A persistent question in early Earth investigations is the depletion history of the mantle as recorded in the isotopic compositions of the oldest rocks and minerals. Here we present new, in situ, ...sensitive high resolution ion microprobe (SHRIMP) U–Th–Pb geochronology, and laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) Lu–Hf tracing data for individual zircons extracted from the oldest rocks of the Antarctic and Australian continents. These are 3.88 and 3.85Ga orthogneisses from the Gage Ridge and Mt. Sones in Enderby Land and a 3.73Ga Meeberrie gneiss from the Yilgarn Craton. The zircons from each sample show complex U–Th–Pb systematics due to the mobile behaviour of Pb, resulting in several age populations with discordant to reversely discordant arrays on concordia plots. In contrast, measured 176Hf/177Hf compositions from the same analysis sites show narrowly defined compositional ranges, with 3 or fewer groups for each sample, reflecting the resistance of 176Hf/177Hf in zircons to disturbance even under high-temperature, granulite conditions. The initial 176Hf/177Hf calculated for the most primitive Hf population for each sample using the best age estimate yields near-chondritic weighted mean compositions of −1.6±0.5, −1.6±0.4 and −0.6±0.7epsilonunits (deviations in parts per 10,000 from a chondritic uniform reservoir (CHUR) reference) at 3877±62, 3850±50, and 3731±4Ma respectively. These results provide no evidence for significant Lu–Hf fractionation on the early Earth expected to result from processes including accretion, intra-mantle differentiation, or massive Hadean crustal growth.
•We report Hf isotopes in zircons from the oldest rocks of the Yilgarn and East Antarctic Cratons (3.73, 3.85 and 3.88Ga).•The oldest zircon populations yield precise chondritic initial Hf isotopic compositions.•No evidence in either Yilgarn or East Antarctic Cratons for Hadean basement.•Eoarchean chondritic Hf compositions permit only limited extent of Hadean continents or intra-mantle Lu/Hf fractionation.
The International Atomic Energy Agency uses the Thermal Ionization Mass Spectrometry Total Evaporation method routinely to analyze the n(U-235)/n(U-238) atom amount ratios in inspection and quality ...control samples. This paper uses a top-down statistical model applied to quality control measurements of certified reference materials (CRM) to study the capabilities of the technique. The technique is shown to produce measurements with a relative standard deviation not more than 0.03% when applied to a wide range of certified reference materials.
The sources and petrogenetic processes that generated some of the Earth’s oldest continental crust have been more tightly constrained via an integrated, in situ (U–Pb, O and Hf) isotopic approach. ...The minerals analysed were representative zircon from four Eoarchaean TTG tonalites and two felsic volcanic rocks, and olivine from one harzburgite/dunite of the Itsaq Gneiss Complex (IGC), southern West Greenland. The samples were carefully chosen from localities with least migmatisation, metasomatism and strain. Zircon was thoroughly characterized prior to analysis using cathodoluminescence, scanning electron, reflected and transmitted light imaging. The zircon from all but one sample showed only minor post-magmatic recrystallisation.
207Pb/
206Pb dating of oscillatory-zoned zircon using SHRIMP RG (
n
=
142) indicates derivation of the felsic igneous rocks from different batches of magma at 3.88, 3.85, 3.81, 3.80 and 3.69
Ga.
Analyses of
18O/
16O compositions of olivine from a harzburgite/dunite (
n
=
8) using SHRIMP II in multi-collector mode, indicate that the oxygen isotopic composition of this sample of Eoarchaean mantle (
δ
18O
Ol
=
6.0
±
0.4‰) was slightly enriched in
18O, but not significantly different from that of the modern mantle. Zircon
δ
18O measurements from the six felsic rocks (
n
=
93) record mean or weighted mean compositions ranging from 4.9
±
0.7‰ to 5.1
±
0.4‰, with recrystallised domains showing no indication of oxygen isotopic exchange during younger tectonothermal events.
δ
18O
Zr compositions indicate that the primary magmas were largely in equilibrium with the mantle or mantle-derived melts generated at similar high temperatures, while calculated tonalite
δ
18O
WR compositions (6.7–6.9‰) resemble those of modern adakites.
LA-MC-ICPMS zircon
176Hf/
177Hf analyses were obtained from six samples (
n
=
122). Five samples record weighted mean initial
ε
Hf compositions ranging from to 0.5
±
0.6 to −0.1
±
0.7 (calculated using
λ
176Lu
=
1.867
×
10
−11
yr
−1), while one sample records a composition of 1.3
±
0.7, indicating the magmas were generated from a reservoir with a time averaged, near chondritic Lu/Hf. The derivation of TTG magmas from a chondritic Lu/Hf source implies either that there was not voluminous continental crustal growth nor major mantle differentiation leading to Lu/Hf fractionation during the Hadean or Eoarchaean, or alternatively that rapid recycling of an early formed crust allowed the early mantle to maintain a chondritic Lu/Hf.
Previous studies have demonstrated that ancient TTG rocks were mostly produced by dehydration melting of mafic rocks within the stability field of garnet, probably in flatly-subducted or buried oceanic crust. The oxygen isotopic signatures measured here at high spatial resolution allow the source materials to be better defined. Melting of a mixed mafic source consisting of ∼80% unaltered gabbro (
δ
18O
WR
=
5.5‰) with ∼20% hydrothermally altered gabbro/basalt (
δ
18O
WR
=
4.0‰) would produce tonalite magmas within the average compositional range observed.
