Mount Pinatubo's Volcanic Lake Geochemistry

A geochemical investigation of Mount Pinatubo's volcanic lake indicates mixing between neutral chloride spring discharge from springs in the crater walls and meteoric waters from precipitation and runoff. I originally hypothesized that the lake water is a mixture of deep hydrothermal acid sulfate fluid representative of magmatic input into the hydrothermal system, neutral chloride fluid representative of the hydrothermal input directly into the lake via crater wall springs, and meteoric water. Lake water was periodically sampled and analyzed for temperature, pH, major element concentration, and isotopic composition. This study utilizes lake samples collected in 1992, 1994, 1999, 2000 and 2001. Solution speciation and mixing models do not support the mixing scenario originally hypothesized. Further investigation of major element concentrations and isotopic composition supports mixing between the neutral chloride spring discharge and meteoric water. A series of Cl vs. major element plots of end member solutions and sampled lake waters delineate a mixing line between the meteoric water and the neutral chloride spring discharge. For conservative elements such as Li, Na, and K, lake samples plot on or near this mixing line. For less conservative elements such as Mg, Ca, Fe, Mn, and SO₄^2-, lake samples plot between the two end members but above or below the mixing line indicating enrichment or depletion in the lake. The significant enrichment of Ca and SO₄^2- in the 1992 lake sample and less-enriched later samples can be explained by initial anhydrite dissolution in the early lake followed by elimination of source anhydrite and dilution from meteoric water in more recent lake samples. A strong correlation between Ca and SO₄^2- supports this hypothesis. Further, the slope of a regression line on a Ca vs. SO₄^2- plot of the lake samples is ~2.8 in good agreement with the expected SO₄^2- to Ca ratio of ~2.5 in anhydrite. The slight deviation can be accounted for via substitution of Mg for Ca. This is supported by a similar enrichment of Mg and a strong, positive correlation between Ca and Mg. Redox conditions in the lake are such that Fe and Mn depletions can likewise be explained through precipitation of secondary Fe and Mn oxides. A Cl vs. δ D plot also delineates a mixing line between meteoric water and neutral chloride spring discharge and, along with δ O-18 vs. δ D data, negates any acid sulfate hydrothermal influence on the lake chemistry. Although this mixing scenario can reasonably explain the major element concentrations and isotopic composition of the lake water, it does not account for periods of extreme acidity and temperature fluctuations. The pH fluctuated from an initial 6.0 in October 1991 to its most acidic, 1.9, in December 1992 before neutralizing to 6.2 by January 2001. There is evidence of a gas phase bubbling through Pinatubo's volcanic lake. I observed surface bubbling, an area of upwelling, and an extreme inverted temperature profile over the submerged dome in August 2000. Modeling with previously published CO₂, SO₂, H₂O, and Cl gas ratio and flux data will illuminate the feasibility of explaining pH and temperature fluctuation via volcanic gas flux through the lake.