Heterogeneously entrapped, vapor-rich melt inclusions record pre-eruptive magmatic volatile contents

Abstract

22Silicate melt inclusions (MI) commonly provide the best record of pre-eruptive H2O and CO2 23 contents of subvolcanic melts, but the concentrations of CO2 and H2O in the melt (glass) phase 24 within MI can be modified by partitioning into a vapor bubble after trapping. Melt inclusions 25 may also enclose vapor bubbles together with the melt (i.e., heterogeneous entrapment), affecting 26 the bulk volatile composition of the MI, and its post-entrapment evolution. Here, we use 27 numerical modeling to examine the systematics of post-entrapment volatile evolution within MI 28 containing various proportions of trapped vapor from zero to 95 volume percent. Modeling 29indicates that inclusions that trap only a vapor-saturated melt exhibit significant decrease in CO2 30 and moderate increase in H2O concentrations in the melt upon nucleation and growth of a vapor 31 bubble. In contrast, inclusions that trap melt plus vapor exhibit subdued CO2 depletion at 32 equivalent conditions. In the extreme case of inclusions that trap mostly the vapor phase (i.e., 33 CO2-H2O fluid inclusions containing trapped melt), degassing of CO2 from the melt is negligible. 34In the latter scenario, the large fraction of vapor enclosed in the MI during trapping essentially 35 serves as a buffer, preventing post-entrapment modification of volatile concentrations in the 36 melt. Hence, the glass phase within such heterogeneously entrapped, vapor-rich MI records the 37 volatile concentrations of the melt at the time of trapping. These numerical modeling results 38 suggest that heterogeneously entrapped MI containing large vapor bubbles represent amenable 39 samples for constraining pre-eruptive volatile concentrations of subvolcanic melts. 40

Deep CO2 in the end-Triassic Central Atlantic Magmatic Province

“…The elemental carbon, which is likely present as a thin film coating the inner spherical surface of the bubbles, replaces CO 2 in some samples, probably due to a change in the oxidation state within MIs, for instance related to a diffusive loss of oxygen from the bubbles to the melt, when the latter crystallized oxides during cooling. The large variability in volume and number of bubbles observed in coexisting MIs (ranging from 1 to 25 bubbles per MI, approximately occupying from <0.1 to >0.5 of the MI volume, as optically estimated in thin and thick sections) reveals heterogeneous entrapment of MIs 27,36 . Therefore, the bubbles within MIs are interpreted as gas exsolution bubbles, formed during exsolution of a CO 2 -rich fluid phase likely from the silicate melt prior to, or during, their entrapment.…”

Highly explosive basaltic eruptions driven by CO2 exsolution

“…One additional factor that can affect the size of MI bubbles is H + diffusion out of the MI during pre-eruptive (subsurface) cooling. This process results in a lower partial molar volume of the MI, which can lead to contraction of the MI and formation of a bubble 31 . This process cannot be solely responsible for the differences in bubble sizes between Group I and Group II MIs given the similar H 2 O contents of all MIs (Fig.…”

“…We can estimate the original dissolved CO 2 content at entrapment for MIs with a co-entrapped exsolved phase by approximating the proportion of dissolved MI CO 2 that sequesters into Group II bubbles during post-entrapment processes. According to models 31 , co-entrapped bubbles suppress exsolution of CO 2 from the melt because they counter some of the pressure loss due to shrinkage of the MI. The degree of suppression of CO 2 exsolution not only depends very strongly on the initial pressure but also on the initial bubble volume fraction and magma composition 31 .…”

Hot, volatile‐poor, and oxidized magmatism above the stagnant Pacific plate in Eastern China in the Cenozoic

“…Melt inclusions with vapor bubbles that make up large proportions of the inclusion space (i.e., vapor bubbles that make up >~8–10 vol%), as some in the phenocrysts of our samples (Figures c and b and Table S3), may record the entrapment of two‐phase liquids (silicate melt and an exsolved vapor phase), but they may also record decrepitation or leakage (cf. Bodnar & Student, ; Moore et al, ; Steele‐MacInnis et al, ). The presence of high‐density CO 2 (~0.39–0.75 g/cm 3 ) in some of the high‐volume vapor bubbles (Figure b and Table S3) suggests that a two‐phase liquid was indeed locally trapped (cf.…”

“…The presence of high-density CO 2 (~0.39-0.75 g/cm 3 ) in some of the high-volume vapor bubbles (Figure 6b and Table S3) suggests that a two-phase liquid was indeed locally trapped (cf. Steele-MacInnis et al, 2017), and thus that the Menluhe, Xunke, and Wuchagou systems were volatile saturated during phenocryst crystallization. Bubbles in melt inclusions that occupy only~1-6 vol% and on average~3 vol% of the melt inclusion space, in contrast, are interpreted to have formed by postentrapment cooling (cf.…”