Palaeotemperature determinations for the 1.8-ka Taupo ignimbrite, New Zealand, and implications for the emplacement history of a high-velocity pyroclastic flow
Palaeotemperature determinations for the 1.8-ka Taupo ignimbrite, New Zealand, and implications for the emplacement history of a high-velocity pyroclastic flow
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新西兰 1.8-ka 陶波熔结岩的古温度测定及其对高速火山碎屑流侵位历史的影响
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发表时间：
2004
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通讯作者：
Leon Bardot
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作者：
E. Mcclelland;C. Wilson;Leon Bardot
从新西兰1.8 ka Taupo褐煤第1层和第2层的46个地点收集的岩石碎屑的古地磁数据被用来确定该矿床在其位置的古温度和热结构。距离喷口不到30-40公里的站点的平衡温度为150-300℃，而距离更远的站点的平衡温度高达400-500℃。这种变化在第1层和第2层沉积物中都可以看到，第1层的值稍冷一些，其温度的升高发生在距离喷口较远的地方。在离喷口约50公里处的最高温度与浮石的粉红色热氧化着色带相吻合，先前推断浮石反映了较高的温度。我们和其他人从浮石碎屑中收集的其他古地磁数据显示出类似的温度变化，但这里显示的这些温度估计是由于化学残留物造成的，对于准确的温度估计是不可靠的。火成岩近端温度较低，与火山爆发前的陶波湖（Taupo Lake）与源处bb0 ~ 20%的低温岩屑混合作用相一致。中间层和远端层1和2的平衡温度的相似但抵消的增加与两层来自同一流沉积一致。然而，较晚、较热的物质留下的近端沉积物一定是后来被侵蚀了，或者太薄了，以至于我们的采集无法对它们进行采样。平衡温度的径向不对称以及其他物理参数表明，沉积物就位温度主要是在源处确定的，而不是在运输过程中与空气的相互作用。这些资料支持了以往的解释，即集中的基底流在陶波褐煤的侵位和沉积中起主导作用。
Palaeomagnetic data from lithic clasts collected at 46 sites within layers 1 and 2 of the 1.8-ka Taupo ignimbrite, New Zealand, have been used to determine the palaeotemperatures and thermal structure of the deposit on its emplacement. Equilibrium temperatures from sites less than 30–40 km from vent are 150–300 °C, whereas at greater distances site equilibrium temperatures increase up to 400–500 °C. This variation is seen in both layer 1 and 2 deposits, with values for layer 1 being somewhat cooler, and with its increase in temperature occurring at a greater distance from vent. A temperature maximum at ~50 km from vent coincides with a zone of pink thermal-oxidation colouration of pumices previously inferred to reflect higher emplacement temperatures. Additional palaeomagnetic data collected by us and others from pumice clasts show comparable temperature variations, but these temperature estimates are shown here to be due to a chemical remanence and unreliable for accurate temperature estimates. Cooler temperatures in proximal parts of the ignimbrite are consistent with admixture of >20% cold lithic clasts at source and interaction with the pre-eruption Lake Taupo. The similar, but offset, increases in equilibrium temperatures for medial and distal layers 1 and 2 are consistent with both layers being deposited from the same flow. However, any proximal deposits left by the later, hotter material must have been subsequently eroded, or be so thin that our collection failed to sample them. Radial asymmetries in equilibrium temperatures as well as other physical parameters suggest that the deposit emplacement temperature is primarily determined at source, rather than by interaction with air during transport. These data support previous interpretations that a concentrated basal flow played a dominant role in emplacement and deposition of the Taupo ignimbrite.