Chapter 12 X-Ray fluorescence of obsidian: approaches to ...

Chapter 12

X-Ray fluorescence of obsidian: approaches to calibration and the

analysis of small samples Jeffrey R. Ferguson

Introduction

For decades, archaeologists have used a number of techniques to dClcnnine the

compositions of geological sources of chemically homogenous materials and then

attempted to match artifacts to sources to understand trade and exchange of material

objects. Such provenance research is common with glassy rhyolites (obsidian) that

were lIsed wherever available as a source of raw material for flaked slone artifacts

such as projectile points and cutting tools. Handheld X-ray fluorescence. of all of

the analytical techniques available for obsidian compositional analysis, has the

potential to make the greatest impact by combining non-destructive analysis with

rapid results, relatively low equipment and analysis cost, and the option of in-field

analysis. While handheld XRF is a powerful research tool, its successful use in

obsidian provenance research requires an understanding of X-ray physics. igneous

petrology, the calibration process, and the ability 10 test a sufficient variety of

homogeneous and well-characterized reference materials suitable for developing

a valid calibration curve. The potential for "point and shoot" handheld XRF is

hampered in pan by the all of lhese factors.

This chapter addresses precision requirements of a matrix-specific calibration,

the choice and analysis of reference standards, spectral normalization to account

ror differences in sample geometry and size, and the use of secondary targets to

fine tune the spectra to maximize sensitivity to the elements of interest. While the

focus here is on obsidian provenance research, many of the same basic principles apply to almost all applications of XRF.

Compositional studies of anifacts have been an integral pan or archaeological

"cientific investigations for decades, providing insight into many aspects of past

human behavior, and obsidian sourcing studies are often presented as the "poster

child" of adherence to the provenance postulate (Weigand, et al. 1977; Neff 1998).

The requirements for materials to permit successful provenance studies as stated in the provenance postulate are that measurable intersource variability must exceed

intrasource variability. Obsidian is a nearly ideal material for provenance studies

402 Jeffrey R. Ferguson

because it occurs in a relatively limited number of geologic contexts and typically

exhibits uniform chemistry within particular outcrops. There are numerous

problematic cases of sources with complex and even overlapping chemistry

(Braswell and Glascock 1998; Glascock et al. 1999: Ambroz et al. 200 1), but in

general obsidian studies are extremely successful at linking artifacts with geologic

sources wherever it was used in the past.

Obsidian is a glassy rhyolitic rock, meaning that it has high silica content and

a disordered atomic structure (Shackley 2005). The disordered atomic structure

allows obsidian to fracture predictably and with extremely sharp edges. Obsidians

arc formed on ly in specific volcank events when magma of the right chemistry

is cooled under specific conditions making it a remarkably rare material. The

fracturing ability and limited natural outcrops made obsidian a valuable resource

that in many cases was carried or exchanged across great distances.

Provenance studies are particularly appealing to archaeologists because they

can document migrations. logistical movements, exchange relationships, and

tool production , use. and discard in a spatial context (Duff 1999; Shackley 2002:

Carter et al. 2007: Eerkens el al. 2007; Ferguson et al. 2010) . Obsidian has the

additional benefit of potentially providing direct temporal informalion through the

measurement of water absorption; a process known as obsidian hydration (Friedman

and Trernbour 1978, ] 983; Origer 1989). When coupled with temporal information

acquired either through obsidian hydration analysis or by other means . such

temporally-sensitive artifact types , stratigraphic sequences, or direct association

with dated materials, it is possible to examine changes in obsidian procurement

patterns over time spans ranging from decades to hundreds of thousands of years.

No doubt the reasonable cost of obsidian sourcing relative to the valuable cultuml

information obtained has entrenched obsidian provenance in the world of contract

archaeology, driving further research in the development of analytical techniques.

geologic characterization, and the cultural interpretation of sourcing results.

This chapter is aimed at providing a technical background for the analysiS

of obsidian using XRF, with a particular focus on proper calibration techniques

and the analysis of small artifacts. This chapter is included in a volume focusing

on handheld XRF because handheld XRF users tend to be most likely to

underestimate the complexity of proper analysis. The analytical issues presented

here are common to both handheld and lab-based XRF. This paper is not intended

to discuss the geologic context of obsidian in detail , proper methods of source

sampling and characterization, or the anthropological implications of obsidian

provenance studies. There are a number of excellent publications on these topics.

starting with volumes by Shackley (l998a, 2005) and edited volumes by Glascock

and others (Taylor 1976; Hughes 1984, 1989; Glascock 2002; Glascock el al.

X-ray fluorescence of obsidian 403

2(07) and these volumes contain references to many of the hundreds of relevant

publications.

