Petrological Monitoring of the AD 2011–2012 Volcanic Ash from Sakurajima Volcano, Southern Kyushu, Japan

geosciences

Article Petrological Monitoring of the AD 2011–2012 Volcanic Ash from Sakurajima Volcano, Southern Kyushu, Japan

Idham Andri Kurniawan 1,2,*, Masayuki Sakakibara 1,† and Emmy Suparka 2,†

1 Department of Earth Science, Graduate School of Science and Engineering, Ehime University, Matsuyama 790-8577, Japan; [email protected] 2 Department of Geology Engineering, Faculty of Earth Science and Technology, Bandung of Institute Technology, Bandung 40123, Indonesia; [email protected] * Correspondence: [email protected]; Tel.: +81-89-927-9649; Fax: +81-89-927-9640 † These authors contributed equally to this work.

Academic Editor: Ian Coulson Received: 4 November 2015; Accepted: 15 February 2016; Published: 24 February 2016

Abstract: Sakurajima in Japan is one of the world’s most active volcanoes. This paper presents the results of a petrological study of the Showa Crater volcanic ash samples ejected from January 2011 to November 2012 from Sakurajima. The aim of this paper is to reconstruct the evolution in time of the conduit magma system, based on the compositions and physical properties of the studied volcanic ash. We analyzed the composition of interstitial glass and microcrystal of Black Volcanic Rock (BVR) and Black Vesicular Volcanic Rock (BVVR) in order to estimate the magma ascent rate. The results show that SiO2 content of interstitial glass and crystallinity of the BVR is generally higher than for BVVR. The different types of juvenile material likely resulted from cooling-induced crystallization and decompression-induced crystallization in the conduit. The conditions of magma transit within the conduit from 2011 to 2012 differ: in 2011, the decompression rate and magma ascent rate of BVR were higher than for BVVR, but, in 2012, the decompression rate and magma ascent rate between BVR and BVVR were similar. As such, monitoring the petrological features of dated eruptive materials could provide useful information for evaluating ongoing eruptive activity.

Keywords: Sakurajima; juvenile materials; interstitial glass; magma ascent rate

1. Introduction The nature of magma ascent through a conduit is directly linked to magmatic processes including rheology, composition of magma, geometry of the plumbing system, and the variability in eruption style of active volcanoes. However, the mechanisms that control the magnitude of Vulcanian explosions, which are typical of intermediate to silicic volcanoes, remain enigmatic [1,2]. Magmatic properties evolve in space and time, reﬂecting the complexity of underground magmatic plumbing systems and the processes by which those systems are recharged and emptied [3,4]. Therefore, to understand these processes better, and to understand the changes that occur during volcanic eruptions, it is important to quantify the rates at which magma rises to the surface [5]. It has been suggested that horizontal gradients in the ascent rate may be involved, since magma viscosities are expected to be higher along the conduit walls due to cooling, degassing, and crystallization [2,6–10]. Previous petrological monitoring studies of explosive eruption sequences have included a variety of techniques [11–18]. The studies have focused on tracking changes in interstitial glass composition of juvenile materials [12,14], calculation of diffusion timescales across compositional zones in crystals as

Geosciences 2016, 6, 12; doi:10.3390/geosciences6010012 www.mdpi.com/journal/geosciences Geosciences 2016, 6, 12 2 of 12 indicators of magma recharge [11], determination of differences in crystal and bubble sizes and number densities as signs of change in magma supply rate [17–19], and characterization of melt inclusion compositions to better understand magma evolution and residence [15]. Sakurajima in southern Kyushu, Japan, is one of the most active volcanoes in the world. The styles of eruption and the locations of active craters have changed over time. Previous observations have reported daily and inter-event variations in the eruptive process of small events, Vulcanian, phreatic, and Strombolian styles have all been reported on the basis of petrological analyses of the volcanic ashes [12–14,19–23]. The volcanic ash of Sakurajima has been classified by many workers using various methods as either juvenile or accidental material [9,12–14,19,21]. Previous studies have documented the magmatic processes beneath Sakurajima volcano [9,12–14,19]. In the 1974–1987 period, the textural variation data analysis determined that the observed textural differences are not caused by changes in SiO2 wt. % or magma temperature but instead relate to differences in the water exsolution process; correlation shows that the amplitude explosion earthquake becomes large when water exsolution is high [19]. Since the formation of the Showa crater, temporal changes in eruptive style and glass composition have been recorded, and observations indicate that the eruptive activity since AD 2006–2010 has not been directly affected by the addition of mafic components into the magma system, and has likely originated from silicic (andesitic) magma in a shallow subsurface reservoir [12]. From January 2011 until May 2012, the interstitial glass composition of ash fluctuated in ca. 100-day cycles, and the variations within each cycle can be explained by the crystallization of microlites in the groundmass; this indicates that the fractionation occurred while the magma was ascending within Sakurajima volcano [14]. Previous studies have discussed the magmatic processes and the variations in activity at Sakurajima [9,12–14,19,21]. On the basis of these studies, we present here analysis of the magmatic processes in Sakurajima using volcanic ash samples erupted in AD 2011–2012. The aim is to reconstruct the evolution in time of the conduit magma system, based on the compositions and physical properties of the volcanic ash. The compositions of the interstitial glass and microcrystals are used to estimate the decompression rate and ascent rates of the magma in the conduit over time, from which we propose the conditions attending magma transit within the conduit magmatic system of Sakurajima volcano during the AD 2011–2012 eruptive cycle.

