Comparison between major and trace element ...

Spectrochimica Acta Part B 58 (2003) 699–709

Comparison between major and trace element concentrations in garnet performed by EPMA and micro-PIXE techniques夞 G. Vaggellia,*, A. Borghib, R. Cossioc, C. Mazzolid, F. Olmia a CNR, Istituto di Geoscienze e Georisorse, Sezione di Firenze. Via La Pira, 4 I-50121 Firenze, Italy Dipartimento di Scienze Mineralogiche e Petrologiche, Universita` di Torino. Via V. Caluso, 37, I-10100 Torino, Italy c Dipartimento di Scienze della Terra, Universita` di Torino. Via V. Caluso, 37, I-10100 Torino, Italy d Dipartimento di Mineralogia e Petrologia – Corso Garibaldi 37, Universita` di Padova, I-53100 Padua, Italy

b

Received 16 October 2001; accepted 16 July 2002

Abstract This paper deals with the application of the electron and proton microprobe (EPMA and micro-PIXE) in order to determine major and trace elements in metamorphic garnet samples. The selected garnet samples come from a metapelitic rock belonging to the tectonic unit of Monte Rosa Nappe (Italy). Quantitative spot analysis profiles and compositional X-ray maps of both major and trace (yttrium) elements are reported. Major elements show a smoothed and continuous compositional zoning characterized by concentric variations from core to rim. Yttrium displays a strong enrichment in the core and a flat pattern at the rim. No correlation is shown between major and yttrium distribution. Only a rough correlation may be supposed with manganese. A good agreement was found between EPMA and micro-PIXE yttrium data, for values higher than 80–100 ppm. For lower values micro-PIXE technique is strongly recommended. The combination of X-ray two-dimensional maps and quantitative microanalyses allowed to evaluate the distribution of major and trace elements in a petrologically significative rock-forming mineral, to define the type of the chemical zoning and finally to attest difference in diffusivity between major and trace elements. Because of the slow diffusivity of yttrium, its quantitative determination in garnets is fundamental to reconstruct the temperature path suffered by polymetamorphic garnet-bearing rocks. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: EPMA; Micro-PIXE; Garnet; Yttrium; Trace elements

1. Introduction

夞 This paper was presented at the 16th International Conference on X-Ray Optics and Microanalysis (ICXOM-XVI), held in Vienna, Austria, July 2001, and is published in the Special Issue of Spectrochimica Acta Part B, dedicated to that conference. *Corresponding author. Tel.: q39-55-275-6203; fax: q3955-2903-12. E-mail address: [email protected] (G. Vaggelli).

In mineralogy and petrology it is important to understand and to quantify the distribution of elements in rocks and minerals. In fact, the chemical composition of rock-forming minerals reflects their crystallization history and provides information on the temperature and pressure conditions during their formation w1x. Thus, the determination of major and trace element content in mineral phases has long been regarded as a useful tool in

0584-8547/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0584-8547(02)00278-1

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G. Vaggelli et al. / Spectrochimica Acta Part B 58 (2003) 699–709

understanding petrologic processes as they can be used to reconstruct equilibrium and pressure–temperature conditions of crystallization and growth. In the past only major element composition was able to be performed by non-destructive analytical procedures. However, at high temperature major element growth zoning may be significantly modified by intra-crystalline diffusion process w2x. Consequently, the study of distribution of trace elements (i.e. from a geological perspective, those occurring in abundance -1000 ppm), which may be less susceptible to diffusion modification w3x, becomes of fundamental importance. Partitioning of trace elements between mineralogical phases is controlled by physical, chemical and kinetic conditions of crystallization and differentiation. In particular, the capability of trace elements to record physical changes and their resistance to diffusive re-equilibration are well known w4x. For this reason, in the Earth Science field it is nowadays fundamental to perform spot analyses with good spatial resolution and with a detection limit of the order of ppm. The easiest and more commonly used way to carefully determine the spatial distribution of major elements is represented by X-ray compositional maps performed by EPMA w1x. However, the statistical error greatly increases at low concentrations, and detection limit is usually greater than few hundreds of parts per million w5x. Therefore, the abundance of trace elements is generally assessed by means of secondary ion mass spectrometry. Although this analytical tool is extremely powerful both in terms of detection limit and dwell time, the main disadvantage is related to the dimension of the spot, which is generally in tenths of micrometer. Proton induced X-ray emission microanalysis (micro-PIXE) combines the ability to quantitatively determine trace element concentrations with a high spatial resolution w6x. In addition, micro-PIXE is capable of simultaneous, multielementary analysis, by means of an energy dispersive spectroscopy system, with better signal-to-noise ratio and lower minimum detection limit (MDL) for most elements, compared to EPMA. These major advantages of the proton microprobe mainly lie in its

