G 3. AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society

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1 Geosystems G 3 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Technical Brief Volume 7, Number 4 13 April 2006 Q04005, doi: /2005gc ISSN: Improved dissolution and chemical separation methods for Lu-Hf garnet chronometry J. N. Connelly Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA (connelly@mail.utexas.edu) [1] Garnet-based Lu-Hf geochronology using conventional (HF-HNO 3 ) dissolution methods may be compromised by full or partial digestion of Hf-rich zircon inclusions. This study integrates two complimentary methods to substantially reduce zircon digestion while assuring complete digestion of garnet and sample-spike equilibration. Handpicked garnet fractions are heat treated to >1000 C in an evacuated silica glass ampoule to anneal zircon inclusions and then dissolved with 12 M HCl at 210 C and cold 28 M HF. Analyses of heat-treated garnet (and their high-temperature breakdown products orthopyroxene-spinel-quartz) from Gore Mountain, New York, demonstrate the method is capable of complete dissolution of garnet and routinely achieving sample-spike equilibration. Independent analyses of annealed and unannealed zircons dissolved by HCl cold HF demonstrate the benefit of heat treatment prior to dissolution. Analyses of zircon-rich garnets from peletic paragneisses of Labrador, Canada, show the potential of this method by returning Lu-Hf ratios commensurate with the degree of heat treatment. Finally, a complementary chemical separation method for Lu and Hf is presented that supports analysis of garnet by MC-ICP-MS. Components: 5912 words, 3 figures, 5 tables, 1 dataset. Keywords: Lu-Hf; Hf isotopes; garnet; geochronology; boric acid; HCl dissolution. Index Terms: 1040 Geochemistry: Radiogenic isotope geochemistry; 1115 Geochronology: Radioisotope geochronology; 3652 Mineralogy and Petrology: Pressure-temperature-time paths. Received 19 July 2005; Revised 28 November 2005; Accepted 1 February 2006; Published 13 April Connelly, J. N. (2006), Improved dissolution and chemical separation methods for Lu-Hf garnet chronometry, Geochem. Geophys. Geosyst., 7, Q04005, doi: /2005gc Introduction [2] There remains a long-standing desire to better link geochronologic (t) and geothermobarometric (PT) estimates to define accurate PTt excursions for metamorphic rocks. Metamorphic garnet provides qualitative and quantitative information about the PT path of its host rock and may possess appropriate concentrations of radiogenic and daughter elements to constrain time. The main obstacles to using garnet to accurately define t include uncertain diffusion parameters, the relatively large amount of garnet required for isotopic measurements and the general inability to avoid digestion of near ubiquitous inclusions with elemental concentrations that may overwhelm the relevant elemental budgets of garnet. Whereas the U-Pb system showed early promise for dating garnet [Mezger et al., 1989; Frei and Kamber, 1995; Vance et al., 1998 and references therein], Sm-Nd and Lu-Hf geochronology have become more routinely applied [Mezger et al., 1992; Duchêne et al., 1997; Scherer et al., 2000; Blichert-Toft and Frei, 2001; Lapen et al., 2004]. However, the common occurrence of zircon inclusions in garnet has proven especially problematic for Lu-Hf chronology given that zircon typically represents the main Hf-bearing phase (1% Hf) and is unlikely to be in isotopic equilibrium with metamorphic garnet. Attempts to limit Hf from zircon by selective Copyright 2006 by the American Geophysical Union 1 of 9

2 dissolution of garnets have been encouraging [Anczkiewicz and Thirlwall, 2003; Anczkiewicz et al., 2004] but are dependant on the degree of metamictization of the zircon inclusions. [3] This paper presents new data demonstrating that Hf from zircon inclusions in garnet can be minimized by annealing the zircon and dissolving the garnet in 12 M HCl at 210 C and cold 28M HF. The paper also presents a simplified chemical separation method for the concentration of Lu and Hf from garnet matrixes suitable for analysis by MC-ICP-MS. Recognizing Scherer et al. s [2000] comprehensive review of the potential effects of zircon contamination in Lu-Hf analysis of garnet, only a brief review of the motivation for this study is presented here. Furthermore, given the abundance of papers evaluating different approaches to analyzing Lu and Hf by MC-ICP- MS, this study has not focused on this important step in deriving precise and accurate ages [see Blichert-Toft et al., 1997; Chu et al., 2002; Lapen et al., 2004; Albarède et al., 2004 and references therein). 