Quantitative Methods for Electron Microprobe Analysis of Sodium In

American Mineralogist, Volume 66, pages 547-552, 1981

Quantitative methodsfor electron microprobeanalysis of sodiumin natural and synthetic glasses

CHaRI-Bs H. NInI-sBN'aND HAnaLDUR SIGURDSSoN GraduateSchool of OceanograPhY University of RhodeIsland Narragansett,R. I. 02882

Abstract

Two methodshave been developedfor the microprobe analysisof Sodiumin natural and synthetic glassesthat exhibit time-dependentelement migration during electron bombardment. A cryogenicmethod is basedon the cooling of the sampleto -90oC at which temperature sodium diffusion rate from the excitation volume is reducedto z€ro. An empirical correction method determinesthe shapeof the sodium decaycurve as a function of time during electronbombardment, which under normal operating conditions,gives initial sodium concentration by extrapolation.

Introduction et al., 1978).We describehere two analytical meth- that addressthis problem. One approsgl slimi- The precise determination of sodium concentra- ods natesthe lossof sodium and potassiumduring analy- tion in hydrous and rhyolitic synthetic and natural sis by a cryogenic lsshniqu€, while the other more glassesis one of the principal problems in quan- practical approachprovides a quantitative correction titative electronmicroprobe analysis due to the time- procedure. dependentloss in intensity of the Na K line during continuous electron bombardment.Concurrent with Previouswork the alkali loss one may observea correspondingin- creasein counts and, hence,apparent concentrations Typically 0.02 pA at l5kv representsreasonable of the non-mobile elementsin the phase.This phe- working conditions for quantitative analysisof geo- nomenon has particularly presentedproblems in ex- logic materials. For many natural and synthetic perimental petrology in the analysis of hydrous glassesconsiderable structural damage is incurred glassycharges (Helz, 1976;Kushiro, 1972).A loss of even at theselevels due to localizedheating from the over 50Voof the sodium counts and someloss in po- focusedbeam after the first few secondsof exposure. tassiumwas observed,for example,during the analy- The most notable of the beam-induced composi- sis of glassy basaltic quench products with a beam tional changesis a reduction in concentrationof mo- current of as low as 0.01p.Aat l5kV and a 5 g,mspot bile alkali metal ions. The effect is most prominent size(Mysen and Kushiro,1975). The increasedappli- for sodium and to a lesserextent for potassium,cal- cation of the electronprobe microanalyzer(nrue) to cium and barium (Borom and Hanneman,1967). Re- the study of terrestrial and deep-seatephra deposits ports of alkali losscan be found extensivelythrough- has further emphasizedthe problem of sodium loss. out the literature, for example,see Varshneya et al. For example,the sodium loss during analysisof the (1966), Vassamilletand Caldwell, (1969), Goodhew Minoan tephra from Santorini volcano was mea- and Gulley, (1974),and Watkins et al. (1978)' sured as 5OVoof the original concentrationduring 30 Depletion of alkali ions from glassduring micro- secondsanalysis time at a beam current of 0.0125pA probe analysisis most likely due to the processfirst at 15kV and a beam diameterof 5 to l0 pm (Watkins describedby Lineweaver (1962). A fraction of incident electrons separatesthe alkali atoms from the bridging oxygenatoms forming two ions. The oxygen as a gas I Presentaddress: JEOL (USA) INC., ll Dearborn Road, Pea- ions migrate to the surfaceand may appear body, Massachusetts01960. bubble trapped under the carbon coating.The alkali 00n3-00/'X/ 8r / 0506-0547$02.00 NIELSEN AND SIGURDSSON: METHODS FOR MICR2PR1BE ANALYSIS oF S1DIT]M

