Isotopic microanalysis of seawater strontium in biogenic calcite to assess subsequent rehomogenisation during metamorphism

Precise LAM-MC-ICPMS in situ microanalyses of 87Sr/86Sr ratios in biogenic calcite of the bivalve ‘Atomodesma’ are reported for 14 horizons in low-grade metasediments of late Early Permian to late Late Permian age in South Island, New Zealand. ‘Atomodesma’ shells (and associated skeletal echinoderm calcite) in zeolite and possibly to lower prehnite-pumpellyite mineral facies material yield isotopic data that are interpreted as primary seawater 87Sr/86Sr values. Data for higher metamorphic grade (probable upper prehnite-pumpellyite to lawsonite-albite facies) samples consistently show evidence of Sr isotopic homogenisation between ‘Atomodesma’ calcite and host matrix and thus seawater 87Sr/86Sr values are not retained in shell debris under any circumstances.