2. Lu-Hf Geochronology Using Garnet [4] 176 Lu breaks down to 176 Hf with half-life of 37 Gyr [Scherer et al., 2001; Söderlund et al., 2004; Amelin, 2005]. Some garnets have 176 Lu/ 177 Hf ratios as high as 30 [Anczkiewicz et al., 2004], but most ratios reported are lower than 0.5, many lower than These variations in Lu/ Hf ratios reflect the relative availability of rare earth elements (REEs) and Hf during garnet formation but may also depend upon varying degrees of contamination by Hf-rich phases, most notably zircon. [5] If zircon and garnet were not in isotopic equilibrium at t o, incorporation of zircon will displace garnet analyses off their rightful isochron line and yield incorrect ages (see discussion by Scherer et al. [2000]). In cases where zircon and garnet are in isotopic equilibrium at t 0, inclusion of zircon will lower the Lu/Hf ratio of garnet analyses, thereby resulting in correct, but less precise ages. The common occurrence of zircon as microinclusions in garnet typically precludes avoidance by picking. 3. Methods [6] Experiments to develop new methods of garnet-based Lu-Hf geochronology required identification of natural garnet of known age that would be relatively easy to process. Almandine-pyrope-rich garnet from the Gore Mountain amphibolite [Mezger et al., 1992] from New York was selected for its very coarse grain size, general lack of inclusions and the availability of published isotopic ages and data. These metamorphic garnets formed during the Grenville Orogeny and have yielded Lu- Hf and Sm-Nd ages ranging from Ma [Mezger et al., 1992; Scherer et al., 2000; Lapen et al., 2004]. A metamorphic zircon age of 1055 ± 2 Ma was also determined by Mezger et al. [1992] for the Gore Mountain amphibolite. [7] A second sample comes from the pelitic Tasiuyak Gneiss of northern Labrador, where metamorphism occurred at circa 1.85 Ga during the Torngat Orogeny [Wardle et al., 2002]. This sample was selected for its abundance of large, pink garnets (>1 cm; 70% almandine, 26% pyrope, 3% grossular and 2% spessartine) with well-characterized zircon inclusions and a published U-Pb metamorphic zircon age of 1861 ± 8 Ma [McFarlane et al., 2004, 2005] Dissolution of Garnet by HCl and Cold HF [8] Accepting that zircons are effectively dissolved only by concentrated HF-HNO 3 mixtures, tests were conducted to determine whether other acids would dissolve garnet. Tests with 12 M HCl in a pressurized bomb in a 210 C oven for 24 hours were encouraging but the dissolution reaction failed to proceed to completion even if acids were replenished and/or the bombs were left in the oven for longer periods. At the point when dissolution stalled, the undissolved grains of garnet were fully rimmed by a clear, raspberry-textured material inferred to be HCl-insoluble silica that was forming a physical barrier inhibiting further reaction. For dissolution to proceed, it was necessary to remove the 12 M HCl from the Savillex dissolution capsule and expose the sample to a small volume ( mm) of cold 28 M HF or larger volume (1 ml) of warm 1 M HF for min. Dissolution of exposed pristine garnet would resume after the silica rims were dissolved, the HF removed and the sample placed back in the oven at 210 with fresh 12 M HCl. Pristine garnet can be fully dissolved in 2 3 cycles of 12 M HCl at 210 in 3 ml Savellix capsules inside a 125 ml Teflon Parr TM bomb for 24 hours interspersed with short intervals (10 15 min) of cold 28 M HF to dissolve silica rims. Experiments with finer grain sizes of 2of9

