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of granites from the same field area yielded Micro-Characterization of Zircon Crystals: Insights into Deep Time and the Process of Mountain Building Final Report of Student-Faculty Collaborative Challenge Grant Results Claire Kauffman (mentor: Dr. Paul Tomascak, Department of Earth Sciences) small but interpretable concentration variations. Incorporation of Ti in zircon is known to be temperature sensitive, and we interpret the variation in concentrations to reflect a group of granites with distinct intrusion temperatures, consistent with bulk elemental and isotopic data from those rocks. We had originally proposed to measure oxygen isotope ratios of these crystals, but Project Summary The purpose of this project was to characterize the internal structure and chemical variation of crystals of the mineral zircon (ZrSiO4) from granitic rocks of the Appalachian mountain belt in southwestern Maine. The study is a segment found that the sample size was too small for application of the instrumentation at McGill University. We were fortunate that we could accomplish similar research objectives by employing a different analytical technique in the same laboratory. of a larger project involving students from Oswego and Buffalo State College. Our findings include the following: (1) The fine-scale internal chemical structure of zircon crystals from contrasting granite types were captured using an electron probe microanalyzer (EPMA). Secondary electron and cathode luminescence images revealed a variety of features, most interpreted as igneous (“primary”). In one sample complex internal zonation patterns suggest older, inherited cores. The data allow for the selection of appropriate grains for continuing U-Pb geochronology of these samples. (2) Analyses by laser ablation inductivelycoupled plasma source mass spectrometry of the titanium (Ti) concentrations of these zircon samples plus zircon from a related suite Introduction In southwestern Maine, a variety of granitic rocks crop out around Sebago Lake (Figure 1). Many of these rocks have been studied previously for elemental and isotopic geochemistry (Tomascak et al., 1996), but only recently have the migmatitic rocks of the area begun to receive detailed study. The principal goal of these studies is to understand the nature of crustal melting by defining the relationships between different generations of granitic rocks, the primary record of crustal growth in the continents. One of the most critical steps in defining ‘genetic’ relations amongst spatially associated groups of granitic rocks is through precise geochronological constraints. The mineral zircon is the primary igneous geochronometer used in (Speer, 1982; Cherniak and Watson, 2001). As studies of ancient rocks. In order to achieve high technology advances, tools become more precision geochronology, internal complexities widespread to examine physical and chemical of zircon crystals need to be mapped out. features at the micrometer (0.001 mm) scale in Additionally, the trace element budget of zircon minerals. Modern geochronologic analysis of single can yield information about the chemical and thermal environment of the magma from which zircon crystals has pointed out that individual the zircon crystallized, information that can grains are frequently internally complex. This compliment studies of bulk compositions of complexity can manifest itself in complicated, rocks. difficult to interpret, geochronologic data (Bickford et al., 1981; Mezger and Krogstad, 1997). To this end, the characterization of zircon grains by a micro-chemical technique has become an important first step in the analytical process. The approach of choice here is examination by electron beam methods (EPMA), which combine submicrometer-scale spatial resolution with essentially non-destructive measurement (Hanchar and Miller, 1993). Using EPMA, individual crystals can be inspected for the presence of “inherited” cores or recrystallization features that do not Figure 1. Geological map of the study area in southwestern Maine. The locations of the three samples analyzed by EPMA are noted with x symbols. The location of the set of pluton margin samples is noted by the shaded oval. reflect the igneous history that we endeavor to date. Hence, potentially aberrant age data can be viewed in the context of the pre-igneous history of the rock, yielding further valuable insight into Zircon is a common accessory mineral in the mountain-building process. granitic rocks. Uranium-lead (U-Pb) dating of When zircon crystallizes from a magma, it zircon permits ages precise to better than 1 Myr incorporates numerous minor and trace for rocks in the 300-400 Myr range. Zircon is elements. The incorporation of these elements additionally notable for its enormous resistance depends somewhat on the activity of the to both high temperature alteration and diffusion elements in the magma as well as on the 2 temperature, with higher crystallization mineral surface, which generates an array of temperature translating to “freer” inclusion of secondary exotic elements. Among the elements that may bombarded atoms. The first mode of analysis is substitute into zircon by this process is Ti, which for secondary electron imaging. This is not a has identical electrical charge to Zr and is chemical technique, but allows the user to similar in ionic radius. It has been observed that evaluate the texture