CO2 Diffusion in Dry and Hydrous Haplobasaltic Melts

玄武岩喷发脱气是地幔气体进入大气的主要途径，也是早期大气形成演化的主要来源。二氧化碳（CO2）是大洋中脊和海洋岛屿玄武岩岩浆中含量第二丰富的气体成分，仅次于水（H2O）。此外，由于CO2在熔体中的溶解度较H2O低，因此在海底玄武岩脱气过程中，CO2是气相中最主要的成分。因此，海底玄武岩熔体中的气泡生长和脱气在很大程度上受CO2控制。直接测量海底玄武岩玻璃中的CO2浓度，往往显示CO2过饱和，这意味着玄武岩脱气不是一个平衡过程，而是由CO2扩散和输运控制的。因此，了解玄武岩熔体中CO2的扩散对于量化CO2气泡生长和脱气以及地幔的挥发收支至关重要。采用扩散偶实验研究CO2在干湿单玄武岩熔体中的扩散。对于每一对的两半，化学成分（包括H2O含量）将是相似的，但一半的二氧化碳浓度将为零，另一半的二氧化碳浓度约为1000ppm。实验过程与我们之前关于水和氩扩散的扩散偶实验类似。实验条件为1300-1700℃，0.5-1.5 GPa， 0-7重量% H2O。实验结束后，将使用显微镜傅里叶变换红外光谱仪测量CO2浓度分布。用理论解拟合剖面，得到扩散系数。新的数据将与以前的数据相结合，以评估二氧化碳扩散率对温度、压力和水含量的依赖。根据从这项拨款获得的扩散数据，玄武岩熔体中的气泡生长将使用最近开发的模型进行建模。此外，多组分气泡生长将被解决。因此，这项工作将为海洋中脊、海洋岛屿和岛弧环境下玄武岩岩浆的CO2扩散、气泡生长、脱气和气体组分的动力学分馏提供基本的认识。

Degassing during basalt eruption is the major pathway for mantle gases to enter the atmosphere and was also the major source for the formation and evolution of the early atmosphere. Carbon dioxide (CO2) is the second most abundant gas component in basaltic magmas at mid-ocean ridges and ocean islands, after water (H2O). Moreover, due to the low solubility of CO2 in melts compared to H2O, CO2 is the most major component in the gas phase during degassing of submarine basalts. Hence, bubble growth and degassing in submarine basaltic melts are largely controlled by CO2. Direct measurement of CO2 concentrations in submarine basaltic glasses often show oversaturation of CO2, meaning that basalt degassing is not an equilibrium process, but also controlled by CO2 diffusion and transport. Therefore, understanding CO2 diffusion in basaltic melt is essential to quantifying CO2 bubble growth and degassing, as well as the volatile budget of the mantle.Diffusion couple experiments will be carried out to investigate CO2 diffusion in dry and wet haplobasaltic melts. For the two halves in each couple, the chemical compositions (including H2O content) will be similar, but CO2 concentration will be zero in one half and about 1000 parts per million in the other half. The experimental procedures will be similar to our previous diffusion couple experiments on H2O and Ar diffusion. The experimental conditions will be 1300-1700°C, 0.5-1.5 GPa, and 0-7 weight% H2O. After the experiments, CO2 concentration profiles will be measured using a microscope Fourier transform infrared spectrometer. The profiles will be fit by the theoretical solution to obtain diffusivity. The new data will be combined with previous data to assess the dependence of CO2 diffusivity on temperature, pressure, and H2O content. From diffusion data obtained from this grant, bubble growth in basaltic melts will be modeled using recently developed models. Furthermore, multicomponent bubble growth will be tackled. Hence, this work will provide a fundamental understanding to CO2 diffusion, bubble growth, degassing, and kinetic fractionation of gas components of basaltic magma at mid-ocean ridge, ocean islands and island arc settings.