Appendix A: Analytical techniques

Electron Probe Micro-Analysis (EPMA)Laser Ablation Inductively Coupled Plasma Mass Spectrometry

(LA-ICP-MS)X-Ray Fluorescence Spectroscopy (XRF)

Stable isotope determinationsAsh leachate compositions

200

Appendix A

ELECTRON MICROPROBE ANALYSIS

All electron microprobe analyses presented in this thesis were carried out using the Jeol Superprobe at the Department of Earth Sciences at Utrecht University. The microprobe is equipped with wavelength-dispersive spectrometers (WDS) and one energy-dispersive spectrometer (EDS). All measurements were performed using WDS, whereas the EDS was used for reconnaissance only.

. Accessory minerals

Accessory minerals (zirconolite, zircon, baddeleyite, perrierite, monazite) were analyzed using an acceleration voltage of 1 kV, a specimen current of nA, and a diameter of the electron beam of 1 µm. Pure metals (for Sc, Ti, Cr, Mn, Ni, Zn, Nb, Mo, Zr, Ta andW), glass standards (for rare earth elements (REE) and Y; Drake and Weill, 1) and natural minerals (for the remaining elements) were used as standards. During calibration on glass standards the spot size was set at 20 µm to prevent decomposition of the glass. Pr, Gd and Dy were determined on the L�₁ peaks (instead of L �) to avoid interference with other REE peaks. Two positions for background determinations were selected on both sides of each peak, based on wavelength spectra of the natural mineral. Corrections for interferences on net peak signals were applied to Sr (from Zr-L�), Ba (from Ti-K�), Sm (from Ce-L�₂₁₅), Eu (with Pr-L�₂₁₅ and Nd-L�₃), Gd (with Ho-L �₁) and W (with Si-SK� and Zr-L�₂). A data collection time of seconds for both peak and backgrounds was used for Ba, Sm, Eu, Gd, Dy, Er, Yb, Hf, Ta, W and Pb, 1 seconds for Na, Sc, Mn, Fe, Sr, Nb, Mo, La, Pr, Nd and U, and 1- seconds for the remaining elements. Detection limits (based on three times the standard deviation of the background at the peak position) of HFSE, REE, Th and U range from . to . wt.% . Raw data were corrected by a ZAF correction algorithm; in the case of monazite, however, a (�) algorithm was used because the ZAF procedure proved inadequate for La and Ce in combination with a phosphate-rich matrix.

. Glass inclusions and groundmass glass

Slightly different procedures were used for the analysis of groundmass glass and glass inclusions in Chapters and other chapters, and will be described separately below.

. Analyses presented in Chapter (Galunggung degassing)The analyses were performed using an accelerating voltage of 1 kV and a beam current

of nA. Counting times ranged from 1 seconds for major elements to seconds for S and Cl. Replicate runs on glass standards showed good agreement with data reported by other authors (Table A1). Detection limits for S and Cl were ppm and ppm, respectively. A ZAF-correction procedure was used for matrix corrections. The sulfur

201

Analytical techniques

peak position was updated before each analysis, if permitted by sulfur concentration (see S-K� peak shift). The analyses were performed by Govert Koetsier.

. Analyses presented in other chapters

The analyses were performed using an accelerating voltage of 1 kV and a beam current of 1 nA, and spot sizes of 1 µm. Counting times were seconds for Mg, Al, Si, and Ca, s for Ti and Fe, 1 s for K and s for P, S, Cl and Mn, whereas the counting time for Na was 40 s in total during the Na-volatilization correction procedure (see below). Elements were calibrated on in-house jadeite (Na), corundum (Al), diopside (Si, Ca, Mg), KTiPO₅ (K, Ti, P), chalcopyrite (S), halite (Cl), manganese metal (Mn) and hematite (Fe) standards. Halite was moved under the beam during Cl calibration to prevent decomposition of the standard. Raw counts were corrected using a (�) algorithm supplied by Noran. The S-K� peak position was updated before each analytical run of samples from each volcano by a Gaussian fit on the peak shape and considered to be constant for all melt inclusions (in samples from one volcano). Calibration was monitored on VG-A99 and VG-2 basaltic glasses (Table A). Replicate analyses on the glass standards indicate that the precision of S analysis was % at 1 ppm but decreased to 1 % at ppm. Detection limit for S was ppm.

