The intramolecular kinetic oxygen isotope fractionation between CO2 and
CO32- during reaction of phosphoric acid with natural smithsonite
(ZnCO3) and cerussite (PbCO3) has been determined between 25 and
72degreesC. While cerussite decomposes in phosphoric acid within a few
hours at 25degreesC, smithsonite reacts very slowly with the acid at
25degreesC providing yields of CO2 < 25% after 2 weeks. The low yields
result in a low precision for oxygen isotope measurements of the
acid-liberated CO2 ( +/-1.65parts per thousand, 1sigma, n = 9). The
yield and reproducibility of oxygen isotope values of the acid-liberated
CO2 from smithsonite can be improved, the latter to similar
to+/-0.15parts per thousand, by increasing the reaction temperature to
50degreesC for 12 h or to 72degreesC for I h. Our new phosphoric acid
fractionation factor for natural cerassite at 25degreesC deviates
significantly from a previously published value on synthetic material.
The temperature dependence of the oxygen isotope factionation factor,
alpha between acid-liberated CO2 and carbonate at 25 to 72degreesC is
given by the following equations
10001nalpha(phos) (CO2-cerrusite) = 5.13 (+/-0.15) + 4.79 (+/-0.15) x
10(5)/T-2
10001nalpha(CO2-smithsonite)(phos) = 3.96 (+/-0.21) + 6.69 (+/-0.22) x
10(5)/T-2
with temperature T in kelvin. A comparison with published equations of
the form 10001nalpha(CO2-carbonate)(phos) = A + B x 10(5)/T-2 for other
divalent metal carbonates shows that the factors B of slowly-reacting
carbonates from the rhombohedral calcite group (magnesite, siderite,
smithsonite and rhodochrosite) are very similar (6.7 +/- 0.2) and
distinct from those of fast-reacting minerals witherite, cerussite, and
strontianite of the orthorhombic aragonite group (4.5 +/- 0.3) and
calcite (5.6 +/- 0.1). These differences indicate a crystallographic
control on the temperature dependence of the kinetic oxygen isotope
fractionation between phosphoric acid liberated CO2 and carbonate.
Copyright (C) 2003 Elsevier Ltd.