Oxygen isotope and trace element compositions of platiniferous dunite pipes of the Bushveld Complex, South Africa – Signals from a recycled mantle component?
Oxygen isotope and trace element compositions of platiniferous dunite pipes of the Bushveld Complex, South Africa – Signals from a recycled mantle component?
复制标题
DOI：
10.1016/j.lithos.2018.04.017
复制
发表时间：
2018-06
期刊：
影响因子：
3.5
通讯作者：
T. Günther;K. Haase;M. Junge;T. Oberthür;Dominic Woelki;S. Krumm
中科院分区：
文献类型：
--
作者：
T. Günther;K. Haase;M. Junge;T. Oberthür;Dominic Woelki;S. Krumm
含铂白云石管产于Bushveld大火成岩省（LIP）的Rustenburg层状套下部基性/超基性部分。这些管道中的橄榄石成分从橄榄石（Fo） 80到35 mol%不等，表明在高温（~1200 °C）下不同演化的岩浆结晶。最原始的橄榄石来自一个砧木单元，Ni含量最高（>0.15 wt%）， Mn含量最低（<0.3 wt%）。辉石质寄主岩的分异结晶和部分熔融作用是形成含锰量高（>0.3 wt%）的铁质橄榄石的重要原因。橄榄石（5.7 ~ 7.0‰）和辉石（6.7 ~ 7.4‰）的高δ18O值与Bushveld岩体下临界带的δ18O值相似，表明两者具有共同的成因。橄榄石中恒定的高O同位素比值和可变的Fo含量，不同于同化-分级结晶形成的岩浆中橄榄石斑晶的趋势。我们认为，最原始的泥质岩的高δ18O反映了Bushveld管道的初始熔体，表明在管道形成之前地壳的大量同化或次大陆岩石圈地幔（SCLM）中海洋地壳的再循环贡献。原始管状橄榄石的高Ni/Mn比值可能继承自富辉石源的熔融。
Platiniferous dunite pipes occur in the lower mafic/ultramafic portion of the Rustenburg Layered Suite of the Bushveld large igneous province (LIP). Olivine compositions in these pipes range from forsterite (Fo) 80 to 35 mol% and suggest crystallization from variably evolved magmas at high temperatures (~1200 °C). The most primitive olivines are from a stock unit and have the highest contents of Ni (>0.15 wt%) and lowest contents of Mn (<0.3 wt%). Fractional crystallization and partial melting of pyroxenite host rock play a significant role in the formation of the fayalitic olivines with its high Mn contents (>0.3 wt%). High δ18O values of olivine (5.7–7.0‰) and pyroxene (6.7–7.4‰) are akin to those of the Lower and Critical Zone of the Bushveld intrusion suggesting a common origin. The constant high O isotope ratios with variable Fo contents in the olivines are unlike trends observed in olivine phenocrysts in magmas forming by assimilation-fractional crystallization. We suggest that the high δ18O in the most primitive dunites reflect that of the primary melt of the Bushveld pipes, indicating either a bulk assimilation of crust prior to pipe formation or a contribution from recycled oceanic crust in the sub-continental lithospheric mantle (SCLM). The latter scenario is supported by the high Ni/Mn ratios in primitive pipe olivine that might be inherited from melting of a pyroxene-rich mantle source.