Advanced in situ geochronological and trace element microanalysis by laser ablation techniques

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Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was developed in 1985 and the first commer-cial laser ablation systems were introduced in the mid 1990s. Since then, LA-ICP-MS has become an important analytical tool in the
  Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was developed in 1985 and the first commer-cial laser ablation systems were introduced in the mid 1990s.Since then, LA-ICP-MS has become an important analyticaltool in the earth sciences. Initially, the main interest for geol-ogists was in its ability to quantitatively determine the con-tents of a wide range of elements in many minerals at very low concentrations (a few ppm and below) with relatively high spa-tial resolution (spot diameters of typically 30–100 µm). Thepotential of LA-ICP-MS for rapid in situ U–Th–Pb geo-chronology was already realised in the early to mid 1990s.However, the full potential of LA-ICP-MS as the low-costalternative to ion-microprobe techniques for highly preciseand accurate in situ U–Th–Pb age dating was not realiseduntil the relatively recent advances in laser technologies andthe introduction of magnetic sectorfield ICP-MS (SF-ICP-MS) instruments. In March 2005, the Geological Survey of Denmark and Greenland (GEUS) commissioned a new laserablation magnetic sectorfield inductively coupled plasma mass spectrometry (LA-SF-ICP-MS) facility employing a ThermoFinnigan Element2 high resolution magnetic sector-field ICP-MS and a Merchantek New Wave 213 nm UV laserablation system. The new GEUS LA-SF-ICP-MS facility is widely used on Survey research projects in Denmark andGreenland, as well as in collaborative research and contractprojects conducted with partners from academia and indus-try worldwide. Here, we present examples from some of thethese ongoing studies that highlight the application of thenew facility for advanced geochronological and trace element in situ microanalysis of geomaterials. The application of LA-SF-ICP-MS based in situ zircon geochronology to regionalstudies addressing the Archaean geology of southern WestGreenland is presented by Hollis et al  . (2006, this volume). Zircon U–Pb geochronology using LA-SF-ICP-MS In situ U–Th–Pb geochronology was developed in the mid-80s with the introduction of ion-microprobe techniques, mostcommonly referred to as secondary ion mass spectrometry (SIMS) and sensitive high resolution ion microprobe(SHRIMP). The advantage of in situ U–Th–Pb geochronol-ogy over conventional chemical dating by isotope dilutionthermal ionisation mass spectrometry (ID-TIMS) is the capa-bility to analyse different domains in heterogeneous singlezircons with high spatial resolution (spot diameters of typi-cally 10–30 µm). This allows resolution of igneous and meta-morphic events separated by intervals of only a few tens of million years from polychronic zircons. The disadvantages of ion-microprobe techniques are the very high purchasing andoperating costs for the instrument. The rapid improvementsin laser based U–Th–Pb geochronology makes it now possi-ble to obtain in situ U–Th–Pb geochronological data withcomparable spatial resolution as well as analytical precisionand accuracy at only a fraction of the costs of ion-microprobetechniques (e.g. Jackson et al  . 2004; Janouˇsek  et al  . 