Neodymium isotope equilibration during crustal metamorphism revealed by in situ microanalysis of REE-rich accessory minerals
Neodymium isotope equilibration during crustal metamorphism revealed by in situ microanalysis of REE-rich accessory minerals
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DOI：
10.1016/j.epsl.2014.02.018
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发表时间：
2014-04
影响因子：
5.3
通讯作者：
J. Hammerli;A. Kemp;C. Spandler
中科院分区：
文献类型：
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作者：
J. Hammerli;A. Kemp;C. Spandler
放射性同位素广泛用于研究地壳演化过程，然而最近关于深熔过程中 Nd 和 Sr 同位素不平衡的说法质疑此类信息的可靠性。我们对不同变质级别的变质沉积岩中的磷灰石、铜铁矿、钛铁矿、磷钇矿和独居石进行了原位Sm-Nd同位素研究，以测试变质过程中的Nd同位素平衡。我们的结果表明，无论岩石内的结构背景如何，磷灰石在同位素均质化之前至少在 500°C 之前都保留了原始的、可能是碎屑的、高度可变的 Nd 同位素特征。一旦达到平衡，磷灰石在整个深熔过程中都会保留其 Nd 同位素特征。相比之下，铜铁矿和钛铁矿在低至 350–400° C 的温度下达到平衡。高品位岩石 (∼ 600° C) 中富含 REE 的副矿物在变质作用时表现出非常相似的初始 ε Nd 值。我们的结论是，在这些变质条件下，地壳熔体和变沉积岩来源之间不太可能出现 Nd 同位素不平衡。独居石晶内Nd同位素分带表明，部分熔融是开放系统，涉及外部熔融物注入混合岩中。这一过程可能在深熔地体中很常见，但在手工标本规模或大块岩石地球化学数据中并不总是明显，可能会产生同位素变化，可能会被误解为熔体与其原岩之间的不平衡。
Radiogenic isotopes are widely used to investigate crustal evolutionary processes, however recent claims of Nd and Sr isotope disequilibrium during anatexis question the reliability of such information. We have conducted an in situ Sm–Nd isotope study of apatite, allanite, titanite, xenotime and monazite in metasedimentary rocks of different metamorphic grade to test Nd isotope equilibrium during metamorphism. Our results show that apatite retains an original, probably detrital, highly variable Nd isotopic signature until at least 500° C before being isotopically homogenised, irrespective of textural context within the rock. Once equilibrated, apatite retains its Nd isotope signature throughout anatexis. In contrast, allanite and titanite are equilibrated at temperatures as low as 350–400° C. REE-rich accessory minerals in high-grade rocks (∼ 600° C) show very similar initial ε Nd values at the time of metamorphism. We conclude that under these metamorphic conditions Nd isotope disequilibrium between crustal melts and metasedimentary sources is unlikely. Intra-grain Nd isotope zoning of monazite indicates that partial melting was open system, involving the injection of externally-derived melt into migmatites. This process, likely to be common in anatectic terranes but not always obvious at hand-specimen scale or from bulk rock geochemical data, can produce isotope variation that could potentially be misinterpreted as disequilibrium between the melt and its protolith.