Calcium Isotope Fractionation in Hydrothermal Systems

We present measurements of stable Ca isotopes (δ44/40Ca) from hydrothermal fluids (Long Valley, California) and epidote from fossil hydrothermal systems (Troodos and Betts Cove ophiolites). Hydrothermal fluids in Long Valley show a progressive increase in the heavy isotopes of Ca (δ44/40Ca +0.56‰ relative to the initial thermal fluid) with decreasing temperature, Ca concentration, Ca/Sr and CO2 concentration. The increase in the δ44/40Ca along the hydrothermal fluid flow path is potentially consistent with the precipitation of hydrothermal calcite (which would be isotopically lighter) or mixing between the thermal fluid and meteoric water. We favor the former explanation because non-linear relationships between CO2 concentrations and noble gasses suggest that decreasing CO2 concentrations are primarily due to reservoir degassing, which would likely drive calcite precipitation (e.g. Arnorsson cont. min. pet, 1978). Epidote mineral separates from the Betts Cove (Newfoundland, early Ordovician) and Troodos (Cyprus, Cretaceous) ophiolites are isotopically light relative to bulk silicate earth (δ44/40Ca ranges from -0.7 to 0.0‰). The epidote δ44/40Ca is not correlated with calculated fluid temperatures or 87Sr/86Sr measured in the epidote but is negatively correlated with the epidote Sr/Ca. Black smoker fluids, which are thought to be related to epidote formation in ophiolites, have δ44/40Ca of about 0-0.2‰, meaning that epidote Ca is consistently lighter than the inferred fluids from which they precipitate (Amini et al, GCA, 2008). To explain the complimentary Long Valley hydrothermal fluid and fossil epidote data there must be a mechanism for fractionating Ca isotopes at hydrothermal temperatures. Equilibrium fractionation of Ca isotopes should be close to 0‰ at high temperatures (100-400°C), implying that any Ca isotopic fractionation between fluid and hydrothermal minerals is likely a kinetic effect. Experimental data suggest that, for example, epidote equilibrium dissolution rates are about 0.3um/y (Wood and Walther, Science, 1983) while observations of epidote growth rates in geothermal systems are 73 um/y (Browne et al, Am Min, 1989). DePaolo (2009 Goldschmidt abstract) calculated that kinetic isotope fractionation between fluids and minerals should occur if net precipitation rates are greater than the equilibrium dissolution rate. If the estimates for dissolution and net precipitation rates for epidote are representative of most hydrothermal systems then epidote Ca should commonly have δ44/40Ca lighter than the associated fluid. Ocean ridge hydrothermal fluids should be slightly enriched in the heavy isotopes of Ca compared to oceanic crust as recently observed (Amini et al, GCA, 2008).