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Li Isotopes and Trace Elements As a Petrogenetic Tracer in Zircon: Insights from Archean Ttgs and Sanukitoids

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Li Isotopes and Trace Elements As a Petrogenetic Tracer in Zircon: Insights from Archean Ttgs and Sanukitoids Contrib Mineral Petrol (2012) 163:745–768 DOI 10.1007/s00410-011-0697-1 ORIGINAL PAPER Li isotopes and trace elements as a petrogenetic tracer in zircon: insights from Archean TTGs and sanukitoids Anne-Sophie Bouvier • Takayuki Ushikubo • Noriko T. Kita • Aaron J. Cavosie • Reinhard Kozdon • John W. Valley Received: 7 March 2011 / Accepted: 14 September 2011 / Published online: 2 October 2011 Ó Springer-Verlag 2011 Abstract We report d7Li, Li abundance ([Li]), and other granitoids is significantly higher than from zircons in trace elements measured by ion probe in igneous zircons primitive magmas in oceanic crust. TTG zircons have d7Li from TTG (tonalite, trondhjemite, and granodiorite) and (3 ± 8%) and d18O in the range of primitive mantle- sanukitoid plutons from the Superior Province (Canada) in derived magmas. Sanukitoid zircons have average d7Li order to characterize Li in zircons from typical Archean (7 ± 8%) and d18O higher than those of TTGs supporting continental crust. These data are compared with detrital genesis by melting of fluid-metasomatized mantle wedge. zircons from the Jack Hills (Western Australia) with U–Pb The Li systematics in sanukitoid and TTG zircons indicate ages greater than 3.9 Ga for which parent rock type is not that high [Li] in pre-3.9-Ga Jack Hills detrital zircons is a known. Most of the TTG and sanukitoid zircon domains primary igneous composition and suggests the growth in preserve typical igneous REE patterns and CL zoning. [Li] proto-continental crust in magmas similar to Archean ranges from 0.5 to 79 ppm, typical of [Li] in continental granitoids. zircons. Atomic ratios of (Y ? REE)/(Li ? P) average 1.0 ± 0.7 (2SD) for zircons with magmatic composition Keywords Zircon Á Trace elements Á Lithium isotopes Á preserved, supporting the hypothesis that Li is interstitial SIMS Á Jack Hills Á TTG Á Sanukitoid and charge compensates substitution of trivalent cations. This substitution results in a relatively slow rate of Li diffusion. The d7Li and trace element data constrain the Introduction genesis of TTGs and sanukitoids. [Li] in zircons from Zircons are a retentive accessory mineral in many rocks. They are extensively used for U–Pb geochronology, giving Communicated by F. Poitrasson. useful information about tectonic events and related pro- Electronic supplementary material The online version of this cesses (e.g., Davis et al. 2003). Zircons also give valuable article (doi:10.1007/s00410-011-0697-1) contains supplementary geochemical information from oxygen isotope ratios and material, which is available to authorized users. trace elements (Hoskin and Schaltegger 2003; Valley 2003). Magmatic d18O values are generally preserved in A.-S. Bouvier (&) Á T. Ushikubo Á N. T. Kita Á R. Kozdon Á J. W. Valley non-metamict zircons, even through high-grade metamor- WiscSIMS, Department of Geoscience, University of Wisconsin, phism and anatexis (Page et al. 2007; Valley et al. 2005; Madison, WI 53706, USA Watson and Cherniak 1997). Grimes et al. (2007) used e-mail: [email protected] trace elements to show that zircons from continental crust Present Address: can be distinguished from oceanic crust. Recently, Li A.-S. Bouvier content ([Li]) and d7Li in zircons have been reported Swedish Museum of Natural History, 10405 Stockholm, Sweden (Grimes et al. 2011; Li et al. 2011; Ushikubo et al. 2008) showing that Li is useful for characterizing a zircon’s A. J. Cavosie Department of Geology, University of Puerto Rico, Mayagu¨ez, parent rock. However, interpreting the significance of [Li] PR 00681, USA and d7Li in zircons requires better understanding of the 123 746 Contrib Mineral Petrol (2012) 163:745–768 possible effects of alteration in Li and d7Li distribution in Speer 1982). Possible substitutions are summarized in zircon, Li substitution mechanisms, and Li diffusion rates Table 1. Some substitutions are simple, such as tetravalent in zircon. Hf, U, Th, Ti, and Sn ions that could directly substitute for Here, we report the analysis of d7Li, [Li], and other trace Zr?4, whereas other elements require a coupled substitution elements in zircons coupled to imaging for well-studied for charge balance. Finch et al. (2001) and Hanchar et al. igneous suites. This is the first study reporting analyses of (2001) suggested that Li substitutes on an interstitial site, d7Li, associated with 23 trace elements (Li, P, Ca, Ti, V, charge-balancing REE and Y (see Table 1), leading to a Fe, Y, REE, U, and Th) in single analysis pits in zircon. couple substitution as follows: Due to intracrystalline zoning, such correlated analyses are Li1þ þ ðÞY þ REE 