much lower X-ray background, due to a ‘bremsstrahlung’ effect, which is lower by several orders of magnitude, leading to MDL approximately 2 ppm for LILE and HFSE elements. For this reason this technique has great potentiality in the solution of many different problems in the field of the Earth Sciences, although it has been rarely applied by geologists w7,8x. The potential of PIXE microanalysis in geosciences is enhanced when the proton microprobe data are integrated with those obtained by electron microprobe and petrographic microscope. In this paper we report a comparison between major and trace element concentrations in garnet, a typical metamorphic rock-forming mineral. The compositional zoning of garnet, which is a solid solution of various mineralogical end-members wA3B2(SiO4)3, AsCa, Mg, Fe2q, Mn2q; BsAl, Cr3q, Fe3q)x, is a key piece of information in metamorphic petrology because the chemical zoning preserved in the garnet porphyroblasts potentially records the changes in the petrogenetic history of the hosting rock w9x. We focused our attention on possible relationships between the zoning patterns for the four major elements involved in the exchange processes in the structural A site and Y distribution. Thus, we present quantitative spot analyses and X-ray maps of both major elements and Y trace element, performed by electron and proton microprobe. 2. Methodology The study in thin sections of the considered samples by means of an optical microscope allowed to identify garnet crystals with suitable microstructural features and to select the sites for the required microprobe analyses. Samples were then coated with a carbon film to ensure reliable beam current integration and analyzed by EPMA, in order to determine major and trace element distribution and to provide an indication on the heterogeneity of the crystals. Finally, micro-PIXE spot analyses were performed at selected sites on the same garnet grains to obtain trace element data. For both electron and proton microprobe analyses, rock samples were prepared as polished 50mm thin sections.


2.1. EPMA analyses Electron microanalyses of major and trace elements were performed at the Istituto di Geoscienze e Georisorse, Sezione di Firenze, with a Jeol JXA 8600 microprobe equipped with four wavelength dispersive spectrometers (WDS). As the MDL of a given element mainly depends on the counting statistics, appropriate instrumental settings have been chosen for the different analyzed elements on the basis of the expected concentration. More in detail, the following analytical conditions have been applied (Table 1): an accelerating potential of 15 kV, a Faraday cup current of 500 nA, a defocused beam of 10 mm in diameter and acquisition times (at the peak and background position) of 120 s for Y, 10 s for Si and Al, and 5 s for the other major elements. Si and Al were collected using Ka II order X-ray lines in order to prevent the detector saturation. Y was acquired measuring YLa X-ray line with a PET crystal analyzer. With these analytical conditions a MDL for Y approximately 50 ppm was achieved. ZAF matrix correction routine was used to convert counts into concentration by means of the TN-56 TRACOR-NORTHERN software package. Natural silicates and oxides were used as primary standards. In particular, for quantitative Y analyses a synthetic Y-garnet (Y3Al5O12) was used as primary standard. 2.2. X-ray maps As regards X-ray bidimensional maps, they were collected with the same Jeol JXA 8600 microprobe by means of an automated mapping program. A framework of spot peak counts was collected using a remote control system on the stage movements and keeping spectrometers at fixed positions. Operating conditions include an accelerating potential of 15 kV, a Faraday cup current of 2 mA, a defocused beam spot of 10 mm in diameter and a dwell-time of 2 sypixel with step size of 10 mmy pixel. For a spatial resolution of 128=128 pixel, a total acquisition time of approximately 48 h was required. The results consist of a set of numerical matrixes containing the X-ray counts collected at their peak