3 Geosystems G 3 connelly: garnet-based lu-hf geochronology /2005GC Table 1. Amount of Hf Released From per ug of Zircon Unannealed#1 Unannealed#2 Annealed#1 Annealed#2 12M HCl - bomb, 210 C, 48 hours ng ng ng ng 28M HF - cold, 10 min ng ng - 1M HF - 90 C, 2 hours ng ng % Total Hf dissolved a 9.10% 6.28% 0.20% 0.18% a Total Hf released in both steps assuming zircons contain 1% Hf by weight. garnet were not successful in avoiding use of HF or reducing the number of cycles required for dissolution. For isotope dilution analyses, all solutions must be carefully combined prior to the first dry down step to ensure sample-spike equilibration Dissolution of Zircon by HCl [9] Zircon is one of the most resilient common silicates with respect to traditional acid digestion techniques. The time required for complete dissolution of zircon in HF-HNO 3 mixtures in pressurized Teflon TM bombs in an oven at 220 C depends on their size and degree of crystallinity. Tests were made here to determine the extent to which zircon could be dissolved or leached by the 12M HCl-based dissolution methods defined above for garnet. [10] Tests were conducted on handpicked fractions (18 25 mg) of large (+150 mm), clear zircons grains (U-100 ppm) from a circa 1.4 Ga granite in the Wet Mountains of Colorado. Two fractions were first subjected to 12 M HCl in a 3 ml Savillex TM capsule in a Parr TM bomb at 210 C for 48 hours. This 12 M HCl step yielded and ng of Hf per microgram of zircon for the two fractions analyzed (Table 1) or % of the total available Hf (assuming 1% Hf). The zircon fractions exposed to 12M HCl were subsequently subjected to either cold concentrated HF or warm 1M HF. Neither method liberated significant amounts of Hf from the zircon ( and ng Hf, respectively, per microgram of zircon). [11] Assuming that neither the temperature nor duration of the 12 M HCl step or the duration of the cold 28 M HF step could be significantly reduced when dissolving garnet, the effect of decreasing the solubility of zircon by increasing its crystallinity was investigated. This follows the work of Mattinson [2005], who has demonstrated that metamict portions of zircons are made more crystalline by annealing radiation-damaged zircon at high temperatures. Two fractions of zircon from the same 1.4 Ga granite sample were heated to 1000 C for 24 hours in alumina crucibles. Subjected to the same dissolution steps as outlined above, the annealed grains released only ng of Hf per microgram of zircon in the 12 M HCl solution (Table 1), or about 2% of the Hf released by the unannealed grains. The cold and hot HF steps released and ng of Hf per microgram of zircon, respectively, or about 50% of that from the unannealed grains during this step Effects of Heat Treating Garnets [12] Having confirmed the benefit of heat treating zircon at temperatures between C, a series of tests were conducted to document the effect of this heat treatment on the prospective host garnet. All heat treatment tests at 1 atm caused garnet test fractions to turn dark red to black, presumably related to the oxidation of Fe +2 in garnet. In extreme cases, this caused the 12 M HCl dissolution acid to become murky during the 210 C dissolution step. This required the sample to be centrifuged prior to the removal of the 12 M HCl and the subnate to be transferred back to the dissolution capsule before addition of cold 28 M HF. Provided all the subnate was successfully transferred, this complication did not preclude successful analysis (see analysis M12 Garnet 1 below). However, recognizing the risk related to transferring the sample prior to sample-spike equilibration, a means was sought to prevent oxidation during the heat treatment. Test fractions of garnet were heated to different temperatures in sealed, evacuated (1E 4 torr) ampoules made by heat sealing silica (quartz) glass tubes. Heating Gore Mountain and Tasiuyak Gneiss garnets up to temperatures of 1000 C produced no oxidation and had no visible effect on the garnet when viewed with a binocular microscope. At temperatures in excess of 1025 C, garnet was transformed to orthopyroxene + spinel (as determined by X-ray diffraction patterns) in a reaction that was complete in less than 12 hours at 1050 C. A model reaction is proposed in which garnet ¼ orthopyroxene þ spinel þ quartz: 3of9

