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Progress of quantitative micro-PIXE applications in geology and mineralogy
Nuclear Instruments North-Holland
and Methods
in Physics Research
B75 (1993) 403-410
NlMilB
Beam Interactions with Materials&Atoms
Progress of quantitative and mineralogy
micro-PIXE
applications
in geology
S.H. Sie Heavy Ion Analytical Facility, CSIRO Division of Exploration Geoscience, P.O. Box 136, North Ryde, NSW 2113, Australia
The scope and depth of micro-PIXE applications in geology and mineralogy are increasing steadily, including those with direct relevance to the minerals industry. The power of micro-PIXE as a quantitative, in-situ nondestructive multielement microanalyzer addresses a whole range of problems, from cosmochemistry to ore mineralogy, from the basic geosciences to minerals exploration problems. Despite the progress of other microbeam techniques, micro-PIXE maintains its advantage on a number of points, including ease of quantification and compatibility with a number of ion beam analytical techniques that complement its capabilities. Alone, or in combination with other IBA techniques, micro-PIXE has a secure place as a viable tool for the geosciences and the minerals industry alike.
1. Introduction Much of the progress of geological and mineralogical applications of PIXE has been achieved by using microbeams (henceforth referred to as micro-PIXE) and has been reviewed extensively [l-14]. Industrial and commercial applications of PIXE in geology have been attempted since the earliest days [15] and although their true economic viability is still well in the future, they are becoming a reality in some laboratories [16,17]. While micro-PIXE appears to be more widely used now, promising bulk PIXE applications in geology continue to develop, e.g. in analysis of emanations from orebodies [ 181. However, in geological applications, neither PIXE nor micro-PIXE have a captive market, since other analytical methods, particularly microbeam methods, are making steady progress. These include laser ablation microanalysis combined with inductively coupled plasma techniques, with atomic emission spectrometry (ICP-AES) [19] and mass spectrometry (ICP-MS) detection systems [20]. Synchrotron radiation probes X-ray fluorescence (SXRF) have been applied successfully e.g. in study of fluid inclusions [19,21,22] and meteorites [23]. More established methods such as EMP (electron microprobe) and SIMS (secondary ion mass spectrometry) are continually being improved, with the EMP now competing with micro-PIXE for the detection of trace elements in the regime of 50-100 ppm concentrations. Quantification problems besetting earlier SIMS application in trace analysis is solved by ion implantation of the sample with a known dosage of the element of interest for calibration [24,2.5]. 0168-583X/93/$06.00
0 1993 - Elsevier
Science
Publishers
Nevertheless, micro-PIXE maintains its advantage in readily quantified, standardless, nondestructive, insitu multielement analytical capability with ppm sensitivity and microns spatial resolution, and with high reliability and accuracy. With appropriate design of hardware and software, the method can be applied equally well to survey type of problems involving analyses of large number of samples, as well as to precision studies of intricate and microns size samples.
2. Micro-PIXE For I*.-PIXE analysis, rock and ore samples are prepared as polished thin (typically SO-100 km) sections or polished blocks similar to that required for EMP analysis. Special sample preparation may be required for study of fluid inclusions [26-291, to ensure that inclusions of interest are within 5-10 km below the surface to prevent rupture under beam bombardment while minimizing X-ray absorption by the host mineral. Virtually all geological applications of micro-PIXE have been carried out with 2-4 MeV proton microbeams. Some have been carried out with high energy protons [30], or with beams other than protons, the latter usually as part of experiments designed to detect specific light elements, e.g. H [31] and C [32], by nuclear reaction analysis (NRA). Micro-PIXE is most effective when complemented by EMP analysis for major (light) elements, and for yield normalization when the integrated beam charge is not reliably known.
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VI. GEOLOGICAL
SAMPLES
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S.H. Sie / p-PlXE applications in geology and mineralogy
Light elements (e.g. F, Na and Al) can be obtained simultaneously with gamma-ray measurements (PIGE) [27,28]. Combined ion beam techniques including channelling can provide unique new information, such as location of trace elements in the mineral [33]. MicroPIXE can be combined with cathodoluminescence imaging to delineate zoning features [34]. The X-rays are detected in energy dispersive spectrometers (EDS), commonly Si(Li) detectors. Ge detectors improve the detection efficiency of K lines of REE and heavier elements, but their sensitivity to gamma rays degrades the peak to background ratio. A wavelength dispersive spectrometer (WDS) with its better energy resolution can in principle solve this particular problem, but its low efficiency, even when position sensitive detectors are used [35,36], precludes its use on a routine basis. A number of features are essential for geological applications of a proton microprobe [5,7]: 1) Sample viewing at > 100 x magnification is needed to enable viewing of microscopic features of the sample. A reflected geometry and normal takeoff angle is preferred to enable viewing of thick specimen or opaque minerals, ideally with the optical axis colinear with the beam [37,38]. For thin, transparent specimens, normal viewing is achieved readily using microscopes at 0”. 2) Micrometer driven stages for positioning the sample accurately are required, preferrably computer driven, to enable preprogramming of sample coordinates. This and a quick sample loading system are essential to achieve high throughput, especially in the context of a commercial operation. 3) Readily selectable X-ray absorbers (filters) are essential to control the major element count rate (say < 4000 c/s) in order to minimize pileup effects and optimize detection of the elements of interest. Pileup effects can mask lines of interest (e.g. U and Rb by Fe K pileups). Absorption edges can be exploited to enhance detections of lines of interest (e.g. RbCl filter to absorb the Zr lines in order to improve Pb line detection). To achieve minimum detection limits (MDL) in the ppm range, a few micro-coulomb of charge is typically required, assuming a detector solid angle in the 50-150 msr range typical of most setups. Many proton microprobes can achieve spatial resolution < 1 pm [39,40] but source brightness limitations result in low beam currents (< 1 nA), and unviable counting times for the desired MDL [38]. The large effective depth of analysis (20-50 urn) implies that grains > 50 pm should be selected for analysis, to avoid contribution from the substrate or overlapping grains. Thus probing with lo-30 u.rn beam is optimum, leaving ample scope for increased beam currents. The scanning mode with smaller beam sizes can be used effectively in geological studies particularly where quantification is not essential [41,42,43].
3. Data analysis programs The main features of data analysis programs required for geological applications are common to all PIXE applications, namely a facility to deconvolute complex overlapping peaks of an EDS spectrum, and conversion of the peak areas to concentrations using calculated X-ray yields for thick specimens [10,44-511. The yields depend strongly on the type of minerals, reflecting the matrix effects on stopping power and self-absorption and are calculated generically for stoichiometrically simple minerals, or by using electron microprobe data for more complex ones. Parametrized or tabulated ionization cross-sections based on the binary encounter approximation (BEA) [52] or ECPSSR theory [53,54] are used in the calculation, supplemented with fluorescence yields [55] and both theoretical [56] and empirical relative intensities [57,58], including radiative Auger and Coster-Kroenig transitions. Special features, e.g. the treatment for layered samples [59,60] are essential for the analysis of thin samples on substrates, and for the analysis of special samples such as fluid inclusions. Secondary fluorescence effects [60,61] by major lines can be significant in a number of samples, e.g. the effect of Fe on Cr in chromites ((Fe,Mg)(Cr,Al),O,), of Zn on Fe in sphalerite (ZnS) and of Au on PGE (platinum group elements) in Au grains. Treatment of pileup peaks [62] should include higher orders [lo]. Various treatments for the continuum background have been adopted, some based on some representation (e.g. polynomials) of theoretical expectations of the shape of the continuum background, others based on strictly empirical approach such as digital filters [45] and peak filing techniques [44,63]. Details for accurate quantification of the results, including detector response functions and efficiencies [64], absorber thickness and relative intensities can be determined iteratively by analyzing pure elemental or stoichiometrically simple samples. Ultimately, the validity of the different approaches must be tested against well known geological standards [65] (e.g. AGV, BCR, GSP), or special standards [66], as well as against one another through intercomparisons [67]. Extensive tests at CSIRO [68] appears to support the consensus that 3-5% accuracy is possible for major and minor elements, and better than 10% for trace elements.
