INTRODUCTION Papandayan is a 2665 m high stratovolcano located 180 km southeast of the capital Jakarta. During its historic activity, the eruptive centers moved to the Northeast with the formation of successive new craters. The last magmatic eruption of Papandayan volcano occurred in 1772 and created a horseshoe- shaped crater open to the Northeast, Papandayan volcano has an elliptical depression (1×1.5 km) and until November 2002, its activity was centered in two craters with multiple fumaroles (stable maximum temperatures of about 300°C), sulfur-mud pools and hot springs.
| ||
Location map of Papandayan volcano |
Image ASTER (08/22/2003) courtesy NASA (USA), http://asterweb.jpl.nasa.gov/.Image processing: A. Bernard |
|
ERUPTION OF NOVEMBER 2002 In early October 2002, the seismicity of the volcano increased and small phreatic explosions occurred in the crater . On 11 November 2002, a phreatic eruption from the Southeast of the crater caused a landslide that became a lahar. On 15 November 2002, a phreatic eruption ejected steam and an ash plume up to 4000 m above the crater. Earth Probe TOMS (Total Ozone Mapping Spectrometer) instrument detected a small amount of SO2 on November 13 (<1500 tons). On November 15, SO2 was again detected, in a small plume extending west of Papandayan. The estimated SO2 mass was 7000 tons. Between 16 and 20 November 2002, explosions ejected ash plumes up to 700 m high and were followed by landslides and lahars.
Eruption of Papandayan volcano and new craters |
||
MINERALOGY OF ERUPTIVE PRODUCTS Products of the 2002 eruption consist of grey ashes and altered blocks. These products are derived from highly altered lavas of andesitic to dacitic compositions. Some samples show complete silicification. The mineral phases were studied by X-Ray diffraction and SEM/EDS. They are composed of amorphous silica, cristobalite, quartz, pyrite, marcasite, pyrophyllite (AlSi2O5OH) and alunite (KAl3(SO4)2(OH)6). Anhydrite or gypsum and kaolinite (Al2Si2O5(OH)4) are sporadic phases. Pyrite is very widespread and is well crystallized in altered rocks. On 20 November, another major eruption took place in the northwest zone of the crater. Ash plume reached 1500 m above the vent and the cloud drifted to the northeast. The eruption also produced a directed blast traveling as far as 2 km and left blocks and smaller fragments of altered rocks and a 4-8 cm thick deposit of wet ashes. Breadcrust bombs with maximum diameter of 50 cm were found. During the next month the activity decreased and today only fumarolic manifestations are present. New vents have been created with one containing a small lake, and another a mud-pool and new hot springs appeared. SEM images:
|
||
Quartz
|
Pyrite
|
Anhydrite |
Natroalunite ((Na0.5, K0.5) Al3 (SO4)2 (OH)6)
|
Pyrophyllite
| |
Exemple of X-Ray diffraction patterns. Al: Natroalunite, Gy: Gypsum, Ka: Kaolinite, Po: Pyrophyllite, Py: Pyrite, Qz: Quartz, S: Sulfur.
|
| |
COMPOSITION OF HOT SPRINGS Based on chemical composition, two groups of hot springs can be distinguished. The first group of springs has outlet Temperatures between 40 and 94°C and pH varying from 1.6 to 3.8. Their compositions are typical of acid sulfate-chloride waters. These fluids are likely to be formed by the direct absorption of magmatic gases such as SO2 and HCl into the hydrothermal aquifer. The second group is acid sulfate waters that contain high SO4 but low amounts of chlorides. The pH of these springs ranges from 1.6 to 2.5 and temperatures range from 22 to 91°C. The composition of these fluids is typical of steam condensates that are formed by a boiling acid aquifer at depth. To understand the origin of these fluids, we studied the δ34S of dissolved sulfates. A positive correlation is observed between δ34S values and the chloride contents. δ34S tends to decrease with Cl concentration. The high δ34S values (9-15 ‰) observed in acid sulfate-chloride waters strongly suggest that dissolved sulfates are mainly formed by the disproportionation of magmatic SO2 . The disproportionation of SO2 leads to a large isotopic fractionation between dissolved sulfates and H2S or elemental sulfur. On the other hand, the low δ34S values observed in chloride-depleted waters suggest that the origin of dissolved sulfates for these waters is the surficial oxidation of hydrogen sulfide. These waters are likely to be formed by the condensation of steam rising from a boiling aquifer.
