major element and rare earth elements investigation of ... merapi, sumbing, sindoroand slamet...

Procedia Earth and Planetary Science 6 ( 2013 ) 202 – 211

1878-5220 © 2013 The Authors. Published by Elsevier B.V.Selection and/or peer review under responsibilty of Institut Teknologi Bandung and Kyushu University.doi: 10.1016/j.proeps.2013.01.027

International Symposium on Earth Science and Technology, CINEST 2012

Major Element and Rare Earth Elements Investigation of Merapi Volcano, Central Java, Indonesia

Tri Wulaningsih1, Hanik Humaida2,3, Agung Harijoko4, Koichiro Watanabe1

1Departement of Earth Resources Engineering, Faculty of Engineering, Kyushu University, Japan

2Department of Chemistry, Faculty of Mathematics and Natural Science, GadjahMada University, Indonesia 3Geological Agency, Centre for Volcanology and Geological Hazard Mitigation Indonesia

4Department of Geological Engineering, Faculty Engineering, GadjahMada University, Indonesia

Abstract

Major element and rare earth element (REE) investigations of 2010 Merapi eruption are newly reported. Several samples from Merapi summit were analyzed. SiO2 and K2O contents range from 55.2 to 60.3 wt% and 1.99 to 2.99 wt%, respectively. The increase in silica and potassium contents newly obtained in this study indicates the difference of magma composition compare to previous geochemical data. Chondrite normalized REE is characterized by enrichment of light rare earth element (LREE) and slightly depleted in heavy Rare Earth Element (HREE). This characteristic of the pattern commonly occurs in subduction bearing volcanic rocks with no Eu anomaly. Chondrite normalized La/Yb ratio ranges from 6.3 to 7.6 showing high potassium content. Increasing volatile components indicates degassing and subsequent saturation of the volatiles which may trigger such eruption. Positive correlation between La/Yb and SiO2 shows that garnet is rarely major fractionating phase, while negative correlation between Dy/Yb and SiO2 indicates amphibole fractionation. Therefore, amphibole fractionation with high concentration of water may lead fluid saturation which may trigger strong explosive eruptions.

1. INTRODUCTION

Merapi volcano is regarded as one of active volcanoes in Indonesia. It is located in one of the most densely populated areas of the major city of Yogyakarta, Indonesia. Van Bemmelen (1949) reported that Merapi Volcano is characterized by eruption with frequency range from 1 to 7 years activity which is followed by periods of 1 to 12 years dormancy. The eruption further characterized by low gas pressure and lava dome growth and collapse to

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203 Tri Wulaningsih et al. / Procedia Earth and Planetary Science 6 ( 2013 ) 202 – 211

Figure 1. Topographic sketch map of Merapi Area, showing major towns, Merapi volcano hazard map and debris avalanches deposits, red squareshows research location (Camus et al, 2000 with modification). Three different colors represent 2010 Merapi Hazard map (Sayudiet al, 2010).

Since 2006 eruption, the direction of pyroclastic flows of Merapi volcano has been shifted to the south flank result aloose deposit of about 13.3 x 106 m3 of volcanic material and spread out across 4.7 km2 areas(Charbonnier&Gertisser, 2008). In 2010 eruption, Merapi volcano produced ten times volumes of about 130 x 106

m3 pyroclastic materials (Aisyah, et al, 2010). It was an explosive eruption and quiet different from previouseruption. This eruption was occurred without volcanic precursor of lava dome. It followed by formation of largeeruption columns which produced pyroclastic flows down the slope of volcano with Volcanic Explosivity Indexww(VEI) 4 (CVGHM). In this study, several samples were collected from PasarBubar, about 300 m north of Merapi summit (Fig1), where the pyroclastic flow were mostly deposited. The samples were analyzed to determine geochemical content of 2010eruption. Geochemical data has been vital to understand the origin of magma and evolution of volcano particularly.Expectedly, this research will be able to interpret the behavior of volcanic complex systems.

