DOC PREVIEW
UW-Madison G 777 - Correction of secondary X-ray fluorescence near grain boundaries in electron microprobe analysis

This preview shows page 1-2-3 out of 10 pages.

Save
View full document
View full document
Premium Document
Do you want full access? Go Premium and unlock all 10 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 10 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 10 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 10 pages.
Access to all documents
Download any document
Ad free experience

Unformatted text preview:

American Mineralogist, Volume 88, pages 121–130, 20030003-004X/03/0001–121$05.00 121INTRODUCTIONElectron microprobe analysis (EMPA) is a reliable, non-destructive technique for the quantitative analysis of rock-form-ing minerals (see e.g., Reed 1993). The technique is based onthe measurement of characteristic X-ray intensities emitted bythe elements that make up the mineral when it is bombardedwith a focused electron beam. For each element, the ratio ofthe X-ray intensity emitted from an unknown to that emittedfrom a standard of known composition, measured under thesame analytical conditions is, to a first approximation, propor-tional to the ratio of elemental concentrations. A more accuratevalue of the concentration is obtained by correcting for the dif-ferences in electron and photon transport between the unknownmineral and the standard, using the so-called “matrix correc-tion” procedures.Matrix correction procedures usually assume that the ana-lyzed sample region from which X-rays are emitted has a ho-mogeneous composition. The volume of this region can beestimated by calculating the effective penetration range of in-cident electrons. For keV electron beams, this range is typi-cally of the order of several micrometers, depending on thematerial and the analytical conditions. Accordingly, EMPA isadequate for analyzing homogeneous mineral grains that havea diameter larger than several micrometers. However, the rangeof characteristic X-rays and Bremsstrahlung that are generatedby incident electrons, is typically one to two orders-of-magni-tude greater, namely several tens or hundreds of micrometers.Consequently, although the electron beam impacts quite a dis-tance from the grain boundary, X-rays may reach a neighbor-ing phase and induce further ionizations (if the photon energyis larger than the binding energy of the considered atomic shell)causing the emission of characteristic X-rays, and this processis usually referred to as secondary fluorescence (SF). As ma-trix corrections assume chemical homogeneity, the contribu-tion of SF coming from the adjacent mineral will not be takeninto account and will lead to an erroneous concentration. For-tunately, this effect is generally small and it can be disregarded.However, in certain situations, SF coming from a neighboringphase should be taken into consideration carefully. One suchsituation is the analysis for a minor or trace element in amineral that coexists with another mineral containing theelement of interest. In this case, even if we analyze the min-eral relatively far from the boundary, SF may affect the re-sults not only qualitatively but also quantitatively. Examplesof interest in geology include the analysis for Ti in chromitein contact with ilmenite (Maaskant and Kaper 1991), Ca inolivine close to clinopyroxene (Dalton and Lane 1996), orTi in garnet close to rutile or ilmenite inclusions (Feenstraand Engi 1998).The contribution of SF can be minimized by using L-linesinstead of K-lines in the quantitative procedure (Pouchou 1996).* E-mail: [email protected] of secondary X-ray fluorescence near grain boundaries in electron microprobeanalysis: Application to thermobarometry of spinel lherzolitesXAVIER LLOVET1,* AND GUMER GALAN21Serveis Cientificotècnics, Universitat de Barcelona. Lluís Solé i Sabarís, 1-3. 08028 Barcelona, Spain2Departament de Geologia, Universitat Autónoma de Barcelona. Edifici C (S). 08193 Bellaterra, Barcelona, SpainABSTRACTA correction procedure is proposed to account for the effect of secondary X-ray fluorescencenear grain boundaries in electron microprobe analysis. The procedure is based on the Monte Carlosimulation method, which is used to calculate the X-ray spectrum emitted by the mineral couple(i.e., the mineral of interest with the neighboring mineral). The contribution of secondary fluores-cence from the neighboring mineral, which appears in the simulated spectrum in a natural way, isthen subtracted from the measured k-ratio and thereafter conventional matrix corrections are ap-plied. The Monte Carlo simulation algorithm used is largely based on the general-purpose simu-lation package PENELOPE. In order to assess the reliability of this code, simulated “apparent”element profiles are compared with electron microprobe measurements found in the literature, inwhich the effect of secondary fluorescence was characterized and the reliability of the differentassumptions underlying the proposed procedure is discussed.Finally, the procedure is used to assess data from the olivine-clinopyroxene thermobarometer ina spinel lherzolite xenolith. The application of the secondary fluorescence correction leads to: (1)higher systematic pressure estimations than those obtained from uncorrected data, with lower uncer-tainties; and (2) a better agreement between the olivine-clinopyroxene temperature estimations andthose estimated using the two-pyroxene thermometer. Estimated P-T conditions indicate adecompressional path with a slight decrease in temperature from core to crystal rim. However, if theeffect of secondary fluorescence is not taken into account, an apparent heating event is observed.LLOVET AND GALAN: CORRECTION OF SECONDARY FLUORESCENCE122However, measurement and quantification using L-lines is dif-ficult and may lead to larger uncertainties. Besides, for someelements such as Ca, the use of the L-line is not a realistic al-ternative. Several methods have been proposed for correctingSF near phase boundaries (see Dalton and Lane 1996 and ref-erences therein). For example, Bastin et al. (1983) developed anumerical method to correct for SF effects in metal couplesthat can be incorporated into a matrix correction procedure.Myklebust and Newbury (1994) used the Monte Carlo (MC)simulation of electron transport in metal couples to predict SFinduced at phase boundaries. These numerical corrections wereapplied mainly to single-element materials and, moreover, SFinduced by the continuum was approximated by very crudemethods or even neglected.In this work, a new correction method is proposed to ac-count for the effect of SF near phase boundaries. The methodis based on the simulation of the full X-ray spectrum emittedby the mineral couple, using the MC simulation method. TheMC algorithm used is the one developed by Acosta et al. (1998),which is based on a modified version of the general-purposesimulation package PENELOPE (Baró et al. 1995). This codeallows the simulation of coupled


View Full Document

UW-Madison G 777 - Correction of secondary X-ray fluorescence near grain boundaries in electron microprobe analysis

Documents in this Course
Load more
Download Correction of secondary X-ray fluorescence near grain boundaries in electron microprobe analysis
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Correction of secondary X-ray fluorescence near grain boundaries in electron microprobe analysis and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Correction of secondary X-ray fluorescence near grain boundaries in electron microprobe analysis 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?