. Na-volatilization correction

A major concern during the analysis of glasses is volatilization of alkali metals, especially sodium, and water. These effects can be most severe in silica-rich hydrous glasses (Morgan and London, 1). To correct for sodium loss, each analysis started with four repeated measurements of Na of 1 seconds each, and the initial Na signal (at t = ) was calculated assuming an exponential decrease of the signal with time (Nielsen and Sigurdsson, 11). Background signals were measured after peak count rates had been determined. The

Table A1. Glass standard test runs by EPMA with 1�standard deviation

S (ppm) Cl (ppm)SRM-610 a (n=6) 403 ± 44 416 ± 45

VG-A99 b (n=15) 132 ± 32 238 ± 91

VG-2 c (n=19) 1441 ± 83 297 ± 60a NIST SRM-610; Devine et al. (1) reported 1± ppm S, whichis somewhat higher than ppm ±-% S determined by LIMS(Rocholl et al. 1); ±1 ppm Cl by EMPA was reported byPearce et al. (1). b,c see caption Table A2.

202

Appendix A

restored signal was subsequently used in (�) correrections. The effect of Na loss in water-poor basaltic glasses of Galunggung was small, adding up to ca. 1-1% relative of initial signals.

. S-K� peak shift

The peak position of S-K� radiation is dependent on the oxidation state of sulfur, with a maximum peak shift of ~×1³ Å between sulfate (S⁶+) and sulfide (S²). Failure to take a potential S-K� peak shift into account may lead to underestimation of sulfur concentrations up to % (relative). However, the peak shift can be used determine the oxidation state of sulfur species and the magma (Carroll and Rutherford, 1). The shift can be defined as:

Table A2. EPMA analyses of VG-A99 and VG2 reference materials

VG-A99 a VG2 b

cert. value mean (n=6) cert. value mean (n=10)

major elements (wt.% oxide)

SiO₂ 50.94 51.05 ±0.19 50.81 50.63 ±0.15TiO₂ 4.06 4.13 ±0.07 1.85 1.92 ±0.02Al₂O₃ 12.49 12.56 ±0.06 14.06 13.97 ±0.11FeO e 13.70 13.56 ±0.09 11.84 11.88 ±0.10MnO 0.15 0.20 ±0.01 0.22 0.22 ±0.01MgO 5.08 4.98 ±0.13 6.71 6.72 ±0.12CaO 9.30 9.32 ±0.07 11.12 11.12 ±0.05Na₂O 2.66 2.66 ±0.04 2.62 2.61 ±0.11K₂O 0.82 0.83 ±0.01 0.19 0.19 ±0.01P₂O₅ 0.38 0.39 ±0.01 0.20 0.19 ±0.01Total 99.58 99.67 99.62 99.46

volatile elements (ppm)

S 240 c 177±21 1350 d 1416±36Cl 200 c 212±31 300 d 303±56a VG-A99: basaltic glass, Makaopuhi lava lake, Hawaii, USNM 113498/1 (Jarosewich et al. 1); Sand Cl are not certified. b VG2: basaltic glass, Juan de Fuca ridge, USNM 111240/52 (Jarosewich et al.1); S and Cl are not certified. c Thordarsson et al. (1) reported two averaged measurements:1± ppm S and ± ppm Cl (Cameca trace routine) and ± ppm S and ± ppm Cl(CSIRO trace routine); Dixon et al. (11) reported 1± ppm S. d Metrich et al. (1) reported anaverage of 1± ppm S and ± ppm Cl; Thordarsson et al. (1) reported 1± ppm S and1± ppm Cl (Cameca trace routine) and 1± ppm S and 1±1 ppm Cl (CSIRO traceroutine); Dixon et al. (11) reported 1± ppm S. e All Fe as FeO.

