ISOTOPIC STUDIES OF SULFUR-BEARING MINERALS IN THE CLASTS AND MATRIX OF HOLOCENE DEBRIS FLOWS FROM MOUNT RAINIER, WASHINGTON

The isotopic composition of sulfur-bearing minerals in the clasts and matrix of major Holocene lahars (Osceola, Electron, and Paradise) on Mount Rainier was used to estimate conditions of hydrothermal alteration prior to edifice collapse.  Mineral assemblages in the clasts were organized into a sequence of increasingly altered rock: smectite + pyrite ® alunite + kaolinite ± pyrite ® jarosite ® gypsum/anhydrite + sulfur.  The relative abundance of clasts with each mineral assemblage decreased as the level of alteration increased.  Water-soluble Fe and Al sulfates occur in the matrix (< 2 mm fraction) along with pyrite, jarosite, and minor alunite.  Elemental sulfur, water-soluble (w.s.) sulfates (gypsum, Fe and Al sulfates), acid-soluble (a.s.) sulfate (jarosite and alunite), and pyrite were extracted sequentially for isotopic analysis using a chemical speciation scheme.

The isotope composition of each sulfur form is consistent among the lahars.  d³⁴S_sulfur (-2 to 10‰) increases with decreasing amounts of elemental sulfur.  The d³⁴S_w.s. (0 to 12‰) covers the range of all sulfur forms.  d³⁴S_a.s. (3.5 to 10 ‰) is generally similar to d³⁴S_sulfur.  The maximum difference of 7 ‰ between d³⁴S_a.s. and  d³⁴S_sulfur is in contrast to many volcanic settings where d³⁴S_sulfur << d³⁴S_sulfate.  d³⁴S_pyrite (–2 to 3.2 ‰) does not vary systematically whether the associated minerals include smectite, alunite+kaolinite or gypsum+sulfur.  d³⁴S_Ssulfur of the parent hydrothermal fluids generally increased through the sequence of alteration minerals.  d³⁴S and d¹⁸O data support a hydrothermal origin for pyrite, sulfur, anhydrite, alunite and most jarosite. Weathering of pyrite in the debris flows formed some water-soluble sulfates and some jarosite.  The positive correlation of d¹⁸O_w.s. (-7 to 15 ‰) with d³⁴S_w.s. is attributed to mixing of hydrothermal sulfate with sulfate from sulfide weathering.  The results can not be explained by a simple model of alteration.  Observations and isotope data are compatible with depletion of sulfur in underlying crystallizing magma, increasingly oxidized hydrothermal fluids, multiple episodes of alteration, and disequilibria among sulfur and mineral species.