IMPROVING GEOCHRONOLOGY FOR PRECISE GEOHISTORY DETERMINATION
June 19th, 2014
Sm-Nd ID-TIMS technique
Re-Os ID-TIMS technique for silicate rocks, sulphides and hydrocarbons and black shales
U-Pb ID-TIMS technique for zircons
Our TritonTM TIMS instruments features a multicollector system consisting of nine Faraday cups and a secondary electron multiplier, multiple ion counting, and a 21-sample turret. The isotope dilution technique used is a ‘definitive’ method of analysis producing highest achievable accuracy and precision. TIMS represents the gold standard for isotopic analysis, reaching a precision of 0.001% on isotopic ratios. The history of TIMS-based research at Curtin can be traced back to the 1960s, when the isotope study group carried out Rb-Sr and Lu-Hf chronological studies. The facility has recently installed a Thermo Scientific TritonTM mass spectrometer, facilitating a new range of geochemical, geological and environmental research applications.
The TIMS instrumentation, in combination with isotope dilution methodology, is widely used in chemical metrology for the calibration of isotopic standards, and the calculation of isotopic abundances and atomic weights.
The TIMS Facility at Curtin University has contributed to accurately measuring the concentrations of various elements in international reference materials. The currently accepted atomic weights of Cd, Te and Sn are based on measurements made at Curtin. The present applications of TIMS are mainly in the broad field of geochronology involving Re/Os, U/Pb, Pb/Pb and Sm/Nd isotope systems, and in isotope fingerprinting in the study of ore- and petroleum formation processes.
Sample preparation and chemical separation of elements is carried out in the recently purpose-build Acid Digest Laboratory with clean air environment designed for ultra-trace analyses. It is equipped with ULPA-Filtered air to reduce particle levels, ducted fume cupboard and deionized distilled ultra-pure water.
The Clean Lab represents the latest evolution in the design of clean labs for processing ultra-trace amounts of heavy metals.
The present applications of TIMS are mainly in the broad field of geochronology involving Re/Os, U/Pb, Pb/Pb and Sm/Nd isotope systems, and in isotope fingerprinting in the study of ore- and petroleum formation processes.
• Fundamental and applied study of the mineral systems evolution
• Sources and age of volcanic rocks from the underexplored Western Australian ‘greenfields’ regions specifically targeting mineral exploration
• Geochronology and sources of hydrocarbons (oil and black shales)
• High-precision U-Pb dating capable of resolving crystallisation histories within magmatic systems
• Determining the ages of units within sedimentary sequences with high resolution
Direct dating of the timing of oil generation and that of its source rock is crucial for petroleum exploration. Accurate chronology of oil generation and migration in a petroleum basin can provide essential information for reliable petroleum resource evaluation, reserve calculation and predictive drilling, especially for shale oil plays, in which oil is stored in situ in the source rocks. Combining Re-Os isotope dating and PGE abundance analysis can provide a powerful tool not only for constraining the age of oil generation and charging events, but also for tracing oil-source correlations.
Pilot Re-Os study on crude oil samples from Canning basin in Western Australia (WA) has been performed in order to test the suitability of Re-Os isotope system for geochronology and sources fingerprinting. The samples come from Dodonea, Sundown and Blina fields within Canning basin. The reservoir ages for these fields range from Ordovician to Carboniferous. The asphaltene fraction was extracted from crude oil samples in order to pre-concentarte the Re and Os, and then these fractions were used for Re-Os chemistry using Carious tube tecnique, followed by N-TIMS analysis.
From twelve samples of WA oil studied, only three returned the results because of quite low Re-Os abundances in separated asphaltines, in a range of up to 1 ppb for rhenium and tens of ppt for osmium. This is quite low compare to Canadian oils, which usually show about an order of magnitude higher abundances of these metals. Higher Re and Os contents in Canadian oils correlate with markedly higher nickel and vanadium abundances.
Our preliminary data show a good fit with the basin ages using geologically reasonable initial Os ratios varying from depleted mantle-like ratio of 0.12 for Ordovician samples to the more evolved ratio of 0.4, corresponding to the Os isotopic composition of Late Devonian seawater. Interestingly, the Ordovicial oil from Canning basin show more depleted source according to carbon isotope composition (~-31‰δ¹³C) as well, trending towards more enriched marine compositions for Devonian-Carboniferous parts of the basin (-27 to -29‰δ¹³C). As a conclusion, the study involving more samples shows a good potential for both geochronology and source fingerprinting of WA oils.
This project seeks to utilize the Re-Os isochron method on sulphides to determine precise ages for two classes of metal deposits in WA: i.e. VHMS Zn-Pb-Cu deposits and orogenic gold deposits. Selected VHMS deposits from the Murchison (Golden Grove, Scuddles), eastern Yilgarn (Jaguar, Nimbus) and Pilbara (Sulphur Springs) will be studied as part of this project. The ages of mineralization for only two orogenic gold deposits in WA have been determined using the Re-Os isochron method on sulphides: at Sunrise Dam and Tropicana. The former example states an age uncertainty of +/-6 Ma and provides encouragement that ages, with precision similar to that gleaned from the U-Pb and Ar-Ar methods, are achievable. This project will seek Re-Os isochron ages on sulphides for undated deposits in the western Yilgarn (e.g. Boddington, Murchison), to build a comparative database to the eastern Yilgarn: i.e. ∼20 dated deposits using mainly ore-associated phosphates. The Re-Os ages will be complemented by U-Pb and Ar-Ar data to establish a 4D framework, where appropriate.
As part of the development of the Re-Os method in the John de Laeter Centre, we have recently determined a Re-Os age for molybdenite from the Central Bore gold prospect within the Yamarna terrain, in the eastern Yilgarn craton (Fuller et al., AESC proceedings, Newcastle, 2014). Here, gold is closely associated with molybdenite and is found in veins and inclusions in molybdenite. The Re-Os molybdenite age was 2620±10 Ma, which is compatible with U-Pb hydrothermal titanite ages from the same samples and slightly younger than zircon ages of hosting felsic volcanics (2677±7 Ma) and a minimum age for sandstone (2682±5 Ma) from the Yamarna terrain. It is also close to the timing of the gold deposition event (2640-2630 Ma) recorded at Kalgoorlie (Vielreicher et al., 2010), which is the youngest known mineralization event in the EGP (Vielreicher et al., in prep.). This pilot study suggests confirmation of the robust nature of Re-Os geochronometer in molybdenite and its potential application in WA, where molybdenite occurs.
Moreover, we recently performed a collaborative project with Russian Academy of Sciences in order to date the Olimpiada gold deposit using arsenopyrites. The reasonable isochron model age of 689±28 Ma was obtained (Borisenko et al., Acta Geol Sin, 2014), which is compatible with host rocks geochronology. PPrevious work of Dr Tessalina included determining isochron model ages on pyrite separates for several Urals VHMS deposits (e.g., Gannoun et al., 2003 Chem Geol; Tessalina et al, 2008 OGR). In addition, JdLC Director Professor Brent McInnes was previously a Project Leader on AMIRA P563 Re-Os Geochronology of Ore Systems (1999-2002: $320,000), an industry-ARC sponsored project that investigated the application of Re-Os isotope systematics in major ore deposits around the globe (McInnes et al., 1999, 2004 and 2008). This project generated research results for the Mount Morgan Cu-Au deposit in Queensland (Unpublished AMIRA P563 Final Report 2003).