ions remain in place until a particular temperatureis Methods reachedat which time they begin to ditruse through Rhyolitic glass fragments (KN-18) containing the sample.A spacecharge layer, formed severalmi- 5.687oNarO and4.39VoKrO and ranging in size from crons beneath the surface where the incident elec- 20 to 200 pm in diameter were mounted in a small trons cometo rest within the sampleacts as a sink for brass disc with epoxy. The mount was polished and thesemobile alkali ions. The mobile ions diffuse out coatedwith approximately200 angstromsof carbon. of the excitedvolume toward the spacecharge layer, The loss of sodium from the rhyolitic glass during resultingin a decreasedalkali X-ray production. The electron bombardmentwas monitored both at 25oC thermal characterof the processaccounts for the ex- under vacuum, and under cryogenic conditions. A istenceof what Vassamilletand Caldwell (1969)have cryogenic cold-finger cooled the sample through a termedthe incubation period, i.e., anearly flat region copperbraid to a final temperatureof -110'C in a in the beginning of the decay curve where little, if specimenchamber under vacuum of 5 x 10-6Torr. any, depletion takes place. The incubation period, During cooling the rate of sodium loss was mon- therefore,represents the time required for the sample itored every lOoC for a 60 second period. Counts to attain the temperaturenecessary for noticeable al- were automaticallyaccumulated in 2 kali diffusion. secondintervals from samplesexposed to a focused,0.02p.A, 15 kV Severalattempts have been made to eliminate al- electron beam. The EIMA (JEoLJxA-5OA) was stan- kali loss by altering the analytical conditions. Mov- dardized using basaltic glass VG-2 and a synthetic ing the sampleduring the analysisallows collection mineral composition of 85Vodiopside-l5Zo jadeite. of X-rays beforethe material has a chanceto heat up. The standardswere mounted in a brassdisc with the Rastering or defocusingthe beam over a large area unknowns so that standardizationcould be checked also has the efect of reducingthe heating rate. These without having to warm the sample. Quantitative methodsare effectiveonly when large (-100 pm) ho- nrue analyseswere performed on unexposedsam- mogeneoussamples are available which is generally ples of known composition at a temperature of not the casefor silicic glassytephra or experinental -l10oC, with countingtimes of 30 secondsper ele- charges.Goodhew (1975)has shown that by increas- ment. Correctionswere made on-line with a Bence- ing the acceleratingvoltage the spacecharge layer is Albee routine supplied by Krisel Control. formed at a greaterdepth in the sample.This reduces the electric field to which the alkali ions in the ex- cited volume are subjected and hence slows the decay rate. The combination of increasedaccelera- Results ting voltage, reduced beam current and energy dis- The decaycurves of Na and K X-ray intensitiesat persive spectrometersprovides reasonablequan- room temperatureare presentedin Figure l. The er- titative results(Goodhew, 1975).Unfortunately, this ror bars indicate plus or minus one standard devia- method requires acceleratingvoltages much higher tion uncertainty in the peak rate from simple count- than those compatiblewith the Bence-Albeecorrec- ing statistics.The drarratic slope of the Na curve tion schemeand relies on energy-dispersivedetectors illustrates the severity of the problem of alkali-loss which limit the accuracyof the analysis. which reducesthe initial count rate by 50Vowithin In order to utilize wavelengthdispersive spectrom- the first l0 secondsof exposureto the beam. The eters(WDS) for quantitative analysisof sodium and shapeofthe sodium curve is very nearly exponential potassiumin natural and synthetic glasses,we have and seemsto reach a steadystate condition after ap- found it necessaryto developa samplecondition that proxinately 45 seconds.Observed loss of potassium eliminatesor at least minimizes alkali mobility dur- counts is much lessand nearly linear. ing exposureto the beam. Vassamilletand Caldwell The decay rate, as measuredby the steepnessof (1969) have shown that for potassium the diffusion the curve, variesas a function ofseveral physical pa- rate increaseswith increasinginitial specimentem- rameters.For instance,the sample size is important perature.It should follow, therefore,that by reducing becausethe smallerthe samplethe fasterthe temper- the specimentemperature sufficiently the diffusion ature will rise for similar materials.Other factors in- processshould cease.If so, the initial concentration clude the embedding material, the thicknessof the can then be measuredat a temperaturewhere diffu- conductive coating, the thermal conductivity of the sion is negligible. sampleand the alkali and water contentsof the glass. NIELSEN AND SIGURDSSON:METHODS FOR MICROPROBE ANALYilS OF SODIUM

-d too

Y90 X o \80 o KinKN- F70 a \ z. o H60 z o E. >

5 rO 15 20 25 30 35 40 45 50 55 60 TIME(SECONDS) Fig. L sodium and potassium loss from natural rhyolitic glass during electron microprobe analysis under normal operating conditions.

