Geochemical Evolution of the Young Crater Lake of Kelud Volcano in Indonesia

7/21/2019 Geochemical Evolution of the Young Crater Lake of Kelud Volcano in Indonesia

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1 INTRODUCTION

Kelud is one of the most active volcanoes of Javawith a record of 29 eruptions since 1311 and with

repose intervals between eruptions ranging from 1 to75 years. After the last eruption that occurred inFebruary 1990, a new lake rapidly filled the crater ofthis volcano. Persistent degassing from subaqueousfumaroles and hot springs discharging into the lakemaintain water temperatures between 31-50°C,higher than the ambient temperature of 19°C. Thelake is shallow with a maximum depth of 34 m and avolume estimated at 2.1 million m3.A tunnel was drilled through the crater wall in 1926to drain the lake and keep its volume constant. Highoverflows through the drainage tunnel (between 300and 500 kg s-1) are typically observed even duringthe dry season. This high overflow combined withthe small volume of the lake make the residencetimes of the elements in the lake particularly short,around 80-100 days. The temperature and geochem-istry of the lake has been regularly monitored since1993 in order to understand the processes leading tothe re-equilibration of the lake-hydrothermal systemafter the February 1990 magmatic eruption.

2

GEOCHEMISTRY OF THE LAKE WATERS

The composition of the lake is rather unusual for anactive volcano and corresponds to a near-neutral pH

(~6) relatively diluted water with a TDS of a fewg.kg-1 (Table1).

This neutral composition contrasts with thehighly acidic waters (pH = 0-1) most frequently ob-

served in active crater lakes where the discharge ofmagmatic volatiles (SO2, HCl, HF) directly into thelake or into the sub-surface hydrothermal system

produces acid sulfate-chloride waters with elevatedAl and Fe contents.

Table 1. Selected compositions of Kelud crater lake. Data arein mg.kg

-1. __________________________________________________

KCL3 KCL9 KCL37 KCL56Date 18 Dec 93 1 Aug 94 24 Sept 02 4 Sept 03 __________________________________________________

Temperature 42.8 42.1 33.2 30.7(°C)Depth 15 32 0 0(m) pH 5.9 6.3 6.5 6.5TDS 3.2 3.7 2.2 2(g/kg) Na 700 1020 342 271K 92 102 39 30Ca 105 130 135 147Mg 55 67 80 78Si 109 133 129 111B 11 14 4 3

F 6 7 <1 <1Cl 1300 1300 290 200SO4 631 692 670 679HCO3

- 238 207 435 472 __________________________________________________

Geochemical evolution of the young crater lake of Kelud volcano inIndonesia

A. Bernard & A.Mazot BRUEGEL (Brussels unit for Environmental, Geochemical and Life Sciences Studies), Université Libre de Bruxelles, Brussels, Belgium

ABSTRACT: After the February 1990 plinian eruption of the Kelud volcano, a new lake rapidly filled its cra-ter. This young lake offers the opportunity to study the evolution and re-equilibration of a volcanic-hydrothermal system after a magmatic eruption. Geochemical and thermal changes of the crater lake have

been recorded during the period 1993-2003 in which a decrease in temperature and ionic concentrations of the

lake was observed. The initial lake chemistry (1993-1997) was dominated by Na-K chloride waters. Today,Ca-Mg sulfate waters are the main component in the lake. The temporal evolution in the chemistry of the lakewaters suggests the presence of two distinct hydrothermal systems feeding the lake: a deep system at hightemperature (250°C) with neutral alkali-chloride fluids and a shallow aquifer at lower temperature dominated

by Ca/Mg-sulfate waters.

Water-Rock interaction (WRI-11) 2004. Wanty & Seal II eds. A.A Balkema Publishers.



The evolution of temperature and geochemistryduring the period 1993-2003 is shown on Figure 1.The general trend is toward a decrease in tempera-ture and TDS, the lake becoming more diluted withtime. The main geochemical change observed is adecrease in Na+K chlorides, sulfates and Ca/Mg re-maining almost constant during this period (Table1).

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

T e m p e r a t u r e ° C

25

30

35

40

45

50

55

Date

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

0

1

2

3

TDS (g.kg -1 )

Cl/SO4

Na/SO4

K/SO4

Figure 1. Temporal evolution of the temperature and chemistryof the lake during 1993-2003. Arrows represent the start of aheating episode. TDS is the Total Dissolved Solids.

