Incorporation of rare earth elements (REE) in garnet enables garnet chronology (Sm-Nd, Lu-Hf), and imparts a garnet-stable signature on cogenetic phases, which allows petrochronology and general petrogenetic tracing of garnet stability in minerals and melts. Constraints on the uptake and redistribution mechanisms, as well as on the diffusive behaviour of REE in garnet are required for allowing accurate interpretation of REE signatures and ages. Garnet REE profiles are often measured to gain insight into the nature and cause of REE zoning. Interpretation of such profiles is nevertheless complicated by poor constraints on the extent of diffusive relaxation. This is especially relevant for Lu, which, according to experiments, has a relatively high diffusivity and thus may re-equilibrate with possible consequences for Lu-Hf chronology. To provide new insight into the REE systematics of garnet, we applied quantitative trace-element mapping of garnet grains from metamorphic rocks that record peak temperatures above 750°C and cooling rates as low as 1.5°C Ma⁻¹. Garnet in all samples preserves Rayleigh-type or oscillatory growth zoning with sharply defined interfacial angles that match the garnet habit. Re-equilibration of REE compositions appears restricted to domains with nebulous and patchy zoning, which likely form by interface-coupled dissolution and re-precipitation reactions mediated by fluids or melts, rather than REE volume diffusion. The possible effect of Lu diffusion in the analysed grains was investigated by comparing the observations to the results from 2D numerical modelling using Lu diffusivities from recent diffusion experiments. This test indicates that Lu diffuses significantly slower in natural garnet than experiments predict. The retentiveness of REE in garnet demonstrates the reliability of REE signatures in magmatic tracing and petrochronology and establishes Lu-Hf chronology as a robust means of dating garnet growth and recrystallization in metamorphic rocks, including those that underwent high- or ultrahigh-temperature conditions.

中文翻译：

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石榴石中稀土元素的保留性及其对石榴石 Lu-Hf 年代学的影响

在石榴石中加入稀土元素 (REE) 可以实现石榴石年代学（Sm-Nd、Lu-Hf），并在同生相上赋予石榴石稳定特征，从而可以对矿物和熔体中的石榴石稳定性进行岩石年代学和一般岩石成因追踪。为了准确解释 REE 特征和年龄，需要对吸收和再分配机制以及 REE 在石榴石中的扩散行为进行限制。通常通过测量石榴石稀土元素剖面来深入了解稀土元素分区的性质和原因。然而，由于对扩散弛豫程度的限制不佳，对这种分布的解释变得复杂。这对于 Lu 来说尤其重要，根据实验，Lu 具有相对较高的扩散率，因此可能会重新平衡，从而可能对 Lu-Hf 年代学产生影响。为了提供对石榴石稀土元素系统学的新见解，我们对变质岩中的石榴石颗粒进行了定量微量元素测绘，这些颗粒记录了高于 750°C 的峰值温度和低至 1.5°C Ma ^-1的冷却速率。所有样品中的石榴石都保留了瑞利型或振荡生长区带，具有与石榴石习性相匹配的清晰界面角。稀土元素成分的重新平衡似乎仅限于具有云状和斑片状分区的区域，这些区域可能是由流体或熔体介导的界面耦合溶解和再沉淀反应形成的，而不是稀土元素体积扩散。通过将观察结果与使用最近扩散实验中的 Lu 扩散率进行二维数值建模的结果进行比较，研究了 Lu 扩散在分析的颗粒中可能产生的影响。该测试表明，Lu 在天然石榴石中的扩散速度比实验预测的要慢得多。石榴石中 REE 的保留性证明了 REE 特征在岩浆示踪和岩石年代学中的可靠性，并将 Lu-Hf 年代学确立为测定变质岩（包括那些经历高温或超高温条件的变质岩）中石榴石生长和再结晶的可靠手段。