2. Experimental Section

2.1. Materials We collected 32 samples (Table1) of volcanic ash during the eruptions of January 2011 to November 2012. The ash samples were recovered by using a simple container that was placed in the area where it was predicted ash would fall, according to the prevailing wind direction. The sampling locations are shown in Figure1, and the distances from Showa crater are 2.8–10 km in various directions. In addition, ash samples from June–November 2012 eruptions were obtained from the collection of the Japan Meteorological Agency in Kagoshima.

Table 1. Summary of the volcanic ash samples from January 2011 until November 2012 eruptions (*: ash samples from Japan Meteorological Agency in Kagoshima).

No. Eruption Date Collection Point (Distance and Direction from Crater) 1 9 January 2011 6.7 km; SE 2 22 January 2011 6.7 km; SE 3 31 January 2011 6.7 km; SE 4 15 March 2011 6.7 km; SE 5 9 April 2011 6.7 km; SE 6 25 April 2011 5.7 km; SE 7 29 May 2011 2.8 km; S 8 8 June 2011 5.7 km; SE Geosciences 2016, 6, 12 3 of 11

10 6 September 2011 5.7 km; SE 11 30 September 2011 5.7 km; SE 12 25 October 2011 5.7 km; SE 13 14 November 2011 5.7 km; SE 14 6 January 2012 2.8 km; S 15 23 January 2012 5.7 km; SE 16 10 February 2012 5.7 km; SE 17 27 February 2012 5.7 km; SE 18 6 March 2012 5.7 km; SE 19 19 March 2012 5.7 km; SE 20 1 April 2012 2.8 km; S Geosciences 2016, 6, 12 3 of 12 21 12 April 2012 2.8 km; S 22 3 May 2012 10 km; SSE 23 18 May 2012 Table 1. Cont. 10 km; SSE 24 12 June 2012 11.7 km; SW * 25No. 19 June Eruption 2012 Date Collection Point 11.7 (Distance km; SW and * Direction from Crater) 26 4 July 2012 11.7 km; SW * 9 29 July 2011 5.7 km; SE 27 4 August 2012 11.7 km; SW * 10 6 September 2011 5.7 km; SE 28 29 August 2012 11.7 km; SW * 11 30 September 2011 5.7 km; SE 2912 16 September 25 October 2012 2011 11.7 km; 5.7 SW km; * SE 3013 26 September 14 November 2012 2011 11.7 km; 5.7 SW km; * SE 3114 15 October 6 January 2012 2012 11.7 km; 2.8 SW km; * S 3215 16 November 23 January 2012 2012 11.7 km; 5.7 SW km; * SE 16 10 February 2012 5.7 km; SE 2.2. Methods17 27 February 2012 5.7 km; SE 18 6 March 2012 5.7 km; SE The 19ash samples are 19 March clean, 2012 and were sieved to 0.125–1 mm 5.7 km;which SE is used for petrological analysis. 20The ash components 1 April 2012(i.e., the different clast types) were 2.8 quantitatively km; S analyzed using a 21 12 April 2012 2.8 km; S stereoscopic22 microscope, 3and May we 2012 counted at least 300 grains per sample. 10 km; SSE Both juvenile and accidental materials23 were then picked 18 May from 2012 the volcanic ash, set in resin, 10 km;polished, SSE and carbon coated in preparation24 for scanning electron 12 June 2012 microscope (SEM) and Energy Dispersive 11.7 km; SWSpectrometry * (EDS) analyses. The 25SEM–EDS analyses 19 June were 2012 made using a JEOL JSM-6510LV 11.7 km; Scanning SW * Electron Microscope 26 4 July 2012 11.7 km; SW * (SEM) and X-max 50 mm2 Oxford electron microprobe at Ehime University, Matsuyama, Japan. The 27 4 August 2012 11.7 km; SW * quantitative28 crystal textural 29 August analyses 2012 were conducted on 11.7 backscattered km; SW * SEM images; the magnification29 varied between 16 September 500–1000 2012 times depending upon the 11.7 microlite km; SW *size and number density. Plagioclase30 microlites 26 in September the groundmass 2012 were outlined manually 11.7 km; and SW the * crystal measurements calculated31 using image 15 J October(version 2012 1.48); at least 100 crystals were 11.7 measured km; SW * for each sample over an 32 16 November 2012 11.7 km; SW * area generally ranging from 2000 to 25,000 µm2.

