Revista Geológica de América Central, 57, 149-159, 2017 doi: 10.15517/rgac.v0i57.30336 ISSN: 0256-7024 PETROGRAPHIC ANALYSIS OF THE VOLCANIC BOMBS AND BLOCKS FROM POÁS VOLCANO: APRIL-JUNE 2017 ERUPTIVE PERIOD ANÁLISIS PETROGRÁFICO DE LAS BOMBAS Y BLOQUES VOLCÁNICOS DEL VOLCÁN POÁS: PERIODO ERUPTIVO DE ABRIL A JUNIO DE 2017 Pilar Madrigal* y Oscar H. Lücke Escuela Centroamericana de Geología, Universidad de Costa Rica, Costa Rica, P.O. Box 214-2060, San Pedro, Costa Rica Centro de Investigaciones Geológicas, Universidad de Costa Rica, Costa Rica *Autora para contacto: mariadelpilar.madrigal@ucr.ac.cr (Recibido: 24/06/2017; aceptado: 23/08/2017) Abstract: During the first semester of 2017, Poás Volcano, in the Central American Volcanic Arc (CAVA) initiated a period of volcanic unrest that included high energy phreatomagmatic and magmatic eruptions. The most notable erup- tions occurred on April 14th and April 22nd of 2017, which produced abundant ashes and ballistic materials in the form of blocks and bombs. Here, we present results from the petrographic analyses conducted in the collected material from the largest eruptions of April 2017. Mineral textures observed on the petrographic analyses show evidence of reactivation and fragmentation of a crystal mush in the magma chamber, triggering re-melting episodes, volatile exsolution, and an increase in the pressure of the system, all of which are expected conditions during an eruption episode. Our analyses done on juvenile and non-juvenile material suggest that processes of magma mingling and injections of new batches of material of different compositions have played an important role throughout previous eruptions and likely in the current phase of volcanic activity in Poás. Keywords: Poás, volcanic activity, magmatic processes, igneous petrography, crystallization. Resumen: Durante el primer semestre del 2017, el volcán Poás situado en el arco volcánico centroamericano (AVCA) inició un periodo de actividad volcánica que incluyó erupciones freatomagmáticas y magmáticas. Las erupciones más destacadas ocurrieron los días 14 y 22 de abril del año 2017. Estas erupciones produjeron la dispersión de ceniza y abundantes materiales balísticos en la forma de bloques y bombas. En este trabajo, presentamos los resultados de los análisis petrográficos realizados en los productos eruptivos de Poás para el mes de abril de 2017. Las texturas minerales observadas a partir del análisis petrográfico muestran la evidencia de la reactivación y fragmentación de un agregado cristalino en la cámara magmática y el disparo de episodios de fusión, exsolución de volátiles y un incremento en la presión del sistema. Todas estas condiciones son de esperar en un proceso eruptivo. Los análisis realizados en material Madrigal, P. y Lücke, O. H. (2017). Petrographic Aanalysis of the Volcanic Bombs and Blocks from Poás Volcano: April-June 2017 Eruptive Period. Revista Geológica de América Central, 57, 149-159. doi: 10.15517/rgac.v0i57.30336 150 REVISTA GEOLÓGICA DE AMÉRICA CENTRAL juvenil y no juvenil sugieren que los procesos de mezcla de magmas e inyección de volúmenes nuevos de material con distinta composición han tenido una influencia importante en procesos eruptivos anteriores y en la nueva fase de actividad volcánica en Poás. Palabras clave: Poás, actividad volcánica, procesos magmáticos, petrografía ígnea, cristalización. INTRODUCTION During the first semester of 2017 Poás vol- cano initiated a period of volcanic unrest charac- Poás volcano is a complex stratovolcano terized by vigorous eruptions (ejecting bombs and that rises 2708 m.a.s.l. within the northern sec- blocks), ash spewing episodes, and nearly con- tion of the Central Cordillera of Costa Rica, and stant degassing. This current episode is accom- constitutes one of the most active volcanoes in panied by persistent seismic activity, which en- the Central American Volcanic Arc (CAVA). It is tails mainly low-frequency tremors and volcano- characterized by gentle slopes and a wide sum- tectonic earthquakes (Mora, 2017a). Furthermore, mit caldera, where