Felsic Plutonic Rocks from IODP Hole 1256D, Eastern Pacific: Implications for the Nature of the Axial Melt Lens at Fast-Spreading Mid-Ocean Ridges
Felsic Plutonic Rocks from IODP Hole 1256D, Eastern Pacific: Implications for the Nature of the Axial Melt Lens at Fast-Spreading Mid-Ocean Ridges
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DOI：
10.1093/petrology/egx064
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发表时间：
2017-08
影响因子：
3.9
通讯作者：
Chao Zhang;J. Koepke;L. France;M. Godard
中科院分区：
文献类型：
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作者：
Chao Zhang;J. Koepke;L. France;M. Godard
尽管快速扩张的洋中脊下的轴向熔融透镜（AML）已被地震反射探测了几十年，但其性质及其在下洋壳增生和洋中脊玄武岩（MORB）演化和喷发中的作用仍缺乏充分的认识。由含石英辉长岩、闪长岩和英云闪长岩组成的深成岩，可能代表一个结晶AML的上部，首次由综合大洋钻探计划（IODP）1256 D孔从一个完整的快速扩张洋壳剖面中发现。全岩主要元素显示出宽而连续的成分范围（例如Mg# 24-70），并且在中等MgO含量（4- 6wt%）下明显富集Ti和Fe。微量元素特征是一致的，由岩相学和矿物模式定义的不同岩性组，即辉长岩，单斜辉石丰富的闪长岩，角闪石丰富或氧化物丰富的闪长岩和英云闪长岩。辉长岩和闪长岩是一致的MORB分离结晶，由混合熔体和堆积在不同的比例模型的产品。建模的微量元素（特别是相对于Eu）支持的模型，其中英云闪长岩起源于低程度的部分熔融的片状岩脉覆盖的AML，而不是极端的分离结晶。辉长岩和闪长质侵入岩中的单斜辉石中稀土元素（REE）的富集表明富稀土英云闪长岩组分被演化的MORB岩浆强烈同化。热液蚀变是普遍的深成岩系统，这可以追溯到岩相学，矿物成分和岩石地球化学。AML的上部，主要由低密度和高粘度的长英质岩浆，可能作为一个屏障，喷发MORB熔体在AML的下部。
Although the axial melt lens (AML) beneath fast-spreading mid-ocean ridges has been detected by seismic reflection for decades, its nature and role in the accretion of lower oceanic crust and the evolution and eruption of mid-ocean ridge basalts (MORB) are still poorly constrained. Plutonic rocks consisting of quartz-bearing gabbros, diorites and tonalites, which might represent the upper part of a fossilized AML, have for the first time been recovered from an intact fast-spreading oceanic crust section by Integrated Ocean Drilling Program (IODP) Hole 1256D. Whole-rock major elements show a wide and continuous compositional range (e.g. Mg# 24–70) and apparent enrichments in Ti and Fe at intermediate MgO contents (4–6 wt %). Trace element characteristics are coherent for the different lithology groups defined by petrography and mineral modes; that is, gabbro, clinopyroxene-rich diorite, amphibole-rich or oxide-rich diorite and tonalite. The gabbros and diorites are consistent with modeled products of MORB fractional crystallization, composed of mixed melt and cumulate in varying ratios. Modeled trace elements (especially with respect to Eu) support a model in which the tonalites originated from low-degree partial melting of the sheeted dikes overlying the AML, rather than extreme fractional crystallization. Enrichments in rare earth elements (REE) in clinopyroxenes from the gabbroic and dioritic intrusive rocks suggest strong assimilation of REE-rich tonalitic components by evolved MORB magmas. Hydrothermal alteration was pervasive during cooling of the plutonic system, which can be traced by petrography, mineral compositions and bulk-rock geochemistry. The upper part of AML, largely composed of low-density and high-viscosity felsic magmas, may serve as a barrier to eruptible MORB melts in the lower part of AML.