With the exception of the 1993 data, there are positive correlations between chloride and the alkalimetals or with B and Li but no correlation existswith Ca or Mg (Figs. 2 and 3).

During the 10-year monitoring period, two nota- ble events were observed in 1996 and 2001 when asudden increase in lake temperatures and bubblingoccurred. In both events, the lake temperature

peaked at 50°C. Heating episodes, sometimes cyclic,are relatively frequent in some crater lakes and re-flect changes in the flow rate or in the enthalpy ofhot fluids entering the lake. These heating episodesalways represent an alarming situation because anincreasing lake temperature can be a precursory sig-nal for the renewal of magmatic activity as was ob-

served 3 months before the 1990 eruption (Vande-meulebrouck et al. 2000).

The temperature increases observed in 1996 and2001 were accompanied by rapid changes in lake-water composition with a net increase in Na+K chlo-rides, B and Li. These heating episodes could be theconsequence of the (re)opening of fractures at depthleading to an increase in the contribution from a

deep hydrothermal aquifer. This evolution suggeststhat two distinct hydrothermal systems are feedingthe crater lake. The first aquifer has neutral alkalichloride waters enriched in B and Li typical of thedeepest part of hydrothermal systems and the secondshallow system is dominated by Ca-Mg sulfate wa-ters.

Cl (mg/kg)

200 400 600 800 1000 1200 1400

N a + K ( m g / k g )

200

400

600

800

1000

1200

1400

Cl (mg/kg)

200 400 600 800 1000 1200 1400

B ( m g / k g )

0

2

4

6

8

10

12

14

1993

1993

Figure 2. Evolution of chloride versus Na+K and boron during1993-2003.

3 TEMPERATURE ESTIMATION OF THEDEEP HYDROTHERMAL SYSTEM

The presence of a deep alkali chloride system sug-gests the use of Na-K and K-Mg as geoindicators.Equilibrium temperatures for Na/K and K/Mg and

based on Giggenbach (1988) are presented in Figure4.The lake water compositions are far from the

equilibrium with a mineral assemblage as expectedfor waters resulting from the mixing between two




Li (µg/kg)

200 300 400 500 600 700

N a ( m g / k g )

200

300

400

500

600

Figure 3. Evolution of Li versus Na during 1993-2003.

Log Na / K (mg/kg)

0.0 0.5 1.0 1.5 2.0

L o g K 2 / M g ( m g / k g )

1

2

3

4

5

6

7 1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

3 5 0TNa-K °C

100

150

200

250

300

350

TK-Mg °C

F u l l E q u i l i b r i u m I m

m a t u r e w a t e r s

Figure 4. .K-Mg and Na-K equilibrium temperatures.

Log Na / K (mg/kg)

0.5 1.0 1.5 2.0

L o g N a / L i ( m o l e s / k g )

2.0

2.5

3.0

F u l l

e q u i l i b

r i u m

100

150

200

250

300

350

1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

3 5 0

TNa-K °C

TNa-Li °C

Figure 5. Na-Li and Na-K equilibrium temperatures.

types of fluids. However, the Na/K remained almostconstant during the entire period despite a large de-crease in their concentrations. This suggests that thecomposition of the deep neutral-chloride hydrother-mal system remained stable during this period. Thedeep hydrothermal system most probably existed be-fore the 1990 eruption and its composition was notsignificantly affected by the relatively brief (1 hour)

plinian eruption. The average log Na/K value around0.9 could reflect equilibrium temperatures close to250°C in the deep hydrothermal system. On Figure5, these Na/K equilibrium temperatures are com-

pared to the Na-Li geothermometer of Verma &Santoyo (1997) assuming a saline brine.

4

STABLE ISOTOPE COMPOSITIONS

The hydrogen (D/H) and oxygen (18O/16O) isotoperatios also show changes clearly correlated withchanges in temperature or chloride contents. Figure6 shows the strong positive correlation betweenchloride and oxygen isotopic composition, exceptfor the 1993 data. The same positive trend exists be-tween chloride and (D/H). This trend can be inter-

preted as a mixing line between two end members.The first is represented by the deep neutral-chloridefluids with a heavier isotopic composition and the

second by the shallow sulfate fluids with an isotopiccomposition identical to meteoric fluids (spring).