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positions. Raw peak counts are corrected by subtracting the background counts calculated using an estimation of the background level based on the average atomic number of the garnet w10x. The net peak counts are then computed by means of PETROMAP, a PC-dedicated software program allowing quantitative ZAF correction w11x. In this way, each pixel of the map represents a ZAFprocessed WDS quantitative analysis, with its own precision and accuracy. 2.3. Micro-PIXE facility Y quantitative trace analyses were performed by a 2.5 MeV Van de Graaff accelerator, with a highresolution proton scanning microprobe consisting of a triplet of magnetic quadrupoles, installed at the ‘Laboratorio Nazionale di Legnaro’ (LNL, Padova, Italy) of the ‘Istituto Nazionale di Fisica Nucleare’ (INFN). We refer to Ref. w12x for details on the equipment. The system is equipped with an energy dispersive Si(Li) Oxford Pentafet detector with a nominal resolution of 140 eV at Mn Ka energy, a nominal surface area of 80 mm2 and a thickness of 4.5 mm. To satisfy the requirement of high data rates, a close detection geometry is used with detector mounted at 458 to the beam. The PIXE X-ray spectrum of mineralogical samples is generally dominated by the intense low energy lines of major elements. To significantly reduce their contribution, a 250-mm thick Al filter is interposed between the sample and the detector. Consequently, beam current can be greatly increased from the usual hundreds of picoampere to 5–8 nA at 2 MeV of proton beam energy, in order to increase the count rate, reduce the counting time and enhance the fraction of counts in the energy window of the trace element emission range. In this way, the detection limit for many trace elements significantly improves, allowing the measurement of their concentration with accuracy better than 10% w13x. The use of such a filter for optimizing the determination of trace elements with Z)26, precludes the accurate analysis of minor elements with Z-26 because their X-rays are heavily attenuated or entirely removed. This is not a drawback,

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Table 1 Summarizing sketch of analytical conditions used for EPMA and micro-PIXE analyses

Analyzing crystals with the detected elements

Correction method Micro-PIXE analyses Micro-PIXE X-ray detector Proton beam energy Proton beam current Probe diameter Total acquisition time Total collected X-ray counts Processing program X-ray maps Electron microprobe Accelerating voltage Beam current Beam spot diameter Step size Spatial resolution Counting times at peak position Total acquisition time Analyzing crystals with the detected elements

Correction method

Jeol JXA-8600 15 kV 500 nA (monitored on a Faraday cup) Defocused beam (10 mm) 120 s for Y 10 s for Si and Al 5 s for Ca, Fe and Mn TAP: Mg Ka; Si Ka (II order) and Al Ka (II order) PET: Y La, Ca Ka LiF: Fe Ka and Mn Ka ZAF Van de Graaff accelerator Si(Li) Oxford Pentafet resolution of 140 eV 2 MeV 5–8 nA 10 mm 1200 s f500.000 GUPIX

Jeol JXA-8600 15 kV 2 mA (monitored on a Faraday cup) defocused beam (10 mm) 10 mmypixel 128=128 2 sypixel 48 h TAP: Mg Ka and Ca Ka PET: YLa LiF: Fe Ka or Mn Ka ZAF


WDS analyses Electron microprobe Accelerating voltage Beam current Beam spot diameter Counting times at peak position (the same times at both backgrounds, as total)


because these elements can be determined with easy accuracy by EPMA. These analytical conditions imply a loss of spatial resolution, corresponding to a probe diameter of 10 mm. However, this fact does not represent a problem in most of the geological applications. In fact, the penetration depth of the proton beam is approximately 50 mm for silicate minerals, although the effective depth of analysis is less (f25 mm) because of the self-absorption effects and the rapidly decreasing excitation energy, as soon as the beam loses energy with depth. It is, therefore, desirable to select larger crystals ()30 mm) for analysis to avoid the possible contribution of adjacent crystals which may be present at shallow depth underneath the analyzed spot. The spot size has been determined by evaluating the elemental map on a copper mesh and found to be around the expected value. For each spot analysis an acquisition time of 1200 s, corresponding to approximately 500.000 X-ray counts, was used for trace element acquisition. The level of beam current is controlled to maintain the count rate in the detector at approximately 2000 cps, in order to reduce pile-up effects which may obscure X-ray peaks of interest. In addition, line profiles across the garnet sample were acquired with a spatial step of 20 mm, using a dwell time corresponding to 60 sypoint. 3. Standardization reduction