4 a first-stage cation resin column [Patchett and Tatsumoto, 1980] and a second-stage TODGA resin from Eichrom Industries [Horwitz et al., 2005; Connelly et al., 2006]. The first stage separates the high field strength elements (HFSE) and REEs from the matrix in preparation for the secondstage chemistry, which purifies Hf (with Zr) and Lu (with Yb). Figure 1. Flowchart of the chemical separation methods recommended for Lu-Hf analyses of garnet by MC-ICP-MS. As an additional benefit to enhanced annealing of zircon at these elevated temperatures, dissolution of the fine-grained orthopyroxene-spinel-quartz intergrowth that replaced garnet required only one cycle of 12 M HCl at 210 C and cold HF. At temperatures in excess of approximately 1100 C, garnets from the Tasiuyak Gneiss began to fuse together and reacted with the quartz glass. Despite garnet grains clumping together and sticking to the inner glass wall, partial melting did not preclude successful Lu-Hf analysis (see M12 Garnet 2 and 3). This material was also readily dissolved in one cycle of 12M HCl at 210 C and cold HF. [13] These tests indicate that temperatures for heat treating should be above that required for the conversion of garnet to orthopyroxene + spinel + quartz (for efficient annealing of zircon inclusions and ease of subsequent dissolution) but below that which induces melting. Temperatures of 1050 C were optimal for garnets used in these experiments. The recommended heat treatment and dissolution procedures are described in detail in the Auxiliary Material Chemical Separation Methods [14] Lu and Hf are isolated from the matrix elements with two ion exchange steps (Figure 1), including 1 Auxiliary material are available at ftp://ftp.agu.org/apend/gc/ 2005gc First-Stage Chemistry: 50W 8 Cation Resin ( Mesh) [15] The first-stage 2 ml cation column provides sufficient capacity for at least 100 mg of garnet. Samples are loaded in 0.5 M HCl and the HFSEs are eluted immediately in 0.5 M HCl M HF, followed by the removal of matrix elements in 1.5 M HCl and, finally, the elution of the REEs in 6 M HCl (Table 2). The HFSE and REE separates are evaporated to dryness and redissolved in 3.5 M HNO 3 + boric acid (HFSE) and 1 M HNO 3 (REE). Using a calibrated cation column of appropriate geometry, Lu and Yb could be isolated in the first stage [Patchett and Tatsumoto, 1980] Second-Stage Chemistry: TODGA [16] The second-stage chemistry (Table 3) concentrates Hf (and Zr) from the HFSE separate and heavy REEs (HREE) from the light and middle REEs (LREEs and MREEs) using Eichrom Industries TODGA resin loaded into a disposable 0.2 ml resin bed volume column made from an Eppendorf TM -type pipette tip. A top frit is not strictly necessary but the low density of the TODGA resin results in suspended resin when solutions are loaded if unconfined. The addition of 200 ml of saturated boric acid to the 3.5M HNO 3 loading Table 2. Step Cation Chemical Separation Procedure Procedure 1 Fill 2 ml BioRad polypropyene column (0.8 4 cm) with mesh cation resin (50W 8). 2 Clean with 20 ml 6 M HCl. 3 Precondition with 1 ml and 5 ml 0.5 M HCl. 4 Load sample in 2 ml 0.5 M HCl. 5 Collect 10 ml 0.5 M HCl M HF (HFSE). 5.1 Drydown. 5.2 Add 0.75 ml 3.5 M HNO ml boric acid (0.047 g boric crystals/ml) (to TODGA). 6 Wash 36 ml 1.5 M HCl (matrix). 7 Collect 15 ml 6 M HCl (REE). 7.1 Drydown. 7.2 Add 0.5 ml 1 M HNO 3 (to TODGA). 4of9

5 Table 3. TODGA Chemical Separation Procedure for Lu and Hf Step Procedure 1 Fill column a with 0.2 ml of TODGA. 2 Remove organics with 2 ml 10.5 HNO 3. 3 Clean with 6 ml 1 M HNO M HF. 4 Clean with 3 ml 0.05 M HCl. 5 Remove HF with 1 ml 3.5 M HNO 3 saturated with boric acid (0.028 g/ml). 6 Precondition with 2 ml 3.5 M HNO 3. 7 Load HFSE sample in 0.75 ml 3.5 M HNO ml boric acid (0.047 g boric crystals/ml). 8 Wash 3.5 ml 3.5 M HNO 3 (Ti). 9 Collect 6 ml 1 M HNO M HF (Zr-Hf). 9.1 Drydown. 9.2 Convert to nitrate form by adding 50 ml concentrated HNO Drydown. 9.4 Add 0.2% HNO M HF ready for Hf isotopic analyses. 10 Load REE fraction in 0.5 ml 1 M HNO Wash 3 ml 1 M HNO M HF (Ca, Fe, ±Zr, Hf). 12 Collect 5 ml 0.5 M HCl (L-MREE) - save for Sm-Nd analyses. 13 Collect 2 ml 0.05 M HCl (HREE) Drydown HREE Convert to nitrate form by adding 50 ml concentrated HNO Drydown. 14 Add 0.2% HNO 3 ready for Lu isotopic analyses. a Column made from eppendorf-type pipette with 2 3 mm of tip removed. solution is essential to retain Hf and Zr on TODGA resin in HNO 3. This method returns yields of >95% of Hf loaded and approximately natural relative abundances of Yb/Lu, which is acceptable for analysis of garnet by ICP-MC-MS (<10 [Blichert- Toft et al., 1997; Ulfbeck et al., 2003]). To reduce the amount of MREEs in the Lu separate to acceptable levels, only approximately 30% of the Lu is intentionally collected. 