| The solubility of CO2 in basaltic liquids is dependent on pressure. The CO2 content of
olivine-hosted melt inclusions therefore provides a potential source of information about the
depths of magma storage. Such petrological constraints on magmatic histories provide
context for the interpretation of geophysical volcano monitoring data. However, when
multiple petrological barometers are available for the magmatic suites, pressure
estimates from melt inclusion CO2 contents appear to be systematically low, even
when saturation has occurred. In this contribution I will explore the role of the
post-entrapment pressure-temperature-time (P-T-t) path of the host crystal in controlling
the relationship between observed melt inclusion CO2 contents and entrapment
pressures.
A global compilation of 2878 melt inclusions from mafic volcanics of mid-ocean ridges,
ocean islands and continental rift zones reveals some unexpected features. First, the
distribution of estimated saturation pressures is not strongly dependent on the methods of
correction of measured inclusion compositions for post-entrapment crystallisation and bubble
growth. Second, the different tectono-magmatic settings show similar distributions of
estimated saturation pressures. Third, 80% of the recovered saturation pressures are <100
MPa and much more than half correspond to pressures that are lower than those of the
shallowest reservoir in the system as constrained by geophysical observations. Finally, in all
settings, 5-10% of the inclusions record saturation pressures >200 MPa, with an upper limit
at ∼500 MPa.
A model was developed of the evolution of the pressure and distribution of CO2
inside inclusions as their olivine hosts travel through an imposed P-T-t history.
Model results indicate that the dominance of low saturation pressures in the melt
inclusions and the systematic difference between these pressures and the independent
estimates of magma storage depths are likely to be caused by decrepitation: previous
experimental studies have found that the maximum pressure difference that can
be maintained between large fluid inclusions and the external melt is ∼200-300
MPa.
A fraction of the inclusions record higher apparent pressures of saturation. These
bubble-bearing inclusions may have experienced significant post-entrapment cooling and
crystallisation, which acts to reduce pressure in olivine-hosted inclusions. Such
inclusions provide the most useful barometric constraints for magmatic histories. |