2006). Dating of magmatic and metamorphic events The capabilities of LA-SF-ICP-MS for the precise and accu-rate U–Pb age dating of relatively young igneous zircons aredemonstrated by the analysis of a population of 40 zirconsextracted from a gabbro from the Coastal Cordillera at Tre-gualemu, central Southern Chile. The gabbro is believed tohave been formed by regional extension during the late Trias-sic to early Jurassic (Charrier 1979). Three zircons proved tobe too small for analysis (< 30 µm). The results for the remain-ing 37 zircons (Fig. 1) define a highly precise igneous con-cordia age of 203 ±2 Ma (2 σ  ; MSWD = 1.7) and indicate a Late Triassic (Rhaetian) intrusion age of the gabbro. An even Advanced in situ geochronological and trace element microanalysis by laser ablation techniques Dirk Frei,Julie A.Hollis,Axel Gerdes,Dan Harlov,Christine Karlsson,Paulina Vasquez,Gerhard Franz,Leif Johansson and Christian Knudsen © GEUS, 2006. Geological Survey of Denmark and Greenland Bulletin 10, 25–28. Available at:  25 240220 200 180160 0.0250.0270.0290.0310.0330.0350.0370. 207 Pb/ 235 U    2   0   6    P   b   /    2   3   8    U GEUS LA-SF-ICP-MS:Igneous Concordia age203 ± 2 Ma (95% conf.)MSWD: 1.7 Gabbro, central southern Chile (Sample PV 4-22) Fig. 1. Concordia diagram for igneous zircons from a gabbro from theCoastal Cordillera in central Southern Chile.  younger igneous age of 158 ±2 Ma was recently obtained fora zircon xeno- or phenocryst derived from a newly discoveredcarbonatite in southern West Greenland (Steenfelt et al  . 2006,this volume).The high sensitivity of the Element2 SF-ICP-MS allowsU–Pb zircon age dating with a laser spotsize of 30 µm or less,depending on the Pb content of the zircons. This makes itfeasible to analyse different age domains in polychronic zir-cons, e.g. igneous cores and metamorphic rims. For example,zircons from an orthogneiss from the Nuuk region, southern West Greenland, display characteristic textures in back-scat-tered electron (BSE) and cathodoluminescence (CL) picturesthat are interpreted as igneous cores surrounded by rims grownduring a metamorphic event (see inset in Fig. 2). The U–Pbage data of the cores suggest an emplacement of the igneousprotolith at c  . 3660 Ma, while the rim data indicate meta-morphism close to 2700 Ma (Fig. 2). Dating of detrital zircons  Analyses of the crystallisation ages of detrital zircons in clas-tic sediments are a powerful tool in sedimentary provenanceanalysis. Accurate and precise U–Pb ages of >100 detrital zir-con grains in a sample are needed to detect all major sedi-mentary source components with statistical confidence (cf.Vermeesch 2004; and references therein). The relatively highcosts and the limited capacities of ion microprobe techniques( c  . 75 zircon age analyses per day) impose restrictions on thenumber of samples that can be studied. Because LA-SF-ICP-MS provides very high capacities (in excess of 300 zircon ageanalyses per day) without compromising accuracy and preci-sion, it constitutes the economic method of choice for prove-nance studies based on detrital zircon U–Pb ages. An example for detrital zircon age data obtained by LA-SF-ICP-MS is shown in Fig. 3, where the 207 Pb– 206 Pb agedistribution for a population of 100 zircon grains separatedfrom a Cambrian sandstone from Torekov, southern Sweden,are shown in a combined histogram and probability density distribution (PPD) diagram. The concordance filtered zircons(dark shaded area; 90–110% concordance, defined as 100*[ 206 Pb– 238 U age / 207 Pb– 206 Pb age]) show a polymodal agedistribution with a minor peak at ~1000 Ma and two majorpeaks at c  . 1150 Ma and 1650 Ma. The presence of two oldersedimentary sources ( c  . 2150 Ma and c  . 3050 Ma) is indica-ted by discordant grains (lighter shaded grey areas) that mostlikely suffered lead loss during their petrogenetic evolution. Figures of merit The short and long term precision and accuracy of LA-SF-ICP-MS for U–Pb zircon age dating has been assessed using two zircon reference materials, Plesovice (with an ID-TIMSage of 338 ±1 Ma; Aftalion et al  . 