4. Applications 4.1. Cosmochemistry The nondestructiveness of micro-PIXE holds a special attraction for the study of the relatively rare ex-
405
S.H. Sk / CL-PIXE applications in geology and mineralogy
traterrestrial material such as meteorites and lunar samples. Meteorites are classified into major groups of stone, iron and stone-iron meteorites. The first group includes chondritic and achondritic types. The elemental abundance in chondrites appears to be similar to that observed in the solar photosphere, and thus are thought to represent the “primordial” abundances in the solar system. Micro-PIXE and other complementary techniques were used to test the smoothness of the elemental abundances in CI chondrites [69,70]. Ca-Al-rich inclusions (CAI) in chondritic meteorites are thought to be among the oldest materials in the solar system, on the basis of Sr and Mg isotopic data. Wark et al. [71] studied such an inclusion in the Allende meteorite with concentric layers indicative of stages of condensation at high temperatures. Bajt and Traxel [72] studied trace element distribution of ohvine ((Mg,Fe),SiO,) in chondrites. Some isolated crystals showed pronounced zoning profiles, which were analyzed in terms of diffusion models. Following an earlier study [73], Makjanic et al. [74,7.5] discovered C-rich rims around troihte (FeS) grains in chondrites using combined nuclear reaction analysis and PIXE methods with microbeams of deuterons. Using micro-PIXE, and a complement of other microanalytical techniques Kracher et al. [76] studied the partitioning of trace elements between the metal, oxide and sulfide phases in achondritic meteorites, which can be used to infel the conditions during differentiation of the parent bodies. Lunar samples from the Apollo program were some of the earliest material studied by micro-PIXE [77-821. Partitioning of trace elements Zr and Nb were mesured between various pairs of coexisting opaque phases in lunar basalt samples which include armalcolite ilmenite (FeTiO,), ulvospinel (@k,Fe)Ti,O,), (Fe,TiO,), and chromite. A recent study of melt inclusions in a lunar olivine sample demonstrated that lunar magma undergoes similar differentiation and mixing processes as that in the Earth’s mantle [83]. 4.2. Indicator
minerals
and diamond
exploration
In more readily obtained samples of terrestrial ilmenites from kimberlites, the effects of crystallization sequence can be observed in more detail [3,5,84,85], not only through Nb and Zr, but in Ni which appears to be absent in lunar ilmenites. Precipitation of olivine from the magma depletes the melt of Ni, and a similar effect is seen for Zr due to zircon precipitation. Suites of ilmenites precipitating from various kimberlites show similar trends of behaviour but the actual concentrations will depend on the composition of the parent magmas and the crystallization history. This property has implications in one method of diamond explo-
ration designed to locate kimberhtes, the main source of diamonds. The method is based on collection of garnets minerals (ilmenites, refractory [(Mg,Fe),(A1,Cr),Si,0,,], chromites) in stream and soil samples [84]. At CSIRO trace element distribution characteristics in these minerals is being used to discriminate their source rocks, to assess diamond prospects, and to study the processes affecting diamond formation [87-911. 4.3. Geothermobarometry Partitioning of elements, both major and trace between coexisting phases can depend very sensitively on pressure and temperature, and this property is exploited in geothermobarometry, especially for relatively simple (few end members) assemblage such as mantle rocks. Development of thermobarometry was spurred by the advent of microbeam techniques, that enabled calibration through high P, T laboratory experiments involving small samples. Electron microprobe analysis has been used extensively in establishing various geothermometers [92-961 using the major and minor elements. More sensitive techniques such as SIMS led to the beginning of thermometry based on trace elemcnts e.g. Mn for clinopyroxene-garnet [97,98] and Ni for olivine-orthopyroxene [99]. Micro-PIXE can be a very effective method for such a development, as evidenced in the discovery of the Ni thermometer [loo] for peridotitic garnets, based on the partition between garnet and olivine. With mantle olivine representing a reservoir of Ni (- 3000 ppm), the Ni content in the garnet alone can be used as a thermometer. This is now used effectively in assessing the prospectivity of kimberhtes and lamproites, for likely diamond grades based on garnet concentrates alone [86-891. 4.4. Xenoliths
and metasomatism
Kimberlites and alkali-basalt magmas originate from the base region of the crust and upper mantle, and often carry fragments of mantle rock called xenoliths. Just as chondrites are time capsules of the solar system formation, xenohths are those of the Earth’s mantle. Studies of these xenoliths, complemented by experimental petrology and seismic geophysics, have revealed the mantle composition and stratigraphy, and even effects of geodynamics. Convection of the upper mantle led to mid-ocean rifting and subduction of the oceanic plate under the continental crust. Melt and fluid extraction from the subducted plate led to complex alteration processes (metasomatism) of the the mantle rocks [101,102]. Previously depleted mantle rocks can be re-enriched with incompatible elements such as Ti, Zr and Y. Evidence of metasomatism through melt or fluid infiltration can be found in VI. GEOLOGICAL
SAMPLES
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S.H. Sie / pPIXE
applications in geology and mineralogy
xenoliths both in the presence of certain minerals in the assembly (e.g. hydrous phases) or as zoning of the minerals. Zoning however could also be due to exchange reactions between coexisting phases due to changes in P and T. Micro-PIXE can be used to delineate these effects. Griffin et al. [103] studied trace element zoning in garnets from sheared xenoliths from the Frank Smith mine, shown previously to be zoned in major and minor elements [104] attributed to melt infiltration. Using diffusion models and estimated diffusion coefficients for Zr, it was concluded that infiltration occurred shortly ( < 300 yr) before eruption. A similar study on garnets from xenoliths from a minette volcanic intrusion in Colorado [105], led to the conclusion that the metasomatism was preceded by heating prior to melt infiltration and again that the process was rapid (< 15 000 yr). A micro-PIXE study of spine1 lherzolite xenoliths from West Victoria [106] metasomatized by fluids released by crystallizing silicate melt, delineated the trace element distribution. The study showed that the bulk of the fluid introduced trace elements reside in specific minerals (e.g. REE in apatite, Ba in mica) and not in grain boundaries. This contrasts with a scanning micro-PIXE study by Fraser et al. [42], which showed the presence of trace Sr on grain boundaries in a serpentinized garnet lherzolite. Another study of West Victorian xenoliths from a different source about 100 km away [107] showed characteristics consistent with mainly basaltic melt infiltration, demonstrating the heterogeneous nature of these processes and their effects on the rock composition. 4.5. Mineral and melt inclusions Minerals often contain inclusions, which are fragments of minerals or portions of the melt or fluid trapped during crystallization. Isolated from further evolution, they preserve the chemical composition and conditions of the system at the moment of trapping. Micro-PIXE is ideally suited to study these typically minute (few tens of microns or less) inclusions. Diamonds are one example of minerals produced in the upper mantle at a depths greater than N 150 km. Trace element data obtained in a micro-PIXE study of inclusions in diamonds from West Australia [108] indicated disequilibrium conditions, leading to the conclusion that the diamonds grew in an open system. A similar study of garnet inclusions from African diamonds [109] showed evidence of metasomatic events, as well as a wide a range of formation temperatures using the Ni garnet thermometer. Systematic studies of diamond inclusions can be used to deduce mantle stratigraphy, and palaeogeotherms in cratonic areas that produce these diamonds, and refine the application of the Ni thermometry m diamond exploration in general [11,86-901.
