Glass inclusions and melt volatile contents at Parícutin Volcano, Mexico

Abstract.

Olivine-hosted glass inclusions were investigated from tephra samples erupted at Parícutin volcano on four different dates: May 26 and August 1, 1943; January 23, 1945; and March 31, 1948. These dates span the first two thirds of the 9 year eruption, during which time the tephra/lava mass-eruption rate fell dramatically. They also span the strong whole-rock compositional shift of 1947, attributed to the increased importance of crustal contamination. Nine of the 26 analyzed glass inclusions have lower SiO₂ contents than any previously analyzed Parícutin lava sample, ranging to below 53 wt%. These silica-poor glasses are found in olivines erupted in 1943 and 1945, and provide evidence for melts that are parental to the main Parícutin lava suite. Total water contents in the glass inclusions measured by Fourier transform infrared (FTIR) spectroscopy vary considerably in all individual samples, with a total range of 1.8–4.0 wt%. Total water contents are not correlated with SiO₂ of the glass, Mg# of the adjacent host olivine, or eruption date. Only two glass inclusions have carbonate contents (248 and 296 ppm CO₂) above the FTIR detection limit of ~50 ppm CO₂; importantly, these inclusions also have the highest total water contents and among the highest SO₃ ^t values. These two inclusions were trapped at minimum depths of 9.0–9.6 km beneath the volcano. Thus, early degassing likely stripped most carbon from Parícutin melts at mid-crustal levels. Other glass inclusions yield minimum entrapment depths of 1.3–5.1 km based on water solubility limits. Total sulfur (0.30 to 0.01 wt% SO₃) declines as SiO₂ contents increase from 52.7 to 60.5 wt%. This trend and the wide range of glass inclusion total water contents are interpreted to reflect degassing accompanied by fractional crystallization and assimilation at upper crustal levels.