Caminite: An underestimated metastable Mg reservoir in hydrothermal recharge zones?

Mid-Oceanic ridge hydrothermal systems are in general significantly enriched in Ca due to leaching from the oceanic basement (albitization of anorthite), while Mg and SO4 are quantitatively removed by Mg-rich smectite and anhydrite formation respectively. During Meteor expedition 141 in September 2017 we sampled, close to a volcanic outcrop in the Terceira Rift (Azores, Central North Atlantic Ocean), pore fluids with a significantly different composition compared to typical Mid-Oceanic hydrothermal systems. Pore water Mg, SO4, and total alkalinity (TA) concentrations are significantly higher compared to seawater and a nearby reference core, while Ca concentrations stay at low values. The most straightforward way of interpreting the observed deviations is the dissolution of the prior hydrothermally formed high temperature (> 240°C) mineral caminite (MgSO4·0.25Mg(OH)2·0.2H2O). This interpretation is corroborated by a thorough investigation of fluid isotope systems (δ26Mg, δ30Si, δ34S, δ44/42Ca, and 87Sr/86Sr). Caminite is known from mineral assemblages with e.g. anhydrite and forms only under specific conditions such as high fluid temperatures and in altered oceanic crust with only little fresh basaltic glass present, which are generally met in the Terceira Rift. To date, no signs of extensive caminite formation and/or dissolution have been reported, caminite has only been once described before in the nature by Haymon and Kastner (1986). Our study is the first indication of an abundant occurrence of caminite. The results imply that element recycling through caminite might play a presently unrecognized role in element budgets of hydrothermal systems. Haymon, R. M., and Kastner, M., 1986, Caminite; a new magnesium-hydroxide-sulfate-hydrate mineral found in a submarine hydrothermal deposit, East Pacific Rise, 21 degrees N: American Mineralogist, v. 71, no. 5-6, p. 819-825