Zircon LA-ICPMS Hf and SIMS O isotopic data for granitic gneiss of the Billabong Complex, Tanami Region
Whelan, JA; Woodhead, JD; Cliff, J; Whelan, JA; Woodhead, JD; Cliff, J
Northern Territory Geological Survey; Northern Territory Geological Survey
E-Publications; E-Books; PublicationNT; NTGS Record
This record presents new laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) Lu Hf isotope results and secondary-ion mass spectrometry (SIMS) O isotope data for zircon from a granitic gneiss of the Billabong Complex, Tanami Region.'; This record presents new laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) Lu Hf isotope results and secondary-ion mass spectrometry (SIMS) O isotope data for zircon from a granitic gneiss of the Billabong Complex, Tanami Region.'
Made available via the Publications (Legal Deposit) Act 2004 (NT); Available from GEMIS - Geoscience Exploration and Mining Information System; Made available via the Publications (Legal Deposit) Act 2004 (NT)
Geochronology; Isotope Geochemistry; Tanami Region
Northern Territory Government; Northern Territory Government
9780724572717 (CD-ROM); 9780724572724 (Web); 9780724572717 (CD-ROM); 9780724572724 (Web)
Attribution International 4.0 (CC BY 4.0)
Northern Territory Government; Northern Territory Government
https://geoscience.nt.gov.au/gemis/ntgsjspui/handle/1/82465 [GEMIS]; https://geoscience.nt.gov.au/gemis/ntgsjspui/handle/1/82465
1 INTRODUCTION This technical note presents new laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) Lu-Hf isotope results and secondary-ion mass spectrometry (SIMS) O isotope data for zircon from a granitic gneiss of the Billabong Complex, Tanami Region. The sample was obtained during a joint Northern Territory Geological Survey (NTGS)Geoscience Australia (GA) geochronology project in October 2011. The samples were collected in support of ongoing NTGS geoscientific investigations in the Tanami Region. SHRIMP U-Pb zircon data and methodologies, including sample preparation, instrument setup and data reduction, are reported in Kositcin et al (2013), a summary of which is provided here as well as information on sample location and geological context, for the Hf and O results. Zircon is the major reservoir of Hf in most crustal rocks. Numerous studies during the past decade (eg Amelin et al 1999, 2000, Bodet and Schaerer 2000, Griffin et al 2000, 2002, Wilde et al 2001) demonstrate that the Hf isotopic composition of zircon can be used to better understand various crustal processes and to provide age constraints on the growth of continental crust. The advent of multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) in the early 1990s has made it possible to obtain high-precision Hf-isotope data on individual zircon grains (Griffin et al 2000, Thirlwall and Walder 1995), while U-Pb ages can be obtained from the same grains by either SHRIMP or LA-ICPMS techniques. In situ analysis by laser ablation of Hf isotopes in zircon is increasingly used in the geological community because it provides the precise spatial control required for analysis of zircons with complex internal morphology (Griffin et al 2000, 2002). The basis of the Hf isotopic system is the decay of 176Lu to 176Hf, whereas 177Hf is a stable isotope. During mantle melting, Hf is partitioned more strongly into melts than Lu. Over time, the 176Lu/177Hf ratio evolves to higher values in the mantle than in crustal rocks. During the production of granitoid magmas, high values of 176Lu/177Hf (ie Hf > 0) indicate juvenile mantle input, either directly via mantlederived mafic melts, or by remelting of young mantlederived mafic lower crust. Low values of 176Lu/177Hf (Hf < 0) provide evidence for crustal reworking. 176Lu/177Hf values for zircon can also be the product of mixed sources with distinct 176Lu/177Hf ratios. This leads to the concept of average crustal residence ages. Two types of model ages can be calculated from Hf-isotope data. TDM model ages are calculated using the measured 176Lu/177Hf ratio of the zircon grain, and give a minimum age for a potential juvenile source of the host magmatic rock. A more realistic crustal model age (TDM C) can be calculated, assuming that the source of the magma had the 176Lu/177Hf ratio of average continental crust (0.015; Griffin et al 2000). In both cases, the resultant ages are model ages only and the latter can be considered an average crustal residence age, or an average of the time since the source material was extracted from the mantle to enter the crust. All calculations given in this report use the 176Lu decay constant of Blichert-Toft et al (1997). Although Hf isotopic data can be used to extract important information on the nature of the source rocks of the melt from which a particular zircon grain grew, it provides only an average crustal residence age, rather than a true estimate of the time since the source was extracted from the mantle. This is particularly problematic for detrital zircons (compared with magmatic rocks), for which wholerock geochemical and isotopic data can provide further (but limited) constraints. Zircon stable oxygen isotope data can be useful to further constrain the nature of the sources for the following reasons. 1. 18O values are time independent. 2. Diffusion rates of O in zircon are negligible (Watson and Cherniak 1997, Peck et al 2003, Zeb Page et al 2007). 3. 18O in a source that could be melted to yield zircon can be changed only by low temperature and surficial processes. 4. 18O of zircon derived from mantle melts is tightly constrained and distinct from rocks that have experienced a sedimentary cycle or seafloor alteration (Cavosie et al 2005, Valley et al 2005). 5. 18O can be measured in situ with high precision by ion microprobe (Valley 2003). 18O values for specific zircons (or groups of zircons of similar morphology) can be used to identify grains that have grown from a mantle-derived melt. The corresponding Hf data should give a true indication of the timing of extraction from the mantle. Conversely grains with 18O values above the range typical for mantle-derived melts (Cavosie et al 2005, Valley et al 2005) indicate derivation from melting of crust that has been weathered or hydrothermally altered at the seafloor. Analytical Procedures SHRIMP U-Pb zircon analyses were undertaken at the GA in Canberra and are reported in Kositcin et al (2013). Hf isotopic data were collected using a Nu Plasma MCICPMS coupled with a HelEx 193 nm ArF Excimer laser ablation system at the School of Earth Sciences, University of Melbourne (Woodhead et al 2004). O isotopic data were collected using a CAMECA IMS 1280 multi-collector ion probe at the Centre for Microscopy, Characterisation and Analysis, University of Western Australia using conditions similar to (Kita et al 2009) Instrument setup and analysis Hf isotope measurements by LA-ICPMS Measurements of Hf isotopes were conducted by laser ablation multicollector inductively coupled plasma mass spectrometry using a Nu Plasma MC-ICPMS coupled with a HelEx 193 nm ArF Excimer laser ablation system (Woodhead et al 2004) at the School of Earth Sciences at the University of Melbourne. Laser ablation analyses were performed on the same locations in grains previously analysed for U-Pb dating by SHRIMP. Hf-isotope analyses reported in this manuscript were obtained using spots of 47 m in diameter. The following description of instrument set-up and analysis is taken from Woodhead et al (2004).