Olivine is the most abundant mantle mineral, and those with the high Mg content are believed to crystallize from primitive magma. Inclusions of melts trapped in such olivine can therefore be assumed to represent primitive mantle. Micro-PIXE has been applied to study melt inclusions in olivine from several tectonic environments and lunar material [83]. The heterogeneity of primitive mantle is evident in the wide range of the ratio of LILE (large-ion lithophile elements, e.g. Rb, Sr, Ba) to HFSE (high field strength elements, e.g. Ti, Zr, Nb, Ta) groups of geochemical marker elements. Ratios of elements between these groups are known to be distinctive in different tectonic environments [llO]. 4.6. Fluid inclusions Fluids trapped during the formation of minerals in hydrous environments (e.g. hydrothermal systems, or during fluid metasomatism) as well as in magmatic systems form fluid inclusions, which can often include vapor phases and precipitated daughter minerals. The large penetration of proton beam can be exploited to permit analysis of the fluid composition without decrepitating the inclusion. Micro-PIXE has been applied to study fluid inclusions in quartz from hydrothermal systems [ 111,26,27,112] and in combination with PIGE [28,27]. PIXE allows the determination of elements typically heavier than S, while PIGE is used to determine F, Na and Al. Ryan et al. [26] used the K,/K, ratio of Cl from the saline fluid to determine the depth to within k 1.5 km. Laser ablation with ICP techniques [20] and SXRF [20,22] have also been applied to fluid inclusion studies. 4.7. Experimental
petrology
Detailed modelling of magma extraction from primitive mantle relies on accurate knowledge of partitioning of elements between coexisting phases. The partition coefficients can be derived empirically from naturally occurring assemblages reliably only if the system was closed and equilibrated. Laboratory simulations provide the alternative, where conditions are controllable. With typically small quantities of samples involved, micro-PIXE is an ideal analytical tool, offering the prospect of systematic studies of partitioning of the geochemical marker elements at levels comparable to natural systems. With micro-PIXE, MDL in mafic silicates of around 2 ppm are obtained readily (5-10 min with a 15-20 km beam for a collected charge of 3 PC) for the LILE and HFSE, but for the REE the detection limit is poor at around 50-200 ppm because of the limitations of the EDS system. An experimental study of partitioning of Nb and Ta between basaltic melt and the crystallizing phases has
407
S. H. Sie / p-PIXE applications in geology and mineralogy
suggested large fractionation effects in the partitioning of Nb and Ta, hitherto assumed to be geochemically coherent [113]. Similarly, systems of mixed material (e.g carbonatite and silicate) [114,115], the presence of volatiles and fluids [116] and effects of low partial melting [117] can now be studied at levels close to natural systems. 4.8. Sulfide mineralogy Micro-PIXE has been used to determine the trace element distribution in sulfide ore samples [118-1201 and mill concentrate [S], with emphasis on residence of precious metal for beneficiation applications. For example, gold occurs mainly as free gold, but in refractory ore it is locked in solid solution or as submicroscopic inclusions in other phases normally relegated to the tailings e.g. pyrite (FeS,) and arsenopyrite (FeAsS). Micro-PIXE can determine Au with MDL of 5 ppm in pyrite, but in arsenopyrite interference with the “tail” of As increases the MDL to _ 20 ppm. Cabri et al. [121,122] made a comparison of the effectiveness of SIMS and micro-PIXE, concluding that SIMS [122] is more sensitive although the quantification is more elaborate requiring implanted standards. SXRF has also been used in a similar study [123], but its routine use in an industrial context may not be practical. Similar to gold, platinum group elements (PGE: Pt, Pd, Rh, Ir, Ru, Re, OS) occur mainly as readily separable free PGM (platinum group minerals) but sulfides often present in the deposit can be significant carriers [4,124-1271. For example, a micro-PIXE study of the Merensky Reef ore showed pentlandite ((Fe,Ni),S,) as a significant carrier of Ru, Rh, Pd at levels up to few hundred ppm, and pyrrhotite (FeS) as carrier of Ru (8-12 ppm) [4], with typical MDL values of 2-6 ppm. 4.9. Sulfide ore genesis The use of trace element distribution by micro-PIXE in ore genesis studies is beginning. A detailed study of the trace element distribution in zoned pyrite in an epithermal system [128] revealed a broad correlation of Cu, Sb, Se with As, and with Ag-Au grades. Many significant massive sulfide deposits were formed initially by volcanic-exhalative processes followed by various physico-chemical reactions associated with hydrothermal activities and possibly deformation which remobilize and concentrate the metals. Pyrite in these volcanogenic massive sulfide (VMS) deposits can be used as a guide to locate mineralized regions, and its morphology and trace elements (Cd, Sb, Sn, In, Se, Te) distribution carry the signature of ore genesis. Huston et al. [129] have determined such distribution in a selection of VMS deposits using EMP for analysis of Ag, and Hg content of Au, and micro-PIXE to
determine the Au level in pyrite and arsenopyrite. systematic, spatially controlled micro-PIXE study Zn-Pb rich and Cu-rich VMS deposits is reported these Proceedings [ 1301. 4.10. Trace element
A of in
in Au
Free Au is nearly universally alloyed with Ag, at proportions controlled by the ambient temperature, activity of S and salinity of the mineralizing fluid as well as the source rocks. Elements occurring in the source rock (Fe, Cu, Zn, Hg, Pb) are also transported and may coprecipitate at trace levels with Au. Their distribution in gold can reflect genetic associations and may thus be used as discriminants in gold exploration programs which involve the collection of detrital Au grains. A preliminary systematic micro-PIXE study on bedrock and alluvial Au grains from several prospects in Tasmania show indications of differences among different deposit types, styles of mineralization and mineralogical associations [3]. This study higlights the limits of the EDS system: the MDL for most elements adjacently below Au are > 0.1%. Elements immediately above Au (Hg, Pb, Bi) can be detected through their Ly lines with MDL values of * 200 ppm. A WDS system will significantly improve the detection limits.
5. Summary and conclusion Significant progress and mature methodologies have been achieved by micro-PIXE in a number of areas of applications: - In studies involving mafic silicates and other mantle minerals, geochemical marker trace elements such as LILE (Rb, Sr, K,) and HFSE (Zr, Nb, Ta) can be detected with ppm sensitivity with good efficiency. Other trace elements such as Ni, Mn and Zn can be used in geothermobarometry. Ni garnet thermometry is now used extensively in diamond exploration to assess prospectivity of kimberlites. Thermoand barometry based on other trace elements are being developed. - Zoning features in mantle minerals reveal geodynamic effects and details of metasomatism, and are used to deduce timescales of geological events. Zoning in minerals in hydrous environment reveals deposition history. - Precious metal distribution in sulfides has direct applications in industry for devising beneficiation strategy. While in specific cases other methods (e.g. SIMS in Au determination) may be more sensitive, microPIXE is still competitive for large scale applications and convenient for quantification. - Systematic application of trace element distribution to ore genesis studies with implications in exploration programs have begun. VI. GEOLOGICAL SAMPLES
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S.EI. Sie / F-PIXE applications in geology and mineralogy
