To understand the processes that lead to the formation of the oceanic crust, one must know the composition and the depth at which primary melts originate. Towards this end, this dissertation focuses on plagioclase-hosted melt inclusions from plagioclase ultraphyric basalts (PUBs). Plagioclase is usually considered to be the second phase, after olivine, in the crystallization sequence of mid-ocean ridge basalts. However, the relatively low density of plagioclase makes it possible for melts to carry them from great depths. Volatiles were analyzed in plagioclase-hosted melt inclusions from PUBs from the Blanco Transform Fault using a new approach that analyzes the CO2 content trapped in both the glass and vapor bubbles of melt inclusions. This technique revealed that 60 to 90% of the CO2 is trapped within vapor bubbles. More importantly, minimum entrapment pressures calculated from CO2+H2O analyses demonstrated upper mantle origins of the plagioclase crystals with pressures averaging 3.5 to 6.5 kbars.
To obtain reliable analyses, our samples had to be homogenized to dissolve post entrapment crystallization (PEC) back into the melt of melt inclusions. The homogenization temperature used was 1230ºC for the specific sample studied, based on the anorthite content of the plagioclase crystals An83±2. These experiments, performed on a one-atmosphere vertical furnace, revealed a more primitive composition of the melt inclusions compared to typical MORB glasses (Mg# 67.53±2.48 2σ). Time-series heating experiments performed from 30 minutes to 4 days documented systematic changes in the composition of the melt inclusions. This illustrated the importance of re-equilibration processes, as well as the importance in understanding the effects of heating-experiments of melt inclusions. After 4 days, the melt inclusions experienced an average of 15% increase in CO2 in the vapor bubble. With time, the melt inclusion compositions drift toward higher SiO2 and MgO, and lower Al2O3, CaO and Na2O. The increase in MgO in the melt inclusion is paired with diffusion of MgO from the plagioclase crystals to the melt inclusions. These diffusion profiles exist in the longer run time experiments. Crystal/melt partition coefficients for Mg (DMg) calculated in our 30-minute and 4-day runs demonstrated an anti-correlation exists between DMg and plagioclase anorthite content in the 30-min runs, and in the 4-day runs the anti-correlation is annihilated, and the DMg decreased by 25%.
The chemistry drifts and increasing CO2 partitioning can be attributed to re-equilibration processes. Crystal relaxation corresponds to a volume increase of a mineral’s structure by remaining at high temperatures for an extensive amount of time. Crystal relaxation likely induced a pressure drop within the melt inclusions, which explains the increase in CO2 partitioning within vapor bubbles and enhanced diffusive exchange between the plagioclase crystals and their melt inclusions. Performing heating-stage experiments on a Vernadsky disk we observed that plagioclase-hosted melt inclusions behave differently from olivine-hosted melt inclusions. Instead of having the vapor bubbles disappearing past the homogenization temperature, the melt inclusions experienced episodes of bubble nucleation. At each increment of temperature starting about 40ºC above the homogenization temperature, a new episode of bubble nucleation would occur. These episodes are likely the results of a pressure gradient forming between the still pressurized melt inclusions and their relaxed host-plagioclase. Both the melt inclusions and vapor bubbles increased in volume. The melt inclusions volume increase is limited by the low coefficients of expansivity of their host-plagioclase, whereas the original amount of volatile dissolved within the melt inclusions constitutes the vapor bubbles’ expansion threshold.