the main crater lies, along with the hyper acidic crater lake has disappeared and the Botos and Von Frantzius cones. All the identi- an asymmetric cinder cone or tuff ring has formed fied volcanic foci associated with the massif are in the southern edge of the active crater (Fig. 1). aligned with N-S volcano-tectonic fracture zones. Its proximity to densely populated areas and The last significant eruptive episodes are the popularity of Poás Volcano as one of the most those of 1910 and 1953. According to (Casertano, visited national parks in Costa Rica justifies the Borgia, & Cigolini, 1983) both of these episodes importance of monitoring the current activity and were phreatomagmatic where the 1910 eruption studying the magmatic products of the current dispersed fine juvenile material in a single episode eruption of Poás volcano. This work aims to as- whereas the 1953 eruptions lasted several months sess the petrographic characteristics of the blocks with strombolian type activity, the creation of an and bombs ejected during the recent eruptions of intra-crateric dome by effusion of lava and the Poás Volcano in order to infer crystallization and disappearance of the crater lake for a period of fragmentation processes involving the underlying close to a decade. Phreato-magmatic eruptions, magmatic system. according to (Browne & Lawless, 2001), involve the ejection of juvenile igneous material thus di- rectly involving magma as a source of ejecta. ACTIVITY RECORDED IN THE FIRST Since the early 1950’s, the activity of Poás SEMESTER OF 2017 volcano has been limited to phreatic events in which ash, mud, lapilli and blocks has been dis- During the period of January to April of 2017 persed, mainly within the borders of the active cra- Poás Volcano presented a significant increase in ter’s caldera. According to (Barberi, Bertagnini, seismological activity which at the end of March, Landi, & Principe, 1992), phreatic eruptions are resulted in persistent volcanic tremors, low fre- processes of fragmentation of pre-existing rock quency events, and volcano-tectonic earthquakes and ash due to explosions of steam. These erup- (Mora, 2017a). On April 13th, a small phreatic tions do not directly involve magma as a source eruption dispersed sediments from the crater’s lake of material. In phreatic eruptions, magma acts as around the area inside the main active caldera. the source of energy by heating and flashing wa- From April 12th to 14th the eruptive sequence ter from a shallow aquifer (Browne & Lawless, initiated with an eruption that eroded the cen- 2001; Rouwet et al., 2014). tral section of the inner dome and fractured Revista Geológica de América Central, 57, 149-159, 2017 / ISSN: 0256-7024 Madrigal y Lücke: Petrographic Aanalysis of the Volcanic Bombs and Blocks... 151 Fig. 1. Caldera containing the active crater of Poás Volcano. A: Structure of the 1953 lava dome located on the southern edge of the crater as seen in February of 2017. B: Structure of the southern edge of the active crater as seen on August 22nd, 2017, note the absence of the dome structure. Image from the National Seismological Network’s (RSN) camera, courtesy of Mauricio Mora. the structure through its SE wall. According to pyroclastic cone or tuff ring formed. This new (Mora, 2017a) the eruption was accompanied by feature rose 15 m above the bottom of the cra- a 40 minute long tremor at frequencies near 2.0 ter’s edge with an aperture of 80 m approximately Hz. On April 13th two intense eruptions occurred (OVSICORI, 2017). The external wall appeared within the hyperacid lake which ejected blocks to be formed by reddish scoria fragments. and sediments that reached a minimum radius Toward the end of the first semester of 2017, of 2 km surrounding the active crater. The frag- volcanic unrest continued in the form of intense ments erupted belonged mainly to non-juvenile degassing with occasional dispersion of ash material from the affected dome. and incandescent ballistic fragments. The last The eruptive activity continued until April significant eruption during this