EDS

Chemical Geology, 95 (1992) 283-297 Elsevier Science Publishers B.V., Amsterdam

283

[61

Accurate and precise measurement of rock varnish chemistry using SEM/EDS Paul R. Bierman and Scott M. Kuehner Department of Geological Sciences, University of Washington, Seattle, WA 98195, USA (Received December 18, 1990; revised and accepted July 16, 1991 )

ABSTRACT Bierman, P.R. and Kuehner, S.MI, 1992. Accurate and precise measurement of rock varnish chemistry using SEM/EDS. Chem. Geol., 95: 283-297. We describe important refinements to the Harrington-Whitney technique by which an energy-dispersive spectrometer attached to a scanning electron microscope (SEM/EDS) is used to measure the chemical composition of rock varnish, a Si-/Al-/Fe-/Mn-rich material coating rock surfaces in arid regions. In particular, we have developed a calibration procedure which generates accurate, quantitative SEM/EDS analyses. This procedure differs from that typically followed for SEM/EDS because the reference spectrum for each element is calibrated using multiple standards. Analyses of 27 standards of varied composition, including three synthetic rock varnish standards, demonstrate the accuracy of our measurement technique. Repeated analyses of the same sample quantify the precision of our measurements and suggest that, if a high beam current and pulse processing rate are used, 1000 s is the optimal acquisition time for material similar in composition to rock varnish. Our data show that a spectral analysis program, using stored reference spectra, is capable of deconvoluting the severely overlapping Ba-L,~ and Ti-K,~ X-ray peaks and accurately determining the abundance of Ti and Ba, both of which are present in rock varnish. The accuracy and precision of SEM/EDS analyses of intentionally roughened synthetic varnish standards compare favorably with those of polished standards. This finding indicates that surface roughness does not introduce a significant error into SEM/EDS analyses of in situ rock varnish. Our data imply that carefully calibrated SEM/EDS can accurately measure rock varnish chemistry and determine varnish cation ratios, (Ca + K ) / T i , which have been used as a calibrated geochronometer by previous investigators.

1. Introduction

Rock varnish, a thin ( < 200/tm) dark-colored coating rich in Si, A1, Fe and Mn, is found on rock surfaces in arid and semi-arid regions. Formation of rock varnish is probably biologically mediated with eolian fallout providing detrital material for incorporation into the coating (see Staley et al., 1991 ). The chemistry of rock varnish, in particular the ratio ( C a + K ) / T i , has been used as a calibrated geochronometer (see Dorn, 1983, 1989a). However, accurately and precisely determining rock varnish composition has proven to be difficult and controversial (Harrington and Whitney, 1987; Dorn, 1989a, b; Dorn et al., 0009-2541/92/$05.00

1990; Bierman and Gillespie, 1991; Harrington et al., 1991 ) due to the particular physical and chemical properties of this material and because until recently, standards of varnish-like composition were not available (Bierman and Gillespie, 1991 ). In this paper, we review the theory and geologic application of rock-varnish cation ratio dating, consider the characteristics of rock varnish which influence the accuracy of chemical analyses, and review techniques used previously to measure varnish composition. We detail a methodology by which a scanning electron microscope with an energy-dispersive spectrometer (SEM/EDS) can be effectively and reproducibly calibrated, and present data

© 1992 Elsevier Science Publishers B.V. All rights reserved.

284

constraining the accuracy and precision of our SEM/EDS measurements. We use SEM/EDS and not wavelength-dispersive spectroscopy (WDS) to analyze varnish because the area from which X-rays are collected in S E M / E D S is not constrained to be focussed on the Roland circle ( < 50-/lm beam diameter). Thus, S E M / E D S allows us to measure the variance of varnish chemistry as a function of analytic area, a prerequisite to developing unbiased sampling strategies and rigorously evaluating rock-varnish dating methods (Bierman et al., 1991 ).

1. I. Geologic applications ofrock varnish chemical analyses Interest in rock varnish chemistry has recently increased because differences in varnish composition may provide a means for determining the time at which a rock surface was first exposed. Such exposure ages are particularly valuable in arid regions where few other numerical dating techniques are applicable. Bard (1979) first proposed that varnish' composition (Mn and Ba concentrations) might be a proxy for varnish age. Dorn ( 1983 ) later identified the ratio ( C a + K ) / T i (defined by him as the "rock varnish cation ratio" or CR) as most indicative of varnish age and suggested that this ratio decreased with time due to the preferential leaching of "more mobile" cations (Ca, K, Na, Mg) from the varnish. Dorn and coworkers have since published log-normal calibration curves relating rock-varnish cation ratios to the age of surfaces independently constrained by such techniques such as K / A r and 14C (see Dorn, 1989a). Two other investigations have reported that varnish cation ratios decrease as a function of time (Harrington and Whitney, 1987; Pineda et al., 1988). Although numerical rock-varnish cation ratio dating has been used to revise the Sierra Nevada glacial chronology (California, U.S.A., Dorn et al., 1990), to challenge accepted pe-

P.R. BIERMAN AND S.M. KUEHNER

troglyph chronologies (Nobbs and Dorn, 1988) to constrain erosion rates in the Rio Grande rift (New Mexico, U.S.A.; Dethier et al., 1988 ), and to characterize seismic and volcanic hazards near the proposed Yucca Mountain high-level waste repository (Nevada, U.S.A.; Dorn, 1989b), no chemical or physical model has been proposed that rigorously explains and predicts the reasons for, and magnitude of, cation ratio changes. Reneau et al. (1990) found no evidence of the elemental leaching proposed by Dorn (1983, 1989a): rather, they suggested that the observed trends in cation ratios may result from unintentional inclusion of substrate in varnish analyses. Bierman and Gillespie ( 1991 ) suggested that observed trends in cation ratios with time may be due, at least in part, to inaccurate varnish analyses, in particular, to interference of Ba in proton-induced X-ray emission spectroscopy (PIXE) and SEM/EDS analyses of Ti (see also Harrington et al., 1991 ).