Figure 1. (A) Index map Japan; (B) location sampling map (solid rectangle). Figure 1. (A) Index map Japan; (B) location sampling map (solid rectangle).

2.2. Methods

The ash samples are clean, and were sieved to 0.125–1 mm which is used for petrological analysis. The ash components (i.e., the different clast types) were quantitatively analyzed using a stereoscopic microscope, and we counted at least 300 grains per sample. Both juvenile and accidental materials were then picked from the volcanic ash, set in resin, polished, and carbon coated in preparation for scanning electron microscope (SEM) and Energy Dispersive Spectrometry (EDS) analyses. The SEM–EDS analyses were made using a JEOL JSM-6510LV Scanning Electron Microscope (SEM) and X-max 50 mm2 Oxford electron microprobe at Ehime University, Matsuyama, Japan. The quantitative crystal textural analyses were conducted on backscattered SEM images; the magniﬁcation varied between 500–1000 times depending upon the microlite size and number density. Plagioclase Geosciences 2016, 6, 12 4 of 11

The threshold size for discriminating microlite from phenocryst is here taken as 100 µm. The 3D of mean crystal area, the crystal volume fraction, and the crystal dimensions (short and long axes) of microlitic plagioclase were measured using a CSD software package that employed a best-fit ellipsoid Geosciences 2016, 6, 12 4 of 12 method [24]. The Microlite Number Density (MND) was calculated by dividing the number of whole crystals (plagioclase, pyroxene and Fe-Ti oxide) in the analyzed area by the area of the groundmassmicrolites in (mm the groundmass−2). were outlined manually and the crystal measurements calculated using imageThe J (versionmajor element 1.48); at compositions least 100 crystals of the were glasses measured and microcrystals for each sample were over determined an area generallyusing an accelerationranging from voltage 2000 to of 25,000 15 kV,µ ma nominal2. sample current of 0.8 nA, and a working distance of 10 mm. The spotThe size threshold of glasses size and for discriminatingmicrocrystals (plagioclase microlite from and pyroxene) phenocryst were is here determined taken as using 100 µm. 10 and The 43D µm, of respectively. mean crystal The area, glass the compositions crystal volume were fraction, normalized and theto 100% crystal and dimensions calculated using (short the and INCA long quantitativeaxes) of microlitic analysis plagioclase program. were A range measured of glass using standards a CSD software(K2O glass, package obsidian, that employedand Na glass) a best-ﬁt and silicateellipsoid standards method (enstatite, [24]. The clinopyr Microliteoxene, Number and K-feldspar) Density (MND) were wasused calculatedfor calibration by dividingof the spectrometers. the number of whole crystals (plagioclase, pyroxene and Fe-Ti oxide) in the analyzed area by the area of the 3.groundmass Results (mm´2). The major element compositions of the glasses and microcrystals were determined using an 3.1.acceleration Ash Component voltage of 15 kV, a nominal sample current of 0.8 nA, and a working distance of 10 mm. TheBased spot size up ofon glassesour analysis and microcrystals of the components (plagioclase and BSE and images pyroxene) observations were determined of the 1–0.25 using mm 10 sizedand 4 volcanicµm, respectively. ash, the volcanic The glass ash compositionsis made up of were eight normalized characteristic to100% materials: and calculatedBlack Vesicular using Volcanicthe INCA Rock quantitative (BVVR), analysis Black Volcanic program. Rock A range (BVR), of