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Assessing Changes in Elemental Mass as a Result of Chemical Weathering of Granodiorite in a Mediterranean (Hot Summer) Climate

¹ Department of Geological Sciences, San Diego State University, San Diego, California 92182, U.S.A.
² Department of Geological Sciences, San Diego State University, San Diego, California 92182, U.S.A.
³ Department of Geological Sciences, San Diego State University, San Diego, California 92182, U.S.A.
⁴ Department of Geological Sciences, San Diego State University, San Diego, California 92182, U.S.A.
⁵ Department of Geological Sciences, San Diego State University, San Diego, California 92182, U.S.A.
⁶ Department of Geological Sciences, San Diego State University, San Diego, California 92182, U.S.A.
⁷ Department of Geological Sciences, San Diego State University, San Diego, California 92182, U.S.A.
⁸ Department of Geological Sciences, San Diego State University, San Diego, California 92182, U.S.A.
⁹ GeoAnalytical Laboratory, Washington State University, Pullman, Washington 99164-2812, U.S.A.
¹⁰ Department of Geological Sciences, San Diego State University, San Diego, California 92182, U.S.A.
¹¹ Department of Geological Sciences, San Diego State University, San Diego, California 92182, U.S.A.

In ascending order, relatively unweathered granodiorite, 1.97 m of friable granodiorite, and 0.91 m of very coarse loamy sand make up a well developed weathering profile that developed on a Cretaceous granodiorite lying within a Mediterranean (hot summer) climatic belt in the Peninsular Ranges of southern California, U.S.A. Textural, modal, and chemical data indicate that in the friable granodiorite and in the very coarse loamy sand, plagioclase was leached at a rate faster than was K-feldspar while quartz remained little changed. In addition, molecular Ca and Na were probably leached at a faster rate from plagioclase and titanite than K was leached from K-feldspar and biotite.

Mass-balance calculations show that 3.4% (± 2.0%) of the original bulk mass of the friable granodiorite was removed during its development. This loss of bulk mass was accommodated through the removal of 4%, 9%, 18%, 11%, 17%, 29%, and 14% of the masses of Si, Ca, K, Na, P, Ba, and Sr respectively. In addition, the masses of Fe and Y were increased by 12% and 52% respectively. Of the 3.4% loss of original bulk mass, 99% is due to the loss of Si, Ca, K, and Na mass.

The severity of chemical alteration is greatest in samples analyzed from very coarse loamy sand. This conclusion is supported not only by textural relationships, but also by mass balance relationships that show that 11.2% (± 2.1%) of the original bulk mass of the very coarse loamy sand was removed during its development. This loss in mass was accommodated through the removal of 12%, 12%, 42%, 14%, 41%, 80%, 21%, 14%, 10%, and 40% of the masses of Si, Mn, Ca, K, Na, P, Ba, Zn, Rb, and Sr, respectively. In contrast, during development of the very coarse loamy sand the masses of Ti and Fe were increased by 17% and 22%, respectively. Of the 11.2% loss of original bulk mass, 99% is attributable to the removal of Si, Ca, K, and Na mass.

Data summarized above, and in work by others, imply that plutoniclastic sediment derived from sources lying within a wide range of climatic conditions, do not reflect directly the bedrock from which they were derived, but instead will mirror the compositions of the weathering profile.