Fig. 1. Cross-section of rock varnish cut normal to the rock surface and polished. SEM photograph is backscattered electron image in which lighter tones represent high average atomic number material (varnish) and darker tones represent low atomic number substrate (rhyolite tuff). Bar represents 10 ~m.

MEASUREMENTOF ROCK VARNISH CHEMISTRY USING SEM/EDS

I. 2. Physical and chemical characteristics of rock varnish Rock varnish ranges in thickness from < 1 to > 200/xm, is rich in Si, A1, Mn and Fe, and contains lesser amounts of Ti, Ca, K, P, Na, Mg, S, C and Ba. SEM observations indicate rock varnish is physically (see Krinsely et al., 1990) and chemically heterogeneous on a variety of length scales (see Bierman et al., 1991 ).

2~ 5

The thickest varnish is commonly found in depressions on the rock surface (Fig. 1 ). Varnish in these depressions is often chemically and texturally stratified (Perry and Adams, 1978; Raymond et al., 1991 ) and appears to contain layers of detrital fragments (S. Reneau, pers. commun., 1990). The physical properties of rock varnish are difficult to characterize because the coating is so thin and discontinuous. Reneau et al.

Fig. 2. SEM secondary electron images of three natural v a r n i s h e d surfaces and of one synthetic glass varnish standard roughened to simulate surface roughness of natural varnish. All images acquired at 400 x magnification and represent an area a b o u t 200 × 220/~m. Natural varnish collected from boulders near Lone Pine fault scarp, Lone Pine, California. A-C. Surface textures o f natural varnish. D. Surface of synthetic varnish s t a n d a r d roughened to simulate natural varnish surface.

286

(1991) state that varnish density is poorly known and propose an average density of 3.1 g c m - 3; there are no published measurements of rock varnish porosity. The F e / M n matrix is either microcrystalline or amorphous (Potter and Rossman, 1978; Raymond et al., 1990); however, we and others, using an electron microscope (Perry and Adams, 1978; Harrington, 1988; Krinsely et al., 1990), have identified, in rock varnish, detrital grains of specific mineralogy such as magnetite, barite, sphene and quartz. The texture of the rock varnish surface is highly variable (Fig. 2 ). SEM observations using secondary and backscattered electron detectors often reveal protruding mineral grains and areas of thin or discontinuous varnish on what, in hand specimen, appear to be well-varnished rocks.

1.3. Published analyses of rock varnish chemistry Rock varnish chemistry has been measured using a variety of techniques. Some workers have scraped varnish from the underlying substrate and analyzed the scrapings using: wet chemistry (Engle and Sharp, 1958), inductively coupled plasma emission spectroscopy (ICP) (Dorn et al., 1990), neutron activation analysis (NAA) (Bard, 1979), proton-induced X-ray emission spectroscopy at the University of California, Davis (PIXE UCD, e.g., Dorn, 1989a), wavelength-dispersive SEM (Dorn et al., 1990) and X-ray fluorescence spectroscopy (XRF) (Bard, 1979; Dorn, 1983). Others have analyzed varnish in situ using an ion microprobe (O'Hara et al., 1989), a scanning electron microscope (SEM) equipped with energy-dispersive (EDS) and wavelength-dispersive spectrometers (WDS) (Hooke et al., 1969; Perry and Adams, 1978; Harrington and Whitney, 1987; Dragovich, 1988; Reneau et al., 1990; Raymond et al., 1991 ), or PIXE (Pineda et al., 1988).

P.R. BIERMAN AND S.M. K U E H N E R

I

~,e--- L0~(4.465 Ba

keY)

~!,e--TiK(7" (4.508keV)

INCREASING keV

Fig. 3. Ti-K and Ba-E peaks overlap in energy-dispersive

X-ray spectra. If Ba peaks are not deconvolved,apparent Ti-K, (4.508 keV) and-K~ (4.931 keV) peaks shift to a lower energies as they are superimposed upon Ba-L, (4.465 keV) and -L~ (4.828 keV) peaks. Improper or inadequate deconvolutionwill result in incorrect Ti concentrations and inaccuratecation ratios.

A particular concern in the analysis of rock varnish is the accurate measurement of Ti because: ( 1 ) the accuracy and precision of the Ti analysis strongly influence the certainty of the cation ratio - - the basis of the dating method; (2) if Ba is also present, the concentration of Ti is difficult to measure accurately by energydispersive X-ray fluorescence techniques such as SEM/EDS, X R F / E D S and PIXE because of peak overlaps (Fig. 3; Cahill, 1975; Goldstein et al., 1981 ); and (3) published analyses of rock varnish, made by methods known to measure Ba and Ti accurately, indicate that they are both present in varnish at concentrations of 5,000 to 15,000 ppm ( Engle and Sharp, 1958; Bard, 1979; Dragovich, 1988; Dorn et al., 1990; Harrington et al., 1991 ). Bierman and Gillespie ( 1991 ) and Harrington et al. ( 1991 ) present data indicating that most previous measurements of rock varnish chemistry are inaccurate because they were made by methods incapable of accurately detecting both Ba and Ti (non-quantitative SEM/EDS and PIXE UCD). Ba does not affect Ti measurements made by NAA, ICP, SEM/WDS and X R F / WDS.

287

MEASUREMENT OF ROCK VARNISH CHEMISTRY USING SEM/EDS

2. Methods

stant detector-sample distance by arbitrarily focussing the samples to the Roland circle.