and

data

processing

The analytical micro-PIXE spectra were processed by GUPIX, a dedicated software developed at the University of Guelph (Ontario, Canada) w14,15x. This program performs standardless analyses by means of a sophisticated peak-stripping and modeling routine based on a judicious use of a well-known atomic physics database. Standardization in micro-PIXE system for trace element analysis is usually accomplished by one of the two following methods: (a) the PIXEderived concentrations can be normalized to the known concentration of a major or a minor element in the specimen, independently measured by another method (i.e. EPMA) w16x; (b) the system can be calibrated on the basis of an external standard

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Fig. 1. GUPIX reduced micro-PIXE spectrum from inox steel AISI 316S containing 1.98 wt.% of molybdenum, used for H value determination. The model spectrum of the characteristic X-rays produced by GUPIX is shown as a continuous curve.

whose elemental composition is specified by an accredited agency w17,18x. Czamanske et al. w13x described an alternative variant to the latter approach in the specific context of measuring trace element concentrations in mineral phases. It is based on the accurate determination of the instrumental constant H, which represents the product of detector solid angle times a calibration factor including all system corrections related to the charge measurement. It follows that if the detector and absorber are well characterized, the instrumental constant H can be determined with a minimal set of standards. Following recommendations reported by previous authors w18,13x a Mo-bearing standard reference material was selected for calibrating the micro-PIXE system, because the energies of the Mo Ka lines are sufficiently high that their attenuation by the Al absorber is small. A concentration of a few percent is enough to obtain adequate spectrum intensities and corresponding statistical counting uncertainty. For this study the H value was determined on the basis of 1.98 wt.% Mo contained in the standard reference inox steel AISI 316 S, provided by the Dipartimento di Meccanica (University of Firenze, Italy). Five spots were analyzed over an area of 100=100 mm2 to average heterogeneity. Characteristic X-ray intensities for Fe, Ni, Cu, Mo and other minor elements obtained by means of the GUPIX program are reported in Fig. 1. The average H value for each element was then obtained from equation (1) of w18x. In obtaining the initial set of H values, the nominal thickness of 250 mm of the Al


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Fig. 2. GUPIX reduced micro-PIXE spectrum from the glass standard NIST 610. The model spectrum of the characteristic X-rays produced by GUPIX is shown as a continuous curve. The peaks corresponding to elements reported in Table 2 are labelled.

absorber was adopted. These H values resulted to be different for Fe and Mo, indicating that the nominal thickness value of the absorber needed to be adjusted. The known Fe content allowed the refinement of the Al absorber thickness to 257 mm for an H value of 0.035 srad. In addition, in order to check the accuracy and precision of the Legnaro micro-PIXE facility for trace element determination in silicate materials, we also measured the standard glass NIST 610, which is a silicate glass containing 61 trace elements with a nominal concentration of 500 ppm. Four different spots on the same sample were analyzed. Measurements were performed in two

stages: (a) major elements were determined by electron microprobe; (b) trace elements were evaluated by micro-PIXE. In Fig. 2 a spectrum of the standard glass NIST 610 is reported. In Table 2, the average value of four spot analyses, comprehensive of S.D. and MDL values, are reported and compared with the certified values only for the elements where NIST supplies a recommendation for content. We found a good agreement between reported vs. certified composition for elements with Z)29 (Cu). The relative percent precision for any single elements (S.D.y mean=100) is reported. The last column shows the dispersion on the element concentration and allows verifying their consistency with the certified data. Therefore, the reported data confirm that the micro-PIXE technique shows a good precision for elements between Zn (Zs30) and Cd (Zs48), and a good consistency with other micro-PIXE data on the standard glass NIST 610 reported by other authors w13,19x. 4. The geological problem The application of a multianalytical approach (optical microscopy, EPMA, micro-PIXE) to a specific geological problem, allowed to overcome the limitations of each of the considered analytical techniques.

