SEGUAM ISLAND, ALASKA: OBSERVATIONS BV NOMARSKI CONTRAST INTERFERENCE

• Main Authors: Bradley S. Singer, Thomas H. Pearce
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Keyvords; plagioclase; interferometry; Nomarski method; Aleutian island arc; Seguam Island; Alaska- SoMtr{arne; Aleutian Island; Alaska
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.502.8244; http://rruff.info/doclib/cm/vol31/CM31_459.pdf

Plagioclase zoning in lavas from the Seguam volcanic center, Aleutian arc, has been studied using the Nomanki Differential Interference Contrast (NDIC) imaging technique on etched polished sections. Plagioclase in basalts records a complex history including periods of oscillatory euhedral growth intemrpted repeatedly by patchy cellular growth, and less commonly, by dissolution evenn, whereas andesites, dacites, and rhyodacites crystallized oscillatory euhedral zoned plagioclase with only rare evidence of subtle dissolution events. The complexity of plagioclase in basalts and the simplicity of plagioclase in the evolved lavas at Seguam contrast with NDIC observations from previously studied orogenic lavas. These first-order observations uggest that the intensive parameten T, P, and X of equilibrium crystallization for the basaltrc magmas may have been disturbed by thermal effects of magma mixing or convection, or by decompression during magma ascent. Andesitic to rhyodacitic magmas apparently were less strongly affected by thermal, baric, or compositional shifts. The contrasts in plagioclase zoning between Seguam and other orogenic volcanic centers reflect the difference between systems dominated by closed-system fractionation (Seguam) and more tlpical open systems where magma rnixing may be common.