2.1. SEM/EDS equipment and software

2.2. Calibration procedure SEM/EDS analyses were performed at the University of Washington on a JEOL ® 733 superprobe using a 15-kV accelerating voltage. Beam current and alignment were electronically stabilized and the stage on which the samples were mounted was computer controlled. Spectra were collected using a fixed Tracor Northern ® detector with a model 5500 analyzer and stored on disk. SEM/EDS analyses were made in January, July and November 1990. January 1990 analyses were made using a 10 Na beam current, a high pulse-processing rate (time constant ~ 2/ts, FWHM MnK , = 174 eV), and a dead time of~- 25%. July and November 1990 analyses were made using a 15 Na beam current, a high pulse-processing rate, and a dead time o f ~ 45%. We used the Tracor Northern ® FIT program (version IE) to deconvolve the energy-dispersive spectra and make quantitative analyses. All data were reduced using the Tracor Northern ® ZAF routine. Uncertainties for SEM/ EDS analyses were reported as the sample standard deviation (lcr) of multiple (n) analyses. The uncertainty is usually limited by counting statistics but for some standards, uncertainty reflects simple inhomogeneity. Reference spectra for each element were collected by rastering a focused electron beam at a magnification of 1000×(scan area= 150X 110/tin) for 1000 s. Three spectra were collected from different locations on each geologic standard at a magnification of either 1000 × or 2000 X using an acquisition time of 500 s. Eight different locations on each of three roughened and polished synthetic varnish standards were analyzed using a magnification of 400X and an acquisition time of 1000 s. Precision tests were also made using 400X magnification. All reference and standard spectra acquisitions were obtained at a con-

The calibration procedure we used to make the SEM/EDS measurements reported in this paper is shown graphically in Fig. 4. Before spectra were collected, gain on the multichannel analyzer was calibrated, the electron beam aligned and the SEM allowed to stabilize for at least one hour. Reference spectra for each element were collected so as to include at least 300 eV of interference-free background on either side of the peak(s) of interest (Table I ). We collected reference spectra from phases selected specifically because the element of interest was present in high concentration and because no elements with interfering X-ray lines were present. For example, we collected Mn from Mn metal because we could not obtain a Mnrich silicate phase which did not also contain Fe. In cases such as the Ba/Ti, M n / F e and K / Ca overlap, correct deconvolution occurred only if the region of interest included in each reference spectrum was widened to include most if not all peaks of the other, interfering element. Correction factors are used in Tracor Northern ® software to represent the apparent concentration of the pure element in its reference spectrum. Correction factors used in the January analyses were calculated using Tracor Northern ® software and the nominal composition of reference phases. Correction factors used in the November analyses were modified in an iterative fashion after the spectra of 24 geologic standards of various composition were processed and the observed (non-normalized) concentration for each element was compared to the nominal value. A similar, but less rigorous procedure, was followed in July. We find that two or three iterations are usually sufficient to generate slopes between 0.9995 and

288

P.R. BIERMANAND S.M KUEHNER SELECT REFERENCE AND STANDARD PHASES I [

SELECT SEM OPERATING CONDITIONS

]

I COLLECT REFERENCE SPECTRUM FOR EACH ELEMENT COLLECT 3 SPECTRA FROM EACH STANDARD I STORE SPECTRA ON DISK

I SETUP REFERENCE SPECTRA AND INPUT THEORETICAL CORRECTION FACTORS

I PROCESS STORED STANDARD AND REFERENCE SPECTRA /

I TRANSFER NON-NORMALIZED CONCENTRATIONS TO SPREADSHEET

REGRESS ANALYSIS VALUES AND NOMINAL VALUES FOR EACH ELEMENT

]

I PE

~

MODIFY CORRECTION FACTOR FOR EACH ELEMENT ON THE BASIS OF THE SLOPE CALCULATED BY REGRESSION

YES

SET SEM TO SAME OPERATING CONDITIONS USED TO COLLECT STANDARDS AND REFERENCES

t ANALYZE UNKNOWNS

Fig. 4. Schematic diagram o f procedure used to calibrate S E M / E D S analyses.

TABLE I Parameters for SEM/EDS calibration Element

Si Ti AI Fe Mn Mg Ca Ba Na K P S

Region of interest (keV) min.

max.

0.80 3.40 0.80 5.80 5.20 0.70 2.70 3.40 0.50 2.70 1.40 2.00

2.50 6.30 2.00 7.60 7.00 1.60 4.90 6.30 1.50 4.30 2.55 2.85

Standard

SiO2 TiO2 A1203 almandine Mn metal MgO anorthite barite NaC1 orthoclase apatite barite

Correction factor* Tracor ®

Jan. 1990

Jul. 1990

Nov. 1990

0.4044 0.5556 0.4283 0.3017 1.0000 0.4668 0.1270 0.4933 0.2438 0.1076 0.1526 0.1204

d.s. 0.5556 0.4283 0.3017 1.0000 0.4668 0.1270 d.s. n.a. 0.1076 n.a. n.a.

0.3926 0.5282 0.4348 0.2623 1.0000 0.4668 0.1270 0.4424 0.2438 0.1076 0.1526 0.1204

0.4037 0.5564 0.4284 0.2996 0.9273 0.4676 0.1292 0.4872 0.2684 0.1091 0.1526 0.1204

n.a. = not included in analyses; d.s. = different standard used for this element. *Apparent concentration of pure element in standard.

