Heat-induced alteration of glauconitic minerals in the Middle Stone Age levels of Blombos Cave, South Africa: Implications for evaluating site structure and burning events
Heat-induced alteration of glauconitic minerals in the Middle Stone Age levels of Blombos Cave, South Africa: Implications for evaluating site structure and burning events
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DOI：
10.1016/j.jas.2017.06.008
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发表时间：
2017-10-01
影响因子：
2.8
通讯作者：
Henshilwood, Christopher S.
中科院分区：
文献类型：
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作者：
Haaland, Magnus M.;Friesem, David E.;Henshilwood, Christopher S.
在本文中，我们进行地球化学和色度测量的微形态薄片从中石器时代的网站，南非布隆伯斯洞穴，钙钛矿颗粒，调查的形成，内部结构和改造的热暴露的洞穴沉积物，与史前燃烧事件。控制加热实验首先进行了富镁方解石松散沉积物和块体样品，这两者都是从布隆伯斯洞穴基岩收集。然后对对照样品进行傅里叶变换红外光谱（FTIR）、显微傅里叶变换红外光谱（micro-FTIR）和岩相-比色分析。对比实验表明，当温度高于摄氏度时，绿柱石矿物会经历一个渐进的、系统的颜色变化。300-400摄氏度，主要是由于脱水和铁氧化。当温度高于摄氏度时，它们也会发生明显的结构变化。550摄氏度由于脱羟基和矿物转化。通过评估热诱导的光学和分子蚀变的性质和程度，我们展示了薄片中的钙钛矿颗粒如何根据它们所暴露的温度（20-400摄氏度，>400摄氏度，>600摄氏度和>800摄氏度）进行分类。为了评估我们的受控加热实验的考古学相关性，我们将这种钙钛矿分类方案应用于在中石器时代（MSA）燃烧特征的三个微形态薄切片中发现的>200个颗粒。这些颗粒是单独的地理参照布隆伯斯洞穴的地方坐标系内，通过一个薄的部分为基础的地理信息系统绘图程序。随着空间控制的改善，我们能够研究未蚀变和热蚀变的钙钛矿颗粒在其原始沉积环境中的一般分布，以及计算覆盖整个采样段的热分布模型。这种结合地球化学，光学和空间背景的方法提供了更难以捉摸的方面的MSA网站结构和燃烧事件，如热强度，燃烧频率，温度分布，内部炉膛结构和沉积后改造的见解。我们提出的工作流程可以很容易地实施和适应其他考古背景和类似的沉积材料，显示可比的热致蚀变模式。(C)2017年，作者。爱思唯尔有限公司出版
In this paper we conduct geochemical and colourimetric measurements of glauconite grains in micro morphological thin sections from the Middle Stone Age site of Blombos Cave, South Africa, to investigate the formation, internal structure and reworking of heat-exposed cave deposits that are related to prehistoric burning events. Controlled heating experiments were first carried out on glauconite-rich loose sediments and block samples, both of which were collected from the Blombos Cave bedrock. The control samples were then subjected to Fourier transform infrared spectrometry (FTIR), microscopic Fourier transform infrared spectrometry (micro-FTIR) and petrographic-colourimetric analyses. The control experiment shows that glauconitic minerals undergo a gradual and systematic colour change when temperatures reach higher than c. 300-400 degrees C, primarily due to dehydration and iron oxidation. They also undergo clear structural changes when temperatures reach higher than c. 550 degrees C due to dehydroxylation and mineral transformation. By assessing the nature and degree of heat-induced optical and molecular alteration in glauconitic minerals, we demonstrate how glauconite grains in thin sections can be classified by the temperature to which they were exposed (20-400 degrees C, >400 degrees C, >600 degrees C and >800 degrees C). To assess the archaeological relevance of our controlled heating experiment, we applied this glauconite classification scheme to >200 grains found in three micromorphological thin sections of a Middle Stone Age (MSA) combustion feature. These grains were individually geo-referenced within the local coordinate system of Blombos Cave, through a thin-section-based GIS mapping procedure. With improved spatial control, we were able to study both the general distribution of non-altered and heat altered glauconite grains in their original sedimentary context, as well as to calculate heat distribution models that cover the entire sampled section. This combined geo-chemical, optical and spatio-contextual approach provides insights into more elusive aspects of MSA site structure and burning events, such as heat intensity, burning frequency, temperature distribution, internal hearth structure and post-depositional reworking. The workflow we propose may easily be implemented and adapted to other archaeological contexts and to analogous sedimentary materials that show comparable heat-induced alteration patterns. (C) 2017 The Authors. Published by Elsevier Ltd.