Petrology of the 1952 Eruption and Ancient Lava and Pyroclastic Flows of Krenitsyn Peak Volcano, Onekotan Island, Kuril Arc, Russia

Abstract

Krenitsyn Peak is one of the two active volcanoes on Onekotan Island (Greater Kuril Ridge). The inaccessibility of the island, along with the volcano being situated within a sizeable (7 km in diameter) and cold (3.7 °C) caldera lake, has led to minimal research on the area. We present the first detailed characterization of the rocks from the only historical eruption of Krenitsyn Peak (November 1952) and a brief description of the ancient lava and pyroclastic density current (PDC) deposits that make up the building of the volcano. The 1952 eruptive products are represented by two-pyroxene andesites (59.2–63.3 wt.% SiO₂), and the older lava and pyroclastic flow rocks consist of two-pyroxene andesites and dacites (62–67.6 wt.% SiO₂). Almost all samples belong to the calc-alkaline, medium-K, and medium-Fe series, and the pumiceous lapilli from the 1952 eruption fall into the low-Fe series. The minerals exhibit signs of magma mingling, including relic high-Ca (up to An₉₂) plagioclase cores with signs of dissolution and recrystallization, and oscillatory-zoned pyroxene.

1. Introduction

The Kuril–Kamchatka island arc (Figure 1a) is one of the most active arc regions in the world. However, a combination of complex weather conditions, a lack of infrastructure, poor transport options, and limited foot accessibility has complicated access to many geological sites. As a result, active research on numerous volcanoes primarily took place between the 1950s and 1980s. Consequently, the available data on these sites are limited to the methodologies accessible then, which has particularly affected the study of the Kuril Islands. Onekotan Island, located in the northern Greater Kuril Ridge, has two active volcanoes: Krenitsyn Peak and Nemo Peak (Figure 1b). Their eruptive activity was noted between the 18th and 20th centuries. The island also features older volcanic buildings and large calderas, including the Tao-Rusyr and Nemo calderas, inside which the young volcanoes Krenitsyn Peak and Nemo Peak were formed, respectively [1,2,3]. Tephrochronological studies have been conducted on the island to determine the ages of the most significant caldera-forming eruptions [4,5]. Additionally, detailed geochemical studies have been carried out on the caldera-forming eruptions of Nemo Peak Volcano [6,7]. Research on other volcanoes in the region has been limited to brief descriptions or mentions in studies discussing broader global geochemical patterns [8,9,10,11,12,13,14,15,16,17]. In these cases, the volcanoes were considered only part of the Kuril Island arc, limiting the study of individual sites to just one or two samples and specific methods (e.g., B or Li isotopes [9,18]). This situation also applies to Krenitsyn Peak volcano, for which geochemical and petrological data are restricted to only a few measurements of major elements, whole-rock composition of products from the 1952 eruption, and descriptions of the optical properties of the minerals [12,14,15,17].

In this work, we present the results of our study on the rocks of the Krenitsyn Peak volcano. This includes the petrological–geochemical characteristics of the products from its last eruption in 1952 and data on the composition of older lava and pyroclastic flows that contributed to the building of the volcano. In addition, we provide a description of the current state of the volcano, including results of thermal imaging surveys that revealed a localized thermal anomaly at the base of the 1952 dome.

2. Geological Setting

Krenitsyn Peak is an active stratovolcano located inside the large Tao-Rusyr caldera on Onekotan Island (Figure 1). Onekotan is part of the northern group of islands in the Greater Kuril Ridge, which was formed by the subduction of the Pacific Plate beneath the Okhotsk Plate at a rate of 8.6 cm/year [19]. The depth of the slab beneath the island ranges from 125 to 130 km [9].

Before the caldera-forming eruption, the volcano Tao-Rusyr was a large stratovolcano with effusive–explosive eruption types. The lava and less abundant PDC deposits exposed on the caldera walls consist of basalt, basaltic andesite, and andesite (50.6–59.9 wt.% SiO₂) [11,14]. The catastrophic eruption of the volcano Tao-Rusyr occurred 7500 ± 80 years ago (¹⁴C, [5,20]) and resulted in the formation of a caldera measuring 7.8 × 7 km and the ejection of up to 60 km³ of juvenile and accidental (products of the destroyed volcanic building) material [4]. The products of the catastrophic eruption, including PDC and ignimbrite sequences composed of andesitic and dacitic material (61.8–67.2 wt.% SiO₂ [14,21]) occupy a significant portion of the eastern coastline of the island [22]. The caldera of the volcano Tao-Rusyr is filled with Lake Koltsevoe (“Ring Lake”), and the water level is currently 400 m above sea level. The building of the volcano Krenitsyn Peak (Figure 2a) is slightly eccentric in the northwest part of the caldera, rising approximately 900 m (1324 m above sea level) above the lake’s surface. The diameter of the base of the building at lake level is up to 4 km.

