Isotopic equilibrium between mantle peridotite and melt: Evidence from the Corsica ophiolite
Isotopic equilibrium between mantle peridotite and melt: Evidence from the Corsica ophiolite
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DOI：
10.1016/j.epsl.2009.10.024
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发表时间：
2009-11
影响因子：
5.3
通讯作者：
E. Rampone;A. Hofmann;I. Raczek
中科院分区：
文献类型：
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作者：
E. Rampone;A. Hofmann;I. Raczek
地幔源区与地幔熔体之间存在同位素平衡是地幔地球化学和大洋部分熔融理论中广泛采用的假设。然而，最近的扩散研究和同位素调查蛇绿岩，深海橄榄岩和相关的MORB提出了质疑，通过提供证据的同位素不平衡残留橄榄岩和MORB之间的假设。在这里，我们提出的Sr和Sm-Nd同位素数据地幔橄榄岩和辉长岩侵入体从山。Maggiore（法国阿尔卑斯科西嘉）特提斯蛇绿岩，它记录了Nd同位素的均匀性，意味着同位素平衡，在1公里的规模。橄榄岩记录了岩石圈不同深度的多阶段熔体-岩石相互作用和熔体侵入。研究的样品是残余的贫cpx尖晶石二辉橄榄岩，反应尖晶石方辉橄榄岩，浸渍斜长石橄榄岩和相关的辉长岩细脉，后来的辉长岩脉。橄榄岩和辉长岩中的锶同位素是高度可变的，由于与海水衍生流体的相互作用，并且不能用于测试熔体残余物同位素平衡。相比之下，Nd同位素不受海水蚀变的影响。橄榄岩显示出现今较高的147 Sm/144 Nd（0.49-0.59）和143 Nd/144 Nd（0.513367-0.513551）比值，残余和反应尖晶石橄榄岩之间以及尖晶石和斜长石橄榄岩之间没有明显差异。辉长岩脉具有现代MORB的典型Nd同位素组成（143 Nd/144 Nd =0.513122-0.513138）。橄榄辉长岩脉和辉长质细脉的Sm-Nd等时线测得的侏罗纪年龄（橄榄辉长岩为162±10 Ma和159± 15 Ma，辉长岩为155± 6 Ma），初始εNd=8.9-9.7，指示MORB型源区。橄榄岩的Sm-Nd同位素组成符合辉长岩的线性排列，其初始ε Nd值（160 Ma）为7.6-8.9，与MORB源区一致。这种Sm-Nd同位素均匀性可能表明地幔中1公里尺度上的固熔平衡接近，或者它可能代表地幔源的继承性同位素均匀性。在任何一种情况下，熔融和熔体-岩石相互作用是最有可能产生橄榄岩和熔体之间几乎完全平衡的过程。与最近积累的明显的熔体源同位素不平衡的证据，一般在几公里的尺度上，在这里，我们显示了一个设置源岩和熔体之间的同位素均匀性达到。这一结果能否适用于其他地区，将取决于其他地区的进一步详细研究。
A widely used assumption of mantle geochemistry and the theory of partial melting at oceanic settings is the existence of isotopic equilibrium between mantle source and melt. Yet, recent diffusion studies and isotopic investigations of ophiolites, abyssal peridotites and associated MORBs have cast doubts on this assumption, by providing evidence for isotopic disequilibrium between residual peridotites and MORBs. Here we present Sr and Sm–Nd isotope data on mantle peridotites and gabbroic intrusions from the Mt. Maggiore (Alpine Corsica, France) Tethyan ophiolite, which document Nd isotopic homogeneity, implying isotopic equilibrium, on a 1-kilometer scale. The peridotites record multi-stage melt–rock interaction and melt intrusion occurring at different lithospheric depths. Samples studied are residual cpx-poor spinel lherzolites, reactive spinel harzburgites, impregnated plagioclase peridotites and related gabbronoritic veinlets, later gabbroic dykes. Strontium isotopes in peridotites and gabbros are highly variable, due to interaction with sea-water derived fluids, and cannot be used to test melt-residue isotopic equilibrium. In contrast, Nd isotopes are unaffected by sea-water alteration. Peridotites display present-day high147Sm/144Nd (0.49–0.59) and143Nd/144Nd (0.513367–0.513551) ratios, with no appreciable differences between residual and reactive spinel peridotites, and between spinel and plagioclase peridotites. Gabbroic dykes have present-day Nd isotopic compositions typical of MORB (143Nd/144Nd=0.513122–0.513138). Internal (plag–whole rock–cpx) Sm–Nd isochrons for olivine gabbro dykes and a gabbronoritic veinlet yield Jurassic ages (162±10 and 159±15Ma in ol-gabbros, 155±6Ma in gabbronorite), and initial εNd=8.9–9.7 indicative of a MORB-type source. Sm–Nd isotopic compositions of peridotites conform to the linear array defined by the gabbroic rocks, and yield initial (160Ma) εNdvalues of 7.6–8.9, again consistent with a MORB source. This Sm–Nd isotopic homogeneity may indicate a close approach to solid-melt equilibration on a 1-kilometer scale in the mantle, or it may represent an inherited isotopic homogeneity of the mantle source. In either case, melting and melt–rock interaction were the most likely processes to produce nearly complete equilibration between peridotite and melt. In contrast with recently accumulated evidence of apparent melt-source isotopic disequilibrium, generally on scales of several kilometers, here we show a setting where isotopic homogeneity between source rock and melt was attained. Whether this result can be applied elsewhere will depend on additional detailed studies in other regions.