Controversial Pb-Pb and Sm-Nd isotope results in the early Archean Isua (West Greenland) oxide iron formation: preservation of primary signatures versus secondary disturbances

Frei, Robert; Bridgwater, D.; Rosing, Minik T.; Stecher, Ole

doi:10.1016/s0016-7037(98)00290-7

Cited by 90 publications

(83 citation statements)

References 45 publications

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“…Clermont-Ferrand II 22) 0.511849ῌ0.000034 Max Planck 23) 0.511860ῌ0.000021 CNRS 24) 0.511852ῌ0.000010 Univ. Copenhagen 25) 0.511847ῌ0.000011 Univ. Arizona 26) 0.511869ῌ0.000015 Univ.…”

Section: Isotope Measurement Of Trace Amount Of Ndunclassified

Precise Isotope Determination of Trace Amounts of Nd in Magnesium-rich Samples

Shibata

Yoshikawa

2004

J. Mass Spectrom. Soc. Jpn.

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We developed a technique combining mass spectrometry with chemical separation to analyze Sm῎Nd isotopic compositions of ultramafic rock samples, which have extremely high Mg and low Nd concentrations and are always di$cult to analyze. In the case of ultramafic rocks, abundant Mg forms fluorides during decomposition by HF, and minor amounts of Sm and Nd are strongly partitioned into fluorides, which would be a significant reason of low recovery yields of these elements and obstructed to determine a reliable isotopic ratio of Nd and Sm. We introduced pre-decomposition by HCl prior to the conventional method by HF and HClO 4 that made it possible to elevate recovery yield to 85ῑ. On the other hand, replicate measurements of Nd standard solution (La Jolla) by changing the ion current of Nd ῌ and the amount of Nd indicate that reliable isotopic ratios are measured under the condition of ῐ0.04῎10῍11 A of 144 Nd ῌ ion current and ῐ2 ng of Nd. By applying our method, we analyzed 143 Nd/ 144 Nd ratio and Sm and Nd concentrations of PCC-1, which is an ultramafic standard rock sample of United States Geological Survey. The results of our analysis were 0.512226῍0.000026, 0.00551 ῍0.00041, and 0.0278῍0.0011 ppm, respectively. These results are the first report of 143 Nd/ 144 Nd ratio for PCC-1.

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Section: Isotope Measurement Of Trace Amount Of Ndunclassified

Precise Isotope Determination of Trace Amounts of Nd in Magnesium-rich Samples

Shibata

Yoshikawa

2004

J. Mass Spectrom. Soc. Jpn.

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“…Nevertheless, due to small K/Ca ratios of the plagioclase minerals, an age difference of 150 Ma (e.g., 3.8 Ga as formation age for the Amîtsoq gneisses proposed by Nutman et al, 1996Nutman et al, , 1999 would decrease the 40 Ca/ 44 Ca of the plagioclase separates by less than 0.1 Ca (with the exception of sample 125540 which would show a ⌬ Ca ϭ 0.4, however, this sample yields straightforward ϳ3.65Ga ion-microprobe zircon ages: Whitehouse et al, 1999). The three mafic rocks from the Akilia association as well as the four samples from the ISB are corrected using the age of 3.7 Ga and 3.71 Ga, respectively (e.g., Nutman et al, 1997b;Frei et al, 1999Frei et al, , 2002.…”

Section: Resultsmentioning

confidence: 99%

“…This in turn implies that the radiogenic Ca isotope signatures documented in the plagioclases of the mafic Akilia enclaves were present at 2.7 Ga (Fig. 5), presumablyas a result of 40 Ca ingrowth since extensive K metasomatism at ϳ3.6 Ga Rosing et al, 1996;Frei et al, 1999;Fedo and Whitehouse, 2002). The 2.7 Ga metamorphic event plausibly reduced the K/Ca ratio of the whole rock, due to biotite dehydration, 'freezing' in this radio- genic signature in both whole rock and plagioclase (e.g., SM/ GR/97/17, Fig.…”