2. GEOLOGICAL BACKGROUND

Merapi Volcano is located at 25 km north of the Yogyakarta, resulted from the south north subduction of the Indiannoceanic plate beneath Java arc system (Hamilton, 1979). This subduction creates line of active volcanoes fromSumatra, Java, Bali, and continues until Nusa Tenggara Island. This volcanoes chain is regarded as the most activeof the pacific Ring of Fire. This Mesozoic subduction has formed a trench of 6 to 7 km deep. It was formed in LatePleistocene to Early Holocene. It is located at the intersection between two major alignments of volcanoes. The firstvolcanic belt consist of Ungaran, Telomoyo, Merbabu and Merapi from north to south and the second one consist of Lawu, Merapi, Sumbing, Sindoro and Slamet volcano from east to west direction.Merapi volcano has typical characteristic with collapsing and constructing lava dome in each period indicated by theaffinity of differentiating magma basaltic into andesitic (Newhall et al, 2000). Predictably, constructing Merapistarted in Pleistocene at GunungTurgo, Plawangan and GunungBibi which produced basaltic lava (Newhall, et al,2000). Recently Merapi volcano precursor is characterized by extrusion of viscous lava forming lava domes in the summit area and continued by gravitational instability and collapse of these lava domes to form pyroclastic flows ataaregular interval of a few years (Gertisser, et al 2003).

3. ERUPTION ACTIVITY OF MERAPI VOLCANO


Previous studies of Merapi volcano show that history of Merapi volcano is complex. Detail distributions regarding Merapi history still require further investigation. According to Andreastutiet al, (2000), Merapi volcano is composed by basaltic andesite, characterized by explosive and typical of arc stratovolcano. It has several small-scale eruptions with several years eruption period. Historically recorded, the modern Merapi Volcano started since the eruption in 1786. During this time, there was a changing of eruption type of Merapi Volcano. The eruption has initial precursor by the effusive growth of viscous lava domes and lava tongues, followed by gravitational collapse of disequilibrium domes then produce pyroclastic flows, commonly called as Merapi type (Voight, et al., 2000). Mostly, since 1913 (Nandaka, et al 2006), Merapi volcano pyroclastic flow has been flown down in southwest direction. However, the 2010 Merapi eruption was shifted to south direction going to Kaliadem area. CVGHM recorded that last explosive eruption was occurred in 1872 with VEI 4. 4. SAMPLES AND ANALYTICAL METHOD

Major elements and REE analysis have been carried out in this study. Whole-rock compositions were obtained from fresh interior slabs cut from the samples and crushed to <200 mesh (~74 μm) using CMT vibrating sample mill T1-100. Major elements (SiO2, TiO2, Al2O3, FeO, MnO, MgO, CaO, Na2O, K2O, P2O5) and trace elements (S, V, Cr, Co, Ni, Cu, Zn, Pb, As, Mo, Rb, Sr, Ba, Y, Zr, Nb) were determined by X-ray fluorescence (XRF) at Dept. of Earth Resources Engineering, Kyushu University using XRF Rigaku RIX-3100. Volatile compounds determined by LOI analysis. In order to measure LOI, samples were dried at about 100oC for 1.5 hours; continue heated by 500oC for 1 hour and ignition at 900oC for 1 hour. Eight selected samples for 2010 Merapi eruption were determined by inductively coupled plasma mass spectometry (ICP-MS) in order to analyze REE (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu ) at Central Laboratory in Kyushu University using Agilent 7500c. The 100 mg samples were decomposed and dissolved by using acid dissolution technique with MARS CEM system at Kyushu University. Japan andesite standard (JA-3) is used as the standard sample. 5. GEOCHEMICAL VARIATION 5.1. Major Element Geochemistry

Volatile components which represented as LOI content within 0.1 to 2.68 wt% indicating that the rocks samples are fresh and initial chemical composition of the rocks is still preserved from weathering processes (Le Maitreet al, 2002).

