Bubble formation and decrepitation control theCO₂content of olivine‐hosted melt inclusions

Abstract

The CO 2 contents of olivine-hosted melt inclusions have previously been used to constrain the depth of magma chambers in basaltic systems. However, the vast majority of inclusions have CO 2 contents which imply entrapment pressures that are significantly lower than those obtained from independent petrological barometers. Furthermore, a global database of melt inclusion compositions from low H 2 O settings, indicates that the distribution of saturation pressures varies surprisingly little between mid-ocean ridges, ocean islands, and continental rift zones. 95% of the inclusions in the database have saturation pressures of 200 MPa or less, indicating that melt inclusion CO 2 does not generally provide an accurate estimate of magma chamber depths. A model of the P-V-T-X evolution of olivine-hosted melt inclusions was developed so that the properties of the inclusion system could be tracked as the hosts follow a model P-T path. The models indicate that the principal control on the saturation of CO 2 in the inclusion and the formation of vapor bubbles is the effect of postentrapment crystallization on the major element composition of the inclusions and how this translates into variation in CO 2 solubility. The pressure difference between external melt and the inclusion is likely to be sufficiently high to cause decrepitation of inclusions in most settings. Decrepitation can account for the apparent mismatch between CO 2 -based barometry and other petrological barometers, and can also account for the observed global distribution of saturation pressures. Only when substantial postentrapment crystallization occurs can reconstructed inclusion compositions provide an accurate estimate of magma chamber depth.

Spreading rate‐dependent variations in crystallization along the global mid‐ocean ridge system

“…At MORs, the magmatic plumbing system is shallower than at ocean islands such as Iceland, and the majority of the melt inclusions are entrapped at pressures <2.5 kbar with a large fraction corresponding to the shallow melt lens (Figures e–h). Moreover, prolonged storage and cooling in the warm oceanic crust and melt lens—as suggested by petrologic studies [e.g., Sinton and Detrick , ; Rubin and Sinton , ] and the wide range of Mg#s in the host‐olivine (see below)—decrease the probability of decrepitation [ Maclennan , ]. Thus, we argue it is unlikely that a majority of melt inclusions from MORs undergo significant decrepitation, and therefore that the measured CO 2 contents preserve the true pressure of melt entrapment.…”

“…The overall range of VSPs in melt inclusions is remarkably similar across all spreading rates, with maximum pressures up to ∼3 kbars bsf (Figure c). We note that over 97% of our melt inclusions have entrapment pressures below the minimum threshold of decrepitation (2.5 kbar; supporting information Figure S3) defined by Maclennan []. Minimum melt inclusion VSPs vary slightly between ridges, but are generally greater than or equal to local seafloor depths.…”

“…While melt inclusions studies from ocean island basalts have difficulty reconciling VSPs with geophysical constraints of crustal structure [e.g., Maclennan , ], results of our VSPs and seismic studies at MORs are remarkably consistent. We find that when present, the peaks in melt inclusion VSPs coincide with the depth of the seismically imaged melt lens (Figures e–h).…”

“…Recent studies from Iceland indicate that olivine-hosted melt inclusions can undergo significant decrepitation (or rupturing), where the CO 2 content of the inclusion is reset to be in equilibrium with the pressure at which decrepitation occurs [Maclennan, 2017]. Typically, decrepitation requires inclusions to undergo a relatively large pressure change (2.5 kbar), and it is less likely to occur in melt inclusions formed at shallow pressures or in inclusions that undergo prolonged cooling histories.…”

Reequilibration Processes Occurring in Plagioclase‐Hosted Melt Inclusions From Plagioclase Ultraphyric Basalts

“…However, the composition of MI is often modified by post entrapment crystallization (PEC), element diffusion at the host/MI interface, and the formation of vapor bubbles ( Figure 1). These processes can occur during magma transport and storage, and after eruption (Danyushevsky et al, 2000(Danyushevsky et al, , 2002Lowenstern, 1995;Maclennan, 2017;Roedder, 1979;Schiavi et al, 2016;Steele-MacInnis et al, 2011;Toplis, 2005).…”