当前位置: X-MOL 学术Chem. Geol. › 论文详情
Our official English website, www.x-mol.net, welcomes your feedback! (Note: you will need to create a separate account there.)
Pathways for 39Ar loss during step-heating of alkali feldspar megacrysts from the Shap granite (UK): Combined evidence from diffusion experiments and characterisation of heating-induced texture modifications
Chemical Geology ( IF 3.6 ) Pub Date : 2020-08-01 , DOI: 10.1016/j.chemgeo.2020.119677
Daniil V. Popov , Richard A. Spikings , Kalin Kouzmanov

Abstract This study aims to identify the dominant fast pathways for 39Ar loss during step-heating 40Ar/39Ar analysis of alkali feldspar from the Shap granite (UK). In our analysis we combined step-heating 40Ar/39Ar data acquired from variably sized feldspar fragments (0.4 to 2.2 mm in the shortest dimension) and optical and electron microscopy observations of microtextural modifications caused by heating in a muffle furnace in air. Our optical and electron microscopy results suggest that laboratory heating of the Shap feldspar causes it to fracture. The resulting cracks form an interconnected network, which may link with some of previously documented nanoscale defects such as nanotunnels, slots, pull aparts and bubbles. Our step-heating 40Ar/39Ar analyses yielded non-linear Arrhenius trajectories of 39Ar release. The non-linearity is best explained by heating-induced reduction of the effective diffusion length due to fracturing, likely with some contribution from nanotunnels, pull-aparts, slots and bubbles. Incoherent grain boundaries apparently played a minor role in providing fast pathways for 39Ar loss, if any. Importantly, unambiguous evidence for a reduction of the diffusion length during step-heating was only observed in experiments with large fragments (≥1 mm in the shortest dimension). The Arrhenius trajectory obtained from the smaller fragments (≤0.5 mm in the shortest dimension) appears to be consistent with the presence of variably-sized non-interacting intra-grain diffusion domains that existed prior to mineral separation. Heating-induced fracturing must be taken into account when interpreting 40Ar/39Ar dates of alkali feldspar, with particular focus on the following two points. First, this process is clearly inconsistent with the underlying assumptions of commonly utilised multi-diffusion domain (MDD) theory, although it can result in 39Ar release patterns that appear consistent with them. Therefore, it is important to identify on a case by case basis the dominant fast pathways for 39Ar loss from alkali feldspar during step-heating before applying MDD theory. Second, if heating-induced fracturing is ubiquitous in alkali feldspar and affects even gem-quality samples, then presently available diffusion parameters of Ar in alkali feldspar structure may be inaccurate to some degree, since all of them were obtained without considering the presence of cracks.

中文翻译:

来自 Shap 花岗岩(英国)的碱性长石巨晶在逐步加热过程中 39Ar 损失的途径:来自扩散实验的综合证据和加热诱导的织构改变特征

摘要 本研究旨在确定在阶梯加热 40Ar/39Ar 分析来自 Shap 花岗岩(英国)的碱性长石期间 39Ar 损失的主要快速途径。在我们的分析中,我们结合了从不同尺寸的长石碎片(最短尺寸为 0.4 至 2.2 毫米)中获得的步进加热 40Ar/39Ar 数据以及在马弗炉中在空气中加热引起的微观结构变化的光学和电子显微镜观察。我们的光学和电子显微镜结果表明,夏普长石的实验室加热会导致其断裂。由此产生的裂缝形成一个相互连接的网络,这可能与一些先前记录的纳米级缺陷有关,例如纳米隧道、狭缝、拉裂和气泡。我们的步进加热 40Ar/39Ar 分析产生了 39Ar 释放的非线性 Arrhenius 轨迹。非线性最好通过加热引起的有效扩散长度减少来解释,这可能是由于纳米隧道、拉开、槽和气泡的一些贡献。如果有的话,不连贯的晶界在为 39Ar 损失提供快速通道方面显然起次要作用。重要的是,只有在大碎片(最短尺寸≥1 mm)的实验中才能观察到在步进加热过程中扩散长度减少的明确证据。从较小碎片(最短尺寸≤0.5 mm)获得的阿伦尼乌斯轨迹似乎与矿物分离之前存在的大小不一的非相互作用晶粒内扩散域的存在一致。在解释碱长石的 40Ar/39Ar 日期时必须考虑加热致裂,特别注重以下两点。首先,这个过程显然与常用的多扩散域 (MDD) 理论的基本假设不一致,尽管它可能导致 39Ar 释放模式看起来与它们一致。因此,在应用 MDD 理论之前,根据具体情况确定逐步加热期间碱长石中 39Ar 损失的主要快速途径是很重要的。其次,如果碱长石普遍存在加热致裂现象,甚至影响宝石级样品,那么目前可用的碱长石结构中Ar的扩散参数在一定程度上可能不准确,因为所有这些参数都是在没有考虑裂缝存在的情况下获得的. 这一过程显然与常用的多扩散域 (MDD) 理论的基本假设不一致,尽管它可能导致 39Ar 释放模式与它们一致。因此,在应用 MDD 理论之前,根据具体情况确定逐步加热期间碱长石中 39Ar 损失的主要快速途径是很重要的。其次,如果碱长石普遍存在加热致裂现象,甚至影响宝石级样品,那么目前可用的碱长石结构中Ar的扩散参数在一定程度上可能不准确,因为所有这些参数都是在没有考虑裂缝存在的情况下获得的. 这一过程显然与常用的多扩散域 (MDD) 理论的基本假设不一致,尽管它可能导致 39Ar 释放模式与它们一致。因此,在应用 MDD 理论之前,根据具体情况确定逐步加热期间碱长石中 39Ar 损失的主要快速途径是很重要的。其次,如果碱长石普遍存在加热致裂现象,甚至影响宝石级样品,那么目前可用的碱长石结构中Ar的扩散参数在一定程度上可能不准确,因为所有这些参数都是在没有考虑裂缝存在的情况下获得的. 在应用 MDD 理论之前,根据具体情况确定逐步加热期间碱长石中 39Ar 损失的主要快速途径是很重要的。其次,如果碱长石普遍存在加热致裂现象,甚至影响宝石级样品,那么目前可用的碱长石结构中Ar的扩散参数在一定程度上可能不准确,因为所有这些参数都是在没有考虑裂缝存在的情况下获得的. 在应用 MDD 理论之前,根据具体情况确定逐步加热期间碱长石中 39Ar 损失的主要快速途径是很重要的。其次,如果碱长石普遍存在加热致裂现象,甚至影响宝石级样品,那么目前可用的碱长石结构中Ar的扩散参数在一定程度上可能不准确,因为所有这些参数都是在没有考虑裂缝存在的情况下获得的.
更新日期:2020-08-01
down
wechat
bug