|
|
CHEMICAL EVOLUTION OF THE HOT SPRINGS SINCE NOVEMBER 2002 ERUPTION The chemistry of samples collected two months before the November 2002 eruption shows no detectable changes suggesting that no significant modification occurred in the pre-eruptive degassing. The chemistry of samples collected after the eruption reveal however notable changes. The most significant changes are an increase in SO4/Cl and Mg/Cl. This could result from the intrusion of fresh magma at shallow depth. In the case of Papandayan, the acid sulfate waters that are likely to have been generated at subsurface show the same increase in Mg/Cl than the acid chloride-sulfate waters. A more likely explanation for the changes observed in the 2003 data is the opening of new fractures where unaltered or (less altered) volcanic rocks were bring in contact with the ascending acid waters. To confirm this hypothesis, we evaluated the degree of water-rock interaction by calculating the percentage of residual acidity (PRA) introduced by Varekamp et al. (2000) for crater lake studies. The PRA is defined as the residual acidity left after the neutralization of a fluid by water-rock (WR) interaction. Since acidity is consumed by hydrolysis reaction during WR interaction, PRA reflects the relative rate of acid volatiles input versus the rate of WR interaction. PRA is independent of dilution or boiling. The pure acid or initial acidity was computed for each sample from their Cl and SO4 concentrations with PHREEQC. The y axis corresponds to the easily leached element Mg on a molar basis relative to the sum of major rock forming elements (RFE = Na + K + Mg + Ca + Al + Fe). The data show a net decrease in the residual acidity for samples collected in 2003-2005 accompanied by an increase in the relative Mg concentrations. The low δ34S of sulfates in chloride-depleted water collected before the eruption suggests that the origin of sulfates for this water is the surficial oxidation of hydrogen sulfide. This Cl-depleted water has been formed by the condensation of steam rising from the boiling hydrothermal system to the subsurface above the water table.
|
|---|---|
WATER-ROCK INTERACTION In the stability diagrams , pyrophyllite and kaolinite coexist at 250°C. Experimental studies by Hemley et al. (1980) show that kaolinite and pyrophyllite can coexist at temperature around 270°C at quartz saturation. The stability of kaolinite is pH dependent and for waters with pH < 2 and temperature of 250°C, kaolinite is not stable anymore. Pyrophyllite is a mineral that may form at high temperature in geothermal systems (Reyes, 1990; Nogami et al., 2000; Hedenquist et al., 1998), in many ore deposits districts (Marumo, 1989; Hedenquist et al., 1994; Yilmaz, 2003) and on active volcanic sites (Christenson and Wood, 1993; Nogami et al., 2000). Alunite can also coexist with kaolinite and pyrophyllite between 250-300 °C (Hedenquist et al., 1998). For the fluids studied, alunite is a stable phase between 200 and 150 °C. |
|
HYDROTHERMAL SYSTEM OF PAPANDAYAN VOLCANO The November 2002 eruption produced notable changes in the chemistry of the hot springs, especially the evolution in sulfur isotopic compositions suggests a decrease in the magmatic contribution to the chemistry of the hydrothermal system. Most of the observed chemical changes can be related to a modification in the dynamic circulation of the hydrothermal fluids to the surface by the opening of new fractures. Furthermore, no evidence of intrusion of fresh magma in the upper part of the hydrothermal system has been found. The hydrothermal zones are likely restrained to faults or permeability structure. The acid fluids ascend to the surface and lead to the formation of hot springs or descend along the structures to deep level and react with the host rock. This interpretation may explain why closely different fluids can cause different types of advanced argilic alterations. |
 |
References:
Christenson, B. and Wood, C.P., 1993. Evolution of a vent-hosted hydrothermal system beneath Ruapehu Crater Lake, New Zealand. Bulletin of Volcanology, 55: 547-565.
Hedenquist, J.W., Arribas, A.J. and Reynolds, T.J., 1998. Evolution of an intrusion-centered hydrothermal system: Far Southeast-Lepanto porphyry and epithermal Cu-Au deposits, Philippines. Economic Geology, 93(4): 373-404.
Hedenquist, J.W., Matsuhisa, Y., Izawa, E., White, N.C., Giggenbach, W.F. and Aoki, M., 1994. Geology, geochemistry, and origin of high sulfidation Cu-Au mineralization in the Nansatsu District, Japan. Economic Geology, 89(1): 1-30.
Hemley, J.J., Montoya, J.W., Marinenko, J.W. and Luce, R.W., 1980. Equilibria in the system Al2O3-SiO2-H2O and some general implications for alteration/mineralization processes. Economic Geology, 75: 210-228.
Marumo, K., 1989. Genesis of kaolin minerals and pyrophyllite in Kuroko deposits of Japan: implications for the origins of the hydrothermal fluids from mineralogical and stable isotope data. Geochemica et Cosmochimica Acta, 53: 2915-2924.
Nogami, K., Hirabayashi, J.-I., Ohba, T. and Yoshiike, Y., 2000. The 1997 phreatic eruption of Akita-Yakeyama volcano, northeast Japan: Insight into the hydrothermal processes. Earth Planets Space, 52: 229-236.
Reyes, A.G., 1990. Petrology of Philippine geothermal systems and the application of alteration mineralogy to their assessment. Journal of Volcanology and Geothermal Research, 43: 279-309.
Varekamp, J.C., Pasternack, G.B. and Rowe, G.L., 2000. Volcanic lake systematics II. Chemical constraints. Journal of Volcanology and Geothermal Research, 97: 161-179.
Yilmaz, H., 2003. Exploration at the Kuscaryiri Au (Cu) prospect and its implications for porphyry-related mineralization in western Turkey. Journal of Geochemical Exploration, 77: 133-150.
Université Libre de Bruxelles. Last modification: December 08, 2006 |
|---|