period occurred 14th when two more episodes occurred. During on June 11th, with a concentrated ash plume these events, a gas and ash column formed, reach- dispersed toward the SSE. ing heights of up to 3 km and new ejecta covered the main crater, including blocks (10 cm up to 1 m in diameter) and sediments (Mora, 2017b; METHODS OVSICORI, 2017). However, this material was still dominated by non-juvenile components. The collection of block and bomb samples Fifteen more small-scale eruptions occurred dur- took place at the southern edge of the active cra- ing the twenty-four hour interval following the ter’s caldera, around the tourist lookout of the main event. These small-scale eruptions showed national park where the ballistic impacts left cra- phreatic to phreatomagmatic characteristics. ters of several decimeters in diameter (Fig. 2A). After the April 12th – 14th activity the dome Ash samples were collected on the vicinity of the disappeared due to the continuous eruptions and lookout from the solar panels of the RSN seismo- only a remnant structure of the southeastern edge logical station VPS6 (Fig. 2B) and from the roof of the former dome remains visible. In addition, of the visitors’ center. the main crater’s lake started to dry significantly Thin sections prepared for the bomb and allowing the fumarole degassing to occur directly block samples were observed on a polarizing mi- to the atmosphere. Some of these vents displayed croscope (Nikon Eclipse LV100N POL) and pho- Sulphur-rich gas and water vapor fumaroles that tographed for image stacking using the software presented a yellow hue. Combine ZP (www.hadleyweb.pwp.blueyonder. On April 22nd, vigorous activity resumed, co.uk). Ash samples were sifted and submitted to with ash and gas eruptions during which a new ultrasonic baths using distilled water. Preferably, Revista Geológica de América Central, 57, 149-159, 2017 / ISSN: 0256-7024 152 REVISTA GEOLÓGICA DE AMÉRICA CENTRAL Fig. 2: A: collection of block samples from the April 12th to 14th eruptions, note the crater pierced on the solid concrete by the block’s impact. B: Ash covered solar panels of the seismological station VPS6 of the RSN, photos taken on April 15th, 2017. the fractions from 2.7 ϕ to 2 ϕ (0.15 to 0.25 mm) Clinopyroxene phenocrysts (6%) appear unal- were observed under non-polarized light on a ste- tered (Figs. 3C and 3D). Olivine phenocrysts reo microscope (Nikon SMZ1270). (2%) appear opacitized and show kelyphitic rims composed of orthopyroxene (Figs. 3C and 3D). Orthopyroxene as phenocrysts are scarce (<1%). RESULTS Plagioclase phenocrysts (29%) show a poorly developed sieved texture and zoning, some are Blocks arranged as radiated laths showing incipient vari- olitic texture (Figs. 3E and 3F). Opaque minerals On April 14th, 2017 and subsequent episodes, account for 2% of the phenocrysts. The intersertal Poás volcano dispersed large blocks that left sig- matrix (60%) is composed mainly of plagioclase nificant impact craters around the upper caldera (35%) with interstitial tan colored (CMYK: 0, 14, as shown in figure 2. These blocks may have 26, 18) glass (21%), opaque minerals (3%) and been part of the lava dome formed in 1953 on clinopyroxene (<1%). the southern end of the active crater and bordered the intracrateric lake. From this date forward, the Bombs dome appeared severely eroded with only the eastern part protruding from the crater’s edge as The bombs have a hypocrystalline, porphy- a remnant. ritic, highly vesicular (50%) texture with plagio- Overall, the blocks show a hypocrystalline, clase, clinopyroxene, orthopyroxene, and olivine porphyritic texture with plagioclase, clinopy- phenocrysts in an intersertal matrix (Figs. 4A roxene, orthopyroxene, and olivine phenocrysts and 4B). Plagioclase phenocrysts (20%) show in an intersertal matrix (Figs. 3A and 3B). clearly defined sieved textures in the center of Revista Geológica de América Central, 57, 149-159, 2017 / ISSN: 0256-7024 Madrigal y Lücke: Petrographic Aanalysis of the Volcanic Bombs and Blocks... 