18O-enriched components such as altered shallow basaltic oceanic crust and pelagic or continental sediments were not present in the sources of these TTG melts. The absence of high
18O signatures may indicate either the rarity of low temperature altered sediments, or their effective removal from the down-going slab.
A nuclear forensics investigation involving a uranium ore concentrate relies on accurate and precise analysis of impurities. Analytical data defensibility requires the use of reference materials as ...part of quality control. This study presents a compilation of trace element concentration results of the CUP-2 Uranium Ore Concentrate Standard measured by 11 different laboratories. The laboratories employed various dissolution methods, analytical preparation methods, and instrumental platforms. The data presented here contain concentrations of 66 impurities with up to 138 individual data points for each impurity. Consensus values have been assigned to each impurity following a statistical analysis of the data set.
This paper presents an electron microprobe study of inclusions within igneous zircons from ca. 3800 Ma metatonalite sample G97/18. Crystalline inclusions are K-feldspar, plagioclase, quartz, ...hornblende, biotite, ilmenite and apatite. Additionally, one globular polymineralic inclusion interpreted as crystallised melt occurs at the surface of a polished grain mount. This consists of plagioclase + quartz + K-feldspar around a biotite lath. Other similar, but rare globular inclusions were seen buried within other zircons below the polished surface. These inclusions, particularly the presence of K-feldspar and plagioclase in broadly equal amounts, entail crystallisation of the zircons from a (residual) granitic (sensu-stricto) melt rather than a tonalitic melt. SHRIMP (Sensitive High Resolution Ion MicroProbe) U/Pb dating of zircons with inclusions indicates that they grew at ca. 3800 Ma (the accepted igneous age of the rock).
This inclusion suite is compatible with the low Ti-in-zircon temperatures for G97/18 igneous zircon (
Hiess et al., 2008). Therefore, low Ti-in-zircon temperatures and “granitic” inclusions found in Hadean detrital zircons do not necessitate crystallisation from low temperature granites, because such features would also be found if Hadean zircons had crystallised (late) from higher temperature zircon-undersaturated melts of tonalitic composition.
The present-day (238)U/(235)U ratio has fundamental implications for uranium-lead geochronology and cosmochronology. A value of 137.88 has previously been considered invariant and has been used ...without uncertainty to calculate terrestrial mineral ages. We report high-precision (238)U/(235)U measurements for a suite of uranium-bearing minerals from 58 samples representing a diverse range of lithologies. This data set exhibits a range in (238)U/(235)U values of >5 per mil, with no clear relation to any petrogenetic, secular, or regional trends. Variation between comagmatic minerals suggests that (238)U/(235)U fractionation processes operate at magmatic temperatures. A mean (238)U/(235)U value of 137.818 ± 0.045 (2σ) in zircon samples reflects the average uranium isotopic composition and variability of terrestrial zircon. This distribution is broadly representative of the average crustal and "bulk Earth" (238)U/(235)U composition.
Ti-in-zircon thermometry with SHRIMP II multi-collector has been applied to two well-documented Archean igneous and metamorphic samples from southern West Greenland. Zircons from 2.71 Ga partial melt ...segregation G03/38 formed in a small (<
1 m
3), closed system within a mafic rock under high pressure granulite facies conditions. Results of 14 Ti analyses present a mean apparent zircon crystallization temperature of 679
±
11 °C, underestimating independent garnet-clinopyroxene thermometry by 20–50 °C but consistent with reduced
a
TiO2 in this system. 36 spot analysis on 15 zircons from 3.81 Ga meta-tonalite G97/18, with an estimated magmatic temperature >
1000 °C, yield a low-temperature focused normal distribution with a mean of 683
±
32 °C, further demonstrated by high resolution Ti mapping of two individual grains. This distribution is interpreted to represent the temperature of the residual magma at zircon saturation, late in the crystallization history of the tonalite. Hypothetically, Ti-in-zircon thermometry on Eoarchaean detrital zircons sourced from such a high temperature tonalite would present a low-temperature biased image of the host magma, which could be misconstrued as being a minimum melt granite. Multiple analyses from individual zircons can yield complex Ti distributions and associated apparent temperature patterns, reflecting cooling history and local chemical environments in large magma chambers. In addition to inclusions and crystal imperfections, which can yield apparent high temperature anomalies, zircon surfaces can also record extreme (>
1000 °C) apparent Ti temperatures. In our studies these were traced to
49Ti (or a molecular isobaric interference) contamination derived from the double sided adhesive tape used in sample preparation, and should not be assigned geological significance.
Herein, a nuclear forensics investigation involving a uranium ore concentrate relies on accurate and precise analysis of impurities. Analytical data defensibility requires the use of reference ...materials as part of quality control. This study presents a compilation of trace element concentration results of the CUP-2 Uranium Ore Concentrate Standard measured by 11 different laboratories. The laboratories employed various dissolution methods, analytical preparation methods, and instrumental platforms. The data presented here contain concentrations of 66 impurities with up to 138 individual data points for each impurity. Consensus values have been assigned to each impurity following a statistical analysis of the data set.
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