3þ¼ Zr4þ þ h ð2Þ necessary to test the proposed Li substitutions in zircons. ðinterstitialÞ ðÞinterstitial We chose igneous zircons from TTGs and sanukitoids When converted to atomic proportions, each ppm of Li with ages ranging from 2.7 to 3.0 Ga, which are typical (at. wt * 7 amu) compensates for over twenty times its Archean granitoids (e.g., King et al. 1998). The selected weight in REE (at. wt * 150 amu) in Eq. (2). In this case, TTG and sanukitoid zircons are from the Superior Province if Li and P compensate REE, the ratio defining these two 18 (Canada), and d O(Zrc) and U–Pb ages are reported coupled substitutions, [(Y ? REE)/(Li ? P)]atomic, should IV 1? elsewhere (Davis et al. 2005; King et al. 1998). The use of be 1 (Ushikubo et al. 2008). Because Li has a larger 4? well-described zircons with known parent rocks allows ionic radius than Si (Shannon 1976), Li is unlikely to be VIII VIII testing the potential of [Li] and d7Li as petrogenetic trac- incorporated in Si sites. Alternatively, Li could fit Zr ers. The differences in trace elements, Li, and d7Li between sites, associated with 3 interstitial Li for charge compen- zircons from four TTG and four sanukitoid plutons are sation (Eq. 3). Li could also be 100% interstitial for charge discussed in terms of their different petrogenesis. These compensation of Zr, if an eightfold vacancy can exist data are also compared to Archean detrital zircons from the (Eq. 4). Jack Hills (Western Australia) that are older than 3.9 Ga VIII þ þ VIII 4þ h Li þ 3LiðintersitialÞ ¼ Zr þ 3 ðinterstitialÞ ð3Þ (up to 4.35 Ga) and for which parent rock type is uncertain. þ VIII h 4þ h 4LiðinterstitialÞ þ ¼ Zr þ 4 ðinterstitialÞ ð4Þ Li substitution in zircons Li substitution in natural zircons was recently discovered While Li is a mobile element in many processes, zircon to vary by over 5 orders of magnitude, from \2 ppb to appears to be highly retentive (Ushikubo et al. 2008); 250 ppm (Ushikubo et al. 2008). Zircons from primitive however, Li exchange in zircon is not well understood. The magmas, the mantle, and ocean crust are low in [Li] mechanism of lithium substitution in zircon affects chem- (kimberlite megacrysts \2 ppb, mid-ocean ridge gabbro ical diffusion rates and must be evaluated before it can be \10 ppb, and mid-ocean ridge plagiogranite \ 100 ppb), interpreted as reflecting magmatic compositions. Zircon is whereas zircons from continental crust have significantly higher Li contents (typically 1–100 ppm, Barth and an orthosilicate in which isolated SiO4 tetrahedra shares Wooden 2010; Grimes et al. 2011; Ushikubo et al. 2008). corners and edges with distorted ZrO8 polyhedra. The zircon crystal structure incorporates a wide range of trace elements, including Li, P, Y, Ti, Hf, U, Th, and REE (Finch Li isotopic fractionation et al. 2001; Hanchar et al. 2001; Hoskin and Schaltegger 2003). Incorporation of REE in zircon is commonly Fresh igneous rocks from the mantle have d7Li(whole assumed to be a ‘‘xenotime-type’’ substitution, because of rock) = 3.8 ± 1.5% (Chan et al. 2002a, b; Jeffcoate et al. the high [P] in many zircons and because zircon is iso- 2007; Magna et al. 2006; Seitz et al. 2004; Tomascak et al. 7 structural with xenotime (YPO4). The charge-balanced 2008), whereas seawater has very high d Li (*31%; Chan substitution is as follows: and Edmond 1988; Millot et al. 2004; Tomascak 2004; You and Chan 1996). Li isotope fractionation during ðÞY þ REE 3þþP5þ ¼ Zr4þ þ Si4þ ð1Þ fluid–rock interaction causes up to 50% variation in d7Li, If xenotime-type substitution solely governs the REE with products of subaerial weathering as low as -20% incorporation in zircons, P5? and REE3? compensate and (Chan and Edmond 1988; Chan et al. 1992, 2002a; Pist- [(Y ? REE)/P]atomic should be 1. However, numerous iner and Henderson 2003; Rudnick and Ionov 2007; studies have observed an excess of total REE compared Seyfried et al. 1998; Teng et al. 2008). Continental crust 7 with P (i.e., [(Y ? REE)/P]atomic [ 1), indicating that at has an average d Li(WR) of ?1.7% (Teng et al. 2009), least one other substitutional mechanism occurs (Cavosie making d7Li a potential tracer of continental alteration et al. 2006; Hinton and Upton 1991; Pidgeon et al. 1998; and weathering. 123 Contrib Mineral Petrol (2012) 163:745–768 747 Table 1 Substitution in zircons Element Equations References Xenotime substitution Y3? (Y ? REE)3? ? P5? = Zr4? ? Si4? Simple substitutions (OH)4 (OH)4 = SiO4 Frondel (1953) Hf4? Hf4? = Zr4? Frondel (1953) U4?,Th4?,Ti4?,Sn4? (U, Th, Ti, Sn)4? = Zr4? Frondel (1953) Coupled substitutions Li? 1þ 3þ 4þ Finch et al. (2001), Hanchar et al. (2001) LiðinterstitialÞ þ REE ¼ Zr - n? - 4? 4- a (OH) M ? n(OH) ? (4 - n)H2O = Zr ? (SiO4) Caruba and Iacconi (1983) Mg2?,Fe2? 2þ 3þ 5þ 4þ 4þ Hoskin et al. (2000) ðÞMg; Fe ðinterstitialÞþ3YðÞ; REE þP ¼ 3Zr þ Si Al3?,Fe3? 3þ 3þ 5þ 4þ 4þ Hoskin et al.
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