1989; provided by JanKosler, University of Bergen) and 91500 (ID-TIMS age =1065 ±0.4 Ma; Wiedenbeck et al  . 1995). The PL zircon isroutinely analysed as unknown for quality control purposesin every analytical session in the GEUS laboratory. Theresults for 16 analyses of the zircon from a typical single ana-lytical session are shown in Fig. 4A. They define a concordia age that is in excellent agreement with the ID-TIMS agereported by Aftalion et al  . (1989). Long-term precision (2 σ  )based on 109 analyses of the Plesovice zircon by two differ-ent operators was 2%, 2.3% and 1.1% for the 206 Pb/ 238 U, 207 Pb/ 235 U and 207 Pb/ 206 Pb ratios, respectively. The widely used 91500 zircon has so far only been analysed during oneanalytical session. The results for all seven analyses carried out 26 39003700350033003100290027002500 38003400300026002200 c. 2700 Man=8c. 3660 Man=4    2   0   6    P   b   /    2   3   8    U 207 Pb/ 235 U Fig. 2. Concordia diagram and inset of 207 Pb/ 206 Pb age plot for poly-chronic zircons from an orthogneiss in the Nuuk region, southern WestGreenland.  Inset  CL image shows laser ablation pits (pit diameter = 30µm) in igneous cores and metamorphic rims of polychronic zircons.Note the shallow depth (usually < 20 µm) of the laser ablation pits.  0 4    0    0    8    0    0   1   2    0    0   1    6    0    0   2    0    0    0   2   4    0    0   2    8    0    0    3   2    0    0    3    6    0    0   4    0    0    0    Age (Ma)    P  r  o   b  a   b   i   l   i  t  y 05101520253035 F   r  e   q u en c    y  Fig. 3. Combined display of histogram and probability density distribu-tion (PPD) diagram for zircons from a Cambrian sandstone fromTorekov, south-western Sweden. See text for explanations.  during this session (Fig. 4B) define a concordia age which isin excellent agreement with the ID-TIMS age reported by  Wiedenbeck et al  . (1995). Trace element analysis using LA-SF-ICP-MS Due mainly to the introduction of LA-ICP-MS techniques,the trace element signatures of individual minerals (e.g. gar-net, clinopyroxene, epidote, rutile, calcite) are now frequently being used to deduce the petrogenetic evolution of magmaticrocks, unravel water–rock interactions, identify geotectonicsettings and sediment sources, track down the pathways of potentially health-damaging pollutants, and to unravel thechange of seasurface and atmospheric temperatures.Further-more, LA-ICP-MS is an important tool for the characterisa-tion of synthetically produced geomaterials. Water–rock interaction Fluid-mediated mass transfer during metamorphism andmetasomatism is a hotly debated issue. Mobility of geochem-ically important trace elements during fluid flow has far-reaching implications for e.g. ore-forming processes and masstransfer during subduction. Detailed investigation of the mecha-nisms of element mobility during fluid–rock interaction on a grain-scale are pivotal for an understanding of the processesthat lead to the characteristic element enrichments and deple-tions observed in nature.In the Söndrum stone quarry in Sweden, a localised dehy-dration zone of 2.5 to 3 m width occurs in garnet-bearing granitic gneiss around an approximately 1 m wide pegmatoiddyke. Whole-rock chemistry suggests that the solid-statedehydration of the granitic gneiss to charnokite via low H 2 Oactivity fluids consisting principally of CO 2 and a minorbrine component was predominantly isochemical (Harlov et al  . 