153 Fig. 3: Thin section of a block erupted by Poás volcano on April 14th, 2017 under polarized (A, C, E) and cross-polarized (B, D, F) light. A/B: Overview of hypocrystalline porphyritic texture. C/D: Opacitized olivine (OL) phenocryst with orthopyroxene (OPX) kelyphitic rim. E/F: Radiated plagioclase laths showing incipient variolitic texture. Revista Geológica de América Central, 57, 149-159, 2017 / ISSN: 0256-7024 154 REVISTA GEOLÓGICA DE AMÉRICA CENTRAL Fig. 4: Thin section of a bomb erupted by Poás volcano on April of 2017 under polarized (A, C, E) and cross-polarized (B, D, F) light. A/B: overview showing vesicular, hypocrystalline porphyritic texture. C/D: Plagioclase phenocryst with an inner core showing poikilitic texture with clinopyroxene inclusions as well as sieve texture with glass-filled interstices. The outer rim of the phenocryst lacks these features and shows strong, discontinuous zoning. E/F: Plagioclase phenocryst showing similar textures as figure 4C and 4D and a thicker, more developed zoned rim. the crystals and a rim of strong discontinuous 4C and 4E) and larger phenocrysts show a poiki- zoning (Figs. 4C, 4D, 4E, 4F). Most of the spaces litic texture with clinopyroxene inclusions (Figs. on the sieved texture are occupied by glass (Figs. 4D and 4F). Clinopyroxene phenocrysts (6%) are Revista Geológica de América Central, 57, 149-159, 2017 / ISSN: 0256-7024 Madrigal y Lücke: Petrographic Aanalysis of the Volcanic Bombs and Blocks... 155 Fig. 5: Thin section of a bomb erupted by Poás volcano on April of 2017 under polarized (A, C, E) and cross-polarized (B, D, F) light. A/B: overview showing vesicular, hypocrystalline porphyritic texture. C/D: Plagioclase phenocryst with an inner core showing poikilitic texture with clinopyroxene inclusions as well as sieve texture with glass-filled interstices. The outer rim of the phenocryst lacks these features and shows strong, discontinuous zoning. E/F: Plagioclase phenocryst showing similar textures as figure 4C and 4D and a thicker, more developed zoned rim. arranged as cumulates in a glomeroporphyritic 5D). Orthopyroxenes (3%) appear as euhedral texture (Figs. 5A and 5B) although some appear phenocrysts (Figs. 5C and 5D) and unlike the as individual euhedral phenocrysts (Figs. 5C and orthopyroxene present in the blocks, they do not Revista Geológica de América Central, 57, 149-159, 2017 / ISSN: 0256-7024 156 REVISTA GEOLÓGICA DE AMÉRICA CENTRAL form kelyphitic rims. Olivine phenocrysts (Figs. suggest that olivine might be a xenocryst that is 5E and 5F) account for 1% of the phenocrysts. not in equilibrium with the melt that carried it to The intersertal matrix (20%) is composed of pla- the surface. gioclase (11%) with interstitial reddish brown Centimetric to metric andesitic lava blocks (CMYK: 6, 32, 63, 14) glass (7%), opaque miner- were ejected during the eruption of April 22nd als (1%) and clinopyroxene (<1%). 2017, nevertheless, most of them corresponded to non-juvenile material. We infer that this sample belonged to the eroded dome which disappeared DISCUSSION after the April 2017 eruptions. Blocks Bombs The block collected on April of 2017 con- The textures and zonations in phenocrysts are stitutes a fragment of an andesitic lava that most indicative of processes within the magma cham- likely belonged to the disappeared dome (Fig. 1). ber and conduits of a volcanic system. In an open The alignment of feldspar microlites in the ma- system such as a magma chamber with an active trix indicates a flow vector during the rock’s cool- recharge, processes like degassing, crystalliza- ing history (Fig. 3A). Plagioclase is a dominant tion, magma mixing and mingling, and assimila- mineral phase in the sample; it shows a poorly tion of country rock can happen simultaneously, developed sieved texture in some of the crystals, leaving a trace in the phenocrysts’ textures and zoning, or both in some cases (Figs. 3C and 3D). morphology (Streck, 2008). Both textures are indicative of magmatic differen- Most porphyritic lavas may contain evidence tiation occurring during formation of the dome in of some degree