Date of Award

Fall 12-1-2001

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geography, Geology, and Anthropology

Abstract

The 1600 eruption of Vol can Huaynaputina in Southern Peru was one of the largest historic eruptions (VEl 6). The eruption proceeded in three stages containing the same mineral assemblage of plagioclase>homblende>biotite> Fe-Ti oxides±apatite. Differences in crystal content, vesicularity, volatile content, temperature estimates, and the presence of disequilibrium textures have brought into question the petrogenesis of the 1600 eruption products. This work strives to quantify the pre- and syn-eruption magmatic conditions in order to ascertain the petrogenesis of the eruption products. Stage I was less evolved(~ 73% Si02), volatile rich (:S 5.3% H20), crystal poor (17- 26 wt. %crystals), hotter ( ~ 839 °C) high-K dacite. Stages II and III were slightly more evolved(~ 76% Si02) and crystal rich. Stage II shows bimodal crystal content (31 wt.% crystals up to 56 wt.% crystals), pre-eruption temperatures (773 °C to 847 °C), and biotite Mg numbers (Mg60_66). Stage III has a crystal content of~ 52 wt. % and a preeruption temperature of~ 777 °C. Density and viscosity estimates indicate that Stage I (2.48 g/cm3 ; 4.85 x 107 Pas) was both less dense and viscous than Stage III (2.49 g/cm3 ; 3.22 x 1010 Pas). Four populations of plagioclase were identified on the basis of optical and chemical zonation. Their character and distribution, combined with the other petrologic and chemical data reveal that Stage III magma was a long-lived remnant of earlier IV activity that was intruded by the less evolved Stage I. Temperature differences drove dissolution and volatile exsolution, triggering the 1600 eruption. Stage II, due to mixing constraints imposed by viscosity differences, was a zone of magma mingling between Stage I and Stage III. Stage I magma was removed preferentially due to its greater buoyancy and lower viscosity. Bulk chemistry reveals that Huaynaputina is distinct from CVZ volcanics by producing suites ofhigh-K dacite with distinct Na20/K20 ratios, indicating an independent differentiation path.

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