The rapid resetting of the Ca isotopic signatures of calcite at ambient temperature during its congruent dissolution, precipitation, and at equilibrium

Eric H. Oelkers*, Philip A.E. Pogge von Strandmann, Vasileios Mavromatis

*Corresponding author for this work

Research output: Contribution to journalArticle

Abstract

This study provides direct experimental evidence of the resetting of the calcium (Ca) isotope signatures of calcite in the presence of an aqueous fluid during its congruent dissolution, precipitation, and at equilibrium at ambient temperatures over week-long timescales. Batch reactor experiments were performed at 25 °C in aqueous NaCl solutions; air or CO 2 -gas mixtures were bubbled through this fluid to fix pH. During congruent calcite dissolution, the fluid became enriched in isotopically heavy Ca, and the Ca isotope composition continued to become heavier after the fluid attained bulk chemical equilibrium with the mineral; the δ 44/42 Ca composition of the fluid was up to 0.8‰ higher than the dissolving calcite at the end of the dissolution experiments. Calcite precipitation was provoked by increasing the reactor fluid pH after chemical equilibrium had been attained via dissolution. Rayleigh isotope fractionation effects were observed immediately after the pH was increased and rapid calcite precipitation occurred. However, isotopic exchange continued after the system chemically equilibrated, eradicating this Rayleigh signal. Taken together, these observations 1) confirm dynamic mineral-fluid equilibrium (i.e. dissolution and precipitation occur at equal, non-zero rates at equilibrium), and 2) indicate that isotopic compositions of calcite can readily equilibrate even when this mineral is in bulk chemical equilibrium with its coexisting fluid. This latter observation suggests the preservation of paleo-environmental isotopic signatures in calcite may require a combination of the isolation of the fluid-mineral system from external chemical input and/or the existence of a yet to be defined calcite dissolution/precipitation inhibition mechanism.

Original languageEnglish
Pages (from-to)1-10
Number of pages10
JournalChemical Geology
Volume512
DOIs
Publication statusPublished - 5 May 2019

Keywords

  • Ca isotopes
  • Calcite
  • Fluidmineral interaction
  • Isotope exchange
  • Reaction rates

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology

Fingerprint Dive into the research topics of 'The rapid resetting of the Ca isotopic signatures of calcite at ambient temperature during its congruent dissolution, precipitation, and at equilibrium'. Together they form a unique fingerprint.

Cite this