Section: Metamorphic Overprintmentioning

confidence: 97%

“…Samples from the Zimbabwe Craton consist of four TTG gneisses from the Shabani-Mashaba area (Table 1). Most samples have been previously investigated for geochemistry and several isotope systems (Rb-Sr, Th-U-Pb, Sm-Nd; Re-Os, e.g., Black et al, 1971;Moorbath et al, 1972Moorbath et al, , 1973Moorbath et al, , 1975Moorbath et al, , 1976Moorbath et al, , 1977aMoorbath et al, , 1977bMoorbath et al, , 1986Baadsgaard, 1973;Hawkesworth et al, 1975;Taylor et al, 1984Taylor et al, , 1991Baadsgaard et al, 1986aBaadsgaard et al, , 1986bBennett et al, 1993;Luais and Hawkesworth, 1994;Whitehouse et al, 1999;Frei et al, 1999Frei et al, , 2002Frei et al, , 2004Kamber, 2002, 2003;Frei and Jensen, 2003).…”

Section: Samplesmentioning

confidence: 99%

“…The age of the Akilia association is also controversial (3.7 Ga vs. Ͼ3.85 Ga; Schiøtte and Compston, 1990;Nutman et al, 1997a;Myers and Crowley, 2000) but the rocks are definitively older than the surrounding Amîtsoq gneisses (McGregor and Mason, 1977). The deposition of the volcanic and chemical-sedimentary rocks of the ISB, on the other hand, is comparatively well defined and took place at 3.71 Ga and at 3.81 Ga for some lithologies (Moorbath et al, 1973;Nutman et al, 1997b;Blichert-Toft et al, 1999;Frei et al, 1999;Villa et al, 2001).…”

Section: Samplesmentioning

confidence: 99%

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Ca isotope fingerprints of early crust-mantle evolution

Kreissig

Elliott

2005

Geochimica et Cosmochimica Acta

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Ca ratios indistinguishable from our measured modern MORB value (i.e., Ca (3.6) ϳ 0). Greenland samples, however, are very diverse ranging from Ca (3.7) ϭ 0.1 in mafic pillow lavas and felsic sheets from the Isua supracrustal belt, up to very radiogenic signatures ( Ca (3.7) ϭ 2.9) in both mafic rocks of the Akilia association and felsic TTG from the coastal Amîtsoq gneisses.At face value, these results imply the Zimbabwe crust is juvenile whereas most Greenland samples include an earlier crustal component. Yet the west Greenland craton, as with many Archaean localities, has experienced a complex geological history and the interpretation of age-corrected initial isotope values requires great care. Both felsic and mafic samples from Greenland display Ca (3.7) so radiogenic that they are not readily explained by crustal growth scenarios. The presence of such radiogenic 40 Ca/ 44 Ca found in low K/Ca plagioclases requires Ca isotope exchange between plagioclase and whole rock during later metamorphic event(s). In addition the unexpectedly radiogenic Ca isotope ratios in some mafic samples reflect anomalous K/Ca ratios as a result of intense K-metasomatism ϳ3.6 Ga. Thus Ca isotope measurements are not a robust tracer of crustal growth in the presence of intense tectono-metamorphic processes. Coupled with other isotope data, however, the degree of overprint can be estimated and the 40 Ca/ 44 Ca ratio of a little disturbed sample hints at a small contribution of Hadean protocrust in the coastal part of the Godthåbsfjord area (Southwest Greenland). In the majority of Zimbabwe TTG samples, unradiogenic initial Ca isotope ratios point to very little prior crustal history and minor subsequent disturbance. We thus infer that the modest initial Nd ϳ0.8 of the Zimbabwean samples is representative of the depleted mantle at ϳ3.6 Ga. Furthermore, Ca isotope systematics provide little support for a "steady state" model of crustal growth.

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The Ancient Earth

Beard

Weyer

2020

Iron Geochemistry: An Isotopic Perspective

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Controversial Pb-Pb and Sm-Nd isotope results in the early Archean Isua (West Greenland) oxide iron formation: preservation of primary signatures versus secondary disturbances

Cited by 90 publications

References 45 publications

Precise Isotope Determination of Trace Amounts of Nd in Magnesium-rich Samples

Precise Isotope Determination of Trace Amounts of Nd in Magnesium-rich Samples

Ca isotope fingerprints of early crust-mantle evolution

The Ancient Earth

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