Table 2 Microchemical analyses of NIST 610 glass standard performed by micro-PIXE Element

Mean (ppm)

S.D. (ppm)

MDL (ppm)

ref. (ppm)

26 27 28 29 30 37 38 47 82 90 92

459.8 360.9 411.3 375.7 435.1 422.6 511 228.5 414.7 480.5 521.5

3.63 13.65 7.19 13.18 8.95 6.45 14.03 13.09 9.06 7.13 5.11

37.7 22.5 13.6 14.1 6.6 6.2 2.7 10.3 57.2 38.1 20.6

458 390 458.7 444 433 425.7 515.5 254 426 457.2 461.5

Fe Ka Co Ka Ni Ka Cu Ka Zn Ka Rb Ka Sr Ka Ag Ka Pb La Th La U La

Err. ref. 9 4 4 0.8 0.5 10 1 1.2 1.1

% S.D.ymean

Meanyref.

0.8 3.8 1.7 3.5 2.1 1.5 2.7 5.7 2.2 1.5 1.0

1.004 0.925 0.897 0.846 1.005 0.993 0.991 0.900 0.973 1.051 1.130

Concentration values are expressed as ppm. Mean—average value of four spot analyses; S.D.—standard deviation; MDL— minimum detection limit; ref.—certified value; Err. ref.—certified value range.


The aim of our research is to investigate the distribution of petrologically relevant major and trace elements in garnet porphyroblasts from metamorphic rocks. On the basis of EPMA data from natural samples, in which temperature has been independently determined by means of geothermometers based on exchange reactions among adjacent mineral phases, Pyle and Spear w20x recently calibrated a geothermometer based on the Y content in garnet, in the presence of a buffering phase (i.e. xenotime YPO4), predicting that the maximum amount of Y is a function of temperature and it exponentially decreases with increasing T. For this reason we selected strongly zoned garnet crystals from metapelites outcropping in high pressure and low temperature (eclogite facies) metamorphic terrains from Western Alps (Italy). These samples come from well known geological areas (Monte Rosa Nappe), in which the evolution of the pressure and temperature with time is

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independently constrained on the basis of petrological data w21x. They are made up of quartz, white mica, albite and minor biotite, garnet and chloritoid, and preserve relics of a former metamorphic event developed under eclogite-facies metamorphic conditions at high pressure (1.2–1.4 GPa) and low temperature (480–500 8C), which was pervasively overprinted by a subsequent metamorphic event under amphibolite-facies conditions, along a prograde (i.e. with increasing temperature) and decompressional path, reaching climax conditions at T above 600 8C and pressure under 0.5 GPa. A large garnet crystal (2 mm in diameter), with respect to the grain-size of surrounding mineral phases, was selected. As evidenced by the backscattered image of Fig. 3, it shows an internal microstructure defined by small quartz grains arranged in such a way that an idiomorphic core, which crystallized during the eclogitic event, can

Fig. 3. SEM backscattered digitized image of the selected garnet sample. Quartz inclusions define two garnet generations boundary. Scale bars1 mm. GRT I, first garnet generation; GRT II, second garnet generation. AA, analyzed profile.