According to previous observations, “very uniform pyroxene andesites represent the lavas of Krenitsyn Peak,” and “the eruption of 1952 produced a variety of shaped products: andesitic ash, andesitic pumiceous lapilli, and a lava dome” [14].

1952 Eruption

The only historical eruption of the Krenitsyn Peak volcano occurred in November 1952 (11–19 November). No previous eruptive activity has been recorded, except for minor fumarolic activity in 1846 and 1879. The 1952 eruption led to the formation of a lateral crater on the northeastern slope of the volcano at an elevation of about 900 m asl and an extrusive dome (Figure 3) adjacent to the volcano’s building at the steep eastern shore [12,14,15,17].

On 5 November 1952, a tsunami occurred, sweeping away the town of Severo-Kurilsk and causing irreparable damage to several Pacific settlements [17,23,24]. To assess the impact of the tsunami on the Kuril Islands, an expedition of researchers from the Institute of Volcanology was organized, during which B.I. Peep and A.E. Svyatlovsky were on Onekotan at the moment of the eruption. Below, we present the sequence of events of this eruption according to [17], with times given in Kamchatka time (UTC + 12).

12 November (evening): Beginning of the explosive eruption and probably the start of the formation of a lava dome. The eruptive vent is at the bottom of the lake at the eastern foot of the Krenitsyn Peak volcano. “Heavy clouds of gas and ash” rise above the lake’s surface.

13 November: The explosive phase continues. The thickness of the ash layer that has fallen on the shore of the Sea of Okhotsk (12 km from the volcano) reaches 2 cm.

14 November (around noon): The power of the eruption increases, and the feeding conduit shifts: instead of the lake bottom, explosive activity continues from the crater that has formed on the slope of the volcano. Only clouds of steam now rise from the lake bottom (according to the authors, a lava flow is possibly moving along the bottom of the lake at this time).

15 November: During the paroxysmal phase of the eruption, the ash cloud extends far to the northeast over the surface of the Pacific Ocean. Numerous lightning bolts accompany the formation of the ash cloud.

16 November (day): The eruption begins to weaken.

17 and 18 November: The eruption slightly intensifies and then quickly stops. There was an intense snowstorm on 18–19 November; so, the date of the eruption’s end (the 18th or 19th) is unknown.

3. Materials and Methods

3.1. Materials

Samples of eruptive products from the Krenitsyn Peak volcano were collected during fieldwork by staff of the Laboratory of Volcanochemistry of the Institute of Volcanology and Seismology of the Far Eastern Branch of the Russian Academy of Sciences, conducted in July 2024 as part of the expedition of the Russian Geographic Society “Eastern Bastion. Kuril Ridge-2024” on Onekotan Island. Products from the recent volcanic eruption (11–19 November 1952) were collected, along with older lava and PDCs and subvolcanic bodies, to characterize the volcanic activity.

Samples from the 1952 eruption were collected from the extrusive dome, along with deposits of pumiceous lapilli on the southern slope of the dome. The extrusive dome, which was described immediately after the eruption as a “dark flat-topped lava dome” [12,14,15,17], is currently covered by a thick layer of lichens under which altered rocks of a distinctly reddish-brown color are hidden.

Samples of lava and PDCs formed before 1952 (hereafter referred to as “ancient”) were collected at the water level while conducting a circular catamaran route around the base of the volcano building (see Figure 1b). According to earlier descriptions, at the waterline, “tongues of lava flows occur alternately with pyroclastic debris” [14]. During the circular route around Krenitsyn Peak, we found that a significant part of the slopes, previously identified as PDCs based on visual assessments from the caldera rim, are formed as a result of gravitational processes (see Figure 1b and Figure 4). Uninterrupted PDC deposits are also found at the water level, but these are isolated occurrences.

In the summit area of the volcanic building, lava flows, subvolcanic bodies, and remnants of ancient lava domes were sampled in the eastern and northeastern parts of the crater. Most of the rocks have been significantly altered due to fumarolic activity (from partially altered andesite to hydrothermal clay), with some geological bodies completely covered by a thin layer of lichens, distorting their appearance. For instance, the remnants of the ancient extrusive dome on the eastern edge of the modern crater, described by predecessors as “a large ‘tooth’ of monolithic dark lava that rises sharply like a cone” [14], are composed of light gray rocks profusely covered with black lichens.