289

MEASUREMENTOF ROCK VARNISH CHEMISTRY USING SI~M/EDS TABLE 11

Analyses of standards Mineral

Oxide

Standard

MgO

Analyses Magnitication SiO2 TiO2 A1203 FeO MnO MgO CaO BaO Na20 K2O P205 SO~

Total

SiO2 3 1000 0.51

TiO2 3 1000

100.00

3 1000

100.36 100.00

100.00

AI203

0.21 100.11 0.05

0.12

0.09

100.37

0.05

0.02 0.06

0.15

0.13

0.06 0.11

Sulfate

Silicate

Barite

Sanbornite

3 1000

3 1000

0.04 100.00

100.51 0.08

0.46 0.03 0.10 0.03

0.13

0.04

0.02 0.17 0.10

65.40

64.36 0.71

34.30

34.26

99.70

99.99

2 2000 43.90

42.66 0.14

0.14 56.00

55.68 0.72

0.05 0.02 100.00

101.15

Mineral

Garnet

Standard

Gore Mountain

Analyses Magnitication

100.00

39.01 0.07 22.20 22.05 0.49 11.51 4.21

Total

99.54

100.00

100.63

100.00

101.05

99.90

99.39

Apatite

40.23 0.06 22.16 22.55 0.48 11.17 4.31

Neuvo

707

3 1000

SiO2 TiO 2 A1203 FeO MnO MgO CaO BaO Na20 K20 P205 SO3

100.69

0.04

3 2000 37.03

37.44

20.98 35.10 1.00 4.76 0.83

20.98 35.62 0.72 4.45 0.72

0.26

Ely 2 1000

36.18 0.03 20.18 21.47 21.06 0.32 0.48

0.23

35.86 0.03 19.93 21.43 21.04 0.44 0.39

Almandine 3 1000

Wilberforce

2 1000

36.26

36.54

36.18

37.07

0.13

20.51 31.07 11.44 0.22 0.52

20.57 31.41 11.52 0.29 0.40

20.35 43.54

21.03 43.13

0.02 0.03 0.03 0.01 54.48

0.04 54.14

0.24

0.22

40.71 0.60

40.52 0.04

96.25

95.05

0.22

0.21

0.25

0.29

0.03 101.22

99.70

100.17

99.72

99.37

3 1000

100.02

100.98

100.07

101.73

0.09

First column of each pair is nominal concentration of each standard; second column is average composition determined by SEM/ EDS.

1.0005 when comparing nominal and observed concentrations. In most cases correction factors change by < 10%. By deriving correction factors in this fashion, we minimize systematic bias caused by inaccurate nominal analyses of standards and/or matrix effects. Final correction factors and therefore analyses

of unknowns are based on numerous rather than single standards. The calibration procedure requires,-, 24 hr. of SEM time to collect standard and reference spectra for 13 elements, < 1 hr. to process the spectra for each iteration, and several hours using a spreadsheet to perform regressions and adjust the

290

P.R. BIERMANAND S.M. KUEHNER

Feldspar

Orthoclase, OR-1

Anorthite, Miyake

2 1000

Albite, Tiburon

3 1000

Celsian

3 1000

63.58

44.09

43.41

68.74

68.59

31.53

31.52

18.65 0.01

18.24 0.04

35.46 0.41

19.37 0.05

19.46 0.06

25.42

24.04

0.84 1.21 15.13

19.45 0.01 0.40 0.01 0.02

35.13 0.44 0.02 0.10 19.76 0.04 0.56

0.02 99.71

99.06

2 1000 43.60 19.88

42.51 18.96

11.41

42.33 0.45 0.02

36.63

37.15 0.54

3 2000 34.39 3.07 13.31 30.18 0.38 4.57 0.03 0.18 0.17 9.20

33.78 3.43 13.12 29.84 0.33 4.42 0.15 0.01 0.31 8.86

0.12 99.48

6927 3 1000

Biotite 1

0.16 41.43

11.79

0.03 99.95

99.72

Pyroxene CPXKH 1

Benitoite

0.08

0.03 99.85

Mica

2 2000

64.30

0.02 0.78 1.02 14.93

Silicate

Alpine 3 2000

98.38

98.39

0.10 100.11

99.37

Glass

Amphibole

Basalt, 1921

Kaersutite

3 1000

3 1000

95.48

94.36

Glaucophane 2 1000

2 1000

49.12 0.88 9.11 6.07 0.15 15.74 17.80

49.40 0.89 8.20 6.04 0.15 15.32 18.08

51.75 0.54 5.14 3.81 0.10 16.08 20.98

51.27 0.54 4.45 4.19 0.20 15.42 21.14

50.71 0.53 6.05 14.64 0.22 26.15 1.64

51.62 0.51 5.10 14.49 0.25 25.65 1.53

49.80 2.74 13.00 11.00 0.17 8.10 11.30

49.14 2.72 12.44 10.86 0.15 8.22 11.17

40.37 4.38 14.60 10.95 0.09 12.80 10.10

40.96 4.91 13.95 10.73 0.07 12.76 10.46

56.73 0.09 9.52 12.37 0.19 10.57 1.17

57.93

1.05

1.16

0.90

0.93

0.12

0.29

2.21 0.51

2.27 0.46 0.23 0.04

2.45 2.01

2.74 2.12

6.55 0.07

7.00

97.70

97.75

0.02 0.03 99.92

99.28

0.04 99.30

98.18

100.06

99.47

98.83

9.30 12.15 0.16 10.22 1.14

0.06 0.05 98.75

97.26

97.96

First column of each pair is nominal concentration of each standard; second column is average composition determined by SEM/ EDS.

correction factors during each iteration. We performed the linear regressions used to alter the correction factors both forcing and not forcing the regression line through the origin. For all elements except Na, the y-intercept (measured concentration) is within 0.20 wt.% of the origin and the slopes calculated by the

different methods are very similar. Because there was little difference between the methods and because low concentrations of elements are often not reported in the nominal composition of our standards, we chose to make adjustments to the correction factors by using curves forced through the origin.

