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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Ferropicrites: geochemical evidence for Fe-rich streaks in upwelling mantle plumes
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Ferropicrites: geochemical evidence for Fe-rich streaks in upwelling mantle plumes

机译：铁锰铁矿：地幔柱上升流中富铁条纹的地球化学证据

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A comparison of high-MgO magmas from both oceanic and continental settings reveals that they exhibit wide variations in their bulk-rock contents of FeO~* (9-16 wt% at 15 wt% MgO). The high-FeO~* picrites (ferropicrites) range from Archean to recent in age but are relatively rare at the Earth's surface, typically forming thin isolated flows near the base of thick lava piles in large igneous provinces. They are characterised by high contents of compatible trace elements (e.g. Cr = 400 - 1650 parts per million (ppm) and Ni = 250 - 1050 ppm) and unaltered samples (Parana-Etendeka and Madagascar) have relatively smooth, normalised multi-element patterns that lack significant relative enrichments in strongly incompatible elements (e.g. [Ba/La]_n = 0.5 - 1.0) and [La/Nb]_n = 1.2 - 1.4). The ferropicrites are distinguished from other picritic rocks (e.g. Deccan, Hawaii, West Greenland) by their relatively low abundances of Al_2O_3 (～10 wt%) and heavy rare-earth elements (HREEs, e.g. Lu = < 10 * chondrite). They have ~(87)Sr/~(86)Sr_i ratios of ～0.7048 and ε_(Nd) values of ～+4 that are comparable to those of ocean-island basalts. Modelling calculations of combined Sr, Nd and Pb isotopic ratios indicate that some of the ferropicrites may have assimilated upper or lower crust but this appears to have had little effect on major element abundances. The high-FeO~* contents of world-wide ferropicrites, relative to 'normal' picrites, cannot simply be attributed to variations in degrees of partial melting and/or depth of melt segregation of an anhydrous lherzolite mantle source. Quantitative partition modelling suggests that the contributing parental melts of the ferropicrites were derived by adiabatic decompression melting of a mantle source that was similar in composition to experimentally studied Fe-rich peridotite PHN1611. The parental melts of the Parana-Etendeka ferropicrites appear to have been generated by ～10% partial melting, at high pressures (45-35 kbar) and high mantle potential temperatures (T_p = 1550 ℃). The relatively low volume of world-wide ferropicrites and their association with igneous rocks of 'normal' FeO~* contents in mantle plume-related igneous provinces suggest that the former may be derived from Fe-rich streaks in mantle plume starting-heads.
机译：对来自海洋和大陆环境的高MgO岩浆的比较表明，它们的FeO〜*块状岩石含量表现出很大的差异（MgO为15 wt％时为9-16 wt％）。高FeO〜*的微晶石（铁锰铁矿）的年龄范围从太古宙时代到最近，但在地球表面相对罕见，通常在火成岩大省的厚熔岩桩基部附近形成稀疏的孤立流。它们的特征在于高含量的相容痕量元素（例如Cr = 400-1650百万分之一（ppm）和Ni = 250-1050 ppm）且未改变的样品（Parana-Etendeka和马达加斯加）具有相对平滑的，标准化的多元素图案在强不相容元素中缺乏明显的相对富集（例如[Ba / La] _n = 0.5-1.0和[La / Nb] _n = 1.2-1.4）。铁素体铁矿与其他轻质岩石（如Deccan，夏威夷，西格陵兰）的区别在于其Al_2O_3含量相对较低（〜10 wt％）和重稀土元素（HREE，例如Lu = <10 *球粒陨石）。它们的〜（87）Sr /〜（86）Sr_i比值为〜0.7048，ε_（Nd）值为〜+ 4，与大洋玄武岩相当。结合的Sr，Nd和Pb同位素比的模型计算表明，某些铁锰矿可能具有同化的上，下地壳，但这似乎对主要元素的丰度影响很小。相对于“普通”苦味粉，世界范围内的亚铁盐铁含量较高的FeO〜*含量，不能简单地归因于无水锂铁矿幔源的部分熔融程度和/或熔融偏析深度。定量分配模型表明，铁锰铁矿的母体熔融物是通过地幔源的绝热减压熔融而得到的，其组成与实验研究的富铁橄榄石PHN1611相似。在高压（45-35 kbar）和高地幔电势温度（T_p = 1550℃）下，巴拉那-埃滕德卡铁锰矿的母体熔体似乎是由〜10％的部分熔体产生的。与地幔羽相关的火成岩省中，全世界范围内的亚铁盐铁矿含量相对较低，并与“正常” FeO〜*含量的火成岩相关，这表明前者可能源自地幔羽始端中富含铁的条纹。

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