For further study, rocks from the 1952 eruption and various lava samples and PDCs exposed in coastal cliffs were collected. Among the summit samples, only the least altered samples of ancient lava flows—VKu2405a and VKu2406a (see Figure 2b,c), as well as VKu2408, morphologically overlapping the flows exposed at the shore—were subsequently used.

3.2. Methods

The minerals and glass were analyzed with a Jeol JSM-IT500 scanning electron microscope (20 kV and 0.7 nA) equipped with an Oxford X-MaxN energy-dispersive analytical device at the Department of Petrology, Geological Faculty, Moscow State University. The counting livetime was 100 s with 25–30% of detector deadtime. We used a focused beam to analyze mafic minerals and sulfides and a rasterized beam (spot diameter >122 μm²) to analyze feldspar and glass. Pure synthetic oxides and natural silicates and sulfides were used for calibration, and the accuracy was controlled by an analysis of silicate standards [25]. Scanning electron microscopy was also employed for backscattered electron (BSE) imaging of textures and mineral assemblages (image resolution: 2056 × 1920 pixels).

Bulk rock composition (

Table 1. Whole-rock major element composition in wt.%. LD—lava dome, LF—lava flow, PF—pyroclastic density currents.

Sample	VKu2430	VKu2447	VKu2447b	VKu2405a	VKu2406a	VKu2413	VKu2415
Notes	1952 LD	1952 LD	1952 lapilli	ancient LF	ancient LF	ancient LF	ancient LD
SiO2	62.55	61.49	58.61	63.23	61.38	63.51	61.38
TiO2	0.70	0.69	0.71	0.71	0.73	0.70	0.72
Al2O3	16.06	16.25	17.18	16.33	16.21	16.07	16.29
FeO	6.33	6.85	7.57	6.06	6.61	6.10	6.96
MnO	0.18	0.18	0.19	0.18	0.17	0.18	0.19
MgO	2.14	2.49	3.12	1.85	2.35	1.94	2.43
CaO	5.66	6.1	7.22	5.12	5.62	5.3	6.13
Na2O	3.81	3.63	3.34	4.04	3.64	3.88	3.66
K2O	1.27	1.2	0.96	1.35	1.18	1.27	1.16
P2O5	0.15	0.14	0.14	0.17	0.16	0.16	0.15
Total	98.85	99.02	99.04	99.04	98.05	99.11	99.07
Sample	VKu2408	VKu2443	VKu2443a	VKu2431	VKu2432	VKu2433	VKu2435
Notes	dike	ancient PF (bomb)	ancient PF (ash)	ancient LF	ancient LF	ancient LF	ancient LF
SiO2	62.89	64.60	64.33	64.88	64.89	64.48	64.66
TiO2	0.70	0.69	0.67	0.67	0.68	0.69	0.69
Al2O3	16.23	16.14	16.00	15.90	15.70	15.94	15.96
FeO	6.08	5.49	5.68	5.83	5.42	5.56	5.53
MnO	0.18	0.18	0.18	0.17	0.17	0.18	0.17
MgO	1.89	1.74	1.79	1.43	1.55	1.7	1.64
CaO	5.23	5.08	5.1	4.68	4.71	4.95	4.88
Na2O	3.92	3.96	4.01	4.06	4.22	3.99	4.09
K2O	1.28	1.27	1.33	1.45	1.43	1.34	1.36
P2O5	0.17	0.17	0.17	0.2	0.17	0.18	0.18
Total	98.57	99.32	99.26	99.27	98.94	99.01	99.16
Sample	VKu2436	VKu2437	VKu2439	VKu2440	VKu2441	VKu2442	VKu2444
Notes	ancient LF	ancient LF	ancient LF	ancient LF	ancient LF	ancient LF	ancient LF
SiO2	64.57	64.22	64.30	65.50	66.90	65.32	63.69
TiO2	0.68	0.68	0.67	0.65	0.63	0.66	0.70
Al2O3	15.85	15.89	16.00	15.76	15.32	15.62	16.04
FeO	5.55	5.68	5.89	5.17	4.80	5.22	6.00
MnO	0.17	0.17	0.18	0.17	0.16	0.17	0.18
MgO	1.68	1.75	1.61	1.49	1.23	1.42	1.84
CaO	4.87	4.99	5.02	4.6	4.03	4.46	5.2
Na2O	4.02	4.02	3.96	4.17	4.22	4.19	3.99
K2O	1.37	1.35	1.4	1.43	1.5	1.43	1.31
P2O5	0.17	0.17	0.19	0.18	0.19	0.18	0.18
Total	98.93	98.92	99.22	99.12	98.98	98.67	99.13

