石榴石中稀土元素的保留性及其对石榴石卢-铪年代学的影响

IF 3.5 2区 地球科学 Q1 GEOLOGY
Matthijs A. Smit, Johannes C. Vrijmoed, Erik E. Scherer, Klaus Mezger, Ellen Kooijman, Melanie Schmitt-Kielman, Lorraine Tual, Carl Guilmette, Lothar Ratschbacher
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引用次数: 0

摘要

在石榴石中掺入稀土元素(REE)可以进行石榴石年代学研究(Sm-Nd、Lu-Hf),并在成因相上形成石榴石稳定特征,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学追踪。为了准确解释 REE 特征和年龄,需要对石榴石中 REE 的吸收和再分布机制以及扩散行为进行约束。为了深入了解 REE 分带的性质和原因,通常会对石榴石 REE 剖面进行测量。然而,由于对扩散松弛程度的限制较少,对这些剖面的解释变得复杂。这一点与 "镧 "尤其相关,根据实验,"镧 "的扩散率相对较高,因此可能会发生再平衡,从而对 "镧-铪 "年代学产生影响。为了对石榴石的 REE 系统学提供新的见解,我们对来自变质岩的石榴石晶粒进行了痕量元素定量绘图,这些变质岩的峰值温度超过 750°C,冷却速率低至 1.5°C Ma-1。所有样品中的石榴石都保留了雷利型或振荡型生长分带,具有与石榴石习性相匹配的明确界面角。REE 成分的再平衡似乎仅限于具有云雾状和斑点状分带的区域,这些区域很可能是通过流体或熔体介导的界面耦合溶解和再沉淀反应而形成的,而不是 REE 体积扩散。通过将观测结果与二维数值建模结果进行比较,并使用最近的扩散实验得出的璐扩散系数,研究了璐在分析晶粒中扩散可能产生的影响。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。REE在石榴石中的保持性证明了岩浆追踪和岩石年代学中REE特征的可靠性,并确立了Lu-Hf年代学作为变质岩(包括经历过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测年方法。
本文章由计算机程序翻译,如有差异,请以英文原文为准。

Retentiveness of rare earth elements in garnet with implications for garnet Lu-Hf chronology

Retentiveness of rare earth elements in garnet with implications for garnet Lu-Hf chronology

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.

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来源期刊
CiteScore
6.60
自引率
11.80%
发文量
57
审稿时长
6-12 weeks
期刊介绍: The journal, which is published nine times a year, encompasses the entire range of metamorphic studies, from the scale of the individual crystal to that of lithospheric plates, including regional studies of metamorphic terranes, modelling of metamorphic processes, microstructural and deformation studies in relation to metamorphism, geochronology and geochemistry in metamorphic systems, the experimental study of metamorphic reactions, properties of metamorphic minerals and rocks and the economic aspects of metamorphic terranes.
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