203

Analytical techniques

��(S-K�) = [L sulfate – L sulfide] × 2d/2R

where ��(S-K�) is the peak shift relative to sulfide sulfur in Å, L = peak in mm, 2R = mm (spectrometer radius of JEOL ) and d = . Å (spacing of the PET crystal). Wavelength scans of spectrometer steps of µm (equivalent to 1.×1⁵ sin �) on a PET crystal were smoothed with a Gaussian function, after which the peak shift was determined relative to chalcopyrite and barite standards. The fraction of sulfate sulfur is assumed to linearly increase with ��(S-K�) (Carroll and Rutherford, 1; Wallace and Carmichael, 1), and is log-linearly related to the relative oxygen fugacity, so that

�NNO = [log SO₄ ² /(S² + SO₄ ²) + 0.70]/0.48

valid from NNO-. to NNO+1. (Wallace and Carmichael, 1).

. Restoration of melt-inclusion compositions

After entrapment melt-inclusion compositions may have been changed by crystallization of the host mineral on the inclusion walls, a process frequently observed and discussed in previous studies (e.g., Anderson, 1; Gurenko et al., 11), and usually referred to as post-entrapment crystallization (PEC). In the case of olivine, the calculated forsterite (Fo) content of olivine last in equilibrium with the melt inclusion will be lower than that of the olivine host. Therefore, the composition of the inclusions was corrected for the effect of post entrapment crystallization (PEC) after Sobolev and Shimizu (1), using the Fe²+/Fe³+ ratio calculated from oxidation states of the melt inclusions based on S-K� peak shifts (see above). In essence, fractional crystallization was simulated by incrementally adding small amounts of olivine in equilibrium with the melt to reach olivine-liquid equilibrium, using the equations of Ford et al. (1). This procedure simultaneously solves temperature and K of last equilibration as well. In case of the melt inclusions in Chapter 4, calculated fractions of PEC range from to % , which suggests that changes in melt-inclusion composition after entrapment have been relatively minor. There is no evidence of Fe-Mg exchange of the melt inclusions with the host olivines (Gurenko et al., 11; Danyushevsky et al., ), as FeO contents of the melt inclusions (.-1. wt.%) are comparable to whole rock concentrations (<.1 wt.%). Fe-loss would results in low or even negative PEC corrections. A few melt inclusions with very low FeO contents (~ wt.%) do not have calculated PEC fractions different from other inclusions, and we suspect that low Fe contents are characteristic for their peculiar (CaO-enriched) compositions.

A number of trace elements reported here may be present in olivine in significant (Ni, Cr, Co) or lesser (V, Cu) amounts, and PEC of the host might therefore have depleted the original melt inclusion. Using the calculated fraction of PEC and assuming equilibrium between melt and olivine we calculated how much of each element would have been partitioned in crystallizing olivine. This procedure was not applied to Cr, as this element was depleted in the inclusions after entrapment (see Chapter ).

204

Appendix A

. Minerals

Minerals were analyzed at 1 kV accelerating voltage and a 1- nA beam current. Spot sizes used were 1-1 µm. Raw counts were corrected using a (�) algorithm.

Olivine was the host for most glass inclusions presented in this thesis. Counting times were seconds for Mg, Si and Fe, and seconds for Ca, Mn and Ni. Elements were calibrated on forsterite (Mg, Si), diopside (Ca), pure metal (Mn, Ni) and hematite (Fe) standards. Calibration was monitored on San Carlos olivine Fo₉₀ standard (Jarosewich et al., 1).

205

Analytical techniques

LASER ABLATION ICP-MS

Laser ablation ICP-MS was used to determine trace-element concentrations in glass inclusions of which major and volatile elements had been determined previously by electron microprobe. A detailed description of this relatively new technique follows below.