Figure 2.Classification diagram of K2O vs SiO2 (Le Maitreet al, 2002) for Merapi Volcanic. Data include samples from selected pyroclastic flows

and volcanic ash during 2010 Merapi crisis.


Figure 3. Variation diagram for 2010 Merapi eruptionThe SiO2 content ranged from 55.5 to 60.3 wt%, while K2O content varied between 1.99 to 2.99 wt%.

Correlation diagram of K2O vs SiO2 are illustrated in Fig 2. The samples analyzed clearly belongs to high-K series(Le Maitreet al, 2002). All the samples have moderate alkali contents (K2O + Na2O = 4.73 7 wt%). All analyseswere recalculated to 100 per cent on a volatile free basis. Variation diagram of selected major element and trace element as a function of SiO2 for high-K series arerepresented in Figure 3. With increasing in SiO2 composition, TiO2, FeO, MnO, MgO, CaO decrease, while Na2O and K2K O increase (see Fig 3). Al2O3 contents range from 16.6 to 18.9 wt% and are few scattered with negativecorrelation with SiO2 content. P2O5 contents are fairly constant with range from 0.25 to 0.32 wt%.

5.2. Trace Element and REE Geochemistry

Variations of selected trace elements with SiO2 are shown graphically in Figure 4. Although the trace element graphics are generally scattered compare to major elements, systematic variation with SiO2 are observed. Among thetrace elements, Rb, Ba and Zr generally has positive correlation with SiO2 in appropriate with other incompatibleelements, while abundance content of ferromagnesian-bearing trace element such as V, Sr and Co have negativecorrelation with SiO2 (Fig 4). Concntrations of Cr, Ni, As, Mo, and Nb are generally very low and some cases close to or below detection limit by XRF.


Figure 4.Variation diagram of selected trace elements Ba, Rb, Zr, V, Co, Sr (ppm) vs SiO2 (wt%) determined by XRF analysis.

Figure 5.Chondrite-normalized REE pattern for series rock of Merapi 2010 volcano eruption. Normalized by Sun & McDonough (1989).

Chondrite-normalized REE of the 2010 Merapi eruption rocks are illustrated in Figure 5. Merapi rocks are characterized by fractionated LREE and flat in HREE pattern. This characteristic of the pattern commonly occurs insubduction bearing volcanic rocks with no Eu anomaly. Chondrite normalized La/Yb ratio ranges from 6.3 to 7.6.

6. DISCUSSION6.1. Geochemical Variations of Merapi High-K SeriesAccording to chemical composition, magma composition at Merapi tends to have higher SiO2 and K2K O content.Silica-alkali diagram shows that samples plotted in high-K and display an increasing trend into more acidic compareto previous eruptions (see Fig. 6) (Gertisser (2003).


Figure 6.K2KK O vs SiO2 classification diagram (After Gill, 1981, Le Maitreet al., 2002) for medium-K and High-K (Gertisser, 2003) compare to2006 and 2010 Merapi eruption. The 2006 data reported by Wulaningsih (2009).

The variation diagram of major element (Fig 3) may be in fact partly derivable to the highly phyric character of theMerapi rocks. Geochemical whole-rock compositions also indicate that shifting of the SiO2 content against to othermajor elements reflect differences in the degree of magma differentiation. This shifting has also been mentioned by Gertisser (2003) (after Gill, 1981) that early medium-K series changes toward more high-K series followed at timesby posterior alkaline phase (see Fig 6). The decrease in Al2O3, MgO, FeO and CaO with increasing SiO2 is appropriate with fractional crystallization of plagioclase and mafic phase, such as pyroxene or olivine. It indicatesthat processes of crystal liquid fractionation in generating evolved magma compositions at Merapi are involved.Absent of olivine would increase FeO/MgO ratio and the separation of a calcic plagioclase would cause Al2O3 and CaO to decrease as well (see Fig 3). Clinopyroxene probably also forms and removes calcium at faster rate (Winter,2001). The decrease of TiO2 is probably a result of fractionation of a Fe-Ti oxide.Merapi magma series are belongs to High-K magma series (Fig 2). It has more LREE enriched instead of HREE.LREE enrichment is similar to those highly incompatible elements, and can be achieved by low degrees of partialmelting of a primitive mantle. The HREE part of the curve in Figure 6 are generally flat, represents of no occurrenceof garnet, also be supported by petrographic analysis. It suggests that magma series are not related by deepfractionation processes (Winter, 2001).