Analytical techniques for obsidian

Unlike many other geologically-derived materials typically recovered from

archaeological sites, obsidian sources lend to follow the requirements of the

provenance postulate for major, minor, and trace elements, permitting the

successful application of a variety of analytical techniques. Specific details of

the analytical techniques are readily found in existing publications (i.e. Shackley

1998b) so the discussion here is limited to the differences between the techniques

and their advantages and limitations as they relate to obsidian analysis.

Visual sourcing is occasionally put forth as a more cost-effective method for

examining large assemblages in contexts where all geologic sources are well

known (Weisler and Clague 1998), however the reliability of visual sourcing is

highly questionable (Bettinger et al. 1984; Braswell et al. 2(00). On more than

one occasion 1 have seen archaeologists working in the American Southwest

get excited after finding an artifact with a green translucent color, a feature

characteristic of the Pachuca source in Central Mexico. In one case it turned out

to be from Pachuca, comprising one of the handful of Mexican obsidian artifacts

recovered from north of the Mexican border (except for a small cluster in the

southern tip of Texas [Hester et al. 1991: Ferguson and Skinner 2006]). In the

second , the artifact was from the Antelope Wells source in southwestern New

Mexico that is somewhat similar to the Pachuca source as it also very high in

zirconium and has a green translucent color. While there may be a few cases of

material so visually unique as to aUow some correct visual source assignments ,

there is an error rate most researchers are not comfortable accepting.

Neutron activation analysis (NAA) was one of the first techniques used for the

chemical characterization of archaeological obsidian (Gordus et al. 1968; Glascock

et al. 1994; 1998). NAA allows for the precise determination of a wide range

of clements that can vary depending on the nuclear properties of the elements ,

irradiation conditions, and timing of the gamma count(s), but it is an expensive

technique and typically requires the destructive analysis of at least some portion

of the artifacts (although, as described later, there are options for non-destructive

NAA of small samples). NAA also requires considerable sample preparation time

and cost in the fonn of irradiation and the use of high-purity sample containers.

Another common technique is inductively coupled plasma mass spectrometry

(ICP-MS), often incorporating a laser (LA-ICP-MS) to ablate, or vaporize, an


extremely small ponion of the anifact for analysis in the ICP-MS. LA-ICP-MS

can provide data on a wide range of elemental compositions, comparable to NAA,

but can suffer from short and long term difficulties with analytical accuracy due

to difficulties with consistent calibration (Speakman el al. 2007). LA-ICP-MS is

often louted as a non-destructive analytical method, but almost every instrument

in use today requires the removal of a portion of a sample in order to fit inside the

laser ablation chamber.

Techniques with limited application to the compositional analysis of obsidian

include proton induced X-ray emission (PIXE: Pollard el al. 2007; Bellot-Gurlct

el al. 2008) and electron microprobe analysis (EPMA; Merrick and Brown 1984;

Pollard el al. 2007). These techniques often involve the same basic method, .,

XRF, but they vary in the means of removing the electron from the inner shell of

the atom in order to create the emitted X-ray as a result of outer shell electron~

dropping down to fill the void. Many of the same issues discussed here for XRF

apply to these techniques as well. Additional analytical techniques include, but are

not limited to: natural radioactivity (Leach el al. 1978). obsidian density (Reeves

and Armitage 1973), fission track analysis (Bigazzi el al. 1998), and numerous

chemical methods (Tykot 2004).

XRF offers the potential for totally non-destructive analysis, ponable and

handheld operation , minimal or no sample preparation. rapid results. widely

available analytical instrumentation, low cost. and quantitative analysi, of some

of the most important elements used for discriminating obsidian sources and

providing provenance data. All of the other techniques described above requ ire

large stationary equipment - ranging from the size of a chest freezer to a nuclear

reactor - and require the sample to fil the space requirements of an instrument

sample chamber rather than introducing a smal l instrument to the object of interest

as is possible with handheld XRF. Most of the commercial obsidian provenance

laboratories employ non-ponable energy-dispersive (ED-XRF) as their main

instruments , yet portable instruments can provide high quality data in the lab while

also aJlowing in-field analysis including obsidian artifacts in countries or curation

facilities that will not allow their removal of artifacts. While the decreasing co ... 1.

increasing availability, and portability have all contributed to making XRF a more

attractive analytical technique to a broader spectrum of researchers across man)

disciplines. the fundamental physical principles behind the technique and the need

to understand them have not changed. The following section describe' some of

the most imponant issues in handheld XRF analysis of obsidian, with panicular

attention to some of the issues encountered in the research at the Archaeometr)

Laboratory at the University of Missouri Research Reactor (MURR).


XRF methodology

One of the firM considerations in XRF analysis is whether or not quantitative

analysis is necessary, or even possible, with the sample set under investigation.