. Laser-ablation sample-introduction system

Most ablation systems employ solid-state frequency-quadrupled Nd:YAG lasers that produce a low-UV wavelength at nm. These lasers have proved to be versatile, easy to use and capable of ablating a wide range of materials. However, problems of ablation-related elemental fractionation and poor calibration between samples of different composition (i.e., matrix effects) have reduced the capability for high degrees of accuracy for some elements and sample types (Longerich et al., 1a; Mank and Mason, 1). The absorption of laser radiation by a solid target is proportional to the wavelength of the incident waves and the structure and chemistry of the sample. Improvements in the reproducibility of ablation processes have been found by using frequency-quintupled Nd:YAG lasers at 1 nm (Jeffries et al., 1) or Excimer ArF gas lasers at 1 nm (Günther et al., 1; Eggins et al., 1b; Horn et al., ). For the analysis of glass inclusions described in Chapter , a Lambda Physik Compex 1 Excimer laser at the 1 nm wavelength was used in combination with a Microlas GMBH high-resolution beam handling system (Günther et al., 1). The sys-tem is capable of producing ho-mogenized energy density (<1 to J cm²), independent of cra-ter diameter (-1 µm). High-resolution viewing optics incor-porate a Schwarzschild objective lens that enables imaging of both UV and visible light onto the sample substrate at the same time. A modified petrographic micro-scope with both transmitted and reflected light is used to view the sample. The capability for high-resolution optical viewing was important in this study, as many of the melt inclusions were small (< µm) and difficult to find within the olivine matrix. In

Fig. A1 SEM photograph of ablation craters made by 193 nm ArF Excimer laser. Although small rims of melt around the crater pits are visisble, the relatively small amount of material deposited indicates that the ablation process was very efficient. The light areas surrounding the craters show where the carbon coating of the samples (neccessary for EPMA) was removed

by the power of the laser shots.

206

Appendix A

addition, the system allowed us to determine the subsurface volume of the melt inclu-sions, which is important to select the crater size, and to detect the possible presence of subsurface secondary inclusions or cracks. Ablation was performed in a helium atmosphere in a Teflon and Plexiglas cell capable of holding standard geological thin sections and reference material glasses. Particles were carried by the helium into a simple mixing device where argon was added before ionization in the plasma of the ICP-MS instrument. The benefits of ablation in helium as opposed to argon have been discussed in several recent studies (Eggins et al., 1a; Günther and Heinrich, 1; Mank and Mason, 1) and most significantly include a greater transport efficiency of ablated material from analysis site to plasma. Typical ablation craters are shown in Fig. A1.

. ICP-MS Instrumentation

The chemical composition of the ablated particles was determined using a Platform ICP (Micromass), which is a new design of ICP-MS instrument that incorporates a hexapole device (collision cell) for ion focussing. In solution nebulization mode the instrument gives a typical sensitivity of - million counts per second per µg ml¹ (cps/ppm) for abundant, easily ionized isotopes above m/z=1. The response for a nebulized solution is reported here, as it is a better measure of instrument performance than for particles produced by a laser ablation system. This is because in laser ablation, the signal response is proportional to the amount of material per unit time that is ablated, a parameter that is not easy to measure and therefore to compare between different systems and laboratories. However, instrument performance as measured with a solution nebulization system is not necessarily the same under ‘dry’ plasma conditions. In this study we found that continuum background count rates under independently optimized conditions typically increased from < cps to >1 cps when removing the solvent load from the plasma.

The hexapole cell was pressurized with small amounts of helium and hydrogen. Hy-drogen has been shown to react with argon and argide ions (Eiden et al., 1) and is used in this instrument as a chemical filter to selectively reduce the abundance of these unwanted polyatomic ions (Feldmann et al., 1). Hydrogen can also react with some analyte ions but this did not occur at the low flow rates used in this study. A negative bias was applied between hexapole and quadrupole to prevent some unwanted ionic species that were formed during ion-molecule reactions in the hexapole cell from being measured by the detection system.