glassLight standards Brown Volcanic (K2O glass, Rock obsidian, (LBVR), and White Na Vesicularglass) and Volcanic silicate standardsRock (WVVR), (enstatite, Silicified clinopyroxene, Rock (SR), andOxidized K-feldspar) Rock (OR), were Altered used for Volcanic calibration Rock of (AVR),the spectrometers. and Isolated Phenocrysts (IP) (Figure 2). The juvenile materials are made up of BVVR, BVR, LBVR, WVVR, and IP, whereas the accidental materials consist of SR, OR, and AVR. Juvenile material 3. Results is distinguished by a lack of evidence of secondary alteration, such as a hydrothermal or weathering process.3.1. Ash ComponentFor the present study, only BVR and BVVR were analyzed, because these are the dominant components (22.3%–66.7% and 12.2%–30.9% for BVR and BVVR, respectively) (Figure 3). The contentsBased of upthe on other our analysis components of the componentsare IP (1.3%–38.6%), and BSE images SR (4.1%–16.4%), observations AVR of the (0%–16.8%), 1–0.25 mm sized OR (1.1%–4.9%),volcanic ash, WVVR the volcanic (0%–29.9%), ash is madeand LBVR up of (0%–5.7% eight characteristic); the lattermaterials: two components Black Vesicular were not Volcanic present inRock all the (BVVR), ash samples. Black Volcanic Rock (BVR), Light Brown Volcanic Rock (LBVR), White Vesicular Volcanic RockThe (WVVR), BVR under Siliciﬁed stereoscopic Rock (SR), microscope Oxidized Rock examination (OR), Altered is black Volcanic in color Rock with (AVR), plagioclase and Isolated and pyroxenePhenocrysts phenocrysts, (IP) (Figure 2angular). The juvenile shapes, materials and inters areertal made textures up of BVVR, (Figure BVR, 4A,B). LBVR, The WVVR, groundmass and IP, componentswhereas the accidentalare interstitial materials glass consist (43.1%–64.7%) of SR, OR, andand AVR.microlites Juvenile of material plagioclase is distinguished (22.6%–45.8%), by a pyroxenelack of evidence (5.5%–13.1%), of secondary and ilmenite alteration, (0.6%–3.7%). such as a hydrothermal or weathering process. For the present study,The only BVVR BVR andexhibits BVVR wereangular analyzed, shapes, because a vesicular these are structure, the dominant and components an intersertal (22.3%–66.7% texture (Figureand 12.2%–30.9% 4C,D). It consists for BVR ofand inters BVVR,titial glass, respectively) plagioclase, (Figure orthopyr3). Theoxene, contents clinopyroxene, of the other and components ilmenite. Theare IPmodal (1.3%–38.6%), composition SR (4.1%–16.4%),(Table S1) is AVRplagioclase (0%–16.8%), (9.6%–40.9%), OR (1.1%–4.9%), pyroxene WVVR (4.1%–20.6%), (0%–29.9%), ilmenite and (0.1%–5.2%),LBVR (0%–5.7%); and interstitial the lattertwo glass components (46.8%–74.7%). were not present in all the ash samples.

Figure 2. BSE image of volcanic ash: (A) BVR; (B) BVVR; (C) LBVR; (D) WVVR; (E) SR; (F) OR; (G) AVR;

and (H) IP (plagioclase). Geosciences 2016, 6, 12 5 of 11 Geosciences 2016, 6, 12 5 of 11

GeosciencesFigure2016, 62., 12BSE image of volcanic ash: (A) BVR; (B) BVVR; (C) LBVR; (D) WVVR; (E) SR; (F) OR; 5 of 12 (GFigure) AVR; 2.and BSE (H image) IP (plagioclase). of volcanic ash: (A) BVR; (B) BVVR; (C) LBVR; (D) WVVR; (E) SR; (F) OR; (G) AVR; and (H) IP (plagioclase).

Figure 3. The result of componentry analysis of the volcanic ash from Showa crater during the January Figure 3. The result of componentry analysis of the volcanic ash from Showa crater during the January Figure2011–November 3. The result 2012 of componentry eruptions. analysis of the volcanic ash from Showa crater during the January 2011–November 2012 eruptions. 2011–November 2012 eruptions.

