Carbonate geothermometry is a fundamental tool for quantitative assessment of the
geothermal and geochemical evolution of diagenetic and hydrothermal systems, but it
remains difficult to obtain accurate and precise formation temperatures of low-temperature
calcite samples (below ~ 40 to 60 °C). Here, we apply three geothermometry methods
(∆ 47 -thermometry, nucleation-assisted fluid inclusion microthermometry—hereafter
NA-FIM—and oxygen isotope thermometry) to slow-growing subaqueous calcite spar samples
to cross-validate these methods down to 10 °C. Temperatures derived by NA-FIM and
Δ 47 -thermometry agree within the 95% confidence interval, except for one sample.
Regression analyses suggest that the real uncertainty of ∆ 47 -thermometry exceeds
the 1 SE analytical uncertainty and is around ± 6.6 °C for calcite spar that formed
at 10–50 °C. The application of δ 18 O thermometry was limited to a few samples that
contained sufficient primary fluid inclusions. It yielded broadly consistent results
for two samples with two other geothermometers, and showed higher temperature for
the third spar. We also found that calcite with steep rhombohedral morphologies is
characteristic of low temperatures (11–13 °C), whereas blunt rhombohedra prevail in
the 10–29 °C domain, and the scalenohedral habit dominates > 30 °C. This suggests
that the calcite crystal morphology can be used to qualitatively distinguish between
low- and higher-temperature calcite.