Because optimal conditions for the high mass range (m/z>1) differ from those for the lower mass range (<m/z<1, i.e. the mass range with most chalcophile elements), we preferred to analyze the melt inclusions in two sessions for different groups of elements. Under the conditions used during the first session (Table A) the sensitivity during laser ablation was optimized for heavier masses and was typically 1, cps/ppm for abundant, easily ionized elements with m/z>1 with a µm diameter crater at a J/cm² laser pulse repetition rate of 1 Hz. During the second analytical session a sampling cone with an orifice diameter reduced from 1.1 mm to . mm was used to reduce background

207

Analytical techniques

noise and increase the signal/noise ratio at lighter masses. This method for improvement in the signal/noise ratio was first described on another instrumental set-up by Günther et al. (1) and is of similar benefit in the Platform ICP. Lower backgrounds for Ni were reached by changing the Ni sampling cone and the extraction lens to Al ones, and by lowering potential over the extraction lens from - V to - V.

. Data Collection

Laser ablation was performed at sites where the concentration of internal standard elements was known from previous electron microprobe analysis. The elements, listed in Table A, were measured using short dwell and settle times following optimum data acquisition parameters devised by Longerich et al. (1b). Time-resolved signal response was monitored as the ablation took place. Background count rates were first measured for seconds before ablation commenced and this signal was integrated in order to perform a background correction. By monitoring the response for several trace elements at once, the ablation of melt inclusions could be distinguished from the ablation of the host olivine matrix (Fig. A). Some melt inclusions contained additional inclusions of chromium-

Table A3. ICP-MS instrument operating parameters

ICP-MS Instrument Platform ICP (Micromass)Cool gas flow 13.0 l/minIntermediate gas flow 1.00 l/minCarrier gas flow 0.40 l/min Ar and 0.45 l/min He mixed after the ablation cellRF power 1100 WSampling cone Ni sampler with 1.1 mm orifice

a; Al sampler with 0.8 mm orifice

b

Skimmer cone Ni skimmer with 0.7 mm orificeExtraction lens potential -300 V

a; -100 V b

Hexapole gases 2.0 l/min He and 3.0 l/min H2a; 3.0 l/min He, 2.0 l/min H2

b

Hexapole bias -3.0 VIsotopes ⁴³Ca a,b, ⁵¹V a,b, ⁵²Cr b, ⁵⁵Mn a,b, ⁵⁹Co b, ⁶⁰Ni b

, ⁶³Cu b, ⁶⁵Cu b

, ⁶⁶Zn b,

⁸⁵Rb b, ⁸⁸Sr b

, ⁸⁹Y b, ⁹⁰Zr b

, ⁹³Nb b, ¹³⁷Ba b

, ¹³⁹La b, ¹⁴⁰Ce b

, ²⁰⁸Pb a,

²³²Thb, ²³⁸U b

Dwell time 10 msPoints per peak 1Excimer laser Compex 102 (Lambda Physik)Output energy 200 mJ at 193 nmPulse duration 15 nsEnergy density at substrate 20 J/cm²

Pulse repetition rate 5 Hz a, 10 Hz b

Crater sizes 20, 30 and 40 µma First analytical session;

b second analytical session

208

Appendix A

time (s)

sig

nal

(cp

s)si

gn

al (

cps)

laser on

olivine in

integrated

laser on

olivine in

laser off

melt inclusion outintegrated

a

b

GA21-015

GA22-001a20µ crater

20µ crater

0

5000

10000

15000

20000

80 90 100 110 120

0 20 40 µm

2000

0

4000

6000

8000

10000

50 70 90 110 130 150

43Ca52Cr60Ni63Cu88Sr

0 80 120 µm40

Fig. A2 Time-resolved plots of detector signals for various elements during laser ablation of melt inclusions. The scale bars at the top of the figures show the depth of the ablation crater. (a) Typical example of a relatively thick melt inclusion. Note the increase in Ni when the host olivine starts to be ablated. (b) Typical example of a shallow melt inclusion. The signal representing the melt inclusion is short-lived, which has a negative influence on precision (see Section .). At a depth between 1 and

micrometer, the signal represents a mixture of olivine and melt inclusion.

209

Analytical techniques

rich spinel, which were easily identified and omitted during signal integration. After approximately seconds of ablation, the crater started to penetrate the glass slide of the thin section beneath the host rock and a large increase in Ba or Sr concentration was observed. Ablation of olivine was easily observed by monitoring the Ni signal.

