The influence of melt infiltration on the Li and Mg isotopic composition of the Horoman Peridotite Massif
The influence of melt infiltration on the Li and Mg isotopic composition of the Horoman Peridotite Massif
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DOI：
10.1016/j.gca.2015.05.006
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发表时间：
2015-09
影响因子：
5
通讯作者：
Y. Lai;P. P. V. Strandmann-P.;R. Dohmen;E. Takazawa;T. Elliott
中科院分区：
文献类型：
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作者：
Y. Lai;P. P. V. Strandmann-P.;R. Dohmen;E. Takazawa;T. Elliott
本文对霍罗曼橄榄岩地块的一组样品进行了Li、Mg同位素分析。我们的研究结果表明，霍罗曼橄榄岩的大部分Li和全部Mg同位素组成在连续露头100米以上是恒定的，得到原始地幔的δ 7 Li = 3.8 ± 1.4‰（2SD，n= 9），δ 25 Mg = −0.12 ± 0.02‰和δ 26 Mg = −0.23 ± 0.04‰（2SD，n= 17），与未扰动地幔捕虏体的值保持一致。然而，也有一些异常低的δ 7 Li值（−0.2‰至1.6‰），这与不相容元素富集的位置相吻合，表明之前有小程度的熔体通过。我们认为，Li从高[Li]熔液中扩散到低[Li]矿物中，并因此动力学分馏7 Li/6Li。熔体流动停止后的继续扩散将导致这种同位素轻签名在不到15 Ma的消失。为了保持这一特征，熔体渗透必须是一个晚期事件，并且很可能在侵位期间，Li-Li-F必须随后从峰值温度（950 °C，假设熔体是含水的）冷却到Li闭合温度（700 °C），最大冷却时间超过10.3 Ma。霍罗曼橄榄岩稳定的δ 26 Mg值表明熔体渗透引起的化学势梯度不足以驱动相关的δ 26 Mg分馏大于我们的外部精度0.03‰。
We have analysed the Li and Mg isotope ratios of a suite of samples from the Horoman Peridotite Massif. Our results show that most Li and all Mg isotopic compositions of the Horoman peridotites are constant over 100 metres of continuous outcrop, yielding values for pristine mantle of δ7Li = 3.8 ± 1.4‰ (2SD,n= 9), δ25Mg = −0.12 ± 0.02‰ and δ26Mg = −0.23 ± 0.04‰ (2SD,n= 17), in keeping with values for undisturbed mantle xenoliths. However, there are also some anomalously low δ7Li values (−0.2‰ to 1.6‰), which coincide with locations that show enrichment of incompatible elements, indicative of the prior passage of small degree melts. We suggest Li diffused from infiltrating melts with high [Li] into the low [Li] minerals and kinetically fractionated7Li/6Li as a result. Continued diffusion after the melt flow had ceased would have resulted in the disappearance of this isotopically light signature in less than 15 Ma. In order to preserve this feature, the melt infiltration must have been a late stage event and the massif must have subsequently cooled over a maximum of ∼0.3 Ma from peak temperature (950 °C, assuming the melts were hydrous) to Li closure temperature (700 °C), likely during emplacement. The constant δ26Mg values of Horoman peridotites suggest that chemical potential gradients caused by melt infiltration were insufficient to drive associated δ26Mg fractionation greater than our external precision of 0.03‰.