- Micro-PIXE application in fluid inclusion studies is progressing rapidly, with precision and accuracy improving steadily. Although SXRF is a competing method, quantitative analysis with micro-PIXE is more readily achieved. Combined with PIGE, micro-PIXE might have decisive advantage. - One area developed earlier may need to be revisited: e.g. applications in zircon for geochronology through the U/Pb ratios. Although ion microprobes predominate in this area, micro-PIXE may be used as a screening tool and to calibrate standards.
References [l] H.J. Annegarn HJ and S. Bauman, Nucl. Instr. and Meth. B49 (1990) 264. [2] S.H. Sie, C.G. Ryan and G.F. Suter, Scanning Microscopy 5 (1991) 977. [3] S.H. Sie, W.L. Griffin, C.G. Ryan, G.F. Suter and D.R. Cousens, Nucl. Instr. and Meth. B54 (1991) 284. [4] S.H. Sie, D.R. Cousens, C.G. Ryan and W.L. Griffin, Nucl. Instr. and Meth. B45 (1990) 604. [5] S.H. Sie, C.G. Ryan, D.R. Cousens and W.L. Griffin, Nucl. Instr. and Meth. B40/41 (1989) 690. [6] J.D. MacArthur and X.P. Ma, Int. J. PIXE 1 (1991) 311. [7] L.J. Cabri, Nucl. Instr. and Meth. B30 (1988) 459. [S] T.M. Benjamin, C.J. Duffy and P.S.Z. Rogers, Nucl. Instr. and Meth. B30 (1988) 454. [91 D.G. Fraser, Chem. Geol. 83 (1990) 27. [lOI C.G. Ryan, D.R. Cousens, S.H. Sie, W.L. Griffin, G.F. Suter and E. Clayton, Nucl. Instr. and Meth. B47 (1990) 55. ill1 C.G. Ryan and W.L. Griffin, Proc. 3rd. Int. Conf. on Microprobe Technology and Applications, Uppsala, 1992, to be published in Nucl. Instr. and Meth. B. [121 J.L. Campbell, Nucl. Instr. and Meth. B31 (1988) 518. I131 J.L. Campbell, J.A. Maxwell, W.J. Teesdale, J.X. Wang and L.J. Cabri, Nucl. Instr. and Meth. B44 (1990) 347. [141 H. Blank and K. Traxel, Scanning Electron Microscopy 3 (1984) 1089. [151 L.E. Carlsonn and K.R. Akselsson, Adv. X-ray Anal. 24 (1980) 313. 1161J.L. Campbell, W.J. Teesdale and J.A. Maxwell, Nucl. Instr. and Meth. B56/57 (1991) 694. (1985) 664. [171 S.H. Sie, Nucl. Instr. and Meth. BlO/ll iI81 K.G. Malmqvist, H. Bage, L.E. Carlsson, K. Kristiansson and L. Malmqvist, Nucl. Instr. and Meth.B22 (1987) 386. [19] A.H. Rankin, M.H. Ramsey, B. Coles, F. Van Langevelde and C.R. Thomas, Geochim. Cosmochim. Acta 56 (1992) 67. [20] I. Jarvis and K.E. Jarvis, Chem. Geol. 95 (1992) 1. [21] J.D. Frantz et al., Chem. Geol. 69 (1988) 235. [22] J.B. Lowenstem, G.A. Mahood, M.L. Rivers and S.R. Sutton, Science 252 (1991) 1405. [23] S.R. Sutton, J.S. Delaney, J.V. Smith and M. Prinz, Geochim. Cosmochim. Acta. 51 (1987) 2653. [24] S.L. Chryssoulis, Secondary Ion Mass Spectrometry
(SIMS VII), eds. A. Bennighoven, C.A. Evans, K.D. McKeegan, H.A. Storms and H.W. Werner (Wiley, 1989) p. 405. [25] S.L. Chryssoulis, L.J. Cabri and W. Lennard, Econ. Geol. 84 (1989) 168. [26] C.G. Ryan, D.R. Cousens, C.A. Heinrich, W.L. Griffin, S.H. Sie and T.P. Mernagh, Nucl. Instr. and Meth. B54 (1991) 292. WI C.A. Heinrich, C.G. Ryan, T.P. Mernagh and P.J. Eadington Segregation of ore metals between magmatic brine and vapor: a fluid inclusion study using PIXE microanalysis, Econ. Geol., in press. X.P. Ma, G.R. Palmer, A.J. Anderson Dl J.D. MacArthur, and A.H. Clark, Nucl. Instr. and Meth. B45 (1990) 322. 1291A.J. Anderson, A.H. Clark, X.P. Ma, G.R. Palmer, J.D. MacArthur and E. Roedder, Econ. Geol. 84 (1989) 924. 1301J.S.C. McKee, G.R. Smith, A.A. Mirzai, M.S. Mathur, N.M. Halden, C. Pinsky and R. Bose, Scanning Microscopy 4 (1990) 843. I311 M. Mosbah, R. Clocchiatti, J. Tirira, J. Gosset, P. Massiot and P. Trocellier, Nucl. Instr. and Meth. B54 (1991) 298. 1321J. Makjanic, D. Heymann and R.D. Vis, Nucl. Instr. and Meth. B54 (1991) 325. I331 D.N. Jamieson and C.G. Ryan, Proc. 3rd. Int. Conf. on Microprobe Technology and Applications Uppsala, 1992, to be published in Nucl. Instr. and Meth. B. X.P. Ma, G.R. Palmer and [341 P.L. Roeder, D. MacArthur, A.N. Mariano, Am. Mineral. 72 (1987) 801. Nucl. Instr. and Meth. I351 F. Folkmann and F. Frederiksen, B49 (1990) 126. M. Okane and Y. Yamamoto, Nucl. [361 H. Hamanaka, Instr. and Meth. B45 (1990) 360. [37] S.H. Sie and C.G. Ryan, Nucl. Instr. and Meth. B15 (1986) 664. [38] S.H. Sie, C.G. Ryan, D.R. Cousens and G.F. Suter, Nucl. Instr. and Meth. B45 (1990) 543. [39] F. Watt, G. Grime, G.D. Blower, J. Takacs and D.J.T. Vaux, Nucl. Instr. and Meth. 197 (1982) 65. [40] G.J.F. Legge et al., Nucl. Instr. and Meth. B15 (1986) 669. [411 D.G. Fraser, D.J. Feltham and MI. Whiteman, Sedim. Geol. 65, (1989) 223. [421 D.G. Fraser, F. Watt, G.W. Grime and J. Takacs, Nature 312 (1984) 352. I431 R.A. Wogelius, D.G. Fraser, D.J. Feltham and MI. Whiteman, Geochim. Cosmochim. Acta 56 (1992) 319. I441 E. Clayton, PIXAN - the Lucas Heights PIXE analysis package, Australian Atomic Energy Commission Report AAEC-Ml13 (1986). I451 J.A. Maxwell, J.L. Campbell and W.J. Teesdale, Nucl. Instr. and Meth. B43 (1989) 218. R.K. Akselsson and W.J. Courtenay, [461 H.C. Kaufmann, Nucl. Instr. and Meth. 142 (1977) 251. [471 E. Bombelka, W. Koenig, F.W. Richter and U. Waetjen, Nucl. Instr. and Meth. B22 (1987) 21. [481 I. Orlic, J. Makjanic, G.H.J. Tros and R.D. Vis, Nucl. Instr. and Meth. B49 (1990) 166. [49] A.D. Lipworth, H.J. Annegarn, S. Bauman, T. Molokomme and A.J. Walker, Nucl. Instr. and Meth. B49 (1990) 173.
409
S.H. Sie / t.~-PIXE applications in geology and mineralogy [50] X.P. Ma, G.R. Palmer [51] [52] [53] [54] [55] [56] [57] [58] [59] [60]
[61] [62] [63] [64] [65] [66] [67]
[68] [69]
[70]
[71]
[72] I731 [74]
[75] [761 [77] I781
and J.D. MacArthur,
Nucl. Instr.
and Meth. B22 (1987) 49. C.J. Duffy, P.S.Z. Rogers and T.M. Benjamin, Nucl. Instr. and Meth. B22 (1987) 91. J.D. Garcia, R.J. Fortner and T.M. Kavanagh, Rev. Mod. Phys. 45 (1973) 111. W. Brandt and G. Lapicki, Phys. Rev. 23 (1981) 1717. D.D. Cohen and M. Harrigan, Atom. Data Nucl. Data Tables 33 (1985) 255; 34 (1986) 393. M.O. Krause, J. Phys. Chem. Ref. Data 8 (1979) 307. J.H. Scofield, Phys. Rev. A9 (1974) 1041; A10 (1974) 1507. S.I. Salem, S.L. Panossian and R.A. Krause, Atom. Data Nucl. Data Tables A7 (1 970) 565. W. Bambynek, Rev. Mod. Phys. 44 (1972) 716. J. Rickards and E.P. Zironi, Nucl. Instr. and Meth. 829 (1987) 527. D.R. Cousens, C.G. Ryan, S.H. Sie and W.L. Griffin, Proc. 5th Australian Conf. on Nuclear Techniques of Analysis, ISSN 0811-9422 (Australian Institute of Nuclear Science and Engineering, Lucas Heights, NSW, 1987) p. 58. W. Reuter, A. Lurio, F. Cardone and J.F. Ziegler, J. Appl. Phys. 46 (1975) 3194. G.I. Johansson, X-ray Spectr. 11 (1982) 194. C.G. Ryan, E. Clayton, W.L. Griffin, S.H. Sie and D.R. Cousens, Nucl. Instr. and Meth. B34 (1988) 396. J.L. Campbell and P.L. McGhee, Nucl. Instr. and Meth. A248 (1986) 393. K. Govidaraju, Geostandard Newsletter 8 (1984). J.L. Campbell, L.J. Cabri, P.S.Z. Rogers, K. Traxel and T.M. Benjamin Nucl. Instr. and Meth. B22 (1987) 437. J.L. Campbell, W. Maenhaut, E. Bombelka, E. Clayton, K. Malmqvist, J.A. Maxwell, J. Pallon and J. Vandenhaute, Nucl. Instr. and Meth. B14 (1986) 204. C.G. Ryan, D.R. Cousens, S.H. Sie and W.L. Griffin, Nucl. Instr. and Meth. B49 (1990) 271. D.S. Burnett, D.S. Woolum, T.M. Benjamin, P.S.Z. Rogers, C.J. Duffy and C. Maggiore, Nucl. Instr. and Meth. B35 (1988) 67, Geochim. Cosmochim. Acta 53 (1989) 471. D.S. Woolum, D.S. Burnett, T.M. Benjamin, P.S.Z. Rogers, C.J. Duffy and C.J. Maggiore, Nucl. Instr. and Meth. B22 (1987) 376. D.A. Wark, T.C. Hughes, M. Lucas and C.D. McKenzie, Lunar. Planet. Sci. XI (1980) 1205; D. Wark, Earth Planet. Sci. Lett. 77 (1986) 129. S. Bajt and K. Traxel, Nucl. Instr. and Meth. B54 (1991) 317. R.D. Vis, C.C.A.H. Van der Stap and D. Heymann, Nucl. Instr. and Meth. B22 (1987) 380. J. Makjanic, D. Heymann, C.C.A.H. Van der Stap, R.D. Vis and H. Verheul Nucl. Instr. and Meth. B30 (1988) 466. J. Makjanic, L.R. Touret, R.D. Vis and H. Verheul, Meteoritics 24 (1989) 49. A. Kracher, T.M. Benjamin, C.J. Duffy and P.S.Z. Rogers, Microbeam Analysis (1988) 491. F. Bosch et al., Science, 199 (1978) 765. F. Bosch, A. El-Goresy, W. Herth, B. Martin, R. Nobiling, B. Povh and K. Traxel, Nucl. Sci. Appl. 1 (1980) 33.