While qualitative comparative spectra] overlay can be used to assign individual

obsidian artifacts to particular sources, it is not the most efficient means of

processing large numbers of samples and can lead to difficulties in discriminating

geologic sources with similar elemental ratios (i.e. the shape of the spectra are

similar but the magnitudes differ). Without the ability to extract quantitative

concentrations for certain clements. it is not possible to compare new analyses with

previously published compositional data, limiting comparisons to those samples

previously analyzed using the same equipment. Thus, quantitative analysis is

necessary in obsidian studies. In contrast, qualitative analysis can be highly useful

in. for example. attempting to determine if some large pit features were used to

smelt iron or to fire limestone to produce lime. In this case qualitative spectra]

overlay comparing areas inside the features to areas of the surrounding natural soil

;howed significantly higher calcium and strontium levels in deposits at the bottom

orthe features compared to outside the features and no difference in the amount of

iron. Additional tests may be warranted. but it seems likely thatlhe features were

used as lime ki lns strict ly based on spectral comparisons (Ferguson 20 10).

The main reason for not perfonnjng quantitative analysis on the pit features

described above is I have not yet developed a comprehensive calibration for the

analysis of soils. and the difficulties in properly quantifying such a heterogeneous

material in the field make the process difficult at best. The "point and shoot" claims

by some instrument manufacturers have led to misleading data being published

without a proper matrix-specific calibration (i.e. Morgenstein and Redmount 2005;

Goren el 01. 20 I I). A single calibration is simply incapable of such a broad range

of different matrices, and matrix-specific calibrations are required for accurate

quantitative analysis due to a number of factors, including absorption propenies of

the matrix, secondary fluorescence, and different concentration ranges for specific elements.

Developing accurate quantitative approaches for obsidian analysis allows

the rapid (although not fool-proof) assignment of geologic sources to artifacts

Using a variety of multivariate statistical methods, something not possible with

qualitative spectral overlay. Proper calibration is the key to efficient and accurate

SOurce assignment. MURR has a long history of obsidian analysis by NAA and

consequently a massive source library including almost every known obsidian

SOurce in the world. The MURR handheld XRF generally uses a calibration based

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406 Jeffrey R. Fergu,on

on compatibi lity with NAA data in order to maximize the potential to find sources

not anticipated in a panicular study, and thus not yet fully characterized by XRF.

The most common deviations between M URR XRF data and that produced at other

labs using only XRF are in the iron and zirconium concentrations. Unfortunately

NAA is not as capable of producing rughly accurate zirconium concentration, due

to the production of both "Zr and "Zr during "'u fission. It is possible to correct

for the uranium interference, but the correction has a large uncertainty (Glascock

el al. 1986).

The differences between the iron concentrations reported by the Berkeley

XRF Laboratory and MURR are more complicated to explain, but Mike Glascock

(pers. com. 201 I) provides a possible explanation. The published values for iron

in many USGS standards used by some XRF labs for calibration arc generally

listed as total iron. USGS standards, as a historical convention resulting from the

use of gravometric techniques, report concentrations for multiple iron oxides. It is

possible to convert the weight percent into ppm concentrations, but there is often

the false assumption that all of the iron exists as the single oxide reported. Thi,

can result in inaccurate iron concentrations entered into the calibration standards.

NAA iron concentrations are based on a single 1ST standard (fly ash) and the

concentration in the standard is not reported as an oxide, but as total iron. AA

calibration is linear and only requires a single standard for iron. The MURR

XRF calibration is based on the data derived from AA measurements of the

40 calibration standards and thus is not susceptible to the oxide weight percent

conversion problem.

Obsidian XRF calibration

Proper calibration is not a simple process. It is important to have a broad range

of concentrations in one's standards that cover the full range of concentrations

anticipated in the unknown samples. This was made clear with the initial analysis

of obsidians from the Central Rift Valley of Kenya. The initial calibration included

samples from 40 well-known obsidian sources in the Americas]. For these

obsidians, iron rarely exceeds 6-7 percent. zirconium is rarely above 1,000 ppm,

and niobium is rarely over 2oo ppm. It was clear from the spectral overlay that

many of the Kenyan samples had abnormally high concentrations of these three

elements , yet the concentrations calculated by the calibration produced lower

concentrations as the peak areas increased. Bruce Kaiser (personal communication

2oo9) has termed this phenomenon the "wet noodle effect" in which peak areas for

panicular elements that exceed the range of the standards used in the calibration


can trail off in extreme directions. sometimes leading to inverse relationships

between peak areas and concentrations.

The data from the 2009 Kenya source samples (n:14I , collected by Stanley

Ambrose) serve as an example of the need to include the complete range of

variability for each element. Figure 12.1 is a plot of the NAA-determined

concentrations for iron (X -axis) and the XRF-detcrmined concentrations for

iron (Y-axis) for both the original and extended calibrations. As noted above, the

original calibration produced decreasing iron concentration for larger peaks once

the concentrations exceeded the range in the calibration. The problem was corrected

by including a couple Kenya obsidian samples with high iron , zirconium, and

niobium concentrations. previously analyzed by NAA. The corrected calibration

produces XRF results comparable to NAA .

