (see Figs. 8e, 8f).

The Sr-bearing barite of the composition (Ba0.93–0.95Sr0.05–0.07)[SO4] mostly occurs as homogeneous grains of irregular shapes (4.34 ± 1.45) × (3.4 ± 1.85) µm in size. Some barite grains have an aggregate structure, consisting of numerous tightly coalesced subindividuals of angular oval shapes and being (0.51 ± 0.38) × (0.13 ± 0.05) µm in size.

Magnetite occurs in tephra rather frequently and in various forms, ranging between flaky accumulations of mesonanometer particles (Figs. 9a, 9b) and isolated grains (2.01 ± 2.9) × (1.39 ± 1.87) µm in size (see Figs. 9c, 9d). There are occasional grains 6–8 µm across that show nanoporosity (see Figs. 9e, 9f). The composition of the mineral (Table 7) is described by the formula (Fe0.94–1Mn0–0.06)(Fe1.28–1.99Ti0–0.64Al0–0.29)2O4. Compositional calculations for endmembers leads one to the conclusion that the tephra contains four varieties of magnetite, namely (in the order of decreasing frequency of occurrence), hercynite-magnetite, magnetite, ulvite-magnetite, and magnetite-ulvite.

Fig. 9.
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Exhalations of magnetite varieties (MG) in the tephra. SEM images in secondary (a, c, e) and in elastically reflected (b, d, f) electrons.

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Table 7. The chemical (wt %) and the endmember (mol %) compositions of magnetite
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One characteristic admixture in the tephra consists in the presence of numerous inclusions of native metals (3.9 ± 1.9) × (1.9 ± 1) µm in size whose elongation index is 2.2 ± 0.8 (Fig. 10). The inclusions are dominated by brass of the composition Cu0.55–0.59Zn0.37–0.4 Fe0.02–0.06Pb0–0.01 (Table 8). In addition to brass, one occasionally encounters Cu-Fe-bearing native tin. It should be emphasized that native-metal mineralization is rather characteristic chiefly of the explosive volcanic facies (Karpov et al., 2017). Special importance attaches to the detection of isometric chloride particles in the tephra (see Figs. 3e, 3f), which frequently occur near degassing-induced bubbles. The particles vary in size in the range (3.5 ± 1.57) × (2.7 ± 1) µm. Judging from the composition, these are sylvite solid solutions (see Table 8): (0.77–0.8) KCl + (0.17–0.2) NaCl + 0.03 CaCl2.

Fig. 10.
figure 10

Exhalations of native brass (CuZn) in the tephra. SEM images in secondary (a, c) and in elastically reflected (b, d–f) electrons.

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Table 8. The chemical composition of chloride solid solutions and metal alloys, wt %
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It should be emphasized that the explosive phonolite under study here has not been found to contain nepheline whose presence in the Erebus effusive tephriphonolites and phonolites was previously noted by American volcanologists. It is possible that the sample was too small and contained too much glass.

CARBONACEOUS MATERIAL

The fact of hydrocarbons being detected in the lithogenic gases from the Erebus tephra provides evidence that condensed carbonaceous material may be present in the tephra. Coulometric titration based on the value of pH (An-7529М) showed that the bulk concentration of carbon in the sample under study varied in the range 0.02–0.05 wt %, which is an order of magnitude greater than that of hydrocarbon gases in the tephra. It follows that the Erebus tephra is dominated by an admixture of nongaseous carbonaceous components. An isotope analysis of this component (6 determinations) resulted in the following value of the isotope ratio: δ13CPDB = –26.47 ± 0.34‰. This result lies within our earlier data for the so-called atomic dispersed form of carbon (Haggerty, 1999) on present-day volcanoes of Kamchatka and the Pacific Ring of Fire, as well as for organoids of volcanogenic atmospheric electrogenic origin (Silaev et al., 2016, 2018).

CONCLUSIONS

First multidisciplinary petrologic, mineralogical, and geochemical studies were carried out on tephra sampled from the greatest intraplate Erebus stratovolcano, Antarctica, which has a unique basanite–phonolite effusive lineage. The sample consists of angular lumpy, prismatic, horned, and tabular particles that vary widely in size between 0.3 × 0.1 and 2 × 0.7 mm. There are two characteristic features of the particles, namely, their microvesicularity and shape elongation, giving elongation indices between 1.2 and 8. Obviously enough, this habitus is due to the explosive origin of the tephra. The tephra bulk chemical composition classifies it as an anorthoclase phonolite that has slightly greater alkalinity than lava phonolites. With regard to its phase composition, the tephra largely consists of volcanic glass that shows a widely varying chemical composition, from phonolite to trachyrhyolite.

The sample was found to contain 51 trace elements whose total concentration varies in the range 3765–4425 g/t, with the total lanthanoids being 626 ± 106 g/t. The mean total trace elements in this tephra make it very similar to lava phonolite, while exceeding basanite, phonotephrite–tephriphonolite, and trachytes by factors of 1.34, 1.23, and 1.43, respectively. Its total lanthanoids exceed basanite, phonotephrite, and tephriphonolite by a factor of 1.6 and lava phonolite by 1.3. Viewed within the theory of geochemical differentiation due to Yu.G. Shcherbakov, the tephra fairly well fits the common sequence of magmatic differentiation known as the Erebus lineage, but requires some correction for the explosive origin of the tephra. Several geochemical criteria identify this tephra as volcanic ejecta that were specifically formed in an intraplate geodynamic rift setting.

There were some admixtures to the volcanic glass in the tephra. These were identified and studied, including feldspars of the anorthite–orthoclase–albite and orthoclase compositions; fluochlorohydroxylapatite; La–Ce–Nd monazite; Sr-bearing barite; spinellids whose compositions vary from magnetite to hercynite-magnetite, ulvite-magnetite, and magnetite-ulvite; Fe–Pb-bearing brass, and Cu–Fe-bearing native tin; chloride solid solutions of the hydrophilite-halite-sylvite composition.

The tephra lithogenic gases were found to contain N2, СO, CO2, and С1–4 hydrocarbons. The Erebus tephra gases have a composition and proportions between components to correlate them with products of crust–mantle interactions, rather closer to mantle formations proper. The carbon isotope composition in the tephra lies within the values determined for the so-called atomically dispersed form of carbon in present-day volcanic ejecta at the Pacific Ring of Fire, as well as for organoids of volcanic atmospheric electrogenic origin. Overall, the tephra studied here can be treated as an explosive, almost wholly glassy analogue of the anorthoclase lava phonolites that terminate the continuous sequence of magmatic differentiates known as the Erebus lineage.