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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 >Asthenospheric percolation of alkaline melts beneath the St. Paul region (Central Atlantic Ocean)
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Asthenospheric percolation of alkaline melts beneath the St. Paul region (Central Atlantic Ocean)

机译：在圣保罗地区（大西洋中部）下方的碱液的软流圈渗透

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摘要

Two peridotite suites collected by submersible in the equatorial Atlantic Ocean (Hekinian et al., 2000) were studied for textures, modes, and in situ major and trace element compositions in pyroxenes. Dive SP12 runs along the immersed flank of the St. Peter and Paul Rocks islets where amphibole-bearing ultramafic, mylonites enriched in alkalies and incompatible elements are exposed (Roden et al., 1984), whereas dive SP03 sampled a small intra-transform spreading centre situated about 370 km east of the St. Peter and Paul Rocks. Both suites are characterized by undeformed, coarse-grained granular textures typical of abyssal peridotites, derived from residual mantle after similar to 15% melting of a DMM source, starting in the garnet stability field. Trace element modelling, textures and lack of mineral zoning indicate that the residual peridotites were percolated, reacted and refertilized by similar to 2.6% partially aggregated melts in the uppermost level of the melting region. This relatively large amount of refertilization is in agreement with the cold and thick lithosphere inferred by previous studies. Freezing of trapped melts occurred as the peridotite entered the conductive layer, resulting in late-stage crystallization of olivine, clinopyroxene, spinel, +/-plagioclase. Chondrite-normalized REE patterns in clinopyroxenes from SP03 indicate that they last equilibrated with (ultra-) depleted partial melts. In contrast, REE concentrations in clinopyroxenes from SP12 display U and S shaped LREE-enriched patterns and the calculated compositions of the impregnating melts span the compositional range of the regional basalts, which vary from normal MORB to alkali basalt sometimes modified by chromatographic fractionation with no, or very limited, mineral reaction. Thus the mylonitic band forming the St. Peter and St. Paul Rocks ridge is not a fragment of subcontinental lithospheric mantle left behind during the opening of the Central Atlantic, nor the source of the alkaline basalts as previously suggested. Rather, dive SP12 sampled residual peridotites of normal MORB mantle that were located close to channels transporting alkali basalts. Reacted melts escaping from these channels, infiltrated, and locally equilibrated with, the peridotite matrix by ion exchange reactions. Relicts of the source of the alkaline basalts were not sampled but our study suggests that it was a component of the MORB mantle underlying the St. Paul region.
机译：研究了由潜水器在赤道大西洋中收集的两个橄榄岩组（Hekinian等，2000），研究了辉石中的质地，模式以及原位主要和微量元素组成。潜水SP12沿圣彼得和保罗岩石小岛的浸没面运行，其中暴露有带闪石的超镁铁矿，富含碱的my石以及不相容的元素（Roden等人，1984），而潜水SP03则采样了一个小的转换内扩展中心位于圣彼得和保罗岩石以东370公里处。两种套件均具有深变形橄榄岩典型的未变形，粗颗粒状纹理，其特征是从石榴石稳定性领域开始，其残留的地幔是在约15％的DMM源融化后从残留的地幔中获得的。痕量元素建模，质地和缺乏矿物分区表明，残留的橄榄岩在熔融区最上层被大约2.6％的部分聚集的熔体渗透，反应和渗透。相对大量的化肥与先前研究推断的寒冷而厚实的岩石圈是一致的。当橄榄岩进入导电层时，被困的熔体发生冻结，从而导致橄榄石，斜辉石，尖晶石，+ /-斜长石的后期结晶。 SP03的斜辉石中的球粒晶归一化REE模式表明，它们最后与（超）贫化的部分熔体达到平衡。相比之下，SP12的斜辉石中的REE浓度呈现U和S形LREE富集模式，并且浸渍熔体的计算组成跨越了区域玄武岩的组成范围，从正常的MORB到碱性玄武岩不等，有时通过色谱分离进行改性，没有或非常有限的矿物反应。因此，形成圣彼得和圣保罗岩石山脊的mylonitic带既不是中大西洋开放期间留下的亚大陆岩石圈地幔的碎片，也不是先前提出的碱性玄武岩的来源。相反，潜水SP12采样了正常MORB地幔的残留橄榄岩，该橄榄岩位于靠近运输碱性玄武岩的通道附近。从这些通道逸出的反应熔体通过离子交换反应渗透到橄榄岩基质中并与之局部平衡。没有对碱性玄武岩的来源进行采样，但我们的研究表明，它是位于圣保罗地区的MORB地幔的一部分。

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