15000 +--~-__ -~-~-_--~ lSOOO 25000 lSOOO 4SOOO SSOOO 65000 75000

Iron (ppm) from NM

Figure 12.1. Comparison of the initial and extended MURR XRF calibrations for iron. The lower curve (triangles) are the inItial concentrations reported for a group of 141 source samples from Kenya. The upper linear spread ("x' symbols) are the same analyses recalculated using the extended calibration. The Y-axis values are the XRF concentratIons for Iron and the X-axis values are concentrations determined by NAA.

The current MURR calibration uses either 40 or 50 different obsidian secondary

standards previously characterized by NAA at MURR (the main calibration uses

only 40 standards, and the calibration for high iron , zirconium, and/or niobium

samples uses an additional 10 standards, this is explained in greater detail below).

The calibration produces concentrations for 19 elements (Na. AI, K, Ba, Ti, Mn.

I'


Fe, Co, Ni, Cu, Zn, Ga, Pb, Th, Rb, Sf. Y, Zr, and Nb). Although not a ll these

elements are relevant to the overall composition of obsidian, they are included to

improve the overa ll calibration . Only six elements are routinely used in obsidian

provenance studies. These six elements and their concentration ranges are listed in

Table 12.1. It is necessary to not only bracket the expected range of concentrations,

but to have uncorrelated variation between the elements in the suite of standards.

This multi-elemental variability helps the ca~bration algorithms account for peak

interferences and secondary fluorescence (especially for the elements Rb- 'b

where the spacing of the peaks allows for the K~ peaks from a particular clement

to interfere with the Ka peaks of the element two units greater in atomic number).

It is also important to test calibrations using samples of known composition that

are not included in the calibralion (Mauser 2006).

Mmimum (ppm) Maximum (ppm)

Fe 3554 69319

Rb II 48 1

Sf 0 488

Y 12 662

Zr 61 3455

Nb 4 372

Table 12.1: Minimum and maximum concentrations for standards used in extended MURR XRF calibration.

Unfortunately, concentrations for yttrium and niobium are not avai lable using

NAA. and AA has a relatively high detection limit (approximately 50ppm) for

strontium. Standard values for yttrium, niobium. and low strontium concentrations

were averaged from the ED-XRF analysis of the same standards at twO other

obsidian studies laboratories: Northwest Research Obsidian Studies LaboralOry

in Corvallis Oregon , and the Berkeley Archaeological XRF Lab althe Unive"ity

of California, Berkeley. Methods for analysis and calibration used by these

laboratories arc available through their respective web sites (Northwest Rc!)carch

Obsidian Studies Laboratory 20 II ; Shackley 20 II a).

Shackley (2010) recently commented on the need to use certified international

standards in calibration. This is certainly an ideal goal that would help lO increase

inter-laboratory compatibility, but there are a number of problems with this

approach. First of all, most certified standards are available only as powders.

While the powders can be pressed into pellets there is the potential for analytical

II X-ray Ouorescence of obsidian 409 I'!

~I differences when analyzing solid glass samples using a calibration based on !.

pressed powders. Empirical tests of this powder versus solid sample difference

have shown minimal , if any. effecl. Shackley (201Ib) has analyzed both solid

and pressed pellets from the single boulder used by USGS to create the RGM-

I standard and found very little difference. A recent test at MURR examined a

polished solid piece of obsidian from the Whitewater Ridge source in Oregon and

compared the spectra to loose powder and a pressed pellet from the same rock.

There was a reduction in the backscatter for the powder and pellet samples , but

there was no apparent difference in the peaks of the main elements of interest (Rb-

b). Others have observed morc significant differences as a function of grain size

of powdered samples. Liritzis and Zacharias (20 II) observed differences between

solid and powdered obsidians ranging from up to 5% for K. Ca, Sr, Zr, and Rb,

to 20 and 25% for Fe and Ti respectively. The different matrix effects of different

types of geologic materials (i.e. basalt versus rhyolite) may be more problematic

but are not yet fully studied.

Second, there are very few certifieq,&Iandards for obsidian or compositionally

similar materials . The inter-elemental interactions in XRF are incredibly complex,

and it is desirable to have a large number of calibration standards that not only

bracket the range of concentrations in the samples, but aJso include a large mix

of these elements. The goal of the MURR calibration is to use a large number of

secondary standards in the actual working range for obsidian rather than include a

number of certified standards with concentrations irrelevant for obsidian analysis.

