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Influence of surface-area estimation on rates of plagioclase weathering determined from naturally weathered 3400 y old Hawaiian basalt

Published online by Cambridge University Press:  05 July 2018

M. A. Velbel  and
A. I. Losiak
M. A. Velbel*
Affiliation:
Department of Geological Sciences, Michigan State University, 206 Natural Science Building, East Lansing, MI 48824-1115, USA
A. I. Losiak
Affiliation:
Department of Geological Sciences, Michigan State University, 206 Natural Science Building, East Lansing, MI 48824-1115, USA
*
*E-mail: losiakan@msu.edu
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Abstract

Dimensions of plagioclase (An65) crystals in a polished thin section of naturally weathered 3400 y old Hawaiian basalt were measured by scanning electron microscopy (in backscattered electron imaging mode). The three-dimensional shape, size, volume and surface area of the plagioclase crystals were then estimated using the method of Morgan and Jerram (2006). The weathering rate of plagioclase (mass loss divided by initial geometric surface area during 3400 y) is –12.116 log10 mol m–2 s–1. This weathering rate represents a time span (3.4 ka) intermediate between laboratory experimental timescales and the ages of regoliths in field studies of natural weathering rates. Weathering rates of the same plagioclase are recalculated using different assumptions concerning the method of estimating surface area, including simple geometric surface area and geometric surface area adjusted for surface roughness. Varying assumptions about surface area result in variations of up to two orders of magnitude in rates estimated from the same basic crystal geometry. The slowest rate estimated here approaches but does not fall within the range of previously determined field rates of plagioclase weathering.

Type
Research Article
Information
Mineralogical Magazine , Volume 72 , Issue 1 , February 2008 , pp. 91 - 94
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2008

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