of the surface of grains. The Ti concentrations in igneous zircon can be used energies of the secondary X-rays are measured as a thermometer: concentration is a in a spatially resolved spectrometer, allowing for crystallization temperature proxy (Watson and the fine-scale chemical imaging of crystals. In Harrison, 2005). this mode X-rays (called characteristic back-scattered of the electron imaging, or BSE) enrichment in elements with high atomic number shows up as bright regions, Methods Zircon makes up < 0.001% of most granites. and regions enriched in lighter elements show up Hence, the physical removal and concentration dark. A third mode, cathode luminescence (CL), of these tiny (predominantly < 0.1 mm) crystals allows another window into chemical variations. from rocks is non-trivial. Using methods In this case instead of highlighting variation in established in the Oswego labs, prior to the atomic number, CL shows enrichment in of a beginning of this project Ms. Kauffman made group of “activator elements” such as several of zircon separates from a set of key granite the lanthanides. Images from BSE and CL are samples. For part of the current work her used complimentarily. Under the direction of samples zircon research faculty at Syracuse, Ms. Kauffman was separates from a group of related granites from able to carry out the image acquisition fully elsewhere in the field area in Maine. independently. were supplemented with For in situ analyses, hand-picked zircon Acquisition of precise Ti concentrations in crystals were first encased in an inert epoxy tiny zircon crystals requires a spatially resolved mount and polished to achieve a surface capable method of analysis. Electron probe is generally of being micro-characterized. A benefit to this not sensitive enough to adequately quantify the technique is that individual crystals can later be low levels of Ti that may be present (e.g., < 50 plucked from the mount and dissolved for U-Pb ppm, or parts per million). Thus we elected to geochronology, part of the larger study. examine the selected grains by plasma source The first analytical tool used was the newly- mass spectrometry (ICP-MS), with samples upgraded EPMA at the Department of Earth introduced by ablation from an ultraviolet laser. Sciences at Syracuse University. This instrument Dr. William Minarik of the Department of Earth focuses a beam of electrons on the polished and Planetary Sciences at McGill University 3 gave Ms. Kauffman the necessary tutorial in Crystals examined in this study were the smaller operation of the instrument, and she lead the grains, with an expectation that the larger acquisition of the analyses, with her faculty crystals have a greater likelihood of inheritance advisor present to help. Unlike electron probe and internal complications. Sample SG01-35b is measurement, the laser beam disintegrates a a two-mica granite associated with migmatites in finite volume of sample, with more destruction the central portion of the study area. Zircon from for longer ablation time. The 40 micrometer this sample was mainly in the form of colorless, laser spot was sufficiently large that only one or faceted prisms, often showing igneous zonation some times two analyses per crystal were in visible light (Figure 2). Sample SG01-78 is a possible. muscovite granite from the eastern portion of the study area. Zircon in this sample was in stubbier forms that the other two samples. Results Zircon grains from three samples were Zircon grains from sample SG02-15 are analyzed by EPMA, and images were acquired primarily chemically homogeneous, and many for multiple grains from each sample. Sample are unzoned with minimal CL activation (Figure SG02-15 is a two-mica granite from the western 3). Where cores are apparent, they show simple part of he study area. Its zircon population was (igneous) zonation in CL but not in BSE. composed of both larger slightly dark crystals and smaller more pristine grains (Figure 2). Figure 2. Bulk zircon separates from SG02-15 (left) and SG01-35b (right), viewed in plane polarized light. Scale bars are 100 micrometers long. Note the two distinct size populations in SG02-15. Figure 3. EPMA images from two zircon grains in sample SG02-15 (BSE=top, CL=bottom). 4 Grains from sample SG01-35b show an The three samples documented for internal overall similar lack of zonation to SG02-15, chemical variations were also analyzed for Ti although some grains show rhythmic (igneous) concentration by LA-ICP-MS. In addition to zonation in BSE, although cores are not these, a set zircon grains from five granite apparent. Zircon crystals from sample SG01-78 samples from the northeast margin of the Sebago show a variety of internal features not in pluton were examined, as they were made common with the other two samples. Most available by a separate ongoing student project. grains show a distinct, zoned core. This zoning The overall precision of the method, as defined is in some samples simple and rhythmic, but in by repeat analyses of a benitoite standard, was others it is complex and boundaries between ±4.5% (1 sd). The sample data are reported in zones are cuspate, suggesting resorption during Table 1. metamorphism or detrital transport and later new overgrowth (Figure 4). Table 1. Titanium concentration data for samples from this study mean sample Ti (ppm) ± 1 sd n 2.8 6 33 7 14 9 27 5 12 4 3.6 3 7.0 5 112 6 [range] SG02-15 13.4 [5.5-15.4] SG01-35b 51 [16.5-96.5] SG01-78 11 [1.6-2.7] LL1077 25 [4.7-65.3] LL178 15 [7.2-32.1] LL164-55 5.2 [3.1-9.4] LL153-87 Figure 4. EPMA images of zircon crystal from sample SG01-78 (top=BSE, bottom=CL). Note the complex CL zoning in the core of the crystal, common in grains from this sample. 