Large calcium isotope fractionations by zeolite minerals from Iceland

Claire J Nelson,Gabriella D Kitch,Tobias B Weisenberger,Andrew D Jacobson

doi:10.1038/s43247-021-00274-9

Claire J Nelson, Gabriella D Kitch + Show 2 more

Open Access

https://doi.org/10.1038/s43247-021-00274-9

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Abstract

Zeolites are secondary tectosilicates produced during the hydrothermal alteration of basalt. The minerals serve as major sinks of calcium, which readily exchanges with calcium from surrounding groundwater. However, no studies have specifically investigated the calcium isotope geochemistry (δ44/40Ca) of zeolites. Here, we report δ44/40Ca values for zeolites from East Iceland, where the minerals form during progressive burial of the lava pile. The zeolites show a δ44/40Ca range of 1.4‰, which strongly correlates with average mineral calcium-oxygen bond lengths. As this correlation appears most consistent with equilibrium isotope partitioning, our findings point toward developing a novel geothermometer for studying low-grade basalt metamorphism. The results also have significance for using calcium isotopes to trace basalt weathering, including its role in long-term climate regulation and application in carbon capture and storage, a leading strategy for mitigating anthropogenic climate change.

Highlights

Zeolites are secondary tectosilicates produced during the hydrothermal alteration of basalt
We report a large range of zeolite δ44/40Ca values, which is best explained by equilibrium isotope partitioning, given a strong observed correlation with Ca–O bond lengths
The primary minerals have a narrow range of δ44/40Ca values, which bracket those for bulk basalt

Summary

Introduction

Zeolites are secondary tectosilicates produced during the hydrothermal alteration of basalt. Calcium cycling during basalt weathering in subsurface hydrothermal systems plays a major role in regulating Earth’s long-term carbon cycle[49,50,51] Owing to their properties as ion-exchangers, absorbents, molecular sieves, and catalysts[52], zeolites have numerous environmental, industrial, and medical applications, including drinking water purification[53,54,55], nuclear waste management[56,57], contaminant transport mitigation[58,59,60], automotive emission reduction[61], and cancer treatment[62]

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