法国国家科学研究中心L. Tual团队在研究石榴石的生长上取得新进展。2024年12月16日出版的《地质学》杂志发表了这项成果。

据介绍，矿物反应决定了岩石的物理和流变特性，但这些反应是发生在接近还是远离平衡状态，是连续的还是脉冲的，很难解释清楚。这在确定地壳的热力学性质和行为，以及估计岩石在其构造历史中所经历的压力和温度条件方面，带来了很大的不确定性。

研究人员采用元素制图和高精度的Lu-Hf测年方法，来研究石榴石（岩石圈中压力、温度、变形和时间的最重要的记录者之一）是否和在多大程度上跟上了构造过程。该分析是在意大利Schneeberg杂岩的一块富含碳酸盐的云母片岩中，对单个1.2cm大小的石榴石颗粒进行的。研究结果确定了五个成分不同的区，并分别确定了其年代。以微量元素振荡为特征的四个内区年龄相同，加权平均值为98.4±0.1Ma (2σ)，而最外层区年龄为97.8±0.3Ma。

在第一个生长脉冲期间，石榴石的平均径向生长速率至少为6.2cm m.y.^-1。成核始于平衡条件下，并导致高流体产出，进而又促进了石榴石的生长，有时受到岩石元素传输渗透率的限制。这种脉冲式、超快的石榴石生长，通常发生在非常有限的压力-温度窗口内。这个例子提供了对变质岩中矿物反应的不连续性质的罕见一瞥，并突出了石榴石作为这种岩石推向平衡时发生的过程的独特记录者。

附：英文原文

Title: Garnet growth in a geological blink of an eye

Author: L. Tual, M.A. Smit, J.A. Cutts, K.A. Musiyachenko, E. Kooijman, J. Majka, I. Foulds, A. Ielpi

Issue&Volume: 2024-12-16

Abstract: Mineral reactions determine the physical and rheological properties of rocks, but whether these reactions occur close to or far from equilibrium and whether they are continuous or pulsed is challenging to unravel. This introduces significant uncertainty in determining the thermomechanical properties and behavior of the crust and estimating the pressure and temperature conditions that rocks underwent during their tectonic history. Here, we employ elemental mapping and high-precision Lu-Hf chronology to investigate whether and to what extent garnet—one of the most important recorders of pressure, temperature, deformation, and time in the lithosphere—keeps up with tectonic processes. The analysis was done on a single 1.2-cm-sized garnet grain from a carbonate-rich mica schist from the Schneeberg Complex (Italy). Five compositionally distinct zones were identified and dated separately. The four inner zones, characterized by trace-elements oscillations, yielded identical ages with a weighted mean of 98.4 ± 0.1 Ma (2σ), whereas the outermost zone yielded 97.8 ± 0.3 Ma. During the first growth pulse, garnet grew at an average radial growth rate of at least 6.2 cm m.y.^-1. Nucleation initiated out of equilibrium conditions and resulted in high fluid production that, in turn, boosted garnet growth, episodically limited by the rock’s elements transport permeabilities. This pulsed, ultrafast garnet growth must have occurred over a very limited pressure-temperature window. This example provides a rare glimpse into the discontinuous nature of mineral reactions in metamorphic rocks and highlights garnet as a unique recorder of the processes that occur when such rocks push toward equilibrium.

DOI: 10.1130/G52772.1

Source: https://pubs.geoscienceworld.org/gsa/geology/article/doi/10.1130/G52772.1/650745/Garnet-growth-in-a-geological-blink-of-an-eye