4. Isotopic Results [17] Whereas both garnet samples served a common purpose in heat treating and dissolution experiments, they served different purposes in tests of the Lu-Hf analyses. The abundance of inclusion free garnet from the Gore Mountain amphibolite facilitated the initial tests of sample-spike equilibration, chemistry and mass spectrometry methods. The benefits of the method were better tested using zircon-bearing garnets from the Tasiuyak Paragneiss. Lu and Hf analyses were performed at The University of Texas at Austin using a Micromass (now GV) Isoprobe (see auxiliary material Appendix 2 for mass spectrometry procedures) Gore Mountain Amphibolite [18] Two single, large (1.5 cm diameter), red garnet porphyroblasts were crushed in a mortar and pestle and sieved to mesh size ( mm) to yield two separate aliquots. Of the six fractions processed from the first aliquot, three were heat treated in air whereas three others were not heat treated. One fraction from the second aliquot was heat treated in an evacuated ampoule at 1050 C. All seven of these fractions were dissolved by one or more cycles of 12M HCl at 210 C followed by Table 4. Lu and Hf Isotopic Data for Garnet From Gore Mountain Amphibolite and Estimate of Initial Ratio Sample Lu Hf Lu 176 /Hf 177 2SE Hf 176 /Hf 177 2SE a Model Age, b Ma Initial Ratios as Determined From Literature Lapen et al. [2004] Garnet Fractions Not Heat Treated and Dissolved With HCl ± ± ± 13 Garnet Fractions Heat Treated in Air and Dissolved With HCl ± ± ± 11 Garnet Fractions Heat Treated in Evacuated (1E-4 torr) Silica Glass and Dissolved With HCl ± 11 Garnet Fractions Not Heat Treated and Dissolved With HF/HNO ± 11 a Errors for regression use 2SE listed here or external errors, whichever is larger. b Model ages use initial ratio from Lapen et al. [2004]. 5of9

6 Figure 2. Lu-Hf isochron diagram for garnets from the Gore Mountain amphibolite, New York, USA. cold HF. An eighth fraction of garnet constituted a single large fragment of a third garnet grain and was dissolved by HF-HNO 3 in a Savillex beaker on a hot plate at 140 C. This fraction was processed by the same chemical separation method than those dissolved by HCl, with the exception of a boric acid step after the HF/HNO 3 dissolution and initial dry down to ensure full dissolution of fluorides. [19] The eight garnet fractions yield model ages ranging from Ma (Table 4) using an initial 176 Hf/ 177 Hf ratio of as defined by Lapen et al. [2004]. Using the same initial ratio, all eight analyses define a line corresponding to an age of ± 8.6 but with an MSWD of 2.7 (Figure 2). Regressing the four heat-treated garnets and the HF-dissolved garnet with the lower intercept of yields an isochron corresponding to an age of ± 4.5 Ma (MSWD = 0.1). Regressing only the heat-treated fractions and the lower intercept estimate of yields an isochron corresponding to an age of ± 4.9 Ma (MSWD = 0.1). With the lower intercept unconstrained, the four heat-treated garnets and one HF dissolved garnet define an isochron corresponding to an age of 1046 ± 36 Ma with a lower intercept of ± [20] The best test of the full recommended procedure lies in the isochron defined by the four heattreated fractions (4 7) and the lower intercept of Lapen et al. [2004]. The coincidence of the resulting age of ± 4.9 Ma with published ages and the low MSWD, indicate that these methods facilitate full garnet digestion and sample-spike equilibration Tasiuyak Paragneiss [21] Processing and analyses of these garnets were intended to evaluate the benefit of heat-treating and HCl dissolution to garnets with known zircon inclusions (confirmed by electron microprobe backscatter images [McFarlane et al., 2004, 2005]). Assuming that rutile formed as part of the metamorphic assemblage and in isotopic equilibrium with garnet, two fractions of rutile were analyzed to constrain the lower intercept of the isochron. Analyses of four zircon fractions served to determine the Lu and Hf composition of zircon that may be included in the garnet analysis. [22] Three fractions of garnet were processed by HCl dissolution after varying degrees of heat treatment: one fraction (Garnet 1) was heat treated in a ceramic crucible in air at 1 atm whereas the other two (Garnet 2 and 3) were heat treated in an evacuated (1E-4 torr) ampoule. The times and temperatures of heat treatment varied such that there is a predicted decrease in degree of annealing of zircon inclusions from fractions Garnet 1 to Garnet 3 (see Table 5 for times and temperatures). The fraction heat treated in air turned dark red during heating and required two cycles of 12 M HCl dissolution with intervening cold HF cycles. The fractions heat treated under vacuum turned gray-green, partially fused and required only one cycle of 12 M HCl and cold HF to fully dissolve. 6of9

