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Research article2009Peer reviewed

Oxygen isotopic fractionation during inorganic calcite precipitation ― Effects of temperature, precipitation rate and pH

Dietzel, Martin; Tang, Jianwu; Leis, Albrecht; Koehler, Stephan J.

Abstract

Stable oxygen isotopic fractionation during inorganic calcite precipitation was experimentally studied by spontaneous precipitation at various pH (8.3 < pH < 10.5), precipitation rates (1.8 < log R < 4.4 μmol m− 2 h− 1) and temperatures (5, 25, and 40 °C) using the CO2diffusion technique.

The results show that the apparent stable oxygen isotopic fractionation factor between calcite and water (αcalcite–water) is affected by temperature, the pH of the solution, and the precipitation rate of calcite. Isotopic equilibrium is not maintained during spontaneous precipitation from the solution. Under isotopic non-equilibrium conditions, at a constant temperature and precipitation rate, apparent 1000lnαcalcite–water decreases with increasing pH of the solution. If the temperature and pH are held constant, apparent 1000lnαcalcite–watervalues decrease with elevated precipitation rates of calcite. At pH = 8.3, oxygen isotopic fractionation between inorganically precipitated calcite and water as a function of the precipitation rate (R) can be described by the expressions

1000lnαcalcite–water=-1.102logR+34.56" role="presentation" style="display: inline-block; line-height: normal; font-size: 14.4px; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border-width: 0px; border-style: initial; position: relative;">1000lnαcalcite–water=-1.102logR+34.56
1000lnαcalcite–water=-1.094logR+30.87" role="presentation" style="display: inline-block; line-height: normal; font-size: 14.4px; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border-width: 0px; border-style: initial; position: relative;">1000lnαcalcite–water=-1.094logR+30.87
1000lnαcalcite–water=-0.534logR+26.80" role="presentation" style="display: inline-block; line-height: normal; font-size: 14.4px; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border-width: 0px; border-style: initial; position: relative;">1000lnαcalcite–water=-0.534logR+26.80
at 5, 25, and 40 °C, respectively.

The impact of precipitation rate on 1000lnαcalcite–water value in our experiments clearly indicates a kinetic effect on oxygen isotopic fractionation during calcite precipitation from aqueous solution, even if calcite precipitated slowly from aqueous solution at the given temperature range. Our results support Coplen's work [Coplen T. B. (2007) Calibration of the calcite–water oxygen isotope geothermometer at Devils Hole, Nevada, a natural laboratory. Geochim. Cosmochim. Acta 71, 3948–3957], which indicates that the equilibrium oxygen isotopic fractionation factor might be greater than the commonly accepted value.

Keywords

Calcite; Oxygen isotopes; Isotopic fractionation factor; Precipitation rate

Published in

Chemical Geology
2009, Volume: 268, number: 1-2, pages: 107-115 Publisher: ELSEVIER SCIENCE BV

    UKÄ Subject classification

    Geosciences, Multidisciplinary

    Publication identifier

    DOI: https://doi.org/10.1016/j.chemgeo.2009.07.015

    Permanent link to this page (URI)

    https://res.slu.se/id/publ/89830