[79] H. Blank, R. Nobiling, K. Traxel and A. El Goresy, Lunar. Planet. Sci. 12 (1982) 49. [SO] H. Blank, A. El Goresy, J. Janicke, R. Nobiling and K. Traxel, Lunar. Plan. Sci. 14 (1983) 51. [Sl] H. Blank et al., Nucl. Instr. and Meth. B3 (1984) 681. [82] H. Blank et al., Earth Planet Sci. Lett. (1984) 19. [83] A.V. Sobolev, S.H. Sie and R.A. Binns, Trace elements in primary magmas: proton microprobe analysis of melt inclusions in minerals submitted to Contr. Min. Petr. [X4] R.O. Moore, W.L. Griffin, J.J. Gurney, C.G. Ryan, D.R. Cousens, S.H. Sie and G.F. Suter, Trace element geochemistry of ilmenite megactysts from the monastery kimberlite, South Africa, Lithos 29 (1992). [85] W.L. Griffin, C.G. Ryan and D.J. Schulze, Proc. 5th Kimberlite Conf., Brazil, 1991, ed. H.O.A. Meyer, Brazilian Geological Survey Publication. [86] W.L. Griffin, C.G. Ryan, D.R. Cousens, S.H. Sie and G.F. Suter, Nucl. Instr. and Meth. B49 (1990) 318. [87] W.L. Griffin, C.G. Ryan, J.J. Gurney, N.V. Sobolev and T.T. Win, Proc. 5th Kimberlite Conf., Brazil, 1991, ed. H.O.A. Meyer, Brazilian Geological Survey Publication. [SS] W.L. Griffin, C.G. Ryan, S.Y. O’Reilly, P.H. Nixon and T.T. Win, ibid. [89] W.L. Griffin, S.Y. O’Reilly, C.G. Ryan and M.A. Waldman, ibid. [90] J.X. Zhou, W.L. Griffin, A.L. Jaques, C.G. Ryan and T.T. Win, ibid. [91] W.L. Griffin, N.V. Sobolev, C.G. Ryan, N.P. Pokhilenko, ES. Yefimova, Trace elements in garnets and chromites: diamond formation in the Siberian lithosphere, Lithos. in press. [92] F.R. Boyd, Geochim. Cosmochim. Acta 37 (1973) 2533. [93] A.A. Finnerty and F.R. Boyd, in: Mantle Xenoliths, ed. P.H. Nixon (Wiley, 1980) p. 381. [94] B.J. Wood and S. Banno, Contr. Min. Petr. 42 (1973) 109. [95] H.St.C. O’Neill and B.J. Wood, Contr. Min. Petrol. 70 (1979) 59; 72 (1980) 337. [96] G.E. Adams and F.C. Bishop, Contr. Min. Petr. 94 (1986) 230. [97] N. Shimizu and C.J. Allegre, Contr. Min. Petr. 67 (1978) 41. [98] J.S. Delaney, J.V. Smith, J.G. Dawson and P.H. Nixon, Contr. Min. Petr. 71 (1979) 157. [99] R.L. Hervig, J.V. Smith, I.M. Steele and J.B. Dawson, Earth Planet Sci. Lett. 50 (1980) 41. [lOOI W.L. Griffin, D.R. Cousens, C.G. Ryan, S.H. Sie and G.F. Suter, Contr. Min. Petr. 103 (1989) 199. [loll A.E. Ringwood, Composition and Petrology of the Earth’s Mantle (McGraw-Hill, New York, 1975). [102] D.H.Green and A.E. Ringwood, J. Geophys. Res. 68 (1963) 937. [103] W.L. Griffin, D. Smith, F.R. Boyd, D.R. Cousens, C.G. Ryan, S.H. Sie and G.F. Suter, Geochim. Cosmochim. Acta 53 (1988) 561. [104] D. Smith and F.R. Boyd, in: Mantle Xenoliths, ed. P.H. Nixon (Wiley, 1987) p. 551. ]105] D. Smith, W.L. Griffin, C.G. Ryan and S.H. Sie, Contr. Min. Petr. 107 (1991) 60. [106] S.Y. O’Reilly, W.L. Griffin and C.G. Ryan, Contr. Min. Petr. 109 (1991) 98.
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S.H. Sie / p-PIXE applications in geology and mineralogy
[107] A. Greig, S.H. Sie and I.A. Nicholls, these Proceedings (6th Int. Conf. on PIXE in its Analytical Applications, Tokyo, 1992) Nucl. Instr. and Meth. B75 (1993) 411. [lo81 W.L. Griffin, L. Jaques, S.H. Sie, C.G. Ryan, D.R. Cousens and G.F. Suter, Contrib. Miner. Petr. 99 (1988) 143. [109] W.L. Griffin, J.J. Gurney and C.G. Ryan, Contrib. Min. Petr. 110 (1992) 1. [llO] J.A. Pearce and J.R. Cann, Earth. Planet. Sci. Lett. 19 (1973) 290. [ill] E.E. Horn and K. Traxel, Chem. Geol. 61 (1987) 29. [112] C.G. Ryan, CA. Heinrich and T.P. Mernagh, Proc. 3rd Int. Conf. on Nuclear Microprobe Technology and Applications, Uppsala, 1992, to be published in Nucl. Instr. and Meth. B. [113] T.H. Green, S.H. Sie, C.G. Ryan and D.R. Cousens, Chem. Geol. 74 (1989) 201. [114] T.H. Green, J. Adam and S.H. Sie, Mineral. Petrol. 46 (1992) 179. 11151 R.J. Sweeney, D.H. Green and S.H. Sie, Earth Planet. Sci. Lett. 113 (1992) 1. [116] J. Adam, T.H. Green and S.H. Sie, Contr. Min. Petr., in press. [117] S.H. Sie, R.J. Sweeney, D.H. Green and G.F. Suter, Proc. 7th Australian Conf. on Nuclear Techniques of Analysis, Melbourne, ISSN 0811-9422 (Australian Institute of Nuclear Science and Engineering, 1991) p.126. [118] L.J. Cabri, J.L. Campbell, J.H.G. Laflamme, R.G. Leith,
[119] [120]
[121]
11221 [123] [124] [125] [126] [127] [128] [129] [130]
J.A. Maxwell and J.D. Scott, Can. Mineral. 23 (1985) 133. D.C. Harris, L.J. Cabri and R. Nobiling, Can. Mineral. 22 (1984) 493. K.J. Reeson, C.J. Stanley, C. Jeynes, G. Grime and F. Watt, Nucl. Instr. and Meth. B45 (1990) 327; Geochem. 6 (1991) 225. L.J. Cabri, S.L. Chryssoulis, J.P.R. De Villiers, J.H.G. Laflamme and P.R. Buseck, Can. Mineral. 27 (1989) 353. L.J. Cabri, S.L. Chryssoulis, J.L.Campbell and W.J. Teesdale, Appl. Geochem. 6 (1991) 225. J.R. Chen et al., Nucl. Instr. and Meth. B22 (1987) 394. A.D. Paktunc, L.J. Hulbert and D.C. Harris, Can. Mineral. 28 (1990) 475. W. Przybylowicz, H. Kucha, A. Piestrzynski, K. Travel and S. Bajt, Nucl. Instr. and Meth. B50 (1990) 231. B. Orberger and K. Traxel, Nucl. Instr. and Meth. B54 (1991) 304. L.J. Cabri et al., Can. Miner. 22 (1984) 521. W.L. Griffin, P.M. Ashley, C.G. Ryan, S.H. Sie and G.F. Suter, Can. Mineral. 29 (1991) 185. D.L. Huston et al., Econ. Geol. 87 (1992) 542. D.L. Huston, S.H. Sie, G.F. Suter and C.G. Ryan, these Proceedings (6th Int. Conf. on PIXE and its Analytical Applications, Tokyo, 1992) Nucl. Instr. and Meth. B75 (1993) 531.
and Methods
in Physics Research
B75 (1993) 403-410
NlMilB
Beam Interactions with Materials&Atoms
Progress of quantitative and mineralogy
micro-PIXE
applications
in geology
S.H. Sie Heavy Ion Analytical Facility, CSIRO Division of Exploration Geoscience, P.O. Box 136, North Ryde, NSW 2113, Australia
The scope and depth of micro-PIXE applications in geology and mineralogy are increasing steadily, including those with direct relevance to the minerals industry. The power of micro-PIXE as a quantitative, in-situ nondestructive multielement microanalyzer addresses a whole range of problems, from cosmochemistry to ore mineralogy, from the basic geosciences to minerals exploration problems. Despite the progress of other microbeam techniques, micro-PIXE maintains its advantage on a number of points, including ease of quantification and compatibility with a number of ion beam analytical techniques that complement its capabilities. Alone, or in combination with other IBA techniques, micro-PIXE has a secure place as a viable tool for the geosciences and the minerals industry alike.
1. Introduction Much of the progress of geological and mineralogical applications of PIXE has been achieved by using microbeams (henceforth referred to as micro-PIXE) and has been reviewed extensively [l-14]. Industrial and commercial applications of PIXE in geology have been attempted since the earliest days [15] and although their true economic viability is still well in the future, they are becoming a reality in some laboratories [16,17]. While micro-PIXE appears to be more widely used now, promising bulk PIXE applications in geology continue to develop, e.g. in analysis of emanations from orebodies [ 181. However, in geological applications, neither PIXE nor micro-PIXE have a captive market, since other analytical methods, particularly microbeam methods, are making steady progress. These include laser ablation microanalysis combined with inductively coupled plasma techniques, with atomic emission spectrometry (ICP-AES) [19] and mass spectrometry (ICP-MS) detection systems [20]. Synchrotron radiation probes X-ray fluorescence (SXRF) have been applied successfully e.g. in study of fluid inclusions [19,21,22] and meteorites [23]. More established methods such as EMP (electron microprobe) and SIMS (secondary ion mass spectrometry) are continually being improved, with the EMP now competing with micro-PIXE for the detection of trace elements in the regime of 50-100 ppm concentrations. Quantification problems besetting earlier SIMS application in trace analysis is solved by ion implantation of the sample with a known dosage of the element of interest for calibration [24,2.5]. 0168-583X/93/$06.00
0 1993 - Elsevier
Science
Publishers
Nevertheless, micro-PIXE maintains its advantage in readily quantified, standardless, nondestructive, insitu multielement analytical capability with ppm sensitivity and microns spatial resolution, and with high reliability and accuracy. With appropriate design of hardware and software, the method can be applied equally well to survey type of problems involving analyses of large number of samples, as well as to precision studies of intricate and microns size samples.