It is critical to create a matrix-specific calibration. Finally. it is unlikely given

the compositional variations of obsidians that si mply using a series of common

standards will allow one to match source and artifact data collected at different

laboratories and using different instruments. Proper calibration using appropriate

standards can increase the probability of identifying possible sources for an

unassigned artifact in published data using other laboratories or techniques. but

assigning an artifact to a particular source unless the sample and source material

are analyzed on the same instruments using the same settings and calibration is

unwise. Some other analytical techniques. such as NAA. are much morc stable

due. in part , to linear standardizations and allow for calibration using a small

number of standards. These techniques are not susceptible to matrix effects,

and thus specific procedures to allow for inter-laboratory exchange of data are

warranted (Graham ef al. 1982; Glascock ef al. 1998). but XRF is based on

different excitation and absorption phenomena and requires a more conservative

approach. A recent inter-laboratory comparison of quantitative XRF of copper

alloys found that the accuracy of the data was a function of variables such as the

type of calibration used (fundamental parameters with and without standards and

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empirical calibrations). the number of standards used in the calibralion, bUl other

variables such as delector type and valid count rates, had Iiule apparent impact on

the accuracy (Heginbotham 20 I I) .

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Mule Moun"lns

,,>U 200 jW J~"

Rb (ppm'

Figure 12.2: Bivariate plot of rubidium and niobium for source samples from Mule Creek. The open symbols are source samples analyzed at MURR and the solid symbols are source samples analyzec at the Berkeley XRF Lab. The specific samples are not the same for each lab. Ellipses represent 90 percent confidence Intervals for membership in the group.

The goal of archaeological obsidian provenance stud ies is to match obsidian

artifacts to specific geologic sources of obsidian . While every effort should be

made to detcrmjne accurate elemental concentrations by using appropriate

standards in a robust calibration , consistent separation of sources is the ultimate

goal. Shackley (2005:7) correctly described the balance between precision and the

ultimate goals of obsidian provenance research: "[sjome physicists and chemists in

the past have made what 1 consider the mjstake of focusing on precision versus the

archaeological accuracy we seek in source provenance studies, and archaeologistS

have been guilty of the opposite - ignoring precision and accuracy in provenance

studies by wholly trusting the analyst because they don ' t understand the process."

In the case of XRF at MURR, the calibration is more valuable the closer it matches

the NAA data to facilitate the identification of artifacts from sources not yet

analyzed by XRF. The differences between the MURR calibration and other major

X·ray fluorescence of obsidian 411

labs are relatively minor. For example, Table 12.2 provides a comparison belween

the Be rke le y Lab and M U RR fo r som e o f the m ajor source s in N e w M ex ico. While

the re a re s uffi c ie nt differences to complic ate inte r- labo ratory s haring o f data , both

labs produce quantitalive elemental concentration data capable of discriminating

most sources, including two compositionally-similar sources from Mule Creek

( Figure 12.2).

Fe Rb Sr y Zr Nb n mean s.d mean s.d. mean s.d. mean s.d. mean !..d. mean

EI Rechuclos Bcrkelcy 15 6676 296 152 6 9 3 23 I 77 3 47 Rh)olile I MURR It 3803 114 138 7 I I I 22 2 7 1 4 39

rObSidian Ridge Berkeley 20' 9735 666 207 II 5 3 63 3 183 7 98

MURR 10 7289 185 186 5 6 I 54 4 168 7 89

f(-cITO del Medio Berkeley 2' 103 15 18 17 160 9 10 I 43 I 172 4 54

MURR 22 7053 396 143.6 7 10 I 4 1 3 167 9 50

Bcar Springs Berkeley 24 ' 6593 43 1 116 5 43 4 2 1 2 108 4 53 Peak

M URR 7 4877 206 110 4 51 5 20 I 108 5 47

~ran" Ridge Berkeley 10 8253 474 564 34 4 I 76 2 11 8 4 196

M UR R 10 5016 256 47 1 22 II I 67 5 10 1 5 171

lHomce ~ l e,. Berkeley 12 9568 321 530 17 4 I 87 3 142 4 236

M URR 13 5945 4 12 422 21 13 I 73 5 11 7 5 205

~I ulc Creek Berkeley 21 9619 385 246 15 16 3 42 4 122 7 27 i(AnlelOpe Creek)

I- MURR 54 6441 380 206 10 2 1 3 37 4 106 5 23

IMuie Creck (N.Sawmill )

Berkelcy 34 8423 564 41 1 15 6 2 7 1 2 118 7 120

L MURR 26 5007 300 358 13 12 2 63 4 98 7 101

Mule Creek Berkeley 15 ~M u le

7936 297 184 7 10 2 25 2 120 5 32

Mounta in<;)

L MURR 13 4861 22 1 163 6 13 2 24 2 11 6 5 28

Gwynn Canyon Berkeley 20 85 12 539 226 \3 19 2 3 1 2 156 7 22

I---- MURR 12 6079 266 205 5 21 2 30 2 156 12 22

melope Well ... Berkeley 25 22521 19 14 348 22 7 I 128 6 1255 47 97

M URR 3 18 178 900 271 8 8 I 115 6 1069 50 82

Fewer samples with Iron concentrations.