2006). Exceptions include Y and the heavy rare earth ele-ments (HREE), which are markedly depleted throughout thedehydration zone. In order to assess the mechanism of Y andHREE depletion, the trace element geochemistry of garnet was studied in a traverse across the dehydration zone. Garnetsfrom the pristine, unaltered granitic gneiss are characterisedby a strong negative Eu-anomaly and the steep, HREE en-riched pattern typical for garnet (sample SD45-600 in Fig.5).In contrast, garnets from within the dehydration zone show a less pronounced Eu-anomaly and are characterised by dra-matic Y and HREE depletions, leading to almost flat REEpattern (sample SD9-120 in Fig. 5). This observation pro-vides direct evidence for massive release of Y and HREE fromgarnets, the principal hosts of these elements in the graniticgneiss, via solid-state fluid–rock interaction which was alsoaccompanied by dehydration of hornblende and biotite toortho- and clinopyroxene. Analysis of synthetic geomaterials  Another application of LA-ICP-MS is the trace elementanalysis of synthetic geomaterials, e.g. products of experi-ments carried out for petrogenetical purposes. Since theexper-imentally produced mineral phases are usually very small, insitu microanalysis with high spatial resolution is needed. This was traditionally achieved by SIMS techniques. Because LA-ICP-MS analyses are much cheaper and facilities are much 380360340320300 0.0470.0490.0510.0530.0550.0570.0590.0610.340.360.380.400.420.440.46 GEUS LA-SF-ICP-MS:concordia age339.0 ± 2.7 Ma (95% conf.)MSWD: 2.1 A: Plesovice Zircon Standard ID-TIMS Age: 338 ± 1 Ma 98010201060110011401180 GEUS LA-SF-ICP-MSconcordia age1070 ± 10 Ma (95% conf.)MSWD: 0.27 B: 91500 Zircon Standard ID-TIMS Age: 1065 ± 0.4 Ma 207 Pb/ 235 U 207 Pb/ 235 U    2   0   6    P   b   /    2   3   8    U    2   0   6    P   b   /    2   3   8    U Fig. 4. Concordia diagrams for 16 analysisof the Plesovice zircon reference materialobtained during a single analytical ses-sion (  A  ); and 7 analysis of the 91500 zir-con reference material obtained during asingle analytical session ( B ). 27 SmEuGdTbDyHoErTmYbLu    C   1  n  o  r  m  a   l   i  s  e   d SD45-600SD9-120110100100010000 Fig. 5. Average chondrite-normalised REE-pattern of garnets from theunaffected granitic gneiss (SD45-600) and the dehydration zone (SD9-120). Note the depletion in HREE of garnets from the dehydration zone.  more widely available, LA-ICP-MS is increasingly used forthe trace element analysis of synthetic geomaterials.The inset in Fig. 6, for example, shows a back-scattered elec-tron photomicrograph of an experimental charge containing euhedral clinopyroxene and a coexisting anhydrous silicatemelt. The experimental charge was synthesised in order todetermine the clinopyroxene-melt trace element partition coef-ficients D  i (defined as D  i = [concentration of i] cpx  /[concen-tration of i] melt ), knowledge of which is important forgeochemical melt modelling purposes (Landwehr et al  . 2001).In order to test the ability of laser ablation techniques to cor-rectly determine partition coefficients from such experimen-tal charges, we have analysed clinopyroxenes and coexisting silicate melts from a number of experiments with both SIMS(at the NERC ion-microprobe facility in Edinburgh) andLA-SF-ICP-MS (at GEUS). The resulting partition coeffi-cients determined by SIMS and LA-SF-ICP-MS are graphi-cally compared in Fig. 6 for a representative sample. For allinvestigated samples, the partition coefficients determined by SIMS and LA-SF-ICP-MS are in excellent agreement, clearly demonstrating the reliability of laser ablation techniques forthe characterisation of synthetic geomaterials. Acknowledgements The establishment of the new LA-SF-ICP-MS facility was funded with agrant of the Danish Ministry