OBSERVED (WT% OXIDE)

OBSERVED (WT% OXIDE)

2

e'3 t.~

°o

x

P~

~

=,

o

~

-

\

\\\\

\\

e~

OBSERVED (WT% OXIDE)

OBSERVED (WT% OXIDE)

OBSERVED (WT% OXIDE)

~"

°o

o~

o

\

z

~ -

\

~.

o

:

\

a~

"\, \

x°l

\

~

~~

,~

oO

~

~

OBSERVED (WT% OXIDE)

OBSERVED (WT% OXIDE)

OBSERVED 0NT% OXIDE)

=~

o

m

o

Z~

~o

li ~

OBSERVED (WT% OXIDE)

•

;.,~ ~-~

OBSERVED (WT% OXIDE) o~

0 Z~

O~ r"

o

;.

\\

\ ~ \\

| 6~

'\\"\

S(]3/IA13S 9 N I S f l t ~ I I S I I N 3 H 9

II

\\

\

H S I N ~ I V A HS~O~I zlO 1 N 3 1 N 3 H f1S'¢31~

292

P.R. BIERMAN AND S.M. KUEHNER

TABLE I11 Analyses of synthetic varnish standards Technique Laboratory Analyses Date Surface Total

Mixture

XRF/WDS LANL (N= 1 ) May 1990 Raw

ICP UW (N=6) May 1990 Raw

SEM/EDS UW (N=8) Jan. 1990 Polished 100%

48.36_+0.44 1.85_+0.03 23.11_+0.13 11.96_+0.28 7.76_+0.01 1.89_+0.69 2.38_+0.06 0.54_+0.01 0.10_+0.20 1.95_+0.02

48.64_+ 0.13 1.75_+ 0.01 22.54_+ 0.03 11.71+_ 0.05 7.62_+ 0.05 1.87_+ <.01 2.40_+ <.01 0.54_+ <.01 0.18_+<.01 1.89_+ 0.04

48.12_+0.33 1.95_+0.03 22.67+_0.19 12.07+_0.15 8.39+_0.05 1.70+_0.03 2.45+_0.01 0.54_+0.03 n.a. 2.10_+0.03

99.90

99.14

100.00

SEM/EDS UW (N= 11 ) Jul. 1990 Polished Raw

SEM/EDS UW (N=8) Nov. 1990 Polished Raw

SEM/EDS UW (N=8) Nov. 1990 Rough Raw

SEM/EDS UW (N=8) Nov. 1990 Rough 100%

Standard FV/ (Ba/Ti ratio=0.5 ): SiP2 TiP2 A1203 Fe203 MnO MgO CaP BaO Na20 K20 Total ( K + C a ) / T i ¢*~) ( K + C a ) / T i t°2)

49.54 1.84 22.92 11.83 7.12 1.83 2.31 0.62 0.00 1.99 100.0 3.00

3.00_+0.07 (n=l)

3.13_+ 0.03 (-3)

3.00_+0.05 3.00_+0.03

49.24_+0.24 48.39_+0.30 45.33_+0.76 48.22+0.30 1.81_+0.02 1.78_+0.02 1.75_+0.02 1.87+_0.02 23.52+0.12 22.69_+0.17 21.24_+0.34 22.59_+0.14 11.65_+0.10 11.50_+0.12 11.36_+0.09 12.09_+0.19 7.51 +0.04 7.61 _+0.07 7.46_+0.04 7.93_+0.11 1.94_+0.05 1.91 _+0.04 1.79_+0.11 1.91 _+0.11 2.32+_0.02 2.33-+0.02 2.27-+0.02 2.41 -+0.03 0.63_+0.02 0.66_+0.03 0.63_+0.04 0.67_+0.04 0.21 _+0.03 0.27+0.04 0.25_+0.05 0.27_+0.06 1.98_+0.02 1.96_+0.02 1.91 _+0.02 2.03_+0.02 100.60

98.82

3.05_+0.04 3.05_+0.04

3.09_+0.04 3.09_+0.04

93.75 3.05_+0.04 3.05_+0.04

99.72 3.05+_0.04 3.05+0.04

StandardFV2 (Ba/Ti ratio=0.0): SiO2 TiP2 A1203 Fe203 MnO MgO CaP BaO NazO KzO Total ( K + C a ) / T i ~°~ ( K + C a ) / T i ~'2)

50.96 1.37 22.76 11.74 7.07 1.82 2.30 0.00 0.00 1.98 100.00 4.00 4.00

49.64+_0.44 1.38_+0.03 23.20_+0.13 12.03_+0.28 7.74_+0.01 1.98_+0.69 2.37_+0.06 <0.01 0.04_+0.20 1.90_+0.02

49.58_+ 0.02 1.32+_ 0.01 22.60_+ 0.03 11.65_+ 0.06 7.57+_ 0.07 1.85_+<.01 2.41_+ 0.02 <0.02 0.27_+ 0.20 1.86_+ 0.05

48.98_+1 . 3 2 1.44_+0.03 22.98_+0.78 12.00_+0.28 8.38+_0.11 1.75_+0.14 2.44_+0.03 <0.02 n.a. 2.04_+0.09

50.27_+0.88 1.36+_0.02 23.93_+0.54 11.73_+0.21 7.58+_0.12 1.98_+0.06 2.32_+0.03 <0.03 0.19_+0.03 1.92_+0.07

49.98_+0.47 1.35_+0.02 22.73+_0.27 11.42_+0.12 7.59+0.09 1.93+_0.02 2.33+0.02 <0.02 0.24+0.02 1.95_+0.04

46.69_+1.26 49.69_+0.43 1.31 _+0.02 1.39+0.03 21.18+0.66 22.54+0.33 11.20_+0.07 11.93+0.29 7.42+0.07 7.90_+0.12 1.75+0.13 1.86_+0.14 2.25+0.03 2.39_+0.03 <0.02 <0.02 0.20_+0.08 0.22_+0.09 1.86_+0.03 1.98_+0.03

100.28

99.11

100.01

101.07

99.27

93.66

3.96+_0.10 ( n = 1)

4.13_+ 0.06 (°3)