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Fig. 4. WDS quantitative compositional maps for the selected garnet sample of Fig. 3. Major chemical elements (Mn, Mg, Ca) expressed as wt.% oxide; yttrium (Y) expressed as ppm. The concentration range of each map is divided in discrete pseudocolor classes.

be distinguished from the rim, which grew during the following amphibolitic event. 5. Results and discussion Four major elements (Mg, Ca, Mn, Fe) present in A site have been mapped by EPMA, along with Y, which is present as trace element (Fig. 4). Representative core to rim profiles across garnet crystals and single spot analyses were also collected by EPMA and micro-PIXE (Figs. 5 and 6; Table 3). The considered garnet crystal shows welldefined compositional zoning, characterized by a smooth and concentric variation of the selected elements from core to rim (Figs. 4 and 5). In particular, from the core towards the rim, Fe and Mg contents, recalculated on the basis of 12

oxygens, increase from 2.248 to 2.331 atoms per formula unit (a.p.f.u.) and from 0.319 to 0.599 a.p.f.u., respectively, whereas Mn and Ca concentrations decrease from 0.124 to 0.007 a.p.f.u. and from 0.335 to 0.148 a.p.f.u., respectively (Table 3). This pattern is generally considered a growth zoning developed under low to medium metamorphic grade w9x. In addition, both the map and the profile of Ca show a discontinuous corona of higher concentration relative to adjacent garnet portions, near the rim of the crystal, which is balanced by a decrease in Fe (Figs. 4 and 5). As regards the trace elements distribution, the two-dimensional X-ray map displays a strong Y enrichment in the core, followed by a flat pattern at the inner and outer rim (Fig. 4). It is also evident that the abrupt change in decreasing of Y


Fig. 5. Representative concentration profile of major elements for the selected garnet sample of Fig. 3. (a) Fe content expressed as wt.% oxide. (b) Mg, Mn, Ca expressed as wt.% oxide.

concentration corresponds to the boundary between the two garnet generations previously defined by microstructural evidences. Even the quantitative profile performed along the garnet crystal shows the same Y concentration trend, with a strongly enriched core (corresponding to first garnet generation) and a flat pattern towards the inner and outer rim (corresponding to second garnet generation). In Fig. 6 it is shown the quantitative Y profiles along the selected garnet crystal performed by both EPMA and micro-PIXE. A good agreement is observable between EPMA and micro-PIXE yttrium data for values higher than 80–100 ppm. For lower values micro-PIXE technique is strongly recommended. This pattern is in agreement with what found by w22x for metapelite garnets of low to medium grade metamorphic conditions, where a negative correlation between the Y concentration and temperature is reported.

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Fig. 6. Representative concentration profile of yttrium content acquired by EPMA (a) and micro-PIXE (b). Yttrium content is expressed as ppm.

Y concentration spreads over an interval of two orders of magnitude, from approximately 50 ppm at the rim, to almost 2500 ppm at the core. In Fig. 7 a GUPIX reduced micro-PIXE spectrum of a spot

Fig. 7. GUPIX reduced micro-PIXE spectrum from the garnet spot analysis containing 86 ppm of Y using 2 MeV. The model spectrum of the characteristic X-rays produced by GUPIX is shown as a continuous curve. This spectrum corresponds to analysis 噛7 in Table 3.


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Table 3 Representative microchemical WDS analyses (expressed as wt.% oxide) and cationic calculation (atoms per formula unit based on 12 oxygens) Analysis 噛

1

7

SiO2 Al2O3 FeO MgO MnO CaO Total

37.88 20.44 34.65 4.93 0.12 2.12 100.14

37.95 20.37 33.66 3.81 0.25 4.04 100.08

Si Al Fe Mg Mn Ca Sum cations Y (ppm) MDL (ppm) % error

3.018 1.913 2.331 0.599 0.008 0.157 8.026 56 2.3 6.13

12

3.031 1.917 2.248 0.453 0.017 0.345 8.011 86 2.5 3.93

14

37.28 19.87 33.86 2.68 1.53 3.63 98.85 3.036 1.907 2.306 0.325 0.105 0.316 7.996 1824 4.1 0.89

18

37.45 19.98 33.82 2.69 1.51 3.86 99.31 3.035 1.908 2.292 0.324 0.104 0.335 7.999 1476 3.7 0.99

22

37.29 20.25 33.25 2.7 1.8 3.8 99.09 3.023 1.935 2.254 0.326 0.124 0.330 7.990 2436 4.7 0.77

24

37.65 19.87 33.72 2.75 1.53 3.77 99.29 3.048 1.896 2.283 0.332 0.105 0.327 7.992 1476 3.7 0.99