) was determined by a vacuum sequential X-ray fluorescence spectrometer with a wavelength dispersion Axios mAX (PANalytical, Almelo, The Netherlands) in IGEM RAS (Moscow, Russia). The spectrometer was equipped with a 4 kW X-ray tube with an Rh anode, with a maximum tube voltage of 60 kV and a maximum anode current of 160 mA. Trace element content was determined by inductively coupled plasma mass spectrometry (ICP-MS) using a Thermo Fisher Scientific Element XR spectrometer in GEOKHI RAS (Moscow, Russia). Analytical quality was assessed using USGS international standard samples (BHVO-2 and AGV-2).

4. Results

4.1. Current State of the Volcano

Today, at the northern contact between the extrusive dome and the main edifice of the volcano, several small discharges of thermal waters are observed (Figure 5). The images in Figure 5 were obtained using a DJI Mavic 2 Enterprise drone equipped with a thermal imaging camera. Thermal waters discharge into the lake both from the extrusion itself and from the base of the main volcanic edifice (Figure 5). The capacity of these discharges is relatively low, as the thermal waters form only a thin layer (approximately 5–10 cm) on the surface of the cold lake water. The plumes of heated water extend only 10–20 m from the discharge points before cooling down to the background lake water temperature. The maximum temperature of the thermal waters, measured with a thermocouple, was 25.5 °C, while the background temperature of Lake Koltsevoye was 3.5 °C. No other thermal anomalies were detected on the Krenitsyn Peak volcano.

4.2. Whole-Rock Geochemistry

The rocks of the Krenitsyn Peak volcano (Table 1 and Table S1) belong to the calc-alkaline series and are represented by medium-K andesites and dacites (Figure 6). On discrimination diagrams, the data points of the rocks form a clear linear trend from 59 to 67.6 wt % SiO₂. The most silicic part of the trend is represented by the compositions of ancient lava and PDC deposits (bombs and ash) sampled in the coastal cliffs. In contrast, the composition of the eruption products from 1952 and the ancient lava domes is shifted to the less silicic part of the trend. The products of the 1952 eruption are noticeably heterogeneous in their bulk composition, with a difference of 4 wt.% SiO₂ between the composition of pumiceous lapilli (59.2 wt.% SiO₂) and the composition of the densest rocks of the extrusive dome (63.3 wt.% SiO₂).

The contents of other elements also differ systematically (Figure 7). Unfortunately, the composition of the lava flow from the 1952 eruption (presumably formed at the bottom of Lake Koltsevoe [12]) is unknown. It was not possible to collect various products from the same older eruptions due to the active gravitational processes and thick vegetation cover on the slopes.

The distribution of rare earth elements (

Table 2. Whole-rock trace element concentration in ppm.

Sample	VKu2430	VKu2447	VKu2447b	VKu2443	VKu2405a	VKu2440	VKu2441
Li	6.5	8.6	9.9	9	8.3	9.5	9.1
Be	0.44	0.58	0.56	0.63	0.53	0.69	0.76
Sc	32	30	27	23	25	23	20
V	209	175	140	85	99	68	39
Cr	4.9	5.3	3.8	0.69	4.2	1.9	2.3
Ni	3	2.8	1.9	0.47	3.1	0.8	0.56
Cu	43	48	24	19	20	18	13
Zn	87	82	82	78	81	76	68
Ga	17	16	16	16	16	16	16
Rb	18	22	22	23	22	26	28
Sr	304	279	275	275	268	268	259
Y	27	32	33	33	27	37	34
Zr	84	104	114	95	87	118	102
Nb	1.3	1.6	1.7	1.4	2	1.8	1.5
Mo	2	2.3	2.4	1.5	2.8	2.1	1.3
Sn	0.54	1.2	0.66	0.43	0.72	0.73	0.78
Sb	0.31	0.39	0.44	0.39	0.41	0.44	0.41
Cs	1.6	1.8	1.7	2	0.96	1.8	1.8
Ba	234	284	303	323	316	345	367
La	5.2	6.3	6.7	7.1	5.7	7.7	8
Ce	14	17	18	19	15	21	20
Pr	2	2.5	2.6	2.7	2.1	3	2.9
Nd	10	12	13	13	10	15	14
Sm	3.2	3.8	4	4.1	3.1	4.5	4.2
Eu	1.1	1.2	1.2	1.3	1.3	1.4	1.4
Gd	3.6	4.2	4.5	4.6	3.5	5	4.6
Tb	0.69	0.8	0.85	0.85	0.68	0.95	0.86
Dy	4.7	5.4	5.8	5.7	4.6	6.4	5.7
Ho	1	1.2	1.3	1.2	1	1.4	1.3
Er	3	3.5	3.7	3.6	3	4.1	3.6
Tm	0.45	0.54	0.58	0.54	0.47	0.64	0.56
Yb	3	3.5	3.8	3.6	3.2	4.1	3.6
Lu	0.45	0.54	0.59	0.54	0.5	0.64	0.55
Hf	2.6	3.3	3.6	3.2	3	3.8	3.4
Pb	7.8	8.6	9.2	8.1	9.2	10	9.8
Bi	0.186	0.05	0.031	b.d.l.	b.d.l.	0.021	0.027
Th	1.2	1.6	1.8	1.6	1.4	1.9	1.7
U	0.61	0.77	0.84	0.69	0.54	0.87	0.7