7 Table 5. Isotopic Data for Garnet, Rutile, and Zircon From Sample M-12 a Fraction Method Lu, ppm Hf, ppm Lu 176 /Hf 177 2SE Hf 176 /Hf 177 2SE b Model Age, c Ma Garnet 1 HCl HT (air, 60hrs, 1050 C) ± 9.4 Garnet 2 HCl HT (evac, 24hrs, 1100 C) ± 9.6 Garnet 3 HCl HT (evac, 24 hrs 1050 C) ± 9.7 Rutile 1 HF-HNO 3 bomb Rutile 2 HF-HNO 3 bomb Zircon 1 HF-HNO 3 bomb Zircon 2 HF-HNO 3 bomb Zircon 3 HF-HNO 3 bomb Zircon 4 HF-HNO 3 bomb a HT, heat treated; evac, evacuated (1E-4 torr) silica glass tube. b Errors for regression use 2SE listed here or external errors, whichever is larger. c Model ages using rutile analyses to constrain lower intercept. The rutile and zircon fractions were dissolved by HF-HNO 3 in a Teflon bomb at 210 C and processed using only the second-stage chemistry. [23] Both rutile fractions have 176 Lu/ 177 Hf ratios that overlap zero and have overlapping 176 Hf/ 177 Hf ratios (Table 5). The garnet fractions yielded 176 Lu/ 177 Hf ratios of 0.463, and and Hf isotopic ratios that correspond to model ages of , and Ma, respectively, with the lower intercept constrained by the two rutile analyses (Table 5, Figure 3). These three garnet and two rutile fractions do not define a statistically acceptable line (1847 ± 16 Ma; MSWD = 3.2). The 176 Lu/ 177 Hf ratios are interpreted to reflect varying degrees of zircon dissolution in response to different durations and temperatures of heat treatment. This is supported by the observation that the three garnet fractions lie on a line that passes through the zircon cluster near the lower intercept rather than on an isochron that includes rutile. Inferred to contain the least amount of Hf from zircon, Garnet 1 yields a garnet-rutile age of ± 9.1 Ma that is consistent with 1861 ± 8 Ma metamorphic zircon age from this sample [McFarlane et al., 2005] and the orogen at large [Scott and Machado, 1995; Scott, 1998; Connelly, 2000]. The similar isotopic composition of zircon and rutile from this sample ensures that incorporation of zircon in the garnet analyses minimizes the disturbance of the isochron but substantially decreases the 176 Lu/ 177 Hf ratios of the garnet analyses. 5. Discussion [24] The effectiveness of the heat treating and HCl dissolution method to limit dissolution of zircon inclusions was tested directly by subjecting treated and untreated zircon from a 1.4 Ga granite to the Figure 3. Lu-Hf isochron diagram for garnets from the Tasiuyak Paragneiss, Torngat Orogen, Labrador, Canada. 7of9

8 recommended dissolution methods. Dissolution of treated and untreated zircon by HCl demonstrate that annealing zircon substantially reduces the amount of zircon dissolved by 12 M HCl at 210 C and cold HF. Annealed zircons yielded approximately 0.2% of the total amount of available Hf. These tests are in accord with Mattinson [2005], whose more systematic study demonstrates that annealed zircons are more difficult to leach and dissolve than unannealed grains. It is likely that longer annealing times will further reduce the amount of Hf liberated from zircon during HCl dissolution. [25] Using the initial ratio of Lapen et al. [2004], the four heat-treated fractions of garnet from the Gore Mountain amphibolite define an isochron corresponding to an age of ± 4.9 Ma. This lies within error of the ± 6.6 Ma age determined by Lapen et al. [2004] for garnethornblende mixtures from the Gore Mountain amphibolite. The colinearity of analyses presented here and the coincidence of age between these studies interpreted to indicate that the heat treating and HCl dissolution method provides complete garnet digestion and sample-spike equilibration. Moreover, fraction 7 demonstrates that the formation of more readily dissolved garnet breakdown products orthopyroxene + spinel + quartz is not detrimental. The lack of a strong correlation between Lu/Hf ratio and method employed to dissolve garnet from the Gore Mountain amphibolite, combined with the linearity of the consequent data infer that the measured variations reflect real Lu/Hf