Figure 4. The groundmass BSE image and manually traced gray scale images of microlite textures of BVR (A,B) and BVVR (C,D). Gray scale image (B,D), black = vesicles, dark gray = plagioclase, gray = pyroxene, light gray = glass interstitial, and white = Fe-Ti oxide. Geosciences 20162016,, 66,, 1212 66 ofof 1211

Figure 4. The groundmass BSE image and manually traced gray scale images of microlite textures of TheBVR BVR(A,B) and under BVVR stereoscopic (C,D). Gray microscope scale image (B,D), examination black = vesi iscles, black dark in gray color = withplagioclase, plagioclase gray = and pyroxenepyroxene, phenocrysts, light gray = angular glass inters shapes,titial, and and white intersertal = Fe-Ti oxide. textures (Figure4A,B). The groundmass components are interstitial glass (43.1%–64.7%) and microlites of plagioclase (22.6%–45.8%), pyroxene (5.5%–13.1%),3.2. Geochemical and Differences ilmenite among (0.6%–3.7%). Ash Components The BVVRcompositions exhibits of angular interstitial shapes, glass a vesicular of the BVR structure, and BVVR and anare intersertal markedly texture different (Figure from4C,D). each It consists of interstitial glass, plagioclase, orthopyroxene, clinopyroxene, and ilmenite. The modal other. The SiO2 content of the BVVR glass is lower than that of the BVR glass. The SiO2 composition compositionof the glass shows (Table temporal S1) is plagioclase fluctuations, (9.6%–40.9%), but, overall, pyroxene this increases (4.1%–20.6%), with time ilmenite (Figure (0.1%–5.2%), 5). and interstitial glass (46.8%–74.7%). In 2011 (January–December), the SiO2 contents of the BVR ranged from 70 wt. % to 77 wt. % 3.2.(generally Geochemical higher Differences than the BVVR among values Ash Components of 68 wt. %–74 wt. %). The average SiO2 contents of the BVVR and BVR tended to decrease at first, but then they continually increased over time (Figure 5). In (January–December)The compositions 2012, of interstitialthe BVR had glass SiO of2 contents the BVR ranging and BVVR from are 70 wt. markedly %, to 78 different wt. %, consistently from each other.higher The than SiO in2 thecontent BVVR of (69 the wt. BVVR %–74 glass wt. is %) lower for any than single that of sample. the BVR The glass. average The SiO of 2SiOcomposition2 contents of the glassBVVR shows tended temporal to increase ﬂuctuations, over time. but, overall, this increases with time (Figure5).

Figure 5. (A) the SiO2 content of interstitial glass and (B) An numbers of BVVR (solid rectangle) and Figure 5. (A) the SiO2 content of interstitial glass and (B) An numbers of BVVR (solid rectangle) and BVR (solid circle). In interstitial glass composition, for black solids, both of them are average value. BVR (solid circle). In interstitial glass composition, for black solids, both of them are average value.

The cores of plagioclase microcrystals from two types of juvenile material exhibit a wide range In 2011 (January–December), the SiO contents of the BVR ranged from 70 wt. % to 77 wt. % of An values (100 × Ca/(Ca + Na + K)) (Figure2 5). In 2011, the An numbers for the BVVR and BVR (generally higher than the BVVR values of 68 wt. %–74 wt. %). The average SiO contents of the were 40–84 and 35–59, respectively, and, in 2012, the An numbers were 32–90 and 34–80,2 respectively. BVVR and BVR tended to decrease at ﬁrst, but then they continually increased over time (Figure5). In Despite the wide ranges especially in 2012, the An numbers for the BVVR and BVR in AD 2011–2012 (January–December) 2012, the BVR had SiO contents ranging from 70 wt. %, to 78 wt. %, consistently are similar to each other, and no temporal trends2 were detected. higher than in the BVVR (69 wt. %–74 wt. %) for any single sample. The average of SiO2 contents of the3.3. BVVRPlagioclase tended Microlite to increase Textural over Data time. The cores of plagioclase microcrystals from two types of juvenile material exhibit a wide range of AnThe values BVVR (100 microlitesˆ Ca/(Ca are + slightly Na + K)) acicular (Figure in5). shape In 2011, (short/long the An numbers axis ratio for of the0.34), BVVR and andthe range BVR wereof MND 40–84 is 20,685–48,510 and 35–59, respectively, mm−2 with and,average in 2012, MND the of An 42,459 numbers mm−2 were. The BVR 32–90 microlites and 34–80, are respectively. acicular in Despiteshape (short/long the wide ranges axis ratio especially of 0.27), in and 2012, the the range An numbers and average for the of BVVRMND is and 28,298–84,417 BVR in AD 2011–2012mm−2 and are44,391 similar mm to−2, eachrespectively. other, and Figure no temporal 6 shows trends the MND were detected.of BVR is higher than BVVR. The MND has correlated with the SiO2 content of interstitial glass compositions. BVR has high proportions of MND 3.3.and PlagioclaseSiO2 content Microlite of interstitial Textural glass Data compositions and BVVR vice versa. The BVVR microlites are slightly acicular in shape (short/long axis ratio of 0.34), and the range of MND is 20,685–48,510 mm´2 with average MND of 42,459 mm´2. The BVR microlites are acicular