of magma mixing. As a magma the 1953 eruptive phase. A similar differentiation reservoir gets replenished by new melts, crys- process may be taking place in the current period tals in the crystal-mush will suffer changes due of activity. to the geochemical disequilibrium that entails a The presence of glomeroporphyritic texture new batch of material ascending from the mantle composed by plagioclase aggregates (Figs. 3E wedge (in the case of a subduction zone). and 3F) suggests heterogeneous nucleation of From the samples collected after the April plagioclase in preexisting phenocrysts within 22nd eruptions, the bomb shows characteristics the magma chamber. Additionally, at the time common to juvenile ejecta. It is very vesicular of formation, the magma maintained an ascent and the matrix is intersertal with feldspar mi- velocity that surpassed that of the sedimentation crolites, mainly plagioclase (Fig. 4A and 4B). velocity rate of the aggregates (Lange, Nielsen, Most of the phenocrysts in this vesicular lava Tepley, & Kent, 2013). Since the density of the are plagioclase crystals that show cores with a glomerocrysts is greater than that of individual strong sieved texture that gradually disappears plagioclase crystals, it may play an important towards the rims. Furthermore, the plagioclase role in the fractionation process of plagioclase phenocrysts’ rims show a fine zoning pattern, within a magma chamber or intrusion (Hogan, surrounding the sieved core (Fig. 4C-F). 1993). The sieved texture at the core of the crys- Olivine doesn’t seem to be a mineral phase tals can develop during the residence time of from the magma that formed the rock, since it the mineral in a magma chamber. Changes in is not an abundant mineral in the sample, but, composition during magma recharge or even most importantly, it shows evidence of chemi- temperature changes during reservoir mixing. cal disequilibrium with the surrounding matrix Resorption in plagioclase could result in disso- (Figs. 3C and 3D). The presence of oxidation at lution surfaces and could happen due to rapid the crystal rims, and the formation of a reaction growth. Decompression can produce pervasive halo of orthopyroxene around the phenocrysts resorption as well (Pearce & Kolisnik, 1990). Revista Geológica de América Central, 57, 149-159, 2017 / ISSN: 0256-7024 Madrigal y Lücke: Petrographic Aanalysis of the Volcanic Bombs and Blocks... 157 Additionally, it is worth noting that many incorporation of preexisting xenocrysts is not an of the plagioclase phenocrysts become zoned to- uncommon phenomenon, where the ascension of wards the crystal edges, showing very thin tran- fresh magma can erode the surrounding conduit sition fringes. Nevertheless, in most cases, these walls including crystals from previous magmatic phenocrysts maintain the sieved texture of the episodes. Nevertheless, this interpretation should core. This may imply recharge processes within be confirmed by performing geochemical analy- the magma chamber, where initially the phe- ses to demonstrate if the olivine is in composi- nocrysts crystallized under relatively undisturbed tional equilibrium with the surrounding melt. conditions that allowed the core to undergo re- sorption and probably rapid growth. Afterwards, a new batch of melt was introduced into the res- CONCLUSIONS ervoir, changing the compositional equilibrium and triggering convective currents, resulting in a The material ejected from the recent erup- highly dynamic environment (Couch, Sparks, & tions of Poás Volcano contains petrographic and Carroll, 2001). Under such conditions, preexisting morphological evidence that suggests that the cur- phenocrysts tend to re-equilibrate by crystalizing rent period of unrest is related to the entrainment successive rims of different composition around of new material into the magma chamber below. their lattice, thus forming the zoned edges. The The influx of new batches of magma with a dif- convective currents are likely to be a consequence ferent composition can change the equilibrium of of new injections of