Table 12.2:

Mean and standard deviations (in ppm by XRF) for some of the major sources In New Mexico for both the Berkeley Lab and MURR. Berkeley Lab concentrations taken from the laboratory

webSite (http://www.swxrfiab.netlswobsrcs.htm).

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41 2 Jeffrey R. Ferguson

Maximizing the potential of XRF for obsidian

The goal of XRF analysis is to maximize the nuorcscence of, and thus sensitivity

lO, clements best able to discriminate geologic sources of obsidian. The mOst

useful group of elements that occur in sufficient concentrations for accurate

quantification by EO-XRF are the elements Rb through Nb (atomic numbe"

37-41). These clements generally occur in concentrations of tens to hundreds of

parts per million and the K01

nuorescence energies for each of these elements

are roughly in the middle of the spectrum of energies produced by most handheld

XRF instruments operating at 40kV.

The probability of nuorescing a particular atom is related to the cxcitarion energy of the atom relative to the energy of the incident X-ray. The probability of

fluorescence is greatest when using incident X-rays with energies just above the

exc itation energy of the atom in the sample. For example. iron has a Ka , of 6.4

keY. and there is no chance of K-shell electron nuorescence from incoming X-rays

with energies less than or equal to 6.4 keY, while X-rays lightly higher than 6.4

have the highest probability of fluorescence. The probability of fluorescence

decreases exponentially with higher energy. Thus, the goal should be to produce an

abundance of X-rays with energies just above the absorption edge of the elements

of greatest interest. The elements Rb-Nb have Ka, energies from 13.40 - 16.62

keY. An ideal incoming array would contain X-rays primarily above 17 keY and

tapering off at much higher energies.

Bruce Kaiser (with Bruker Scientific) and R. Jeffrey Speakman (wi th the

Museum Conservation Institute at the Smithsonian) have developed a filter that

acts like a secondary target to absorb the incoming X-rays below about 18 keY

in order to both reduce the background of scattering X-rays at the same energy as

Rb-Nb and to remove thc X-rays of low energy that do not significantly contribute

to the fluorescence of Rb-Nb. The filter consists of thin foils of copper ( 150 Iill')'

aluminum (50 1lJ11) and titanium (300 11m). The copper absorbs the X-ray'

below 18 keY, but the fluorescence of copper produces a large spike of X-ray'

at approximately 8-9 keY. The titanium layer absorbs the 8 .05 keY X-rays. but

produces another peak between 4 .5 and 5 keY as a result of titanium fluorescence.

The aluminum layer absorbs the titanium peak, and the resulting aluminum

fluorescence does not have sufficient energy to impact sample analysis. Figure

12.3 shows the difference in background and increased sensitivity for the same

obsidian sample analyzed under the same conditions with and without the filter.

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16000

14000

4j12000 c c 210000 v

~8000 tl § 6000 0

~ 4000 15 ~ 2000

0

0 S 10 IS 20 2S 30 3S 40 KeV

Figure 12.3: S~tra of the same large flake of obsidian showing the difference in background when using the coppeHitanium-aluminum filter (solid line) versus without the filter (dashed line). The filter reduces the total number ofX:7aYs hitting the sample so the filtered data have been multiplied eight times. Note the minimal background under the peaks between 13 and 17 keV that correspond to the elements rubidium to niobium.

Normalization and small/thin sample analysis

Samples selected for XRF analysis should ideally be infinitely thick fortheelements

of interest. Infinite thickness is defined as the thickness at which additional sample

thickness does not result in additional fluorescent X-rays . lnfinite thickness is

different for each element in direct correlation with the excitation energy and

varies between matrices. although sample matrix differences are not addressed

here because obsidians are relatively similar in their overall high-silica matrix.

For example, 99 percent of the iron fluorescent X-rays (6.4 keY) are from up to

0.1 mm from the surface of a pure silica matrix (obsidian is close to a pure silica

matrix). The X-rays with the highest probability of exciting iron have relatively

low energy and thus arc not capable of penetrating deep into a sample, and even if

they did fluoresce an iron atom deep in the sample, it is unlikely for the low energy

fluorescent X-ray to make it back through a thick sample and into the detector.

While iron peaks in a spectrum result primarily from nuorescence at the surface

of the sample. under the same conditions, zirconium Ka X-rays (15.8 keY) escape

from up to 2.2 mm deep, and barium Ka X -rays (32 .2 keY) from up to 17 mOl

deep.