Cl (mg/kg)

0 200 400 600 800 1000 1200 1400

1 8 O ( ‰ )

-8

-7

-6

-5

-4

-3

-2

-1

1993

8/1996

spring

5/2000

1/2001

Figure 6. Evolution of the δ18

O (‰) isotopic composition ver-sus chloride for the lake.

5

THERMAL REGIME

Like many other active crater lakes, Kelud acts as acalorimeter trapping the heat supplied into the lake

by subaqueous fumaroles and hot springs. A heatand mass balance model was used to evaluate thechanges in the heat supplied to the lake by thehydrothermal fluids. The model is derived from Ste-venson (1992) and assumes that the lake is in steady-state equilibrium, i. e. that the lake temperature isconstrained by the balance between heat input and

output. Heat is derived from the enthalpy of hydro-thermal fluids (brine + steam) entering and mixingwithin the lake and from solar and atmospheric ra-diation (rad ). Heat is lost by evaporation (evap),conduction (cond ), radiation (rad ) and by the over-




flow (over ) of the hot waters through the drainagetunnel. The lake surface radiated a thermal power es-timated at 130 MW for the period 1993-1995 (Fig.7), peaked to 250 MW in 1996 and 2001 when thelake surface reached 50°C and is today close to 80MW.

T : 19°CT : 42°C

air

water

E : 80 MW

M :32 kg.s

evap

evap

-1

E : 7 MWrad

E :15 MWcondE : 29 MW

M : 300 kg.s

over

over

-1

Cl : 33 T.day

S : 6 T.day

-1

-1

E : 130 MWbrine + steam

Figure 7. Heat and mass balance model calculated for a laketemperature of 42°C.

6 CO2 DEGASSING

The CO2 flux emitted by the surface of the lake wasmeasured during 2001-2003 by IR spectrophotome-try using a Dräger Polytron. We modified the tech-nique initially developed for soil gas flux monitoring

by Chiodini et al. (1996) in order to work on a craterlake by using a floating accumulation chamber. Dur-ing each field campaign, about 260 measurementswere obtained to cover the entire lake surface(105,000 m2). Results show a decrease in the CO2 flux from 30,000 T/year in 2001 to 13,000 T/year in2003 following the temperature decrease. These re-sults compared to the CO2 lost as HCO3

- by theoverflow of the lake waters through the drainagetunnel show that most of the CO2 degasses from thelake surface and escapes to the atmosphere. Only asmall fraction (<15%) is trapped and converted as

HCO3-

in the lake waters.

7 CONCLUSION

In the case of Kelud, the lack of acidity and lowfluoride content suggest that the input of magmaticvolatiles to the hydrothermal system is low or lim-ited. Only the samples collected in 1993 are slightlydifferent in Cl and stable isotopic compositions. Thiscould suggest that a small input of magmatic vola-tiles was still present 3 years after the eruption.

The emission of neutral-chloride fluids at the topof a volcanic edifice is a relatively rare situation.These neutral-chloride fluids are generally dis-charged at low elevation on the flanks of the vol-

canic edifice or are only accessible by well-drilling.For Kelud, the 1990 eruption cleared the vent and

permitted the ascent of fluids from the deep neutral-chloride hydrothermal system. Today, this channelopens towards the surface, is progressively cloggedand the contribution from the deep Na/K chloridefluids to the lake is decreasing.

REFERENCES

Chiodini, G., Frondini, F. & Raco, B. 1996. Diffuse emissionof CO2 from the Fossa crater,Vulcano Island (Italy): Bull.Volcanol. 58: 41-50.

Giggenbach, W.F. 1988. Geothermal solute equilibria. Deriva-tion of Na-K-Mg-Ca geoindicators. Geochim. Cosmochim. Acta 52: 2749-2765.

Stevenson, D.S. 1992. Heat transfer in active volcanoes: mod-els of crater lake systems. PhD thesis, The Open University,235p.

Vandemeulebrouck, J., Sabroux, J.C., Halbwachs, M., Surono,

Poussielgue, N., Grangeon, J. & Tabbagh, J. 2000. Hy-droacoustic noise precursors of the 1990 eruption of Kelutvolcano, Indonesia. J. Volcanol. Geotherm. Res. 97: 443-456.

Verma, S.P. & Santoyo, E. 1997. New improved equations for Na/K, Na/Li and SiO2 geothermometers by outlier detectionand rejection. J. Volcanol. Geotherm. Res. 79: 9-23.


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Geochemical Evolution of the Young Crater Lake of Kelud Volcano in Indonesia