Opal-A in Glassy Pumice, Acid Alteration, and the 1817 Phreatomagmatic Eruption at Kawah Ijen (Java), Indonesia

Abstract

At Kawah Ijen (Indonesia), vigorous SO 2 and HCl degassing sustains a hyperacid lake (pH ∼0) and intensely alters the subsurface, producing widespread residual silica and advanced argillic alteration products. In 1817, a VEI 2 phreatomagmatic eruption evacuated the lake, depositing a widespread layer of muddy ash fall, and sending lahars down river drainages. We discovered multiple types of opaline silica in juvenile low-silica dacite pumice and in particles within co-erupted laharic sediments. Most spectacular are opal-replaced phenocrysts of plagioclase and pyroxene adjacent to pristine matrix glass and melt inclusions. Opal-bearing pumice has been found at numerous sites, including where post-eruption infiltration of acid water is unlikely. Through detailed analyses of an initial sampling of 1817 eruption products, we find evidence for multiple origins of opaline materials in pumice and laharic sediments. Evidently, magma encountered acid-altered materials in the subsurface and triggered phreatomagmatic eruptions. Syn-eruptive incorporation of opal-alunite clasts, layered opal, and fragment-filled vesicles of opal and glass, all suggest magma-rock interactions in concert with vesiculation, followed by cooling within minutes. Our experiments at magmatic temperature confirm that the opaline materials would show noticeable degradation in time periods longer than a few tens of minutes. Some glassy laharic sedimentary grains are more andesitic than the main pumice type and may represent older volcanic materials that were altered beneath the lake bottom and were forcefully ejected during the 1817 eruption. A post-eruptive origin remains likely for most of the opal-replaced phenocrysts in pumice. Experiments at 25 • C and 100 • C reveal that when fresh pumice is bathed in Kawah Ijen hyperacid fluid for 6 weeks, plagioclase is replaced without altering either matrix glass or melt inclusions. Moreover, lack of evidence for high-temperature annealing of the opal suggests that post-eruption alteration of pumice is more likely than pre-eruption envelopment of Lowenstern et al. Opal in Glassy Pumice euhedral opal-replaced phenocrysts in dacitic melt. At Ijen and elsewhere, the ascent of magma into hydrous acid-altered mineral assemblages (e.g., opal, kaolinite, alunite) could induce rapid dehydration of hydrous minerals and amorphous materials, generating considerable steam and contributing to magmatic-hydrothermal and phreatomagmatic explosions.

Acid‐Induced Dissolution of Andesite: Evolution of Permeability and Strength

“…However, we highlight that we did not here attempt a quantitative analysis of the spatial evolution of diktytaxitic groundmass, nor can we confirm that the textures we observe in the postimmersion samples were not preexisting (indeed, we suggest that the differing apparent behavior of large plagioclase phenocrysts and microlites in response to acid exposure warrants future investigation). Previous studies have also highlighted the development of interfacial fracture planes along the encroaching silification front, recognized both in experiments (King et al, ) and in natural volcanic samples (Lowenstern et al, ). Such cracks are posited to serve as conduits for acid migration.…”

“…The ASSLs widen over time as the reaction front propagates into the center of the crystals (e.g., Figures e and f), ultimately resulting in almost complete pseudomorphic replacement of the initial mineral by a silica‐rich phase or in residual irregular patches of plagioclase within silica‐rich frameworks that retain the external shapes of relict phenocrysts. Complete or partial pseudomorphic replacement of silicate minerals has been recognized in a broad range of volcanic systems, including in fumarolic deposits from Mount Usu, Japan (Africano & Bernard, ), ash from Sakurajima volcano, Japan (Kawano & Tomita, ), crater lake sediments from Mount Ruapehu, New Zealand (Christenson et al, ), ash from Mount Kiso Ontake, Japan (Minami et al, ) and Cotopaxi volcano, Ecuador (Gaunt et al, ), lavas from Poás, Costa Rica (Rodríguez & van Bergen, ), and in lavas and volcaniclastic deposits from Kawah Ijen, Indonesia (Lowenstern et al, , and references therein), as well as in experimental studies (see Putnis, , and references therein). In each of these cases, feldspar phenocrysts were replaced by amorphous or crystalline silica due to synemplacement or postemplacement interaction with a reactive chemical environment.…”

“…Their existence and persistence requires a regular input of water (e.g., meteoric water) at a rate that exceeds the migration of fluid from the system—for example, due to evaporation or fluid flow through the porous edifice—coupled with a relatively continuous transfer of magmatic volatiles and heat (e.g., Delmelle & Bernard, ). Crater lakes serve to trap reactive magmatic volatiles such as SO 2 , H 2 S, HCl, and HF, commonly resulting in a highly acidic composition: pH values of around 0.6 or lower are common in volcanic lakes around the globe (e.g., Armienta et al, ; Bernard et al, ; Casadevall et al, ; Christenson & Wood, ; Delmelle & Bernard, ; Lowenstern et al, ; Rouwet et al, ; Varekamp et al, ). One of the most prevalent acid agents is sulfuric acid (H 2 SO 4 ), which may be derived either from the disproportionation of volatile SO 2 via hydrolysis (e.g., de Moor et al, ; Kusakabe et al, ): or or from the hydrolysis of elemental sulfur: all of which are common processes in volcanic environments.…”

An Experimental Investigation of Interaction between Andesite and Hyperacidic Volcanic Lake Water

“…Here, we investigate a porous andesite interacting with hyperacidic, high-sulphidation brine from the Kawah Ijen crater lake (Indonesia). This study was motivated by the discovery of abundant opal-rich pumice and scoria in phreato-magmatic deposits of the Kawah Ijen 1817 eruption [11]. This eruption involved interaction with, and eventual expulsion of, a hyperacidic lake with a composition similar to that found in high-sulphidation hydrothermal systems [12].…”

Pleistocene hydrothermal activity on Brokeoff volcano and in the Maidu volcanic center, Lassen Peak area, northeast California: Evolution of magmatic-hydrothermal systems on stratovolcanoes

“…However, detailed studies of hydrothermal alteration on stratovolcanoes remain uncommon. Most studies are examinations of surficial features, such as recent ash deposits, fumaroles, and crater lakes, on active volcanoes (e.g., Hedenquist et al, 1993;Ohba and Kitade, 2005;Minami et al, 2016;Caudron et al, 2017;Lowenstern et al, 2018;Takahashi and Yahata, 2018) or drill-hole studies of geothermal systems on the margins of active stratovolcanoes (e.g., Reyes, 1990;Reyes et al, 1993;Rae et al, 2003;Moore et al, 2008). Other studies examine hydrothermal alteration associated with epithermal and porphyry mineral deposits in more deeply eroded stratovolcanoes and subvolcanic intrusions where it is difficult to link volcanic and hydrothermal activity (e.g., Gustafson and Hunt, 1975;Hedenquist et al, 1998Hedenquist et al, , 2018Longo et al, 2010).…”