Heterogeneous large total CO2 abundance in the shallow magmatic system of Kilauea volcano, Hawaii

摘要

Due to its very low solubility in silicate melts, CO2 concentrations in melt inclusions (Mls) within crystals are commonly orders of magnitude less than the total concentration in the multiphase magma, strongly limiting the possibility to constrain CO2 abundance based on the dissolved quantities. Here we develop a statistical method to process MI data, which allows analytical uncertainties to be taken into account together with the peculiar features of the local saturation surface. The method developed leads to retrieve total H20 and CO, concentrations in magma as well as the gas phase abundance at the time of magma crystallization. Application to a set of 29 high-resolution secondary ion mass spectrometry (SIMS) MI data from a single specimen of the 1842 --1844 eruption of Kilauea, Hawaii, reveals the existence of heterogeneous total CO, abundance, and of at least 2-6 wt % total CO-, in some magma hatches, two orders of magnitude higher than the dissolved amounts and 30- 50 times more abundant than the corresponding total IT20 content. Heterogeneous total volatile concentrations are interpreted as due to a combination of degassing and gas flushinL, in magma subject to convective motion at shallow depth where P < 100 MPa. In such a view, the magma rising to shallow depth in the volcanic system carries initially a total volatile contentwt %, corresponding to the determined low total CO2 population, and consistent with previous global estimates. The high CO, populations correspond to progressive CO, enrichment due to degassing at low P and flushing from a deep CO2-rich gas. A total C:O, content >1 wt % is likely to characterize the >30 km deep magma, not represented in the analyzed inclusions, from which a CO2-rich gas phase exsolves and decouples from the liquid.