26

37.3 20.07 33.87 2.64 1.5 3.69 99.07 3.029 1.922 2.300 0.319 0.103 0.321 7.994 1890 4.2 0.88

37.73 20.27 34.48 3.03 0.78 3.44 99.73 3.038 1.924 2.322 0.363 0.053 0.296 7.998 116 2.6 3.54

31

36

37.78 20.22 33.85 3.39 0.43 4.31 99.98

38.2 20.27 34.87 4.95 0.11 1.74 100.14

3.029 1.911 2.270 0.405 0.029 0.371 8.015 99 2.5 3.81

3.041 1.902 2.322 0.587 0.007 0.149 8.008 67 2.4 5.64

Yttrium micro-PIXE concentration and MDL are expressed as ppm; relative error is expressed as %. 噛 analysis is referred to profile of Figs. 5 and 6.

analysis, which contains approximately 86 ppm of Y (analysis 噛7 in Table 3), on the considered garnet crystal is reported. In addition, a narrow Yrich layer has been detected in correspondence of the boundary between the core and the rim of the garnet. The formation of this kind of oscillatory zoning can be explained by garnet re-absorption and contemporaneous back-diffusion of Y into the garnet crystal, followed by re-incorporation of Y during growth of the new garnet generation of different composition w22,23x. This interpretation is in agreement with our microstructural observation, which suggest the development of two garnet generations under different metamorphic conditions. This corona is not detectable from the major element X-ray maps, with the exception of Ca, where a smoothed enrichment is shown, but it developed in an outer portion of the crystal respect to two garnet generations boundary. Comparing the sharp Y pattern distribution with the smoothed profiles shown by major elements, no correlation can be ascertained (Figs. 5 and 6), if we exclude a weak similarity between Mn and Y pattern. As concerning Ca vs. Y distribution, no correlations have been observed, although some

authors w4,24x tend to link their concentrations. In addition, the absence of an abrupt change in the major elements concentration across the core to rim boundary and the sharp and steep Y profile suggests that the diffusivity of Y in garnet was very slow compared to that of major elements, whose composition was strongly modified by diffusion at high temperature. This observation also indicate that Y zoning pattern was not affected by the subsequent growth, and, therefore, Y concentration can be used to reconstruct the thermal history of the host rock, as suggested by Ref. w20x. 6. Conclusions In conclusion, we can point out that the case reported represents a good example where the contemporaneous use of EPMA and micro-PIXE can successfully resolve a problem of petrological pertinence. In particular – a good agreement was found between the electron microprobe and the micro-PIXE data. Single spot analyses are, indeed, comparable within


–

–

–

–

–

the error range also at low values approximately 100 ppm; the compositional X-ray maps provide a detailed evaluation of the distribution of several elements in a single mineralogical phase and can provide useful information on the type and the entity of the chemical zoning; the combination of X-ray maps with quantitative chemical elements profiles underlines the considerable difference in diffusivity between major elements and Y, and provides pieces of information on the extent of the diffusion processes; the slow diffusivity of Y in garnet allows to recognize petrogenetic events which might not be recorded or preserved by major elements, due to high-temperature diffusive re-equilibration processes; Y content in xenotime-bearing metapelites can be related to temperature changes during garnet growth; Y distribution profiles made up of quantitative spot analyses can be used to reconstruct the temperature path suffered by garnet-bearing rocks during metamorphism.

Acknowledgments This study was carried out with the financial support of M.U.R.S.T., of the C.N.R. Istituto di Geoscienze e Georisorse, Sezioni of Firenze and Torino, and of the Istituto Nazionale di Geofisica e Vulcanologia. The authors thank A.P. Santo (Dipartimento di Scienze della Terra, University of Firenze) and G. Pinto (Dipartimento di Meccanica, University of Firenze) for providing NIST 610 glass and inox steel AISI 316 S standards, respectively. References w1x F.S. Spear, Metamorphic Phase Equilibria and Pressure– Temperature–Time Paths, Mineralogical Society of America, Monograph 1, Washington, 1993, pp. 1–799.

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