Geosciences 2016, 6, 12 7 of 12 in shape (short/long axis ratio of 0.27), and the range and average of MND is 28,298–84,417 mm´2 and 44,391 mm´2, respectively. Figure6 shows the MND of BVR is higher than BVVR. The MND has correlated with the SiO2 content of interstitial glass compositions. BVR has high proportions of MND Geosciencesand SiO2 2016content, 6, 12 of interstitial glass compositions and BVVR vice versa. 7 of 11

Figure 6. Microlite number density of of BVVR (solid rectan rectangle)gle) and BVR (solid circle) over the time in 2011–2012 eruptions.

4. Discussion 4. Discussion

4.1. Melt Melt Composition Composition We interpret this compositional variation in interstitial glass between BVR and BVVR as being thethe result of degree of crystallization (Figure 77AA),), wherewhere variablevariable decompressiondecompression crystallizationcrystallization hashas occurred to enrich the glass in SiO2 [2,4,17,18,25]. To test this, we use melt composition as a proxy for occurred to enrich the glass in SiO2 [2,4,17,18,25]. To test this, we use melt composition as a proxy for whole rock composition prior to the onset of crystall crystallizationization to be calculated calculated to to evaluate evaluate whether whether they they derive from the same source magma or not. Mass-balance calculations employing microlite phases (plagioclase, pyroxene pyroxene and Fe-Ti oxide) and interstitial glass compositions were used to estimate the initialinitial melt melt compositions. compositions. The The interstitial interstitial glass glass compositions compositions on on each each rock rock were were assumed assumed as as the the last melt composition. The The modal modal compositions compositions were were de derivedrived from from conversion of volume microlites and interstitialinterstitial glassglass usingusing densities densities of of minerals minerals and and interstitial interstitial glass. glass. The densitiesThe densities are assumed are assumed as follows: as follows:plagioclase, plagioclase, 2.6; pyroxene, 2.6; pyroxene, 3.3; Fe-Ti 3.3; oxide, Fe-Ti 4.75; oxide, and 4.75; interstitial and interstitial glass, 2.35 glass, [12,15 2.35]. [12,15]. The estimated melt compositions of both types generally (Figure7B, Table S2 and Figure S1) show similar compositions for each of the eruptions, to within 1 wt. %–2 wt. % differences for all major elements, and the sum of the squared residuals (R2) shows high reproducibility. The differences between the BVVR and BVR melt are less than 10% differences for several elements. On plots of all major elements (Al2O3, CaO, NaO, K2O, FeO, MgO, and TiO2) against SiO2, the compositional trends by microlite crystallization correspond to the melt composition (Figure S1). However, in some of the eruptions, we ﬁnd that the results vary by more than 4 wt. % for all major elements and the sum of the squared residuals is more than 10%, which differs for several elements, due to errors from the calculation of modal proportion. Therefore, the result of estimation of melt concluded that the compositional variation of BVR and BVVR reﬂects as similar or the same source, and that the composition of interstitial glass relates to degree of microlite crystallization. Furthermore, from this result, we assume that microlite nucleation occurs as the equivalence between the cooling-induced crystallization and the single event during decompression-induced crystallization [18,19,25].

Geosciences 2016, 6, 12 7 of 11

Figure 6. Microlite number density of BVVR (solid rectangle) and BVR (solid circle) over the time in 2011–2012 eruptions.

