Analytical methods
Seafloor samples were dredged in the central Lau basin during April-June 2008 with
the SS07/2008 voyage of the RV Southern Surveyor (http://www.marine.csiro.au/
nationalfacility/voyagedocs/2008/MNF-SS07-08_sum.pdf) of the Australian Marine National
Facility. All samples have previously been analysed for their major and trace elements and He
isotope compositions [Jenner et al., 2012; Lupton et al., 2009]. For Hf-Nd-Fe isotope
analyses, approximately 100 mg of powdered glass chips was dissolved in 15 ml Teflon vials
in a HNO3-HF acid mixture on a hotplate at ~150°C for 24h. Samples were subsequently
dried down and repeatedly re-dissolved and dried in concentrated nitric acid with traces of
HCl+HF until a clear sample solution was achieved.
For Fe isotope analyses, a 5% aliquot was taken prior to Hf-Nd separation (see below),
dried, and re-dissolved in 10M HCl. Iron was separated from the rock matrix using ~1 ml of
DOWEX AG-MP-1 (200-400 mesh) anion exchange resin. Sample were loaded onto the
columns in 1 ml 9M HCl and matrix elements were washed from the columns with additional
5 ml 10M HCl and subsequent 5 ml of 5M HCl. Purified Fe was eluted from the resin in 4 ml
1M HCl and dried. For isotope analyses, a ~4-6 ppm Fe sample solution was spiked with ~1520 ppm Ni solution to ensure a 1V signal on
61
Ni for mass bias control of Fe. Isotope
measurements were performed on a ThermoFisher Neptune-plus multi-collector mass
spectrometer with an inductively coupled plasma source (MC-ICP-MS) housed at ANU in
high-resolution mode and using Ni H-cones. The sample was introduced with a 50µl
aspirating nebuliser into a cyclone glass chamber in 0.1 M HNO3. The IRMM-014 isotope
reference standard (by definition 57Fe=0‰) was analysed between samples and all samples
were normalised to the average of the bracketing standards analysed prior and after sample
analyses. Masses 54, 56 and 57 were collected and data is presented in the delta notation:
5XFe = [5XFe/54Fe(sample) / 5XFe/54Fe(average IRMM-014) -1] × 1000
where 5X refers to the relative masses 56 or 57. All samples are reported in the 57Fe notation,
but 56Fe is only used for data control to ensure samples lie on a mass dependent fractionation
line. Data was collected in 45 cycles and 4 sec integration time. Possible remnants of
isobarically interfering chromium isotopes (54Cr) from the sample was monitored online
during the measurements, but was always insignificant during analyses. All sample solutions
were analysed at least twice and internal 1 errors (1 standard deviation of the mean) are
listed in the data table. The long-term reproducibility of isotope analyses of IRMM-014
(n>>50) with the standard sample bracketing method and internal mass bias correction using
Ni is 57Fe±0.03 for a period of approximately one year. Our true external reproducibility was
estimated by repeated analyses of standard reference material BHVO-2. Independent
dissolutions analysed at ANU and at the university of Adelaide yield a 2 s.d. of the mean of
±0.03‰ for both isotope compositions. The absolute values are in excellent agreement with
recommended values [Craddock and Dauphas, 2011]. As to the low number of SRM analyses
(n=5), we estimate our reproducibility with ±0.05‰, in line with values reported elsewhere
[Poitrasson and Freydier, 2005; Schönberg and von Blanckenburg, 2006] using the std.sample bracketing technique. Blank levels for Fe were below detection limit and insignificant
throughout the study.
Both, Hf and Nd were sequentially extracted from the rock matrix using a combination
of Eichrom Ln Spec and DOWEX cation resin. First, Hf was separated from the rock matrix
using a procedure described in [Münker et al., 2001]. Afterwards, REE were purified using
True Spec resin [Pin and Zalduegui, 1997]. The clean REE fraction was then loaded on the
Ln-Spec columns and Nd was separated from other REE following the procedure of [Pin and
Zalduegui, 1997]. Hafnium and Nd were analysed on a Thermo-Fisher Neptune MC-ICP-MS
at ANU. Detailed descriptions of the chemical procedures and mass spectrometric
measurements are given elsewhere [Nebel et al., 2009; Nebel et al., 2010]. All samples were
normalised to a JMC-475
143
176
Hf/177Hf=0.282160 [Blichert-Toft et al., 1997] and La Jolla
Nd/144Nd=0.511858 [Lugmair and Carlson, 1978] from standard values during the
analytical sessions of JMC-475=0.282145±9 and 0.282158±5 and La Jolla=0.511829±20 and
0.511848±8, respectively. Blanks for both elements were constantly low during the course of
the study with Hf<10 pg and Nd<30pg.
References Figure FS01: (Figure A-1): [Martinez and Taylor, 2002] [Pearce and Peate, 1995]
[Jenner and O'Neill, 2012];
References Appendix:
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isotope analysis of rock samples by magnetic sector-multiple collector ICP-MS, Contributions to
Mineralogy and Petrology, 127, 248-260.
Craddock, P. R., and N. Dauphas (2011), Iron Isotopic Compositions of Geological Reference Materials
and Chondrites, Geostandards and Geoanalytical Research, 35(3), 101-123.
Jenner, F. E., and H. S. C. O'Neill (2012), Analysis of 60 Elements in 616 Ocean Floor Basaltic Glasses,
Geochemistry Geophysics Geosystems, 13, doi: 10.1029/2011GC004009.
Jenner, F. E., R. J. Arculus, J. A. Mavrogenes, N. Dyriw, O. Nebel, and E. Hauri (2012), Chalcophile
element Systematics in Volcanic Glasses from the Northwestern Lau Basin, Geochemistry Geophysics - Geosystems, 13, doi: doi:10.1029/2012GC004088.
Lugmair, G. W., and R. W. Carlson (1978), The Sm-Nd history of KREEP, paper presented at 9th Lunar
Planetary Science Conference
Lupton, J. E., R. J. Arculus, R. R. Greene, L. J. Evans, and C. I. Goddard (2009), Helium isotope
variations in seafloor basalts from the Northwest Lau Backarc Basin: Mapping the influence of the
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Geoanalytical Research, 33(4), 487-499.
Nebel, O., W. van Westrenen, P. Z. Vroon, M. WIlle, and M. M. Raith (2010), Deep mantle storage of
the Earth’s missing niobium in late-stage residual melts from a Hadean magma ocean, Geochimica et
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rocks, Analytica Chimica Acta, 339(1-2), 79-89.
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resolution MC-ICP-MS, Chemical Geology, 222(1-2), 132-147, doi: 10.1016/j.chemgeo.2005.07.005.
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