Large Ca stable isotopic (δ44/40Ca) variation in a hand-specimen sized spheroidally weathered diabase due to selective weathering of clinopyroxene and plagioclase

The samples with lower δ44/40Ca show higher CIA (Chemical Index of Alteration) values and Al/Ca than the UW diabase while samples having higher δ44/40Ca show lower CIA and Al/Ca than the UW diabase. The weathered sample having the lowest δ44/40Ca (close to the δ44/40Ca of plagioclase) exhibits the highest value of europium anomaly (Eu/Eu*). These observations are explained by the presence of variable relative proportions of residual clinopyroxene and plagioclase in the weathered diabase, due to selective weathering of these minerals. Since plagioclase and clinopyroxene could not be separated from the diabase, we measured the Ca isotopic compositions of a clinopyroxene (δ44/40Ca = 1.06‰) from the San Carlos mantle peridotite and plagioclase from the Chilka anorthosite complex (δ44/40Ca = 0.40‰) from the Eastern Ghats and used these isotopic values as representative of the compositions of these minerals in the diabase. Based on mixing models, using δ44/40Ca, Al/Ca and Sr/Ca of the plagioclase and clinopyroxene end-members, we estimate that the δ44/40Ca of the UW diabase can be explained by 31% clinopyroxene and 69% plagioclase, which is consistent with the modal proportion of these minerals in an average unweathered diabase. Based on mixing calculations, the δ44/40Ca, Sr/Ca and Al/Ca variability of the micro-drilled samples of the weathered diabase are explained by the presence of varying proportions of residual clinopyroxene (20-65%) and plagioclase (80-35%) in the weathered rock. This study demonstrates that selective weathering of major rock-forming minerals in nature can result in significant variation in δ44/40Ca in weathered rocks and that selective weathering of rock-forming minerals should be considered as an additional mechanism to explain the δ44/40Ca variability in rivers.