HR: 0800h
AN: V11B-0590    [Abstracts]
TI: Incipient Dissolution Features of Corestone Pyroxenes in Guatemalan Basalts
AU: * Donatelle, A R
EM: donatel4@msu.edu
AF: Michigan State University, Department of Geological Sciences, 206 Natural Science Building, East Lansing, MI 48824, United States
AU: Velbel, M A
EM: velbel@msu.edu
AF: Michigan State University, Department of Geological Sciences, 206 Natural Science Building, East Lansing, MI 48824, United States
AU: Patino, L C
EM: patinol@msu.edu
AF: Michigan State University, Department of Geological Sciences, 206 Natural Science Building, East Lansing, MI 48824, United States
AB: The earliest stages in the chemical weathering of volcanic rocks are characterized by the redistribution of rare- earth elements (REE). Corestones are ideal for studying the onset of weathering both morphologically and chemically. The fresh core surrounded by adjacent spheroidally weathered rind provides a natural setting in which to study incipient pyroxene alteration. Thin sections of spheroidally weathered basaltic-andesite core-rind pairs from the Tecuamburro volcano in Guatemala were studied using a scanning electron microscope with and energy dispersive spectrometer, microprobe, and a laser ablation inductively coupled mass spectrometer. Backscattered images show the presence of sub-micron to micron en echelon etch pits forming within augite- diopside and enstatite. The etch pits appear to be associated with nearby fractures within the pyroxenes. Sub- micron lamellae, identified in backscattered mode on the scanning electron microscope, are oriented in rows parallel to, and contiguous with, the strings of en echelon etch pits. This suggests that the lamellae are preferentially susceptible to weathering. Electron microprobe data of the augite show a slight increase of Ti in the lamellae, with little difference in major and minor elements when compared to the host. Trace element data for La, Ce, Sm, and Nd show REE enrichment relative to unweathered augites, with several showing a negative Ce anomaly. The greatest degree of enrichment of REE appears to be associated with the most extensive etching of the lamellae. Fine-scale petrologic features of the primary minerals influence weathering textures of volcanic pyroxenes, and influence the contributions of those primary minerals to REE mobilization during incipient weathering.
DE: 1000 GEOCHEMISTRY
DE: 3625 Petrography, microstructures, and textures
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting