Trace element and isotopic characterization of mafic cumulates in a fossil mantle diapir (Oman ophiolite)
Trace element and isotopic characterization of mafic cumulates in a fossil mantle diapir (Oman ophiolite)
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DOI：
10.1016/s0009-2541(96)00087-3
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发表时间：
1996-12
期刊：
影响因子：
3.9
通讯作者：
M. Benoit;M. Polvé;G. Ceuleneer
中科院分区：
文献类型：
--
作者：
M. Benoit;M. Polvé;G. Ceuleneer
本文对阿曼地幔方辉橄榄岩中出现的一套堆积特征进行了探索性的地球化学研究(微量元素、锶、、钕同位素)。堆积物集中分布在化石地幔底辟周围。从底辟的中心到外围，它们的矿物学越来越演化，从流纹岩到橄榄辉长岩和辉长岩，其结构和场特征指向较凉爽的环境中的注入。所有这些堆积物的Nd同位素数据都与地幔起源相一致。地幔源区在小尺度上具有不均一性(6.09；ϵ；ND<；1015)，但这种不均质性和ϵ平均RmND(≈8)可与印度洋中脊玄武岩(MORB‘s)相媲美。这种堆积岩套的起源主要是由具有MORB特征的液体的分离结晶形成的，并得到了主要和相容的微量元素变化的支持。然而，相容的元素表明，所有这些累积物不太可能都是通过不同程度的分离结晶从同一液体中获得的。这一结论得到了不相容的微量元素数据的证实。计算出的与这些堆积物平衡的液体具有类似MORB的稀土配分模式。然而，从一个岩性群到下一个岩性群，以及在每个岩性群内，稀土配分模式的变化意味着平衡液体中稀土元素含量的显著不均一性，这显然不是简单的分离结晶过程的结果。与沸石平衡的液体比与橄榄辉长岩平衡的液体具有更高的[La]n/[Yb]n范围。演化最多的堆积岩(辉长岩)具有REE配分模式，这可能意味着它们的母质熔体有更复杂的来源，包括类似MORB的液体与来自LREE枯竭来源的熔体的混合。
This paper is devoted to an exploratory geochemical study (trace elements, SrNd isotopes) of a suite of cumulate features cropping out in the mantle harzburgites of Oman. The cumulates are concentrically distributed around a fossil mantle diapir. From the centre to the periphery of the diapir, their mineralogy becomes more and more evolved, from troctolite to olivine gabbro and gabbronorite, and their textural and field characteristics point to injection in a cooler environment. Nd isotopic data are consistent with a mantle origin for all these cumulates. The mantle source is heterogeneous on a small scale (6.09 < ϵNd< 1015) but this heterogeneity and the average ϵrmNd(≈8) are comparable to those of Indian Ocean mid-ocean ridge basalts (MORB's). An origin of this cumulate suite, largely by fractional crystallization from liquids with MORB characteristics, is supported by major and compatible trace element variations. However compatible elements show that it is unlikely that all these cumulates are derived by different degrees of fractional crystallization from the same liquid. This conclusion is corroborated by incompatible trace element data. The calculated liquids in equilibrium with these cumulates have MORB-like REE patterns. However, variations in REE patterns from one lithological group to the next, and within each lithological group, imply a pronounced heterogeneity in the REE content of the equilibrium liquids that clearly does not result from a simple fractional crystallization process. Liquids in equilibrium with the troctolites have a higher range in [La]n/[Yb]nthan liquids in equilibrium with the olivine gabbros. The most evolved cumulates (gabbronorites) have REE patterns that might imply a more complex origin for their parental melt involving mixing of MORB-like liquids with melts from a LREE-depleted source.