Ion Microprobe Techniques and Analyses of Olivine and Low-Ca
American Mineralogist, Volume 66, pages 526-546, l98l Ion microprobetechniques and analysesof olivine and low-ca pyroxene I. M. SreBrE, R. L. HnRvIc, I. D. HurcHEoN AND J. V. Snanu Department of Geophysical Sciences, (Iniversity of Chicago Chicago, Illinois 60637 Abstract Experimentalconditions for major, minor, and trace elementanalysis of olivine and low- Ca pyroxene are describedand analytical accuracy tested using suites of natural samples spanninga wide range of Mg/Fe. Special attention was given to sample cleanlinessto avoid contamination, instrumental vacuum to eliminate hydrides in the secondaryion spectrum,and samplepreparation to give good precisionin measuredintensities. Especially bothersome were molecular interferences in the secondaryion spectrumwhich were separatedfrom analytical peaksusing massresolu- tion (M/AM) over 3000.Careful analysisof the secondaryion spectrumallowed choiceof ei- ther high or low massresolution depending on the presenceof interferences.Lithium (0.005), F (l?), Na (0.01),Mg, Al (0.1),Si, P (5), K (0.05),Mn (0.1),Fe, and Co (l) were analyzedat low resolutionwith detectionlimits (ppm) in parentheses.Elements requiring high resolution includeCa (l), Sc (l), Ti (l), V (l), Cr (l) and Ni (5). The secondary-ionintensities for Mg and Si do not correlate linearly with composition whereasFe is nearly linear. The simple relation of the Mgl(Mg + Fe) ratio to the measured secondaryion ratio Mg*/(Mg* + Fe*) enabled major element determination to within +l mol% of forsterite or enstatitecontent. The yield of Mg and Fe secondaryions is a complex function of composition,and Mg*/Si* and Fe*/Si* are not simply related to atomic ratios in the target. To test accuracyof minor elementdetermination as a function of major elementvariation, secondary-ioniatensities were compared with compositionsbased on electron probe mea- surements.Some elements (Al, Cr, Ti, Mn) give a linear relationshipwith no obvious matrix eflect,but Ni, and possiblyCa and Co, definitely dependon the matrix.
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