3.2.a. Petrography and Elemental Geochemistry
We used thin-sections, prepared by Idaho Petrographics (Grangeville, ID), and a Zeiss Axioskop petrographic microscope at Vanderbilt University to examine and characterize the mineral assemblage and textural relations for each sample (Table 1). All samples were analyzed for major and trace element abundances (Table 2) by a combination of wavelength dispersive X- ray fluorescence (WD-XRF) and laser ablation inductively-coupled plasma-mass spectrometry (LA-ICP-MS) at the Michigan State University Geological Analytical Services Laboratory (East Lansing, MI). One sample (IA-G-5) was analyzed at the Peter Hooper GeoAnalytical Laboratory at Washington State University (WSU; Pullman, WA), where major elements were determined
by WD-XRF and trace element concentrations were determined by solution ICP-MS.
International reference materials RGM-1, W-2, JA-2, and BHVO-1 were used for calibration and quality control.
Based on whole-rock major element compositions and Zr concentrations, we calculated model zircon saturation temperatures (ZSTs) for all samples using the formulation of Watson and Harrison (1983) as revised by Boehnke et al. (2013). The saturation equation requires the composition of a melt that was saturated in zircon, and we acknowledge that large uncertainties are introduced because whole-rock compositions do not strictly equal melt compositions, particularly with intrusive rocks. ZSTs provide minima for the initial temperature of the magma represented by the sample, unless inherited and/or accumulated zircon is relatively abundant which would increase calculated model temperatures. For most silicic rocks, calculated ZSTs are likely to be low by a modest amount, whereas for mafic whole-rock compositions ZSTs tend to severely underestimate the temperatures at which zircon actually saturates (e.g. Miller et al., 2003, Harrison et al., 2007; Boehnke et al., 2013; Moecher et al., 2014; McDowell et al., 2014).
Given these uncertainties, we use ZSTs as estimates to generally compare model magmatic temperatures for the silicic units of the Austurhorn system, and do not discuss mafic ZSTs.
3.2.b. Solution MC-ICP-MS Hf Isotope Analyses
We determined whole-rock Hf isotopic compositions of AIC samples at the Radiogenic Isotope and Geochronology Laboratory (RIGL) at WSU (Pullman, WA) by solution-multi- collector (S-MC) ICP-MS (Table 3). Approximately 0.25 g of each powdered sample were dissolved in Teflon vessels in ~7 mL 10:1 HF:HNO3, then immediately dried at 120°C to reduce silica. Following dry down, ~7 mL 10:1 HF:HNO3 was added to the vessels and placed in steel- jacketed Parr bombs at 150°C for 5-7 days. The solutions were dried and re-dissolved overnight
in a mixture of 6M HCl/H3BO3 to convert to chlorides and minimize production of fluoride species. Lastly, samples were dried and re-dissolved in 6M HCl in Parr bombs at 150°C for 24 hours, until sample solutions became clear.
Samples were then dissolved in a mixture of 1M HCl and 0.05M HF. High-field-strength elements (including Hf) were initially separated on single cation exchange columns loaded with AG 50W-X12 resin (200-400 mesh). Following the method of Patchett & Tatsumoto (1981), we eluted Hf at the beginning of the procedure in 1M HCl/0.05M HF. We then removed Ti from the Hf fraction in a second stage chemistry (a crucial step, as excess Ti has been shown to alter the measured Hf isotopic composition; Blichert-Toft et al., 1997). Any remaining Yb and Lu in the Hf aliquot were removed in a third stage of column chemistry using 0.18 mL of AG 50W-X12 resin.
We re-dissolved the purified Hf aliquots in 2% HNO3 to determine their isotopic
compositions on the RIGL ThermoFinnigan Neptune MC-ICP-MS using an Aridus desolvating nebulizer for sample introduction. Samples and standards were analyzed as 25 ppb solutions.
Mass fractionation was corrected using 179Hf/177Hf = 0.7325 and all sample analyses were normalized using the Hf isotope reference material JMC-475 (accepted 176Hf/177Hf = 0.282161;
Blichert-Toft et al., 1997). Analyses of JMC-475 were conducted during the course of this study and yield a mean 176Hf/177Hf of 0.282135 ± 7 (2SD; n=15). Present day εHf values were
calculated using the CHUR parameters reported by Bouvier et al. (2008).
