Discovery and implications of the chaotic attractor from zoned pyroxene in Qixiangzhan comenditic lava, Tianchi volcano, NE China

Abstract

Mineral compositional analysis is a powerful tool for identifying both volcanic and petrogenetic processes. This property is observed because spatial variations in the composition of magmatic minerals record chemical and physical changes in the magma from which they crystallized. Thus, these variations can also be used to decipher the history of a magmatic system and its triggering mechanism prior to eruption. The Tianchi volcano, located in the Changbaishan volcanic region, is one of the most active and dangerous volcanoes in East Asia; however, the eruptional mechanism of most recent eruptions are not well-resolved, especially the comenditic lava flow in the Qixiangzhan stage, Tianchi volcano. We focused on the zoned patterns in pyroxene phenocrysts of the comenditic lava to decipher the magma process and triggering mechanism of this silicic lava eruption. We analyzed major and minor element concentrations by point measurement and line-scan across a zoned pyroxene grain using the electron probe micro-analyzer (EPMA) equipped with the energy-dispersive (ED) spectrum. We also extracted a compositional time series by considering the variation of zone thickness across oscillatory zoning and reconstructed the phase space of the compositional time series using the optimal time delay method. First, we calculated the optimal time delay of phase space reconstruction obtained from the first minimum method of mutual information, and we later determined the embedded dimension. Using two parameters (i.e., time delay and embedded dimension), we subsequently reconstructed the phase space. We also present the Poincare section of the reconstructed system. The results of EPMA point measurement (Table 1 and Figure 4) showed that the zoned pyroxenes have higher MgO (2 wt%–2.12 wt%) and lower Na2O (1.26 wt%–1.39 wt%) in the core compared with the rim (MgO, 1.22 wt%–1.54 wt%; Na2O, 1.71 wt%–1.92 wt%). The results of the line-scan of EPMA-ED showed that the normalized concentration of each minor element (MgO, Na2O, MnO) exhibits an intense oscillatory variation (Figure 4). Meanwhile, in the reconstructed phase space (Figure 5(c)) and the Poincare section (Figure 5(d)), the compositional time series of Mg exhibits a chaotic attractor. This attractor is different from the periodic oscillation attractor and the completely random oscillatory dynamic system (Figure 3). These features indicated that the pyroxene phenocrysts grow in a system with a high degree of freedom and complexity and the magma composition around pyroxene changed notably and quickly during crystallization. These features lead us to believe that the oscillatory zoning observed in the Qixiangzhan comenditic lava is not the result of a simple fractional crystallization process, which generally occurred in the stable magma chamber. Although such a process may lead to compositional zoning in the pyroxenes, it cannot form oscillatory zoning. A simple, closed-system fractional crystallization process will produce a periodic oscillation attractor instead of a strange attractor in their reconstructed phase space. A feasible process that may account for the oscillatory zoning features in the Qixiangzhan comenditic lava is an interaction between two (or more) compositionally distinct magmas (magma mixing). This process can create strong heterogeneities within magma bodies in a short time and on a very short length scale. More importantly, this process can produce a strange attractor in the reconstructed phase space of a zoning mineral. These features are all consistent with what we have obtained from the zoned pyroxene in the Qixiangzhan comenditic lava. Thus, we believe that the Qixiangzhan comenditic lava results from magma mixing or the magma recharge process. The mixing and convection processes of infilled primary magma and evolved magma can accelerate the vesiculation of the magma chamber, thereby causing an eruption. Combined with the frequent occurrence of magma recharge or magma mixing evidence in the latest eruptions of the Tianchi volcano, we believe that the infilling of primitive magmas plays a key role in the triggering mechanism of the Tianchi volcano.