Portal de Periódicos da CAPES

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

2018; Elsevier BV; Volume: 483; Linguagem: Inglês

10.1016/j.chemgeo.2018.02.031

1872-6836

Anupam Banerjee, Ramananda Chakrabarti,

Groundwater and Isotope Geochemistry

Calcium stable isotopic compositions (δ44/40CaSRM 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 ~2.37 Ga old un-weathered 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 cross-cutting micro-fractures are identified from the micro-CT image. The older fracture is sealed with secondary mineral deposits. The δ44/40Ca values of the weathered samples range from 0.42‰ to 0.84‰ with samples showing both higher and lower values than that of the un-weathered diabase dike which shows a δ44/40Ca value of 0.65‰. The variation in δ44/40Ca in the weathered samples is significantly higher than the external reproducibility of our measurements (<0.1‰) based on multiple measurements of Ca isotopic standards SRM 915a, SRM 915b and seawater (NASS-6). Mass balance calculations based on Sr/Ca and δ44/40Ca values rule out the contributions of secondary silicate as well as carbonate minerals in causing the variability of δ44/40Ca in the weathered samples. The positive correlations between δ44/40Ca and Mg/Al, Mg/Na in the weathered samples further negate the possible contribution of secondary carbonates in causing the variability in δ44/40Ca, as Al and Na are unlikely to be present in carbonates. 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.