Table 1. Locations, descriptions, and petrography for samples collected from the AIC
Sample5: Location (UTM)6: Mafic bodies?7
Silicic Rel.
Abundance8 Major9 Mineral Phases (relative abundance) Accessory10 Mineral Phases Northing Easting
IA-NS-1 524180 7146224 none 100 % Qz (~30%), Afs+Pl (~60%), Amph (5-7%), Cpx (~7%), Bt (<2%), Alt (<2%) FeTi, Zrc, Sph, Ap IA-NS-2 524147 7146097 10 cm - 1 m ~75 % Qz (30-35%), Afs+Pl (40-45%), Amph (~10%), Bt (<1%), Alt (8-10%) FeTi, Zrc, Sph, Ap IA-NS-3 524101 7145946 > 1m 60-70 % Pl (~75%), Cpx (~10%), Amph (<5%), Qz (<5%), Alt (5-10%) FeTi, Zrc, Sph IA-NS-4a 522153 7144490 1-10 cm ~95 % Qz (~30%), Afs+Pl (~60%), Amph (<2%), Bt (<1%), Alt (<5%) FeTi, Zrc, Sph
IA-NS-4b* 522153 7144490 none 100 % Qz (~45%), Afs+Pl (~50%), Alt (<5%) FeTi, Zrc, Ap, ±Sph
IA-NS-5 522643 7142846 1 cm - 10 m ~30 % Qz (~35%), Afs+Pl (~45%), Bt (~5%), Amph (~5%), Alt (~15%) FeTi, Zrc, Sph, Ap IA-NS-6 522378 7142211 > 1 m ~50 % Pl±Afs (75-80%), Cpx (~10%), Qz (~5%), Alt (10-15%) FeTi, Zrc, Sph, Ap IA-NS-7 522275 7142118 none 100 % Qz (~20%), Pl+Afs (~45%), Amph (10-15%), Bt (5-10%), Alt (5-10%) FeTi, Zrc, Sph
IA-NS-8 522087 7141856 1-10 cm ~95 % Qz (~35%), Pl (~50%), Alt (~10%) FeTi, Zrc, Sph, Ap
IA-NS-9* 522130 7144385 none 100 % Qz (~45%), Afs+Pl (~45%), Alt (<5%), Amph (<5%) FeTi, Zrc, Ap, ±Sph IA-NS-10* 522083 7144427 none 100 % Qz (~45%), Afs+Pl (~45%), Alt (<5%), FeTi (<5%) Zrc, Ap, ±Sph IA-NS-12* 522115 7144432 none 100 % Qz (~35%), Afs+Pl (~45%), Amph (~10%), FeTi (<10%) Zrc, Ap, ±Sph IA-G-1 521924 7142080 n/a 0 % Pl (~40%), FeTi (~10%), Cpx (~35%), Amph (<5%), Qz (<5%), Alt (<5%) Zrc, Sph, Ap IA-G-3 521149 7142996 n/a 0 % Pl (~45%), Cpx (~30%), FeTi (15-20%), Amph (<5%), Alt (<5%) Zr, Ap IA-G-5 522402 7142368 n/a 0 % Pl (~45%), FeTi (5-10%), Cpx (~35%), Amph (<5%), Qz (<5%), Alt (<5%) Zrc, Ap, ±Sph
5 Asterisks denote HSG zone samples: processed for zircon extraction and U-Pb geochronology, but not discussed in this manuscript (see section 4.1)
6 All coordinates were obtained using the World Geodetic System 1984 (WGS 84), Grid 28W
7 Refers to non-continuous bodies of mafic material surrounded by silicic host (e.g. mafic pillows, enclaves, and clasts; "n/a" = not applicable for mafic units)
8 Relative abundance of silicic (vs. mafic) material within the outcrop where the sample was collected
9 Major Minerals: Qz = quartz; Kfs = alkali feldspar; Pl = plagioclase feldspar; Amph = amphibole; Cpx = clinopyroxene; Bt = biotite; Alt = alteration phases