4. Discussion

4.1. Melt Composition We interpret this compositional variation in interstitial glass between BVR and BVVR as being the result of degree of crystallization (Figure 7A), where variable decompression crystallization has occurred to enrich the glass in SiO2 [2,4,17,18,25]. To test this, we use melt composition as a proxy for whole rock composition prior to the onset of crystallization to be calculated to evaluate whether they derive from the same source magma or not. Mass-balance calculations employing microlite phases (plagioclase, pyroxene and Fe-Ti oxide) and interstitial glass compositions were used to estimate the initial melt compositions. The interstitial glass compositions on each rock were assumed as the last melt composition. The modal compositions were derived from conversion of volume microlites and interstitialGeosciences 2016 glass, 6, 12 using densities of minerals and interstitial glass. The densities are assumed8 of 12as follows: plagioclase, 2.6; pyroxene, 3.3; Fe-Ti oxide, 4.75; and interstitial glass, 2.35 [12,15].

Figure 7. (A) the SiO2 content of interstitial glass vs. microlite crystallinity; and (B) the calculated SiO2 content of melt composition (interstitial glass + microlites). BVVR (solid rectangle) and BVR (solid circle) over the time.

4.2. Magma Ascent Rate We apply the MND water exsolution rate meter proposed by Toramaru et al. [18] to the MND of our volcanic ash samples. We use the anorthite content of the plagioclase microlites as an indicator of the water content at the nucleation. The anorthite content at the plagioclase nucleation depends on the melt composition and water content as shown experimentally (e.g., [18,25]). Equations for the decompression rate (dPw/dz; Equation (1)) and the ascent rate (Vn; Equation (2)) were used to examine the reasons behind the two different crystallization histories. These equations are summarized below; for further details of this method, see [22].

2 dPw c Nv “ 3 (1) » dt b a fi ˆ ˙ — ffi – fl where c is a function of water content, b is a constant (40 for plagioclase), a is the calculation based on melt composition (CSi) and water content (Cw), and N is the microlite number volume,

a “ 3 ˆ 1015˘1`0.345∆CSi´0.65Cw (2)

Water content of 2.6 wt. % as a hypothetical value in microlite nucleation was used for both the BVR and BVVR types [26]; however, microlite number volumes and SiO2 content of melt composition data are the most sensitive factors affecting the decompression rate. From this, the ascent rate can be calculated as follows: Geosciences 2016, 6, 12 9 of 12

1 dP Vn “ (3) ρg dz ˇ ˇz“zn ˇ ˇ ˇ ˇ 3 where ρ = density (melt density is assumed to be 2500ˇ ˇ kg/m ), g = gravity, dPw/dz = decompression rate at a given water-content based on depth (z). The results of estimating decompression rate and magma ascent rates (Figure8, Table2 and Table S3) show that the condition of conduit magma over the time is variable. In 2011, the decompression rate and magma ascent rate of BVR generally was higher than for BVVR. However, in 2012, the decompression rate and magma ascent rate of BVR and BVVR were relatively the same. Geosciences 2016, 6, 12 9 of 11

Figure 8. The result of estimation of (A) decompression rate and (B) magma ascent rate. Solid Figure 8. The result of estimation of (A) decompression rate and (B) magma ascent rate. Solid rectangle rectangle for BVVR and solid circle for BVR. for BVVR and solid circle for BVR.

Our results are consistent with those of a previous study in which the variations in SiO2 content Table 2. The result of estimation of decompression rate (Pa/s) and magma ascent rate (m/s) based of groundmass composition were explained by processes of microcrystal crystallization and on Black Vesicular Volcanic Rock (BVVR) and Black Volcanic Rock (BVR) juvenile materials type in decompression crystallization within the conduit [18]. Consistent with the experiment results that Sakurajima at AD 2011–2012. highlight contrasting ascent vs. cooling [27], cooling is the primary cause of the differences in phase abundanceYear (oxide microlites Type crystallinity Decompression correlates with Rate SiO (Pa/s)2 content). Magma In Ascentcontrast Rate to (m/s)the previous study, our results are inconsistent with the notion in which the increased crystallinity and the max 18879 0.77 BVVR elevated SiO2 contents make themin magma more viscous, 1100 and this, coupled with 0.04 the loss of volatiles, 2011 reduces buoyancy and slows themax ascent of the magma 16689 [2]. 0.68 BVR Futhermore, we note thatmin the average magma 468 ascent rates at AD 2011–2012 0.02 eruptions in Sakurajima volcano are similarmax to those at other Vulcanian 17227 volcanoes (Table 3). 0.70 BVVR min 2257 0.09 2012 max 40019 1.63 Table 2. The resultBVR of estimation of decompression rate (Pa/s) and magma ascent rate (m/s) based on min 954 0.04 Black Vesicular Volcanic Rock (BVVR) and Black Volcanic Rock (BVR) juvenile materials type in Sakurajima at AD 2011–2012.

