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首页> 外文期刊>Contributions to Mineralogy and Petrology >The oxygen isotope composition of Karoo and Etendeka picrites: High delta O-18 mantle or crustal contamination?
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The oxygen isotope composition of Karoo and Etendeka picrites: High delta O-18 mantle or crustal contamination?

机译：Karoo和Etendeka野餐的氧同位素组成：高三角O-18地幔或地壳污染？

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摘要

Oxygen isotope compositions of Karoo and Etendeka large igneous province (LIP) picrites and picrite basalts are presented to constrain the effects of crustal contamination versus mantle source variation. Olivine and orthopyroxene phenocrysts from lavas and dykes (Mg# 64-80) from the Tuli and Mwenezi (Nuanetsi) regions of the ca 180 Ma Karoo LIP have delta O-18 values that range from 6.0 to 6.7 %. They appear to have crystallized from magmas having delta O-18 values about 1-1.5 % higher than expected in an entirely mantle-derived magma. Olivines from picrite and picrite basalt dykes from the ca 135 Ma Etendeka LIP of Namibia and Karoo-age picrite dykes from Dronning Maud Land, Antarctica, do not have such elevated delta O-18 values. A range of delta O-18 values from 4.9 to 6.0 %, and good correlations between delta O-18 value and Sr, Nd and Pb isotope ratios for the Etendeka picrites are consistent with previously proposed models of crustal contamination.Explanations for the high delta O-18 values in Tuli/Mwenezi picrites are limited to (1) alteration, (2) crustal contamination, and (3) derivation from mantle with an abnormally high delta O-18. Previously, a variety of models that range from crustal contamination to derivation from the 'enriched' mantle lithosphere have been suggested to explain high concentrations of incompatible elements such as K, and average epsilon Nd and epsilon Sr values of -8 and +16 in Mwenezi (Nuanetsi) picrites. However, the primitive character of the magmas (Mg# 73), combined with the lack of correlation between delta O-18 values and radiogenic isotopic compositions, MgO content, or Mg# is inconsistent with crustal contamination. Thus, an O-18-enriched mantle source having high incompatible trace element concentration and enriched radiogenic isotope composition is indicated. High delta O-18 values are accompanied by negative Nb and Ta anomalies, consistent with the involvement of the mantle lithosphere, whereas the high delta O-18 themselves are consistent with an eclogitic source. Magma delta O-18 values about 1 % higher than expected for mantle-derived magma are also a feature of the Bushveld mafic and ultra-mafic magmas, and the possibility exists that a long-lived O-18-enriched mantle source has existed beneath southern Africa. A mixed eclogite peridotite source could have developed by emplacement of oceanic lithosphere into the cratonic keel during Archaean subduction.
机译：提出了Karoo和Etendeka大火成岩省（LIP）的苦味粉矿和苦味粉玄武岩的氧同位素组成，以限制地壳污染对地幔源变化的影响。来自大约180 Ma Karoo LIP的Tuli和Mwenezi（Nuanetsi）地区的熔岩和堤坝（Mg＃64-80）的橄榄石和邻苯二甲酚异酚酯的O-18δ值介于6.0至6.7％之间。它们似乎是从δO-18值比完全由地幔来源的岩浆中获得的期望值高约1-1.5％的岩浆中结晶的。来自纳米比亚约135 Ma Etendeka LIP的微晶岩和微晶玄武岩脉石以及来自南极洲Dronning Maud Land的Karoo-age微晶岩脉石没有这种升高的O-18值。 Oten值介于4.9％到6.0％之间，以及Etendeka铅矿的O-18值与Sr，Nd和Pb同位素比之间的良好相关性与先前提出的地壳污染模型一致。图里/姆威尼兹（Tuli / Mwenezi）岩画中的O-18值仅限于（1）蚀变，（2）地壳污染以及（3）三角洲O-18异常高的地幔衍生。以前，已经提出了各种模型，从地壳污染到“富集”地幔岩石圈的衍生，都可以解释高浓度的不相容元素，例如K，以及Mwenezi的平均εNd和εSr值分别为-8和+16。 （努阿内西）野餐。然而，岩浆（Mg＃73）的原始特征，再加上δO-18值与放射性同位素组成，MgO含量或Mg＃之间缺乏相关性，与地壳污染不一致。因此，表明了具有高的不相容的痕量元素浓度和富集的放射性同位素组成的富含O-18的地幔源。高δO-18值伴随着负的Nb和Ta异常，这与地幔岩石圈的侵袭一致，而高δO-18本身与凝结物来源一致。布什维尔德镁铁质岩浆和超镁铁质岩浆的一个特征是岩浆δO-18值比地幔衍生岩浆的预期值高约1％，并且可能存在长寿命的富含O-18的地幔源非洲南部地区。混合的榴辉岩橄榄岩源可能是通过在太古俯冲时期将海洋岩石圈置于克拉通龙骨中而形成的。

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