material that cause magmatic a given magma chamber triggering processes of fractionation and ultimately changes the composi- magma mixing and mingling, which are related to tion of the surrounding liquid, hence, modifying convective currents. the composition of the plagioclase crystal lattices Mechanical mixing resulting from convec- creating successive changes of the anorthite con- tive currents could affect the stability of a previ- tent of the plagioclase fringes. ously stablished crystal mush near the lower sec- The presence of clino- and ortho-pyroxene tions of Poás’ magma chamber. The reactivation cumulates suggests that a process of crystalliza- and fragmentation of this crystal mush can trigger tion/solidification was already active previous to re-melting episodes, volatile exsolution, and an the disequilibrium produced by new magmatic increase in the pressure of the whole system, all of injections responsible for the current episode of which are expected conditions during an eruption unrest. These cumulates represent transported episode (Parmigiani, Huber, & Bachmann, 2014). fragments of the crystal mush that was already Crystal resorption observed in the bombs collect- forming in the magma chamber. The ascent of a ed from the recent eruptions, as well as the brittle new magma batch fragmented the cooling crys- deformation and concoidal fractures found in the tal mush carrying the pyroxene cumulates along mineral crystals from the analyzed ashes may be the flux. Since the crystal mush is already in a an evidence for reactivation of the crystal mush near-solid state, the separation of cumulates oc- due to magmatic recharge. Additionally, signs of curs by brittle breaking of the crystals, which is active magmatic recharge are not only observed evident also in the minerals observed in the ashes in the ejected products but are also evident in the from the eruption in April and May, where many increased volcanic and volcano-tectonic seismic- of the minerals show normal and concoidal mi- ity measured by the local seismic network. crofractures (Huber, Bachmann, & Dufek, 2011; Continued monitoring is necessary to de- Karlstrom, Rudolph, & Manga, 2012). termine if the recharge process develops fur- Few olivine crystals can be identified in the ther. More eruptions can be expected during thin section and they show oxidized rounded the current activity episode at Poás depending borders and alteration to iddingsite, suggest- on the extent of the magma mixing and min- ing that they may be xenocrysts instead of phe- gling, as well as in the bulk composition of nocrysts from the new batch of magma. The the new magma batches ascending from the Revista Geológica de América Central, 57, 149-159, 2017 / ISSN: 0256-7024 158 REVISTA GEOLÓGICA DE AMÉRICA CENTRAL source. We recommend to perform additional geo- Casertano, L., Borgia, A. and Cigolini, C. (1983). chemical analyses from the juvenile ash fragments El volcán Poás, Costa Rica: cronología y (primarily glasses and minerals) of the recent erup- características de la actividad. Geofísica tions to ascertain the compositions of mixing end- Internacional, 22(3), 215-233. members and to achieve a better understanding of the processes associated to the compositional dis- Couch, S., Sparks, R. and Carroll, M. (2001). equilibrium of the Poás magma chamber. Mineral disequilibrium in lavas explained by convective self-mixing in open magma chambers. Nature, 411(6841), 1037-1039. ACKNOWLEDGEMENTS Hogan, J. P. (1993). Monomineralic The authors acknowledge the contribu- Glomerocrysts: Textural Evidence for tion of L.F. Brenes, J.P. Calvo, and G.J. Soto Mineral Resorption during Crystallization in the collection of samples for this study and of Igneous Rocks. The Journal of Geology, Dr. Mauricio Mora and Dr. Javier Pacheco for 101(4), 531-540. doi:10.1086/648245 their contribution with real-time monitoring of seismological activity which allows us to safely Huber, C., Bachmann, O. and Dufek, J. (2011). perform field work near Poás’ active crater. 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