2. Micro-PIXE For I*.-PIXE analysis, rock and ore samples are prepared as polished thin (typically SO-100 km) sections or polished blocks similar to that required for EMP analysis. Special sample preparation may be required for study of fluid inclusions [26-291, to ensure that inclusions of interest are within 5-10 km below the surface to prevent rupture under beam bombardment while minimizing X-ray absorption by the host mineral. Virtually all geological applications of micro-PIXE have been carried out with 2-4 MeV proton microbeams. Some have been carried out with high energy protons [30], or with beams other than protons, the latter usually as part of experiments designed to detect specific light elements, e.g. H [31] and C [32], by nuclear reaction analysis (NRA). Micro-PIXE is most effective when complemented by EMP analysis for major (light) elements, and for yield normalization when the integrated beam charge is not reliably known.
B.V. All rights reserved
VI. GEOLOGICAL
SAMPLES
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S.H. Sie / p-PlXE applications in geology and mineralogy
Light elements (e.g. F, Na and Al) can be obtained simultaneously with gamma-ray measurements (PIGE) [27,28]. Combined ion beam techniques including channelling can provide unique new information, such as location of trace elements in the mineral [33]. MicroPIXE can be combined with cathodoluminescence imaging to delineate zoning features [34]. The X-rays are detected in energy dispersive spectrometers (EDS), commonly Si(Li) detectors. Ge detectors improve the detection efficiency of K lines of REE and heavier elements, but their sensitivity to gamma rays degrades the peak to background ratio. A wavelength dispersive spectrometer (WDS) with its better energy resolution can in principle solve this particular problem, but its low efficiency, even when position sensitive detectors are used [35,36], precludes its use on a routine basis. A number of features are essential for geological applications of a proton microprobe [5,7]: 1) Sample viewing at > 100 x magnification is needed to enable viewing of microscopic features of the sample. A reflected geometry and normal takeoff angle is preferred to enable viewing of thick specimen or opaque minerals, ideally with the optical axis colinear with the beam [37,38]. For thin, transparent specimens, normal viewing is achieved readily using microscopes at 0”. 2) Micrometer driven stages for positioning the sample accurately are required, preferrably computer driven, to enable preprogramming of sample coordinates. This and a quick sample loading system are essential to achieve high throughput, especially in the context of a commercial operation. 3) Readily selectable X-ray absorbers (filters) are essential to control the major element count rate (say < 4000 c/s) in order to minimize pileup effects and optimize detection of the elements of interest. Pileup effects can mask lines of interest (e.g. U and Rb by Fe K pileups). Absorption edges can be exploited to enhance detections of lines of interest (e.g. RbCl filter to absorb the Zr lines in order to improve Pb line detection). To achieve minimum detection limits (MDL) in the ppm range, a few micro-coulomb of charge is typically required, assuming a detector solid angle in the 50-150 msr range typical of most setups. Many proton microprobes can achieve spatial resolution < 1 pm [39,40] but source brightness limitations result in low beam currents (< 1 nA), and unviable counting times for the desired MDL [38]. The large effective depth of analysis (20-50 urn) implies that grains > 50 pm should be selected for analysis, to avoid contribution from the substrate or overlapping grains. Thus probing with lo-30 u.rn beam is optimum, leaving ample scope for increased beam currents. The scanning mode with smaller beam sizes can be used effectively in geological studies particularly where quantification is not essential [41,42,43].
3. Data analysis programs The main features of data analysis programs required for geological applications are common to all PIXE applications, namely a facility to deconvolute complex overlapping peaks of an EDS spectrum, and conversion of the peak areas to concentrations using calculated X-ray yields for thick specimens [10,44-511. The yields depend strongly on the type of minerals, reflecting the matrix effects on stopping power and self-absorption and are calculated generically for stoichiometrically simple minerals, or by using electron microprobe data for more complex ones. Parametrized or tabulated ionization cross-sections based on the binary encounter approximation (BEA) [52] or ECPSSR theory [53,54] are used in the calculation, supplemented with fluorescence yields [55] and both theoretical [56] and empirical relative intensities [57,58], including radiative Auger and Coster-Kroenig transitions. Special features, e.g. the treatment for layered samples [59,60] are essential for the analysis of thin samples on substrates, and for the analysis of special samples such as fluid inclusions. Secondary fluorescence effects [60,61] by major lines can be significant in a number of samples, e.g. the effect of Fe on Cr in chromites ((Fe,Mg)(Cr,Al),O,), of Zn on Fe in sphalerite (ZnS) and of Au on PGE (platinum group elements) in Au grains. Treatment of pileup peaks [62] should include higher orders [lo]. Various treatments for the continuum background have been adopted, some based on some representation (e.g. polynomials) of theoretical expectations of the shape of the continuum background, others based on strictly empirical approach such as digital filters [45] and peak filing techniques [44,63]. Details for accurate quantification of the results, including detector response functions and efficiencies [64], absorber thickness and relative intensities can be determined iteratively by analyzing pure elemental or stoichiometrically simple samples. Ultimately, the validity of the different approaches must be tested against well known geological standards [65] (e.g. AGV, BCR, GSP), or special standards [66], as well as against one another through intercomparisons [67]. Extensive tests at CSIRO [68] appears to support the consensus that 3-5% accuracy is possible for major and minor elements, and better than 10% for trace elements.
4. Applications 4.1. Cosmochemistry The nondestructiveness of micro-PIXE holds a special attraction for the study of the relatively rare ex-
405
S.H. Sk / CL-PIXE applications in geology and mineralogy
traterrestrial material such as meteorites and lunar samples. Meteorites are classified into major groups of stone, iron and stone-iron meteorites. The first group includes chondritic and achondritic types. The elemental abundance in chondrites appears to be similar to that observed in the solar photosphere, and thus are thought to represent the “primordial” abundances in the solar system. Micro-PIXE and other complementary techniques were used to test the smoothness of the elemental abundances in CI chondrites [69,70]. Ca-Al-rich inclusions (CAI) in chondritic meteorites are thought to be among the oldest materials in the solar system, on the basis of Sr and Mg isotopic data. Wark et al. [71] studied such an inclusion in the Allende meteorite with concentric layers indicative of stages of condensation at high temperatures. Bajt and Traxel [72] studied trace element distribution of ohvine ((Mg,Fe),SiO,) in chondrites. Some isolated crystals showed pronounced zoning profiles, which were analyzed in terms of diffusion models. Following an earlier study [73], Makjanic et al. [74,7.5] discovered C-rich rims around troihte (FeS) grains in chondrites using combined nuclear reaction analysis and PIXE methods with microbeams of deuterons. Using micro-PIXE, and a complement of other microanalytical techniques Kracher et al. [76] studied the partitioning of trace elements between the metal, oxide and sulfide phases in achondritic meteorites, which can be used to infel the conditions during differentiation of the parent bodies. Lunar samples from the Apollo program were some of the earliest material studied by micro-PIXE [77-821. Partitioning of trace elements Zr and Nb were mesured between various pairs of coexisting opaque phases in lunar basalt samples which include armalcolite ilmenite (FeTiO,), ulvospinel (@k,Fe)Ti,O,), (Fe,TiO,), and chromite. A recent study of melt inclusions in a lunar olivine sample demonstrated that lunar magma undergoes similar differentiation and mixing processes as that in the Earth’s mantle [83]. 4.2. Indicator
minerals
and diamond
exploration
In more readily obtained samples of terrestrial ilmenites from kimberlites, the effects of crystallization sequence can be observed in more detail [3,5,84,85], not only through Nb and Zr, but in Ni which appears to be absent in lunar ilmenites. Precipitation of olivine from the magma depletes the melt of Ni, and a similar effect is seen for Zr due to zircon precipitation. Suites of ilmenites precipitating from various kimberlites show similar trends of behaviour but the actual concentrations will depend on the composition of the parent magmas and the crystallization history. This property has implications in one method of diamond explo-
ration designed to locate kimberhtes, the main source of diamonds. The method is based on collection of garnets minerals (ilmenites, refractory [(Mg,Fe),(A1,Cr),Si,0,,], chromites) in stream and soil samples [84]. At CSIRO trace element distribution characteristics in these minerals is being used to discriminate their source rocks, to assess diamond prospects, and to study the processes affecting diamond formation [87-911. 4.3. Geothermobarometry Partitioning of elements, both major and trace between coexisting phases can depend very sensitively on pressure and temperature, and this property is exploited in geothermobarometry, especially for relatively simple (few end members) assemblage such as mantle rocks. Development of thermobarometry was spurred by the advent of microbeam techniques, that enabled calibration through high P, T laboratory experiments involving small samples. Electron microprobe analysis has been used extensively in establishing various geothermometers [92-961 using the major and minor elements. More sensitive techniques such as SIMS led to the beginning of thermometry based on trace elemcnts e.g. Mn for clinopyroxene-garnet [97,98] and Ni for olivine-orthopyroxene [99]. Micro-PIXE can be a very effective method for such a development, as evidenced in the discovery of the Ni thermometer [loo] for peridotitic garnets, based on the partition between garnet and olivine. With mantle olivine representing a reservoir of Ni (- 3000 ppm), the Ni content in the garnet alone can be used as a thermometer. This is now used effectively in assessing the prospectivity of kimberhtes and lamproites, for likely diamond grades based on garnet concentrates alone [86-891. 4.4. Xenoliths