The differences in infinite thicknesses are critical 10 an understanding of the

difficulties in analyzing thin samples (I will expand on the analysis of thin samples

in the following seclion), but normalization can help account for some of the issues

related to sample thickness variability. Normalization was initia lly developed as

a means to neutralize the effects of small fluctuations in the output of the X-ray

sources. The method involves selecting a region of the spectrum that does not have

peaks derived from elements of interest in the sample and that directly reneel the

amount and energy of X-rays hitting the sample. The Bruker Tracer lll-V PXRF

in use at MURR uses a rhodium source and thus produces a characteristic peak of

X-rays for Rh Ka at approximately 20.2 keY due to Raleigh scatter of the source

X-rays on the sample. This interaction of Rh X-rays with the sample produces

both elastic and inelastic scatter resulting in both the Raleigh (elastic scalter at -

20.2 keY as mentioned above) and Compton (inelastic scalter at approx imately

18.5-19.5 keY) peaks being produced in the spectrum based on infinitely thick

samples. In order to gel quantitative concentrations for a batch of sarnpl e~ it is

necessary to normalize the peak areas in the rhodium Compton peak and apply the

same correct ion to the other peaks in the spectra. This minimizes the differences

in the incoming X-ray profile.

While the Compton peak serves well for normalization in most obsidians. there

are certain sources with elevated niobium concentrations that have a sufficiently

large niobium K~ peak to interfere with the rhodium Compton peak area . In

these cases, the calibration interprets the additional peak area from niobium as

increased X-ray output requiring comparable reduction in all peaks in the spectra.

Obsidians with such elevated niobium concentrations are rare worldwide. but they

are common for the sources in the Rift Valley of Kenya. For Kenyan obsidian

a slightly modified version of the calibration that normalizes to a portion of the

backscatter in a region above obvious elemental interferences is used. This avoidS

the niobium effects on normalization. but it moves the normalization region

even farther from the peaks for the main elements of interest (Rb-Nb) making

correction for sample size even more difficult. XRF calibration requ ires a series

of compromises to arrive at the best possible data. There is no obvious one-size-

fits-all answer.

The same normalization procedure used to account for tube fluctuations can

also be used , with somewhat less success, to account for variabi lity in sample size.

particularly thickness. Many applications of XRF involve thin samples. such as

the analysis of thin films and filters. This type of thin sample is relatively easy to

correct for if the samples have consistent thicknesses and it is possible lO creat~ comparable calibration standards (Markowicz 2008; Piorek 2008). Although It

is possible to create thin slices of obsidian. artifacts have complex morphology

4

X·ray fluorescence of obsidian 415

and thicknesses are not only inconsistent between samples, but across a single

,ample. By understanding the physics behind the fluorescence of thin samples as

well as the potential errors inherent in normalization it is possible to confidently

determine the source of obsid ian artifacts that are on ly a couple millimeters in

diameter and less than 05mm thick. Figure 12.4 shows two overlaid spectra of a

largelthick flake and a smallithin flake fro m the same obsidian cobble.

700

600

~ 500 c ~

-'= ~ 400 [ ~300

'\' .. ~ 0 ~ 200 , g • , >-'- 100

,

•

. ! ~j

NI ., ~::i

0

0 5 10 15 20 25 30 35 40 KeV

~ igure 12.4: Comparison of large (dashed line) and small (solid line) flakes from the same cobble. The large flake covered the entire beam with a thickness of 7mm. and the small flake measured 4 by 3mm and O.4mm thick.

The current solution to small sample analysis at MURR involves a number of strategies. First of all , we use an instrument with a small beam cross-section .

Figure 125 is a briefly exposed dental X-ray of the Bruker Tracer III-V PXRF

compared to that of the benchtop ElvaX. The Bruker beam focu ses on an area of

approxi mately 2 x 3 mm . Although a narrow beam is more susceptible to var iat ion

in heterogenous samples. obsidian is usually homogenous enough to not create any

problems. The beam in the Tracer III-V is large compared to that of a micro-XRF

and small beam use has great potential for the analysis of obsidian microdebitage.

The small beam improves the analysis of samples with a small diameter by simply

Covering more of the beam. If samples were infirtitely thick, the small sample

diameter would easily be accounted for by the typical normalization process and

no data shift should occur.

416 Jeffrey R Ferguson

I I Smm

Figure 12.5: Dental X-ray film brieOy exposed to the Elva-X (left) and Tracer III-V (right) beams. Note the much smaller beam area of the Tracer.

The real problems arise with the analysis of samples of less than infinite thickness.

For the purposes of this discussion, consider an example in which we are only

concerned with obtaining concentration data for iron and niobium. The niobium

Ka, peak is between 16.3 and 16.9 keY, and this is relatively close to the rhodium

Compton peak used for normalization that is between 18.5 and 19.5 keY. The

closer the two peaks are in energy, the closer they are to having the same infinite

thickness value, thus the two peaks are absorbed at simi lar rates with increases in

sample thickness. When the calibration normalizes to the rhodium Compton peak,

and the same correction factor is applied to the niobium peak the quantitative

results are reasonable. But , iron has an infinite thickness in obsidian measured in

microns , and thus the size of the iron peak does not change as a function of sample

thickness until the samples are extremely thin. This may not seem like a problem,

but the normalization applies the same correction to all peak areas. whether it

is needed or not. Therefore, in a sample thin enough to generale only half the

rhodium Compton peak area. the niobium peak area (and thus the calcu lated

concentration) will be correctly increased by a factor of two, but the iron peak arca

will be significantly over-corrected. The primary elements of interest in obsidian

characterization, Rb-Nb, are all relatively close to the rhodium Compton peak. and

the normalization does a good job of correcting for sample size although there is an

increasing overcorrection moving down in energy. This overcorrection is extreme

for low-energy elements like iron. When analyzing assemblages that include small

samples, it is best to discount data from all elements with fluorescence energie~

below that of rubidium.