) and trace elements in the rocks of the volcano is typical for island arc volcanoes, with significant enrichment of K, Pb, and Sr and depletion of (Ta)-Nb and Ti relative to N-MORB compositions (Figure 8). The europium anomaly is expressed unevenly in different rocks: slightly positive in the lava flow from the summit part of the volcano’s building, slightly negative in the ancient lava flows and the rocks of the extrusive dome from 1952, and completely absent in the compositions of lapilli from 1952 and bombs from ancient PDC deposits.

4.3. Petrography

The rock types of the Krenitsyn Peak volcano are nearly indistinguishable regarding the set of phenocrysts and their characteristics (such as zoning patterns and the composition of phenocryst cores). Some petrographic diversity arises from variations in porosity, groundmass structures, matrix glass composition, microlites, the margins of phenocrysts, and the presence or absence of silica phases.

All studied samples are porphyritic rocks with phenocrysts of plagioclase, orthopyroxene, clinopyroxene, and titanomagnetite (Tables S2–S6). The groundmass of volcanic bombs, lavas, and lapilli comprises microlites of plagioclase, orthopyroxene, clinopyroxene, and titanomagnetite within rhyolitic glass. The presence of cristobalite is characteristic of the most siliceous lava flows and extrusive dome rocks. Cristobalite forms separate crystals in matrix glass, with fibrous aggregates filling the space between other microlites and grains in the pore space (Figure 9d,e).

The plagioclase phenocrysts vary in size, up to 3–4 mm in diameter, although many crystals measure between 0.5 mm and 2 mm (see Figure 9a,b). Most phenocrysts generally exhibit complex zoning, while single crystals with normal zoning can be found in glomeroporphyritic intergrowths. Zoned phenocrysts are characterized by unevenly dissolved Ca-rich cores, displaying spotted or patchy-zoned structures (An_51–90). Numerous melt and fluid inclusions often complicate these structures, leading to spongy or sieve-texture plagioclases (An_80–85). The rims of the phenocrysts exhibit their zoning characteristics. Some rims show rhythmic zoning with An values ranging from An₅₂ to An₉₂, featuring variations in the An component content of 5–25 mol.%, alongside signs of melting at the boundaries. Other rims are normally zoned and nearly homogeneous, with a notable enrichment in sodium compared to the cores (An_50–60 versus approximately An_80–90). Additionally, there are isolated crystals with spongy cores (An₈₅) surrounded by a high-An rim (An₉₂) and a thin rim with increased Na content, similar in composition to microlites. Overall, the composition and zoning characteristics of the phenocryst cores do not show significant variation from sample to sample (Figure 10).

Among the microlites of plagioclase, two generations are distinguished: homogeneous needle-like lathwork (An_40–62) and prismatic and long-prismatic crystals with distinct normal zoning (high-Ca core (An_75–85) and a more sodic rim (An_40–62)), often with traces of skeletal growth. Compositional variations in the microlites and rims of phenocrysts correlate systematically with the bulk composition of the rock; less siliceous rocks are characterized by higher An compositions of microlites and rims (An_55–62). In contrast, more siliceous rocks have less anorthite (up to An₄₀).

Phenocrysts of pyroxenes consist of prismatic crystals up to 0.5 mm long and, less frequently, up to 1 mm long. Rare (Figure 9c) and relatively large clinopyroxene crystals (0.5–1 mm) are characterized by the presence of cores showing signs of resorption (numerous melt and mineral inclusions). The phenocrysts of clinopyroxene (Figure 10) are represented by augite (Mg# 70–75), and orthopyroxene is represented by minerals of the enstatite–ferrosilite series (Mg# 57–71). In some samples, phenocrysts of pyroxenes typically demonstrate simple reverse zoning with jumps in Mg# (Mg# = Mg/(Mg + Fe)) of 2–4 mol. %, while in other samples, the pyroxenes are predominantly homogeneous in composition, but the formation of pigeonite rim around individual clinopyroxene phenocrysts is observed (Figure 9d). All pyroxenes from all samples are characterized by a submicron iron-rich rim forming at the contact with the groundmass.

