Melt inclusion vapour bubbles: the hidden reservoir for major and volatile elements

Swetha Venugopal,Timothy Druitt,Severine Moune,Federica Schiavi,Glyn Williams-Jones,Nathalie Bolfan-Casanova

doi:10.1038/s41598-020-65226-3

Abstract

Olivine-hosted melt inclusions (MIs) provide samples of magmatic liquids and their dissolved volatiles from deep within the plumbing system. Inevitable post-entrapment modifications can lead to significant compositional changes in the glass and/or any contained bubbles. Re-heating is a common technique to reverse MI crystallisation; however, its effect on volatile contents has been assumed to be minor. We test this assumption using crystallised and glassy basaltic MIs, combined with Raman spectroscopy and 3D imaging, to investigate the changes in fluid and solid phases in the bubbles before and after re-heating. Before re-heating, the bubble contains CO2 gas and anhydrite (CaSO4) crystallites. The rapid diffusion of major and volatile elements from the melt during re-heating creates new phases within the bubble: SO2, gypsum, Fe-sulphides. Vapour bubbles hosted in naturally glassy MIs similarly contain a plethora of solid phases (carbonates, sulphates, and sulphides) that account for up to 84% of the total MI sulphur, 80% of CO2, and 14% of FeO. In both re-heated and naturally glassy MIs, bubbles sequester major and volatile elements that are components of the total magmatic budget and represent a “loss” from the glass. Analyses of the glass alone significantly underestimates the original magma composition and storage parameters.

Highlights

Olivine-hosted melt inclusions (MIs) provide samples of magmatic liquids and their dissolved volatiles from deep within the plumbing system
(2) How does the vapour bubble contribute to the total volatile budget of the MI? This was addressed through Raman analyses of the bubble of seven glassy MIs from Mount Meager to examine the migration of C–O–H–S volatiles from the glass to the bubble during natural cooling
Using olivine-melt equilibria[27], entrapment temperatures range from 1090 to 1150 °C for Mount Cayley and 1120 to 1180 °C for Garibaldi Lake; these values are consistent with re-heating experiments, the temperature when the last daughter crystal dissolved (1000 to 1150 °C for all MIs)

Summary

Introduction

Olivine-hosted melt inclusions (MIs) provide samples of magmatic liquids and their dissolved volatiles from deep within the plumbing system. Many of the post-entrapment modifications that occur within olivine-hosted MIs can be corrected using well-constrained exchange coefficients and recently established methods to quantify the amount of volatiles, CO2, sequestered by the bubble These approaches include the use of trace element proxies, such as CO2/ Nb, to determine the pre-eruptive CO2 content of the undegassed melt[8], reheating the MI to resorb the bubble www.nature.com/scientificreports and retrieve the CO2 content of the MI at the time of entrapment[9,10,11], and estimating the mass of CO2 present in the bubble with Raman spectroscopy and adding it back into the glass[4,7,8,10,12,13]. For both questions, selected bubbles were scanned in 3D using Raman spectroscopy

Objectives

Methods

Findings

Conclusion