Trace element concentrations were corrected for variations in ablation efficiency be-tween sample and standard by the use of an internal standard, for which we used Ca. CaO concentrations in the melt inclusions were determined previously by electron microprobe. Calibration was performed against NIST SRM-612 glass with trace elements concentra-tions taken from the compilation by Pearce et al. (1). Trace-element ratios remained constant during ablation, thus laser-related elemental fractionation was not considered to be a limiting factor on accuracy. Detection limits were calculated for each individual melt inclusion as three times the standard deviation on the gas blank signal, normalized to the volume of sample ablated.

. Precision and Accuracy

Because every inclusion was analyzed under unique conditions (integration times ranged from - seconds, crater sizes from - µm, optimization for different mass ranges), and thickness of inclusions varied between 1 and µm, analytical errors in the results must be established for each single analysis. An error analysis for LA-ICP-MS was previously discussed by Norman et al. (1). The authors showed that the error in the result is the combined error of uncertainties in net count rates (ncps) of internal standard and the element in question in both sample and standard, and of the uncertainties in concentrations of internal standard in sample and standard and of the analyzed element in the standard. Because the uncertainty in the concentrations of the internal standard and the analyzed element in the standard are equal for all melt inclusions, and therefore affect accuracy rather than precision, we did not include those uncertainties in the combined error estimate. Uncertainties in both background and gross count rates are described by Poisson distributions, and precisions of net count rates are calculated using standard error propagation techniques, so that:

ncps = � cpsgross/√n + � cpsbkg/√n

These values were combined with the precision of the microprobe determination of the concentration of the internal standard in the melt inclusions based on X-ray counting statistics. The combined relative error in the analyte element concentration is then:

�(cMIis ) (ncpsMI

is ) (ncpsMIi ) (ncpsst

is ) (ncpssti )_____ + ________ + ________ + _______ + _______

cMIis ncpsMI

is ncpsMIi ncpsst

is ncpssti

Replicate analyses on NBS standards and BCR glass show that the calculated errors are comparable to external precision, and therefore this method provides a good means of establishing the precision of measured concentrations of individual melt inclusions. Ac-

210

Appendix A

curacy of the analyses was tested against NIST SRM-614 and BCR-2G glass standards and our data fell within 1% of the recommended USGS values for most elements, although Cu is some % too high (Table A). We suspect that the low Cu concentration in BCR-2G glass ( ppm, Table A) may be the cause of the deviation. As Cu is an important ele-ment in our study, we performed additional accuracy checks against BIR-1 and BHVO-1 glasses, containing 1 and 1 ppm Cu, respectively. Our results were 1 and 1 ppm, respectively, in reasonable agreement with the standards. These concentrations are close to the Cu concentrations in the melt inclusions. In contrast, the measured Zn concentrations were systematically - % higher than recommended values for reference materials. Therefore, Zn data will not be reported here. Repeated analyses on two melt inclusions show excellent agreement within analytical error for most elements. Concentrations fall within the range for some elements (HFSE, LREE) determined by ionprobe for melt inclusions from a sample from the same Galunggung eruption (Sisson and Bronto, 1). Four melt inclusion-hosting olivine crystals were analyzed using the same analytical conditions that were used for melt inclusion analysis, calibrated on NBS 1.

Table A4. LA-ICP-MS analyses of BCR-2 referencematerial (µg/g)

BCR-2 a BCR-2G b BCR-2G

(this study)V 416 414 409 ± 24Cr 18 - 19 ± 2Mn 1520 1500 1577 ± 68Co 37 35.8 46 ± 3Ni - 10.8 15 ± 2Cu 19 19.4 28 ± 1Zn 127 147 185 ± 13Rb 48 49 46 ± 1Sr 346 342 321 ± 21Y 37 35.3 29.3 ± 0.6Zr 188 194 173 ± 15Nb - 12.8 11.0 ± 0.9Ba 683 660 780 ± 38La 25 24.5 24 ± 2Ce 53 50.5 46 ± 2Pb 11 11.5 13 ± 1Th 6.2 6.1 5.8 ± 0.4U 1.69 1.73 1.9 ± 0.3a Recommended values from Wilson (1997), based on varioustechniques on powdered sample; b LA-ICP-MS analysis on BCR-2Gglass (Norman et al. 1998).