23.8 [15.6-32.0] LL300-54 122 [16.9-295] 5 bulk rock Ti concentration can be effectively used to constrain the activity of Ti in the Interpretations The imaging studies of zircon grains suggests magmas from which these crystals derive, the that only a small proportion of grains from variability in concentrations of crystals from the sample SG01-78 will be viable for U-Pb same sample poses a problem for application of geochronology. the geothermometer, as each sample will yield a The Ti analyses produced a number of large range in crystallization temperatures as a conclusions. The samples exhibited a wide range result. This phenomenon, as it applies to other of concentration, from extremely low (3.1 ppm) igneous thermometers, has lead to discussion as to rather high for rocks of these bulk to whether the maximum temperature is most compositions (295 ppm). Indeed, among crystals likely to be “true,” with lower temperatures from a single sample the variability was reflecting variable degrees of re-equilibration, typically large (e.g., standard deviations outside diffusion or other secondary effects (Watson and variation analytical Harrison, 1983). Ultimately we are likely to use uncertainty). The one sample with large enough a model approach that will define temperatures zircon crystals to assess core versus rim relations relative to a variety of geologically reasonable (SG01-78) showed correlated variability: the melt compositions (Ferry and Watson, in press). more recent external zircon growth (“rim”) had That work is ongoing. expected purely from The five samples from the Sebago pluton consistently higher concentrations than the crystal cores; in one instance the offset was by a margin factor of seven. concentrations, despite the fact that they were The three primary samples from this study have broadly Ti spaced no more than several hundred meters concentrations (TiO2 = 2300 ppm in SG02-15, based on mineralogy and texture of the rocks, to 2100 ppm in SG01-35b, and 2000 ppm in SG01- be quite heterogeneous. Bulk rock elemental 78), but they differ somewhat in alumina data are needed from these samples in order to saturation (molar Al2O3/(CaO+Na2O+K2O) = arrive at crystallization temperatures from the 1.52, zircons. 1.06, rock variable apart. The sample compositions are expected, and bulk similarly Ti 0.92 uniform yield respectively). This parameter may be a reasonable means of inferring melt structure, which has an effect on Epilogue Ms. Kauffman presented her results at Quest the viability of temperature calculations. It is in the in 2008 and subsequently completed her B.S. in thermodynamic model for the Ti-in-zircon Geology in August, 2008. She has taken a year thermometer (Watson et al., 2006). Even if the off from school before beginning graduate however not currently considered 6 calibrations for the Ti-in-zircon and Zr-inrutile thermometers. Contributions to Mineralogy and Petrology Tomascak, P.B., Krogstad, E.J., and Walker, R.J. (1996) The nature of the crust in Maine, U.S.A.: evidence from the Sebago batholith. Contributions to Mineralogy and Petrology, v. 125, p. 45-59. Watson, E.B., and Harrison, T.M. (1983) Zircon saturation revisited: temperature and composition effects in a variety of crustal magma types. Earth and Planetary Science Letters, v. 64, p. 295-304. Watson, E.B., and Harrison, T.M. (2005) thermometer reveals minimum melting conditions on earliest Earth. Science, v. 308, p. 841-844. Watson, E.B., Wark, D.A., and Thomas, J.B. (2006) Crystallization thermometers for zircon and rutile. Contributions to Mineralogy and Petrology, v. 151, p. 413433. school, but she continues to work on the zircon project out of pure interest in the research. She currently visits the Isotope Geochemistry Lab at Syracuse University weekly, performing U-Pb chemical separation in their ultra-clean lab, in order to acquire high precision U-Pb dates of the samples she worked on during the SFCCGsupported project. We intend to complete a manuscript for publication of both the geochronologic data and as the Ti concentration results in 2009, and Ms. Kauffman is interested in presenting the results at the Joint Assembly of the American Geophysical Union (May, 2009, in Toronto). References Cited Bickford, M.E., Chase, R.B., Nelson, B.K., Shuster, R.D., and Arruda, E.C. (1981) U-Pb studies of zircon cores and overgrowths, and monazite: Implications for age and petrogenesis of the northeastern Idaho batholith. Journal of Geology, v. 89, p. 433457. Cherniak, D.J. and Watson, E.B. (2001) Pb diffusion in zircon. Chemical Geology, v. 172, p. 5-24. Hanchar, J.M. and Miller, C.F. (1993) Zircon zonation patterns as revealed by cathodoluminescence and backscattered electron images: Implications for interpretation of complex crustal histories. Chemical Geology, v. 110, p. 1-13. Mezger, K. and Krogstad, E.J. (1997) Interpretation of discordant zircon ages: an evaluation. Journal of Metamorphic Geology, v. 15, p. 127-140. Speer, J.A. (1982) Zircon. In: Orthosilicates (ed. Ribbe, P.H.). Mineralogical Society of America Reviews in Mineralogy, v. 5, p. 67112. Ferry, J.M., and Watson, E.B. (in press) New thermodynamic models and revised 7