heterogeneity in the garnet fractions rather than the preferential inclusion or exclusion of zircon in these analyses. The higher Lu/Hf ratios of fractions 7 and 8 are attributed to a higher natural Lu/Hf ratio in these fragments. [26] The benefit of the method was tested using zircon-bearing garnets from the well characterized Tasiuyak Gneiss of Labrador. Three garnet fractions and two rutile fractions do not define an isochron. Instead, the three garnet fractions lie on a line that traverses a cluster of four zircon analyses with isotopic signatures slightly less radiogenic than the two rutile fractions. Furthermore, the Lu/ Hf ratios of the garnet fractions correlate with predicted relative degree of annealing of zircon inclusions caused by different temperatures and durations of heating (see Table 5). This implies that the recommended annealing and dissolution methods are effective in controlling the amount of Hf contributed from zircon inclusions in garnet. Whereas these data fulfill the objectives of this paper, it is clear that the duration of heat treatment for at least fractions Garnet 2 and 3 were insufficient to reduce zircon digestion to an acceptable level. Therefore longer heat treatment sessions (+60 hours) are recommended to more fully anneal zircon inclusions in garnet prior to digestion. 6. Conclusions [27] These data demonstrate that garnet (and its high-temperature breakdown products) can be fully dissolved by one or more alternating cycles of 12 M HCl in an oven in pressurized Parr 2 bombs at 210 C for 48 hours followed by small volumes of cold, concentrated HF to dissolve residual SiO 2. Full dissolution of untreated and heat-treated garnet requires 2 cycles. In contrast, full dissolution of orthopyroxene-spinel-quartz intergrowths, the product of garnet breakdown at temperatures above 1000 C, requires only one dissolution cycle. Experiments with finer grain sizes were not successful in avoiding use of HF or reducing the number of cycles required for dissolution. [28] Analyses of annealed and unannealed zircons that were subjected to the recommended dissolution steps reveal that the release of Hf from zircon is significantly reduced by heat treatment and the use of mainly HCl during dissolution. Heat treating garnets has no adverse effect on the capacity of HCl to dissolve garnet and for sample-spike equilibration to be achieved. To the contrary, heating at temperatures high enough to convert garnet to orthopyroxene + spinel + quartz facilitate dissolution in 12 M HCl and cold HF. [29] The methods presented here have proven successful for garnets from two samples of mafic and pelitic composition. Future work remains to test these methods on a range of garnet compositions and to determine whether Lu-Hf chronology is consistently capable of returning accurate and precise Lu-Hf ages that can be meaningfully interpreted and correlated. This work should include tests of whether annealed zircon can be satisfactorily avoided during dissolution of garnet (or its high temperature breakdown products) by HF- HNO 3 mixtures on a hot plate rather than HCl in a bomb. The benefit of removing phosphate inclusions by leaching with H 2 SO 4 [Anczkiewicz and Thirlwall, 2003; Anczkiewicz et al., 2004] prior to heat treatment should also be investigated. Ultimately, Lu-Hf and Sm-Nd analyses of garnet for 8of9

9 purposes of precise and accurate geochronology may require sample specific methods based on the inclusion assemblage of each sample. Acknowledgments [30] The National Science Foundation (EAR and EAR ), the Danish National Research Foundation, and the Geology Foundation at UT Austin provided financial support for this work. John Lansdown and Todd Housh are thanked for many discussions and their dedication to operating the ICP-MS facilities at UT Austin. Phil Horwitz and Larry Jassin of Eichrom Industries patiently provided technical advice on all aspects of the methods presented. Mark Helper is thanked for discussions about mineral chemistry and for providing X-ray diffraction data. Discussions with Joel Baker, Martin Bizzarro, Kristine Thrane, and David Ulfbeck were very helpful. Comments by J. Patchett and an anonymous journal reviewer improved the manuscript. References Albarède, F., P. Telouk, J. Blichert-Toft, M. Boyet, A. Agranier, and B. 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