and metasomatism
Kimberlites and alkali-basalt magmas originate from the base region of the crust and upper mantle, and often carry fragments of mantle rock called xenoliths. Just as chondrites are time capsules of the solar system formation, xenohths are those of the Earth’s mantle. Studies of these xenoliths, complemented by experimental petrology and seismic geophysics, have revealed the mantle composition and stratigraphy, and even effects of geodynamics. Convection of the upper mantle led to mid-ocean rifting and subduction of the oceanic plate under the continental crust. Melt and fluid extraction from the subducted plate led to complex alteration processes (metasomatism) of the the mantle rocks [101,102]. Previously depleted mantle rocks can be re-enriched with incompatible elements such as Ti, Zr and Y. Evidence of metasomatism through melt or fluid infiltration can be found in VI. GEOLOGICAL
SAMPLES
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S.H. Sie / pPIXE
applications in geology and mineralogy
xenoliths both in the presence of certain minerals in the assembly (e.g. hydrous phases) or as zoning of the minerals. Zoning however could also be due to exchange reactions between coexisting phases due to changes in P and T. Micro-PIXE can be used to delineate these effects. Griffin et al. [103] studied trace element zoning in garnets from sheared xenoliths from the Frank Smith mine, shown previously to be zoned in major and minor elements [104] attributed to melt infiltration. Using diffusion models and estimated diffusion coefficients for Zr, it was concluded that infiltration occurred shortly ( < 300 yr) before eruption. A similar study on garnets from xenoliths from a minette volcanic intrusion in Colorado [105], led to the conclusion that the metasomatism was preceded by heating prior to melt infiltration and again that the process was rapid (< 15 000 yr). A micro-PIXE study of spine1 lherzolite xenoliths from West Victoria [106] metasomatized by fluids released by crystallizing silicate melt, delineated the trace element distribution. The study showed that the bulk of the fluid introduced trace elements reside in specific minerals (e.g. REE in apatite, Ba in mica) and not in grain boundaries. This contrasts with a scanning micro-PIXE study by Fraser et al. [42], which showed the presence of trace Sr on grain boundaries in a serpentinized garnet lherzolite. Another study of West Victorian xenoliths from a different source about 100 km away [107] showed characteristics consistent with mainly basaltic melt infiltration, demonstrating the heterogeneous nature of these processes and their effects on the rock composition. 4.5. Mineral and melt inclusions Minerals often contain inclusions, which are fragments of minerals or portions of the melt or fluid trapped during crystallization. Isolated from further evolution, they preserve the chemical composition and conditions of the system at the moment of trapping. Micro-PIXE is ideally suited to study these typically minute (few tens of microns or less) inclusions. Diamonds are one example of minerals produced in the upper mantle at a depths greater than N 150 km. Trace element data obtained in a micro-PIXE study of inclusions in diamonds from West Australia [108] indicated disequilibrium conditions, leading to the conclusion that the diamonds grew in an open system. A similar study of garnet inclusions from African diamonds [109] showed evidence of metasomatic events, as well as a wide a range of formation temperatures using the Ni garnet thermometer. Systematic studies of diamond inclusions can be used to deduce mantle stratigraphy, and palaeogeotherms in cratonic areas that produce these diamonds, and refine the application of the Ni thermometry m diamond exploration in general [11,86-901.
Olivine is the most abundant mantle mineral, and those with the high Mg content are believed to crystallize from primitive magma. Inclusions of melts trapped in such olivine can therefore be assumed to represent primitive mantle. Micro-PIXE has been applied to study melt inclusions in olivine from several tectonic environments and lunar material [83]. The heterogeneity of primitive mantle is evident in the wide range of the ratio of LILE (large-ion lithophile elements, e.g. Rb, Sr, Ba) to HFSE (high field strength elements, e.g. Ti, Zr, Nb, Ta) groups of geochemical marker elements. Ratios of elements between these groups are known to be distinctive in different tectonic environments [llO]. 4.6. Fluid inclusions Fluids trapped during the formation of minerals in hydrous environments (e.g. hydrothermal systems, or during fluid metasomatism) as well as in magmatic systems form fluid inclusions, which can often include vapor phases and precipitated daughter minerals. The large penetration of proton beam can be exploited to permit analysis of the fluid composition without decrepitating the inclusion. Micro-PIXE has been applied to study fluid inclusions in quartz from hydrothermal systems [ 111,26,27,112] and in combination with PIGE [28,27]. PIXE allows the determination of elements typically heavier than S, while PIGE is used to determine F, Na and Al. Ryan et al. [26] used the K,/K, ratio of Cl from the saline fluid to determine the depth to within k 1.5 km. Laser ablation with ICP techniques [20] and SXRF [20,22] have also been applied to fluid inclusion studies. 4.7. Experimental
petrology
Detailed modelling of magma extraction from primitive mantle relies on accurate knowledge of partitioning of elements between coexisting phases. The partition coefficients can be derived empirically from naturally occurring assemblages reliably only if the system was closed and equilibrated. Laboratory simulations provide the alternative, where conditions are controllable. With typically small quantities of samples involved, micro-PIXE is an ideal analytical tool, offering the prospect of systematic studies of partitioning of the geochemical marker elements at levels comparable to natural systems. With micro-PIXE, MDL in mafic silicates of around 2 ppm are obtained readily (5-10 min with a 15-20 km beam for a collected charge of 3 PC) for the LILE and HFSE, but for the REE the detection limit is poor at around 50-200 ppm because of the limitations of the EDS system. An experimental study of partitioning of Nb and Ta between basaltic melt and the crystallizing phases has
407
S. H. Sie / p-PIXE applications in geology and mineralogy
suggested large fractionation effects in the partitioning of Nb and Ta, hitherto assumed to be geochemically coherent [113]. Similarly, systems of mixed material (e.g carbonatite and silicate) [114,115], the presence of volatiles and fluids [116] and effects of low partial melting [117] can now be studied at levels close to natural systems. 4.8. Sulfide mineralogy Micro-PIXE has been used to determine the trace element distribution in sulfide ore samples [118-1201 and mill concentrate [S], with emphasis on residence of precious metal for beneficiation applications. For example, gold occurs mainly as free gold, but in refractory ore it is locked in solid solution or as submicroscopic inclusions in other phases normally relegated to the tailings e.g. pyrite (FeS,) and arsenopyrite (FeAsS). Micro-PIXE can determine Au with MDL of 5 ppm in pyrite, but in arsenopyrite interference with the “tail” of As increases the MDL to _ 20 ppm. Cabri et al. [121,122] made a comparison of the effectiveness of SIMS and micro-PIXE, concluding that SIMS [122] is more sensitive although the quantification is more elaborate requiring implanted standards. SXRF has also been used in a similar study [123], but its routine use in an industrial context may not be practical. Similar to gold, platinum group elements (PGE: Pt, Pd, Rh, Ir, Ru, Re, OS) occur mainly as readily separable free PGM (platinum group minerals) but sulfides often present in the deposit can be significant carriers [4,124-1271. For example, a micro-PIXE study of the Merensky Reef ore showed pentlandite ((Fe,Ni),S,) as a significant carrier of Ru, Rh, Pd at levels up to few hundred ppm, and pyrrhotite (FeS) as carrier of Ru (8-12 ppm) [4], with typical MDL values of 2-6 ppm. 4.9. Sulfide ore genesis The use of trace element distribution by micro-PIXE in ore genesis studies is beginning. A detailed study of the trace element distribution in zoned pyrite in an epithermal system [128] revealed a broad correlation of Cu, Sb, Se with As, and with Ag-Au grades. Many significant massive sulfide deposits were formed initially by volcanic-exhalative processes followed by various physico-chemical reactions associated with hydrothermal activities and possibly deformation which remobilize and concentrate the metals. Pyrite in these volcanogenic massive sulfide (VMS) deposits can be used as a guide to locate mineralized regions, and its morphology and trace elements (Cd, Sb, Sn, In, Se, Te) distribution carry the signature of ore genesis. Huston et al. [129] have determined such distribution in a selection of VMS deposits using EMP for analysis of Ag, and Hg content of Au, and micro-PIXE to
determine the Au level in pyrite and arsenopyrite. systematic, spatially controlled micro-PIXE study Zn-Pb rich and Cu-rich VMS deposits is reported these Proceedings [ 1301. 4.10. Trace element
A of in
in Au
Free Au is nearly universally alloyed with Ag, at proportions controlled by the ambient temperature, activity of S and salinity of the mineralizing fluid as well as the source rocks. Elements occurring in the source rock (Fe, Cu, Zn, Hg, Pb) are also transported and may coprecipitate at trace levels with Au. Their distribution in gold can reflect genetic associations and may thus be used as discriminants in gold exploration programs which involve the collection of detrital Au grains. A preliminary systematic micro-PIXE study on bedrock and alluvial Au grains from several prospects in Tasmania show indications of differences among different deposit types, styles of mineralization and mineralogical associations [3]. This study higlights the limits of the EDS system: the MDL for most elements adjacently below Au are > 0.1%. Elements immediately above Au (Hg, Pb, Bi) can be detected through their Ly lines with MDL values of * 200 ppm. A WDS system will significantly improve the detection limits.