of Education and Technology to theGeocenter Copenhagen (‘Geocenterbevilling’) and financial support by GEUS. PV thanks the DAAD for awarding a PhD scholarship. References  Aftalion, M., Bowes, D.R. & Vrána, S. 1989: Early Carboniferous U–Pb zirconage of garnetiferous, perpotassic granulites, Blansk´ y les massif, Czechos-lovakia. Neues Jahrbuch für Mineralogie, Monatsheft 4, 145–152.Charrier, R. 1979: El Triásico en Chile y regiones adyacentes de Argentina:Una reconstrucción paleogeográfica y paleoclimática. Comunicaciones 26 , 1–37.Harlov, D.E., Johansson, L., Van Den Kerkhof, A. & Förster, H.-J. 2006:The role of fluid flow and diffusion during localised, solid-state dehy-dration: Söndrum Stenhuggeriet, Halmstad, SW Sweden. Journal of Petrology   47 , 3–33.Hollis, J.A., Frei, D., van Gool, J.A.M., Garde, A.A. & Persson, M. 2006:Using zircon geochronology to resolve the Achaean geology of south-ern West Greenland. Geological Survey of Denmark and GreenlandBulletin  10 , 49–52. Jackson, S., Pearson, N.J., Griffin, W.L. & Belousova, E.A. 2004: Theapplication of laser ablation – inductively coupled plasma – massspectrometry to in situ U–Pb zircon geochronology. ChemicalGeology   211 , 47–69. Janouˇsek, V., Gerdes, A., Vrána, S., Finger, F., Erban, V., Friedl, G. &Braithwaite, C.J.R. 2006: Low-pressure granulites of the Liˇsov massif,Southern Bohemia: Viséan metamorphism of late Devonian plutonicarc rocks. Journal of Petrology 47 , 705–744.Landwehr, D., Blundy, J.D., Chamorro-Perez, E.M., Hill, E. & Wood, B.J.2001: U-Series disequilibria generated by partial melting of spinel lher-zolite. Earth and Planetary Science Letters 188 , 329–348.Steenfelt, A., Hollis, J.A. & Secher, K. 2006: The Tikiusaaq carbonatite andassociated kimberlites: a new alkaline magmatic province in the Nuukregion, southern West Greenland. Geological Survey of Denmark andGreenland Bulletin 10 , 41–44. Wiedenbeck, M., Allé, P., Corfu, F., Griffin, W.L., Meier, M., Oberli, F., vonQuadt, A., Roddick, J.C. & Spiegel, W. 1995: Three natural zircon stan-dards for U–Th–Pb, Lu–Hf, trace element and REE analysis. Geo-standards Newsletters 19 , 1–23. Vermeesch, P. 2004: How many grains are needed for a provenancestudy? Earth and Planetary Science Letters 224 , 441–451.  Authors’ addresses D.F., J.H. & C.Kn., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark  . E-mail:   A.G.,  Institute of Mineralogy, Johan-Wolfgang-Goethe University, Senckenberganlage 28, D-60054 Frankfurt, Germany  .D.H., GeoForschungsZentrum Potsdam, Section 4.1 Experimental Geochemistry and Mineral Physics, Telegrafenberg, D-14473 Potsdam, Germany  .C.Ka. & L.F.,  Department of Geology, University of Lund, Sölvegatan 12, S-22362 Lund, Sweden .P.V. & G.F.,  Institut für Angewandte Geowissenschaften, Technische Universität Berlin, Ernst-Reuter-Platz 1, D-10587 Berlin, Germany  . 28 10 -3 10 -2 10 -1 10 0 10 1 10 -3 10 -2 10 -1 10 0 10 1 LaCeNdSmLuTiZrHf ThU       D    S   I   M   S D LA-SF-ICP-MS 10 -3 10 -2 10 -1 10 0 10 1 10 -3 10 -2 10 -1 10 0 10 1 Experimentally determined Cpx-meltpartition coefficientsLaCeNd   SmLuTiZrHf ThU       D    S   I   M   S D LA-SF-ICP-MS 500  µ m Fig. 6. Comparison of clinopyroxene-melt trace element partition coeffi-cients ( open symbols : REE; solid symbols : HFSE) determined fromsynthetic run products by SIMS and LA-SF-ICP-MS. Inset  shows a BSEphotomicrograph of synthetic, euhedral clinopyroxene coexisting with aglassy melt produced experimentally at high pressure and temperature.Shallow, bright spots are SIMS ablation pits ( c . 20 µm diameter), whiledeeper, dark spots are laser ablation pits ( c . 30 µm in diameter).
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