The diversity and composition of pyroxene in the microlites vary somewhat from sample to sample. In some rocks, the microlites of pyroxene are represented only by clinopyroxene of augitic composition (hereafter augite) and orthopyroxene, while in others, they include augite, pigeonite, and orthopyroxene. Augite is characterized by an increased (10–20 mol. % vs. <10 mol. % for clinopyroxene phenocrysts) content of tschermak and, consequently, Al₂O₃ (up to 5 wt.%), along with decreased CaO (9–18 wt.%). The microlites of orthopyroxene are compositionally similar to the rims of phenocrysts. Pigeonite contains 3–7 wt.% CaO. The presence or absence of pigeonite, in general, does not correlate with the Si or Fe concentration (both in bulk rock and in the glass); however, among the rocks from the 1952 eruption, pigeonite is found only in pumiceous lapilli (the most mafic rocks from the eruption). The crystallization temperature of microlites in rocks containing only augite and orthopyroxene, estimated using a two-pyroxene thermometer, is 980–1000 °C [29], while the rims of phenocrysts are around 970–980 °C. There are only a few pairs that meet the equilibrium conditions, as follows: (K_D(Fe-Mg)^Cpx-Opx = $\left(X_{\mathrm{Fe}}^{Cpx}/X_{\mathrm{Mg}}^{Cpx}\right)/\left(X_{\mathrm{Fe}}^{Opx}/X_{\mathrm{Mg}}^{Opx}\right)$ = 1.09 ± 0.14. Among the microlites of pyroxene, needle-like and skeletal crystals prevail.

Ore minerals are represented by titanomagnetite and pyrrhotite, predominantly by titanomagnetite. In most samples, titanomagnetite is present as homogeneous crystals, while samples from extrusive domes exhibit crystals with signs of exsolution. Unfortunately, the size of the lamellae does not allow their composition to be determined. The microlites of titanomagnetite are exclusively homogeneous and have a higher Ti content than the phenocrysts (11–12 wt.% vs. 7–9 wt.% TiO₂).

Pyrrhotite occurs both as inclusions in minerals and in the glass of the groundmass. The edges of the grains are often oxidized.

The groundmass of the rocks consists of microlites of plagioclase, pyroxene (augite, pigeonite, and orthopyroxene), cristobalite (for dacites), apatite, and titanomagnetite embedded in rhyolitic glass. The glass composition varies from sample to sample—76–78.5 wt.% SiO₂, 2.5–2.7 wt.% K₂O in dacites and andesitic dacites, and 68.5–69.5 wt.% SiO₂ and 1.8–1.9 wt.% K₂O in the more mafic (andesite) pumiceous lapilli. The pressure at the last equilibrium of the melt with microlites, approximately estimated from the glass composition [29,30], corresponds to 0.2–0.3 kbar for lava flow rocks and extrusive domes and 0.5 kbar for pumiceous lapilli from the 1952 eruption.

5. Discussion

5.1. Petrographic Signs of Magma Mingling

Petrographic features in all the volcanic rocks of Krenitsyn Peak indicate the open nature of the magmatic system and the significant role of magma mingling processes throughout its evolution. Such features include relic high-Ca cores in plagioclase phenocrysts and cores of large clinopyroxene phenocrysts; both spongy sieve-texture crystals and patchy-zoned crystals form due to the adiabatic rise of magmas, which accompanies decompression melting and subsequent crystallization of the mineral that is in equilibrium with the new conditions [31,32]. The formation of sharp reverse zoning in pyroxenes may result from a sudden increase in volatile content in the melt or a temperature rise [33,34,35,36]. Additionally, rhythmic zoning at the rims of plagioclase, resorption zones, and partial melting at the zone boundaries are traditionally interpreted as a consequence of mingling processes [37,38,39].

The composition of the relic cores in plagioclase phenocrysts (up to An94) sharply contrasts with the rhyolitic composition of the matrix glass. It suggests that the ascending magma had a significantly more mafic (basaltic) composition. The coexistence of orthopyroxene of the enstatite–ferrosilite series, augite, and pigeonite in the groundmass marks a high-temperature state of the magma before the eruption and relatively reducing conditions—below the Ni-NiO buffer [40]. The stable pyrrhotite–magnetite association also indirectly evidences the low fugacity of oxygen.