3.99+_0.12 3.98_+0.08

4.01 +0.09 4.01 _+0.09

49.83_+0.44 0.47_+0.03 23.20_+0.13 11.95_+0.28 7.74_+0.01 1.79_+0.69 2.35_+0.06 0.55_+0.01 0.08_+0.20 1.92_+0.02

50.09_+ 0.26 0.46_+ 0 . 0 1 22.57+_ 0.33 11.73+_ 0.04 7.58_+ 0.05 1.78_+ <.01 2.39_+ 0.01 0.54_+ <.01 0.26_+ 0.20 1.88_+ 0.04

49.81 _+0.37 0.50_+0.03 22.60+_0.15 12.05+_0.18 8.35_+0.05 1.62_+0.05 2.42+_0.01 0.57+_0.02 n.a. 2.09_+0.03

50.95_+0.24 0.44_+0.02 23.56+_0.08 11.73+_0.10 7.52_+0.03 1.87_+0.08 2.28_+0.01 0.64+_0.05 0.22+-0.04 1.96_+ 0 . 0 1

99.88

99.28

100.01

4.06+0.07 4.06_+0.06

4.02+0.08 4.02_+0.08

99.68 4.02_+0.09 4.02_+0.08

Standard FV~ (Ba/Ti ratio = 2.0): SiP2 TiP2 A1203 Fe203 MnO MgO CaP BaO Na20 K20 Total ( K + C a ) / T i ~°~ ( K + C a ) / T i (°2~

51.11 0.46 22.83 11.78 7.09 1.82 2.31 0.61 0.00 1.98 100.00 12.00

11.63+0.76 (n=l)

11.86_+ 0.28 (*3)

11.57_+0.70 11.64-+0.35

100.95 12.30_+0.53 12.34_+0.55

50.13_+0.27 47.55_+0.74 50.03_+0.25 0.45+_0.02 0.46+0.02 0.49_+0.02 22.65+0.07 21.56_+0.25 22.68_+0.08 11.40+0.24 11.31+0.14 11.90_+0.20 7.57_+0.09 7.45_+0.06 7.84_+0.09 1.83_+0.04 1.75_+0.07 1.84_+0.08 2.29_+0.01 2.23+0.02 2.35_+0.02 0.63_+0.03 0.61 _+0.03 0.64_+0.03 0.25_+0.02 0.22_+0.03 0.24_+0.04 1.96_+0.02 1.89+0.03 1.99_+0.02 98.90

94.82

99.76

12.07_+0.45 12.09_+0.49

11.40+0.46 11.42_+0.52

11.39-+0.52 11.42+0.52

Uncertainties are one sample standard deviation except for XRF/WDS for which long-term average uncertainties are stated, n.a. = not analyzed. *~Cation ratio is average of mean values for K, Ti and Ca; uncertainty calculated using standard error propagation (Bierman et al. 1991 ). "2Cation ratio is average of multiple determinations; uncertainty is one sample standard deviation. *3Data reduction software does not provide results for individual analyses.

293

MEASUREMENT OF ROCK VARNISH CHEMISTRY U S I N G SEM/EDS 14

synthetic varnish standards are available from P.R.B.

12~

2.4. Surface roughness and precision tests

10'

aa

w

zw~wm Z oZ

i

ooZ-*

i )

2i i!

w w wlw

, , . L i i iw i w

.,..i. SYNTHETIC

FIYY FV1 FV2 VARNISH STANDARD

Fig. 6. (Ca + K ) / T i cation ratios measured by SEM/EDS are similar on rough and polished surfaces and are similar using different calibration procedures. Values plotted for FV I and FV2 are in the same order as for FV3. Error bars are 1~. Data summarized in this graph are presented in Table 1II.

2.3. Standards chosen for calibration and testing

To quantify the effect of surface roughness on the accuracy and precision of SEM/EDS measurements, we attempted to mimic the topography of a natural varnish surface by abrading the surface of three synthetic varnish standards using 120 grit (Fig. 2 ). We then analyzed eight different roughened pieces of each synthetic varnish standard. To determine the precision of our measurements as a function of acquisition time and to determine the optimal acquisition time to use for varnish analysis, we made repeated measurements at the same location on a roughened varnish standard. Ten measurements were made at each of six different acquisition times ranging from 60 to 2000 s. 3. Data

To calibrate correction factors for each element and to test the accuracy of our SEM/EDS analyses we selected 24 natural and synthetic standards of differing mineralogy and elemental composition (Table II). All reference materials and standards were fiat, polished samples except the Na reference which was a cleaved fragment of halite. In order to test the ability of SEM/EDS to analyze correctly material of varnish-like composition, we prepared three glass analogs of rock varnish spanning the range of Ba/Ti and cation ratios measured in natural varnish (Bierman and Gillespie, 1991 ). These glass standards have been analyzed by X R F / W D S , ICP, PIXE, S E M / W D S and SEM/EDS. Only methods used to perform S E M / E D S analyses are presented in this paper. Detailed descriptions of the techniques used to prepare the glass and perform the other analyses are presented in Bierman and Gillespie ( 1991 ). Aliquots of the

3.1. Analyses of standards Calibrated SEM/EDS analyses of 24 standards are consistent and are not systematically biased (Fig. 5; Table II). Because some SEM standards have been poorly characterized a n d / or are known to be somewhat inhomogeneous on a large scale, it is often difficult to evaluate the accuracy of SEM/EDS measurements of any single standard. However, the linear nature of the plots in Fig. 5, the high degree of correlation between nominal and observed values (slopes range between 0.9973 and 1.0005 ), and the similarity of measured totals compared to nominal values strongly suggest that our SEM/EDS analyses are accurate over a wide range of composition and mineralogy. The only systematic error in our analyses appears to be a background of between 0.1 and 0.3 wt.% of Na20. This may be due to errors

