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

Abstract

Calcium stable isotopic compositions (delta Ca-44/40(SRM 915a)) as well as selected major and trace element concentrations are reported for micro-drilled samples of a hand-specimen sized spheroidally weathered diabase (synonymously used as dolerite or microgabbro) from southern India. A sample of the similar to 2.37 Ga old unweathered diabase (UW) dike was also analyzed. X-ray micro-CT imaging of a representative portion of the weathered sample shows the presence of two dominant mineral phases, plagioclase and clinopyroxene, with minor proportions of an iron-bearing phase (possibly haematite or ilmenite). Two different generations of crosscutting micro-fractures are identified from the micro-CT image. The older fracture is sealed with secondary mineral deposits. The delta Ca-44/40 values of the weathered samples range from 0.42 parts per thousand to 0.84 parts per thousand with samples showing both higher and lower values than that of the un-weathered diabase dike which shows a delta Ca-44/40 value of 0.65 parts per thousand. The variation in delta Ca-44/40 in the weathered samples is significantly higher than the external reproducibility of our measurements (< 0.1 parts per thousand) based on multiple measurements of Ca isotopic standards SRM 915a, SRM 915b and seawater (NASS-6). Mass balance calculations based on Sr/Ca and delta Ca-44/40 values rule out the contributions of secondary silicate as well as carbonate minerals in causing the variability of delta Ca-44/40 in the weathered samples. The positive correlations between delta Ca-44/40 and Mg/Al, Mg/Na in the weathered samples further negate the possible contribution of secondary carbonates in causing the variability in delta Ca-44/40, as Al and Na are unlikely to be present in carbonates. The samples with lower delta Ca-44/40 show higher CIA (Chemical Index of Alteration) values and Al/Ca than the UW diabase while samples having higher delta Ca-44/40 show lower CIA and Al/Ca than the UW diabase. The weathered sample having the lowest delta Ca-44/40 (close to the delta Ca-44/40 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 (delta Ca-44/40= 1.06 parts per thousand) from the San Carlos mantle peridotite and plagioclase from the Chilka anorthosite complex (delta Ca-44/40= 0.40 parts per thousand) 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 delta Ca-44/40, Al/Ca and Sr/Ca of the plagioclase and clinopyroxene end-members, we estimate that the delta Ca-44/40 of the UW diabase can be explained by 31 parts per thousand clinopyroxene and 69 parts per thousand plagioclase, which is consistent with the modal proportion of these minerals in an average unweathered diabase. Based on mixing calculations, the delta Ca-44/40, 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 parts per thousand) and plagioclase (80-35 parts per thousand) in the weathered rock. This study demonstrates that selective weathering of major rock-forming minerals in nature can result in significant variation in delta Ca-44/40 in weathered rocks and that selective weathering of rock-forming minerals should be considered as an additional mechanism to explain the delta Ca-44/40 variability in rivers.