The SHRIMP ion microprobe has been used to make in situ analyses of the sulfur isotopic composition of individual anhydrite and sulfide crystals in the June 1991 eruption products of Mount Pinatubo. In air­fall pumice from the June 12 eruption, anhydrite crystals exhibit a broad, bimodal distribution of d³⁴S values (46 analyses on 23 crystals; range 3 to 16 per mil; modes of 6.5 per mil and 10.5 per mil). Chalcopyrite crystals have d³⁴S values with a mode of ­1 per mil (5 analyses of 5 crystals, range ­2 to 0 per mil). In crystal­rich ash­flow pumice from the main June 15 eruption, anhydrite crystals exhibit a narrow, unimodal distribution of d³⁴S values (44 analyses on 22 crystals, range 5 to 11 per mil, mode and average of 7 per mil). A single analysis of a chalcopyrite crystal yielded d³⁴S = 0 per mil.

Under the preeruptive temperature and oxygen fugacity conditions of both stages of the eruption, the chalcopyrites are in isotopic equilibrium with the 7 per mil mode of anhydrites, and both phases are likely primary. The isotopically heavier anhydrites in the June 12 pumice may be xenocrysts acquired at a shallow level during this early vent­clearing eruption; the isotopically heaviest anhydrites have d³⁴S values similar to those of hydrothermal vein anhydrites in a drillcore sample recovered from the geothermal system on the flank of Mount Pinatubo (9 analyses of 5 crystals, range 17­22 per mil, average 19 per mil).

The d³⁴S value of primary anhydrite places constraints on the sulfur isotopic composition of SO₂ gas resulting from the eruptions. If magma prior to the eruptions coexisted with an exsolved vapor phase containing SO₂, then isotopic systematics require that the SO₂ had a d³⁴S value of 3.5 per mil. If most of this SO₂ was erupted and quantitatively oxidized to H₂SO₄ in the stratosphere, then Mount Pinatubo aerosols could have a similar sulfur isotopic composition. Given estimates that a significant amount of the total sulfur in the eruption was present as a gas phase prior to eruption, the bulk sulfur isotopic composition of the eruption (crystals + melt + gas) would have been 5.0 to 6.5 per mil.

If, instead, most of the SO₂ was generated by rapid irreversible breakdown of anhydrite during ascent decompression and eruption, then the sulfur isotopic composition of the total eruption and the erupted SO₂ would have been similar to that of the primary anhydrite, 7 per mil. Quantitative conversion of this SO₂ to H₂SO₄ in the stratosphere could have yielded sulfate aerosols with a similar isotopic composition.

Two analyses of a primary chalcopyrite grain in a quenched basalt inclusion from the June 7­14 hybrid andesite dome yielded d³⁴S values of 1 per mil and likely reflect the sulfur isotopic composition of the mafic magma underplating the dacite. Long­term degassing of basalt magma may have been an important ultimate source of reduced sulfur to the dacite. However, Rayleigh effects upon degassing of a mixed SO₂/H₂S vapor from basalt magma would partition bulk sulfur into the dacite with a total d³⁴S value no greater than 3 per mil, significantly less than the 5 to 7 per mil bulk sulfur isotopic composition of the eruption. The further ³⁴S enrichment of the dacite is best explained by passive (noneruptive) steady­state degassing of H₂S and SO₂ from the dacite magma, in dynamic isotopic equilibrium with primary anhydrite, over long time periods.