5. Summary and conclusion Significant progress and mature methodologies have been achieved by micro-PIXE in a number of areas of applications: - In studies involving mafic silicates and other mantle minerals, geochemical marker trace elements such as LILE (Rb, Sr, K,) and HFSE (Zr, Nb, Ta) can be detected with ppm sensitivity with good efficiency. Other trace elements such as Ni, Mn and Zn can be used in geothermobarometry. Ni garnet thermometry is now used extensively in diamond exploration to assess prospectivity of kimberlites. Thermoand barometry based on other trace elements are being developed. - Zoning features in mantle minerals reveal geodynamic effects and details of metasomatism, and are used to deduce timescales of geological events. Zoning in minerals in hydrous environment reveals deposition history. - Precious metal distribution in sulfides has direct applications in industry for devising beneficiation strategy. While in specific cases other methods (e.g. SIMS in Au determination) may be more sensitive, microPIXE is still competitive for large scale applications and convenient for quantification. - Systematic application of trace element distribution to ore genesis studies with implications in exploration programs have begun. VI. GEOLOGICAL SAMPLES
408
S.EI. Sie / F-PIXE applications in geology and mineralogy
- Micro-PIXE application in fluid inclusion studies is progressing rapidly, with precision and accuracy improving steadily. Although SXRF is a competing method, quantitative analysis with micro-PIXE is more readily achieved. Combined with PIGE, micro-PIXE might have decisive advantage. - One area developed earlier may need to be revisited: e.g. applications in zircon for geochronology through the U/Pb ratios. Although ion microprobes predominate in this area, micro-PIXE may be used as a screening tool and to calibrate standards.
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[79] H. Blank, R. Nobiling, K. Traxel and A. El Goresy, Lunar. Planet. Sci. 12 (1982) 49. [SO] H. Blank, A. El Goresy, J. Janicke, R. Nobiling and K. Traxel, Lunar. Plan. Sci. 14 (1983) 51. [Sl] H. Blank et al., Nucl. Instr. and Meth. B3 (1984) 681. [82] H. Blank et al., Earth Planet Sci. Lett. (1984) 19. [83] A.V. Sobolev, S.H. Sie and R.A. Binns, Trace elements in primary magmas: proton microprobe analysis of melt inclusions in minerals submitted to Contr. Min. Petr. [X4] R.O. Moore, W.L. Griffin, J.J. Gurney, C.G. Ryan, D.R. Cousens, S.H. Sie and G.F. Suter, Trace element geochemistry of ilmenite megactysts from the monastery kimberlite, South Africa, Lithos 29 (1992). [85] W.L. Griffin, C.G. Ryan and D.J. Schulze, Proc. 5th Kimberlite Conf., Brazil, 1991, ed. H.O.A. Meyer, Brazilian Geological Survey Publication. [86] W.L. Griffin, C.G. Ryan, D.R. Cousens, S.H. Sie and G.F. Suter, Nucl. Instr. and Meth. B49 (1990) 318. [87] W.L. Griffin, C.G. Ryan, J.J. Gurney, N.V. Sobolev and T.T. Win, Proc. 5th Kimberlite Conf., Brazil, 1991, ed. H.O.A. Meyer, Brazilian Geological Survey Publication. [SS] W.L. Griffin, C.G. Ryan, S.Y. O’Reilly, P.H. Nixon and T.T. Win, ibid. [89] W.L. Griffin, S.Y. O’Reilly, C.G. Ryan and M.A. Waldman, ibid. [90] J.X. Zhou, W.L. Griffin, A.L. Jaques, C.G. Ryan and T.T. Win, ibid. [91] W.L. Griffin, N.V. Sobolev, C.G. Ryan, N.P. Pokhilenko, ES. Yefimova, Trace elements in garnets and chromites: diamond formation in the Siberian lithosphere, Lithos. in press. [92] F.R. Boyd, Geochim. Cosmochim. Acta 37 (1973) 2533. [93] A.A. Finnerty and F.R. Boyd, in: Mantle Xenoliths, ed. P.H. Nixon (Wiley, 1980) p. 381. [94] B.J. Wood and S. Banno, Contr. Min. Petr. 42 (1973) 109. [95] H.St.C. O’Neill and B.J. Wood, Contr. Min. Petrol. 70 (1979) 59; 72 (1980) 337. [96] G.E. Adams and F.C. Bishop, Contr. Min. Petr. 94 (1986) 230. [97] N. Shimizu and C.J. Allegre, Contr. Min. Petr. 67 (1978) 41. [98] J.S. Delaney, J.V. Smith, J.G. Dawson and P.H. Nixon, Contr. Min. Petr. 71 (1979) 157. [99] R.L. Hervig, J.V. Smith, I.M. Steele and J.B. Dawson, Earth Planet Sci. Lett. 50 (1980) 41. [lOOI W.L. Griffin, D.R. Cousens, C.G. Ryan, S.H. Sie and G.F. Suter, Contr. Min. Petr. 103 (1989) 199. [loll A.E. Ringwood, Composition and Petrology of the Earth’s Mantle (McGraw-Hill, New York, 1975). [102] D.H.Green and A.E. Ringwood, J. Geophys. Res. 68 (1963) 937. [103] W.L. Griffin, D. Smith, F.R. Boyd, D.R. Cousens, C.G. Ryan, S.H. Sie and G.F. Suter, Geochim. Cosmochim. Acta 53 (1988) 561. [104] D. Smith and F.R. Boyd, in: Mantle Xenoliths, ed. P.H. Nixon (Wiley, 1987) p. 551. ]105] D. Smith, W.L. Griffin, C.G. Ryan and S.H. Sie, Contr. Min. Petr. 107 (1991) 60. [106] S.Y. O’Reilly, W.L. Griffin and C.G. Ryan, Contr. Min. Petr. 109 (1991) 98.
VI. GEOLOGICAL
SAMPLES
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S.H. Sie / p-PIXE applications in geology and mineralogy
[107] A. Greig, S.H. Sie and I.A. Nicholls, these Proceedings (6th Int. Conf. on PIXE in its Analytical Applications, Tokyo, 1992) Nucl. Instr. and Meth. B75 (1993) 411. [lo81 W.L. Griffin, L. Jaques, S.H. Sie, C.G. Ryan, D.R. Cousens and G.F. Suter, Contrib. Miner. Petr. 99 (1988) 143. [109] W.L. Griffin, J.J. Gurney and C.G. Ryan, Contrib. Min. Petr. 110 (1992) 1. [llO] J.A. Pearce and J.R. Cann, Earth. Planet. Sci. Lett. 19 (1973) 290. [ill] E.E. Horn and K. Traxel, Chem. Geol. 61 (1987) 29. [112] C.G. Ryan, CA. Heinrich and T.P. Mernagh, Proc. 3rd Int. Conf. on Nuclear Microprobe Technology and Applications, Uppsala, 1992, to be published in Nucl. Instr. and Meth. B. [113] T.H. Green, S.H. Sie, C.G. Ryan and D.R. Cousens, Chem. Geol. 74 (1989) 201. [114] T.H. Green, J. Adam and S.H. Sie, Mineral. Petrol. 46 (1992) 179. 11151 R.J. Sweeney, D.H. Green and S.H. Sie, Earth Planet. Sci. Lett. 113 (1992) 1. [116] J. Adam, T.H. Green and S.H. Sie, Contr. Min. Petr., in press. [117] S.H. Sie, R.J. Sweeney, D.H. Green and G.F. Suter, Proc. 7th Australian Conf. on Nuclear Techniques of Analysis, Melbourne, ISSN 0811-9422 (Australian Institute of Nuclear Science and Engineering, 1991) p.126. [118] L.J. Cabri, J.L. Campbell, J.H.G. Laflamme, R.G. Leith,
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