It should now be clear why the calibration discussed previously that account,

for the elevated niobium levels in some East African obsidians is an example of

X~ray fluorescence of obsidian 41 7

a difficult compromise. The East African calibration sacrifices accuracy of small

sample analysis because of the higher energy of the normalization region in order

to avoid the negative effects of high niobium concentrations. In other areas of the

world. namely the western United States, the niobium levels are low enough that

sample size is more important and one should use the calibration that normalizes

to the rhodium Compton region.

The combination of the variably erroneous normalization correction in thin

samples and the overall reduced counts for small/ thin samples causes a slight

shift in the data the correlates with sample size. It can make source assignment

by whatever means (i.e. bivariate plots, cluster analysis, Mahalanobis distance

projection) more difficult. Bivariate plots can show the correlation in the scattered

data that should co-vary with the correct source data across all plots for clements

Rb-Nb. It is very important to confirm the source assignment with direct spectral

overlay, keeping in mind the expected shifts resulting from small samples.

A number of recenr-projects on obsidian from a number of si tes in central and

southern California has recently been conducted at MURR to demonmate that

XRF is effective for small sample source assignment. Of the more than 1,000

artifacts analyzed by XRF, 40 of the smallest and most difficult to assign samples

were selected for additional analysis by short-irradiation NAA . The advantages of

the short NAA are that the samples become only sl ightly radioactive and can be

safely handled after on ly a brief decay period , the technique is a bulk technique that

is not subjected to variations in sample morphology because the samplc rotated ,

and it can produce precise data for about six elements on samples of only a few

milligrams. Of the forty samples, XRF source ass ignment was confirmed for 38

of them . One sample was not assigned to any source by XRF but was assigned by

NAA, and one sample was incorrectly assigned by XRF to a source with similar

composition. This error rate of five percent is higher than one would like, but it

occurs on the smallest and most challenging samples. The error rate for the entire

study, including the larger artifacts, would likely be much lower. Similar tests have

been done with smaller assemblages from the American Southwcst with similarly

encouraging resu lts.

The current approach to small sample analysis by XRF is not yet an ideal

solution. Attempts have been made to create an element-specific correction

factor that uses the rhodium Compton peak as an indicator of sample mass. One

possibil ity is to lise a two-element standard behind the artifact that provides a

good approximation of the thickness of the sample in the beam. The standard

would contain sufficient concentrations of two elements, one with a peak below

the elements of interest and one above. In some preliminary tests using a pressed

pellet containing gallium and indium in which the lower energy element (in this


case gallium) would have a peak area reduced in relation to the higher energy

e lement (indium) in a ratio reOecting sample mass. This method has some promise.

but it is time consuming to keep the standard in proper position and this would

also greatly increase the time required to interpret the final data. It is important to

verify source assignments. particularly with small samples. using other analytical

techniques such as NAA or LA-ICP-MS, but the ultimate solution to small artifact

analysis may involve micro-XRF.

Conclusions

XRF has held a prominent position in the provenance study of archaeological

obsidian for decades, and all indications are that it wil l continue to be a major

analytical technique. Handheld XRF instruments provide some logistical

advantages over lab-based instruments, but aside from a few, relatively minor,

limjtations the two groups of instruments pose the same analytical potential as

well as limitations. Handheld XRF has been the unfair subject of recent skepticism

in the literature (i.e. Shackley 20 I 0). The problems arise not from the portability of

a particular instrument. but from the lower cost of most portable instruments that

places the technology in the hands of researchers with insufficient understanding

of the physics, calibration methods , and analytical limitations. This chapter begi ns

to address some of these problems .

Notes

I . A completely new set of calibration Mandards is in preparation that will cover the kno\\n concentration ranges in obsidian for the elements iron , rubidium , strontium , yttrium. zirconium, niobium and barium. This new calibration set wi ll include 40 obsidian samples from around the world. The concentrations wi ll be derived from a combination NAA, microwave digestion ICP-MS. and XRF analysis of the individual rocks. A publication detailing the concentrations of the originaJ set of caJibrat ion standards in currently in preparation , and considering it will soon be obsolete upon the completion of the new set of calibration standards the values are not reported here.

X-ray fiuorescence of obsidian 41 9

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