211

Analytical techniques

STABLE ISOTOPE DETERMINATIONS

All stable isotope analyses were performed on a Finnigan MAT 252 mass spectrometer at the Stable Isotope Laboratory at the Geological Survey of Canada (GSC) in Ottawa, Canada. The extraction of sulfur from whole-rock powders was done at the Gregory Hatch Stable Isotope Laboratory at the University of Ottawa, Canada.

. Sulfur isotopes

Sulfur has stable isotopes with natural abundances of ³²S: .% , ³³S:.% , ³⁴S: .1% and ³⁶S: .% , of which small variations in their relative proportions are used in sulfur-isotope studies. The most common used ratio is ³⁴S/³²S, which is more conveniently expressed as �³⁴S (‰) = 1 × [³⁴S/³²Sstandard - ³⁴S/³²Ssample] / [³⁴S/³²Sstandard], generally reported relative to troilite (FeS) from the Canon Diablo meteorite (CDT, ³⁴S/³²S= .). However, recently a more accurate Vienna-CDT scale has been calibrated. Accordingly, following the guidelines from the International Atomic Energy Agency (IAEA), �³⁴S-values in this thesis are reported as �-values relative to VCDT, with IAEA-S-1 Ag₂S standard set at -.‰ and acting as the calibration material for the primary reference standard, VCDT (Coplen and Krouse, 1).

Sulfur was extracted from the rocks using the Kiba technique (Sakai et al., 1). Forty gram of rock powder was heated with about 1 ml Kiba reagent (SnCl solution in pure phosphoric acid) to °C for minutes under nitrogen atmosphere (Fig. A). Sulfur is released as H₂S, which is precipitated as ZnS by bubbling through a Zn-acetate solution, and then converted to Ag₂S by adding AgNO₃ solution. Under these conditions, all sulfur species are reduced to H₂S. Zn-acetate rather than Cd-acetate was used to precipitate released H₂S. ZnS was converted to Ag₂S, collected on 1-µm Millipore filters, and subsequently quantitatively recovered by washing with acetone. The total sulfur content of each sample was determined gravimetrically, assuming the product Ag₂S to be stoichiometric. The yield was typically 1- mg of Ag₂S per sample. A blank sample (quartz sand) run trough the Kiba extraction yielded no measurable precipitate.

For the isotope measurement Ag₂S was converted to SF₆ gas following the laser-fluorination procedure (MILES) (Taylor and Beaudoin, 1). Samples of .-1. mg Ag₂S were loaded in nickel crucibles and stored overnight under vacuum. The samples were sequentially heated with a defocused continuous CO₂ laser beam under kPa F₂ atmosphere, upon which the samples react with F₂ to form SF₆ following the reaction

Ag₂S (solid) + F₂ (gas) = SF₆ (gas) + AgF (solid)

To prevent reaction of Ag₂S with fluorine at room temperature, the sample stage was cooled with liquid nitrogen at -1 to -1°C. SF₆ was purified by cryogenic distillation using frozen ethanol and variable temperature traps (- and -1°C, respectively). In this way, contaminant gases (mostly fluorocarbons) were removed.

Samples were analyzed relative to an in-house SF₆ standard with a δ³⁴S value of

212

Appendix A

-.‰ VCDT (B. E. Taylor, personal communication). Sulfur isotopic compositions are reported here relative to VCDT. Replicate analyses of the same Ag₂S sample indicated an uncertainty of .‰. The latter uncertainty is slightly more than three times larger than found for long-term uncertainty on well-characterized internal mineral standards, owing either to small-scale isotopic heterogeneity of the Ag₂S, presence of AgCl, or to minor fractionation during rapid reaction with fluorine. Two aliquots of a sample run independently through the Kiba extraction and the laser-fluorination extraction procedure where within .‰, whereas two samples of the same lava flow sampled at 1m distance from each other differed by .1‰ (see Table .).