A. Empiricalcorrection method for the decay curve, plot the log of intensity versus time, and perform the statistical analysis. Sodium As noted above, the shape of the alkali metal standardsthat are stableunder the beam, such as a decay curves appear to obey some functional rela- solid solution. of 85Vodiopside-l5%o jadeite, wete tionship with time. The function which most closely usedto calibrate the system.To test the routine, sev- fits the decline of sodium intensity with time was em- materials of known composition which experi- pirically determined to be a simple exponential. eral encedsodium depletion were analyzed.Table I lists Hence, a straight-line relationship should be ob- the "true" sodium values together with those ob- tained by plotting the natural log of the intensity tained by our empirical method. The valuesreported data versustime as illustrated in Figure 2 for several representthe averagesand standarddeviations of 25 cases.This approximation can be utilized to estimate measurements.The standard deviations of sodium the initial alkali concentration from the decay curve conc.entrationmeasured by the empirical decay curve by performing a least squaresfit and then determin- method are in all caseswell within experimentaler- ing the intensity intercept on the semi-log plot and ror of the acceptedNaoO concentration. converting to concentration. It must be stressedhere that it is assumedthat the decay begins at t : 0 and B. Cryogeniccorrection method does not go through an incubation period or an initial increasein count rate. Also, this technique ne- In the empirical method describedabove the so- glects the increase in apparent concentration of the dium concentration is measuredwhile the sample un- non-mobile elementspresent in the phase. dergoesa dynamic compositionalchange' [n order to A computer-basedautomation systemwas used to determine the absolute alkali content, however, the standardize the microprobe for sodium, collect so- sample must be analyzed under stable conditions. To dium counts at two-secondintervals, store the data this end cryogenic experiments were performed to 550 NIELSEN AND SIGURDSSON: METHODS FOR MICROPROBE ANALYSIS oF soDII]M

5,2

6 c.v UJ F z > 4.8 E. I X > 4.6 l o U) tL. 4.4 o= KN-18 A= KE-I2 J J 4.2 tr= MtNoAN E :l O= LP F z 4.o

4.8 z 4 6 I tO t2 t4 t6 t8 20 22 ?426 28 30 TIME(SECONDS) Fig. 2. Sodiumdecay plots for four naturalrhyolitic glasses, plotted as the natural log ofX-ray intensityversus tlme.

suppressthe sodium depletion. A natural rhyolitic should be possible.Quantitative microprobe analyses glasssample (KN-18) was cooled to a final temper- were performed on several samples to test the -l ature of l0oC at a rate of I degreeC/rrrrn. Sample method. Calibration of the system was made at temperaturewas monitored with a thermocoupleem- -llO'C using known standards.Two samplesof beddedwith the glassesin the epoxy mount. During known composition (KN-18 and LP) were analyzed the cooling period a seriesof 60-seconddecay curves using the beam conditions stated above. The aver- were accumulatedat l0oC intervals. Results of this agesof l0 analysesof eachsample are given in Table experimentare presentedas a seriesof decay curves 2 along with the acceptedvalues. The low standard in Figure 3. The lowest curve, determined at 20oC, deviations and percent errors demonstratethat the has the highestslope and levelsoffwithin 40 seconds. The curve recordedat a temperatureof -lOoC shows Table a marked reduction in the diffusion rate and does not l. Empirical determination of sodium in rhyolitic glasses under normal operating conditions, i.e., 0.02 pA level offwithin the 60 second beam current. observation.Other data l5kV acceleratingvoltage with a focusedspot. collectedat lower temperaturesyielded decay curves with decreasing -90oC Accepted Na2O Measured Na2O Relative slopes.At approximately the Sample concentration concentration ? error concentration remained stable, within experinental KN-I8 5 68* 5.53+025 2 .58? error, for the entire 60 secondobservation. Cooling KE-I2 5.40x 5.34+0.30 1.r2g to the lowestattainable temperatures of - I lOoC pro- duced resultssimilar to thoseobtained at -90oC. LP 3.22* 3.32+0.|8 3. 05? The results indicate that at a sample temperature Minoan Tephra 5 ll* 4. 98+0.27 2.s7Z -90oC of sodium is stablein the glassfor * at least 60 X-ray.fluorescenceanalysis; S. A. Malik, D. A. Bunsgaard seconds and therefore quantitative determination and R. McDonald, pers. comm. NIELSEN AND SIGURDSSON:METHODS FOR MICROPROBE ANALYSIS OF SODIUM 551

120

ilo tr trtrog tr too tr trtr tr tr 90 D gtr .".96 tr o 80 a t a--..l * o;..,o-r A 70 * F U -^__^ a _^. _^ A trJ ^_ F 60 \ z AA 9 ooo A o 5n o d.. oo E o . I o X 40 o ,o"^o'o f f=+tQoQ o 30 o O = -19o 6 **\+ 20 a=-500c +il**a-.1-#- -9O" lo B= C ++-

o to 20 30 40 50 60 TIME (SECONDS) Fig. 3. The efect of sampletemperature on sodium-lossfrom a rhyolitic glass(KN-18) during electron bombardment. Cryogenic cooling of the sample mouni to -90 degreesC suppressesalkali diffusion in the glass and enables direct quantitative analysis'

-ll0'C Table 2. Microprobe analysesof rhyolitic glassesunder normal beam conditions at sampletemperature.