294

P.R. BIERMAN AND S.M. K U E H N E R

0"51

(A)

61

Si

+

(B)

Ti

0.4 w 0.3

o Q n-

~,/ ~

~0.2-

~

f NORMALIZED 1

0.1

0-~

soo

4.1~

looo 15~o ACQUISITION TIME (S)

2000

0-0

251111

(C) 4.5

5oo

1=~o lsoo ACQUISITION TIME (S)

2ooo

--

2soo

(D)

4.0! _~3.5 =;

3-9i

++

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i 61)

120

2~50

ACQUISITION

5()0 TIME(S)

1~

~

~v,

~2

SYNTHETIVARNI C SH

Fv3

Fig. 7. Measurement precision determined by replicate analyses of synthetic varnish standards• For (A)- (C) mean and/ or sample standard deviation ( 1a) for each acquisition time determined from 10 replicate analyses at the same spot on a roughened varnish standard. For (D), standard deviation determined by analysis of multiple points on each varnish standard. A. Precision-time relationship typical of major elements in varnish such as Si, AI, Fe and Mn. B. Precision-time relationship typical of minor elements in varnish such as Ti, K, Ca and Mg (note that ),-axis scale is ten times that of Fig. 7A). C. Precision-time relationship for the cation ratio (Ca + K)/Ti. Error bars-- I a. D. Uncertainty of rough surface analyses (n= 8 on each synthetic varnish standard) is only slightly greater than thc uncertainty of measurements (n = 8) made on smooth, polished surfaces•

a s s o c i a t e d with spectra fitting at low energies a n d / o r e r r o r s a s s o c i a t e d with o u r c h o i c e o f the N a r e f e r e n c e (NaC1).

3.2. Analyses o f synthetic rock varnish After c a l i b r a t i o n was c o m p l e t e , we reanalyzed the s y n t h e t i c v a r n i s h s t a n d a r d s described in B i e r m a n a n d Gillespie ( 1991 ). Both

e l e m e n t a l c o n c e n t r a t i o n s a n d c a t i o n ratio det e r m i n a t i o n s are in g o o d a g r e e m e n t b o t h with the m i x t u r e values, ICP, a n d X R F / W D S analyses, a n d with S E M / E D S m e a s u r e m e n t s m a d e in July 1990 after a similar b u t less rigorous c a l i b r a t i o n p r o c e d u r e h a d b e e n followed ( T a ble III; Fig. 6). E l e m e n t a l c o n c e n t r a t i o n s m e a s u r e d in July a n d N o v e m b e r 1990 d i f f e r f r o m t h o s e o b t a i n e d in J a n u a r y 1990 because the

295

MEASUREMENT OF ROCK VARNISH CHEMISTRY USING SEM/EDS

January correction factors were not modified by the analyses of multiple standards; however, cation ratios are similar.

3.3. Analyses of rough surfaces Natural varnishes have rough surfaces (Fig. 2) unlike polished probe standards. However, our analyses of roughened synthetic varnish standards show that surface roughness has an insignificant effect on quantitative SEM/EDS analyses of rock varnish standards. Totals on roughened surfaces ranged from 91% to 97%. Cation ratios determined on rough surfaces are slightly lower than those on polished surfaces but are not separable at l a (Fig. 6). When the low totals are normalized, the amount of Al and Na appear to decrease by < 3% relative, the amount of Ti, Ca, K, Mn and Fe appear to increase by several percent relative and the changes in Si and Mg are not systematic (Table III).

3.4. Actual precision of SEM/EDS measurements The precision of SEM/EDS measurements generally improves as acquisition time increases (Fig. 7A and B) however, our measurements suggest that for most elements extending acquisition times beyond 1000 s results in little gain and for major elements such as Si may even lower apparent precision as curve fitting uncertainties becomes more important than uncertainties imposed by counting statistics (e.g., Bower, 1985). Increasing acquisition time improves the precision of cation ratio determinations; however, little is gained by extending acquisition times beyond 1000 s (Fig. 7C). Cation ratios are determined with almost the same precision on roughened surfaces as on polished surfaces (Fig. 7D).

4. Discussion

Our data indicate that carefully calibrated SEM/EDS can accurately analyze materials of wide-ranging chemical composition (Table II ). In particular, analysis of rock varnish standards indicates that SEM/EDS can accurately measure the composition and cation ratios of materials similar in chemistry to natural rock varnish, including materials containing both BaO and TiO2 at concentrations <2 wt.% (Table III). Analysis of varnish standards, roughened to simulate natural varnish, strongly suggests that surface roughness does not significantly bias SEM/EDS measurement of natural rock varnish chemistry and determination of cation ratios (Fig. 6; Table III). The accuracy of these measurements results from determining correction factors on numerous rather than single standards for each element - - in effect, calibrating our measurements to multiple standards. This method of calibration is important because of the difficulty in obtaining, well-characterized, homogeneous SEM standards having a wide range of elemental compositions. Multiple analyses of both smooth and roughsurface standards indicate that rough surfaces increase only slightly the uncertainty of analyses and cation ratio determinations (Table III; Fig. 7D). We have reduced the uncertainty in our measurements by using a high beam current ( 15 Na) and configuring the EDS pulseprocessing system to maximize count rate ( ~ 8 5 0 0 net cps for synthetic varnish standards). Maximizing count rate requires using a short time constant (2/zs) and a high dead time ( ~ 4 5 % ) both of which have the potential to degrade the quality of analyses, in particular, the deconvolution of closely overlapping X-ray peaks. However, our data indicate that any degradation caused by these factors is unimportant (Tables II and III). This study definitively resolves the issue of Ba/Ti deconvolution by SEM/EDS raised by Dorn ( 1989a, b). Data presented in this paper

296

and in Bierman and Gillespie ( 1991 ) demonstrate that carefully calibrated SEM/EDS measurements can accurately and precisely determine the concentration of coexisting Ba and Ti at concentrations typical of rock varnish. The calibration procedure we describe and the generation of quantitative SEM/EDS measurements can only be performed if the electron beam and the SEM are stable over the time period necessary to acquire spectra. We have found that electronic beam stabilization is necessary to produce dependable analyses. In order to perform this calibration and make accurate analyses, all reference, standard, and unknown spectra must be collected under the same operating conditions. To ensure that operating conditions are comparable, we collect all reference and standard spectra sequentially, store them on disk, and do calibration and processing at a later time. Empirical observations on rock varnish standards indicate that once reference spectra have been collected and calibrated they are useful for a period of at least six months before instrumental drift degrades the accuracy of analyses. The similarity between July and November analyses indicates that our calibration procedure is reproducible.