. Hydrogen isotopes

Hydrogen has three naturally occurring isotopes: H, deuterium (D) and tritium (T). Only H and D are stable isotopes, and occur naturally in a ratio of about 1:1. Hydrogen isotope compositions were determined on powdered whole-rock samples. About 1 g powder was degassed overnight at ca. 1°C under vacuum prior to extraction, using a uranium reduction method similar to that of Bigeleisen et al. (1). Measured D/H ratios were normalized to a VSMOW–SLAP scale ( and ‰, respectively), which accounts for H+ ₃ correction and for scale expansion, and are reported as �D = 1

N2

Zn-acetate solution traps

demi watertrap

to fumehood

condenser

1-l vessel

heater (280oC)

coolant (water)

stir

sample with Kiba reagent

gas

Fig. A3 Cartoon of Kiba extraction setup. See text for description.

213

Analytical techniques

× [(D/H) sample / (D/H) ] – 1 with D/H = 1.×1⁴ (De Wit et al., 1). The H₂O content of the samples was determined by the manometrically measured yield of H₂ released during the hydrogen extraction technique. The uncertainty of reported values of �D is ca. ‰, and the uncertainty of H₂O contents is ca. . wt.% .

. Oxygen isotopes

Oxygen isotope analysis of whole-rock samples was carried out following the procedure similar to that of Clayton and Mayeda (1), in which oxygen is extracted from rock powder by reaction with BrF₅. The uncertainty in reported values of �¹⁸O is ca. .‰. Our average value of �¹⁸O for NBS-28 quartz of +.±.‰ is within analytical precision of the recommended value of .‰.

X-RAY FLUORESCENCE SPECTROSCOPY

Whole-rock major and trace-element compositions of the samples in Chapter were determined by conventional XRF methods on fused glass discs and pressed powder pellets, respectively at Utrecht University (the Netherlands) and Canterbury University (New Zealand). Because no inter-laboratory differences were observed, the results are treated as one coherent data set. Analyses were performed by Bronto (Canterbury University), Sriwana and Van Koert (Utrecht University).

Of the samples presented in Chapter and , the samples from Guntur, Batur, Rinjani and Krakatau were analyzed at the Geochemistry Lab at the Geological Survey of Canada in Ottawa, Canada, by conventional XRF methods on fused glass discs. Samples from Werung, Boleng and Lewotolo were analyzed by J. Hoogewerff, and are presented in Hoogewerff (1). The remaining samples (Soputan) were analyzed at the Department of Earth Sciences at Utrecht University, according the following methods:

For major-element analysis, about 1 gram of powder was used to determine loss on ignition (LOI) at 11°C. Glass beads, containing . g of powdered sample and . g LiBO₂-LiB₄O₇ (:1) flux were fused at 11°C in an automated furnace. USGS standards AGV-1, BCR-1, BHVO-1, G-2 and GSP-1 were used to test accuracy and reproducibility. All iron is calculated as Fe₂O₃. Trace elements were measured on pellets made from g powdered sample that was mixed with ml elvacite solution as a binding agent. Pellets were pressed under ton/cm2 for one minute. The standards AGV-1, BCR-1, BHVO-1 and GSP-1 were used to test accuracy and reproducibility.

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Appendix A

ASH LEACHATES

The amounts of material adsorbed on ash particles (described in Chapter ) in the eruption plume were determined by a leaching technique. Tephra samples were leached for four hours in both de-ionized water and in .1 M nitric acid (following Hinkley and Smith, 1). In each case, . g of pristine sample was shaken with ml of leaching medium in a -ml polyethylene bottle for four hours. After filtering, cations and total sulfur were analyzed by ICP-AES, and chlorine and fluorine by a combined ion-selective electrode, using a standard-addition routine. Charge-balance calculations show that most water leachates have <1 % charge excess. Only the low-concentration samples of the Strombolian phase had a larger cation excess. We suspect that this was due to a pH change, which was not monitored during the leaching process.