KN-18 LP Relative Accepted Relative Accepted This work B error Oxide Composition* This work E error Composition*

(.25)** 0.5 76.49 76.9(.30)** 0.66 SiO2 74.6 75.0 o.71 r0.67(.05) 1.3 12,59 12.s0(.lo) At203 10.53 1.09(.os) 3.8 3.s4 3.s2 (.08) 2.3 I .05 FeO* 0.0s(.01) 0.01 0.02(.01) 0. 05 Mgo 0.33(.03) 6.s CaO 0.015 0.17(.01) 4.0 0.31 4.28(.lo) 1'4 5. 58 s.47(.09) 3.7 4.34 Na20 4.so(.lo) 2.0 lt. 39 4.2s(.05) 3.2 4. 59 Kzo 0.10(.01) Tio, 0.18 0.20(.02) ll.l 0. l0 0,01 0.01 Pzo-s 0.06(.01) MnO 0. 05 0.0s(.01) 0.06 99.81 Total 99. 06 99.35 99.60

*X-r:iy fluorescence analysis; S.A. Malik, D.A. Bunsgaard and R. McDonald, pers. comm.

**One standard deviation, based on ten measurments. 552 NIELSEN AND SIGURDSSON:METHODS FOR MICROPROBE ANALYSIS OF SODIUM

precisionand accuracyofthese analysesare as good References as thoseof glasssamples which do not experienceal- kali depletion. Borom, M. P. and Hannerman,R. E. (1967).Local The two major problems encountered with compositional the changes in alkali silicate glasses during electron microprobe cryogenictechnique are specimencontamination and analysis.Journal of Applied Physicis38,2406-2410. the additional time required to prepare and change Goodhew,P. J. and Gulley, J. E. C. (1974).The determinationof samples.Hydrocarbon contamination on the cryo- alkali metals in glassesby electron probe microanalysis. Glass genically cooled sample was experiencedbut could Technology,15, 123-126. Goodhew, P. J. (1975). probe be reducedby increasingthe surface Electron microanalysisof glasses areaof the cold- containing alkali metals. Microstructural Science, finger 3, 63 l-641. and by taking steps to keep the instrument Helz, R. T. (1976). Phase relations of basalts in their melting clean. The time required to cool the sample to the rang€sat PCH2O : 5kb. Part II. Melt compositions.Journal of working temperature results in increased analysis Petrology17, 139-193. time but this can be reduced by mounting several Kushiro, l. (1972).Effect of water on the compositionof magmas samplestogether. formed at high pressures.Journal of Petrology,13, 3l l-334. Lineweaver, J. L. (1962). Oxygen outgassing caused by electron Conclusions bombardmentof glass.Journal of Applied Physicis,34, 1786- 1791. The severeloss of sodium counts durins micro- Mysen, B. O. and Kishiro,l. (1977).Compositional variations of probe analysisof silicateglasses can be corrlcted for coexistingphases with degreeofmelting ofperiodate in the up- or avoided. The migration of sodium due to heating per mantle. American Mineralogist, 62, 843-856. under the electron beam can be arrestedby cooling Varshneya,A. K., Cooper,A. R., and Cable, M. (1966).Changes in composition during electron the sampleto -90oC during analysis.This cryogenic microprobe analysis of K2O- SrO-SiO2glass. Journal of Applied Physics,37, 2199-2202. method involves, however, longer analysistime and Vassamillet,L. F. and Caldwell, V. E. (1969).Electron-probe mi- specialpreparation techniques. A more rapid routine croanalysis of alkali metals in glasses.Journal of Applied phys- of estimatingthe sodium concentrationhas therefore ics,40, 1637-1640. been developed.This empirical technique estimates Watkins, N.D., SparksR. S. J., Sigurdsson,H., Huang, T. C., Fed- erman,A., the initial sodiurr concentration from Carey,S. and Nhkovich, D. (1978).Volume and ex- the rate of t€nt of the Minoan Tephra from Santorini volcano: decay in New evi- sodium counts during continuous electron dencefrom deep-seasediment cores. Nature, 271, 122-126. bombardmentof the sampleand routinely gives ac- curate data. Acknowledgment The authors would like to thank Dr. L. Finger for Manuscript received, May 12, 1980; a prelimfuary review of this work. acceptedfor publicalion, December 29, 1980.