6.2. Volatile abundance in Merapi productMerapi volcano is situated above Sunda arc subduction zone since late Pleistocene - Early Holocene. The volatilecomponents play major role in degassing and differentiation. LOI analysis as a measurement of total volatile isdesign to calculate the amount of moisture or impurities lost when the sample is ignited under the 900oC(Lechler&Desilets, 1987).


Figure 7.Comparison volatile component in Merapi magma between 2010 eruption and previous. Fig 7 shows that volatile component of 2010 eruption product obviously increased compare to 2006 eruption products. Magma in 2010 eruption period is geochemically interesting in terms of crystal fractionation and changing composition of liquid phases. Magma with rich of volatile components will start to degas before reaching volatile saturation, causing explosively erupted. The LOI of Merapi samples has value between 0.1 to 2.68 wt%. It is necessary to stabilize hornblende; a common phenocryst accessory actor such as Na2O content of the coexisting silicate melts (Wallace, et al, 2000). Water is the major constituent of the fluid phase, normally as bubbles which can subsequently become interconnected (Sparks et al, 1977). Some mechanism may lead super saturation of volatiles, consequently can be able to trigger such eruption. 6.3. Amphibole role on explosive eruption Davidson et al., (2007), use La/Yb and Dy/Yb versus SiO2 relationship shows that amphibole is an important mineral during differentiation of arc magma. La/Yb has relatively positive correlation with SiO2 (Fig 8), which indicating that garnet is rarely a major fractionating phases. Most of arc volcanoes have trends of decreasing of Dy/Ybvs SiO2 with any differentiation. Figure 10b shows that Dy/Ybvs SiO2 has slight negative correlation which may be consistent with amphibole fractionation.


Figure 8.Differentiation trends of 2010 Merapi eruption a.)La/Yb versus SiO2. b.) Dy/Yb versus SiO2 (Davidson, et al, 2007). Modelfractionation curves are experimental assemblages of Rapp and Watson, 1995.

Y and Yb are strongly partitioned into garnet and amphibole, and negative trend again also suggests that the mainsource was not deep and garnet bearing. Amphibole fractionation is related to intermediate crustal depths with highconcentration of water contents that characterize primitive magmas at subduction zone (Wallace, 2005). Amphibolefractionation could have implication of achieving fluid saturation and therefore triggering strong explosiveeruptions.

7. SUMMARY & CONCLUSION

The 2010 Merapi volcano eruption was quite different with previous eruptions. Geochemical variations revealed thatSiO2 content was restricted range from 52.64 to 60.3 wt%, but increasingly rise in K2O content from 1.99 to 2.99wt%. The slightly shifting of K2O content which belongs to high-K series has relationship with deeper (subcrustal) processes and derivation of primary magmas. Variation of diagram indicates that magma appropriatewith crystal fractionation. Magmatic series show enrichment in LREE and flat in HREE content. It suggests thatseries are not related by deep fractionation process and accomplished by low degrees of partial melting of aprimitive mantle. High volatile compounds and amphibole fractionation could have implication of achieving fluid saturation and consequently trigger strong explosive eruptions.

ACKNOWLEDGMENTWe would like to acknowledge to GadjahMada University for invaluable support in field work and also VolcanoTechnology Research Center (BPPTK) in Yogyakarta for support and permission to carry out samples and get data about Merapi volcano.

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