10 Accessory Minerals: FeTi = Fe-Ti oxides (e.g. magnetite, ilmenite); Zrc = zircon; Ti = titanite; Sph = sphene
Table 2. Major oxide and trace element compositions of rocks from the AIC
Sample11: NS-1 NS-2 NS-3 NS-4a NS-5 NS-6 NS-7 NS-8 G-1 G-3 G-5
Major Element Oxides (wt.%)12 SiO2 72.4 70.6 63.6 70.4 70.3 61.1 64.9 71.5 47.3 47.5 47.7
TiO2 0.31 0.65 1.1 0.39 0.54 1.4 0.87 0.41 4.1 1.8 2.0
Al2O3 13.5 13.0 15.7 13.5 14.3 14.7 15.0 14.3 13.0 19.9 19.3
Fe2O3 3.6 4.5 6.4 5.0 3.9 7.8 7.1 3.6 16.4 10.0 9.5
MnO 0.07 0.09 0.13 0.15 0.06 0.14 0.15 0.03 0.27 0.11 0.14
MgO 0.11 0.91 1.3 0.12 0.53 2.4 0.92 0.21 4.3 4.4 6.2
CaO 0.96 2.1 3.2 1.6 1.8 5.5 2.7 0.83 8.9 13.1 12.2
Na2O 5.1 4.4 5.2 5.3 4.2 4.4 5.3 5.7 3.5 2.7 2.5
K2O 3.9 3.7 3.2 3.4 4.3 2.5 2.8 3.3 0.89 0.37 0.31
P2O5 0.03 0.07 0.28 0.06 0.10 0.18 0.24 0.06 1.4 0.08 0.24 LOI (%) 1.18 2.16 1.99 2.43 1.77 2.05 1.79 2.62 2.97 2.06 --
Trace Element Concentrations (ppm)
Ni 1 8 3 1 2 17 1 -- 1 55 94
Cu 10 14 23 4 5 26 10 7 33 147 64
Zn 107 102 90 135 34 110 79 25 133 55 81
Rb 74 77 51 63 76 40 51 45 16 5.0 4.7
Sr 90 96 207 146 136 253 243 125 410 462 454
Y 109 106 77 101 83 77 88 77 56 15 23
Zr 874 586 1213 889 607 593 1037 816 214 69 124
Nb 107 95 58 90 69 52 54 74 24 4.7 13
Ba 886 481 763 631 669 440 552 662 158 82 84
V 10 74 44 3.4 28 143 15 7.3 219 355 233
Cr 2.1 18 3.2 0.54 1.2 31 0.53 1.7 1.8 44 108
La 76 70 57 69 63 52 57 66 26 7.4 13
Ce 205 172 127 174 154 116 128 168 60 16 29
Pr 22 19 16 20 17 15 16 18 9.4 2.2 4.0
Nd 87 77 65 79 66 61 69 69 48 10 18
Sm 20 18 15 18 14 14 16 14 13 2.7 4.8
Eu 3.9 2.7 4.1 3.9 2.4 3.5 4.9 3.0 4.6 1.3 1.8
Gd 20 18 15 18 15 14 17 14 13 3.0 5.1
Tb 3.4 3.2 2.5 3.2 2.4 2.4 2.8 2.3 2.0 0.47 0.83
Dy 20 19 14 18 14 14 16 13 11 2.7 4.9
Ho 4.0 3.9 2.8 3.7 2.9 2.8 3.2 2.7 2.0 0.53 0.95
Er 11 11 7.8 10 8.2 7.6 8.9 7.5 5.1 1.4 2.3
Yb 10 10 8.0 10.0 8.1 7.2 8.6 7.4 4.0 1.1 1.8
Lu 1.6 1.5 1.2 1.5 1.2 1.1 1.3 1.1 0.57 0.16 0.28
Hf 21 18 28 23 17 15 24 20 6.1 2.0 3.3
Ta 6.2 6.4 3.5 5.4 4.6 3.4 3.5 4.6 1.8 0.33 0.86
Pb 15 7.1 3.6 6.7 11 5.8 3.6 7.3 1.5 0.91 1.1
Th 9.2 9.6 7.2 8.6 10 5.7 6.0 9.5 1.8 0.98 1.0
U 6.4 4.7 3.0 3.9 4.7 1.9 2.2 4.6 0.56 0.31 0.33
Zr-Temp13 (ºC) 945 886 951 933 905 836 939 942 652 616 660
11 All sample names preceded by the label “IA-” (e.g. “NS-1” = IA-NS-1)
12 Anhydrous basis, normalized
13 Zircon Saturation Temperature: calculated using the formula of Watson & Harrison (1983) (see Section 3.2.a)