5. Conclusions

Data presented in this paper indicate that carefully calibrated SEM/EDS can be used to measure accurately, precisely, and reproducibly the chemical composition of numerous materials, including those similar in composition to rock varnish. Roughened sample surfaces decrease the precision and accuracy of analyses only slightly and do not significantly affect calculated cation ratios of synthetic rock varnish standards. These findings indicate that quantitative SEM/EDS analyses can be a useful tool for determining the chemical composition of natural rock varnish.

P.R. B I E R M A N A N D S.M. K U E H N E R

Acknowledgements Research reported in this paper was performed under U.S. Geological Survey NEHRP grant 14-08-0001 -G 1783 to A.R. Gillespie and P.R.B. We thank G. Ludeman of Los Alamos National Laboratory for performing XRF analyses and A.J. Irving (UW) for providing laboratory facilities and assisting with ICP analyses. Discussions with C. Harrington, R. Raymond, R. Hagan and R.I. Dorn alerted us to the "barium problem". This manuscript benefitted from review by A.R. Gillespie and D.P. Dethier. Suggestions by R.I. Dorn, E. Essene, S. Reneau and G. Remond improved an earlier version of this manuscript. Assistance in drafting figures was provided by F. Bardsley. References Bard, J.C., 1979. The development o f a patination dating technique for Great Basin petroglyphs using neutron activation and X-ray fluorescence analysis. Ph.D. Thesis, University of California, Berkeley, Calif., 409 pp. Bierman, P.R. and Gillespie, A.R., 1991. Accuracy of rockvarnish chemical analyses: Implications for cation-ratio dating. Geology, 19:135-138. Bierman, P.R., Gillespie, A.R. and Kuehner, S.M., 1991. Precision of rock varnish chemical analyses and cation-ratio ages. Geology, 19:196-199. Bower, N.W., 1985. Optimization of precision and accuracy in X-ray fluorescence analyses of silicate rocks. Appl. Spectrosc., 39(4): 697-703. Cahill, T.A., 1975. Ion excited X-ray analysis of environmental samples. In J.F. Ziegler (Editor), New Uses of Ion Accelerators. Plenum, New York, N.Y., pp. 1-67. Dethier, D.P., Harrington, C.D. and Aldrich, M.J., 1988. Late Cenozoic rates of erosion in the western Espanola Basin, New Mexico: Evidence from geologic dating of erosion surfaces. Geol. Soc. Am. Bull., 100: 928-937. Dorn, R.I., 1983. Cation-ratio dating: A new rock varnish age-determination technique; Quat. Res., 20: 49-73. Dorn, R.I., 1989a. Cation-ratio dating of rock varnish: A geographic assessment. Phys. Geogr., 13: 559-596. Dorn, R.I., 1989b. A critical evaluation of cation-ratio dating of rock varnish, and evaluation of its application to the Yucca Mountain repository by the Departmenl of Energy and its subcontractors. In: Nevada Nuclear Waste Site Investigation--Evaluation of the Geologic Relationships and Seismotectonic Stability of the Yucca Mountain Area. Mackay School of Mines, Reno, Nev., Appendix A.

MEASUREMENT OF ROCK VARNISH CHEMISTRY USING SEM/EDS

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297 O'Hara, P.F., Krinsley, D.H. and Anderson, S.W., 1989. Elemental analysis of rock varnish using the ion microprobe. Geol. Soc. Am., Abstr. Prog., 21 (6): A165. Perry, R. and Adams, J.B., 1978. Desert varnish: Evidence for cyclic deposition of manganese. Nature (London), 276: 489-491. Pineda, C.A., Peisach, M. and Jacobson, E., 1988. Ion beam analysis for the determination of cation-ratios as a means of dating southern African rock art. Nucl. Instrum. Methods Phys. Res., B35: 463-466. Potter, S. and Rossman, G., 1978. Desert varnish: the importance of clay minerals. Science, 196:1446-1448. Raymond, R., Bish, D., Guthrie, G.D., Reneau, S.C. and Chipera, S.J., 1990. An amorphous manganese phase with rock varnish. Geol. Soc. Am., Abstr. Prog., 22(7 ): A270. Raymond, R., Reneau, S.L. and Harrington, C.D., 1991. Elemental relationships in rock varnish as seen with scanning electron microscopy and energy dispersive Xray elemental line profiling. Scann. Microsc. Int., 5: 3746. Reneau, S.L., Raymond, R. and Harrington, C.D., 1990. Cation ratio dating of rock varnish: why does it work? Geol. Soc. Am., Abstr. Prog., 22 ( 7 ): A270. Reneau, S.L., Hagan, R.C., Harrington, C.D. and Raymond, R., 1991. Scanning electron microscopic analysis of rock varnish chemistry for cation-ratio dating: an examination of electron beam penetration depths. Scann. Microsc. Int., 5: 47-54. Staley, J.T., Adams, J.B. and Palmer, F., 1991. Desert varnish: A biological perspective. Soil Biochem., 7 (in press).