First oxygen isotopic data of coexisting glauconite and celadonite by NanoSIMS

This study presents the first nanoscale secondary ion mass spectrometry (NanoSIMS) oxygen isotopic analyses in coexisting glauconite and celadonite related to hydrothermal alteration of Jurassic pillow-lavas from the Subbetic (Betic Cordillera, South Spain). The slight differences in Fe and K content of these two micas allowed them to be differentiated by focused ion beam scanning electron microscopy (FIB-SEM) and extract them as separate 10 × 5 μm size and 1 μm thick lamellae of each mineral. This sample preparation eliminated the need of preliminary elemental mapping by NanoSIMS to identify the different minerals within the sample before the isotopic mapping. However, the small thickness of the studied lamellae caused their fast consumption during oxygen isotopic mapping. Together with the H-isotope fractionation that micas can undergo during heating and dehydroxylation this could prevent the later acquisition of deuterium measurements. NanoSIMS reported an average 18 O/ 16 O ratio for glauconite of 2.05 × 10 −3 (±1.14 × 10 −5 ), whereas the average ratio for celadonite was 2.02 × 10 −3 (±1.43 × 10 −5 ). The δ 18 O SMOW values obtained separately for glauconite (29 ‰ ±6) and celadonite (18 ‰ ±6), despite the standard deviations, demonstrate that NanoSIMS could be applicable for analysing the oxygen isotopic composition of coexisting small-size clay minerals within the same sample. According to these preliminary results, the studied glauconite and celadonite could have a partial overlap in their formation conditions, with celadonite beginning to crystallise just as glauconite crystallisation finishes.