Author: Martin J. Whitehouse
Last saved by: Tamzin Anderson
Company: NRM

Supplementary material 

The Akilia Controversy: field, structural and geochronological evidence questions interpretations of > 3.8 Ga life in SW Greenland

MARTIN J. WHITEHOUSE^1, JOHN S. MYERS^2 & CHRISTOPHER M. FEDO^3
^1Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden
(martin.whitehouse@nrm.se)
^2Applied Geology, Curtin University of Technology, Perth, Western Australia 
^3Dept. of Earth & Planetary Sciences, University of Tennessee, Knoxville, Tennessee 37996 USA

Analytical Methods
U-(Th)-Pb analyses of apatite and zircon were performed using a Cameca IMS1270 ion microprobe at the Nordsim facility, Stockholm.  Basic instrument set up parameters for both minerals (ca. 5 nA, 20 µm O_2^- primary beam, secondary mass resolution M/ΔM = 5300, energy window = 50 eV) are similar to those previously described from this laboratory for zircon U-Th-Pb analysis (Whitehouse et al., 1999; Whitehouse & Kamber, 2005).  The mono-collection peak-hopping routine used for zircon and automated centering procedures are described by Whitehouse & Kamber (2005).  Pb/U ratio calibration was carried out using the Geostandards zircon 91500 standard with an assumed age of 1065 Ma (Wiedenbeck et al., 1995).  Common Pb corrections based on the measured ^204Pb signal (when significant relative to background) assume the present day model terrestrial Pb composition of Stacey & Kramers (1975).
For analysis of apatite, oxygen flooding of the sample chamber which enhances Pb^+ yield from zircon, was found to be ineffective and was therefore not employed.  The peak-hopping mono-collection electron multiplier data acquisition routine used for the full U-Pb analyses has been modified from that described by Sano et al. (1999), comprising 12 cycles through the species ^40Ca_2^31P^16O_4 (matrix and centering peak; nominal mass 175), ^141Pr^31P^16O_2 (centering peak; nominal mass 204), ^204Pb, mass 204.3 (background), ^206Pb, ^207Pb, ^208Pb, ^146Nd^31P^16O_2 (centering peak; nominal mass 209), ^208Pb^1H, ^238U, ^232Th^16O, ^238U^16O (centering peak).  Each run consisted of a ca. 2 minute pre-sputter to remove the Au coating, followed by automated centering of the secondary beam in the field aperture, adjustment of the sample high voltage to account for any charging (minimal) and mass centering of selected high count rate species.  Both background and PbH^+ count rates were sufficiently low (average <0.015 counts/sec on both species for both standard and unknown apatites) as to be considered negligible and no corrections were applied.  Calibration of the Pb/U ratios was performed using a linear correlation between ^206Pb/^238U^+ and ^238U^16O/^238U^+ ratios obtained from multiple analyses of the Prairie Lake, Ontario reference apatite “PRAP” (kindly supplied by Y. Sano).  This apatite has relatively low levels of common Pb (average ^206Pb/^204Pb obtained in this study was ca. 1700) and corrected ^207Pb^*/^206Pb^* ratios yield a weighted average age of 1156 ± 15 Ma (2σ, MSWD = 1.1, n = 19) from the first session and 1127 ± 22 Ma (2σ, MSWD = 0.95, n = 9) from the second session.  Both of these ages are in agreement with the ion microprobe age of 1156 ± 45 Ma (2σ) reported by Sano et al. (1999) and a previous ID-TIMS determination of 1155 ± 72 Ma (2σ; Tilton & Kwon, 1990).  For U-Pb calibration, an age of 1156 Ma was assumed for PRAP.  
The separated apatite grains were also analysed for Pb isotope ratios using a multi-collector (electron multipliers) routine at a mass resolution of ca. 4800.  Detector positioning along the focal plain was achieved using the high Pb USGS glass GSE-1G (ca. 370 pm) and BCR-2 was used as a reference against which relative detector gains were normalized using the ratios reported by Woodhead & Hergt (2000).  
	Age calculations use the decay constant recommendations of Steiger & Jäger (1975).  Regression calculations used Isoplot-Ex (Ludwig, 1999). 

References
Ludwig, K.R. 1999. Using Isoplot/Ex, Version 2.01: a geochronological toolkit for Microsoft Excel. Berkeley Geochronology Center Special Publication, No. 1a, 47 pp.
Sano, Y, Oyama, T., Terada, K. & Hidaka, H. 1999. Ion microprobe U–Pb dating of apatite. Chemical Geology, 153, 249–258.
Stacey, J.S. &  Kramers, J.D. 1975. Approximation of terrestrial lead isotope evolution by a 2-stage model.  Earth and Planetary Science Letters, 26, 207-221.
Steiger, R.H. & Jäger, E. 1977.  Subcommission on geochronology; convention on the use of decay constants in geo- and cosmochronology.  Earth and Planetary Science Letters, 36, 359-362. 
Tilton, G.R. & Kwon, S.-T. 1990. Isotopic evidence for crust–mantle evolution with emphasis on the Canadian Shield. Chemical Geology, 83, 149–163.
Whitehouse, M.J. & Kamber, B. 2005. Assigning dates to thin gneissic veins in high-grade metamorphic terranes: a cautionary tale from Akilia, southwest Greenland. Journal of Petrology, 46, 291-318. 
Whitehouse, M.J., Kamber, B.S., & Moorbath, S. 1999.  Age significance of U-Th-Pb zircon data from early Archaean rocks of west Greenland – a reassessment based on combined ion-microprobe and imaging studies.  Chemical Geology, 160, 201-224.
Wiedenbeck, M., Allé, P., Corfu, F., Griffin, W. L., Meier, M., Oberli, F., von Quadt, A., Roddick, J. C. & Spiegel, W. 1995.  Three natural zircon standards for U-Th-Pb, Lu-Hf, trace element and REE analysis.  Geostandards Newsletter, 19, 1- 23.
Woodhead, J.D. & Hergt, J.M. 2000.  Pb-isotope analyses of USGS reference materials.  Geostandards Newsletter, 24, 33-38.