Year Type Decompression Rate (Pa/s) Magma Ascent Rate (m/s) max 18879 0.77 BVVR min 1100 0.04 2011 max 16689 0.68 BVR min 468 0.02 max 17227 0.70 BVVR min 2257 0.09 2012 max 40019 1.63 BVR min 954 0.04

Geosciences 2016, 6, 12 10 of 12

Our results are consistent with those of a previous study in which the variations in SiO2 content of groundmass composition were explained by processes of microcrystal crystallization and decompression crystallization within the conduit [18]. Consistent with the experiment results that highlight contrasting ascent vs. cooling [27], cooling is the primary cause of the differences in phase abundance (oxide microlites crystallinity correlates with SiO2 content). In contrast to the previous study, our results are inconsistent with the notion in which the increased crystallinity and the elevated SiO2 contents make the magma more viscous, and this, coupled with the loss of volatiles, reduces buoyancy and slows the ascent of the magma [2]. Futhermore, we note that the average magma ascent rates at AD 2011–2012 eruptions in Sakurajima volcano are similar to those at other Vulcanian volcanoes (Table3).

Table 3. A comparison of magma ascent rates for volcanoes which display similar Vulcanian activity.

– Ascent Rate (m/s) Method and Reference Sakurajima (explosive) 1.1 ˆ 10´1–3.5 ˆ 10´1 MND water exsolution rate meter [19] 1 ˆ 10´4 dense magma; Colima (effusive/explosive) MND water exsolution rate meter [2] 6.8 ˆ 10´2 vesicular magma 2.7 ˆ 10´1 vesicular magma; Sakurajima (explosive 2011–2012) MND water exsolution rate meter [18]; this study 3.7 ˆ 10´1 dense magma

5. Conclusions

We summarize our results as follows; (1) SiO2 contents of interstitial glass of the BVR are generally higher than for interstitial glass from the BVVR, (2) the crystallinity of BVR is higher than the BVVR, (3) the MND of BVR is higher than the BVVR, (4) an estimation of melt composition shows that the BVR and BVVR parents magma derives from the same source, and differences in SiO2 content of interstitial glass reﬂect the degree of microlite crystallization, and (5) the estimates of decompression rate and magma ascent rate show variation over time. In 2011, the decompression rate and magma ascent rate of BVR were higher than for BVVR, but, in 2012, the decompression rate and magma ascent rate between BVR and BVVR were relatively the same. We conclude that the different types of juvenile material resulted from cooling-induced crystallization and decompression-induced crystallization. The conditions of magma transit in the conduit from 2011 to 2012 differ in some respect; namely, in 2011, the decompression rate and magma ascent rate of BVR were higher than for BVVR, but in 2012 the decompression rate and magma ascent rate between BVR and BVVR were similar. We suggest, therefore, that monitoring of the petrological features of known/dated eruptive materials will provide useful information in evaluating ongoing eruptive activity.

Supplementary Materials: The following are available online at http://www.mdpi.com/2076-3263/6/1/12, Figure S1: Representative plots of oxide against for SiO2 for the melt composition and interstitial glass composition of the BVVR and BVR (BVVR = rectangle and BVR = circle with solid for melt composition and open for interstitial glass), Table S1: Modal composition of BVVR and BVR by area counting analysis (ImageJ 1.48), Table S2: Mass balance calculation of BVVR and BVR for estimate the melt composition, Table S3: The result of estimation of decompression rate and magma ascent rate. Acknowledgments: We express our gratitude to Ryohei Uenosono and the Japan Meteorological Agency in Kagoshima for helping with the collection of samples. The constructive comments of two anonymous reviewers and from our editor considerably improved this manuscript. Author Contributions: All authors contributed to the work presented in the manuscript. Idham Andri Kurniawan as the principal researcher made substantial contributions to this work which was undertaken in association with his PhD program in Ehime University. Masayuki Sakakibara as Ph.D. supervisor and Emmy Suparka provided critical analyses and commentary during development of the manuscript. Conﬂicts of Interest: The authors declare no conﬂict of interest. Geosciences 2016, 6, 12 11 of 12

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