Thus, based on the diversity of mineral compositions and their structural–textural features, we hypothesize that the main trigger for the eruptions of Krenitsyn Peak was the intrusion of basaltic magmas into the feeding system with relatively felsic (dacite–andesite) magma.

5.2. Variations in Lava Composition at Krenitsyn Peak over Time

Our results allow for a general understanding of the eruption mechanism of the Krenitsyn Peak volcano and the main processes that control the evolution of its magmatic system. However, significantly more detailed studies are required to create a complete picture, including geo- and/or tephrochronological investigations. At this stage, we can analyze the compositional evolution of the volcanic rocks solely based on Steno’s laws and general concepts of the temporal relationships of flows (overlapping flows must be younger than the flows beneath them, and the rocks from the 1952 eruption are younger than all other volcanic rocks). Since the flows erupted on different sides of the volcano’s building and are predominantly overgrown with alder and cedar shrubs, this approach does not allow for the establishment of an exact sequence of flow formation. However, we can distinguish three groups of rocks: the oldest flows (exposed in coastal cliffs and overlain by one or more subsequent flows), intermediate (extrusive formations at the summit of the volcano as well as lava flows that cover the “oldest flows”), and modern (products of the 1952 eruption). There is a consistent trend of decreasing silica content from the oldest rocks to the modern ones (and a corresponding change in all other elements, see Figure 7).

Thus, the geochemical and petrographic characteristics of the rocks indicate the important role of mingling and mixing processes in the evolution of the magma plumbing system, particularly the existence of a relatively felsic (dacite/andesite) subsurface chamber fed by magmas of more mafic composition (basalt).

The variations in the ratios of trace element concentrations (e.g., Rb/Sr, La/Nb, Zr/Hf, and Sr/Y) are minor; however, it is clear that fractional crystallization is the dominant, although not the sole, process controlling the composition of the volcanic rocks of Krenitsyn Peak. The mixing of related magmas at different degrees of fractional crystallization would also not affect the distribution of incompatible elements, unlike the assimilation of crustal material. We suggest that the formation of a large magmatic chamber before the caldera-forming eruption of the Tao-Rusyr volcano was accompanied by partial assimilation of crustal material. The Krenitsyn Peak volcano was formed in the Tao-Rusyr caldera. Thus, the eruptions of Krenitsyn Peak are initiated by the intrusion of basaltic magma into an acidic chamber, the magma of which formed through the fractional crystallization of related basaltic magma and the assimilation of surrounding rocks.

5.3. Eruption on 11–19 November 1952

According to eyewitness observations, the eruption began with the squeezing of the lava dome at the bottom of Lake Koltsevoe. The formation of pumiceous lapilli is associated with the climax stage of the eruption, which began after a change in the feeder channel and the relocation of eruptive activity to the slope of the volcano [17]. Samples from different stages of the eruption differ significantly, allowing for tracking the pre-eruptive processes in the magmatic system.

The extrusion of magmas with a relatively high silica content (62–63.5 wt.% SiO₂) contributed to the formation of the lava dome. The formation of cristobalite in the groundmass is typical for extrusive dome rocks and marks crystallization at pressures less than 0.25 kbar [41,42], which aligns with the pressure estimates of the last equilibrium based on the matrix glass composition.

The explosive phase of the eruption began later than the extrusive phase. During the explosion, magmas with a more primitive composition (59 wt.% SiO₂) were erupted, lacking cristobalite and other silica phases. The avalanche-like exsolution of volatiles and fragmentation of the magma during this phase, according to the pressure estimates of the last equilibrium (0.5 kbar), occurred significantly deeper than during the extrusion of the dome. The lapilli formed during the explosive eruption contained pigeonite in the groundmass, which was not found in the other products of this eruption. The preservation of pigeonite corresponds to higher temperature crystallization and rapid cooling, as it breaks down outside its thermal stability range under slow cooling [40].

Based on the literature data on the eruption’s course and the obtained results, we propose the following model of the 1952 eruption. Unfortunately, the eruption products do not contain mineral associations that could serve as reliable geobarometers; thus, the model does not include any data about the possible depth of magma reservoirs.

The long period of dormancy (the 1952 eruption is the only historical eruption) allowed for the gradual cooling of the shallow chamber, which, in turn, led to a gradual increase in the crystallinity of the magma (crystal mush) within it and, consequently, to the evolution of the residual melt’s composition towards an increase in the SiO₂ content. The residual melt, being a lighter and more buoyant phase, tends to gradually segregate in the upper part of the chamber [43,44], leading to a peculiar layering of the magma reservoir into a lower part containing a more crystalline and mafic crystal mush and an upper part “filled” with more siliceous magma [41].

