National Geochemical Survey of Australia: Samarium-Neodymium Isotopes Dataset

Preamble -- The 'National Geochemical Survey of Australia: The Geochemical Atlas of Australia' was published in July 2011 along with a digital copy of the NGSA geochemical dataset (http://dx.doi.org/10.11636/Record.2011.020). The NGSA project is described here: www.ga.gov.au/ngsa. A recent review of the original and ensuing NGSA outputs and impacts can be found in Caritat (2022). The present dataset contains additional geochemical data obtained on NGSA samples: Samarium-Neodymium Isotopes Dataset. 

Abstract -- Ninety three coarse-fraction (<2 mm) Bottom Outlet Sediment (BOS, on average 60 – 80 cm depth) NGSA samples mostly from three continental-scale cross-sections (one central north-south cross-section, one central east-west, and one southern east-west) were analysed for the samarium (Sm) and neodymium (Nd) isotopes 147Sm, 143Nd, and 144Nd to determine the 143Nd/144Nd and 147Sm/144Nd isotope ratios, initial epsilon Nd (εNd0), and single and two-stage depleted mantle model ages TDM and T2DM (in Ga or billion years). Together these samples represent over 490,000 km2 of catchment area sourcing the sampled fluvial/alluvial sediments.

The summary statistics for 143Nd/144Nd and εNd0 results are shown below.

Stats
143Nd/144Nd
εNd0

Min
0.511014
-31.68

25%
0.511550
-21.22

Median
0.511817
-16.02

MAD
0.000232
4.53

Average
0.511802
-16.31

SD
0.000342
6.67

75%
0.512019
-12.07

Max
0.512623
-0.29

The method is briefly summarised below.

Samples were analysed at University of Melbourne (UOM) by Roland Maas and University of Alberta (UOA) by Rob Creaser.

At UOM all analyses were by isotope dilution. Samples were dissolved at high pressure, and Sm and Nd extracted using Eichrom TRU- and LN-resin columns. Isotopic analyses were by Multi Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS), with Nd mass bias corrected by internal normalisation to 146Nd/145Nd = 2.0719425 (equivalent to 146Nd/144Nd = 0.7219). In-run errors (2se) were ± 0.000010 or lower, external precision (2sd) ± 0.000020. All 143Nd/144Nd results have been adjusted to LaJolla=0.511860. The TIMS reference for JNd-1 is 0.512117, and BCR-2 has a nominal 147Sm/144Nd of 0.1382 and 143Nd/144Nd of 0.512640 ± 20.

At UOA rock powders were accurately weighed and totally spiked with a known amount of mixed 150Nd-149Sm tracer solution and dissolved at high pressure for 5 days. Sm and Nd were extracted and separated by conventional cation and HDEHP-based chromatography (Creaser et al., 1997). Chemical processing blanks were < 120 picograms of either Sm or Nd, and are insignificant relative to the amount of Sm or Nd analysed for any rock sample. Isotopic analyses were determined in static mode by MC-ICP-MS (Schmidberger et al., 2007). All isotope ratios were normalized for variable mass fractionation to a value of 146Nd/144Nd = 0.7219. The 143Nd/144Nd ratio of samples are presented here relative to a value of 0.511850 for the La Jolla Nd isotopic standard, monitored by use of an in-house Alfa Nd isotopic standard for each analytical session. Sm isotopic abundances were normalized for variable mass fractionation to a value of 1.17537 for 152Sm/154Sm. The mixed 150Nd-149Sm tracer solution used was calibrated directly against the Caltech mixed Sm/Nd normal described by Wasserburg et al. (1981). Using this mixed tracer, the measured 147Sm/144Nd ratios for the international rock standard BCR-1 ranged from 0.1380 to 0.1382, suggesting a reproducibility for 147Sm/144Nd of ~ ± 0.1 % for real rock powders. The value of 147Sm/144Nd determined for BCR-1 was within the range of reported literature values by isotope dilution methods.

A full report on this dataset will be available shortly as a Geoscience Australia Record.

REFERENCES CITED

Caritat, P. de, 2022. The National Geochemical Survey of Australia: review and impact. Geochemistry: Exploration, Environment, Analysis, 22, geochem2022-032. https://doi.org/10.1144/geochem2022-032

Creaser, R.A., Erdmer, P., Stevens, R.A. and Grant, S.L., 1997. Tectonic affinity of Nisutlin and Anvil assemblage strata from the Teslin tectonic zone, northern Canadian Cordillera: constraints from neodymium isotope and geochemical evidence. Tectonics, 16, 107-121. https://doi.org/10.1029/96TC03317 

Schmidberger, S.S., Heaman, L.M., Simonetti, A., Creaser, R.A. and Whiteford, S., 2007. Lu-Hf, in-situ Sr and Pb isotope and trace element systematics for mantle eclogites from the Diavik diamond mine: evidence for Paleoproterozoic subduction beneath the Slave craton, Canada. Earth and Planetary Science Letters, 254, 55-68. https://doi.org/10.1016/j.epsl.2006.11.020 

Wasserburg, G.J., Jacobsen, S.B., DePaolo, D.J., McCulloch, M.T. and Wen, T., 1981. Precise determination of Sm/Nd ratio, Sm, Nd isotopic abundances in standard solutions. Geochimica et Cosmochimica Acta, 45, 2311-2323.