Assessing changes in elemental mass as a result of chemical weathering of granodiorite in a Mediterranean (hot summer) climate

In ascending order, relatively unweathered granodiorite, similar to 1.97 m of friable granodiorite, and similar to 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 similar to 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 similar to 4%, similar to 9%, similar to 18%, similar to 11%, similar to 17%, similar to 29%, and similar to 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 similar to 12% and 52% respectively. Of the similar to 3.4% loss of original bulk mass, similar to 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 similar to 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 similar to 12%, similar to 12%, similar to 42% similar to 14%, similar to 41%, similar to 80%, similar to 21%, similar to 14%, similar to 10%, and similar to 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 similar to 17% and similar to 22%, respectively. Of the similar to 11.2% loss of original bulk mass, similar to 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.