The intrusion of basaltic magma into the reservoir initiated several processes. The increased temperature and concentration of volatile components caused the partial melting of the crystal mush located in the lower part of the reservoir (we will refer to this as andesitic magma). This resulting magma then rose into the upper part of the reservoir.

As pressure increased, the felsic magma that had accumulated in the upper part was extruded to the surface, leading to the formation of a lava dome. The felsic magma gradually rose to the surface, degassed, and cooled, eventually plugging the feeder conduit completely. When critical pressure was reached in the shallow chamber, the andesitic magma broke through a new feeder conduit that ended in a crater on the slope of the volcano.

5.4. Further Research Directions: Krenitsyn Peak Volcano Resulting from the Post-Caldera Evolution of the Magmatic System of the Tao-Rusyr Volcano

The compositions of the rocks from the Krenitsyn Peak volcano differ significantly from the composition of the rocks of the Tao-Rusyr volcano (Figure 6). The examined rocks belong to two different series (Figure 6): the calc-alkaline series (Krenitsyn Peak) and the tholeiitic series (Tao-Rusyr). Moreover, the oldest and, consequently, the most silicic rocks of the Krenitsyn Peak volcano are richer in SiO₂ than the rocks from the caldera-forming eruption of the Tao-Rusyr volcano [45] and the rocks from the pre-caldera stage of development of the Tao-Rusyr volcano (Figure 6). The concentrations of trace elements in the rocks of the Tao-Rusyr volcano have not been previously studied; however, even from the compositions of the rocks of the Krenitsyn Peak volcano, it is evident that more than one process played a significant role in the evolution of the magmas.

Thus, the early stage of development of the Krenitsyn Peak volcano is associated with the effusion of lavas, which differ markedly in their petrochemical characteristics from all earlier rocks of this long-lived volcanic center. Such a change could have been caused by numerous factors—from the assimilation of previously modified crustal material to changes in the composition of the mantle source or slab fluid.

To date, there are virtually no data on the crustal structure beneath the Krenitsyn Peak volcano. The only known information is that it formed within the Tao-Rusyr caldera, whose walls expose remnants of an ancient pre-glacial volcano [14]. The composition of this pre-glacial volcano remains entirely unknown, while the lavas of the Tao-Rusyr volcano clearly follow fractionation trends distinct from those of Krenitsyn Peak (Figure 5). The presence of secondary alterations typical of large caldera-forming eruptions [46] can only be hypothesized, as the caldera floor is concealed by a deep ice lake (over 360 m deep), and no drilling has been conducted in the area.

In further research, we aim to assess whether partial melting of Tao-Rusyr volcanic rocks (as proposed for the Southern Kuril calderas [47]) and their incorporation into the parental basaltic magma could explain the observed compositional differences or if a more complex mechanism is required. To address this matter accurately, we plan to conduct detailed studies of the behavior of rare and trace elements in the rocks of both volcanoes and, possibly, isotopic studies that go beyond the scope of this work.

6. Conclusions

The Krenitsyn Peak volcano is a typical island arc andesitic volcano whose eruptions are initiated by periodic intrusions of basaltic magma into a relatively felsic near-surface chamber. The rocks of Krenitsyn Peak represent a continuous andesite–dacite series (59.2–67.6 wt.% SiO₂). Based on their geochemical characteristics, the studied rocks significantly differ from the rocks of the Tao-Rusyr volcano, within the caldera of which Krenitsyn Peak was formed.

The presence of relic high-An plagioclase (up to An₉₂) and pigeonite in the groundmass suggests the injection of compositionally and thermally contrasting basaltic magmas into the dacitic reservoir. These observations, together with the compositional differences between the lavas of Tao-Rusyr and Krenitsyn Peak, indicate that the evolution of Krenitsyn Peak magmas was governed not only by fractional crystallization of parental magmas derived from Tao-Rusyr but also by more complex assimilation–recharge–fractionation crystallization processes. Future studies will be aimed at investigating these processes in greater detail.

Recent field surveys using a drone equipped with a thermal imaging camera revealed a small thermal anomaly at the base of the 1952 dome, while no anomalies or signs of active fumarolic activity were detected on the dome itself or at the summit. Nevertheless, old fumarolic mineralization is present both on the 1952 lava dome and at the summit of the Krenitsyn Peak volcano.